Can Ammonium Stress Be Positive for Plant Performance?

Abstract

Ammonium (NH3/NH+4) nutrition is considered as a universal stressful situation (recently reviewed in Li et al., 2014; Esteban et al., 2016; Liu and Von Wirén, 2017). Briefly, the most common symptom of ammonium nutrition is reduced biomass accumulation with respect to non-stressed plants. Growth inhibition has been associated with the high energy cost to control NH3/NH+4 level in tissues. Among others, ammonium stress has been related with deregulation of pH homeostasis, ion imbalance, impaired nitrate signaling, or hormone deregulation (Li et al., 2014; Esteban et al., 2016; Liu and Von Wirén, 2017). Although ammonium stress affects virtually every plant species, the degree of stress it generates is variable and high intraspecific and interspecific variability towards ammonium nutrition has been reported. Some species/genotypes display ammonium preference, while others show extreme sensitivity when growing with ammonium. Regarding the response of a certain genotype, as for almost every stress, there exists a continuum in the response upon ammonium nutrition, which mostly depends on the concentration of NH+4 in the root medium. Overall, ammonium tolerance could be defined as a situation where the plant is somehow sensing and responding towards ammonium stress prior to suffering a serious damage such as chlorosis or cell death. Sole ammonium nutrition is an artificial situation that only takes place when growing plants without soil, either in laboratory conditions or for example when growing crops in pure hydroponics or in inert substrates such as rockwool or perlite. In agricultural fields, exclusive ammonium nutrition does not exist; however, the use of nitrification inhibitors together with ammonium fertilizers or organic fertilizers makes ammonium stable and at high concentrations in the soil for several weeks. From a farmer’s point of view, a potential moderate reduction in yield caused by ammonium stress could be compensated with benefits such as an increase in the resistance of the crop against biotic or abiotic constraints and also with obtaining of products of higher quality (Figure 1). Moreover, the use of ammonium-based fertilizers together with inhibitors of nitrification has been extensively shown to mitigate the impact of nitrogen fertilizers on the environment (Sanz-Cobena et al., 2017). Although sophisticated management would be needed, avoiding ammonium stress could be reached by, for instance, fertigation or frequent additions of small amounts of ammonium-based fertilizers in water delivered through micro-irrigation.