Genotypic variation in response of rainfed lowland rice to drought and rewatering: I. Growth and water use

The lack of information on the dynamics of crop growth and water use has limited the capacity for indirect selection through physiological traits that confer drought tolerance in rainfed lowland rice (Oryza sativa L.). Shoot growth and transpiration in response to drought and rewatering were studied among eight diverse rice genotypes in three sets of pot experiments: one under severe stress development after panicle initiation (average transpiration of 15.0 mm d ; experiment 1), and two under slow and progressive stress development during tillering (average transpiration of 2.1 and 7.6 mm d in experiments 2 and 3, respectively). Higher transpiration generally caused a higher plant growth rate in two periods, though there was some contribution of water use efficiency. The first period was soon after ponded water was drained and watering withheld (early drought phase), when soil became aerobic, soil water was still readily available, and transpiration continued at a rate comparable to the well-watered treatment. The second period was after rewatering when transpiration was again not limited by soil water supply. In experiment 1, the effect of plant size before stress imposition was large and genotypic variation for response to drought and rewatering was small, except for KDML105, which tended to show smaller growth during drought and had a more rapid recovery after rewatering. During early drought phase in experiments 2 and 3, genotypes differed in relative amounts of tiller and leaf area production compared with the well-watered treatment. Genotypes with high seedling vigor before stress imposition and during the early drought phase, such as NSG19, KDML105, Mahsuri and IR58821, produced greater root length during the following more severe drought period and had a larger green leaf biomass at the end of the drought period in experiment 3. In these genotypes, transpiration increased sharply and leaf area expanded rapidly after rewatering, which caused superior drought recovery