Distribution of Nitrogen during Growth of Sunflower (Helianthus annuus L.)

Abstract

The accumulation, distribution and redistribution of dry matter and nitrogen is described for Helianthus annuus L. cv. Hysun 21 grown on 6 mM urea in glasshouse culture. Seed dry matter and nitrogen were transferred to seedlings with net efficiencies of 40 and 86 per cent respectively. At flowering, the stem had most of the plant's dry matter and the leaves most of its nitrogen. About 35 per cent of the plant's nitrogen accumulated after three-row anthesis. The amount of protein in vegetative parts, especially leaves, declined after flowering. Concentrations of free amino compounds also decreased during growth. Mature seeds had 38 per cent of the total plant dry weight and 68 per cent of the total nitrogen. Seeds acquired 33 per cent of their dry matter and nitrogen from redistribution from above-ground plant parts. The stem was most important for storage of carbohydrate, leaves the most important for nitrogen. Over 50 per cent of the nitrogen in the stem and leaves was redistributed. Plants that received 6 mM nitrate accumulated more dry matter than urea-grown plants. Seeds from nitrate-grown plants were heavier (58 mg) than those of urea-grown plants (46 mg), and their percentage oil was greater (50 and 41 respectively). The amount of nitrogen per seed was the same. Little or no urea was detected in xylem sap of plants supplied with 5 mM urea, but it was detected in sap of plants which received 25 mM. Concentrations of urea and amino compounds in the sap decreased up the stem. Plants supplied with nitrate had most of the nitrogen in xylem sap as N03~, suggesting little nitrate reduction in roots. Plants grown on 6 mM nitrate and changed to high levels of urea-nitrogen for 14 days still had high levels of nitrate; little nitrate remained in plants receiving low levels of urea. When urea is applied in irrigation water to field-grown sunflower, the nitrogen is subsequently taken up as nitrate due to rapid nitrogen transformations in the soil.