Proline

Sea cucumber (Acaudina molpadioidea) was purchased from a local market (Qingdao, China) and its water content was determined to be 12.65% per 20 g on average. Hippuryl histidyl leucine (HHL), rabbit-lung ACE and Sephadex G-15 were purchased from Sigma Chemical Ltd (St. Louis, MO, USA). Papain (from papaya) and trypsin (from bovine sources) were purchased from Nanning Pangbo Biological Engineering Ltd (Nanning, China). O-Pthaldialdehyde (OPA) was purchased from Beijing Soularbio Technology Ltd. (Beijing, China). Proline (PubChem CID 145742), valine (PubChem CID 6287), tyrosine (PubChem CID 6057), tryptophan (PubChem CID 6305), leucine (PubChem CID 6106) and phenylalanine (PubChem CID 6140) were purchased from Sinopharm Chemical Reagent Ltd. (Shanghai, China). All other chemicals were obtained from local commercial sources and were of the highest purity available.

Acetone standard grade (AR), ethanol, sulfosalicylic acid, ortho-phosphoric acid, ascorbic acid, and glacial acetic acid were purchased from Xilong Scientific Chemicals (XSC; Shanghai, China); trichloro acetic acid (TCA) was obtained from Thermo Fisher Scientific China (Shanghai chemical); thiobarbituric acid (TBA) was obtained from Sinopharm Chemical reagents; potassium phosphate (K₃PO₄) and potassium iodide (KI) were purchased from Sichuan chemical Reagent (Sichuan, China); toluene was obtained from Shanghai Chemex chemicals; EDTA and riboflavin were obtained from Shanghai Huayi Bio-Lab; nitro blue tetrazolium (NBT) was provided by a lab supplier (Shanghai); GR was obtained from glutathione manufacturers and suppliers; and NADPH, proline, FeSO₄, and HCL were purchased from Sinopharm Chemical Reagent (Shanghai, China). All reagents and chemicals were of analytical grade.

Proline (purity ≥ 98.5%), obtained from Sinopharm, was utilized in this study. To enhance adhesion to wheat leaves, Proline was mixed with 0.1% Tween-20 solution before spraying. The inclusion of Tween-20 aimed to counteract the hydrophobic nature of leaf surfaces, thereby facilitating the effective wetting and penetration of the Proline solution into the leaves [63 ,64 (link)]. A volume of 4.5 L of Proline solution was sprayed per treatment (40 barrels) each time using a 5-L electric watering can. During the stress period, the spraying took place 4 times (21, 23, 24, and 25 April 2022) on windless evenings.