Relationships between CO2 Exchange Rates and Activities of Pyruvate,Pi Dikinase and Ribulose Bisphosphate Carboxylase, Chlorophyll Concentration, and Cell Volume in Maize Leaves

Abstract

The relationships between CO2-exchange rate (CER), DNA and chlorophyll (Chl) concentrations, pyruvate,Pi dikinase (PPDK) and ribulose bisphosphate carboxylase (RuBPCase) activities in 10 maize genotypes were investigated. The in vivo degrees of activation of PPDK and RuBPCase were estimated to make meaningful comparisons with CER. In leaves at a photosynthetic photon flux density (PPFD) of 720 .mu.mol m-2 s-1, in vivo PPDK degree of activation was 80% of that of PPDK fully activated in vitro, whereas RuBPCase could not be further activated in vitro, suggesting that RuBPCase was fully activated in vivo. CER varied .apprx. 50% among the genotypes tested. Significant genetic differences were observed for the average weight of a cell (estimated by fresh wt/mg DNA), but this character was not correlated with CER expressed on a fresh weight basis. CER was correlated with Chl concentration, and with estimates of the in vivo degree of activation of PPDK and RuBPCase. In maize, CER is apparently controlled by the metabolic components of photosynthesis rather than by membrane resistances to CO2. If the latter factor were controlling CER, then smaller cells with higher amounts of exposed cell surface area per unit cell volume would have lower resistance to CO2 diffusion, and therefore higher CER. When data were expressed on a DNA basis (proportional to a per cell basis), results indicated that larger cells (i.e., those with higher fresh wt/mg DNA) have a higher content of Chl, and higher PPDK and RuBPCase activities, resulting in higher CER than in smaller cells.