Physiological changes during development of rice (Oryza sativa L.) varieties differing in salt tolerance under saline field condition

To investigate the changes in physiological parameters in leaves of field-grown rice genotypes differing in their salt tolerance. Thirty rice cultivars classified as tolerant (T; 11), moderately tolerant (MT; 5), moderately sensitive (MS; 7) and sensitive (S; 7) based on the previous screening at the seedling stage were established in a greenhouse. Thirty-day-old seedlings were then transplanted to a rice field, situated in a moderately saline area in northeastern Thailand, where EC slowly increased from 2.03 to 6.46 dS m(-1) from the transplanting date to harvest. Leaf samples (the third leaves from the top or the flag leaves during the vegetative or the reproductive phase, respectively) were collected, at 1 month intervals, when the plants were 60-, 90-, 120- and 150-day-old corresponding to active tillering, early reproductive, late reproductive and harvest stage, respectively. Leaf samples were analyzed for changes in proline, chlorophyll and malondialdehyde (MDA). The harvested panicles were evaluated for the percentage of filled grain weight and the concentration of Na+ and K+ in the top internode. The patterns of change and the mean concentrations of most physiological parameters in rice leaves during the course of development were strikingly similar for the four classes of salt tolerance. Proline concentration remained relatively constant throughout the development and finally showed a dramatic increase in the flag leaves at harvest. MDA concentration tended to increase with age reaching the maximum in the flag leaves at harvest. The chlorophyll concentration was higher during the vegetative stage than the reproductive stages. At harvest corresponding to the time of maximum salinity, the T group tended to contain higher proline and stayed green longer than the other groups. Moreover, the T group showed higher percentage filled grain weight which was associated with lower Na+/K+ ratio in the top internode. The percentage filled grain weight was negatively correlated with Na+ concentration and Na+/K+ ratio in the top internode and proline concentration in flag leaves, but did not correlate with chlorophyll and MDA in flag leaves. Rice cultivars which are tolerant at the seedling stage also showed higher tolerance in the field condition as shown by higher percentage filled grain weight and lower Na+ uptake to the panicles. Tolerant cultivars tended to accumulate less proline in their leaves similar to that found at the seedling stage.