Table 4.
Putative traits for drought tolerance.
| Trait | Proposed function | Comments | Reference |
|---|---|---|---|
| Leaf-rolling score | To reduce transpiration | Used during vegetative stress; high heritability (ca. 0.8), but low/no association with yield. Good as an indicator of stress in an experiment | Courtois et al. (2000) |
| Osmotic adjustment (OA) | To allow turgor maintenance at low plant water potential | Indica types have high OA, japonica types have low OA. This trait has been associated with a yield advantage in wheat, especially in terminal stress environments | Lilley et al. (1996) |
| Deeper, thicker roots | To explore a greater soil volume | There is evidence from MAS that increasing root mass below 30 cm results in greater yield under stress. No evidence on root thickness per se. Large-scale screening is difficult | Yadav et al. (1997) |
| Root-pulling resistance | For root penetration into deeper soil layers | Is correlated with larger root system | Pantuwan et al. (2002b) |
| Greater root penetration ability | To explore a larger soil volume | Most studies use artificial barriers with known mechanical resistance. There is some controversy regarding how well this mimics the soil situation | Ali et al. (2000), Clark et al. (2000) |
| Membrane stability | To allow leaves to continue functioning at high temperature | Genotypic differences are clear. Has been linked to heat tolerance in several species. Link to drought tolerance is less evident | Tripathy et al. (2000) |
| Leaf relative water content (RWC) | Indicates maintenance of favorable plant water status | Trait has rather low heritability; QTLs not repeatable | Courtois et al. (2000) |
| Water-use efficiency (WUE) | Indicates greater carbon gain per unit of water lost by transpiration | Carbon isotope discrimination (Δ13C) provides an integrated measure of WUE over the season. It has been used successfully for crops in more arid climates but has not been applied to rice | Specht et al. (2001) |
Note that QTLs have been identified for these secondary traits. Now they need to be tested for their relationship with performance under drought stress, and suitable high-throughput screening strategies must be developed (Source: Lafitte et al., 2003).