Nitrogen effect on gas exchange characteristics, dry matter production and nitrate accumulation of Amaranthus cruentus L.
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Among the mineral nutrients, nitrogen stands out as an essential constituent of amino acids, nucleic acids and proteins, as well as being part of the photosynthetic apparatus. The present study aimed to evaluate the responses of Amaranthus cruentus L. cv. BRS Alegria to nitrogen availability in terms of biomass production, photosynthetic characteristics, content of photosynthetic pigments, total nitrogen, ammonium and nitrate in the leaves. Plants of A. cruentus were cultivated in plastic pots filled with vermiculite and kept in a greenhouse under natural photo-periodic conditions. The plants were watered with 70% of full strength nitrogen-free Long Ashton solution, containing 1.97, 4.94 or 9.88 kg N ha−1 as ammonium nitrate, three times a week. Increased nitrogen availability resulted in higher shoot dry matter, specific leaf mass, total leaf nitrogen, chlorophyll and carotenoids as well as in higher photosynthetic rates. There was a positive correlation between photosynthesis and leaf nitrogen. Higher nitrogen availability resulted in greater stomatal conductance and transpiration, but the CO2 concentration in the sub-stomatal cavity was not significantly altered. In addition, an increase in ammonium and nitrate content of leaves was observed. The nitrate content reached 176, 193 and 288 mg kg−1 of dry matter in low, medium and high nitrogen supply, respectively. The data show that A. cruentus plants are sensitive to changes in nitrogen availability during the vegetative phase. The limitation of photosynthesis under conditions of low nitrogen supply cannot be fully explained by the reduction in stomatal conductance; non-stomatic limitations are also involved. In addition, the maximum nitrate content observed in leaves was far below the risk level for human health.