CO2 gas exchange and phosphoenolpyruvate carboxylase activity in leaves of Zea mays L.

Important gas exchange characteristics of C4 plants depend on the properties of phophoenolpyruvate carboxylase (PEPC), the enzyme catalysing the primary fixation of CO2 during C4 photosynthesis. In this study, the relationship between intracellular resistance for CO2 fixation (ri) at high photosynthetically active photon flux densities (PPFD) and maximum PEPC activity in vitro (Vpm) was examined in leaves of Zea mays L. The analysis allowed the estimation of the Michaelis constant Kp of the enzyme for CO2 (or the equivalent number for bicarbonate) in vivo. At low PPFD (below 100 μmol m-2 s-1) the initial slopes of the curves describing net CO2 uptake rate A as a function of intercellular CO2 concentration ci increased with increasing PPFD. The increase (i. e. a decrease in ri) was interpreted as due to a reversible activation of PEPC by light. Including this assumption into a model of C4 photosynthesis enabled us to reproduce A(ci) response curves measured at low levels of PPFD. Fitting the model to experimental data resulted in values for KI, the PPFD at which PEPC reaches half of its full activation, of about 200 μmol m-2 s-1. Similar results were derived from the dependence of ri on PPFD. The analysis of the relationships between ri and Vpm and between ri and PPFD, as well as fitting of the model to gas exchange data all gave rise to estimates for the resistance for CO2 transfer within mesophyll cells that are comparable with those known from C3 plants.