Nitrogen is a key component of most organic compounds found in plants, including proteins, amino acids, nucleic acids, and secondary metabolites (Marschner, 1995) and accounts for ≈3% of plant biomass on a dry mass basis. Plant N demand is often higher than that of other nutrients and N deficiency symptoms are frequently reported, resulting in reduced growth and marketability of plant material.
Plant N requirements are species-dependent and may vary within plant parts and the developmental phase. At optimum supplementation, N stimulates growth, delays senescence, modifies plant morphology, increases protein synthesis, and chloroplast formation as well as the concentration of chlorophylls, carotenoids, vitamins, and lipids in leaves (Marschner, 1995). However, suboptimal or excess N levels may result in reduced growth.
Nitrate (NO3–) and NH4+ are the main sources of inorganic N absorbed by roots of higher plants. The uptake of either NO3– or NH4+ affects physiological and metabolic processes as a result of the difference in ion charge (Gerendás et al., 1997) and the proportion of carbohydrates used for plant growth or N metabolism. In that, NH4+ assimilation demands lower energy inputs because its oxidative state eliminates the need for reduction (Marschner, 1995). However, toxicity may develop in plants that receive high concentrations/proportions of NH4+ because it is reduced to ammonia (NH3), which is deleterious for plant growth even at low concentrations (Gerendás et al., 1997). Ammonium toxicity has been attributed to reduced proton extrusion at low external pH, acidification of the cytosol, and growing media (Walch-Liu et al., 2000), reduced content of sugars in the roots, and induced deficiency of cations, particularly K (Marschner, 1995).
Potassium is a highly abundant cation in plants and has a significant effect on N nutrition. Potassium affects NO3– uptake by being a counteranion for NO3– translocation from the root to the shoot (Marschner, 1995). Potassium is also related to NH4+ because they share several similarities: charge type, diameter of hydrated ion, and effect on the electrical potential of cell membrane. High K concentrations, within the luxury consumption zone, have also been reported to have an antagonistic effect on the uptake of other cations such as calcium (Ca), magnesium (Mg), sodium, and NH4+ (Marschner, 1995). The objective of the present study was to delineate the response of Capsicum annuum L. cv. Dársena (bell pepper) plants, a species widely cultivated in protected agricultural and soilless systems in México, to varying proportions of NH4+ combined with increasing concentrations of K in the nutrient solution.
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