Nitrogen nutrition of sunflower (Helianthus annuus L.): Acquisition and partitioning of dry matter and nitrogen by vegetative organs and their relationship to seed yield

Abstract

Abstract The time during plant development when dry matter and nitrogen content of vegetative organs of Helianthus annuus L. cv. Hysun 30 are determined by N supply was identified. Three growth phases were examined: emergence to completion of floret initiation, from then to anthesis and after anthesis. Stem, leaf and capitulum dry matter and N contents were positively influenced by N supply throughout development. However, a low N supply before floret initiation may reduce the potential to respond to a high N supply later. Root dry matter was only influenced before floret initiation despite the fact that most root dry weight accumulation occurred afterwards. Root N-content was influenced in all three phases. The free nitrate content of root, leaf, and capitulum responded positively to N supply between floret initiation and anthesis but the major response was to N supply after anthesis. The harvest index and seed-N% plant N were positively affected by N supply between floret initiation and anthesis but not afterwards. Seed-N% plant N was negatively affected by N supply after anthesis because much of the late-acquired N was stored in the stem. The proportion of the final plant N acquired before floret initiation was twice that of final dry matter acquired before floret initiation. Oil per seed was determined by the number of florets per capitulum, plant dry matter or N accumulated during the period of floret growth, and a base value of 10 mg oil per seed. Floret number was highly correlated with plant dry weight at the end of floret initiation so that seed oil yield can be predicted from plant dry weight data. This could form the basis of an oil yield model. Response to an increased N supply after anthesis may be limited by the size of the seed hull, which is determined before anthesis. These results emphasise the benefits in crop N management of adequate fertilizer application before floret initiation to allow the maximum development of all organs, particularly root mass and floret numbers.