ABA-mediated early stomatal response to salt stress enhances salinity tolerance in rice

Salinity tolerance in rice is complex. It constitutes several physiological mechanisms including the control of salt uptake through the transpiration stream, which can be achieved either through selective ion uptake by roots and/or sensitive stomata that control the flow of water through the plant and consequently, the amount of salt accumulating in plant tissue. The author investigated the latter mechanism using two rice cultivars contrasting in tolerance for salt stress-salt-sensitive IR29 and salt-tolerant IR651. Seedlings were grown on a nylon mesh supported by a Styrofoam float suspended on a nutrient solution. Salt stress was imposed by using 100 mM NaCl at 14 d after sowing. Time course measurements of leaf water potential, stomatal conductance (Gs), and transpiration (T)were taken following the imposition of salt stress. Shoot and root samples were then harvested at the same intervals for monitoring ABA levels to investigate its putative role in mediating stomatal responses to salinity. Unlike in IR29, GS and T of IR651 substantially decreased at the onset of salinity and remained low for sometime before they started to recover. However, the GS of the sensitive cultivar did not decrease until a few days later when its leaf water potential started to decrease. The short-term response of the GS in IR651 to salt stress before its leaf water potential started to decline suggests the involvement of a communication mechanism between root and shoot upon exposure of root to salt stress. ABA concentration in both root and leaves of the tolerant cultivar seem to increase much earlier, upon the onset of stress, and essentially coincide with the reduction in Gs, whereas its increase is delayed in IR29 and seems to coincide with the reduction in leaf water potential. These findings suggests the involvement of ABA as a putative root signal in regulating Gs, under salinity stress. Salt-tolerant cultivars may therefore regulate their water content, and hence, salt uptake upon initial exposure to salt stress to avoid accumulation at toxic levels. The recovery of Gs after a lag period suggests a period of acclimation, whereby tolerant cultivars undergo molecular and physiological changes to prevent salt accumulation and/or mitigate any detrimantal effects. However the processes occuring during its acclimation period warrant further investigation. This mechanism can essentially enhance survival upon transplanting of seedlings in saline soil or when salinity level increase due to sudden drought or salt intrusion in soil or irrigation water, as is commonly experienced in coastal saline areas.