Comparative Effects of Repeated Cycles of Water Stress on Growth and Apparent Dinitrogen Fixation of Ladino and Strawberry Clovers

Water stress is a major environmental factor limiting growth and symbiotic N₂ fixation of legumes. This study was conducted to determine the effects of repeated short-term cycles of water stress on growth and N₂ fixation of Ladino clover (Trifolium repens L.) and strawberry clover (T. fragiferum L.). In separate experiments, stolons of each species were trained into a calcined, montmorillonite clay medium, grown to a LAI of 4.0 ± 0.5 under controlled conditions of ambient air and root temperature, and provided with N-deficient nutrient solution. Visible wilting was induced in treatment plants by withholding solution, and stressed plants were re-irrigated following 10 daylight h of wilting. Additional replicates subsequently were allowed to develop second and third water stress cycles. Canopy-air temperature differentials did not differ between control and nonwatered plants at 48 h prior to visible wilting; at 24 h, they differed for strawberry clover but not for Ladino. Leaf water potentials differed significantly between successive stages of water stress within a stress cycle, but not between given stress stages for successive cycles. Ladino clover visually wilted at −1.65 MPa, with maximum values of −2.4 MPa at the end of the stress cycle. Strawberry clover followed a similar pattern but exhibited a paraheliotropic response prior to wilting. Nodule dry weights were not influenced by three cycles of water stress, but nodule water content was significantly decreased within a stress cycle. Within conditions of the experiments, rehydration of stressed nodules and restoration of acetylene reduction activity to control plant values was achieved following all three stress cycles. However, significant decreases in total plant N and in leaf, stolon and total plant dry matter were observed for both species by the third stress cycle. Dry matter and fixed N₂ yields of stoloniferous forage legumes depend on continued initiation and development of vegetative meristems. The results of these experiments indicate that minor, short-term water .stresses, if repeated, can result in significant decreases in productivity. This emphasizes the importance of continued availability of adequate soil moisture for these crops.