TEMPERATURE-DEPENDENCE OF PHOTOSYNTHETIC CO2 EXCHANGE IN THE LEAVES OF COLD-ACCLIMATED WINTER RYE PLANTS

CO2 gas exchange and transpiration in completely expanded leaves of 11 -week-old hardened (grown at 5 degrees C) and 3-week-old nonhardened ( grown at 25 degrees C, untreated control) plants of winter rye (Secale cereale L.) were followed at varying light levels, temperatures, and CO2 and O-2 concentrations. Stomatal resistance to CO2 diffusion was c alculated together with CO2 concentration in the aqueous phase at the mesophyll cell surface and the values of the apparent carboxylation co nstant and ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) s pecificity factor. By using multiway analysis of variance (ANOVA), we excluded the specific effect of hardening on the carboxylation constan t. The Rubisco specificity factor was 83 at 24 degrees C, notwithstand ing the hardening procedure, and increased with decreasing ambient tem perature, especially in nonhardened plants. At a CO2 concentration tha t saturated photosynthesis (2720 ppm), we did not observe the Warburg effect on the leaf CO2 exchange rate in either nonhardened or hardened plants. However, the ribuloso-1,5-bisphosphate (RuBP) carboxylation r ate in the leaves of hardened plants at 25 degrees C was 25-30% higher than in control plants, both at high and low (21 and 1.5%) oxygen con centration, whereas, at 5 degrees C, the advantage of the hardened pla nts reached 60-65%. Resistance to CO2 diffusion in the hardened plants exceeded that in the control by a factor of 2-4, and, in both cases, resistance indices increased as the leaf temperature changed from 5 to 25 degrees C. The authors conclude that, due to cold-acclimation, rye leaves expanded their intracellular RuBP pool, which, in its turn, pr omoted specific (per leaf) Rubisco activity at higher ambient CO2 conc entration. The capacity to accumulate the major carboxylation substrat e in the leaves of hardened plants seems to be a manifestation of phot osynthesis adaptation to prolonged exposure to low above-zero temperat ures.