Physiological and Biochemical Processes Related to Ammonium Toxicity in Higher Plants

Abstract

Nitrate and ammonium have different effects on many biochemical and physiological processes in plants, and at high concentrations this can lead to markedly different growth responses. Most plant species show reduced growth, smaller leaves and a stunted root system when exposed to high ammonium concentrations, and in severe cases this leads to chlorosis. Although well known, ammonium toxicity is poorly understood and is generally considered to be the result of one or more of the following effects; (i) ammonium‐induced mineral nutrient deficiency, arising from the impaired uptake of metal ions; (ii) secondary growth inhibition arising from the acidification of the rooting medium; (iii) alterations in intracellular pH and osmotic balance; (iv) uncoupling of photophosphorylation from electron transport, following the accumulation of ammonium in leaves; and (v) altered polyamine and phytohormone metabolism. These hypotheses are reviewed in the light of the available literature and experimental evidence from own experiments. It is concluded that no mechanism on its own provides an adequate explanation of the available data.

Ammonium enhances resistance to salinity stress in citrus plants

“…Although other studies have shown that some species develop toxic symptoms when only NH 4 + nutrition is applied (Gerendás et al, 1997;Lasa et al, 2001), the citrange Carrizo N-NH 4 + plants displayed optimal growth, estimated on the basis of biomass production (Fig. 1A).…”

“…The analysis of PAs in our study has determined that N-NH 4 + treatment induces a faster, stronger Put accumulation at 2 h (data not shown) and at 14 d in the absence of salt. It is commonly accepted that some species develop toxic symptoms when NH 4 + nutrition is applied (Gerendás et al, 1997;Lasa et al, 2001), while a negative effect on plant growth has been observed with this kind of nutrition (Claussen and Lenz, 1999;Walch-Liu et al, 2000).…”

“…Gerendás et al (1997) suggest that plants could accumulate PAs to compensate for the lack of some cations other than NH 4 + , hence they could contribute to cellular ionic balance maintenance. Furthermore PAs mainly Put, has an important role in abiotic stress since it reduces salt-induced oxidative damage by increasing the activities of antioxidant enzymes and by lowering lipid peroxidation (Tang and Newton, 2005).…”

Sphagnum can ‘filter’ N deposition, but effects on the plant and pore water depend on the N form

“…NH4 + is preferentially taken up by Sphagnum (Fritz et al, 2014;Liu et al, 330 2013; Wiedermann et al, 2009) because Sphagnum has a high cation exchange capacity 331 (Bates, 1992;Gunnarsson & Rydin, 2000). In addition, NH4 + has greater toxicity 332 (Gerendás et al, 1997;Krupa, 2003;Stevens et al, 2011). DON concentration in pore water (Fig.…”

“…(NH4 + ) versus oxidized (NO3 -) N on the N status of the Sphagnum moss and the (Manninen et al, 2011;Sheppard et al, 2014), possibly due to the greater toxicity of 97 NH4 + (Gerendás et al, 1997;Krupa, 2003;Stevens et al, 2011;Limpens and Berendse, 98 2003) coupled to preferential uptake of NH4 + by Sphagnum (Fritz et al, 2014;Liu et al, 99 2013; Wiedermann et al, 2009). For example, Manninen et al (2011) found that NH4 + 100 addition increased shoot N concentration of Sphagnum and decreased photosynthetic 101 variables (Fv/Fm) and shoot dry weight of Sphagnum.…”