Modification of Carbon Partitioning, Photosynthetic Capacity, and O(2) Sensitivity in Arabidopsis Plants with Low ADP-Glucose Pyrophosphorylase Activity

Wild-type Arabidopsis plants, the starch-deficient mutant TL46, and the near-starchless mutant TL25 were evaluated by noninvasive in situ methods for their capacity for net CO(2) assimilation, true rates of photosynthetic O(2) evolution (determined from chlorophyll fluorescence measurements of photosystem II), partitioning of photosynthate into sucrose and starch, and plant growth. Compared with wild-type plants, the starch mutants showed reduced photosynthetic capacity, with the largest reduction occurring in mutant TL25 subjected to high light and increased CO(2) partial pressure. The extent of stimulation of CO(2) assimilation by increasing CO(2) or by reducing O(2) partial pressure was significantly less for the starch mutants than for wild-type plants. Under high light and moderate to high levels of CO(2), the rates of CO(2) assimilation and O(2) evolution and the percentage inhibition of photosynthesis by low O(2) were higher for the wild type than for the mutants. The relative rates of (14)CO(2) incorporation into starch under high light and high CO(2) followed the patterns of photosynthetic capacity, with TL46 showing 31% to 40% of the starch-labeling rates of the wild type and TL25 showing less than 14% incorporation. Overall, there were significant correlations between the rates of starch synthesis and CO(2) assimilation and between the rates of starch synthesis and cumulative leaf area. These results indicate that leaf starch plays an important role as a transient reserve, the synthesis of which can ameliorate any potential reduction in photosynthesis caused by feedback regulation.