Bull. Soc. bot. Fr., 137, Actual. bot. (1), ...
Bull. Soc. bot. Fr., 137, Actual. bot. (1), 125-137, (IYYUJ.
Genetic of adaptation to drought of cultivated species
and consequences on plant breeding
by Jean-Luc B. KHALFAOUI
Research engineer with the CIRADIIRHO on secondment to ISRA, CNRA, Bambey, Senegal
Summary.- Among various methods used in wrestling with drought. genetic improvement holds
badly exploited cards which should enable it to play one of the leading parts. Among others, improvement based
directly on physiological characters of adaptation to drought is involved. For the most important cultivated species,
informations are gathered in form of the available data concerning genetic variability of these adaptive characters,
the nature of correlations between each other and between them and the production characters, and lastly their
heredity. This stage enables us to point out the blanks in our knowledge of these different components of selection,
to identify the limits of the classical selection method based on production in dry conditions and finally to work out
a more effective approach of genetic improvement of adapted complex ideotype.
This method needs an enlargment of the genetic variability which is studied, the application of
selection based on the various characters of adaptation and production, the parting of long-term improvement led
by recurrent selection with the short-term one led by classical varietal selection, and the parting of selection for
productivity
with selection on adaptive characters.
R&U~&.- Parmi les différentes méthodes utilisées pour lutter contre la sécheresse, l’amélioration
génétique possède des atouts mal exploités qui devraient lui permettre de jouer un rôle de première importance.
Parmi ceux-ci, l’application d’une amélioration directe des caractères physiologiques d’adaptation à la sécheresse
est particulièrement importante. Chez les principales espèces cultivees,
les principales données disponibles sont
présentées sur la variabilité génétique de ces caractères adaptatifs, sur la nature des corrélations existant entre
ceux-ci et avec les caractères de production et enfin sur leur hérédite. Cet examen permet de mettre en évidence
les lacunes dans nos connaissances de ces différentes composantes intervenant dans la selection, de mettre en
évidence les limites des méthodes de sélection classiques basées sur la production en conditions sèches, et
finalement de proposer une approche plus efficace de I’amélioration
génétique des idéotypes d’adaptation
complexes.
Cètte approche repose sur un Qlargissement de la variabilite genétique qui est travaillée, sur
l’application d’une sélection basée sur différents caractères d’adaptation et de production, sur la dissociation de
l’amélioration à long-terme menée par sélection récurrente, de l’amélioration a court-terme menée à l’aide des
méthodes de sélection classiques, et enfin sur ladissociation de la sélection pour la productivité de la sélection des
caractères adaptatifs.
Key words : drought -adaptation - selection - genetic - variability - heredity.
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* *
Q Société botanique de France 1990. ISSN 0181-1789.
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1 2 6
ACTUALITES BOTANIQUES
INTRODUCTION
In order to achieve an improvement of adaptation of cultivated species
to different types of drought they go through, it is clear that the involved methods
- plant breeding, improvement of farming tecnics, among others recourse to irri-
gation, development and management of soils - are to be parts of a whole.
Among these methods, breeding is abble to play a part of major
importance by creating better adapted varieties related to various drought
conditions. Its advantages, which are not sufflciently exploited, lies in the use of
a wide inter- and intra-species variability, in the application of strides of physio-
logy on the understanding of adaptive mechanisms and in the choice of breeding
methods adapted to each case.
Among these advantages, the application of physiology is particularly
neglected. Very few breeding programs from a11 over the world associate physio-
logy to their approach as a part of a whole, although such an association would,
without any doubt, present a great potential. This remark would apply to gcnetic
improvement of characters depending on the whole physiology of the plant. This
is the case of productivity.
The first signs of a reasoned use of “the physiological tool” for breeding
are becoming discernible. This use is one of the ways which should allow genetic
progress ; which has been for more than 60 years by breeding, to rcmain firm.
Other potential approaches, as far as we cari now identify them, deal with a bettcr
long term management of genetic variability and with the use of biotechnologies.
From a genetic point of view, three fondamental components, concer-
ning physiological characters of adaptation to drought Will shape the leading of
breeding. They are :
?? genetic variability of adaptive characters ;
??
nature of correlations between these characters ;
??
their heredity.
l.- GENETIC VARL4BILITIES OF ADAPTIVE CIIARACTER
As Ahmadi (1983) mentions it : “Adaptation to drought appears as the
result of numerous anatomical, morpbological, physiological and biochemical
characteristics, constituent or inductible, which interact in order to make possible
the keeping of growing and development processes under conditions of climates
and soils called drought”. In order to simplify, we Will cal1 “physiological characters
of adaptation” a11 these mechanisms.
In the data-boards presented in appendixes 1,2, 3 (Khalfaoui, 1988),
we list the various physiological characters of adaptation according to Levitt’s
classification (1981) and point out : their influence on productivity, their reversi-
bility, their aptitude for easy selection,
and their effectivencss in relation to
different types of drought. These data-boards extensions from Turner’s one
(1981), with a differcnt opinion concerning the influence of some characters on
productivity and their aptitude for easy selection.
Concerning the main species, a bibliographical study of the works led in
order to point out the existence of genetic variabilities in the manifestation of
adaptive characters, allows us to build the data-boards of appendixes 4,5,6. Their
analyse makes clear a great disparity between the levels of various species and
various adaptive characters, reached through research. For example, the gcnetic
J.L.B. KHALFAOUI
127
Appendix l.- Mechanisms of drought escape
- Earliness
- Shifting of
the sensitive
stage during
the cycle
C-J
difficult
- Developmental
0
easy to
plasticity
difficult
according
to species
-
- - negative effect ; +-= positive effect ; 0 = without effect ; ? = unknown effect ; (
) - uncertainly
established effect ; R = reversible ; I = irreversible ; PR = partially reversible.
variability of adaptive characters of the wheat is pretty well known ; on the
contrary, informations concerning Phaseolus vulgaris L. are very fragmentary. In
the same way, the genetic variabilities of the parameters of root development are
pretty well known for the main species although those concerning the keeping of
photosynthesis have been studied for very few species. Beyond making clear the
adaptive characters of species, this points out the important effort of research that
is still to be done in the field of characterization of their genetic variability.
Conseqnences for breeding
According to the type of drought which the varieties to be created Will
have to be adapted to, the choice of characters, in addition to agronomie charac-
ters, defines an ideotype of selection. The existence of a genetic variability at
disposa1 for each adaptive character is the necessary condition to their genetic
improvement. It is the reason why stomatal control of wheat (Fisher et al., 1979 ;
Sojka et al., 1979 ; Clarke et al., 1982) and rice (Ahmadi, 1983), glucidic reservcs
in the roots of peanut (Khalfaoui, 1988) and osmotic adjustment of rice (Culter et
al., 1980) cannot be improved for lack of sufficient genetic variability although
tbere are physiologically effective for each of these species.
2.- PIIYSIOLOGICAL AND GENETIC CORRELATIONS
BETWEEN CI-IARACTERS
We make a distinction between two kinds of correlations between
adaptive characters and between them and agronomie characters. The flrst one
regroups physiological direct correlations and the second one indirect genetic
correlations.
Direct physiological correlations cannot be modifled by selection. An
1 2 8
ACTUALITES BOTANIQUES
Appendix 2.- Mechanisms of drought avoidance
. I--
1Ste. .0,-.II
T
n. tI
-
.L,
-
111
xl
z
dS0,
Lh
L.,
.d n
Xe,1
m
I.‘
“4
L!E
-
-
+
?
+
+
&
+
?
+
+
+
?
:
+
+
?
+
+
+
+
?
+
+
+
+
0
?
+
+
+
+
0
+
?
+
+
+
(-)
?
+
+
+
(+)
?
+
+
+
0
?
+
+
+
+
?
+
+
+
fO
+
+
to -
?
+
?
-
?
-
-
Fegend : Appendix 1. WR E water reserve of the soil.
example is given by the opposition for tbe restriction of transpiration by closure
of stomats to the keeping of photosynthetic activity (Cowan et al., 1977). From a
genetic point of view, the phenomenon of pleiotropism is involved. It is the kind of
correlation with production under conditions of drought that the characters
studied must show, in order for them to be effcctivcly adaptive.
But, in numerous cases, a genetic indirect correlation betwecn charac-
ters determined by distinct genes is involved. Given expressions may have been
associated in the same genotypes under the effect of natural and human selcction
pressures. If these genes are linked, it Will be very difficult, even impossible, to
dissociate their allels by genetic recombination in segregating material. On thc
other hand, if there is no linkage effect between genes, associations of allels Will be
modified by genetic recombination.
J.L.B. KHALFAOUI
1 2 9
Appendix 3.- Mechanisms of drought tolerance
EFfcct
wdversi- Easiness
Droughl during Lhs
DPOUghf et
o n
bility in
CUItlIre
Produc- o
n
re- scree-
MECHBNISMS
tivity lieF of
ning
ear1y
letc
Stesdy Orought periods
stress
eisges stages smell
during
con-
OF rhe of fhc reins
the Ben- tinge
cu1turc ctl1ture
S‘tiYe
stage
- G e r m i n a t i o n under
high osmotic pressure
0
1
d i f f i c u l t +
0
+
0
0
- Tolerance to d e h y d r a t a t i o n
(and heat) :
7
+
+
+
+
+
s t a b i l i t y o f p r o t o p l a s m i c
membranes
(6)
?
easy
. s t a b i l i t y o f proteins
(among o t h e r s those involved
in photosynthesis)
1
d i f f i c u l t
M a i n t e n a n c e o f turgor :
difficult +
+ + +
+
. osmotic adjustment
R
. tissue efasticity
7
1
M a i n t e n a n c e o f
photosynthesis :
amount o f proteinsfunit o f
leaf are.3
+
1
easy
+
+
+
+
s p i k e l e t s
+
1
easy
; +
+ +
+
Maintenance of harvested
organs production :
0
c h a n g e i n a l l o c a t i o n o f
assimilates
(RI
difficult 0 + + +
+
c o m p e n s a t o r y effect o f
production components
N R
easy
0
0 + + +
Modification of
metabolism
7
R
difficult + + + + +
(plant hormones)
Legend : Appendix 1
This phenomenon has a direct consequence on physiological interpre-
tations. Actually, physiological studies dcal with varietics which have been created
by natural and human sclection.
Some correlations between characters and
between them and productivity may lead to think wong, about physiological direct
correlations, therefore links between causes and effccts, exist although they
actually depend on genetic indirect correlations. The risk of genetic indirect
correlations increases when, willing to present a complete approach of the species
bchaviour, research workers associate in their studies varieties representative of
ideotypes of adaptation very different drom each other. Without any linkage
cffcct, expressions Will be dissociated in the individuals issued from a cross
between these varietics. An example is given by the case of precocity of peanut
(Khalfaoui, 1988). A study of correlation for a group of varieties representative of
the collection has pointed out a very important positive correlation (- 0,98+++)
between flowcring precocity and fruit maturation precocity. After prime analysis,
.
1 3 0
ACTUALITES BOTANIQUES
Appendix 4.- Genetic variability of mechanisms of drought escape for the main studied species
-
- Earl iness
+ +
+ + + + + + + ++
+
+
- S h i f t i n g o f the
sensitive stage
during t h e c y c l e
+
- Developmental
plasticity
+ +
+ + + + + ++
+
+
+ - existence of genetic variability ; + - = lack of genetic variability ; 0 = absence of genetic
variability ; (
) = uncertainly established effect
this correlation seemed to be due to a direct physiological link between the two
characters. But, a study of the F3 generation conducted on segregating material
showed a total absence of phenotypical (0, 01) and genetic cor-relation (- 0, 02).
Thus, it seems important that, when studies are held in order to explain physio-
logical mechanisms involved in the plant behaviour, correlations discovered
between characters of the varieties have to be confirmed using segregating genetic
material before any physiological interpretation.
Consequences
for breeding
The kind of correlation between characters Will either facilitate or
impede breeding of the adaptation ideotype.
Direct physiological cor-relations or genetic ones with a strong linkage
effect cari be directly exploited if they benefit the plant. They allow, among othcr
things, to adopt early tests as done in the case of the link between roots develop-
ment at juvenile and grown up stages observed for many species (IIurd, 1974 ;
Townley-Smith et cd., 1977 ; Ahmadi, 1983 ; de Souza et al., 1984 ; Coleman,
1986 ; Khalfaoui, 1988). On the contrary, if they don’t benefit the plant, they Will
lead to try to find out a compromise. This is the case of physiological adaptive
characters presenting a cost towards productivity. This compromise Will be
obtained by applying a certain selection pressure on each character during the
whole breeding program.
When there is no strong linkage effect, a genetic correlation between
adaptive characters Will lead either to co-Select the allels in order to kcep their
association when one is of some benefit, or to counter-Select them in order to
separate them when the initial association is of negative effect towards the
available varieties.
J.L.B. KHALFAOUI
Appendix 5.- Genetic variabilities of mechanisms of drought escape for the main studied species
3.- IIEREDITY OF ADAPTIVE CIIARACTERS
In appendixes 7 and 8, the principal bibliographical data conccrning
the heredity of avoidance and tolerance characters are presented for some specics
mainly studied. The figures of these different studies are not easily comparable
with one another because of the disparity of genetic materials, of technics, of
gcnerations, of environmental conditions and of models of estimate ; this is the
reason why these figures have been replaced by qualitatives appreciations.
First of all, these data-boards draw the extremcly limited extend of our
knowledge in the field of the heredity of adaptive characters. Few species have
bcen studied and in a very fragmentary way. However, these data are necessary for
optimizing breeding methods adapted to each case. They belong to a field of
research that has to be better explored.
Several adaptive characters seem to depend on a reduce number of
genctic factors (major genes or major groups of genes). For example, the presence
of spiklets on ears @vans et al., 1972) and the accumulation of wax at the cuticle
1 3 2
ACTUALITES BOTANIQUES
Appendix 6.- Genetic variabilities of mechanisms of drought tolerance for the main studied species
MECHANISMS
- G e r m i n a t i o n under h i g h
osmotic pressure‘
- Tolerance to d e h y d r a t a t i o n
(and heat) :
. stability of protoplasmic
membranes
. s t a b i l i t y o f proteins
(among others those invol ved
in photosynthesis)
- M a i n t e n a n c e o f turgor :
. o s m o t i c adjustment
. tissue elasticity
M a i n t e n a n c e o f
photosynthesis :
. amount o f proteins/
u n i t o f l e a f area
. spikelets
M a i n t e n a n c e o f harvested
organs production :
. c h a n g e i n a l l o c a t i o n o f
assimilates
. c o m p e n s a t o r y effect o f
production components
Modification of metabolism
(plant hormones)
Legend : Appendix 4
level (Blum, 1975 ; Richard, 1982 ; Avato et al., 1985). The r-est of the characters
are polygenic heredity type with, in most cases, a marked preponderance of
additive genetic effects over non additive ones (dominante and epistasis). In spite
of a limited genetic complexity, due to additive effects, the estimates of broad and
narrow sense heritabilities are often low or very low because of marked interven-
tion of envir’onment in phenotypical expression and because of an important
interaction between genotypes and environments.
Conscquences
of breeding
When adaptive characters are determined by a very limited number of
genetic factors, it Will be easy to transfer a positive expression to agronomically
interesting varieties. The best adapted breeding method is, in this case, the Back-
cross one. Essentially, it allows transfer of the favourable allels with very littlc
modiftcation of the genotype of the interesting cultivar and thus, if pleiotropism is
J.L.B. KHALFAOUI
Appendix 7.- Heredity of mechanisms of drought avoidance for the main studied species
absent, of its general behaviour. The difficulty which is often met is due to
environmental conditions which affect genes expression. It is the case, for exam-
ple, for the quantity of epicuticular wax in sorghum (Blum et al., 1978 ; Blum,
1985) and the maturity at harvesting in peanut (Khalfaoui, 1988). This phenome-
non dictates to homogenize as much as possible the environmental conditions,
which is made possible by the limited number of plants worked upon by Back-
cross. When the in-vitro technic of genes transfer will be mastered, genetic
engineering will be an effective and rapid method of transfer to efficient varieties
of favourable behaviour determined by monogenic characters.
In the case of adaptive characters with polygenic heredity, the task Will
1 3 4
ACTUALITES BOTANIQUES
Appendix 8.- Heredity of mechanisms of drought toterance for the main studied species
T
LEOUMINWS
OTHER
MAIZE
SOYBEAN
COTTON
- Germination under
high osmotic
pressure
A >> NA
0
f e w g e n
- Tolerance to
d e h y d r a t a t i o n
(and heat) :
. stability of
protoplasmic
m e m b r a n e s
A= NA
A >> NA
1h, = M to H
hb = 1 tO M ’
be more diffmult. Two possibilities may arise. First, the ideotype of adaptation to
be created depends on the expression of a small number of physiological charactcrs
which are found in distinct varieties, agronomically interesting. Breeding cari bc
conducted successfully with classical breeding methods of pedigree type, perfor-
med with crosses between few genotypes. It is necessary to breed the favourable
characters from important effectives and to give preference to choices taking
family performances into account, in order to take into account wcak heritabili-
ties.
Secondly, the more complex ideotype concerns several adaptive mecha-
nisms in order to face the given type of drought in a compatible way with a good
agronomical value. The classical breeding method for adaptation to drought used
until today, consists in an indirect selection according to productivity under dry
conditions (Sullivan, 1972 ; O’Toole et al., 1977), of pedigree type with simple
crosses at its basis. But, in the case of a complex ideotype of adaptation, this
method is greatly limited ; for this reason, a new approach is rccommendcd
(Khalfaoui, 1985). This one is based on four principles :
1 - Extension of the genetic variability which is worked
On the classical breeding method is conducted with simple crosses
bctween unusually not more than three cultivars at its basis. But the number of
characters selected demands a multiplication of genotypes in order to regroup
favourable expression inside the genetic material which is workcd on. Moreover,
an important genetic variability determines, according to the fundamental theo-
rem of natural selection (Fisher, 1930), a high rate of genetic progress as gcnera-
tions follow one another.
2 -Application of direct selection on adaptive characters
The classical breeding mcthod deserves the term of indirect since it does
not estimate the level of adaptation of individuals by a determination of their
behaviours in relation to physiological adaptive characters but by paying atten-
J . L . B . K H A L F A O U I
1 3 5
tion to the effects on productivity under drought conditions of the resultant of the
expressions on adaptive characters. The more productive genotypes are the best
adapted ones. This approach has two drawbacks :
??
First, drought in a given place being qualitatively and quantitatively
variable according to the year, the selection pressure applied on each generation
is variable. Genetic progress is contingent and tends to hit the ceiling. Application
of direct selection on adaptive characters allows the homogenise the selection
pressure and makes steadier and more continuous the genetical progress.
?? The second drawback of indirect selection based on productivity
under drought conditions lies in heredity of adaptive characters. Indeed, we have
seen that most of these characters are polygenic. Heritability is variable according
to characters because of a genetic complexity degree and of an environmental
intervention degree in their expression which are variable. Applying indirect
selection on productivity, which is the resultant of the whole of the adaptive and
productive characters, the selection pressure acts in preference on the more
heritable characters, genetically the less complex. It has little influence on
gcnctically complex characters which tend to escape selection. This phenomenon,
taken into little account, is one of the main reasons of our slack times in genetic
progress that we face nowadays after natural and human selections both related
to productivity. One may suppose that this pause is probably not due to a
physiological definitive limitation but to a temporarly genetic one, because there
is still some potential genetic important spare progress linked to complex polygenic
cbaracters. This potential cari be exploited by applying direct selection on these
characters using methods allowing to work on additive genetic effects and also on
dominante and epistasis effects which are important and even sometimes prepon-
derant. Improvement of this type of genetic effects demands the carrying out of the
largest number of combinations of allels of the different genes in order to keep the
best genic associations. But the classical breeding methods of pedigree type lead to
a very rapid fixing of the combinations by homozygoty which impedes their
multiplication. In addition, the high selection pressure applied puts aside the
favorable combinations which are rare and whose expression is highly under
environmental influences. On the contrary, recurrent selection methods, by a
multiplication genic combinations thanks to repeated intercrosses, allows to a
multiplication of allelic “drawings”
and, thus the best combinations cari be
selected.
3 - Dissociation of long genetic improvemtxt from a short term one
From the initial genetical variability, the classical breeding methods of
pedigree type imposes a high selection pressure on individuals. It leads to a very
rapid restriction of genetical variability and thus to a pause in medium and long
ter-m genetical progress. On the contrary, a low selection pressure would allow to
preserve and optimize genetic variability but would prevent us from obtaining a
high rate of genetical progress and, therefore interesting varieties quickly popu-
larizable. Tndeed, selection pressure is the second component, with genetical
variability, which determines the rate of genetic progress between generation
(Fisher, 1930). Th’ f
1s undamental opposition of long against short term improve-
ment of complex genetic characters leads to dissociate long from short term
inprovement processes. Long tcrm improvement is managed by methods of
1 3 6
ACTUALITES BOTANIQUES
selection of recurrent type applied on populations of wide genetic variability.
Thanks to a moderate selection pressure they enable us to make optimum the
genetic variability. Short term improvement is managed by classical breeding
methods of new varieties taking for basis populations improved by recurrent se-
lection. Thanks to a high selection pressure they enable us to quickly create
improved popularizable varieties.
4 - Dissociation of breeding for productivity from selection based on
adaptive characters
We have seen that lots of adaptive physiological cbaracters create costs
for the plant productivity (Fisher, 1979 ; Richard, 1982) and are certainly a
genetic charge for productivity.
Secandly, conditions of water supply favorable to the breeding of some
pbysiological characters of adaptation to drought are different from conditions of
water supply favorable to the breeding of productivity (Quisenberry, 1983 ;
Richard, 1982). Indeed, physiological characters, completely or partially induc-
tive, need intervention of marked water stresses, as in the case of protoplasmic
resistance and osmotic regulation, while differences between genotypes for pro-
ductivity characters wiU appear at their maximum under optimum conditions of
watcr supply according to the concerned region which correspond to an absence
of hard drought.
Negative physiological correlations between some adaptive characters
and productivity and moreover the opposition of water supply conditions favora-
ble to brecd for adaptive characters to those favorable to breed for productivity,
lead to dissociate breeding for productivity from breeding for adaptive charac-
ters.
Recurrent selection is able to lead both of them according to two
processes which are developpcd at the same time. From a population, one part of
the individuals are screened under controled conditions for their physiological
adaptive characters to drought, after inducing drought. The advantage of its
application lies in its relative reproductibility which makes possible to keep a
constant selection pressure, important factor of selection optimization. A second
part of the individuals is selected for productivity and agronomical characters
according to the result of a comparative essay on the field. A topping-up irrigation
may be added in order to avoid serious stresses. Individuals which have passed the
physiological tests are then intercrossed with those chosen in the agronomie essay
in ordcr to associate thcir respective qualities in the same genotypcs. The result of
these crosses is an improvement population which Will be selectcd by the same
processes. This plan of selection may be called “divergent-convergent”.
Following the principle of dissociation of long from short term genetic
improvement, the improved populations by recurrent selection are used as the
starting point for classical breeding method of new varicties.
BIBLIOGRAPIIY
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L. 1 - Développement du système racinaire. 2 - SensibilitB stomatique au déficit hydrique. Agro.
Trop., 38, 11 O-l 22.
AVATO P., G. BIANCHI and G. MARIANI, 1985.- Epicuticular wax bloom varieties of Sorghum. Effect of plant age
on the enes expression. Genetica Agraia, 39,308-309.
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137
BLUM A., 1975.- Effect of the BM gene on epicuticular wax deposition and the spectral characteristics of Sorghum
leaves. SABRA0 J,, 7. 45-52.
BLUM A., 1985.- Selection criteria for improving drought resistance in Sorghum : a reviw. In : Actes Colloques
“Résistance à la sécheresse en milieu intertropical”. Sept. 1984, Dakar, 273-283.
BLUM A., K.F. SCHERIZ. R. TOLER, R.I. WELCH, D.T. ROSENOW, D.T. JOHNSON and L.E. CLARK, 1978.-
Selection for drought avoidance in Sorghum using infrared photography. Agron. J., 70, 474-477.
CLARKE J.M. andT.N. Mc CAIG, 1982.- Excised leaf water retention capability as an indicatorof drought resistance
of Tricicum genotypes. Can. J. Plant Sci., 62, 571-578.
CUTLER J.M., K.W. SHAHAN and P.L. SPETONKUS, 1980.- Influence of water deficits and osmotic adjustment
on leaf elongation in rice. Crop. SC., 20, 314-318.
COLEMAN W.K., 1986.- Water relations of the potato (Solarium tuberosum L.) cultivars Paritan and Shepody.
Amer. Potato J.. 263-276.
COWAN I.R. and G.O. FARWUHAR, 1977.- Stomatal function in relation to leaf metabolism and environment. In :
Actes Symp. Exp. Biol., 31,471-505.
DE SOUZA J.G., J.A. GILES and M.B. NETO, 1984.- Selection for water stress tolerance in upland cotton in the
northeast of Brasil. Trop. Agric., 61, 2-4.
EVAN L.T., J. BINGHAM, P. JACKSON and P. SUTHERLAND, 1972.- Effect of awns and drought on the supply
pf photosynthate and its distribution within wheat ears. Ann. Appt. Biol, 70, 67-76.
FISHER D.A., 1930.- The genetical theory of natural selection. Oxford Univ. Press, 272~.
FISHER D.A. and J.T. WOCD. 1979.- Drought resistance in spring wheat cultivars. Ill - Yield associations with
morpho-physiological traits. Aust. J. Res., 30, 1001-1020.
HURD E.A., 1974.- Phenotype and drought tolerance in wheat. Agric. Meteo., 14, 39-55.
KHALFAOUI J.L.B., 1985.- Leading of genetic improvement of drought adaptation according to its physiological
mecanisms. Oléagineux, 40, 329-334.
KHALFAOUI J.L.B., 1988.- Approche de I’amelioration genetique de l’adaptation à la sécheresse des espèces
cultivées en zones semi-arides. Application au cas de l’arachide (Arachis hypogaea L.) destinée à
la region seche du SerregaI. Th&e Doct. Science, Univ. Paris-Sud, 297p.
LEVITT J., 1981.- Response of plants to environmental stresses. Acad. Press, New York, London, 697p.
O’TOOLE J.C. and T.T. CHANG, 1977.- Drought resistance in cereals : rice, a case study. Proc. int. Conf. Stress
PhysioL of Plant usefulin FoodProv., Boyce Thompson Inst., Yonkers, N.Y., june 20-30, 1977.
QUISENBERRY J.E., 1983.- Breeding for drought resistance and plant water use efficiency. In : Crop Breeding.
D . R . W o o d ( e d . ) .
SOJKA R.E., L.H. STOLZY and R.A. FISHER, 1979.- Seasonal drought response of selected wheat cultivars.
Agron. J.. 73, 838-845.
SULLIVAN C.Y., 1972.- Mechanisms of heat and drought resistance in Sorghum and methods of measurement.
In : Sorgum in the seventies. N.C. Rao & L.R. House (eds.). Oxford and IBH Publishing CO., New
Delhi, 247-264.
DJRNER N.C., 1981.- The role of shoot characteristics in the drought resistance of trop plants. ln : Actes
Symposium “Principles and methods of trop improvement for drought resistance with emphais on
rice”. IRRI, Los Banos, may 1981, Philippines.
TOWNLEY-SMITH T.F. and E.A. HURD, 1977.-Testing and selection for drought resistance in wheat. In : Stress
Physiology in Crop Plants”. H. Musse1 & RC. Staples (eds.). Wiley-Interscience,
New York, 447-464.
WINTER S.R., J.T. MUSICK and K.B. PORTER, 1988.-Evaluation of screening techniques for breeding drought-
resistant winter wheat. Crop Sci, 28, 512.516.
Genetic of adaptation to drought of cultivated species
and consequences on plant breeding
by Jean-Luc B. KHALFAOUI
Research engineer with the CIRADIIRHO on secondment to ISRA, CNRA, Bambey, Senegal
Summary.- Among various methods used in wrestling with drought. genetic improvement holds
badly exploited cards which should enable it to play one of the leading parts. Among others, improvement based
directly on physiological characters of adaptation to drought is involved. For the most important cultivated species,
informations are gathered in form of the available data concerning genetic variability of these adaptive characters,
the nature of correlations between each other and between them and the production characters, and lastly their
heredity. This stage enables us to point out the blanks in our knowledge of these different components of selection,
to identify the limits of the classical selection method based on production in dry conditions and finally to work out
a more effective approach of genetic improvement of adapted complex ideotype.
This method needs an enlargment of the genetic variability which is studied, the application of
selection based on the various characters of adaptation and production, the parting of long-term improvement led
by recurrent selection with the short-term one led by classical varietal selection, and the parting of selection for
productivity
with selection on adaptive characters.
R&U~&.- Parmi les différentes méthodes utilisées pour lutter contre la sécheresse, l’amélioration
génétique possède des atouts mal exploités qui devraient lui permettre de jouer un rôle de première importance.
Parmi ceux-ci, l’application d’une amélioration directe des caractères physiologiques d’adaptation à la sécheresse
est particulièrement importante. Chez les principales espèces cultivees,
les principales données disponibles sont
présentées sur la variabilité génétique de ces caractères adaptatifs, sur la nature des corrélations existant entre
ceux-ci et avec les caractères de production et enfin sur leur hérédite. Cet examen permet de mettre en évidence
les lacunes dans nos connaissances de ces différentes composantes intervenant dans la selection, de mettre en
évidence les limites des méthodes de sélection classiques basées sur la production en conditions sèches, et
finalement de proposer une approche plus efficace de I’amélioration
génétique des idéotypes d’adaptation
complexes.
Cètte approche repose sur un Qlargissement de la variabilite genétique qui est travaillée, sur
l’application d’une sélection basée sur différents caractères d’adaptation et de production, sur la dissociation de
l’amélioration à long-terme menée par sélection récurrente, de l’amélioration a court-terme menée à l’aide des
méthodes de sélection classiques, et enfin sur ladissociation de la sélection pour la productivité de la sélection des
caractères adaptatifs.
Key words : drought -adaptation - selection - genetic - variability - heredity.
*
* *
Q Société botanique de France 1990. ISSN 0181-1789.
-
--
1 2 6
ACTUALITES BOTANIQUES
INTRODUCTION
In order to achieve an improvement of adaptation of cultivated species
to different types of drought they go through, it is clear that the involved methods
- plant breeding, improvement of farming tecnics, among others recourse to irri-
gation, development and management of soils - are to be parts of a whole.
Among these methods, breeding is abble to play a part of major
importance by creating better adapted varieties related to various drought
conditions. Its advantages, which are not sufflciently exploited, lies in the use of
a wide inter- and intra-species variability, in the application of strides of physio-
logy on the understanding of adaptive mechanisms and in the choice of breeding
methods adapted to each case.
Among these advantages, the application of physiology is particularly
neglected. Very few breeding programs from a11 over the world associate physio-
logy to their approach as a part of a whole, although such an association would,
without any doubt, present a great potential. This remark would apply to gcnetic
improvement of characters depending on the whole physiology of the plant. This
is the case of productivity.
The first signs of a reasoned use of “the physiological tool” for breeding
are becoming discernible. This use is one of the ways which should allow genetic
progress ; which has been for more than 60 years by breeding, to rcmain firm.
Other potential approaches, as far as we cari now identify them, deal with a bettcr
long term management of genetic variability and with the use of biotechnologies.
From a genetic point of view, three fondamental components, concer-
ning physiological characters of adaptation to drought Will shape the leading of
breeding. They are :
?? genetic variability of adaptive characters ;
??
nature of correlations between these characters ;
??
their heredity.
l.- GENETIC VARL4BILITIES OF ADAPTIVE CIIARACTER
As Ahmadi (1983) mentions it : “Adaptation to drought appears as the
result of numerous anatomical, morpbological, physiological and biochemical
characteristics, constituent or inductible, which interact in order to make possible
the keeping of growing and development processes under conditions of climates
and soils called drought”. In order to simplify, we Will cal1 “physiological characters
of adaptation” a11 these mechanisms.
In the data-boards presented in appendixes 1,2, 3 (Khalfaoui, 1988),
we list the various physiological characters of adaptation according to Levitt’s
classification (1981) and point out : their influence on productivity, their reversi-
bility, their aptitude for easy selection,
and their effectivencss in relation to
different types of drought. These data-boards extensions from Turner’s one
(1981), with a differcnt opinion concerning the influence of some characters on
productivity and their aptitude for easy selection.
Concerning the main species, a bibliographical study of the works led in
order to point out the existence of genetic variabilities in the manifestation of
adaptive characters, allows us to build the data-boards of appendixes 4,5,6. Their
analyse makes clear a great disparity between the levels of various species and
various adaptive characters, reached through research. For example, the gcnetic
J.L.B. KHALFAOUI
127
Appendix l.- Mechanisms of drought escape
- Earliness
- Shifting of
the sensitive
stage during
the cycle
C-J
difficult
- Developmental
0
easy to
plasticity
difficult
according
to species
-
- - negative effect ; +-= positive effect ; 0 = without effect ; ? = unknown effect ; (
) - uncertainly
established effect ; R = reversible ; I = irreversible ; PR = partially reversible.
variability of adaptive characters of the wheat is pretty well known ; on the
contrary, informations concerning Phaseolus vulgaris L. are very fragmentary. In
the same way, the genetic variabilities of the parameters of root development are
pretty well known for the main species although those concerning the keeping of
photosynthesis have been studied for very few species. Beyond making clear the
adaptive characters of species, this points out the important effort of research that
is still to be done in the field of characterization of their genetic variability.
Conseqnences for breeding
According to the type of drought which the varieties to be created Will
have to be adapted to, the choice of characters, in addition to agronomie charac-
ters, defines an ideotype of selection. The existence of a genetic variability at
disposa1 for each adaptive character is the necessary condition to their genetic
improvement. It is the reason why stomatal control of wheat (Fisher et al., 1979 ;
Sojka et al., 1979 ; Clarke et al., 1982) and rice (Ahmadi, 1983), glucidic reservcs
in the roots of peanut (Khalfaoui, 1988) and osmotic adjustment of rice (Culter et
al., 1980) cannot be improved for lack of sufficient genetic variability although
tbere are physiologically effective for each of these species.
2.- PIIYSIOLOGICAL AND GENETIC CORRELATIONS
BETWEEN CI-IARACTERS
We make a distinction between two kinds of correlations between
adaptive characters and between them and agronomie characters. The flrst one
regroups physiological direct correlations and the second one indirect genetic
correlations.
Direct physiological correlations cannot be modifled by selection. An
1 2 8
ACTUALITES BOTANIQUES
Appendix 2.- Mechanisms of drought avoidance
. I--
1Ste. .0,-.II
T
n. tI
-
.L,
-
111
xl
z
dS0,
Lh
L.,
.d n
Xe,1
m
I.‘
“4
L!E
-
-
+
?
+
+
&
+
?
+
+
+
?
:
+
+
?
+
+
+
+
?
+
+
+
+
0
?
+
+
+
+
0
+
?
+
+
+
(-)
?
+
+
+
(+)
?
+
+
+
0
?
+
+
+
+
?
+
+
+
fO
+
+
to -
?
+
?
-
?
-
-
Fegend : Appendix 1. WR E water reserve of the soil.
example is given by the opposition for tbe restriction of transpiration by closure
of stomats to the keeping of photosynthetic activity (Cowan et al., 1977). From a
genetic point of view, the phenomenon of pleiotropism is involved. It is the kind of
correlation with production under conditions of drought that the characters
studied must show, in order for them to be effcctivcly adaptive.
But, in numerous cases, a genetic indirect correlation betwecn charac-
ters determined by distinct genes is involved. Given expressions may have been
associated in the same genotypes under the effect of natural and human selcction
pressures. If these genes are linked, it Will be very difficult, even impossible, to
dissociate their allels by genetic recombination in segregating material. On thc
other hand, if there is no linkage effect between genes, associations of allels Will be
modified by genetic recombination.
J.L.B. KHALFAOUI
1 2 9
Appendix 3.- Mechanisms of drought tolerance
EFfcct
wdversi- Easiness
Droughl during Lhs
DPOUghf et
o n
bility in
CUItlIre
Produc- o
n
re- scree-
MECHBNISMS
tivity lieF of
ning
ear1y
letc
Stesdy Orought periods
stress
eisges stages smell
during
con-
OF rhe of fhc reins
the Ben- tinge
cu1turc ctl1ture
S‘tiYe
stage
- G e r m i n a t i o n under
high osmotic pressure
0
1
d i f f i c u l t +
0
+
0
0
- Tolerance to d e h y d r a t a t i o n
(and heat) :
7
+
+
+
+
+
s t a b i l i t y o f p r o t o p l a s m i c
membranes
(6)
?
easy
. s t a b i l i t y o f proteins
(among o t h e r s those involved
in photosynthesis)
1
d i f f i c u l t
M a i n t e n a n c e o f turgor :
difficult +
+ + +
+
. osmotic adjustment
R
. tissue efasticity
7
1
M a i n t e n a n c e o f
photosynthesis :
amount o f proteinsfunit o f
leaf are.3
+
1
easy
+
+
+
+
s p i k e l e t s
+
1
easy
; +
+ +
+
Maintenance of harvested
organs production :
0
c h a n g e i n a l l o c a t i o n o f
assimilates
(RI
difficult 0 + + +
+
c o m p e n s a t o r y effect o f
production components
N R
easy
0
0 + + +
Modification of
metabolism
7
R
difficult + + + + +
(plant hormones)
Legend : Appendix 1
This phenomenon has a direct consequence on physiological interpre-
tations. Actually, physiological studies dcal with varietics which have been created
by natural and human sclection.
Some correlations between characters and
between them and productivity may lead to think wong, about physiological direct
correlations, therefore links between causes and effccts, exist although they
actually depend on genetic indirect correlations. The risk of genetic indirect
correlations increases when, willing to present a complete approach of the species
bchaviour, research workers associate in their studies varieties representative of
ideotypes of adaptation very different drom each other. Without any linkage
cffcct, expressions Will be dissociated in the individuals issued from a cross
between these varietics. An example is given by the case of precocity of peanut
(Khalfaoui, 1988). A study of correlation for a group of varieties representative of
the collection has pointed out a very important positive correlation (- 0,98+++)
between flowcring precocity and fruit maturation precocity. After prime analysis,
.
1 3 0
ACTUALITES BOTANIQUES
Appendix 4.- Genetic variability of mechanisms of drought escape for the main studied species
-
- Earl iness
+ +
+ + + + + + + ++
+
+
- S h i f t i n g o f the
sensitive stage
during t h e c y c l e
+
- Developmental
plasticity
+ +
+ + + + + ++
+
+
+ - existence of genetic variability ; + - = lack of genetic variability ; 0 = absence of genetic
variability ; (
) = uncertainly established effect
this correlation seemed to be due to a direct physiological link between the two
characters. But, a study of the F3 generation conducted on segregating material
showed a total absence of phenotypical (0, 01) and genetic cor-relation (- 0, 02).
Thus, it seems important that, when studies are held in order to explain physio-
logical mechanisms involved in the plant behaviour, correlations discovered
between characters of the varieties have to be confirmed using segregating genetic
material before any physiological interpretation.
Consequences
for breeding
The kind of correlation between characters Will either facilitate or
impede breeding of the adaptation ideotype.
Direct physiological cor-relations or genetic ones with a strong linkage
effect cari be directly exploited if they benefit the plant. They allow, among othcr
things, to adopt early tests as done in the case of the link between roots develop-
ment at juvenile and grown up stages observed for many species (IIurd, 1974 ;
Townley-Smith et cd., 1977 ; Ahmadi, 1983 ; de Souza et al., 1984 ; Coleman,
1986 ; Khalfaoui, 1988). On the contrary, if they don’t benefit the plant, they Will
lead to try to find out a compromise. This is the case of physiological adaptive
characters presenting a cost towards productivity. This compromise Will be
obtained by applying a certain selection pressure on each character during the
whole breeding program.
When there is no strong linkage effect, a genetic correlation between
adaptive characters Will lead either to co-Select the allels in order to kcep their
association when one is of some benefit, or to counter-Select them in order to
separate them when the initial association is of negative effect towards the
available varieties.
J.L.B. KHALFAOUI
Appendix 5.- Genetic variabilities of mechanisms of drought escape for the main studied species
3.- IIEREDITY OF ADAPTIVE CIIARACTERS
In appendixes 7 and 8, the principal bibliographical data conccrning
the heredity of avoidance and tolerance characters are presented for some specics
mainly studied. The figures of these different studies are not easily comparable
with one another because of the disparity of genetic materials, of technics, of
gcnerations, of environmental conditions and of models of estimate ; this is the
reason why these figures have been replaced by qualitatives appreciations.
First of all, these data-boards draw the extremcly limited extend of our
knowledge in the field of the heredity of adaptive characters. Few species have
bcen studied and in a very fragmentary way. However, these data are necessary for
optimizing breeding methods adapted to each case. They belong to a field of
research that has to be better explored.
Several adaptive characters seem to depend on a reduce number of
genctic factors (major genes or major groups of genes). For example, the presence
of spiklets on ears @vans et al., 1972) and the accumulation of wax at the cuticle
1 3 2
ACTUALITES BOTANIQUES
Appendix 6.- Genetic variabilities of mechanisms of drought tolerance for the main studied species
MECHANISMS
- G e r m i n a t i o n under h i g h
osmotic pressure‘
- Tolerance to d e h y d r a t a t i o n
(and heat) :
. stability of protoplasmic
membranes
. s t a b i l i t y o f proteins
(among others those invol ved
in photosynthesis)
- M a i n t e n a n c e o f turgor :
. o s m o t i c adjustment
. tissue elasticity
M a i n t e n a n c e o f
photosynthesis :
. amount o f proteins/
u n i t o f l e a f area
. spikelets
M a i n t e n a n c e o f harvested
organs production :
. c h a n g e i n a l l o c a t i o n o f
assimilates
. c o m p e n s a t o r y effect o f
production components
Modification of metabolism
(plant hormones)
Legend : Appendix 4
level (Blum, 1975 ; Richard, 1982 ; Avato et al., 1985). The r-est of the characters
are polygenic heredity type with, in most cases, a marked preponderance of
additive genetic effects over non additive ones (dominante and epistasis). In spite
of a limited genetic complexity, due to additive effects, the estimates of broad and
narrow sense heritabilities are often low or very low because of marked interven-
tion of envir’onment in phenotypical expression and because of an important
interaction between genotypes and environments.
Conscquences
of breeding
When adaptive characters are determined by a very limited number of
genetic factors, it Will be easy to transfer a positive expression to agronomically
interesting varieties. The best adapted breeding method is, in this case, the Back-
cross one. Essentially, it allows transfer of the favourable allels with very littlc
modiftcation of the genotype of the interesting cultivar and thus, if pleiotropism is
J.L.B. KHALFAOUI
Appendix 7.- Heredity of mechanisms of drought avoidance for the main studied species
absent, of its general behaviour. The difficulty which is often met is due to
environmental conditions which affect genes expression. It is the case, for exam-
ple, for the quantity of epicuticular wax in sorghum (Blum et al., 1978 ; Blum,
1985) and the maturity at harvesting in peanut (Khalfaoui, 1988). This phenome-
non dictates to homogenize as much as possible the environmental conditions,
which is made possible by the limited number of plants worked upon by Back-
cross. When the in-vitro technic of genes transfer will be mastered, genetic
engineering will be an effective and rapid method of transfer to efficient varieties
of favourable behaviour determined by monogenic characters.
In the case of adaptive characters with polygenic heredity, the task Will
1 3 4
ACTUALITES BOTANIQUES
Appendix 8.- Heredity of mechanisms of drought toterance for the main studied species
T
LEOUMINWS
OTHER
MAIZE
SOYBEAN
COTTON
- Germination under
high osmotic
pressure
A >> NA
0
f e w g e n
- Tolerance to
d e h y d r a t a t i o n
(and heat) :
. stability of
protoplasmic
m e m b r a n e s
A= NA
A >> NA
1h, = M to H
hb = 1 tO M ’
be more diffmult. Two possibilities may arise. First, the ideotype of adaptation to
be created depends on the expression of a small number of physiological charactcrs
which are found in distinct varieties, agronomically interesting. Breeding cari bc
conducted successfully with classical breeding methods of pedigree type, perfor-
med with crosses between few genotypes. It is necessary to breed the favourable
characters from important effectives and to give preference to choices taking
family performances into account, in order to take into account wcak heritabili-
ties.
Secondly, the more complex ideotype concerns several adaptive mecha-
nisms in order to face the given type of drought in a compatible way with a good
agronomical value. The classical breeding method for adaptation to drought used
until today, consists in an indirect selection according to productivity under dry
conditions (Sullivan, 1972 ; O’Toole et al., 1977), of pedigree type with simple
crosses at its basis. But, in the case of a complex ideotype of adaptation, this
method is greatly limited ; for this reason, a new approach is rccommendcd
(Khalfaoui, 1985). This one is based on four principles :
1 - Extension of the genetic variability which is worked
On the classical breeding method is conducted with simple crosses
bctween unusually not more than three cultivars at its basis. But the number of
characters selected demands a multiplication of genotypes in order to regroup
favourable expression inside the genetic material which is workcd on. Moreover,
an important genetic variability determines, according to the fundamental theo-
rem of natural selection (Fisher, 1930), a high rate of genetic progress as gcnera-
tions follow one another.
2 -Application of direct selection on adaptive characters
The classical breeding mcthod deserves the term of indirect since it does
not estimate the level of adaptation of individuals by a determination of their
behaviours in relation to physiological adaptive characters but by paying atten-
J . L . B . K H A L F A O U I
1 3 5
tion to the effects on productivity under drought conditions of the resultant of the
expressions on adaptive characters. The more productive genotypes are the best
adapted ones. This approach has two drawbacks :
??
First, drought in a given place being qualitatively and quantitatively
variable according to the year, the selection pressure applied on each generation
is variable. Genetic progress is contingent and tends to hit the ceiling. Application
of direct selection on adaptive characters allows the homogenise the selection
pressure and makes steadier and more continuous the genetical progress.
?? The second drawback of indirect selection based on productivity
under drought conditions lies in heredity of adaptive characters. Indeed, we have
seen that most of these characters are polygenic. Heritability is variable according
to characters because of a genetic complexity degree and of an environmental
intervention degree in their expression which are variable. Applying indirect
selection on productivity, which is the resultant of the whole of the adaptive and
productive characters, the selection pressure acts in preference on the more
heritable characters, genetically the less complex. It has little influence on
gcnctically complex characters which tend to escape selection. This phenomenon,
taken into little account, is one of the main reasons of our slack times in genetic
progress that we face nowadays after natural and human selections both related
to productivity. One may suppose that this pause is probably not due to a
physiological definitive limitation but to a temporarly genetic one, because there
is still some potential genetic important spare progress linked to complex polygenic
cbaracters. This potential cari be exploited by applying direct selection on these
characters using methods allowing to work on additive genetic effects and also on
dominante and epistasis effects which are important and even sometimes prepon-
derant. Improvement of this type of genetic effects demands the carrying out of the
largest number of combinations of allels of the different genes in order to keep the
best genic associations. But the classical breeding methods of pedigree type lead to
a very rapid fixing of the combinations by homozygoty which impedes their
multiplication. In addition, the high selection pressure applied puts aside the
favorable combinations which are rare and whose expression is highly under
environmental influences. On the contrary, recurrent selection methods, by a
multiplication genic combinations thanks to repeated intercrosses, allows to a
multiplication of allelic “drawings”
and, thus the best combinations cari be
selected.
3 - Dissociation of long genetic improvemtxt from a short term one
From the initial genetical variability, the classical breeding methods of
pedigree type imposes a high selection pressure on individuals. It leads to a very
rapid restriction of genetical variability and thus to a pause in medium and long
ter-m genetical progress. On the contrary, a low selection pressure would allow to
preserve and optimize genetic variability but would prevent us from obtaining a
high rate of genetical progress and, therefore interesting varieties quickly popu-
larizable. Tndeed, selection pressure is the second component, with genetical
variability, which determines the rate of genetic progress between generation
(Fisher, 1930). Th’ f
1s undamental opposition of long against short term improve-
ment of complex genetic characters leads to dissociate long from short term
inprovement processes. Long tcrm improvement is managed by methods of
1 3 6
ACTUALITES BOTANIQUES
selection of recurrent type applied on populations of wide genetic variability.
Thanks to a moderate selection pressure they enable us to make optimum the
genetic variability. Short term improvement is managed by classical breeding
methods of new varieties taking for basis populations improved by recurrent se-
lection. Thanks to a high selection pressure they enable us to quickly create
improved popularizable varieties.
4 - Dissociation of breeding for productivity from selection based on
adaptive characters
We have seen that lots of adaptive physiological cbaracters create costs
for the plant productivity (Fisher, 1979 ; Richard, 1982) and are certainly a
genetic charge for productivity.
Secandly, conditions of water supply favorable to the breeding of some
pbysiological characters of adaptation to drought are different from conditions of
water supply favorable to the breeding of productivity (Quisenberry, 1983 ;
Richard, 1982). Indeed, physiological characters, completely or partially induc-
tive, need intervention of marked water stresses, as in the case of protoplasmic
resistance and osmotic regulation, while differences between genotypes for pro-
ductivity characters wiU appear at their maximum under optimum conditions of
watcr supply according to the concerned region which correspond to an absence
of hard drought.
Negative physiological correlations between some adaptive characters
and productivity and moreover the opposition of water supply conditions favora-
ble to brecd for adaptive characters to those favorable to breed for productivity,
lead to dissociate breeding for productivity from breeding for adaptive charac-
ters.
Recurrent selection is able to lead both of them according to two
processes which are developpcd at the same time. From a population, one part of
the individuals are screened under controled conditions for their physiological
adaptive characters to drought, after inducing drought. The advantage of its
application lies in its relative reproductibility which makes possible to keep a
constant selection pressure, important factor of selection optimization. A second
part of the individuals is selected for productivity and agronomical characters
according to the result of a comparative essay on the field. A topping-up irrigation
may be added in order to avoid serious stresses. Individuals which have passed the
physiological tests are then intercrossed with those chosen in the agronomie essay
in ordcr to associate thcir respective qualities in the same genotypcs. The result of
these crosses is an improvement population which Will be selectcd by the same
processes. This plan of selection may be called “divergent-convergent”.
Following the principle of dissociation of long from short term genetic
improvement, the improved populations by recurrent selection are used as the
starting point for classical breeding method of new varicties.
BIBLIOGRAPIIY
AHMADI N., 1983.7 VariabilitB et hérédit6 de mécanismes de tolérance à la skheresse chez le riz, Oriza safiva
L. 1 - Développement du système racinaire. 2 - SensibilitB stomatique au déficit hydrique. Agro.
Trop., 38, 11 O-l 22.
AVATO P., G. BIANCHI and G. MARIANI, 1985.- Epicuticular wax bloom varieties of Sorghum. Effect of plant age
on the enes expression. Genetica Agraia, 39,308-309.
J.L.B. KHALFAOUI
137
BLUM A., 1975.- Effect of the BM gene on epicuticular wax deposition and the spectral characteristics of Sorghum
leaves. SABRA0 J,, 7. 45-52.
BLUM A., 1985.- Selection criteria for improving drought resistance in Sorghum : a reviw. In : Actes Colloques
“Résistance à la sécheresse en milieu intertropical”. Sept. 1984, Dakar, 273-283.
BLUM A., K.F. SCHERIZ. R. TOLER, R.I. WELCH, D.T. ROSENOW, D.T. JOHNSON and L.E. CLARK, 1978.-
Selection for drought avoidance in Sorghum using infrared photography. Agron. J., 70, 474-477.
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