High irradiance increases NH₄ ⁺ tolerance in Pisum sativum: Higher carbon and energy availability improve ion balance but not N assimilation
Ariz, Idoia | Artola, Ekhiñe | Asensio, Aaron Cabrera | Cruchaga, Saioa | Aparicio-Tejo, Pedro María | Moran, Jose Fernando
The widespread use of NO₃ ⁻ fertilization has had a major ecological impact. NH₄ ⁺ nutrition may help to reduce this impact, although high NH₄ ⁺ 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 NH₄ ⁺ 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 NH₄ ⁺ 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 NH₄ ⁺ content regulation (probably NH₄ ⁺ influx/efflux) and to an improvement of the cell ionic balance. The limited C availability at lower irradiance (LI) and high NH₄ ⁺ 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 NH₄ ⁺ and LI is related to their inability to avoid large-scale accumulation of the NH₄ ⁺ ion.
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