Plants under Climatic Stress: VI. Chilling and Light Effects on Photosynthetic Enzymes of Sorghum and Maize.
The activity of several photosynthetic enzymes was unaltered by exposure of sorghum or maize to low temperatures (10 C) and light (170 w m(-2)). Two light-activated C(4)-pathway enzymes, NADP-malate dehydrogenase and pyruvate Pi dikinase, were reduced in activity, and this was largely attributable to a loss of enzyme rather than to incomplete enzyme activation. Loss of NADP-malate dehydrogenase was more marked in sorghum than in maize, and in both species no loss occurred at 10 C when light levels were reduced from 170 to 50 w m(-2). A light-dependent, low temperature-induced loss of catalase activity was also observed in maize leaves.The rate of in vivo activation of pyruvate Pi dikinase following illumination was reduced at 10 C compared with that at 25 C, but no immediate effect of low temperature on the in vivo activation of NADP-malate dehydrogenease could be measured. A similar differential effect of temperature on the rates of activation of these two enzymes was found in vitro. Arrhenius type plots of pyruvate Pi dikinase from sorghum and maize demonstrated a further sensitivity to low temperature. A sharp increase in the activation energy of this enzyme was observed below 12 C, both in the presence and absence of Triton X-100. No change in the activation energy of maize leaf malic enzyme, NADP-malate dehydrogenase, fructose-1, 6-diphosphate aldolase, or NADP-glyceraldehyde 3-P dehydrogenase occurred over a temperature range of 6 to 30 C.The postillumination time course of pyruvate Pi dikinase activation, net photosynthesis and stomatal opening was followed. Reduction in the rate of response that occurred with decreasing temperature was similar in all cases, and at any one temperature, pyruvate Pi dikinase activation slightly preceded increasing photosynthesis rates. Causal relationships could not, however, be proved.