Enhancing chilling stress tolerance of pepper seedlings by exogenous application of 5-aminolevulinic acid

In this study, the possibility of enhancing chilling stress tolerance of pepper ( Capsicum annuum L.) during early growth stages by exogenous application of 5-aminolevulinic acid (ALA) was investigated. To improve chilling tolerance during seedling stage, ALA was applied in various concentrations (0, 1, 10, 25 and 50 ppm) through three different methods (seed soaking, foliar spray, or soil drench). After ALA applications, the plants were subjected to chilling stress at 3 °C for 2 days. Although all ALA application methods improved chilling stress tolerance in pepper seedlings, seed soaking and foliar spray provided better protection against chilling stress compared to soil drench. Exogenous application of ALA provided significant protection against chilling stress compared to non-ALA-treated seedlings, significantly enhancing plant mass and chlorophyll, sucrose, and proline contents. ALA pre-treatment also increased relative water content, stomatal conductance and superoxide dismutase (SOD) enzyme activity and reduced membrane permeability. Of the ALA concentrations, the highest chilling tolerance was obtained with 25 ppm ALA pre-treatment. Results indicate that ALA which is considered as an endogenous plant growth regulator could be used effectively to protect pepper seedlings from damaging effects of chilling stress without any adverse effect on seedling growth.