Simple Relationships Describing the Responses of Leaf Growth to Temperature and Radiation in Sunflower

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Abstract
In attempts to determine simple relationships describing the responses of leaf expansion to temperature and radiation, sunflower plants were grown in glasshouses under five temperature and two natural radiation regimes. Crops were also grown in the field at three radiation levels. All leaves, regardless of their position on the plant, fitted a single relationship of relative leaf expansion rate versus leaf age for each regime. These relationships showed that, whilst increased temperature increased the rate of expansion of a leaf at day 1 on our growth scale (approximately unfolding), and reduced the period for which the leaf expanded, each leaf expanded for the same period in day-degree terms regardless of its temperature regime. However this expansion period was modified by radiation such that, for each 1 MJ m-2 day-1 increase, the day-degree sum was reduced by approximately 5°C. The strict inverse relationship between the initial rate of expansion and the expansion period, expressed in days, suggests that at least in some cases the final area of a leaf is largely determined by the stage of 'unfolding'.