Table 6.

An assessment of selected secondary traits expected to be of value in some drought-tolerance breeding programs.

Trait	Relationship to stress yield	Growth stage for selection	Earliest generation for selection	Technical difficulty of selection	Heritability
Flowering/maturity date (Babu et al., 2003)	Depends on reliability of stress timing; effective for predictable and terminal stress	Flowering	Single plants at F2	Easy	High heritability (ca. 0.9)
Flowering delay (Kumar et al., 2007)	High for stress at flowering	Flowering	When available, seed is sufficient for a small plot	Easy if water can be controlled to provide uniform stress	Moderate heritability (ca. 0.6)
Percent fertile spikelets (Babu et al., 2003; Kumar et al., 2007)	High for stress at flowering	At or near maturity	Single plants at F2	Labor-intensive; error-prone; requires control of water	Moderate heritability (ca. 0.6)
Leaf-rolling score (Babu et al., 2003)	Negative and moderate	Vegetative	Single plants at F2	Easy if water can be controlled to provide uniform stress	High heritability (ca. 0.8)
Leaf-death score (Yue et al., 2005)	Negative and moderate	All stages	Single plants at F2	Easy if water can be controlled to provide uniform stress	Moderate heritability (ca. 0.7)
Canopy temperature (Yue et al., 2005)	Negative and fairly high if maximum stress occurs near flowering	Pre-flowering during full ground cover	When available, seed is sufficient for a small plot	Medium	Fairly low heritability (ca. 0.2) unless climate is very stable and vapor pressure deficit (VPD) is large

Source of assessment: Lafitte et al., 2003; more detail assessments of the heritability for each trait is found in the reference for each trait. Practical instructions on the measurement of these traits are provided in Measurement of Secondary Traits: Some Practical Considerations.