Grain number responses to temperature during floret differentiation in sunflower

The number of florets per head in sunflower is a function of the integral o f the generative area of the inflorescence primordium during the phase of f loret differentiation (FS5-FS8). To understand the effects of temperature d uring early reproductive stages on grain number, we quantified the effect o f temperature on the duration of the FS5-FS8-period, on the maximum generat ive area, on floret number and viability, and on grain number. Plants of th e sunflower (Helianthus annuus L.) inbred line HA89 were exposed to daily t emperatures between 14 and 38 degreesC for 14 days from FS4. Increasing tem perature significantly reduced the integral of the generative area between floral stages 5 and 8, due to reductions in both the maximum size and the d uration of the generative area. These variations in generative area integra l translated into significant proportional variations in florets per head. However, the high floret numbers evoked by low temperatures did not result in a high number of filled grains per head. The latter variable exhibiting a convex response to temperature with an optimum at 25 degreesC. Residual e ffects of early exposures to low and high temperatures significantly reduce d the number of functional florets, filled grain, and increased unfilled gr ain (flats). The curvilinear response of grain numbers per head can be unde rstood as the result of opposing effects of temperature on floret different iation and functionality and on continuation of grain growth after flowerin g. Early exposure to low or high temperature also reduced leaf area and dry matter per plant at anthesis, which may have influenced the observed reduc tion in grain number. Caution is necessary in using the photothermal quotie nt as an estimator of grain number at low temperature. (C) 2001 Published b y Elsevier Science B.V.