Why abaxial illumination limits photosynthetic carbon fixation in spinach leaves

Limitations of carbon fixation within spinach leaves due to light and CO2 were investigated. Under equivalent photon fluxes, carbon fixation was higher when leaves were irradiated adaxially compared to abaxially. Maximal carbon fixation occurred in the middle of the palisade mesophyll under adaxial illumination, whereas, maximal carbon fixation occurred in the spongy mesophyll under abaxial illumination. Total carbon fixation and the pattern of carbon fixation across leaves were similar, when leaves were irradiated with 800 mu mol quanta m(-2) s(-1) either adaxially alone or adaxially plus abaxially (1,600 mu mol quanta m(-2) s(-1)). In contrast, when both leaf surfaces were irradiated simultaneously with 200 mu mol quanta m(-2) s(-1), total carbon fixation increased and carbon fixation in the middle of the leaf was higher compared to leaves irradiated unilaterally with the low Light. Feeding (CO2)-C-14 through either the adaxial or abaxial leaf surface did not change the pattern of carbon fixation across the leaf. Increasing (CO2)-C-14 pulse-feeding times from 5 s to 60 s allowed more (CO2)-C-14 to be fixed but did not change the pattern of (CO2)-C-14 fixation across the leaf We concluded that in spinach, the distribution of both light and Rubisco activity within leaves has significant effects on the patterns of carbon fixation across leaves; whereas CO2 diffusion does not appear to affect the carbon fixation pattern within spinach leaves.