Chlorophyll a Fluorescence and Carbon Assimilation in Developing Leaves of Light-Grown Cucumber

The development of photochemical activity and carbon assimilation in light-grown cucumber (Cucumis sativus L. cv Natsusairaku) leaves was studied to determine the pattern of acquisition and its relationship to leaf growth and expansion. Measurements of chlorophyll a fluorescence showed that leaves acquire photochemical function over a period of 6 or more days, and gas exchange studies showed increases in carbon assimilation over a parallel time period. As leaves expand and mature, they undergo a sequential, three-step series of changes in fluorescence response. The initial kinetics show the absence of wholly functional quenching mechanisms. Dynamic imaging of fluorescence kinetics showed that a temporal series of changes occurred within defined areas of individual developing leaves. The spatial acquisition of photochemical activity in leaves was basipetal as is their directional expansion, development of air spaces and stomata, and the cessation of imported carbon.