Decreased Solution pH and Increased K+ Uptake Are Related to Ammonium Tolerance in Hydroponically Cultured Plants

Jinnan Song,Byoung Ryong Jeong,Jingli Yang

doi:10.3390/horticulturae8030228

Abstract

The ammonium (NH4+) tolerance of plants is an important issue in agriculture, associated with several plant characteristics. So far, plant tissue acidification has been shown as the primary cause of NH4+ toxicity. Suppressed plant growth caused by excess NH4+ can be counteracted by potassium (K+) application. However, the effects of NH4+ tolerances on the differences regarding pH changes together with K+ uptake remain to be determined. Here, we performed an 84 h hydroponic cultivation of five species with different NH4+ tolerances, subjected to three NH4+:NO3− solutions (0:100, 50:50, or 100:0), to investigate the pH changes and ion uptakes. Consequently, the solution pH was lowered over time to varying extents in the presence of NH4+. The NH4+-tolerant ageratum and lettuce, shown to be tolerant to NH4+ in this trial, rapidly lowered the solution pH, whereas extremely NH4+-sensitive salvia and cabbage only gave a minor decrease in the solution pH when grown with 100:0 NH4+:NO3−. Additionally, the increased external NH4+ level led to a substantial decline in the net cation influxes (K+, Ca2+, and Mg2+). As compared to solely NH4+-fed salvia and cabbage, solely NH4+-fed ageratum and lettuce ultimately showed a relatively greater net K+ influx. Taken together, this study discusses how the decreases in pH and K+ are related to NH4+ tolerance in five hydroponically cultured species.

Highlights

As two predominant inorganic nitrogen (N) forms for higher plants absorption and assimilation, ammonium (NH4 + ) and nitrate (NO3 − ) show varying preferences by plants, and affect the plant growth and physiology in a distinct manner [1–3]
When the comparisons were conducted on identical species, the pH of the hydroponic solutions were remarkably affected by the NH4 + :NO3 − ratios, regardless of the species (Figure 3)
The pH of hydroponic solutions supplied with 50:50 NH4 + :NO3 − and 100:0 NH4 + :NO3 − substantially declined, whereas the pH of hydroponic solutions supplied with 0:100 NH4 + :NO3 − were detected to show varying degrees of increase

Summary

Introduction

As two predominant inorganic nitrogen (N) forms for higher plants absorption and assimilation, ammonium (NH4 + ) and nitrate (NO3 − ) show varying preferences by plants, and affect the plant growth and physiology in a distinct manner [1–3]. The use of NH4 + appeared to be superior to that of NO3 − , because NH4 + was more metabolically efficient than NO3 − , requiring relatively less energy for assimilation [4,5]. Plants cultured exclusively with millimolar NH4 + often developed ammonium toxicity symptoms, as characterized by stunted plant growth accompanied by leaf chlorosis and necrosis [6,7]. NH4 + toxicity causes: carbohydrate limitation for efficient detoxification; futile transmembrane NH4 + cycling; disorders of photosynthesis and electron transport homeostasis; extracellular acidification shifting to an intolerable level; and disruption of the plant ion balance [8–12].

Methods

Results

Discussion

Conclusion