Alleviating the Effects of High-Temperature Stress on Parsley Plants by Foliar Application of Proline, Glycine Betaine, and Salicylic Acid

Low temperatures and high relative humidity are ideal for parsley's growth and high quality, making it a distinctive herbaceous vegetable with medicinal properties. Despite the growing local and export demands, meeting these conditions, during the summer season, in Egypt is challenging. Therefore, this study aimed to alleviate the harmful effects of high temperatures stress on the growth and quality of Balady variety of parsley by foliar application of proline, glycine betaine, and salicylic acid. Two field experiments were conducted under a sprinkler irrigation system, in Wadi El-Natroun-Beheira Governorate, during the summer seasons of 2020 and 2021. The following seven treatments were studied: proline (Pr1;2.5 mM l −1 ),(Pr2; 5 mM l −1 ), glycine betaine (GB1;40 mM l −1 ), (GB2; 60 mM l −1 ), salicylic acid (SA1;5 µM l −1 ), (SA2;10 µM l −1 ), and control (distilled water) as a foliar spray. The findings indicated that the optimal treatments for enhancing vegetative growth (leaf fresh weight every cut, leaf dry weight every cut, total plant fresh and dry weight per season, and the total yield of plant fresh and dry weight per M 2 ) were Pr2, GB2, and SA1, in progressive order. Conversely, the least effective treatments were the control, followed by SA2 treatment, in both seasons. The application of SA and GB treatments resulted in enhancements in the content of ascorbic acid, chlorophyll, nitrogen, phosphorus, potassium, protein, and total oil in the parsley leaves. Where, SA1 and the GB2 treatments exhibited the most pronounced impact. At the same time, the Pr2 treatment showed the most significant effect on the proline and oil contents compared to the control, in both seasons. The phenols and fibers exhibited contrasting characteristics, with the control treatment giving the highest values, followed by SA2. Generally, the results indicate that the most effective treatments for enhancing the growth and quality of parsley plants grown under high-temperature stress conditions were with the foliar application of glycine betaine at a concentration of 60 mM l −1 and salicylic acid at a concentration of 5 µM l −1 .