Breeding for increased grains/m2 in wheat crops through targeting critical period duration: A review

Abstract

Context: Continuing to raise the potential yield of wheat through breeding is essential for global food security.Past progress has largely been associated with greater grains/m(GN), the critical period for the determination ofwhich relates to spike growth, with GN often closely related to spike dry weight at anthesis (g/m2).Objective/Methods: This focussed review outlines the importance of the critical period duration (Ds, in days or2◦Cdays) and questions how it may be increased genetically, relying partly on the long involvement of the authorsin this field, primarily with lower latitude crops of spring-type wheat. Ds is further defined as the interval betweenflag leaf emergence and first anthesis, each across 50 % of the culms in any crop, a period encompassingmost of the accumulation of spike dry matter, in turn determining floret survival and final fertile floret numbers/m2.Results: Natural temperature variation and temperature manipulation, particularly in field crops, confirm thedependence of Ds on temperature, which in◦Cdays varies from about 300–500, depending on photoperiod.Evidence points to a stronger influence of night than day temperature on Ds, while maintaining the close positiveDs relationship to GN. However, genetic variation in the response of Ds to temperature appears very minor. Ds isinversely related to photoperiod, again with the expected effects on spike dry weight, fertile florets and GN.Extended photoperiod during the critical period showed the greatest reduction in GN per day advance inanthesis. Ds responses can be related to the major photoperiod sensitivity alleles present. A field experiment witha unique reduced photoperiod treatment demonstrated a strong positive effect on Ds, fertile florets and GN,especially in the fully recessive photoperiod-sensitive isoline. While more recent varieties tend to have a longerDs, experiments targeting selection for Ds or closely related intervals have delivered little change in Ds, whichoften showed low heritability.Conclusion: Field photoperiod shortening studies needs further testing as proof of concept. Better selection studiesare also needed. At the same time, knowledge of the molecular basis of the leaf response to photoperiod is suchthat gene editing is surely ripe to tackle the challenge of down-regulating this response only during the criticalperiod, thereby increasing Ds and GN. Exogenous application of plant development regulators at the appropriatestage may also provide a way forward.