Control of photosynthesis in barley leaves with reduced activities of glutamine synthetase or glutamate synthase

Abstract

Heterozygous plants of barley (Hordeum vul- gare L. cv. Marls Mink) with activities of chloroplastic glutamine synthetase (GS) between 47% and 97% of the wild-type and ferredoxin-dependent glutamate synthase (Fd-GOGAT) activities down to 63% of the wild-type have been used to study the control of photosynthetic fluxes. Rates of CO 2 assimilation measured over a range of intercellular CO 2 concentrations and photon flux den- sities (PFDs) were little different in the wild-type and a mutant with 47% GS, although total activities of ribu- lose-l,5-bisphosphate carboxylase/oxygenase (Rubisco) decreased by about 20% with a decrease in GS to 50% of the wild-type. The quantum efficiencies of photosystem II electron transport (q~PSII) and CO2 assimilation (q5CO2) were determined. ~PSII was lower than expected in mu- tants with 50% less GS under conditions which enhance the photorespiratory flux, but were identical to the wild- type under non-photorespira tory conditions, suggesting that at high rates of photorespiration the electron re- quirement for net CO 2 assimilation declines in plants with decreased GS. This discrepancy in the electron re-