Vegetative propagation studies of gum arabic trees. ...
Vegetative propagation studies of gum arabic trees.
2. The vegetative propagation of adult Acacia senegal
P. DANTHU.‘” J. M. LEBLANC,’ S. BADJI’.” and J. P. COLONNA
’ Institut Sénégalais de Recherches Agricole.~ (ISRA), Direction des Recherches .sur les Produc-
tions Forestières ID. R. P. F.), R.P. 2312. Dakur Senegal;
j Centre ORSTOM, Dakar Senegal;
’ Direction des Eaux et Fort%, Dakar Senegal; * researcher at CIRADK.T.F.T., detached to
ISRAiD. R. P. F.
Keg words:iicaciu Aenegal, -cutting. adult trees
Abstract. Vegetative propagation of Aurcia senegul is possible from branch fragments taken
from thr crown of mature trees (13 years old), without causing ortet destruction.
The most responsive planting stock is taken from cuttings 15 cm long (with 12 to 15
nodes) and a diameter of 10 + 6 mm (wood of 2 to 4 years old). With this material, the
ablation of the leaves prior to planting is a factor which furthers development of the cutting’s
root system.
The development of the root system is also highly influenced by the time at which the
cutting is taken: results vary from 10% during the dry season to more than 70% during the
rainy season (June-Qctober).
Cuttings cari be taken trom branches 50 cm long. It is thus possible to preserve the
planting stock for 8 days in a continually humidified jute cloth and then to make the cuttings
at the time of estahlishment in a nursery. without changmg their rhizogenic aptitude.
The cuttings show characteristics of maturity (slow growth, plagiotropy, fructification). It is
nevertheless possible to preserve the collected genotypes in the field, or better, in planting
pots in the nurserg. SO as to further the process of physiologie rejuvenation with the aim of in
vitro cloning.
Résumé
Lc bouturage d’Acacia senegaf est possible à partir de fragments de rameaux prélevés dans le
houppier d’arbres adultes (13 ans). sans destruction de ceux-ci.
Le matériel végétal le plus réactif est constitué par des boutures de 15 cm de long (12 à 15
noeuds) et de 10 + 6 mm de diamètre (bois de deux j quatre ans). L’ablation des feuilles
avant mise en culture est un facteur favorable à l’enracinement des boutures.
Celui-ci est aussi très influencé par la date de prélèvement des boutures et il varie de lO’%
en saison sèche à plus de 709/0 en saison des pluies (juin-octobre).
La collecte de boutures peut se faire sous forme de fragments de rameaux de SO cm de
I~I$. Il est alors possible de conserver ce matériel pendant huit jours dans une toile maintenue
hutiide et d’y découper les boutures au moment de la mise en place en pépinière, sans altérer
leur aptitude rhizogène.
Les boutures présentent des caractères de maturité (croissance lente, plagiotropie, fructifi-
cation). II est cependant possible de conserver les génotypes ainsi mobilisés au champ, ou
mieux. dans des pots en pépinière. afin d’amplifier le processus de rajeunissement physiolo-
-: ,.__^^_....^ J.I-..~~.1--- ‘.
16
1. Introduction
Acacia senegul (L.) Willd. is a shrub-like species growing in the Sahelian
zone. It is adapted to an annual rainfall of 250 to 750 mm and eight consecu-
tive months of drought [Giffard, 19661. It fixes atmospheric nitrogen due to
its symbiosis with Rhizobium [Dreyfus and Dommergues. 198 11. Some
authors believe that it thus improves soi1 fertility [Gerakis and Tsangarakis.
19701. It has an anti-erosive action provides firewood. aerial fodder and
most importantly, gum arabic [Giffard, 1966: Cheema and Qadir. 19731.
As a result of the recurrent droughts in the Sahel since 1970. human and
animal pressure on this species have increased. The stands of gum trees are
disappearing. Gum procluctior. is decreasing to a point where Senegalese
exports which reached 9200 rons in 1971. are now at 800 tons [Mbaye.
19881.
TO re-establish acceptable levels of production which are capable of
generating foreign currency. and to reconstitute vegetative caver. reforesting
with A. senegul should be promoted. The search for high-yielding and
drought-resistant gum trees. which are also able to reproduce these charac-
teristics therefore constitutes a program of fundamental economic and
ecologic importance.
The selection of trees, based on gum yield characteristics. may be done
only once they have attained maturity. However. these mature trees do not
fully retain their ability to vegetatively reproduce. In particular, the ability of
the cutting to root decreases with the age of the ortet from which it was taken
[Franclet, 198 1; Bonga, 19821. The collection in proximity to the laboratory
of the first vegetative specimens of selected trees (genotypes) and then their
progressive rejuvenation thus constitutes a precondition for clonal multiplica-
tion
The aim of the present study is to refine these methods. It follows up the
work of Badji, Ndiaye, Danthu and Colonna (19911, taking into account the
various material difficulties imposed by field conditions.
Since the stump shoot possesses juvenile characteristics which favor the
attainment of our objectives pranclet, 19811, it requires that the tree be
thinned but this poses several constraints. One cari note that: (i) selected
trees must be tut while they are being exploited by the local population. (ii)
difficult and uncertain growth of the stumps IGiffard. 19661. (iii) necessary
protection of the stump from browsing livestock, (iv) the necessity of re-
peated site visits (prospection,, thinning, surveillance. harvesting). One must
thus focus on small branches which cari be harvested at one time, without
causing destruction to the ortet
This study therefore focuses on planting stock production from lengths of
small branches taken from the crowns of selected trees.
-
1
Il,-,,,
--:--- Ll-- .--T.-L --".---.-:.,, ..ln"t;nrr &firL. (lr ,l,@ll gp
17
Given that the prospection of the best subjects must be conducted
throughout the country and beyond, the time which the branches are in
transit between the sampling site and the cutting nursery could be quite long.
It would therefore be useful to determine if the selected planting stock cari be
harvested and conserved for several days prior to its planting, without losing
its capacity to vegetatively propagate.
Finally, survival of rooted cuttings should be tested to improve the long-
term possibilities for conservation of selected genotypes.
2. Materials and methods
Parts of branches were taken from the crowns of 13-year-old trees at eye
level. The trees were located at the Bandia Research Station (17”Ol’W,
14”34’N) in Senegal. These samples, except as otherwise indicated in the text,
were taken in May 1990. The tropical ferruginous soils found at the station
are depleted and degraded. The average annual rainfall is approximately
450 mm. A substantial deficit was registered in 1990, when incident pre-
cipitation was only 299 mm.
The samples were taken from pieces of branches 50 cm long and con-
served in a jute cloth which was kept humid. These branches were pruned to
produce cuttings 15 cm long (except as otherwise indicated in the text)
immediately prior to their placement in perforated polyethylene potting bags
(12 X 25 cm, when flat) which contained a substratum composed of basalt
chips (< 5 mm), Sand from Mbao and soi1 rich in humus from Mbao, in
volume proportions of 9/2/1. The cuttings were planted at a depth of 3 to 5
cm in the substratum and placed in a Gulfy cold frame [Grolleau, 19891,
under an awning. The unregulated interior temperature varied from 20 “C to
38 “C according to the season, the time of day and the amount of shade.
Atmospheric humidity in the cold frame was maintained at saturation point
by a twice daily spraying with water. A weekly anti-fungal treatment was also
applied: benlate (70 mg 1-l) and aliette (1 g 1-l) alternately.
Uniform hormonal treatments were applied before planting: the bases of
the cuttings were coated with talcum powder containing 4% ,8-indolyl butyric
acid (Rhizopon AA 4). This dosage was selected after trials covering a
spectrum of concentrations ranging from 0 to 8%.
The purpose of the first series of experiments was to determine which
planting stock was best adapted to the objectives enumerated above; the
effects of the following five factors were studied:
- The age of the branches, as indicated from the diameter of the cuttings.
Four classes were compared ( < 4 mm, 4-9.9 mm, lO- 15.9 mm and 16-
3 5 mm) frnm cgmnlpc takpn rl~srinn h/rcarr frnm whirh thn lon.,nr h-J L---
18
The cuttings were taken in May, had diameters of 10 to 16 mm and their
leaves had been removed at establishment time.
- The influence of leaves remaining on the cuttings. Cuttings (of 15 cm in
length. diameters of 1 O-l 6 mm, taken in May) with four leaves remaining
were compared with those which had been subjected to a complete
clenudation of the leaf system.
- The date of collection. Cuttings were taken at eight different times:
January, March, April, May, July, September, October and December.
The cuttings were 3 5 cm in length, had diameters of 1 O-1 6 mm and had
their leaves removed at the time of establishment.
- From the planting stock defined above, conservation trials were con-
ducted on pieces of branches which were maintained in a permanently
humid jute cloth. The objective was to determine how long it was possible
to preserve the cuttings before they lost their capacity to survive or
produce roots. The duration of conservation was examined over six time
periods: 0,2,4,6, and 8 days.
The objective of the final experiment was to study the behaviour and
survival of rooted cuttings after transplanting. This was done in order to gain
an idea of the possibilities for long-term conservation of the genotypes (head
of clone) to be propagated. Two conservation methods were compared: (i) in
20 litre clay pots containing a mixture (li2. v/v) of soi1 rich in humus and
sancl from Mbao which was watered daily, (ii) in the field, after plantation in
small pit-holes (20 X 20 X 30 cm) at the Randia Station, during the early
part of the rainy season (August 14, 1990). These were planted after cumula-
tive precipitation reached 80 mm. The plants thus received only natural
precipitation (2 19 mm).
The parameters measured during the experiments were:
- The survival of the cuttings. A cutting was considered viable if it had at
least one green leaf.
- The number of cuttings which sprouted at least one shoot.
- The number of cuttings which took root. Le. having at least one root
visible through the polyethylene bag.
The experimental design was full randomization, in each of two separate
blocks, each of which contained 28 to 30 cuttings for each protocol described
above. The analyses of variante and the comparisons of means were con-
ducted after angular transformation of frequencies. In the tables (for each
parameter) and figures, the values having the same index (a, b or c) belong to
the same homogeneous group ;as established by the Newman-Keuls test at a
5% level of significance. The confidence interval of the means is 95%.
19
3. Results
3.1. Definition qf‘the most suitahle plant@ stock
Diumeter qf rumets
When the cuttings have a diameter of less than 4 mm (sprouted during the
year of sampling), their survival rate after 90 days is zero (Table 1). Half of
the large diameter cuttings (> 16 mm) are viable three months after esta-
blishment. A large majority of these cuttings produced at least one branch,
however none of them produced roots.
Tuble 1. Survival. ramification and rooting of Acwiu srnegal cuttings, as a function of the
diameter of the ramet, 90 days after planting (cuttings of 15 cm. taken in May from which the
leaves had been removed at establishment).
Diameter of
Number of
Number of
Number of
cuttings
surviving
cuttings with
rooted cuttings
cuttings
new shoots
(mm)
(TO)
( %, j
(%J
<4
o c
-
-
4-9.9
Y3a
30a
15a
IO-15.9
68b
36a
3Ya
16-25
5Ob
43a
Oh
For the two intermediate classes (4 to 9.9 mm and 10 to 16 mm), the
survival rate is significantly lower for cuttings of larger diameters. However,
in the two cases, approximately a third of the cuttings produced at least one
root and one new shoot.
Length of cuttings
The study of the influence of cutting length, demonstrates the poor perform-
ance of long cuttings (23 cm) which dry out very rapidly and die; only 7% of
them survived and had set roots after 90 days (Table 2). Cuttings of 7 cm
show a good capacity to set roots but sprout few shoots. The best com-
promise is obtained with cuttings of 15 cm, as a11 those which established
roots three months after planting (approximately 1/3) also produced new
shoots.
Presence of leaves
Table 3 illustrates that the rooting and survival rates are significantly higher
when the leaves are removed from the cuttings. After leaf removal, 68% of
4L,. ,...A+:--- -..-_. :..- ..-2 1no1 ^_A .--- I^ rl...-. --- - -rl-~
P1
11.1
20
Tub/e 2. Survival, ramification and rooting of Acaria scnegul cuttings, as a function of their
length. 90 days after planting (cuttings 1 5 cm long. diameter 1 O- 15.9 mm. taken in May from
which the leaves had been removed at establishment).
Length of
Number of
Number of
Number of
cuttings
surviving
cuttings with
rooted cuttings
cuttings
new shoots
(cm)
(%)
(“h)
(%)
7
57a
7b
47a
15
68a
36a
39a
23
7b
l b
70
Table .3. Effect of ablation of leaves on the survival, ramification and rooting of A. sencgal
cuttings 90 days after planting (cuttings 15 cm long, diameter 1 O- 15.9 mm, taken in May).
Condition of
Number of
Number of
Numher of
foliage at
surviving
cuttings with
rooted cuttings
planting time
cuttings
new shoots
VI
@)
v4
No leaves
68a
36a
39a
With leaves
39b
30a
llb
Period of collection
Cuttings were planted which were taken from the same tree at different times
of the year. The eight dates selected represent four distinct climatic periods:
the peak dry season (January-March), the peak rainy season (June-Octo-
ber) and two intermediary dates.
Rooting was optimal (more than 70%) when cuttings were taken during
the rainy season (Fig. 1) and minimal but net zero (about 10%) during the
dry season. During the month of May, the rooting rate was 30 to 4O%, which
is illustrated in the results contained in Tables I,2, and 3.
Three months after collection, regardless of the date on which the sample
was taken, the cuttings manifest a root system having on average 25 f 7
roots and a length of 7.2 $I 3.1 cm without callus (Photograph 1). Their
tropism was very variable: the same cutting possesses horizontally oriented
roots (angle in relation to the vertical of 2 60’) and geotropic roots (angle
G 30”); none of them had developed a vertical system.
The cuttings produced on average 1.5 f 0.4 new shoots. Only one shoot
,...+ ,.ç G-7 _..A.., ,T~+h,+,,..-.:,,ll., J nmml~ fr\\rmnA ..Ath th, T,P,+;cQ~ nf -< -in’\\ anrl
80
70
60
50
T
L 40
c”
.3e 30
2
20
10
0
A
B
I
C
t
D
J
FM
A M J
I
J
A
S
O
N
D
Month
Fig. 1. Rcoting success of A. senegal cuttings (15 cm long, diameter 10-15.9 mm) 90 days
after planting as a lünction of the month or season in which the cuttings were collected (A:
mid dry season, B: end dry season. C: rainy season, D: early dry season).
explant or from the newly formed shoots (Photograph 3). These may develop
into pods containing seeds (Photograph 4).
Impact of storage time on sections of shoots taken j?om the field
From 35% to 46% of the cuttings made from sections of branchlings (diame-
ter of 10-15.9 mm) kept in a humid cloth during 2, 4, 6, or 8 days rooted
and produced new shoots branchlings after 90 days. These results do not
show a significant difference from the rate of root establishment measured in
the control group (cuttings established directly following sampling): 36%.
3.2. Conservation of clonal stock plant
Phorqrqh 1. Ro«ted A . senegol cutting showing ne\\+ shoot r~~rowth. threc months aftcr
planting (X0.3).
Photogruph 2. One-year-old A. .rene~ul cutting. conservrd in a pot (XO.OS).
fhotograph 3. A ilowering A. .seryq~rl cutting. threc months after planting (X0.25).
f’hotogruph 4. Fruiting A. .scneq~/ cuttiq. four months nt’tcr plantinp (XC)..;).
field remained alive (Table 4). They thus tolerated a period of seven months
without water, in addition to a rainy season which was well below average
(see ‘Materials and methods’). Moreover. they were sutjected to attacks by
herbivores such as hares and myriapods. However, the cuttings which
7uhle 4. Survival percentage of I-year-old cuttings from Acacia smegd as a function of
method of conservation (cuttings taken in May 19YO).
Conservation method
Waler source
Survival after 1 year (TO)
In nursery
Daily watering
8Sa
In field
Rainfall
33b
4. Discussion and conclusion
This study shows that it is possible to propagate mature A. senegal(l3 years of
age when the life-span of this species doesn’t excess 25 years [Aubreville,
19501) from sections of Iigneous shoots taken from the crowns of selected
trees without provoking destruction of the ortet. It confirms the results
obtained from four year old trees of the same species [Badji et al., 19911 and
is similar to work conducted on adult Fuidherbia albida [Danthu, 19911
branch fragments and roots.
The optimal planting stock is composed of sections of branchlings of
10 f 6 mm in diameter and 15 cm in length (having 12 to 15 nodes).
The rooting success after 90 days of establishment was very dependent on
the time at which the cuttings were taken. It varied between 10% for those
cm during the dry season, to 70% for those tut during the rainy season, thus
confirming the observations of Badji et al. 119911. Two variables might
explain these differences: the physiological state of the planting stock at the
time the sample was harvested and the cultural conditions, in particular, the
temperature inside the cold frame. However, it is not possible under our
experimental protocol to disassociate these two factors.
The success of root establishment of our cuttings was significantly higher
when their leaves were removed prior to planting. This result does not corres-
pond with those of numerous other studies of ligneous propagation [Reuveni
and Raviv, 198 1; Leakey et al., 1982; Oduol and Akunda. 19SSl nor is it in
accord with those results reported by Badji et al. 119911 which treated small
diameter cuttings (<4 mm) from Acacia senegul. After 60 days, these
authors obtained: (i) a survival rate equal to 1 1.5% and a zero rate of root
establishment for completely defoliated cuttings; (ii) a survival rate of 62%
and a 12% rate of root establishment for cuttings which retained four leaves.
This contradiction may be less real than apparent. It is probable that small
diameter cuttings contain insufficient reserves. Their metabolic functions
cannot continue for two to three months without the contribution of photo-
synthesis. Moreover, one cari see in Table 1 that no cutting with a diameter
1
.l
1
1. ,\\A 1
r
,
A..
24
four times greater contain sufficient reserves to survive and also develop a
root system due to the effect of rhizogenic substances.
One cari also note that the works of Paton et al. [1971] showed that the
influence of mature leaves of Eucalyptus grandis presented an impediment to
root establishment. The results obtained here could also therefore be ex-
plained by the possible inhibiting influence of the mature leaves.
From a practical viewpoint, this capacity of large diameter cuttings to root
in the absence of leaves allows their conservation in a humid cloth at least
one week before use. During such a period of conservation, the leaves die
and it is necessary to remove them before planting without altering the
rhizogenic capacity of the cuttings. Thus, one has eight days to transfer the
pieces of small branches from the sampling sites to the nursery.
Once rooted, the cuttings cari be kept during a long period of time in the
field or in pots in the nursery. The latter solution is preferable as plant
survival is improved: it is possible to supplement natural precipitation and
more effectively protect the plants from parasites and herbivores.
The growth of rooted cuttings is generally slow. They are often plagiotropic
and show, in numerous cases, inflorescence. Both observations illustrate the
state of physiological maturity of this planting stock [Franclet, 19811 and the
necessity to initiate a process of rejuvenation in order to conduct E’n vitro
cloning of this material. Studies in this area are presently being undertaken.
Acknowledgements
This study was financially supported by the C.R.D.I. (The Senegal Vegetative
Multiplication Project, n” 3-P-57-0267), the CIRAD (“FAC-Binôme” no
467%650), and by CEE (contract no TSL” 0169). The authors would like to
thank J. Rousse1 and the late A. Grolleau for their knowledgeable advice, A.
Sarr and M. Sagna for their technical assistance and L. Livingston for the
translation of this article.
References
Aubreville A (1950) Flore forestière soudano guinéenne. Société d’Editions Géographiques
Maritimes et coloniales, Paris, 523 pp
Badji S, Ndiaye 1, Danthu P and Colonna JP (1991) Vegetative propagation studies of gum
arabic trees. 1. Propagation of Acacia senegul (L.) Willd. using lignified cuttings of small
diameter with eight nodes. Agroforestry Systems 14: 183- 1 Y 1
Bonga JM (1982) Vegetative propagation in relation to juvenility, maturity and rejuvenation.
In: Bonga JM and Durzan DJ, eds. Tissue Culture in Forestry, Martinus Nijhoff/Dr W.
r....,. D..Ll:,.L-..- T-L- LT ^^..^ -- 707 A,?
25
Danthu P (1991) Mobilisation de Fuidherbiu alhidu adulte par bouturage de rameaux et de
racines. Atelier Faidherbiu albidu. Niamey, 22-26 avril, 199 1
Dreyfus BL and Dommergues YR (1981) Nodulation of Acaciu species by fast- and slow-
growing strains of Rhizobium. Applied Environmental Microbiology 4 1: 97-9Y
Franclet A (19X I ) Rajeunissement et propagation végétative des ligneux. Annales AFOCEL
1980: 1 l-4 1
Gerakis PA and Tsangarakis CZ (1970) The influence of Acaciu senegul on the fertility of a
sand sheet (‘GO~‘) soi1 in the central Sudan. Plant and Soi1 33: 8 l-86
Giffard PL (1966) Les gommiers: Acacia senegrrl Willd.. Acaciu iueta R. Br. Bois et Forêts des
Tropiques 105: 2 l-32
Grolleau A (1989) Contribution à l’étude de la multiplication végétative par greffage du karité.
Bois et Forêts des Tropiques 222: 3X -40
Leakey RRB. Chapman VR and Longman KA ( 19X 1) Physiological studies for tropical tree
improvement and conservation. Factors affecting root initiation in cuttings of Triplochiton
scleroxylon K. Schum. Forest Ecology and Management 4: 53-66
Mbaye 1 (1989) Situation et organisation du commerce international de la gomme arabique.
In: ISRA. ed, SYGGA III-Troisième symposium sous regional sur lc gommier et la gomme
arabique. 25-28 octobre. 1988, Saint-Louis, Senegal, pp 247-255
Oduol PA and Akunda E (198X) Vegetative propagation of Sesbania Sesban by cuttings.
Agroforestry Systems 6: 283-28X
Paton DM, Willing RR, Nicholls W and Pryor LD (1970) Rooting of stem cuttings of
Eucalyptus: a rooting inhibitor in adult tissue. Australian Journal of Botany 18: 175-l X3
Reuveni 0 and Raviv M (198 1) Importance of leaf retention to rooting of avocado cuttings.
Journal of the American Society for Horticultural Science 106: 127- 130
2. The vegetative propagation of adult Acacia senegal
P. DANTHU.‘” J. M. LEBLANC,’ S. BADJI’.” and J. P. COLONNA
’ Institut Sénégalais de Recherches Agricole.~ (ISRA), Direction des Recherches .sur les Produc-
tions Forestières ID. R. P. F.), R.P. 2312. Dakur Senegal;
j Centre ORSTOM, Dakar Senegal;
’ Direction des Eaux et Fort%, Dakar Senegal; * researcher at CIRADK.T.F.T., detached to
ISRAiD. R. P. F.
Keg words:iicaciu Aenegal, -cutting. adult trees
Abstract. Vegetative propagation of Aurcia senegul is possible from branch fragments taken
from thr crown of mature trees (13 years old), without causing ortet destruction.
The most responsive planting stock is taken from cuttings 15 cm long (with 12 to 15
nodes) and a diameter of 10 + 6 mm (wood of 2 to 4 years old). With this material, the
ablation of the leaves prior to planting is a factor which furthers development of the cutting’s
root system.
The development of the root system is also highly influenced by the time at which the
cutting is taken: results vary from 10% during the dry season to more than 70% during the
rainy season (June-Qctober).
Cuttings cari be taken trom branches 50 cm long. It is thus possible to preserve the
planting stock for 8 days in a continually humidified jute cloth and then to make the cuttings
at the time of estahlishment in a nursery. without changmg their rhizogenic aptitude.
The cuttings show characteristics of maturity (slow growth, plagiotropy, fructification). It is
nevertheless possible to preserve the collected genotypes in the field, or better, in planting
pots in the nurserg. SO as to further the process of physiologie rejuvenation with the aim of in
vitro cloning.
Résumé
Lc bouturage d’Acacia senegaf est possible à partir de fragments de rameaux prélevés dans le
houppier d’arbres adultes (13 ans). sans destruction de ceux-ci.
Le matériel végétal le plus réactif est constitué par des boutures de 15 cm de long (12 à 15
noeuds) et de 10 + 6 mm de diamètre (bois de deux j quatre ans). L’ablation des feuilles
avant mise en culture est un facteur favorable à l’enracinement des boutures.
Celui-ci est aussi très influencé par la date de prélèvement des boutures et il varie de lO’%
en saison sèche à plus de 709/0 en saison des pluies (juin-octobre).
La collecte de boutures peut se faire sous forme de fragments de rameaux de SO cm de
I~I$. Il est alors possible de conserver ce matériel pendant huit jours dans une toile maintenue
hutiide et d’y découper les boutures au moment de la mise en place en pépinière, sans altérer
leur aptitude rhizogène.
Les boutures présentent des caractères de maturité (croissance lente, plagiotropie, fructifi-
cation). II est cependant possible de conserver les génotypes ainsi mobilisés au champ, ou
mieux. dans des pots en pépinière. afin d’amplifier le processus de rajeunissement physiolo-
-: ,.__^^_....^ J.I-..~~.1--- ‘.
16
1. Introduction
Acacia senegul (L.) Willd. is a shrub-like species growing in the Sahelian
zone. It is adapted to an annual rainfall of 250 to 750 mm and eight consecu-
tive months of drought [Giffard, 19661. It fixes atmospheric nitrogen due to
its symbiosis with Rhizobium [Dreyfus and Dommergues. 198 11. Some
authors believe that it thus improves soi1 fertility [Gerakis and Tsangarakis.
19701. It has an anti-erosive action provides firewood. aerial fodder and
most importantly, gum arabic [Giffard, 1966: Cheema and Qadir. 19731.
As a result of the recurrent droughts in the Sahel since 1970. human and
animal pressure on this species have increased. The stands of gum trees are
disappearing. Gum procluctior. is decreasing to a point where Senegalese
exports which reached 9200 rons in 1971. are now at 800 tons [Mbaye.
19881.
TO re-establish acceptable levels of production which are capable of
generating foreign currency. and to reconstitute vegetative caver. reforesting
with A. senegul should be promoted. The search for high-yielding and
drought-resistant gum trees. which are also able to reproduce these charac-
teristics therefore constitutes a program of fundamental economic and
ecologic importance.
The selection of trees, based on gum yield characteristics. may be done
only once they have attained maturity. However. these mature trees do not
fully retain their ability to vegetatively reproduce. In particular, the ability of
the cutting to root decreases with the age of the ortet from which it was taken
[Franclet, 198 1; Bonga, 19821. The collection in proximity to the laboratory
of the first vegetative specimens of selected trees (genotypes) and then their
progressive rejuvenation thus constitutes a precondition for clonal multiplica-
tion
The aim of the present study is to refine these methods. It follows up the
work of Badji, Ndiaye, Danthu and Colonna (19911, taking into account the
various material difficulties imposed by field conditions.
Since the stump shoot possesses juvenile characteristics which favor the
attainment of our objectives pranclet, 19811, it requires that the tree be
thinned but this poses several constraints. One cari note that: (i) selected
trees must be tut while they are being exploited by the local population. (ii)
difficult and uncertain growth of the stumps IGiffard. 19661. (iii) necessary
protection of the stump from browsing livestock, (iv) the necessity of re-
peated site visits (prospection,, thinning, surveillance. harvesting). One must
thus focus on small branches which cari be harvested at one time, without
causing destruction to the ortet
This study therefore focuses on planting stock production from lengths of
small branches taken from the crowns of selected trees.
-
1
Il,-,,,
--:--- Ll-- .--T.-L --".---.-:.,, ..ln"t;nrr &firL. (lr ,l,@ll gp
17
Given that the prospection of the best subjects must be conducted
throughout the country and beyond, the time which the branches are in
transit between the sampling site and the cutting nursery could be quite long.
It would therefore be useful to determine if the selected planting stock cari be
harvested and conserved for several days prior to its planting, without losing
its capacity to vegetatively propagate.
Finally, survival of rooted cuttings should be tested to improve the long-
term possibilities for conservation of selected genotypes.
2. Materials and methods
Parts of branches were taken from the crowns of 13-year-old trees at eye
level. The trees were located at the Bandia Research Station (17”Ol’W,
14”34’N) in Senegal. These samples, except as otherwise indicated in the text,
were taken in May 1990. The tropical ferruginous soils found at the station
are depleted and degraded. The average annual rainfall is approximately
450 mm. A substantial deficit was registered in 1990, when incident pre-
cipitation was only 299 mm.
The samples were taken from pieces of branches 50 cm long and con-
served in a jute cloth which was kept humid. These branches were pruned to
produce cuttings 15 cm long (except as otherwise indicated in the text)
immediately prior to their placement in perforated polyethylene potting bags
(12 X 25 cm, when flat) which contained a substratum composed of basalt
chips (< 5 mm), Sand from Mbao and soi1 rich in humus from Mbao, in
volume proportions of 9/2/1. The cuttings were planted at a depth of 3 to 5
cm in the substratum and placed in a Gulfy cold frame [Grolleau, 19891,
under an awning. The unregulated interior temperature varied from 20 “C to
38 “C according to the season, the time of day and the amount of shade.
Atmospheric humidity in the cold frame was maintained at saturation point
by a twice daily spraying with water. A weekly anti-fungal treatment was also
applied: benlate (70 mg 1-l) and aliette (1 g 1-l) alternately.
Uniform hormonal treatments were applied before planting: the bases of
the cuttings were coated with talcum powder containing 4% ,8-indolyl butyric
acid (Rhizopon AA 4). This dosage was selected after trials covering a
spectrum of concentrations ranging from 0 to 8%.
The purpose of the first series of experiments was to determine which
planting stock was best adapted to the objectives enumerated above; the
effects of the following five factors were studied:
- The age of the branches, as indicated from the diameter of the cuttings.
Four classes were compared ( < 4 mm, 4-9.9 mm, lO- 15.9 mm and 16-
3 5 mm) frnm cgmnlpc takpn rl~srinn h/rcarr frnm whirh thn lon.,nr h-J L---
18
The cuttings were taken in May, had diameters of 10 to 16 mm and their
leaves had been removed at establishment time.
- The influence of leaves remaining on the cuttings. Cuttings (of 15 cm in
length. diameters of 1 O-l 6 mm, taken in May) with four leaves remaining
were compared with those which had been subjected to a complete
clenudation of the leaf system.
- The date of collection. Cuttings were taken at eight different times:
January, March, April, May, July, September, October and December.
The cuttings were 3 5 cm in length, had diameters of 1 O-1 6 mm and had
their leaves removed at the time of establishment.
- From the planting stock defined above, conservation trials were con-
ducted on pieces of branches which were maintained in a permanently
humid jute cloth. The objective was to determine how long it was possible
to preserve the cuttings before they lost their capacity to survive or
produce roots. The duration of conservation was examined over six time
periods: 0,2,4,6, and 8 days.
The objective of the final experiment was to study the behaviour and
survival of rooted cuttings after transplanting. This was done in order to gain
an idea of the possibilities for long-term conservation of the genotypes (head
of clone) to be propagated. Two conservation methods were compared: (i) in
20 litre clay pots containing a mixture (li2. v/v) of soi1 rich in humus and
sancl from Mbao which was watered daily, (ii) in the field, after plantation in
small pit-holes (20 X 20 X 30 cm) at the Randia Station, during the early
part of the rainy season (August 14, 1990). These were planted after cumula-
tive precipitation reached 80 mm. The plants thus received only natural
precipitation (2 19 mm).
The parameters measured during the experiments were:
- The survival of the cuttings. A cutting was considered viable if it had at
least one green leaf.
- The number of cuttings which sprouted at least one shoot.
- The number of cuttings which took root. Le. having at least one root
visible through the polyethylene bag.
The experimental design was full randomization, in each of two separate
blocks, each of which contained 28 to 30 cuttings for each protocol described
above. The analyses of variante and the comparisons of means were con-
ducted after angular transformation of frequencies. In the tables (for each
parameter) and figures, the values having the same index (a, b or c) belong to
the same homogeneous group ;as established by the Newman-Keuls test at a
5% level of significance. The confidence interval of the means is 95%.
19
3. Results
3.1. Definition qf‘the most suitahle plant@ stock
Diumeter qf rumets
When the cuttings have a diameter of less than 4 mm (sprouted during the
year of sampling), their survival rate after 90 days is zero (Table 1). Half of
the large diameter cuttings (> 16 mm) are viable three months after esta-
blishment. A large majority of these cuttings produced at least one branch,
however none of them produced roots.
Tuble 1. Survival. ramification and rooting of Acwiu srnegal cuttings, as a function of the
diameter of the ramet, 90 days after planting (cuttings of 15 cm. taken in May from which the
leaves had been removed at establishment).
Diameter of
Number of
Number of
Number of
cuttings
surviving
cuttings with
rooted cuttings
cuttings
new shoots
(mm)
(TO)
( %, j
(%J
<4
o c
-
-
4-9.9
Y3a
30a
15a
IO-15.9
68b
36a
3Ya
16-25
5Ob
43a
Oh
For the two intermediate classes (4 to 9.9 mm and 10 to 16 mm), the
survival rate is significantly lower for cuttings of larger diameters. However,
in the two cases, approximately a third of the cuttings produced at least one
root and one new shoot.
Length of cuttings
The study of the influence of cutting length, demonstrates the poor perform-
ance of long cuttings (23 cm) which dry out very rapidly and die; only 7% of
them survived and had set roots after 90 days (Table 2). Cuttings of 7 cm
show a good capacity to set roots but sprout few shoots. The best com-
promise is obtained with cuttings of 15 cm, as a11 those which established
roots three months after planting (approximately 1/3) also produced new
shoots.
Presence of leaves
Table 3 illustrates that the rooting and survival rates are significantly higher
when the leaves are removed from the cuttings. After leaf removal, 68% of
4L,. ,...A+:--- -..-_. :..- ..-2 1no1 ^_A .--- I^ rl...-. --- - -rl-~
P1
11.1
20
Tub/e 2. Survival, ramification and rooting of Acaria scnegul cuttings, as a function of their
length. 90 days after planting (cuttings 1 5 cm long. diameter 1 O- 15.9 mm. taken in May from
which the leaves had been removed at establishment).
Length of
Number of
Number of
Number of
cuttings
surviving
cuttings with
rooted cuttings
cuttings
new shoots
(cm)
(%)
(“h)
(%)
7
57a
7b
47a
15
68a
36a
39a
23
7b
l b
70
Table .3. Effect of ablation of leaves on the survival, ramification and rooting of A. sencgal
cuttings 90 days after planting (cuttings 15 cm long, diameter 1 O- 15.9 mm, taken in May).
Condition of
Number of
Number of
Numher of
foliage at
surviving
cuttings with
rooted cuttings
planting time
cuttings
new shoots
VI
@)
v4
No leaves
68a
36a
39a
With leaves
39b
30a
llb
Period of collection
Cuttings were planted which were taken from the same tree at different times
of the year. The eight dates selected represent four distinct climatic periods:
the peak dry season (January-March), the peak rainy season (June-Octo-
ber) and two intermediary dates.
Rooting was optimal (more than 70%) when cuttings were taken during
the rainy season (Fig. 1) and minimal but net zero (about 10%) during the
dry season. During the month of May, the rooting rate was 30 to 4O%, which
is illustrated in the results contained in Tables I,2, and 3.
Three months after collection, regardless of the date on which the sample
was taken, the cuttings manifest a root system having on average 25 f 7
roots and a length of 7.2 $I 3.1 cm without callus (Photograph 1). Their
tropism was very variable: the same cutting possesses horizontally oriented
roots (angle in relation to the vertical of 2 60’) and geotropic roots (angle
G 30”); none of them had developed a vertical system.
The cuttings produced on average 1.5 f 0.4 new shoots. Only one shoot
,...+ ,.ç G-7 _..A.., ,T~+h,+,,..-.:,,ll., J nmml~ fr\\rmnA ..Ath th, T,P,+;cQ~ nf -< -in’\\ anrl
80
70
60
50
T
L 40
c”
.3e 30
2
20
10
0
A
B
I
C
t
D
J
FM
A M J
I
J
A
S
O
N
D
Month
Fig. 1. Rcoting success of A. senegal cuttings (15 cm long, diameter 10-15.9 mm) 90 days
after planting as a lünction of the month or season in which the cuttings were collected (A:
mid dry season, B: end dry season. C: rainy season, D: early dry season).
explant or from the newly formed shoots (Photograph 3). These may develop
into pods containing seeds (Photograph 4).
Impact of storage time on sections of shoots taken j?om the field
From 35% to 46% of the cuttings made from sections of branchlings (diame-
ter of 10-15.9 mm) kept in a humid cloth during 2, 4, 6, or 8 days rooted
and produced new shoots branchlings after 90 days. These results do not
show a significant difference from the rate of root establishment measured in
the control group (cuttings established directly following sampling): 36%.
3.2. Conservation of clonal stock plant
Phorqrqh 1. Ro«ted A . senegol cutting showing ne\\+ shoot r~~rowth. threc months aftcr
planting (X0.3).
Photogruph 2. One-year-old A. .rene~ul cutting. conservrd in a pot (XO.OS).
fhotograph 3. A ilowering A. .seryq~rl cutting. threc months after planting (X0.25).
f’hotogruph 4. Fruiting A. .scneq~/ cuttiq. four months nt’tcr plantinp (XC)..;).
field remained alive (Table 4). They thus tolerated a period of seven months
without water, in addition to a rainy season which was well below average
(see ‘Materials and methods’). Moreover. they were sutjected to attacks by
herbivores such as hares and myriapods. However, the cuttings which
7uhle 4. Survival percentage of I-year-old cuttings from Acacia smegd as a function of
method of conservation (cuttings taken in May 19YO).
Conservation method
Waler source
Survival after 1 year (TO)
In nursery
Daily watering
8Sa
In field
Rainfall
33b
4. Discussion and conclusion
This study shows that it is possible to propagate mature A. senegal(l3 years of
age when the life-span of this species doesn’t excess 25 years [Aubreville,
19501) from sections of Iigneous shoots taken from the crowns of selected
trees without provoking destruction of the ortet. It confirms the results
obtained from four year old trees of the same species [Badji et al., 19911 and
is similar to work conducted on adult Fuidherbia albida [Danthu, 19911
branch fragments and roots.
The optimal planting stock is composed of sections of branchlings of
10 f 6 mm in diameter and 15 cm in length (having 12 to 15 nodes).
The rooting success after 90 days of establishment was very dependent on
the time at which the cuttings were taken. It varied between 10% for those
cm during the dry season, to 70% for those tut during the rainy season, thus
confirming the observations of Badji et al. 119911. Two variables might
explain these differences: the physiological state of the planting stock at the
time the sample was harvested and the cultural conditions, in particular, the
temperature inside the cold frame. However, it is not possible under our
experimental protocol to disassociate these two factors.
The success of root establishment of our cuttings was significantly higher
when their leaves were removed prior to planting. This result does not corres-
pond with those of numerous other studies of ligneous propagation [Reuveni
and Raviv, 198 1; Leakey et al., 1982; Oduol and Akunda. 19SSl nor is it in
accord with those results reported by Badji et al. 119911 which treated small
diameter cuttings (<4 mm) from Acacia senegul. After 60 days, these
authors obtained: (i) a survival rate equal to 1 1.5% and a zero rate of root
establishment for completely defoliated cuttings; (ii) a survival rate of 62%
and a 12% rate of root establishment for cuttings which retained four leaves.
This contradiction may be less real than apparent. It is probable that small
diameter cuttings contain insufficient reserves. Their metabolic functions
cannot continue for two to three months without the contribution of photo-
synthesis. Moreover, one cari see in Table 1 that no cutting with a diameter
1
.l
1
1. ,\\A 1
r
,
A..
24
four times greater contain sufficient reserves to survive and also develop a
root system due to the effect of rhizogenic substances.
One cari also note that the works of Paton et al. [1971] showed that the
influence of mature leaves of Eucalyptus grandis presented an impediment to
root establishment. The results obtained here could also therefore be ex-
plained by the possible inhibiting influence of the mature leaves.
From a practical viewpoint, this capacity of large diameter cuttings to root
in the absence of leaves allows their conservation in a humid cloth at least
one week before use. During such a period of conservation, the leaves die
and it is necessary to remove them before planting without altering the
rhizogenic capacity of the cuttings. Thus, one has eight days to transfer the
pieces of small branches from the sampling sites to the nursery.
Once rooted, the cuttings cari be kept during a long period of time in the
field or in pots in the nursery. The latter solution is preferable as plant
survival is improved: it is possible to supplement natural precipitation and
more effectively protect the plants from parasites and herbivores.
The growth of rooted cuttings is generally slow. They are often plagiotropic
and show, in numerous cases, inflorescence. Both observations illustrate the
state of physiological maturity of this planting stock [Franclet, 19811 and the
necessity to initiate a process of rejuvenation in order to conduct E’n vitro
cloning of this material. Studies in this area are presently being undertaken.
Acknowledgements
This study was financially supported by the C.R.D.I. (The Senegal Vegetative
Multiplication Project, n” 3-P-57-0267), the CIRAD (“FAC-Binôme” no
467%650), and by CEE (contract no TSL” 0169). The authors would like to
thank J. Rousse1 and the late A. Grolleau for their knowledgeable advice, A.
Sarr and M. Sagna for their technical assistance and L. Livingston for the
translation of this article.
References
Aubreville A (1950) Flore forestière soudano guinéenne. Société d’Editions Géographiques
Maritimes et coloniales, Paris, 523 pp
Badji S, Ndiaye 1, Danthu P and Colonna JP (1991) Vegetative propagation studies of gum
arabic trees. 1. Propagation of Acacia senegul (L.) Willd. using lignified cuttings of small
diameter with eight nodes. Agroforestry Systems 14: 183- 1 Y 1
Bonga JM (1982) Vegetative propagation in relation to juvenility, maturity and rejuvenation.
In: Bonga JM and Durzan DJ, eds. Tissue Culture in Forestry, Martinus Nijhoff/Dr W.
r....,. D..Ll:,.L-..- T-L- LT ^^..^ -- 707 A,?
25
Danthu P (1991) Mobilisation de Fuidherbiu alhidu adulte par bouturage de rameaux et de
racines. Atelier Faidherbiu albidu. Niamey, 22-26 avril, 199 1
Dreyfus BL and Dommergues YR (1981) Nodulation of Acaciu species by fast- and slow-
growing strains of Rhizobium. Applied Environmental Microbiology 4 1: 97-9Y
Franclet A (19X I ) Rajeunissement et propagation végétative des ligneux. Annales AFOCEL
1980: 1 l-4 1
Gerakis PA and Tsangarakis CZ (1970) The influence of Acaciu senegul on the fertility of a
sand sheet (‘GO~‘) soi1 in the central Sudan. Plant and Soi1 33: 8 l-86
Giffard PL (1966) Les gommiers: Acacia senegrrl Willd.. Acaciu iueta R. Br. Bois et Forêts des
Tropiques 105: 2 l-32
Grolleau A (1989) Contribution à l’étude de la multiplication végétative par greffage du karité.
Bois et Forêts des Tropiques 222: 3X -40
Leakey RRB. Chapman VR and Longman KA ( 19X 1) Physiological studies for tropical tree
improvement and conservation. Factors affecting root initiation in cuttings of Triplochiton
scleroxylon K. Schum. Forest Ecology and Management 4: 53-66
Mbaye 1 (1989) Situation et organisation du commerce international de la gomme arabique.
In: ISRA. ed, SYGGA III-Troisième symposium sous regional sur lc gommier et la gomme
arabique. 25-28 octobre. 1988, Saint-Louis, Senegal, pp 247-255
Oduol PA and Akunda E (198X) Vegetative propagation of Sesbania Sesban by cuttings.
Agroforestry Systems 6: 283-28X
Paton DM, Willing RR, Nicholls W and Pryor LD (1970) Rooting of stem cuttings of
Eucalyptus: a rooting inhibitor in adult tissue. Australian Journal of Botany 18: 175-l X3
Reuveni 0 and Raviv M (198 1) Importance of leaf retention to rooting of avocado cuttings.
Journal of the American Society for Horticultural Science 106: 127- 130