Regulation of photosynthesis in leaves of C 4 plants following a transition from high to low light

Abstract

The aim of this work was to investigate the factors whichregulate photosynthetic carbon assimilation in leaves of maize ( Zea mays ) and Amaranthus edulis following a decrease in photon flux density. The principal observations in maize were that, after a decrease in photon flux density; (i) there was a transient inhibition of the rate of carbon assimilation; (ii) the pool of phosphor enol pyruvate rose while the flux decreased, indicating rapid regulation of the activity of phosphor enol pymvate carboxylase; (iii)the triose- P /glycerate-3- P ratio declined drastically; at the same time the pool of pyruvate increased several-fold; (iv) the activation state of NADP + -malate dehydrogenase declined rapidly, whilethe activation states of pyruvate, P i dikinase, fructose-1,6-bisphosphatase and sucrose phosphate synthase changed relatively slowly; (v) therewas a rapid decrease in non-photochemical quenching of chlorophyll fluorescence and a rapid oxidation and re-reduction of Q A followed by a slow oxidation of Q A ; (vi) in A. edulis the content of aspartate and of the total pool of C 4 -cycleintermediates increased. The observations are discussed in terms of the regulation of C 4 photosynthesis and in terms of the supply of, and demand for, the products of electron transport by carbon metabolism. It is suggested that the transient inhibition of photosynthetic carbon assimilation in maizeleaves is accompanied by a high demand for ATP which is met by cyclic electron transport