Rice breeding

Rice, which is grown on ~150 million ha worldwide, is the major staple food in Asia, where 90% of the crop is produced. IRRI leads an Asia-based rice research and breeding programme which tackles all aspects of adaptation including to drought and heat stress. While most rice is grown under monsoonal rains, a substantial area is also grown under dryland conditions with the possibility of water deficit. Evolving from a semi-aquatic ancestor, rice is generally more susceptible to water-limited conditions compared to other cereals crops (Wassmann et al., 2009). None the less genetic variation has been exploited and under upland field conditions, increased root diameter, and depth and/or branching of root systems have been associated with decreased plant water stress and increased grain yield under severe stress (Lafitte and Courtois, 2002). QTLs associated with constitutive and adaptive root growth under drought have been identified (Khowaja et al., 2009). As proof of concept, near-isogenic lines (NILs) were developed by introgressing four root QTLs on chromosomes 2, 7, 9 and 11 from the variety Azucena into the Indian upland variety Kalinga III, and evaluated in the field and on-farm trials; NILs with root QTLs outperformed Kalinga III for grain and straw yield (Steele et al., 2007).

Under drought stress, decreased peduncle elongation reduces panicle extrusion and florets that remain in the flag leaf sheaf are usually completely sterile, severely reducing grain yield in cultivars that are prone to reduced extrusion (O'Toole and Namuco, 1983). Selection for continued peduncle elongation under reproductive stage drought stress in rice is being used to increase grain yield under field drought stress. Rice peduncle elongation is partially controlled by local gibberellin levels (Kaneko et al., 2004). Repression of cell-wall invertase genes and the cell-wall loosening genes for xyloglucan endotransglycosylase/hydrolase (XTH) have also been linked to reduced peduncle elongation (Ji et al., 2005).

Comparatively less research has been conducted on heat tolerance in rice (Wassman et al., 2009). Genetic variation for high temperature tolerance per se has been observed in rice, with flowering being the most sensitive stage. Variation for the time of day of flowering is an important mechanism of heat avoidance (Jagadish et al., 2008). Oryza glaberrima flowers earlier in the day than Oryza sativa and crosses between them have shown that earlier flowering can be easily selected for (Prasad et al., 2006).

For further details on breeding of cereals for adaptation to low-yielding stress environments, readers are referred to chapters in Drought Adaptation in Cereals (Ribaut, 2006).

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