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Elevated CO2 Compensates for Growth Declines Induced by Ammonium Toxicity in Sweet Pepper Plants

Paper Reviewed
Piñero, M.C., Pérez-Jiménez, M., López-Marín, J., Varó, P. and del Amor, F.M. 2018. Differential effect of the nitrogen form on the leaf gas exchange, amino acid composition, and antioxidant response of sweet pepper at elevated CO2. Plant Growth Regulation 86: 37-48.

In discussing the background for their work, Piñero et al. (2018) note that ammonium (NH4+) is a source of nitrogen (N) whose oxidation state "does not need to be reduced in plant cells," adding that "NH4+ is an intermediate in many metabolic reactions and fundamental processes of plants, such as NO3- reduction, photorespiration, degradation of amides, and catabolism of proteins." Therefore, as they continue, the use of NH4+ is "a good alternative to nitrate-based fertilizer" in agriculture; but its use does not come without a price. When applied as the sole source of N fertilizer, NH4+ can be toxic and reduce plant growth and yield. Therefore, some combination or ratio of NO3-/NH4+ fertilization is often used to prevent ammonium toxicity and improve crop yields.

Unfortunately, little is known about the interactive effects of different N forms under elevated atmospheric CO2 conditions. Thus, it became the objective of Piñero et al. to "investigate the extent to which the optimal form of the N-supply can increase growth at an elevated CO2 concentration."

To accomplish their objective, the team of five Spanish researchers grew sweet pepper (Capsicum annuum, cv. Melchor) plants under ambient or elevated CO2 concentrations with nutrient solutions of different NO3-/NH4+ ratios. Not surprisingly, under ambient CO2 conditions, ammonium toxicity decreased plant growth relative to control conditions (100% N from NO3-). In contrast, elevated CO2 benefited plant growth, increasing shoot dry weights from 6% to 33% among the different N form treatment ratios as illustrated in Figure 1), while also improving water use efficiency and chlorophyll fluorescence. What is more, elevated CO2 was able to totally compensate for the dry weight declines experienced by ammonium toxicity, indicated by the fact that all of the dry weights in the elevated CO2 treatments were significantly higher for each N form ratio and none were significantly lower than the plant dry weight experienced under control conditions of 100% N from NO3-.

In the end, the best treatment regime to optimize plant growth was a low dose of NH4+ fertilizer (10% NH4+ and 90% NO3-) under elevated CO2 (800 ppm) conditions. Commenting on these several findings, Piñero et al. say that "the data acquired in this study will be useful to achieve better N-fertilizer management for this important crop, especially under climate change."

Figure 1. Effects of the different nitrogen ratios and urea (NO3-/NH4+: 100/0, 100/Urea, 90/10, 50/50, and 25/75), and elevated CO2 concentration on sweet pepper plants: (A) net photosynthesis rate; (B) shoot dry weight. Vertical bars represent the mean values ± SE; values with the same letter are not significantly different at P < 0.05.Source: Piñero et al. (2018).

Posted 15 November 2018