C4 Photosynthesis: Activities of Photosynthetic Enzymes in a Virescent Mutant of Maize Having a Low-Temperature Induced Chloroplast Ribosome Deficiency

A virescent mutant of maize (v16/v16), which has a low temperature induced deficiency in 70S ribosomes, was used to examine whether the enzymes of the C4 pathway and other photosynthetic enzymes are synthesised on chloroplast ribosomes. The mutant and control plants were grown at 20°C and 30°C and the rates of photosynthesis and enzyme activities were compared. There was no photosynthesis in v16/v16 grown at 20°C (pale yellow), while plants grown at 30°C (normal green) had rates equivalent to the wild type and normal enzyme activities. On a leaf area basis, with v16/v16 grown at 20°C, the activity of ribulose 1,5-bisphosphate (RuP2) carboxylase was only 2% that of the wild type grown at this temperature, while the activities of enzymes of the C4 cycle were much higher (as a percentage of the wild type activity: pyruvate,PI dikinase, 22%; NADP-malate dehydrogenase, 35%; NADP-malic enzyme, 47%; and phosphoenolpyruvate (PEP) carboxylase, 68%). In another experiment v16/v16 plants were grown initially at 30°C, then transferred to 20°C. After transfer to 20°C leaves previously formed under 30°C remained green and had normal rates of photosynthesis and enzyme activities, but newly formed leaves were pale yellow (only 11% as much Chl) and had low photosynthesis rates (2% of normal) and RuP2 carboxylase activity (5% of control). However, there was high activity of eight enzymes of the C4 cycle. The low activity of RuP2 carboxylase in the mutant grown at 20°C is consistent with the requirement of 70S ribosomes for its synthesis, while the high activities of enzymes of the C4 cycle, including those which are chloroplastic, suggest their synthesis is nuclear-encoded.