Effects of water stress on photosynthesis in relation to diurnal accumulation of carbohydrates in source leaves

Net photosynthetic rates, stomatal and mesophyll conductances to CO2 uptake, water soluble and total nonstructural carbohydrates contents, specific leaf weights of fully expanded source leaves, and elongation rates of rapidly expanding leaves were measured on 2 days during a period of water stress in soybean and sunflower plants in a controlled environment. Compared with control plants, elongation rates of expanding leaves and translocation rates of dry weight from source leaves in the light were more reduced by stress than were net photosynthetic rates of source leaves. Over the 8-h light period, the dry weight increase of source leaves was up to 23 mg dm−2 (1.5 × control) higher in stressed plants, but was not in all cases higher in stressed than control plants. In stressed plants a smaller fraction of the increase in dry weight in source leaves in the light was in nonstructural carbohydrates. At the end of the light period, water soluble and total nonstructural carbohydrates were up to 9 mg dm−2 higher in stressed than control leaves in sunflower, but were not higher in soybean. No differences in carbohydrate contents at the end of the light period were found in sunflower between the 2 days of stress, although stress became more severe in terms of lower rates of photosynthesis, translocation, and leaf elongation. The approximately threefold reductions in net photosynthetic rates in stressed leaves were related to both lower stomatal and lower mesophyll conductances. Mesophyll conductances of stressed leaves were not significantly correlated with water soluble carbohydrate content, total nonstructural carbohydrate content, or specific leaf weight in either species.