Differential responses of saltbush Atriplex halimus L. exposed to salinity and water stress in relation to senescing hormones abscisic acid and ethylene

Drought and salinity induce water deficit, but may also have distinct effects on plant metabolism. To compare their impact on leaf senescence in relation to ABA and ethylene synthesis, young plants of Atriplex halimus L. were exposed to iso-osmotic concentrations of NaCl (160 mM) or PEG (15%) in nutrient solution. Plant growth and development were more affected by PEG than by NaCl. Stressed plants remained able to reduce their osmotic potential, but the nature of accumulated organic osmocompatible solutes varied according to the stressing agent. Glycinebetaine accumulated to a greater extent in salt-treated plants than in water-stressed plants. Sodium chloride induced the accumulation of non-reducing sucrose, while PEG-treated plants mainly accumulated reducing glucose and fructose. Abscisic acid (ABA) accumulated in response to salt, while ethylene was synthesized mainly by PEG-treated plants and was involved in the induction of early senescence processes characterized by synthesis of reactive oxygen species, peroxidation of membrane lipids and a decrease in chlorophyll content. ABA sensitivity of stressed tissues was markedly different in response to salt and in response to non-ionic osmotic stress, and exogenous ABA (50 μM) had contrasting effects on most physiological parameters depending on the stressing agent. Exogenous ABA induced a decrease in root and shoot growth and sucrose content, and an increase in reactive oxygen species content in salt-stressed plants. In contrast, exogenous ABA increased growth in PEG-treated plants in relation to an improvement of water use efficiency resulting from a more efficient stomatal control. Exogenous ABA increased ethylene synthesis in salt-treated plants, but had only marginal impact on PEG-treated ones. The xero-halophyte A. halimus thus responds in a contrasting way to salt and water stress, through accumulation of distinct osmocompatible solutes and hormonal compounds such as ethylene and ABA could play distinct roles in stress-induced senescence processes.