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African Journal of Agricultural Marketing ISSN 2375-1061 Vol. 8 (10), pp. 001-007, October, 2020. Available online at 
www.internationalscholarsjournals.org © International Scholars Journals 

 

Author(s) retain the copyright of this article. 
 
 

Full Length Research Paper 

 

Seed germination and viability in two African Acacia 
species growing under different water stress levels 

  
Amelework Kassa1*, Ricardo Alía1,2

, Wubalem Tadesse3, Valentin Pando1 and Felipe Bravo1,2
 

 
1
Sustainable Forest Management Research Institute, University of Valladolid-CIFOR INIA, Avda. de Madrid s/n, 34004, 

Palencia, Spain. 
2
(Agraria y Alimentaria) INIA-CIFOR. Avda. A Coruña, Km. 7,5. P. O. Box 28040 Madrid. Spain. 

3
Ethiopian Institute of Agricultural Research, P. O. Box 2003, Addis Ababa, Ethiopia. 

 
Accepted 13 August, 2020 

 
Acacia species are important in forestation programs and for producing non-timber forest products in arid and 
semiarid zones, but few studies have been carried out concerning the effects of drought in the germination in order 
to understand the regeneration process of the species. In this paper, we studied the morphology and the 
germination pattern under different water stress of Acacia senegal and Acacia seyal. Seeds were subjected to a 
water stress test for 45 days with four levels of water potential achieved by different concentrations of polyethylene 
glycol 6000. The germination process was studied by adjusting a Gompertz function, and obtaining related 
parameters of the curve (total germination, maximum germination rate and the t value corresponding to the 
inflection point of the curve, and time in reaching the, 50 and 90% of the total germination). The germination process 
in these species was rapid; there were no significant differences in any of the parameters of the curves depending 
on the stress treatment except for the total germination. Total germination was higher in A. senegal, and this 
species was more sensitive to the water availability than A. seyal, as deduced from the reaction norms in the two 
environments. The probability of germination was also modeled by a logistic regression, indicating the higher 
values for non stressed seeds. A consistent pattern is detected among the treatments. The results presented in this 
paper could be applied in forestation programs to improve germination in nurseries, and by incorporating the 
logistic models in more complete models describing the dynamic of regeneration under natural conditions. 
 
Key words: Acacia, water stress, Logistic regression, germination, regeneration. 

 
INTRODUCTION 

 
Different factors are affecting recruitment in forest 
species and the establishment of new forest areas, 
among which dispersal, predation and germination are 
essential to many species (Blate et al., 1998). One of the 
main processes is germination, because we can infer 
some information on the strategy of the species to cope 
with drought (avoidance, tolerance), and also, knowledge 
has important implications in the management of the 
seed during nursery (Kozlowski and Pallardy, 2002; 
Kozlowski, 2002; Choinski and Tuohy, 1991; Boydak et 
al., 2003; Sy et al 2001)  

Africa is one of the most vulnerable continents to  
 
 
 
*Corresponding author. E-mail: mlkassa@yahoo.com. Tel: +34 
979108424. Fax: +34 979 108440. 

 
 
 

 
climate change and climate variability, this vulnerability 
aggravated by the interaction of multiple factors, including 
the growing deforestation and water stress occurring at 
various levels, and low adaptive capacity. The defores-
tation and his consequences are serious problems in 
developing countries, given that their economies are 
based on agriculture, and climate changes impose 
additional pressure on water demand in Africa (Boko et 
al., 2007). The lack of systematic efforts to conserve and 
manage resources is a major concern, and in few cases, 
efforts have been made to cultivate species that yield non 
timber forest product (Michael and Tadesse, 2004). For 
many African species with high importance in aforestation 
or management program, we lack information in regard to 
the drought stress impact, specially, on Acacia senegal 
and Acacia seyal species.  

Non-timber forest products (NTFP) play an important 

file:///C:\Users\user\Documents\REPUBLICATION\AGRICULTURAL%20SCIENCES\AppData\Local\Temp\www.internationalscholarsjournals.org


 
 
 

 

role both in the rural economy and population of the 
African country, in Ethiopia, it play a vital role in the live-
lihoods of communities contributing to food security and 
household income (Michael and Tadesse, 2004). Among 
the species that produce these products; A. senegal and 
A. seyal produces gum Arabic and gum talha, respect-
tively. The Arabic gum obtained from species A. senegal 
is used in the food industries and gum talha obtained 
from A. seyal is used in non-food industries, besides the 
importance of their products, these species adapt to very 
erratic weather conditions and is considered as species 
protective environmental conditions (Eisa et al., 2008) 
and its wood are highly preferred by rural populations as 
source of fuel and to construct agricultural implements 
etc.  

We have selected two Acacia species to address this 
question, differing in size, dormancy and other life-history 
traits. A. senegal (Linne) and A. seyal (Del) produces 
valuable NTFP. Given the importance that have both 
national and international level, these species in Ethiopia, 
due to overexploitation and the problems associated with 
the process of installing and growing plantations on the 
extreme dry conditions, is believed to be in danger of 
extinction in the future (Personal observation). Our 
hypothesis is that species under arid conditions will show 
a clear strategy for avoidance (by a very fast germination) 
or tolerance (by a low germination process).  

The objective of the study is to analyze the germination 
curves and the probability of germination under different 
water stress conditions of A. senegal and A. seyal. The 
analysis of this process will allow us to infer the strategy 
of the species in relation to drought stress, and also to 
provide models of germination under different water 
stress conditions to be included in general models for the 
regeneration process of the two species. 
 

 
MATERIALS AND METHODS 
 
Description of species, their importance and precedence 
 
Two Acacia species has been used in this study. A. senegal is a 
small tree (10-15 m height) distributed from Senegal to red sea, 
east and South Africa b/n 11 and 16° North from 500 - 1700 
m.a.s.l., poor natural regeneration. It is resistant to drought (500 - 
1000 mm of rainfall). The species produces Arabic gum (used as a 
stabilizer in food and pharmaceutical industries and in printing and 
textile industries) and the wood is highly valued by rural populations 
as fuel for firewood and charcoal. A. seyal is a smaller tree (up to 9 
m height), distributed from Senegal to the entire Sahal, Sudan and 
Egypt, East Africa from Somalia to Mozambique from 0 - 2100 m 
a.s.l. It is also resistant to drought, with a broader niche (250 - 1000 
mm of rainfall) than A. senegal. A. seyal produces Talha gum (used 
as a stabilizer in non-food industries) of lower quality than Arabic 
gum, and the wood is also used as fuel for firewood and charcoal 
(Argaw et al., 1999)  

One population group from each of the species was sampled in 
southwest Ethiopia (Langano and Shala populations) in the East 
Shewa region which is classified as a semi-arid area. Langano 
population (7° 26’ N-38° 47’ E), is located at 1749 m a.s.l., with a 
rainfall of 500 - 600 mm, and minimum and maximum monthly 

 
 
 
 

 
mean temperature of 13.8 and 38°C. Shala population (7º 32’ N-38º 
40’ E) is located under similar conditions: 1620 m a.s.l., 600 mm of 
rainfall, and minimum and maximum monthly mean temperature of 
13.8 and 38°C. Seeds were provided by the National Forest 
Research Institute of Ethiopia from commercial seed-lots, and 
stored in a cold chamber at 6°C. The A. senegal seeds are bigger 
(8.62 gr/100 seeds, or 11.600 seeds/kg, length: 8.76 ± 0.95 mm, 
width: 7.67 ± 0.82 mm) than the A. seyal seeds (5.18 gr/100 seeds 
or 19.305 seeds/kg, length: 6.77 ± 0.77 mm, width: 4.01 ± 0.49 
mm). Seeds of the two populations are bigger in comparison to the 
mean values of the species (18.000 seeds/kg for A. senegal, and 
22.000 seeds/kg for A. seyal), indicating the arid conditions of the 
two populations. 

 

Germination process under different drought stress treatments 
 
Germination was tested in a factorial design with Species (2 levels) 
and drought stress (4 levels) as factorial treatments. The four levels 
of water potential ( =0 or control, - 4, -8 and -12 bars respectively) 
were simulated by adding different amount of Polyethylene Glycol 
6000 (PEG), to obtain different water stress level (Michel and 
Kaufmann, 1973; modified by Michel, 1983). These treatments 
mimic from no stress to very severe water stress (-12 bars).  

For each species and drought stress, 100 seeds were used, for a 
total of 400 seeds per species, and 800 seeds in total. The 
experimental unit consisted of 25 seeds in Petri dishes (10 cm-
diameter on a filter paper) and randomly arranged in four replicates 
in a germination chamber IBERCEX V-900-D. The conditions for 
the germination test were: 30°C temperature, a constant relative 
humidity of 80% and a photoperiod of 12 h. Germination was 
checked daily, and a seed was considered as germinated when the 
radicle emerged at least 1 mm from the integument. The test lasted 
for 45 days from 21/01/2009 until 03/03/2009 (but no germination 
was recorded after day 20). A Tetrazolium viability test was 
performed to all the non-germinated seeds in order to quantify the 
no germinating seeds under drought stress. 

The A. seyal seeds were submitted to a pre- germination treat-
ment with boiled water (at 100°C) and then left at room temperature 
for 24 h (Forest Research Directorate, 2000, internal document) to 
break the dormancy, which is the regular method used for this 
species. Otherwise dormancy could hide drought stress impact. 
Seeds were disinfected by applying the methodology proposed by 
Villamediana et al. (2007) before starting the germination analysis. 
The solution and the filter paper were changed every four days, to 
maintain the water potential constant throughout the entire duration 
of the experiment and prevent hyper-concentration processes, in 
agreement with Falleri (1994) and Bravo et al. (2010). 
 

 
Data analysis 
 
Germination at a given day t was adjusted for each experimental 
unit (Petri dish) by non-lineal regression to a Gompertz function 
(Draper and Smith, 1981) with three parameters: c, the predicted 
germination (asymptote of the curve); b, closely related 
(proportional) to the maximum germination rate standardized by the 
total germination (b germinationmax/c); m, the t value corresponding 
to the inflection point of the curve (date at which maximum growth 
rate is reached).  
The initial parameters (c, m and b) were estimated based on the 
optimization of the sum of squares of the residues minimized by the 
Levenberg-Marquardt algorithm (Wolfram, 1999). To test the 

precision of the model, the value of pseudo-R
2
 was calculated for 

each fit. Three other variables were derived for the germination 
data: b, t50 and t90: being, respectively, the date (in days) 
corresponding to germination of 50 and 90% of the total. An 



 
 
 

 
Table 1. Descriptive values for the different variables derived from the germination of each experimental unit curve.  

 
 

Variable 
 A. senegal   A. seyal  

 

 
mean std max. min. mean std max. min.  

  
 

 c 16.84 6.56 25.15 6.00 8.81 4.00 15.28 2.00 
 

 b 2.79 5.79 17.75 0.15 3.32 4.51 14.64 6.31 
 

 m 0.11 3.53 3.19 0,36 2.42 0.50 3.19 1.16 
 

 t50 0.83 3.01 3.58 0,45 2.73 0.52 3.63 2.00 
 

 t90 4.55 2.15 8.73 0.96 4.33 1.79 9.55 2.32 
 

 R
2
 93.8 8.4 100 69.6 94.8 7.1 100 74.36 

 

 
c: total germination, b: maximum germination rate, m: date at which maximum growth rate is reached t50 and t90: date at which 

germination reaches the 50 and 90% of the total germination. R
2
: regression coefficient of the adjusted curves. 

 
 
 

Table 2. Analysis of variance of the variables describing the germination process in A. senegal and A. seyal under 

different watering regimes. Mean squares and significance values of the F-test.  
 

 
Variable 

Species  Drought stress Interacción 
 

 
Squ.of mean p value Squ. of mean p value Squ. of mean p value  

  
 

 germination 457.53 < 0.0001 162.78 < 0.0001 44.78 0.0403 
 

 c 558.18 < 0.0001 131.54 0.0006 32.64 0.1357 
 

 b 2.78 0.776 75.17 0.1104 21.7 0.598 
 

 m 35.56 0.025 16.34 0.06 12.68 0.115 
 

 t50 0.45 0.9017 52.62 0.1768 43.09 0.2498 
 

 t90 701.82 0.3311 719.93 0.4057 610.68 0.4771 
  

c: total germination, b: maximum germination rate, m: date at which maximum growth rate is reached t50 and t90: date at which 

germination reaches the 50 and 90% of the total germination. R
2
: regression coefficient of the adjusted curves. *** P < 0.001; ** 

0.01 > P >0.001; * 0.05 > P > 0.01; n.s. 0.05 > P 

 
 

 
analysis of variance was performed to the different variables (c, b, 
m, t50 and t90) according to the following factorial model (Equation 
1): 
 
Yijk=  +  j +  i +     ji +  ijk (1) 

 
where: Yijk: Value of the variables for the i

th
species under the j

th
 

water stress treatment and the k
th

, and Petri dish; µ: grand mean; j:  
drought stress (j = 1 to 4); i: species (l = 1 to 2) and the error term; jk 

N (0, 2
 j). A Tukey-Kramer multiple comparison test was applied to 

the main factors if they were statistically significant and the 
interaction was graphically analyzed.  

In the second step, the probability of germination (P) was 
determined by independent logistics models for each species. Two 

explicative variables were used: Drought stress (expressed as 
water potential in MPa), the time and the interaction (Equation 2): 
 

P 
  1  

 

  n 
(2) 

 

   
− ( β 0β i χ i ) 

 

1 e 
 

 

i 1  
 

    
 

 
The models were tested using the change in the value of -2 log of 
the likelihood between the model with and without explicative 
variables (Hosmer and Lemeshow, 1989). Starting from the results 
of the logistics regression, the probability of germination was 
analyzed graphically for each species in a specified time, for each 

 
 
 

 
treatment. The value of the area below the receiver operating 
characteristic curve (ROC curve) was used to estimate the 
precision of the models adjusted. All the analysis was conducted by 
using the SAS software (SAS Institute Inc, 2004). 
 

 

RESULTS 

 

Seed germination occurred after 4.5 days in A. senegal 
and 4.3 days in A. seyal. Ninety percent of the total 

germination was reached and no new germination was 
observed after 20 days of the experiment (Table 1). 

The Gompertz model was very precise in describing the 

germination process (R
2
 > 90% in both species). The 

analysis of variance of the variables showed significant 
differences in total germination and c Gompertz model 
parameter. P-values for total germination were under 
0.0001 for species and drought stress and equal to  
0.0403 for the interaction. The p-values for the c para-
meter were under 0.001 for species and drought stress 
and not significant for the interaction, p-value equal to 
0.01357 (Table 2). However, there were no significant 
differences in b, t50 and t90 for any factors.  

The germination of A. senegal was greater (mean value 

99.0 ± 2.00 in the control treatment) than that of A. seyal 



 
 
 
 

-4 MPa     
 

 80     
 

p
ro

b
a
b

il
it

y
 

60     
 

40 
    

 

G
e

rm
in

a
ti

o
n

 

    
 

20   
Acacia senegal 

 
 

    
 

     
 

    Acacia seyal  
 

 0     
 

 0 5 10 15 20 
 

   Days   
  

 
Figure 1. Germination curve at A. senegal and A. seyal. 

 

 

Table 3. Logistic regression results to the species of A. senegal and A. seyal under water stress conditions.  
 

Parameter DF Estimate Standard error Wald chi-square Pr > chi-square 

   A. senegal   

Independent term 1 -0.4891 0.1550 9.9559 0.0016 

Drought 1 0.1007 0.0254 15.6682 <.0001 

Day 1 -0.1418 0.0385 13.5816 0.0002 

Drought*Day 1 0.0195 0.00660 8.7125 0.0032  
 

A. seyal   
Independent term 1 -1.6820 0.2011 69.9480 <.0001 

Drought 1 0.0627 0.0301 4.3302 0.0374 

Day 1 -0.2551 0.0391 42.5996 <.0001 

Drought*Day 1 0.000478 0.00577 0.0069 0.9339 
 

 

(44.0 ± 17.59). The norm of reaction of the germination 
according to water stress can be seen in Figure 1. The 
comparison of these values in respect to the value 
obtained in the control treatment showed that A. seyal did 
not decrease its percentage until a value of -8 MPa, more 
quickly than in A. senegal, with a decrease in germination 
for the most severe treatment were very different: for A. 
senegal the germination is only 35% of the control (35.0 ± 
12.38) in comparison to a higher value (59%) of the 
control (26.0 ± 17.74) in A. seyal. The intermediate treat-
ments (-4 and -8 MPa) showed a response in the middle 
of the two severe treatments (mean value of 73.0 ± 
16.45% and 57.0 ± 17.09, respectively in A. senegal, and  
46.0± 13.27% and 29.0 ± 13.61). The viability test applied 

 

 

to the non-germinated seeds showed that for A. senegal, 

the seeds that did not germinate were not viable at the 
end of the test, but for A. seyal, 50% of the non-
germinated seeds were viable. 

The logistic regression analysis showed that seed ger-
mination in function of a specific time period was lower as 
time increased, yielding a decreasing tendencies in each 
treatment. In A. senegal drought stress and time, and 
their interaction, had a significant effect on germination. 
However, in the case of A. seyal, the interaction had no 

significantly influence on germination (Table 3 and Figure 
2), while the results for the rest of the independent 
variables were the same as in the previous case. The 
value of the area under the ROC curve were c = 0.85 and 



  
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 

Figure 2. Germination for each species and treatment (a) and for each species in relation 

to the control b. 
 

 

and c = 0.81 for the species A. senegal and A. seyal 

respectively. 
 
 

DISCUSSION 

 

Different studies focused on the effect of bush burning on 
the germination of various species of the genus Acacia 
(Danthu et al., 2003). However, they do not provide 
information on the effect of water stress (an essential 
environmental factor in the distribution range of the 
species) on the germination of A. sengal and A. seyal 
species. This paper, analyses four potential effects 
depending on the intensity of water stress conditions 
obtained with Polyethylene glycol, including control, on 
the germination process of this two Acacia species.  

Germination was different depending on the species, 
and also a different pattern of viability of non-germinated 
seeds was found at the end of the experiment. The higher 
percentage of germination of the species A. senegal in 
comparison to A. seyal has been previously reported 
(Argaw et al., 1999; Teketay, 1996; Zida, 2007), but in 
our study, the difference in viable seeds among the two 
species (no viable seeds detected at the end of the 
experiment in A. senegal, and a value of 50% for non- 

 
 

 

germinated seeds for A. seyal) indicate that this pattern is 
caused by a heavy induced dormancy in the latter. Most 
Acacia species are characterized by a very hard and 
impermeable seed coat, which result in temporary 
dormancy and influences the germination process (Aref, 
2000; Argaw et al., 1999; Owens et al., 1995). In A. seyal, 
the application of severe treatments (stronger than the 
applied in our study) is needed to break the dormancy 
and speedup the process of germination. The very hard 
and impermeable seed coat could act as a protection 
against water stress, with little germination under water 
stress conditions to prevent problems during the embryo 
development.  

The size of the seeds (measured as the number of 
seeds/kg) of the two Acacia species are larger than those 

indicated in previous studies (Argaw et al., 1999), 
suggesting the more arid conditions of the analyzed 
populations to those previously reported, as shown in 
Acacia nilotica (Miller et al., 2002, Mahamood et al., 
2005); Cordia africana (Loha et al., 2008), or among 
populations of the same Acacia species (Mahamood et 
al., 2005). Therefore, we can interpret the result as 
derived from populations adapted to drought conditions, 
as a view to explore strategies of the species under 
stressful conditions. 



 
 
 

 

The analysis of variance showed a decrease in germi-
nation as the degree of water stress increases for the two 
species. This is much more pronounced in the extreme 
water stress in the case of A. senegal (reduction of 
germination of 64.6% in the level at -12 MPa with respect 
to the control) than in A. seyal (reduction of 40.9% with 
respect to the control) . This could be related to greater 
tolerance to drought in A. seyal, as shown in leguminous 
species from sub- Saharan areas where a clear reduction 
of germination is found depending on the water stress (Sy 
et al., 2001, in Cassia abtasifolia, C. occidentalis, 
Indigofera senegalensis, I. astragalina, I. tinctoria, 
Sesbania pachycarpa and Tephrosia pururea).  

No significant differences have been found in most of 
the parameter estimated from the Gompertz model (both 
the inflexion point and the rapidity of growth, as well as 
the time to reach different germination rates). The 
analyzed Acacia species reached a total germination over 
90% in only 5 days. No differences between species and 
treatment were found. This might be an avoidance 
strategy to water stress, related to the environment in 
which the plant will develop.  

Under arid condition, these species take advantage of 
favorable conditions (humidity and temperature) to germi-
nate rapidly. However, we can distinguish two different 
water -stress avoiding strategies: A. senegal produces a 
rapid germination of all the viable seeds, but for A. seyal, 
the germination under stressful conditions is more hetero-
geneous (higher std) and is more higher in comparison to 
the control, and it is limited by an induced dormancy; 
these non-germinated seeds would germinate under 
more favorable conditions. Rapid seed germination in arid 
and semi-arid areas seems to be frequent. This was 
demonstrated in the study performed with some species 
(Spartium junceum L.) in which germination began at 4 
days with 5% and reached a total of 67% over the 18 
days of the experiment (Travlos et al., 2007). In Pines 
and Mediterranean Oaks under arid conditions, the 
process is not so rapid, but we can find the strategy of a 
more irregular and extended germination over time 
(Boydak et al., 2003).  

The probability of germination indicates a rapid decay 
when the stress increases, and also the large differences 
among the two species, with implications in the regenera-
tion process. In A. senegal the probability of germination 
under non-stress conditions is quite high and therefore, 
this species could regenerate easily in such conditions. 
However, for A. seyal, the germination is highly 
depending on the conditions: The covers protect the 
embryo from the drought, but it leads to a low probability 
of germination even under no stress conditions (similar to 
the values reached by A. senegal under the -12.0 Pa 
treatments. 

It can be concluded that water stress has a negative 
effect on Acacia germination in arid and semiarid 

environments, but the reduction is not enough to impede 

germination of the seeds if the drought conditions are not 

prolonged over time. We also detect two different 

 
 
 
 

 

avoiding water stress strategies. All viable seeds of A. 
senegal geminates quickly, in order to be installed as 
soon as possible based on the seed reserves. However, 
A. seyal with smaller seeds and a heavy and imper-

meable coat, reacts by inducing dormancy under the 
heaviest water stress conditions. The analysis of the 
Gompertz functions as well as the logistic model, describe 
the process of germination under water stress conditions, 
and these statistical tools could be included in restoration 
and management programs in order to favour the 
conservation and sustainable use under drought 
conditions. 
 

 

ACKNOWLEDGEMENTS 

 

This project was financed by the AECID (Spanish Inter-
national Cooperation Agency for development) under the 
grant programme of a doctoral thesis to the senior author 
(A. Kassa). Thanks to Girmay Fitiwi, Kiros Woldearegay 
and Teklehaimanot Negatu from the Ethiopian Natural 
Gums Company for provide useful information for this 
paper. And also to Encarna Rodríguez, Antonio Sanz 
Ros, María Rosario Núñez, Celia Herrero, Stella Bogino, 
Iñaki Etxebeste, Claudia Escudero, Gonzalo Álvarez and 
Irene Ruano for providing technical support during the 
laboratory work. 
 

 
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