Chilling Injury in Mature Leaves of Rice. I. Varietal Differences in the Effects of Chilling on Canopy Photosynthesis Under Simulated 'Dry Cold Dew Wind' Conditions Experienced in South-East China

Abstract

Canopy temperature and incident photosynthetically active radiation were measured in experimental rice plots in Guangzhou, South China, during conditions which approximated those of the 'dry cold dew wind', a chilling phenomenon which is responsible for substantial yield reduction in rice. These field conditions were then reproduced in controlled environment photosynthetic measurement chambers. Mature plants of five varieties of rice were arranged to simulate a canopy in these chambers which was then exposed to a 3-day chilling treatment (21°C day/10°C night) 14 days after anthesis. Canopy photosynthesis was measured before, during and after the chilling period. A Japonica-like cultivar, Hungarian-1 from Central Europe, and an Indica-like cultivar, Lemont from Texas, were markedly less sensitive to a chilling-dependent reduction in photosynthesis than were cultivars Gui Chao-2, Er Bai Ai and IR-8 from South-East China and The Philippines. The extent of chilling-dependent reduction in photosynthesis was greater if roots of plants in the simulated canopy were allowed to track air tempera- ture at night. This led to a 5-7°C differential between root and air temperature in the morning and was associated with a midday depression of photosynthesis in all varieties. If root temperature was kept at 20°C during the day and night and air temperature controlled at 21°C day/ 10°C night, the effects of chilling on photosynthesis were much less pronounced.

These changes in canopy photosynthesis following chilling treatments were correlated with changes in leaf level parameters. The level of soluble sugars and starch in leaves was found to remain high during the first cool night and to increase further during the next day. In cultivars which showed pronounced inhibition of canopy photosynthesis throughout the chilling treatment (Gui Chao-2, IR-8), soluble sugars remained at high levels until temperatures were returned to control values. In the less sensitive cultivar, Hungarian-1, soluble sugars declined throughout the chilling treatment. These results suggest inhibition of photosynthesis may be associated with sugar retention in leaves of rice at low temperatures, and imply differences between cultivars in this response. Other photosynthetic parameters monitored included the maximum rate of light and CO₂ saturated photosynthesis, the quantum yield of photosynthesis, and F_o and F_v/F_m of chlorophyll fluorescence at 77K. Surprisingly, Hungarian-1, the cultivar which showed least impairment of canopy photosynthesis showed greater reduction in quantum yield and larger changes in fluorescence parameters during the chilling treatments. These changes were readily reversible and may be related to the more horizontal leaf habit of this cultivar compared with the more vertical orientation of leaves in the other cultivars. These observations imply that assimilate regulation of photosynthetic metabolism, rather than light dependent damage to primary photosynthetic processes, underlies low temperature effects on yield of rice during grain filling.