Olive trees are remarkably drought-tolerant, despite a substantial variability between cultivars. We first investigated the response of mature olive trees to severe dehydration during the long Israeli summer. The trees acclimated to stem water potential (Ψstem) as low as -8.1 MPa over seven dry months through a significant increase in leaf osmotic concentration. Despite likely xylem cavitation, they sustained stomatal conductance and positive assimilation, fully recovering after winter rains.
Then we compared ‘Barnea’ (productive) and ‘Souri’ (drought-adapted) olive cultivars in a pot experiment. Surprisingly, ‘Souri’s leaf hydraulic conductivity was more vulnerable to low Ψstem than ‘Barnea’s. However, ‘Souri’ compensated with higher osmotic content and cell rigidity, allowing it to maintain optimal photochemical potential at lower Ψstem. Souri’s lower transpiration enabled it to withstand a longer drought while avoiding low θ. Barnea’s larger xylem vessels and hydraulic conductivity supported higher stomatal conductance and assimilation rate, which nurtured its higher productivity but resulted in quick depletion of θ. These results suggest that hydraulic resistance or the ability to sustain low θ do not set the tradeoff between productivity and drought adaptation in olive leaves.
