Chilling Injury in Mature Leaves of Rice. II. Varietal Differences in the Response to Interactions Between Low Temperature and Light Measured by Chlorophyll Fluorescence at 77k and the Quantum Yield of Photosynthesis

Characteristics of photosynthetic CO2 exchange and 77K chlorophyll fluorescence in attached and detached leaves of rice were used to examine differences between five varieties exposed to a combination of bright light and low temperature. Preliminary studies established that attached leaves of varieties did not differ in the light dependence of photosynthesis at CO2 saturation or fluorescence properties when grown in full sunlight and measured under comparable conditions. All varieties showed a depression in maximum photosynthetic rate, but not in quantum yield or chlorophyll fluorescence when examined in the late afternoon after a period of active photosynthesis in air at 25°C. Attached leaves of all varieties showed depression of the maximum rate of photosynthesis and of quantum yield as well as an increase in Fo and a decrease in Fv/Fm of chlorophyll fluorescence at 77K following horizontal exposure to bright light in air at 25°. Similar results were obtained with detached leaves exposed to bright light while floating on water at 25°C. The rate of photosynthetic CO2 fixation in air as a function of temperature was examined in attached leaves of two representative varieties, Japonica-like cv. Hungarian- 1, and Indica-derived cv. IR-8. The former was capable to rates of photosynthesis at 10°C which were twice those of the latter, although the rates at 25°C were similar in both varieties. When detached leaves of five varieties of rice were floated on water at 10°C while exposed to 700 �mol photons m-2 s-1 of white light, consistent differences in the response of quantum yield, light and CO2 saturated O2 evolion and 77K fluorescence properties were found. Exposure to bright light at low temperature exaggerated the effects of bright light on these parameters, and larger effects were found in the Indica-derived varieties than in Japonica-like cv. Hungarian-1. These treatments at 10°C did not influence the extent of increase in Fo, but led to a greater decline in Fv/Fm of 77K chlorophyll fluorescence compared with treatments at 25°C. Varieties in which there was a larger decline in Fv/Fm showed slower recovery of variable fluorescence in the dark at 25°C, and especially at 10°C. These responses to in vitro treatments seem capable of distinguishing varieties such as cvv. Hungarian-1 and Er Bai Ai, which are believed to be more chilling tolerant by other criteria, from chilling sensitive varieties such as Gui Chao-2 and IR-8. Even though there is potential for application of these methods in screening programs, there is little evidence that the lesions in primary photosynthetic processes indicated by these methods are significant determinants of photosynthesis under field conditions.