Stomatal and non-stomatal limitations of photosynthesis under water stressin field-grown grapevines

Long-term induced water stress in field-grown grapevine leads to a progress ive decline of stomatal conductance, accompanied by a decrease in CO2 assim ilation (40%). The apparent quantum yield also decreases (59%), which may r eflect a relative increase in alternative processes for electron consumptio n. There is also a shift to non-stomatal regulation, as judged from signifi cant depletions (37%) in maximum photosynthesis rate at saturating CO2 rela ted to limited ribulose biphosphate (RuBP) regeneration, whereas small, non -significant effects are observed on carboxylation efficiency. A high corre lation (87%) between photosynthesis and stomatal conductance is observed fo r all experimental data and declines in intercellular CO2 concentration par allel reductions in stomatal conductance. The data show that field response of grapevines to increasing soil water deficit involves stomatal and non-s tomatal effects but, due to gradually induced drought, regulation mechanism s able to adjust mesophyll capacity to the average CO2 supply. The non-stom atal adjustment seems to be exerted mainly in metabolic pathways related wi th the RuBP regeneration. Contrasting characteristics were observed for bot h cultivars. Tempranillo exploited the non-stressful conditions successfull y, whereas Manto Negro, responding to its reputation as more drought resist ant, showed a higher intrinsic water use efficiency, particularly for low w ater availability. This advantage seems to be due to lower non-stomatal lim itations.