Peng Bai ¹ Junwen Wang ¹ Yongmei He ¹ Junfang Feng ¹ Juanli Li ¹ Xingpan Shang ¹ Yue Wu ^1* Jihua Yu ^1,2* Zhongqi Tang ¹ Jianming Xie ¹

• ¹ College of Horticulture, Gansu Agricultural University, Lanzhou, Gansu Province, China
• ² State Key Laboratory of Aridland Crop Science, Gansu Agricultural University, Lanzhou, Gansu Province, China

Tomato fruit are rich in γ-aminobutyric acid (GABA), which lowers blood pressure and improves sleep. An increase in GABA content is important for enhancing the nutritional quality of tomato fruit. To investigate the effects of 5-aminolevulinic acid (ALA) on fruit quality and GABA synthesis in greenhouse tomatoes, the tomato cultivar (Solanum lycopersicum cv. ‘184’) was used as an experimental material. During the fruit-setting period, root drenching with 0, 10, and 50 mg·L-1 ALA, foliar spraying with 0, 50, and 100 mg·L-1 ALA, and fruit surface spraying with 0, 100, and 200 mg·L-1 ALA were applied. The study investigated the application of exogenous ALA to different parts of the plant to determine the optimal ALA concentrations for each application site (10 mg·L-1 for root application, 100 mg·L-1 for foliar application, and 100 mg·L-1 for fruit surface application). Using the selected optimal ALA concentrations, tomatoes were used to study the effects of exogenous ALA application at different sites on fruit quality and GABA synthesis of greenhouse tomatoes. The results demonstrated that exogenous ALA application to different parts of greenhouse-grown tomato plants substantially increased single- fruit weight by more than 42.37%, soluble sugar content by over 78.51%, soluble solids by more than 9.09%, soluble protein by over 82.71%, and vitamin C content by more than 1.31% in mature tomatoes. Additionally, it reduced the organic acid content of the fruit by more than 12.81%. Moreover, it substantially enhanced the amino acid content by 11.22%, umami amino acid content by 7.26%, and GABA content by 214.58% across the different developmental stages of the fruit. Exogenous ALA application at different sites regulates the activity of the key enzyme for GABA synthesis pathway glutamate acid decarboxylase (GAD) and increases the upstream precursor glutamate content during tomato fruit development, thereby affecting GABA content.