Abstract
The yield of rice after 5 days of complete submergence at different growth stages was compared between the high-yielding varieties (HYVs), Tap Giao 4 and CR203, and a local variety, Moc Tuyen, cultivated extensively in the Red River Delta, Vietnam, to assess the difference in the rice flooding tolerance before and after the introduction of HYVs, which are generally considered to be susceptible to flooding. Although the variety with the most severe yield reduction was different depending on he growth stages when submerged, the yield of Moc Tuyen was the lowest among the three varieties except for submergence at the tillering stages. Based on these results and the pattern of severe flooding occurrence, it appears that flooding damage to rice production was not anticipated to increase by the introduction of these two HYVs in this region. Then, we tested further aspects of the effect of submergence at different growth stages for different durations of 2, 5 and 8 days, and water depths, namely submergence of three-quarters or full submergence of the plants, using CR203, to detect critical conditions, under which yield reduction would be enhanced. When the plants were submerged at the tillering stages with three-quarters of plants height, the yields hardly decreased even in the 8 days of submergence, whereas in the plants with complete submergence, the yields decreased severely by submergence lasting for more than 5 days. It could thus be possible to mitigate the yield reduction of CR203 at the tillering stages by limiting the duration of complete submergence to less than 2 days.
