Interaction of drought and 5-aminolevulinic acid on growth and drought resistance of Leymus chinensis seedlings

Leymus chinensis is a dominant grass species in the Inner Mongolia steppes owing to its high vegetative productivity, good palatability for cattle, and abundant horizontally creeping rhizomes. Drought is generally regarded as one of the main environmental issues that is becoming a daunting challenge to the growth of plants, and ultimately results in land degradation. Drought stress influences plant growth and development by modulating physiological and biochemical events; however, some growth substances such as 5-aminolevulinic acid (5-ALA) show potential to ameliorate the damaging effects of drought. The objective of this study was to evaluate the response and mechanism of L. chinensis seedlings under drought stress, the effects of 5-ALA application, and the interaction of drought stress and 5-ALA application. Healthy and uniform-size seeds of L. chinensis were collected in a natural community of the Ecological Experimental Station in the Xilingole grassland. A potting experiment was carried out to determine the influence of exogenously applied 5-ALA at various concentrations (10 mg/L, 50 mg/L, and 100 mg/L) under different soil water regimes (50% and 80% soil relative water content) on the morphological and physiological attributes of L. chinensis plants from June to November 2014. The seeds were grown in a biochemical incubator, and then the seedlings were transferred to pots. Water and Hoagland nutrient solution was applied to ensure an adequate nutrient supply at 5-day intervals. When seedlings attained a height of 18–21 cm, 5-ALA was applied at different concentrations. Water spray was applied to L. chinensis plants as a control treatment. A second and third spray of 5-ALA was applied at 1-day intervals to exploit the full potential of 5-ALA application. Simultaneously, drought stress treatment was imposed using 50% soil relative water content; 80% soil relative water content was treated as the control. Therefore, there were eight treatments with three replications established in a random complete block design. The sampling for morphological, physiological, and biochemical attributes was conducted after 15 days of drought stress, respectively. The results showed that 5-ALA could promote the growth of L. chinensis seedlings, including plant height, leaf area, plant water content, biomass, root activity, and photosynthetic pigments (chlorophyll a, chlorophyll b, and carotenoid), under a soil relative water content of 80%. Among all concentrations, 10 mg/L 5-ALA proved to have the best effect on growth. Drought stress (50% soil relative water content) hampered plant growth. However, treatments of 5-ALA ameliorated the damaging effect of drought stress on seedlings, and improved the morphological index, biomass, plant water content, root activity, photosynthetic pigments, osmotic adjustments, and antioxidase activities viz. peroxidase (POD), superoxide dismutase (SOD), catalase (CAT), glutathione reductase (GR), and ascorbate (APX), but reduced malondialdehyde (MDA) content and electrical conductivity. Among all concentrations, 50 mg/L 5-ALA proved to have the best effect under drought stress. In summary, 5-ALA application improved the performance of L. chinensis by modulating growth and other morphological and physiological traits. However, the effect of 5-ALA on L. chinensis was concentration-dependent. Furthermore, significant interactions between drought stress and 5–ALA treatment (at a given spraying concentration) were observed with respect to leaf area, leaf width, fresh weight, root activity, chlorophyll b, carotenoids, MDA, proline, soluble protein, SOD, APX, and GR.