Consequences of an excess Al and a deficiency in Ca and Mg for stomatal functioning and net carbon assimilation of beech leaves

Stomatal function and photosynthesis were investigated in beech seedlings s ubmitted to excess Al, or/and to a deficiency in Ca and Mg. Excess Al in th e nutrient solution promoted a decrease of Ca and Mg leaf contents, while K was increased. Stomatal responses to darkness, ABA and ambient CO2 remaine d normal. In contrast, steady-state stomatal conductance in light was signi ficantly smaller and correlated to a lower accumulation of K in the guard c ells. Similar stomatal responses were observed for Ca-Mg deficient plants. In response to combined Al stress and low Ca and Mg nutrition, stomata rema ined almost insensitive to the different stimuli. The constancy in K guard cell concentration revealed a disturbance in K fluxes. Lower CO2 assimilati on rates and chlorophyll contents, on a leaf area basis, were recorded in r esponse to all treatments. In conclusion, excess Al associated to low Ca an d Mg nutrition lead to a strong stomatal dysfonction and reduced photosynth esis of beech seedlings.