MEASUREMENTS OF MESOPHYLL CONDUCTANCE, PHOTOSYNTHETIC ELECTRON-TRANSPORT AND ALTERNATIVE ELECTRON SINKS OF FIELD-GROWN WHEAT LEAVES

Photosynthetic electron transport drives the carbon reduction cycle, t he carbon oxidation cycle, and any alternative electron sinks such as nitrogen reduction. A chlorophyll fluorescence- based method allows es timation of the total electron transport rate while a gas-exchange-bas ed method can provide estimates of the electron transport needed for t he carbon reduction cycle and, if the CO2 partial pressure inside the chloroplast is accurately known, for the carbon oxidation cycle. The g as-exchange method cannot provide estimates of alternative electron si nks. Photosynthetic electron transport in flag leaves of wheat was est imated by the fluorescence method and gas-exchange method to determine the possible magnitude of alternative electron sinks. Under non-photo respiratory conditions the two measures of electron transport were the same, ruling out substantial alternative electron sinks. Under photor espiratory conditions the fluorescence-based electron transport rate c ould be accounted for by the carbon reduction and carbon oxidation cyc le only if we assumed the CO2 partial pressure inside the chloroplasts to be lower than that in the intercellular spaces of the leaves. To f urther test for the presence of alternative electron sinks, carbon met abolism was inhibited by feeding glyceraldehyde. As carbon metabolism was inhibited, the electron transport was inhibited to the same degree . A small residual rate of electron transport was measured when carbon metabolism was completely inhibited which we take to be the maximum c apacity of alternative electron sinks. Since the alternative sinks wer e small enough to ignore, the comparison of fluorescence and gas-excha nge based methods for measuring the rate of electron transport could b e used to estimate the mesophyll conductance to CO2 diffusion. The mes ophyll conductance estimated this way fell as wheat flag leaves senesc ed. The age-related decline in photosynthesis may be attributed in par t to the reduction of mesophyll conductance to CO2 diffusion and in pa rt to the estimated decline of ribulose 1,5-bisphosphate carboxylase a mount.