Proline synthesis, physiological responses and biomass yield of eggplants during and after repetitive soil moisture stress

Eggplants (Solanum melongena L. cv. Senryo No. 2) were grown at different levels of soil moisture stress in pots under glasshouse conditions in two separate years. Stress was applied at short-term repetitive (T1), long-term repetitive (T2), and prolonged severe stress (T3) during different growing periods compared with a control (T0). The volumetric water content (VWC) of soil, leaf water potential (Ψp), proline content, transpiration rate (Tr), stomatal conductance (GS) and photosynthesis rate (Pn) of leaves and biomass yield were investigated to verify the extent of injury caused during and after moisture stress. The Ψp decreased in response to stress increase, and it increased to the initial level after stress recovery. Leaf proline synthesis as a compatible solute greatly increased by intensified stress either in the short-term or long-term; the highest value was 51.6 mg g−1 fresh weight (FW) recorded for T3 plants that showed irreversible wilting at 115 times greater than the initial value (0.45 mg g−1 FW). T1 and T2 plants showed a reduced pattern of proline synthesis as they produced 18.8 and 39.9 mg g−1 FW, respectively, which were 42 and 89 times greater than the initial level; but the proline synthesis in these plants was markedly reduced to 1.1 and 5.6 mg g−1 FW, respectively, within a day of stress recovery by rewatering. The Tr, GS and Pn were significantly reduced and varied among the eggplants with stress severity and duration. GS and Pn decreased because of stress and increased again after stress, but not necessarily fully, and did not return to previous levels within a day due to stress injury. Biomass yield was also significantly decreased as the moisture stress retarded the physiological functions. Under short-term and long-term stress conditions, eggplants synthesized proline significantly, but in contrast the net photosynthesis remained less affected and maintained its activity. The results of this study suggest that proline synthesis increases during stress increase and returns to the initial level after stress recovery, which seems to act as part of a survival mechanism. Therefore, eggplants (Solanum melongena L. cv. Senryo No. 2) have an adaptive potential to acclimate under stress conditions.