High irradiance increases NH(4)(+) tolerance in Pisum sativum: Higher carbon and energy availability improve ion balance but not N assimilation.

The widespread use of NO(3)(-) fertilization has had a major ecological impact. NH(4)(+) nutrition may help to reduce this impact, although high NH(4)(+) concentrations are toxic for most plants. The underlying tolerance mechanisms are not yet fully understood, although they are thought to include the limitation of C, the disruption of ion homeostasis, and a wasteful NH(4)(+) influx/efflux cycle that carries an extra energetic cost for root cells. In this study, high irradiance (HI) was found to induce a notable tolerance to NH(4)(+) in the range 2.5-10mM in pea plants by inducing higher C availability, as shown by carbohydrate content. This capacity was accompanied by a general lower relative N content, indicating that tolerance is not achieved through higher net N assimilation on C-skeletons, and it was also not attributable to increased GS content or activity in roots or leaves. Moreover, HI plants showed higher ATP content and respiration rates. This extra energy availability is related to the internal NH(4)(+) content regulation (probably NH(4)(+) influx/efflux) and to an improvement of the cell ionic balance. The limited C availability at lower irradiance (LI) and high NH(4)(+) resulted in a series of metabolic imbalances, as reflected in a much higher organic acid content, thereby suggesting that the origin of the toxicity in plants cultured at high NH(4)(+) and LI is related to their inability to avoid large-scale accumulation of the NH(4)(+) ion.

Short term physiological implications of NBPT application on the N metabolism of Pisum sativum and Spinacea oleracea.

The application of urease inhibitors in conjunction with urea fertilizers as a means of reducing N loss due to ammonia volatilization requires an in-depth study of the physiological effects of these inhibitors on plants. The aim of this study was to determine how the urease inhibitor N-(n-butyl) thiophosphoric triamide (NBPT) affects N metabolism in pea and spinach. Plants were cultivated in pure hydroponic culture with urea as the sole N source. After 2 weeks of growth for pea, and 3 weeks for spinach, half of the plants received NBPT in their nutrient solution. Urease activity, urea and ammonium content, free amino acid composition and soluble protein were determined in leaves and roots at days 0, 1, 2, 4, 7 and 9, and the NBPT content in these tissues was determined 48h after inhibitor application. The results suggest that the effects of NBPT on spinach and pea urease activity differ, with pea being most affected by this treatment, and that the NBPT absorbed by the plant caused a clear inhibition of the urease activity in pea leaf and roots. The high urea concentration observed in leaves was associated with the development of necrotic leaf margins, and was further evidence of NBPT inhibition in these plants. A decrease in the ammonium content in roots, where N assimilation mainly takes place, was also observed. Consequently, total amino acid contents were drastically reduced upon NBPT treatment, indicating a strong alteration of the N metabolism. Furthermore, the amino acid profile showed that amidic amino acids were major components of the reduced pool of amino acids. In contrast, NBPT was absorbed to a much lesser degree by spinach plants than pea plants (35% less) and did not produce a clear inhibition of urease activity in this species.