Photosynthesis, Leaf Resistances, and Ribulose-1,5-Bisphosphate Carboxylase Degradation in Senescing Barley Leaves

The relationship between loss of ribulose-1,5-bisphosphate carboxylase (RuBPCase) and the decline in photosynthesis during the senescence of barley primary leaves was assessed. Loss of RuBPCase accounted for about 85% of the decrease in soluble protein. RuBPCase was highly correlated with in vitro RuBPCase activity (r = 0.95) and gross photosynthesis (r = 0.96). However, the rate of photosynthesis per milligram RuBPCase increased during the early stages of leaf senescence. The concentration of nonreducing sugars was negatively correlated (1% level) with photosynthesis. Free alpha-amino N, in contrast to nonreducing sugars, declined markedly during senescence. A decrease in chlorophyll and an increase in in vitro protease activity was observed, but these changes did not appear to be closely related to the decline in photosynthesis and RuBPCase. Mesophyll resistance increased at the same rate that photosynthesis and RuBPCase declined. Stomatal resistance increased more rapidly than mesophyll resistance and accounted for about 24% of the total increase in resistance to CO(2) diffusion. The concentration of CO(2) in the intercellular air spaces decreased during the last stage of senescence. Although loss of RuBPCase probably is the primary event responsible for the decline in photosynthesis during leaf senescence, other factors such as in vivo regulation and stomatal aperture must also be considered.