Phosphorus Limitation Improved Salt Tolerance in Maize Through Tissue Mass Density Increase, Osmolytes Accumulation, and Na+ Uptake Inhibition.

Hongliang Tang,Yinglong Chen,Xinying Chen,Le Niu,Jing Wei

doi:10.3389/fpls.2019.00856

Abstract

Low phosphorus (P) availability and salt stress are two major constraints for maize (Zea mays L.) growth in north China. A combination of salinity and high P rather than low P is more detrimental to the growth of maize. However, little is known about the mechanisms by which P nutrition modifies the salt tolerance and P uptake of maize. The present study aimed to investigate the combined effects of salinity and P on maize growth and P uptake, and to address the physiological mechanisms of salt tolerance influenced by P availability in maize. Seedlings of a local maize cultivar XY335 were grown hydroponically for 35 days under low (5 μM) or sufficient P supply (200 μM) with or without 100 mM NaCl. Root morphological traits, tissue mass density, leaf osmolytes (sugars and proline) accumulation, and Na+/K+ ratio were measured to allow evaluation of the combined effects of salinity and P on maize growth and P uptake. Both P deficiency and salinity markedly reduced the growth of maize. However, P deficiency had a more pronounced effect on shoot growth while salinity affected root growth more prominently. Combined effects of P deficiency and salinity on total root length, root surface area, and average root diameter were similar to that of plants grown under salt stress. The combination of P deficiency and salinity treatments had a more pronounced effect on tissue mass density, leaf proline and soluble sugars compared to individual treatment of either low P or NaCl. When exposed to salt stress, maize plants of sufficient P accumulated greater amount of Na+ than those under P deficit, but similar amounts of K+ were observed between the two P treatments. Salt stress significantly increased shoot P concentration of maize with sufficient P (P < 0.01), but not for P-deficient plants. In sum, shoots and roots of maize exhibited different responses to P deficiency and salinity, with more marked effect of P deficiency on shoots and of salinity on roots. P deficiency improved salt tolerance of maize plants, which was associated with the increase of tissue mass density, accumulation of osmolytes, reduction of Na+ accumulation, and selective absorption of K+ over Na+.

Highlights

Phosphorus (P) is an essential macro-element required by plants involved in many metabolic processes including energy transfer, signal transduction, biosynthesis of macromolecules, photosynthesis, and respiration (Raghothama and Karthikeyan, 2005)
Plants exposed to high salinity often suffer from osmotic stress, toxicity of Na+ and Cl− ions, nutritional disorders, and oxidative stress, which lead to reduction in photosynthesis and inhibition of plant growth (Hasegawa et al, 2000; Munns and Tester, 2008)
Maize was grown for 35 days under low P (5 μM) and high P (200 μM), with or without 100 mM NaCl

Summary

Introduction

Phosphorus (P) is an essential macro-element required by plants involved in many metabolic processes including energy transfer, signal transduction, biosynthesis of macromolecules, photosynthesis, and respiration (Raghothama and Karthikeyan, 2005). Some mechanisms that enable plants to survive under high salinity have been identified, including ion compartmentalization, osmolytes accumulation, osmotic adaptation, selective transport and uptake of ions, ion homeostasis, and leaf salt excretion (Flowers and Colmer, 2008). Proline plays an important role as osmolyte, contributes to scavenging ROS, stabilizing membrane and proteins, buffering cellular redox potential, and inducing the expression of salt stress-responsive genes (Carillo, 2018). These soluble substances are capable of mediating osmotic adjustment, protect subcellular structures, and mitigate oxidative damage caused by free radicals in response to salt stresses (Singh et al, 2015). Manipulating NHX1 and SOS1 genes in plants is an important strategy to maintain ionic homeostasis and cope with salt stress (Yamaguchi et al, 2013)

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