Role of Avoidance of and Tolerance to Water Deficits in Seedling Survival of Diverse Rice(Oryza sativa L.) Cultivars

The role of avoidance of and tolerance to drought in seedling survival of rice cultivars was tested both in smaller and larger pot experiments. Smaller pot experiment examined the tolerance ability of the cultivars under limited rooting environment. Larger pot experiment was suitable for the study of the avoidance of drought due to large soil volume compatible for continuous root growth. Leaf water potential declined similarly in all the four rice cultivars in the smaller pot experiment after suspension of irrigation. This response was in contrast to the cultivar difference in leaf death in response to drought : leaf death in all the three dryland cultivars was approximately double that of wetland Nipponbare in the smaller pot experiment. Therefore, the cultivar difference in leaf death under drought could arise due to their difference in tolerance to severe drought. In the larger pot experiment, cultivars differed widely in the development of midday leaf water potential, although soil water content decreased almost similarly in all the cultivars after the suspension of irrigation. Dryland cultivars in general and IRAT 13 in particular had much higher midday leaf water potential compared to wetland cultivars : midday leaf water potential in dryland cultivars were within the range of - 1.6 to - 2.1 MPa compared to -2.8 to -3.1 MPa in wetland cultivars. Plant survival in IRAT 13 was relatively better than other cultivars presumably due to its ability to maintain high water potential under drought. Tolerance of the wetland cultivars enabled them to survive either similar to or better than dryland cultivars. It appears from the results of the experiment that avoidance mechanism may be important even in the seedling stage under condition where extent of root development is not physically limited.