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High-light effects on CO2 fixation gradients across leaves

Sun, J., Nishio, J.N., Vogelmann, T.C.
Plant, cell and environment 1996 v.19 no.11 pp. 1261-1271
Spinacia oleracea, photosynthesis, carbon dioxide, leaves, light intensity, chlorophyll, fluorescence, photoinhibition, spatial variation
Chlorophyll fluorescence and internal patterns of 14CO2 fixation were measured in sun and shade leaves of spinach after treatment with various light intensities. When sun leaves were irradiated with 2000 micromoles m-2 s-1 for 2h, Fv/FM decreased by about 15%, but 14CO2 fixation was unaffected, whereas shade leaves exhibited a 21% decrease in Fv/FM and a 25% decrease in 14CO2 fixation. Irradiation of sun and shade leaves with 4000 micromoles m-2 s-1 for 4 h decreased Fv/FM by 30% in sun leaves and 40% in shade leaves, while total 14CO2 fixation decreased by 41% in sun leaves and 55% in shade leaves. After light treatment, gradients of CO2 fixation across leaves were determined by measuring 14CO2 fixed in paradermal leaf sections after a 10s pulse of 14CO2. Gradients of 14CO2 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 micromoles PAR m-2 s-1 for 2 h did not alter patterns of carbon fixation across sun leaves, but slightly altered the pattern in shade leaves. In contrast, treatment of sun and shade leaves with 4000 micromoles 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.