Effects of temperature on the regulation of photosynthetic carbon assimilation in leaves of maize and barley.

The aim of this work was to examine the effect of temperature in the range 5 to 30 ° C upon the regulation of photosynthetic carbon assimilation in leaves of the C4 plant maize (Zea mays L.) and the C3 plant barley (Hordeum vulgare L.). Measurements of the CO2-assimilation rate in relation to the temperature were made at high (735 μbar) and low (143 μbar) intercellular CO2 pressure in barley and in air in maize. The results show that, as the temperature was decreased, (i) in barley, pools of phosphorylated metabolites, particularly hexose-phosphate, ribulose 1,5-bisphosphate and fructose 1,6-bisphosphate, increased in high and low CO2; (ii) in maize, pools of glycerate 3-phosphate, triose-phosphate, pyruvate and phosphoenolpyruvate decreased, reflecting their role in, and dependence on, intercellular transport processes, while pools of hexose-phosphate, ribulose 1,5-bis phosphate and fructose 1,6-bisphosphate remained approximately constant; (iii) the redox state of the primary electron acceptor of photosystem II (QA) increased slightly in barley, but rose abruptly below 12° C in maize. Non-photochemical quenching of chlorophyll fluorescence increased slightly in barley and increased to high values below 20 ° C in maize. The data from barley are consistent with the development of a limitation by phosphate status at low temperatures in high CO2, and indicate an increasing regulatory importance for regeneration of ribulose 1,5-bisphosphate within the Calvin cycle at low temperatures in low CO2. The data from maize do not show that any steps of the C4 cycle are particularly cold-sensitive, but do indicate that a restriction in electron transport occurs at low temperature. In both plants the data indicate that regulation of product synthesis results in the maintenance of pools of Calvin-cycle intermediates at low temperatures.