Effects of a calcium deficiency on stomatal conductance and photosynthetic activity of Quercus robur seedlings grown on nutrient solution

The effects of a calcium deficiency on stomatal functions and photosynthesis were investigated in Quercus robur seedlings grown on a nutrient solution. A severe calcium deficiency did not perturb stomatal reactivity to abscisic acid, and stomatal aperture in darkness was only slightly increased. On the other hand, stomatal conductance under full light, and net CO2 assimilation rates decreased to one-half of the controls. A slowdown of stomatal opening during dark-light transitions was detected in the deficient leaves. Low Ca2+ availabilty could reduce the light activation of chloroplastic enzymes involved in organic osmoticum production in the guard cells. The reduction of net CO2 assimilation was associated with a maintenance of the CO2 mole fraction in the substomatal spaces and with a stability of the photochemical efficiency of photosystem II (PS II) in dark-adapted leaves. Combined measurements of gas exchange and photochemical efficiency allowed the computation of the CO2 mole fraction at the site of carboxylation in the chloroplast, which decreased significantly in the Ca-deficient leaves. This result suggests that a lower CO2 availability at the carboxylation site was the major factor limiting CO2 assimilation under calcium deficiency.