Effect of concentration and application methods of 5-aminolevulinic acid on inducing cold resistance of Soybean (Glycine max L.)

Compound of 5-aminolevulinic acid (5-ALA) has herbicide behavior in high concentration. While, it acts as growth regulator at low concentrations and causes resistance to various stresses incluing cold and salibity. Therefore the exogenous application of 5-aminolevulinic acid was investigated to induce cold stress tolerance in young soybean plants (Glycine max L.). The experiment was conducted as randomized complete block design at Tarbiat Modares University, with three replications in 2010-2011. 5-ALA was applied at different concentrations (0, 0.3, 0.6 and 0.6 mM) by seed priming and foliar application at V4 stage methods. Two temperatures (T1=10, T2=25ºC) were added to the experiment as temperature treatments. After 5-ALA application, the plants were in V4 stage subjected to cold stress at 10 ± 0.5°C for 72 h. Cold stress significantly decreased plant growth, relative water content, chlorophyll, photosynthesis and stomatal conductivity while increased electrolyte leakage and proline accumulation. 5-ALA at low concentration (0.3 mM) protected plants against cold stress, enhancing plant height, shoot fresh and dry weight, chlorophyll content, photosynthesis, stomatal conductivity as well as relative water content. Superoxide dismutase and catalase activities also increased by application of 5-ALA especially under cold stress condition. In most cases,5- ALA applied by foliar application method was better than seed priming method. Results show that 5-ALA, which is considered as an endogenous plant growth regulator, could be used effectively to protect soybean plants from the damaging effects of cold stress without any adverse effect on the plant growth.