Grain number responses to temperature during floret differentiation in sunflower

Abstract

The number of florets per head in sunflower is a function of the integral of the generative area of the inflorescence primordium during the phase of floret differentiation (FS5–FS8). To understand the effects of temperature during early reproductive stages on grain number, we quantified the effect of temperature on the duration of the FS5–FS8-period, on the maximum generative area, on floret number and viability, and on grain number. Plants of the sunflower ( Helianthus annuus L.) inbred line HA89 were exposed to daily temperatures between 14 and 38°C for 14 days from FS4. Increasing temperature significantly reduced the integral of the generative area between floral stages 5 and 8, due to reductions in both the maximum size and the duration of the generative area. These variations in generative area integral 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°C. Residual effects of early exposures to low and high temperatures significantly reduced the number of functional florets, filled grain, and increased unfilled grain (flats). The curvilinear response of grain numbers per head can be understood as the result of opposing effects of temperature on floret differentiation and functionality and on continuation of grain growth after flowering. Early exposure to low or high temperature also reduced leaf area and dry matter per plant at anthesis, which may have influenced the observed reduction in grain number. Caution is necessary in using the photothermal quotient as an estimator of grain number at low temperature.