Phenology, leaf gas exchange, growth and seed yield in Medicago polymorpha L. populations affected by water deficit and subsequent recovery

Abstract The aims of this study were to investigate the effect of prolonged water deficit stress and recovery on phenology, net photosynthesis, leaf features, growth, and seed yield in three Medicago polymorpha populations from dry (Mp-173), middle (Mp-345) and wet (Mp-664) climate regions of Tunisia. After ample irrigation for 30 days, plants of each population were divided into two lots: the first lot was irrigated at 100% field capacity (FC), the second at only 45% FC. After 30 days of treatment, one lot of dehydrated plants was rewatered at 100% FC, while the other was maintained at 45% FC. Under water deficit, compared with Mp-664, Mp-345 had lower reduction in total biomass production and leaf gas exchange properties, but greater reduction in leaf dry weight, leaf area and specific leaf area. The response of the leaf morphological features to water shortage in Mp-345 can be considered a significant trait to overcome drought. In contrast, the population from the dry climate responded to water deficit with the highest reductions in all growth traits (except number of nodes), leaf gas exchange properties and leaf features. The slow growth in Mp-173 under water deficit was associated with a lower metabolic impairment of photosynthesis and relatively higher leaf relative water content and greater decrease of osmotic potential. For plants of all populations stomatal limitation proved to be the main cause of decreased net photosynthesis. The results suggest that plants with low values (leaf dry weight, leaf area and specific leaf area) for leaf features are likely to maintain higher leaf relative water content under water deficit. The number of seeds per pod was an important trait in sustaining seed yield in M. polymorpha populations from an arid climate. The variation in seed yield of water-deficit stressed populations was well explained by variation in number of seeds per plant and to a lesser extent by changes in number of pods per plant. The data clearly indicated that Mp-173 showed a higher degree of drought tolerance. Water shortage tolerance of Mp-173 was found to be associated with lower intrinsic growth rate, maintenance of relatively higher leaf relative water content and a lower metabolic impairment of photosynthesis. Results suggest that Mp-173 has higher competitive potential in response to drought stress, which is expected to occur with the ongoing global climatic changes.