FIELD INVESTIGATIONS OF PHOTOSYNTHETIC ACTIVITY, GAS-EXCHANGE AND WATER POTENTIAL AT DIFFERENT LEAF AGES IN WELWITSCHIA-MIRABILIS DURING A SEVERE DROUGHT

Drought stress amplified the age-dependent gradient in photosynthetic activity in leaves of Welwitschia mirabilis which has been previously demonstrated in well-watered plants. However, neither predawan nor day time leaf water potential changed with leaf age under conditions of li mited soil water availability. During the course of a day the combinat ion of increasing leaf water deficit, high temperature, high water vap our deficit of the air and high radiation load seriously disrupted car bon metabolism. This led to CO2 loss and, thus, a negative daily carbo n balance with largest losses in the older part of the leaf. Stomatal conductance was greatly diminished most of the day except in the early light period. Reduction of photosynthetic efficiency was preceded by photoinhibition as indicated by a decrease in the photochemical effici ency of photosystem II at the highest leaf temperature. Again, this re duction was more pronounced in the older leaf sections. Additionally, it was demonstrated that the estimation of bulk leaf water potential u sing leaf slices of Welwitschia mirabilis with the pressure chamber ga ve meaningful results; this was the most useful method for field inves tigations of leaf water potential on such a difficult material.