EFFECTS OF LEAF AGE, BASAL MERISTEM AND INFECTION WITH POWDERY MILDEWON PHOTOSYNTHESIS IN BARLEY GROWN IN 700-MU-MOL MOL(-1) CO2

The rate of net photosynthesis in the second leaf of barley was higher in 700 than 350 mu mol mol(-1) CO2 when measured in the CO2 concentra tion in which the plants were grown, but the magnitude of this differe nce decreased as the leaf aged. Infection by powdery mildew accelerate d the decline in net photosynthesis of leaves grown in either 350 or 7 00 mu mol mol(-1) CO2. A/C-i curves allowed the reduction in net photo synthesis of plants exposed to 700 mu mol mol(-1) CO2 or after infecti on by powdery mildew to be related to changes in the carboxylation eff iciency or in the regeneration of ribulose 1,5-bisphosphate. The carbo xylation efficiency declined in plants exposed to 700 mu mol mol(-1) C O2. In plants infected with powdery mildew, the reduction in net photo synthesis was associated with both reduced carboxylation efficiency an d reduced ability to regenerate ribulose 1,5-bisphosphate. Reduced car boxylation efficiency of the second leaf of plants grown in 700 mu mol mol(-1) CO2 was not associated with a reduction in the concentration of rubisco within the leaf. In contrast to the presence of a close exo genous sink, leaf age had large effects on the acclimation of photosyn thesis to 700 mu mol mol(-1) CO2.