Abstract: Drought stress is an environmental constraint with many ramifications, restricting Leymus chinensis growth and productivity. A pot experiment was undertaken to investigate the influence of foliar applied 5-aminolevulinic acid (ALA), on dry matter accumulation, enzyme activities, hormone concentrations and the transcriptome of L. chinensis under drought (soil water content 50% of field capacity) and well-watered conditions (soil water content 80% of field capacity). Concentrations of ALA tested were: 10, 50, and 100 mg·L^-1, with a pure water treatment as a control. Healthy and uniform-sized seeds of L. chinensis were collected and germinated in a biochemical incubator, and then the seedlings were transferred to pots after one week. When seedlings attained a height of 18-21 cm, ALA was applied at the concentrations indicated above, with a repeat application 7 days later to exploit the full potential of ALA application. The sampling for morphological, physiological, and biochemical attributes was conducted 7 days after the second application. As expected, it was found that drought stress caused a reduction in growth and development of the plant as compared to well-watered conditions. Nonetheless, application of ALA improved plant height, fresh weight, and root activity, and effectively alleviated the damage of drought stress to L. chinensis. The most effective concentration of ALA was 50 mg·L^-1. Furthermore, production of sugars, proteins and N, P and K concentrations in plant tissues were enhanced. These changes might reflect increased activities of metabolic enzymes, such as nitrate reductase, malate dehydrogenase and acid phosphatase. Endogenous hormone concentrations (indole acetic acid, gibberellins, zeatin riboside and abscisic acid) were also increased. The effect of the plant growth regulator ALA on the drought resistance of L. chinensis, and the changes in the physiological indices were mirrored by changes in gene expression. Under drought stress 1373 genes exhibited significantly different expression compared to well-watered plants, with 733 genes down-regulated, and 640 genes upregulated. Comparing the drought stressed control plants with drought stress + ALA treatment, 1315 genes displayed significantly different expression, among which 676 genes were up-regulated. The mechanism of ALA modulation of L. chinensis growth, physiological processes and gene expression needs further research.

Key words: Leymus chinensis, 5-aminolevulinic acid (ALA), drought stress, hormone, transcriptome