Physiological and Biochemical Characterization of the GABA Shunt Pathway in Pea (Pisum sativum L.) Seedlings under Drought Stress

The physiological and biochemical role of the γ-aminobutyric acid (GABA) shunt pathway in green pea seedlings (Pisum sativum L.) was studied in response to soil water holding capacity levels: 80%, 60%, 40%, 20%, and 10% grown under continuous light at 25 °C for 7 days and 14 days, separately. Characterization of seeds germination pattern, seedlings growth (plant height, fresh and dry weight, and chlorophyll contents), GABA shunt metabolite (GABA, glutamate, and alanine) levels, total protein and carbohydrate levels, and oxidative damage (MDA level) were examined. Data showed a significant effect of drought stress on seed germination, plant growth, GABA shunt metabolites level, total protein and carbohydrate contents, and MDA level. A significant decline in seed germination percentage was recorded at a 20% drought level, which indicated that 20% of soil water holding capacity is the threshold value of water availability for normal germination after 14 days. Seedling fresh weight, dry weight, and plant height were significantly reduced with a positive correlation as water availability was decreased. There was a significant decrease with a positive correlation in Chl a and Chl b contents in response to 7 days and 14 days of drought. GABA shunt metabolites were significantly increased with a negative correlation as water availability decreased. Pea seedlings showed a significant increase in protein content as drought stress was increased. Total carbohydrate levels increased significantly when the amount of water availability decreased. MDA content increased slightly but significantly after 7 days and sharply after 14 days under all water stress levels. The maximum increase in MDA content was observed at 20% and 10% water levels. Overall, the significant increases in GABA, protein and carbohydrate contents were to cope with the physiological impact of drought stress on Pisum sativum L. seedlings by maintaining cellular osmotic adjustment, protecting plants from oxidative stress, balancing carbon and nitrogen (C:N) metabolism, and maintaining cell metabolic homeostasis and cell turgor. The results presented in this study indicated that severe (less than 40% water content of the holding capacity) and long-term drought stress should be avoided during the germination stage to ensure proper seedling growth and metabolism in Pisum sativum L.