Genotypic Variation in Response of Rainfed Lowland Rice to Drought and Rewatering. III. Water extraction during the drought period

Soil water extraction was examined in relation to root system development and leaf osmotic adjustment in a pot experiment with eight rice genotypes (Oryza sativa L.). The time course of cumulative soil water extraction from layers between 5 and 45 cm estimated by time domain reflectometry (TDR) was similar to that of cumulative transpiration estimated from pot weighing for each genotype. The level of the TDR-estimated water extraction was 75% of the cumulative transpiration, and their coefficient of determination between the two was 96%. There was a 5-day difference (18 to 23 d) in transpiring 4 kg of water from the pots among genotypes, and the variation in daily transpiration rate was related to the extraction rate in the top-20 cm soil layers during the early half of the drought period, and to the extraction rate in the below-30 cm soil layers during the latter half of the drought period. The extraction rate in the subsoil was positively correlated with the average root length density at the corresponding depth during the latter half of the drought period, explaining 66% and 58% of the variation around the 30 cm and 40 cm depth, respectively. Mahsuri and IR58821 had higher water extraction rate from these subsoil layers than IR20 and IR62266 during the late drought period. Osmotic adjustment was higher in the genotypes that had a slower rate of transpiration and a lower pre-dawn leaf water potential at the end of the drought period. Among genotypes that extracted water rapidly, KDML105 had the highest osmotic adjustment. IR58821 and CT9993, known to have a deep and thick root system under well-watered conditions, had the lowest levels of osmotic adjustment. This study demonstrated under the simulated rainfed lowland conditions genotypic variation in water extraction rate from the deep soil layers during the late drought period, which was primarily related to proliferation of roots in these layers