Abstract
Ammonium (NH4+) and nitrate (NO3−) are the two forms of inorganic nitrogen essential for physiological and biochemical processes in higher plants, but little is known about how the NH4+:NO3− ratio may affect nitrogen metabolism. This study determined the effect of NH4+:NO3− ratios on plant growth, accumulation, and distribution of nutrient elements, fruit quality, enzyme activity, and relative expression of genes involved in nitrogen (N) metabolism in pepper (Capsicum annuum L.). In a pod experiment, the NH4+:NO3− ratios of 0:100, 12.5:87.5, 25:75, 37.5:62.5, and 50:50 were arranged in a complete randomized design with three replicates. The application of NH4+:NO3− at 25:75 resulted in highest dry matter and N, phosphorus (P), and potassium (K) accumulation. Pepper treated with 25:75 ratio increased root length, surface areas, and root volume and tips. The contents of vitamin C, soluble sugar, soluble protein, total phenols, flavonoids, and capsaicinoids in the fruits were significantly higher with the NH4+:NO3− ratio of 25:75 compared with 0:100 treatment, while lowering nitrate content was found in NH4+:NO3− ratios of 25:75, 37.5:62.5, and 50:50 treatments. Activity of glutamine synthetase (GS), glutamate synthases (GOGAT) enzyme and the levels of relative expression of genes coding these enzymes were superior when the NH4+:NO3− ratio of 25:75 were applied. Therefore, an appropriate ratio of NH4+:NO3− (25:75) in nitrogen application can stimulate root development, promote enzyme activities, and enhance the productivity and fruit quality in pepper.
Highlights
From the Green Revolution, new agricultural methods and technology rapidly increased application of synthetic fertilizer in crop production compared to the prior time
The amounts of dry matter in the shoot (Figure 1A) and root (Figure 1B) significantly increased from 0 to 120 days after transplantation (DAT) (p < 0.05) and the trend of change varied with the NH4 + :NO3 − ratios
The rate of dry matter accumulation significantly increased (p < 0.05) with days after transplantation for all treatments, and they reached the peak at Day 90 except T5 (NH4 + :NO3 − = 50:50) which had the peak value at Day 120 (Figure 1C)
Summary
From the Green Revolution, new agricultural methods and technology rapidly increased application of synthetic fertilizer in crop production compared to the prior time. The national average fertilizer application rate in China was 147 kg N ha−1 year−1 in 2016, which was. It is anticipated that the fertilizer use for agriculture worldwide will increase continuously to meet the food demands for growing population. Minimizing nitrogen (N) fertilizer inputs [3], and improving Nitrogen Utilization Efficiency (NUE) are a major challenge worldwide [4]. Ammonium (NH4 + ) and nitrate (NO3 − ) are the two predominant forms of inorganic nitrogen absorbed by the roots of higher plants in agricultural ecosystems [5], and these nitrogen forms exert different effects on biochemical processes in higher plants or soil environments. The absorption of nitrate nitrogen is accompanied by the uptake of inorganic cation and the release of OH− , whereas the ammonium nitrogen is accompanied by the absorption of inorganic anion and the release of H+
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