The Effects of Night-time Temperatures on Physiological and Biochemical Traits in Rice

High nighttime temperatures impair rice yield. Additionally, heat stress periods have increased during the last years in the rice areas of the tropics. The aim of this study was to physiologically characterize six genotypes of rice (a commercial cultivar (`F60') and five selected lines (IR 1561', 'FLO 2764', 'LV447-1', `CT19021', and `LV1401') subjected to two nighttime temperatures (24 and 30 degrees C), based on different physiological traits. When the collar formed on leaf 6 of the main stem, one group of six plants in each genotype was subjected to 30 degrees C from 18:00 to 24:00 hours for eight days, while the other group remained at 24 degrees C. Differences were found in the interaction between genotype and nighttime temperatures, where a high night temperature reduced leaf photosynthesis by approximately 50% in all genotypes compared to the controls (20 mu mol vs. 10 mu mol CO2 m(-2) s(-1), respectively). In general, higher plant respiration was also observed in almost all genotypes when the plants were exposed to 30 degrees C. However, rice plants of the genotype 'F60' showed a constant respiration under two different night temperatures. A high nighttime temperature increased the electrolyte leakage and malondialdehyde content only in the `LV1401' plants. Plant growth and F-v/F-m ratio were separately conditioned by the night temperature or the genotype factor. A lower total plant dry weight was found at 30 degrees C (620.36 mg) than in rice plants exposed to 24 degrees C (254.16 mg). The F-v/F-m ratio was slightly diminished at a high nighttime temperature. These results suggest that physiological variables, such as leaf photosynthesis, plant respiration, malondialdehyde content and leaf photosynthetic pigments, can be considered markers for characterizing tolerant genotypes in earlier growth phases during plant breeding programs.