CARBON FIXATION GRADIENTS ACROSS SPINACH LEAVES DO NOT FOLLOW INTERNAL LIGHT GRADIENTS

In situ measurements of C-14-CO2 incorporation into 40-mum paradermal leaf sections of sun. and shade-grown spinach leaves were determined. Chlorophyll, carotenoid, and ribulose-1,5-bisphosphate carboxylase/oxy genase (Rubisco) content in similar 40-mum paradermal leaf sections wa s also measured. The carbon fixation gradient did not follow the leaf internal light gradient, which decreases exponentially across the leaf . Instead, the C-14-CO2 fixation was higher in the middle of the leaf. Contrary to expectations, the distribution of carbon fixation across the leaf showed that the spongy mesophyll contributes significantly to the total carbon reduced. Approximately 60% of the carboxylation occu rred in the palisade mesophyll and 40% occurred in the spongy mesophyl l. Carbon reduction correlated well with Rubisco content, and no corre lation between chlorophyll and carotenoid content and Rubisco was obse rved in sun plants. The correlation among chlorophyll, carotenoids, Ru bisco, and carbon fixation was higher in shade leaves than in sun leav es. The results are discussed in relation to leaf photosynthetic and b iochemical measurements that generally consider the leaf as a single h omogeneous unit.