Growth of florets of sunflower ( Helianthus annuus L.) in relation to their position in the capitulum, shading and nitrogen supply

Abstract

Florets at different positions in the capitulum of sunflower grow to different sizes and produce seeds with different weights. Competition between florets during their growth is thought to contribute to size differences and this has been studied in relation to changes in environmental conditions experienced by the plants. The imposition of shading (20% full sunlight) at the end of floret initiation decreased dry matter accumulation by the whole plant, although teh areas of leaves still expanding increased. The distribution of dry matter between plant organs was changed by shading, and competition between the capitulum and leaves was apparent. This, and the decrease in dry matter accumulation, decreased the receptacle area available for floret growth, with the result that florets in all positions were smaller. Changes in N supply had smaller effects on organ growth than shading. Nitrogen concentrations in leaves, capitulum bracts, receptacle and florets were increased by shading but were not affected by N supply rates. Attempts to change dry matter distribution between capitulum and leaves by removing half the leaves as they unfolded were not successful, but th experiment did confirm the apparent competition between the capitulum and the leaves. There were no direct effects of the environmental factors on floret growth but it was controlled by the available receptacle area which was affected by plant dry matter accumulation and the internal competition between the capitulum and leaves. Competition between florets in different positions remained unchanged in changed environmental conditions. It is suggested that the competitive hierarchy between florets is a consequence of their sequence of initiation; the outer florets are initiated before the inner ones and whilst they are growing always have a superior competitive ability.