The critical period for yield determination in common buckwheat (Fagopyrum esculentum Moench)

•Critical period for yield determination was identified in buckwheat, having a duration of 390 °C days.•The critical period extended from the first open flowers to the first brown fruit stages.•Crop yield was closely associated with the number of achenes.•The number of achenes per raceme was highly sensitive to treatments.•Achene weight was not modified by treatments even when the number of achenes was reduced. The aim of this study was to identify the critical period for yield determination in buckwheat by applying successive shading periods (of 14-days duration) throughout the crop cycle, from nine days after emergence to crop maturity. Experiments were carried out under field conditions during the 2017 and 2018 growing seasons. The critical period was defined as the interval when crop yield was significantly reduced by the shading treatments. This period extended from 74 °C days after the first open flowers to the first brown fruits stage (i.e. 390 °C days duration). Aerial biomass and its partitioning to the achenes (i.e. harvest index) were also reduced by shading. Achene number was reduced in a similar way to that of yield, and crop yield was highly associated with this yield component, while the shading treatments did not affect achene weight. The main cause of achene number reduction was the lower number of achenes per raceme, as the number of racemes was not affected. It is possible to speculate that radiation restriction imposed during the critical period could have increased floret mortality and thereby the number of achenes per raceme. However, further studies are required to determine the causes of reductions in the number of achenes per raceme during the critical period.