Leaf Co2 Exchange Rates in Winter Rye Grown at Cold-Hardening and Nonhardening Temperatures

CO2 gas exchange measurements were performed on cold-hardened and unhardened Puma rye (Secale cereale L.) leaves at 10 and 20 °C in the presence of 2 and 21% O2 and as a function of irradiance and CO2 concentration. A decrease in O2 concentration from 21 to 2% appeared to result in a differential stimulation of photosynthetic rates in cold-hardened and unhardened rye leaves. Under light saturating conditions the former exhibited photosynthetic rates that tended to be 1.4- to 1.5-fold higher in the presence of 2% O2 than 21% O2 when measured at either 10 or 20 °C. In contrast, unhardened rye leaves exhibited photosynthetic rates that tended to be about 1.3-fold higher in the presence of 2% O2 than 21% O2 when measured at 10 °C but about 1.8-fold higher when measured at 20 °C. Similarly, at high CO2 concentrations, leaves of unhardened plants exhibited a greater temperature-dependent stimulation of photosynthetic rates by low O2 than leaves of hardened plants. An increased capacity for CO2 utilization in cold-hardened rye could be observed when photosynthetic rates were monitored at 2% O2. Differences in transpiration rates were insufficient to account for these results. The increased capacity for CO2 utilization observed in vivo is discussed with respect to a recent report which described an increased capacity for photosynthetic electron transport in vitro in cold-hardened rye thylakoid membranes. However, photosynthetic acclimation to the contrasting growth temperatures could only be observed when CO2 exchange was measured at 2% O2. We conclude that photosynthetic acclimation in vivo may be severely limited due to the restrictions imposed by photorespiration.