COLD-HARDENING RESULTS IN INCREASED ACTIVITY OF ENZYMES INVOLVED IN CARBON METABOLISM IN LEAVES OF WINTER RYE (SECALE-CEREALE L)

Light- and CO2-saturated photosynthesis of nonhardened rye (Secale cer eale L. cv. Musketeer) was reduced from 18.10 to 7.17 mu mol O-2.m(-2) .s(-1) when leaves were transferred from 20 to 5 degrees C for 30 min. Following cold-hardening at 5 degrees C for ten weeks, photosynthesis recovered to 15.05 mu mol O-2.m(-2).s(-1), comparable to the non-hard ened rate at 20 degrees C. Recovery of photosynthesis was associated w ith increases in the total activity and activation of enzymes of the p hotosynthetic carbon-reduction cycle and of sucrose synthesis. The tot al hexose-phosphate pool increase by 30% and 120% for nonhardened and cold-hardened leaves respectively when measured at 5 degrees C. The la rge increase in esterified phosphate in cold-hardened leaves occurred without a limitation in inorganic phosphate supply. In contrast, the m uch smaller increase in esterified phosphate in nonhardened leaves was associated with an inhibition of ribulose-1,5-bisphosphate carboxylas e/oxygenase and sucrose-phosphate synthase activation. It is suggested that the large increases in hexose phosphates in cold-hardened leaves compensates for the higher substrate threshold concentrations needed for enzyme activation at low temperatures. High substrate concentratio ns could also compensate for the kinetic limitations imposed by produc t inhibition from the accumulation of sucrose at 5 degrees C. Nonharde ned leaves appear to be unable to compensate in this fashion due to an inadequate supply of inorganic phosphate.