Priming with 5-aminolevulinic acid (ALA) attenuates UV-B induced damaging effects in two varieties of Cajanus cajan L. seedlings by regulating photosynthetic and antioxidant systems

Abstract

The objective of this study is to investigate whether pre-treatment (priming) of chlorophyll precursor 5-aminolevulinic acid (ALA) assumes a huge part in moderating harms caused by UV-B exposure on two varieties of Cajanus cajan L. seedlings namely BAHAR and KB-17. Healthy seeds were primed with two doses of ALA (lower dose: ALA1; 25µM and higher dose: ALA2; 100 µM) for 24 h at 25±2 °C and thereafter 17-day old seedlings were exposed to an enhanced dose of UV-B radiation i.e. 2.2 kJ m−2d−1. Results showed that UV-B exposure significantly declined growth attributes, photosynthetic pigments (chlorophyll a, b and carotenoids contents), photosynthetic gas exchange characteristics (A, Ci, Gs, E, and WUE). This decrease was also accompanied by the accumulation of reactive oxygen species: superoxide radical (SOR; O₂˙ˉ), hydrogen peroxide (H2O2), and lipid peroxidation (MDA contents). The antioxidative enzyme activities: catalase (CAT), superoxide dismutase (SOD), glutathione-S-transferase (GST), and peroxidase (POD) and non-enzymatic antioxidants: total phenolics content (TPC) and total flavonoids content (TFC) were increased. On contrary to this, damaging effects of UV-B radiations were markedly alleviated with the exogenous application (priming) of ALA by enhancing plant growth and growth-regulating parameters. Furthermore, ALA application decreased the UV-B induced oxidative stress in Cajanus cajan L. seedlings by further improving the activity of enzymatic and the contents of non-enzymatic antioxidants of test plant defense system. Furthermore, priming with ALA decreased the germination time and increased the germination index which additionally served as a significant purpose behind the improvement in the harvest quality and amount. The BAHAR variety of Cajanus cajan was discovered to be more tolerant against UV-B stress due to its more efficient defense system with a higher rate of antioxidant enzyme activities. On the other hand, KB-17 variety of Cajanus cajan was discovered to be sensitive against UV-B stress due to its feeble defense system with higher accumulation of reactive oxygen species. Thus, these findings suggest that priming of seeds with ALA act as a sustainable remedial strategy in reducing the hazardous effects of UV-B induced stress on crop productivity of Cajanus cajan seedlings by triggering the up-regulation of photosynthesis and antioxidant system to counterbalance the ROS-mediated damage to macromolecules.