Evaluation of heat tolerance of rice genotypes at the seedling stage by growth performance and physiological changes

The basis for heat tolerance of rice results from complex physiological mechanisms that vary among different varieties. To identify the relationships between growth performance and physiological characteristics of rice under heat stress, 29 rice genotypes at the seedling stage were evaluated for their heat (45 °C) tolerance for 5 days. Increasing proline content was most strongly significantly correlated with malondialdehyde (MDA) content. Proline, MDA and total chlorophyll contents were included in a multivariate analysis to classify the rice genotypes into four heat tolerance groups: tolerant (T), moderately tolerant (MT), sensitive (S), and highly sensitive (HS). The means of the proline content, MDA content, visual symptom score, and upper epidermal layer thickness (UET) were significantly different among the four tolerance groups and tended to increase and then decrease with decreasing levels of tolerance. Together, these findings indicated that these parameters were the most reliable indices for identifying heat tolerance. In addition to the commonly used visual symptom score and growth performance, proline content and UET could be two other useful and less expensive physiological and anatomical index measurements to differentiate heat-tolerant rice from heat-sensitive rice. The results of this study also showed that several local Thai cultivars have high heat tolerance and could be used for obtaining heat tolerance alleles for breeding programs.