Physiological interventions in breeding for adaptation to abiotic stress

Abstract

The physiological-trait-based breeding approach has merit over breeding for yield per se because it increases the probability of crosses resulting in additive gene action. While considerable investment in germplasm characterization is required, conceptual models of crop genotypes can be employed as research tools to quantify likely genetic gains associated with specific trait or in defining traits that may have generic value across different stresses. For example, deeper root growth that permits better access to soil water has obvious benefit under drought, while under hot, irrigated conditions permits leaf canopies to match the high evaporative demand associated with hot, low-relative-humidity environments, resulting in higher leaf gas-exchange rates and heat escape through evaporative cooling. Although improvement in adaptation to abiotic stress may occur as a result of transgressive segregation, exotic parents can be used to increase total allelic diversity for such traits. The bread-wheat-breeding programme at CIMMYT is exploiting new genetic diversity using inter-specific hybridization of the ancestral genomes of bread wheat. Novel genetic diversity is also being accessed more directly by crossing adapted germplasm with landrace accessions originating in abiotically stressed environments that have become isolated from mainstream gene pools. Through studying these genetic resources it has been possible to calculate the theoretical impact of combining their best values of trait expression into the check cultivar to gain some insight into which traits may hold most promise in terms of genetic enhancement. It was apparent that the genetic diversity found for water use efficiency offers the greatest and most consistent opportunity for increasing yield, while increasing stem carbohydrates and access to water at depth also shows some potential. Direct physiological interventions in breeding include (i) characterization of potential parents for more strategic crossing; (ii) early-generation selection; and (iii) evaluation of promising genetic resources in pre-breeding. The early-generation selection trait ‘canopy temperature’ (measured with an infrared thermometer) has been readily adopted since measurement is quick, easy and inexpensive. Although genetic markers are not currently used in selection for complex traits, as technology advances and combines with gene discovery approaches, more quantitative trait loci (QTLs) associated with adaptation to complex environments will emerge. A multi-staged approach to identifying molecular markers may be the best approach where QTLs for generic traits – i.e., valid across a range of environments – are identified in well controlled field environments and used to optimize germplasm. Subsequently, environment-specific models would be used to factor in additional traits commonly found in a specific region that may not be directly related to moisture stress, factors such as nematodes or microelement deficiency or toxicity that are exacerbated under drought