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

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/m2 (GN), the critical period for the determination of which relates to spike growth, with GN often closely related to spike dry weight at anthesis (g/m2). This focussed review outlines the importance of the critical period duration (Ds, in days or °Cdays) and questions how it may be increased genetically, relying partly on the long involvement of the authors in this field, primarily with lower latitude crops of spring-type wheat. Ds is further defined as the interval between flag leaf emergence and first anthesis, each across 50 % of the culms in any crop, a period encompassing most of the accumulation of spike dry matter, in turn determining floret survival and final fertile floret numbers/m2. Natural temperature variation and temperature manipulation, particularly in field crops, confirm the dependence 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 positive Ds relationship to GN. However, genetic variation in the response of Ds to temperature appears very minor. Ds is inversely 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 in anthesis. Ds responses can be related to the major photoperiod sensitivity alleles present. A field experiment with a 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 longer Ds, experiments targeting selection for Ds or closely related intervals have delivered little change in Ds, which often showed low heritability. Field photoperiod shortening studies needs further testing as proof of concept. Better selection studies are also needed. At the same time, knowledge of the molecular basis of the leaf response to photoperiod is such that gene editing is surely ripe to tackle the challenge of down-regulating this response only during the critical period, thereby increasing Ds and GN. Exogenous application of plant development regulators at the appropriate stage may also provide a way forward. •Grains per m2 (GN)