The critical period for yield and grain protein determination in quinoa (Chenopodium quinoa Willd.)

Context or problem: Despite its global significance as a highly nutritious food, the critical period for grain yield and grain protein determination of quinoa has not yet been identified.Objective or research question: This study aimed to determine the critical period of grain number, yield, and grain protein of quinoa in contrasting environments of South America.Methods: Five experiments were conducted in contrasting South American environments, including Argentina, Chile, Ecuador, and Peru, from latitudes 4 degrees S to 39 degrees S, and between 19 and 2165 m.a.s.l. Treatments consisted of the factorial combination of two quinoa cultivars (except in Peru, where only one genotype was used) and 4-9 shading treatments (50-85% shade) applied over 10-20 days throughout the crop cycle, depending on the site and the length of the crop cycle. We evaluated how GY, its components, and grain protein responded to the reduction of the source-sink ratio at different phenological stages.Results: Yield of unstressed controls varied from 2.2 t ha-1 in Lima to 9.8 t ha-1 recorded in Valdivia. The grain protein concentration ranged between 12.6% in Lima and 16.8% in Loja. The most sensitive yield-determining period was between -26 and 445 degrees Cd from flowering across genotypes and environments. Grain protein con-centration increased by up to 19% when the source of assimilates was reduced after anthesis; however, this was insufficient to compensate for the protein yield that was affected by the reduction in grain yield due to shading.Conclusions: The critical period for quinoa grain yield was determined throughout the flowering and grain-filling stages across genotypes and environments, mainly due to a decrease in both grain number and above-ground biomass.Implications or significance: Identifying the critical period provides a useful target for breeding and management strategies to increase the quinoa crop yield.