Milkvetch (A. hamosus, L.) and Scorpion vetch (C. scorpioides L.)
are widely distributed in Tunisia and in other parts of the world (Zoghlami
and Zouaghi, 2003). Both these annual legumes are neither well documented
nor used as cultivated crops, although their potential for pasture use was recommended
since more than fifty years ago (Gounot, 1958). They
are often found growing together. A. hamosus is a hard drought-tolerant
plant potentially useful for soil protection in Mediterranean areas (Patane
and Gresta, 2006).
Legume seeds are known for their rigid seed coat (Kozuharova
et al., 2010). In most commercial forage and pasture legumes, seed
germination has been a problem for years. Temperature appears to be an important
factor for seed germination (Brar et al., 1991).
Species which germinate readily over a range of temperatures should be easier
to establish than those with highly specific temperature requirements (Towsend
and McGinnies, 1972).
In our forage and pasture genetic resources programme we are evaluating species for which there is little information on many characteristics, the most important being temperature requirement for seed germination. The purpose of this study is to determine the response of seed germination of local populations of A. hamosus and C. scorpioides under different temperature treatments, in order to promote a more successful use of these species in the improvement of natural pasture of Tunisia, as well as their integration in crop rotation system.
MATERIALS AND METHODS
Seed germination of A. hamosus and C. scorpioides populations
was tested under dark conditions at four temperature treatments: three alternating
temperatures of 5-20, 15-25 and 20-35°C and one constant temperature of
20°C. The 20-35°C treatment was not tested on C. scorpioides
because of insufficient seeds. Duration of each alternating temperature
was 12 h and the regime of moisture was maintained constant between 70 and 80%.
A. hamosus and C. scorpioides were represented by 10 and 5 populations,
respectively seeds of both species were originated from previous collecting
missions conducted conjointly between INRAT, ICARDA and CLIMA/Australia in 1992
and 1994 in order to conserve the local Tunisian forage and pasture legume germplasm.
|| Code and characteristics of sites of origin of A. hamosus
and C. coronilla populations used in the experiment
A seed increase programme was then initiated for all germplasm in the subsequent
years. However, seeds of tested species were approximately the same age and
were stored in room conditions for several years followed by cold storage more
recently. The characteristics of the collection sites of the studied populations
are presented in Table 1.
For each species and population, one hundred seeds (replicated 3 times) were scarified with sandpaper in order to enhance germination and then placed in Petri dishes on imbibed filter paper. The dishes were placed in a dark germinator at the desired temperature.
Germination counts were made daily over four weeks. For alternating temperatures, heating phase starts each day early in the morning (6 AM) and the cooling phase commences early in the afternoon (18 PM) so as to ensure a duration period of 12 h for each phase.
For both species and for each species separately, analysis of variance was
carried out on the angles resulting from the arcsin transformation of the percentages
using the SAS version 8 (SAS, 1998). Differences between
mean values were evaluated for significance using the Duncan test at 0.05 levels.
For both species together, all main effects and interactions among species,
temperatures and accessions were highly significant (p<0.001).
||Seed germination dynamics of A. hamosus populations.
Percentage of all germinated seeds during each week at each temperature
For each species separately, a high significant effect of temperature, accession
and their interaction on total germination was observed (p<0.001).
For A. hamosus, the effect of temperature was higher than that of accession
or the interaction accession x temperature. However, alternating temperatures
(mainly that of 20-35°C) was the most favourable treatment for total germination
of this species compared to the constant 20°C which markedly reduced its
germination (Table 2). Whatever the temperature, the seed
dynamic germination increased with time. The interaction between accession and
temperature for total germination was large: 8 accessions were sensitive to
the temperature treatment of 20-35° while 2 accessions were sensitive to
the temperature treatment of 15-25°C. Forty percent of accessions exceeded
50% of germination at only 20-35°C treatment by the first week. Variation
among populations of A. hamosus for total germination was high at all
regimes especially 35-20°C. There was only one accession (315) which did
not germinate at 20°C and only one accession (348) which did not germinate
at 20-35°C regime. Since the germination occurred at all regimes and increased
with increasing temperature, A. hamosus can be considered as temperature-tolerant
species (Fig. 1a-f). Furthermore, almost
all populations germinated better at 20-35°C regime, except 348 and 335
which did not germinate accidently or germinated weakly.
|Fig. 1 (a-j):
|| Germination of 10 populations (38-89-105-181-196-214-276-315-335-348)
of A. hamosus at four time periods and four temperature regimes
||Average seed germination percentage of C. scorpioides
populations at four time periods and four temperature regimes
More than 60% of seed germination was recorded at 20-35°C for populations
89 and 196 and slightly for 214. The temperature regime of 5-20°C seemed
to be unfavorable for all populations except 105 (Fig. 1).
This result did not agree with Brar et al. (1991)
who found that 20°C was the recommended temperature for all the tested forage
legumes except Mount Barker subterranean clover.
For C. scorpioides, total germination was relatively very low at all regimes and the effect of temperature was not significant (p>0.001) (Table 3).
Variation among accessions was relatively low; the highest total germination
didnt exceed 10% for populations 210 (Fig. 2a-e).
|Fig. 2 (a-e):
||Germination of 5 populations (19-149-210-224-285) of C.
scorpioides after four time periods and four temperature regimes
Seed germination at fluctuating temperature treatments, 5-20, 15-20, 20-35
and constant 20°C temperature was tested for two annual pasture legumes
from the genera Astragalus and Coronilla. Both species are widely
distributed in Tunisia, mainly in arid and semiarid areas where they exhibit
a large morphological variation and high seed yields (Zoghlami
and Zouaghi, 2005, 2003). The ability of both species
to survive over a range of climatic conditions suggests that hard seededness
(seed coat impermeability) is an important germination regulating mechanism.
Temperature requirements for seed germination of some species explain why they
are relatively easy or difficult to establish in field conditions. According
to Towsend and McGinnies (1972) and Revell
et al. (1998), species with very small seeds-A. hamosus in
our case-germinate well at most temperatures. These species are often difficult
to establish although this behaviour will greatly increase their chance of survival
for the long term.
In the present study, temperature regime is a principal factor affecting the
seed germination of A. hamosus. Alternating temperature treatment of
20-35°C was the most favourable temperature on total germination. This
result agrees with that of Quinlivan (1961) who demonstrated
that seeds of Astragalus sinicus became permeable under the influence
of temperature fluctuation between 20 and 40°C. Germination of almost all
populations at 15-25 and 20-35°C was completed by the 7th day (Fig.
1). Early high germination across a large range of temperatures may be beneficial
for rapid establishment of the pasture crop in semiarid warm regions where soil
moisture in the upper soil surface is available for only a short period (Brar
et al., 1991). In our conditions, this temperature range can be encountered
in early autumn (September-October) where sporadic and intense rain may occur
Populations of A. hamosus which germinated well at most temperatures
will be easier to establish in the field than those which required specific
temperatures. The difficulty in establishing this species could be related to
its small seed size rather than to temperature requirements (Towsend
and McGinnies, 1972).
For C. scorpioides, germination was very low and was not affected by temperature. Eighty percent of seeds were rotten and then died.
In conclusion, temperature had a strong effect on germination of A. hamosus populations and genetic variation is probably important. We think that higher temperature than 20-35°C could be tested for improving germination of this species.