Photosynthetic activity of rice sister lines with contrasting cold tolerance.

Incidence of cold during early stages is an obstacle for the growing progress of rice plants. Cold stress has strong negative effects on photosynthetic activity. Previously, our group evaluated plant survival of 90 rice genotypes after cold treatment. Two sister lines were characterized as cold-tolerant and cold-sensitive. Transcriptomic analyses of the same genotypes had indicated differential expression of genes related to photosynthesis. Previous work with rice had suggested that cold sensitivity was more related to photosystem II (PSII) than to photosystem I (PSI). Using our previously identified contrasting genotypes, we investigated the role of specific steps of the photosynthetic process in cold tolerance/sensitivity of rice plants during and after (recovery period) cold exposure. During both cold treatment and recovery period, the photochemical activity (including PSII and PSI) presented higher levels in the low temperature-tolerant genotype, when compared with the sensitive one. The higher photochemical efficiency during the cold treatment appears to be related to a lower fraction of reduced Q in PSII. We also observed lower transpiration rates and higher water use efficiency in the cold-tolerant genotype, due to stomatal closure. After the recovery period, the higher efficiency in the cold-tolerant genotype seems to be related to a lower fraction of reduced Q and a larger pool of final electron acceptors at the PSI. This work uncovered changes in photosynthetic performance including both photosystems and improved water use efficiency which may be important components of cold tolerance mechanisms in rice.