THE PLANT CELL, Vol 5, Issue 8 953-961, Copyright © 1993 by American Society of Plant Biologists
Carbon Fixation Gradients across Spinach Leaves Do Not Follow Internal Light Gradients
J. N. Nishio, J. Sun and T. C. Vogelmann
Department of Botany, University of Wyoming, Laramie, Wyoming 82071
In situ measurements of 14C-CO2 incorporation into 40-[mu]m paradermal leaf
sections of sun- and shade-grown spinach leaves were determined.
Chlorophyll, carotenoid, and ribulose-1,5-bisphosphate
carboxylase/oxygenase (Rubisco) content in similar 40-[mu]m paradermal leaf
sections was also measured. The carbon fixation gradient did not follow the
leaf internal light gradient, which decreases exponentially across the
leaf. Instead, the 14C-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 occurred in
the palisade mesophyll and 40% occurred in the spongy mesophyll. Carbon
reduction correlated well with Rubisco content, and no correlation between
chlorophyll and carotenoid content and Rubisco was observed in sun plants.
The correlation among chlorophyll, carotenoids, Rubisco, and carbon
fixation was higher in shade leaves than in sun leaves. The results are
discussed in relation to leaf photosynthetic and biochemical measurements
that generally consider the leaf as a single homogeneous unit.
