Abstract
The objectives of this study were to determine whether foliar application of a chlorophyll precursor, 5-aminolevulinic acid (ALA), could mitigate salinity stress damages in perennial grass species by regulating photosynthetic activities, ion content, antioxidant metabolism, or metabolite accumulation. A salinity-sensitive perennial grass species, creeping bentgrass (Agrostis stolonifera), was irrigated daily with 200 mM NaCl for 28 d, which were foliar sprayed with water or ALA (0.5 mg L−1) weekly during the experiment in growth chamber. Foliar application of ALA was effective in mitigating physiological damage resulting from salinity stress, as manifested by increased turf quality, shoot growth rate, leaf relative water content, chlorophyll content, net photosynthetic rate, stomatal conductance and transpiration rate. Foliar application of ALA also alleviated membrane damages, as shown by lower membrane electrolyte leakage and lipid peroxidation, which was associated with increases in the activities of antioxidant enzymes. Leaf content of Na+ was reduced and the ratio of K+/Na+ was increased with ALA application under salinity stress. The positive effects of ALA for salinity tolerance were also associated with the accumulation of organic acids (α-ketoglutaric acid, succinic acid, and malic acid), amino acids (alanine, 5-oxoproline, aspartic acid, and γ -aminobutyric acid), and sugars (glucose, fructose, galactose, lyxose, allose, xylose, sucrose, and maltose). ALA-mitigation of physiological damages by salinity could be due to suppression of Na+ accumulation and enhanced physiological and metabolic activities related to photosynthesis, respiration, osmotic regulation, and antioxidant defense.
Highlights
Salinity stress is becoming a major factor limiting plant growth in areas with increasing use of non-potable water sources for irrigation
turf quality (TQ) began to decline significantly below the non-salinity, untreated level at 7 d in plants exposed to salinity alone without application of ALA or with ALA (Fig. 1), but the decline was less pronounced for salinity+ALA plants than salinity alone treatment after 14 d
The greater relative water content (RWC) suggested that ALA facilitated the maintenance of cellular hydration despite osmotic stress under salinity
Summary
Salinity stress is becoming a major factor limiting plant growth in areas with increasing use of non-potable water sources for irrigation. The positive effects of ALA on salinity tolerance have been associated with the reduction in Na+/K+ ratio both in roots and leaves [8], increases in chlorophyll content and photosynthetic rate [8, 15], elevated activities of enzymatic or nonenzymatic antioxidant systems providing significant protection to membranes against harmful reactive oxygen species within tissues [11], or reducing stomatal limitation to gas exchange [7]. Despite the knowledge of the beneficial effects of ALA on stress tolerance, there is lack of comprehensive understanding of physiological and biochemical factors responsive to ALA that may mediate salinity tolerance, such as metabolic changes, in addition to changes in photosynthesis, ion content and antioxidant enzymes. The effective doses for promoting perennial grass tolerance to salinity have yet to be determined
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