Progress in drought avoidance of rainfed lowland rice

摘要

Rainfed lowland rice is grown in bunded fields covering more than 36 million hectares of South and South-east Asia. Drought stress is common, with yields averaging only 2.3 t ha~(-1). The ecosystem is characterised by fluctuating water, with soil hydrology ranging from flooded and anaerobic to droughted and aerobic. Root systems have to cope with too much and too little water at different growth stages. Drought stress may be encountered during panicle development, and especially during grain filling. These patterns vary in intensity with location, season, soil type and topographic position. Our previous research has demonstrated that cultivars differed in their patterns of adaptation to various types of rainfed environments, and this was associated in part with patterns of root behaviour during droughts. In simulated rainfed lowland conditions in the greenhouse, cultivars differed in the extent to which they could proliferate roots in, and extract water from, deeper soil layers as drought progressed. In the field, cultivars differed in the ability of their roots to penetrate hardpans and proliferate in deeper soil layers. Our most recent experiments have demonstrated that cultivars differed in the amounts of water extracted from deeper soil layers during drought. Greenhouse and field experiments have now indicated that root signals are strong in rainfed lowland rice, and are associated with increasing levels of abscisic acid in leaves as drought progresses. Cultivars varied in stomatal closure as drought progressed, with implications for rate and extent of water loss. Molecular studies have identified consistent responses in protein regulation during drought and after rewatering. In the field, the soil water from above and below the hardpan has been shown to differ in hydrogen isotope composition, raising the prospect that analysis of xylem water could be used to identify lines with a greater capacity to extract water from deeper soil layers, without destructive measurements of the root system. The implications of these results for the selection of cultivars with more efficient water use and their agronomic requirements for producing higher and more stable yields in rainfed lowland conditions are discussed.