High‐light effects on CO₂ fixation gradients across leaves

Abstract

ABSTRACT Chlorophyll fluorescence and internal patterns of 14 CO 2 fixation were measured in sun and shade leaves of spinach after treatment with various light intensities. When sun leaves were irradiated with 2000μmol m −2 s −1 for 2h, F V / F M decreased by about 15%, but 14 CO 2 fixation was unaffected, whereas shade leaves exhibited a 21% decrease in F v / F M and a 25% decrease in 14 CO 2 fixation. Irradiation of sun and shade leaves with 4000μmol m −1 for 4 h decreased F V / F M by 30% in sun leaves and 40% in shade leaves, while total 14 CO 2 fixation decreased by 41% in sun leaves and 55% in shade leaves. After light treatment, gradients of CO 2 fixation across leaves were determined by measuring 14 CO 2 fixed in paradermal leaf sections after a 10s pulse of 14 CO 2 . Gradients of 14 CO 2 fixation in control sun and shade leaves were identified when expressed on a relative basis and normalized for leaf depth. Treatment of leaves with 2000 μmol PAR m −2 s −1 for 2h did not after patterns of carbon fixation across sun leaves, but slightly altered the pattern in shade leaves. In contrast, treatment of sun and shade leaves with 4000μmol m −2 s −1 for 4h decreased carbon fixation more in the palisade mesophyll cells than in the spongy mesophyll cells of sun and shade leaves, and fixation in medial tissue of shade leaves was dramatically decreased compared to the adaxial and abaxial tissue. The interaction between leaf anatomy and biochemical parameters involved in tolerance to photoinhibition in spinach is discussed.