Embryo-growth rate and duration in sunflower as affected by temperature

The thermal regime of the inflorescence has been shown to affect grain-filling dynamics and grain weight in several crop species, but temperature response functions for sunflower grain have not, as yet, been developed. We explored the effects of daily mean temperatures in the 12-40 degreesC range during the embryo growth phase on embryo growth characteristics in cv. HA89 by growing plants in temperature-controlled glasshouse modules between the start of linear embryo growth and physiological maturity. Grain was harvested at regular intervals during the growth period and rates and durations of embryo growth derived from bi-phasic linear regression models fitted to the resulting embryo weight dynamics data. Embryo filling duration showed a curvilinear response to temperature with a minimum close to 34 degreesC. In contrast, the curvilinear response of rate of embryo filling peaked at 25 degreesC. The opposing effects of these two responses produced a continuously decreasing embryo size with temperature (P < 0.0001) (average decrease 1.2% per <degrees>C) over much of the temperature range explored. A plot of the inverse of embryo growth duration against temperature served to estimate base and optimum temperatures of -1 and 34 degreesC, respectively. The response functions and cardinal temperatures defined in this experiment are expected to improve the capacity of current crop simulation models to describe grain-filling in sunflower. (C) 2001 Elsevier Science B.V. All rights reserved.