Is thermal time adequate for expressing the effects of temperature on sunflower leaf development?

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Abstract
We have tested whether the effects of temperature on sunflower leaf growth could be documented by using thermal time. The rates of leaf expansion and of cell division were analysed in leaves located at two positions on the stem, and a spatial analysis of expansion rate was carried out. Experiments were performed in growth chamber (stable conditions), in the field or in a greenhouse (fluctuating conditions). We compared three methods for characterizing the rate and the duration of expansion. Responses to leaf temperature were consistent only when expansion was characterized as a two‐phase process — a period of exponential expansion (constant relative expansion rate, RER) followed by a decrease in RER. RER and relative cell division rate (RDR) responded linearly to temperature with a common response curve for all studied conditions. This response curve was also common to all studied zones within a leaf and to leaves at two positions on the stem. The reciprocals of the durations of the periods of exponential expansion, non‐zero expansion and non‐zero division were also linearly related to leaf temperature with common response curves in a given leaf zone. The x‐intercepts of all these response curves and of the response curve of leaf initiation rate to temperature did not significantly differ in an analysis of covariance, with a common value of 4·8 °C. The expression of time in cumulative degree days, with a base temperature of 4·8 °C, resulted in a unique time course of RER and cell division rate regardless of temperature. These results suggest that a powerful ‘program’ of leaf development exists in a sunflower plant.