EFFECT OF 700-MU-MOL MOL(-1) CO2 AND INFECTION WITH POWDERY MILDEW ONTHE GROWTH AND CARBON PARTITIONING OF BARLEY

The dry weight of barley plants in 700 mu mol mol(-1) CO2 was increase d by 19 d after planting relative to plants grown in 350 mu mol mol(-1 ) CO2. Infection of the second leaf by powdery mildew led to reduced g rowth rates in both 350 and 700 mu mol mol(-1) CO2, but the reduction in growth was transitory in 350 mu mol mol(-1) CO2. Neither the alIome tric coefficient k between shoot and root, nor the leaf weight ratio, was altered by growth in 700 mu mol mol(-1) CO2 or by infection with p owdery mildew. The number of tillers produced increased per plant but not per unit d. wt in 700 mu mol mol(-1) CO2. The growth response of b arley to increased concentrations of CO2 and/or to infection with powd ery mildew was not associated with alterations in net carbon partition ing, so a change in the ratio of photosynthetic to non-photosynthetic tissue, contributed to neither the growth response of barley to 700 mu mol mol(-1) CO2 nor to infection with powdery mildew. The increase in the growth rate of barley in 700 mu mol mol(-1) CO2 and the reduction in the growth rate after infection occurred at the same time as incre ased and reduced rates of net photosynthesis respectively.