Nitrate and ammonium nutrition in ryegrass: Changes in growth and chemical composition under hydroponic conditions

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Abstract
Nitrogen (N) is one of the most critical elements affecting grass seed yield. In soil and cropping conditions of Western Oregon, ammonium‐N may play an important role in the growth and development of ryegrass grown for seed. Our objectives were to determine the physiological and biochemical effects of ammonium and nitrate nutrition on ryegrass vegetative growth and subsequent expression of floral tillers, and changes in plant reduced‐N and carbohydrate composition. Plants were grown in hydroponics (active pH‐controlled) and fed nitrate and ammonium alone or in combination at 10 mM total N. The experiment was terminated at anthesis, which occurred at 68 days after planting (DAP). Changes in plant tiller number, height, dry weight, reduced‐N, and water‐soluble carbohydrates (WSC) were recorded. Vegetative growth rate of plants receiving lower ratios of nitrate/ammonium was up to twice those receiving total nitrate or 75/25 nitrate/ammonium. Total primary tiller number at 68 DAP (anthesis) was 30% greater in plants receiving 25/75 and 0/100 nitrate/ammonium. Floral tiller number increased up to 55% with increasing ammonium‐N to nitrate‐N, while vegetative tiller number was not significantly affected. Partitioning of dry matter, WSC, and reduced‐N among vegetative and reproductive organs differed among N treatments. Data indicated that differences in plant growth and development resulted from greater N use efficiency from mixtures of ammonium and nitrate nutrition. Data support previous reports of ryegrass preference for ammonium nutrition over nitrate.

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