Exogenously Used Proline Offers Potent Antioxidative and Osmoprotective Strategies to Re-balance Growth and Physio-biochemical Attributes in Herbicide-Stressed Trigonella foenum-graecum

The present study was performed to verify that the exogenous application of proline as an antioxidant can effectively reduce the damage of the herbicide Basagran® on fenugreek (Trigonella foenum-graecum). Sterilized healthy seeds were soaked in proline (7 mM) with or without the herbicide Basagran® (10–4 M) applied as a foliar spray. Performance was evaluated based on the plant’s physiological and biochemical attributes. Results revealed that herbicide stress caused seedling growth inhibition, which could be due to hydrogen peroxide (H2O2) accumulation with an increase in malondialdehyde (MDA) level and electrolyte leakage (EL). As a consequence, proline metabolism was affected, including the activity of proline dehydrogenase (PRODH) and pyrroline-5-carboxylate synthetase (P5CS), and levels of pyrroline-5-carboxylate (P5C) and proline (Pro) as well as glutathione (GSH) level, total antioxidant capacity (TAC), and activity of catalase (CAT) and glutathione-s-transferase (GST) were affirmatively influenced. However, the exogenous application of proline attenuated the harmful effects of Basagran® by improving growth performance, which might be related to enhanced antioxidant activities, total chlorophyll, relative water content (RWC), and GSH levels. Proline treatment also reduced the damage caused by increased MDA levels and reactive oxygen species (ROS) by regulating the enzymatic and non-enzymatic defense systems. Principal component analysis showed that increased oxidative damage and water imbalance were the most important contributors to herbicide stress-induced damage; however, the proline-mediated antioxidant defense was the crucial determinant of herbicide tolerance in fenugreek. Collectively, findings obtained from this study revealed that externally used proline protects against herbicide stress by enhancing cellular defense mechanisms. These results indicate the capacity of proline to improve the tolerance of fenugreek plants subjected to herbicide constraints.