Abstract
Helianthus annuus L. plants were germinated and grown under environmentally controlled conditions in Long Ashton nutrient medium using nitrogen supplied in the form of 2 mM nitrate, 2 mM ammonium or 1 mM nitrate + 1 mM ammonium and harvested after either 4 or 10 weeks of growth. Ammonium oxidation was prevented by the inclusion of nitrapyrin to the nutrient solutions. After 4 weeks of growth the mixed feed plants had 18% greater leaf area than the nitrate-only and 23% greater leaf area than the ammonium-only fed plants. Both the 4- and 10-week-old mixed-feed plants had greater fresh and dry mass, lower transpiration rates and greater water use efficiency than plants of either of the other two nutrient regimes. The fruit mass for the mixed feed plants was 25% greater than the nitrate-only and more than double that of the ammonium-only fed plants. The bound nitrogen content of both the nitrate-only and mixed-feed plants was over 30% greater than that of the ammonium-only fed plants, although individual seed nitrogen concentration was the same. Combined ammonium + nitrate is therefore considered to be a more efficient nitrogen source for sunflower production than either nitrate or ammonium fed singly.
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