Seed number in sunflower as affected by light stress during the floret differentiation interval

Grain number in sunflower responds to variations in radiation over a broad developmental window running from floral initiation through to grain setting, encompassing several sequential processes (i.e. floret differentiation, micro- and megasporogenesis, flowering, fertilization and grain set). We examined the effect of radiation stress during the floret differentiation process (a period of 5-8 days) using shades (80% interception) to reduce radiation incident on field-grown crops. Light stress reduced the integral of the generative area, inflorescence floret number and grain number per plant, without altering grain set. The effects of stress on floret number were mediated by changes in floret differentiation rate rather than by variations in duration of the floret differentiation interval. These responses were linked to changes in total apex area and dry weight, such that floret cross-section and associated dry weight at the end of floral differentiation were reduced by stress. These differences between treatments tended to become smaller towards anthesis. Although stress reduced plant and organ dry weight, there was little change in biomass partitioning to organs other than the apex. There was no measurable compensation between grain size and grain number, so that yield was determined by grain number. We conclude that light stress during the floret differentiation interval can irreversibly reduce grain number and yield in this species. (C) 2002 Elsevier Science B.V. All rights reserved.