Faba bean breeding for drought-affected environments: A physiological and agronomic perspective

The faba bean (Vicia faba L.) crop often experiences drought during its growth and development such that soil moisture deficits constrain its production. As droughts are predicted to increase in both frequency and intensity due to climate change, a better understanding of drought response patterns and associated traits is essential for obtaining yield stability in water-limited environments. This review deals with adaptation mechanisms associated with drought avoidance, escape and tolerance, with an emphasis on physiological traits such as stomatal conductance, carbon isotope discrimination and leaf temperature. Leaf temperature is considered an effective surrogate measure for other measures of stomatal characteristics. Drought tolerance through osmotic adjustment has not yet been demonstrated in faba bean although it is found in many other legumes including chickpea and pea. Deeper root growth, leading to uptake of otherwise unavailable water, helps the plant to avoid drought by delaying dehydration, but genetic variation and heritability of the trait are essentially unknown for faba bean. Crop management strategies, such as early planting, and appropriate phenology, are particularly important for drought escape in regions where terminal drought is common. Disease resistance is especially important in drought-prone areas to reduce the need for expensive control measures when yields are uncertain. The relevance of soil fertility status and nutrient availability are also covered. Drought escape and ascochyta blight resistance are important breeding objectives for terminal drought regions. Some form of drought resistance is necessary for the transient droughts experienced in most regions, and drought avoidance can be screened by a combination of leaf temperature or other rapid test of stomatal characteristics followed by carbon isotope discrimination in the most valuable materials. No single trait is adequate to improve yield in drought-prone environments, rather, a combination of characteristics is needed.