LEAF CO₂ EXCHANGE RATES IN WINTER RYE GROWN AT COLD-HARDENING AND NONHARDENING TEMPERATURES

Abstract

CO 2 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% O 2 and as a function of irradiance and CO 2 concentration. A decrease in O 2 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% O 2 than 21% O 2 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% O 2 than 21% O 2 when measured at 10 °C but about 1.8-fold higher when measured at 20 °C. Similarly, at high CO 2 concentrations, leaves of unhardened plants exhibited a greater temperature-dependent stimulation of photosynthetic rates by low O 2 than leaves of hardened plants. An increased capacity for CO 2 utilization in cold-hardened rye could be observed when photosynthetic rates were monitored at 2% O 2 . Differences in transpiration rates were insufficient to account for these results. The increased capacity for CO 2 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 CO 2 exchange was measured at 2% O 2 . We conclude that photosynthetic acclimation in vivo may be severely limited due to the restrictions imposed by photorespiration.Key words: Cold-hardening, winter rye, CO 2 exchange, photosynthesis