Changes in metabolite contents of garlic treated with foliar application of glutamic acid under drought and high temperature at different growth stages

Heat and drought stress driven by climate change severely affect garlic growth and bulb quality. To enhance stress resilience, it is crucial to understand garlic's metabolic responses under these conditions. This study examined metabolite changes and the mitigating effects of foliar-applied glutamic acid (Glu) across different growth stages. "Namdo" garlic was planted in plastic containers filled with a mixture of commercial soil and topsoil on September 9, 2021, and cultivated for 225 days. Stress treatments were applied for 7 days at 175 days (clove initiation) and 218 days (clove enlargement). During clove initiation, treatments included control (15/10 °C) and high-temperature (25/11 °C) conditions, along with control (−30 kPa) and drought (−300 kPa) soil moisture levels. For clove enlargement, treatments included control (20/8 °C), moderate (28/16 °C), and extreme (36/24 °C) temperatures. Foliar Glu applications of 0 and 169 mg·L⁻¹ were applied during clove initiation, and 0 and 80 mg·L⁻¹ during clove enlargement. Under stress conditions, abscisic acid (ABA) content increased by approximately 46 %. Glu treatment did not significantly alter this increase during clove initiation. During clove enlargement, glucose, fructose, ABA, and related metabolites (dihydrophaseic acid, 7′‑hydroxy-ABA, ABA-glucose ester) significantly increased with rising temperatures. Glu application suppressed ABA, phaseic acid, and 7′‑hydroxy-ABA under moderate high-temperature conditions (28/16 °C), while those were less effective under extreme temperatures (36/24 °C). These results indicate that foliar Glu application mitigates stress induced by moderately high temperatures and drought during clove initiation, along with moderately high temperatures during clove enlargement. However, its effectiveness is limited under severe conditions.