IMPACT OF DROUGHT STRESS ON THE PHENOLOGY OF 2 SOYBEAN (GLYCINE-MAX LMERR) CULTIVARS

Drought stresses modify the phenology of plants, and thus affect the y ield components. The impact of various periods of water stress on the organ appearance rate and on the durations of the main vegetative and reproductive periods was studied under, greenhouse conditions on soybe an cultivars differing by type of growth. The two cultivars, Weber and Spot, maturity group I, indeterminate and determinate respectively, w ere subjected to drought stress (30% PAW) during a main developmental stage: i.e., vegetative, flowering, pod lengthening or seed filling. D rought stress seemed to trigger a signal that caused an early switch o f plant development from vegetative to reproductive. Appearance of nod es initiated during stress was delayed, resulting in a small number of nodes produced, whereas flower and pod appearance were hastened. Each reproductive phase was shorter under stress, mainly because of the ap pearance of new organs that prevented the emergence of organs belongin g to the earlier ontogenetic phases. The seed-filling stage and the fi nal stage in seed abortion began earlier in stressed plants and the du ration of the maturation period was significantly reduced by stress du ring seed filling, leading to accelerated senescence. The cultivars di ffered mostly with respect to their strategies of partitioning between the main stem and branches. Under stress, the duration of the main re productive phase was increased on the main stem but reduced on the bra nches in cultivar Weber, whereas the opposite response was noted in cu ltivar Spot.