Effets de la salinité et du stress hydrique sur la germination des graines de Hedysarum flexuosum (Fabaceae) ; Effects of salt and water stress on seed germination of Hedysarum flexuosum (Fabaceae)

L’objectif de ce travail est d’étudier les effets des stress salin et hydrique sur la germination des graines de Hedysarum flexuosum, légumineuse spontanée fourragère et pastorale d’origine méditerranéenne. Les résultats montrent que le taux de germination (TG) des graines diminue en réponse à l’augmentation de la concentration de NaCl. Le taux de réduction de la capacité germinative est d’autant plus élevé que la pression osmotique est plus forte. En effet, à partir de 2 g/l l’influence de la salinité est significativement perceptible avec un taux de réduction de 18.7%. La concentration induisant la diminution de la germination de 50 % (DL50), se situe entre 4 et 6 g/l de NaCl. La germination est pratiquement inhibée à la concentration la plus élevée (10 g/l). Le temps moyen de germination (TMG) est allongé significativement avec l’augmentation de la salinité du milieu. Chez les témoins, le TMG est de 2,37 jours, il s’allonge jusqu’à 8 jours à la dose de 10 g/l de NaCl. Nos résultats montrent que le TG le plus élevé (94 %) est obtenu chez les graines témoins. Le TG diminue avec l’augmentation de la pression osmotique du milieu d’imbibition des graines. À la pression osmotique de -1 bar, la capacité germinative est réduite de 60,63 % par rapport au témoin et atteint 90,42% à -3 bars. La germination est complètement inhibée à la pression osmotique de -9,25 bars. Comme pour le stress salin, le TMG obtenu chez les graines de H. flexuosum augmente avec la pression osmotique. Celui-ci est triplé (-7 bars) par rapport au témoin. H. flexuosum paraît être plus sensible à la salinité comparativement à plusieurs cultivars tunisiens de H. coronarium chez lesquels la DL 50 est estimée à -2,11 bars. ; Hedysarum flexuosum L., commonly called Sulla has been the subject of numerous studies which have demonstrated its forage and pastoral potential. Previous researches on the biochemical composition of this species have revealed a high nutritional value and positioned it as an alternative to the imported food for goat and sheep raising. The authors mention the richness of Sulla in proteins (22.5 % g/Kg of dry matter), lipids and minerals. They also note a digestibility rate very similar to those of other forage legumes grown such as alfalfa. The species H. flexuosum or Sulla is reported in Algeria and Morocco on marl and marl-limestone substrates in regions with average rainfall greater than 550 mm. This study consists of an ecophysiological evaluation of the seed germination phase of H. flexuosum through the effects of salt and water stress on the germination rate (GT) of this species. H. flexuosum pods were collected at maturity at a station in Ouadhia commune in northern Algeria. The seeds extracted from the pods were treated with 5 % sodium hypochlorite for 5 minutes and then rinsed three times with distilled water. In order to avoid any integumentary dormancy, a slight scarification of the seed surfaces was carried out. They were then watered with increasing concentration NaCl solutions (2, 4, 6, 8, and 10 g/l), for the salinity test and by solutions of polyethylene glycol 6000 (PEG -6000) corresponding to pressures of: -1 bar, -3 bars, -7 bars, -9.25 bars respectively for the water stress. A batch of control seeds was watered with distilled water. The results of the germination rate (GT) and mean germination time (MGT) were analysed by an ANOVA using the stat box software (one parameter analysis: NaCl concentration for salt stress and PEG concentration for water stress). The ANOVA analysis was completed by the Newman and Keuls test when it reveals a significant variation. Results show that GT variation as a function of NaCl concentrations is highly significant (P < 0.001). Seed GT decreases in response to increased NaCl concentration in the imbibitions substrate. The higher osmotic pressure is, the greater the rate of reduction of the germinative capacity increases. The influence of salinity is in fact significantly perceptible from 2 g/l with a GT of 74 % corresponding to a reduction rate of 18.7 %. The concentration inducing the reduction of the germination of 50 % (LD 50) is between 4 and 6 g/l of NaCl. The germination is practically inhibited at the highest concentration tested (10 g/l). Analysis of the variance of the MGT factor of H. flexuosum shows that it increases significantly with elevated environmental salinity. In the controls, MGT is 2.37 days but extends to 8 days at the dose of 10 g/l of NaCl. The effect of salinity on germination kinetics is manifested by the germination retardation caused by the increasing concentrations of NaCl and PEG substrate (first phase of germination). It could be explained by the prolongation of the latency phase corresponding to the time required for the imbibition of the seeds to trigger the germination process. Our results show that the highest GT is obtained in the control seeds (94 %). At the osmotic pressure of -1 bar, the germinative capacity is reduced by 60.63 % compared to the control. GT decreases with increasing osmotic pressure of the seed imbibitions substrate. At the pressure of -3 bars the germination drop reached 90.42 %. The germination is completely inhibited at the pressure of -9.25 bars. Like GT, the MGT required for H. flexuosum seeds increases with osmotic pressure. This is tripled at -7 bars pressure compared to the controls. H. flexuosum appears to be more sensitive to salinity compared to several Tunisian cultivars of H. coronarium (LD 50 to -2.11 bars for water stress).