Leaf water status, osmoregulation and secondary metabolism as a model for depicting drought tolerance in Argania spinosa

The present investigation was undertaken to characterise and to distinguish four contrasting Argania spinosa ecotypes in terms of drought tolerance by exploring the changes of leaf water status, osmoregulation and secondary metabolism. A. spinosa plants corresponding to four contrasting ecotypes (Lks, Alz, Rab and Adm) were subjected to drought stress. The results exhibited that there was a significant decrease in predawn leaf water potential (Psi(pd)), stomatal conductance (g(s)) and leaf relative water content under the influence of the intensity and duration of drought stress. Negative and significant correlations were recorded between epicuticular wax load (EWL) and residual transpiration rate. Electrolyte leakage (EL) increased significantly in leaves of plants under drought stress treatment compared to control plants. Furthermore, our data revealed that drought stress can induce shikimate and phenylpropanoid pathways in A. spinosa. A significant induction of phenylalanine ammonium lyase (PAL), shikimate dehydrogenase (SKDH) and cinnamate 4-hydroxylase (C4H) enzymes and an increase in polyphenol content were recorded, of which Lks showed the highest induction and accumulation among ecotypes. Accumulation of polyphenols was positively correlated with the SKDH, PAL and C4H activities. The strong induction of secondary metabolism in Lks might be linked to its better ability of drought tolerance. The proline and soluble sugar content in leaves of all ecotypes increased substantially in parallel with the severity of stress-induced. According to canonical discriminant analysis of our data, the four ecotypes were separated by the following physiological and biochemical parameters: EL, g(s), EWL, soluble sugars and polyphenols.