Glucosinolates Are Enhanced by Controlled Application of Abiotic Stresses in Broccoli (Brassica oleracea var. Italica) during Postharvest Storage

Glucosinolates are amino acid-derived health-promoting phytochemicals related to plant defense in Brassicas including broccoli. The major glucosinolates in broccoli are aliphatic glucoraphanin and indole-type glucobrassicins. It is highly perishable and the glucosinolate content decreases during storage along with quality. Given that plants produce secondary metabolites as weapons to combat stresses, it is conceivable that the plant responses can well be tuned to achieve an enhancement of phytochemicals that offer plant protection and/or health benefits and acceptability of the produce by exposing them to controlled doses of stressors. The objective of this work was to enhance the levels of glucosinolates in broccoli florets by low doses of abiotic stresses without compromising the quality during the postharvest storage. Five abiotic stresses were applied to broccoli florets and stored at 4°C for over 21 days. Gene expression and glucosinolate content were monitored during the storage period. The applied stresses were UV-B at 1.5 and 7.2 kJ.m^-2; UV-C at 1.2 and 3.0 kJ.m^-2; ozone (O₃) at 5ppm for 60 and 720 min; hydrogen peroxide (H₂O₂) 1.25 and 5mM for 180 min and heat at 41°C/180 min and 47°C/12 min. Following the treatment, the overexpression of tryptophan N-hydroxylase 2 (CYP79B3) was observed in broccoli florets exposed to UV-B, O₃ and hydrogen peroxide; while high doses of UV-B and O₃ had a strong influence on the overexpression of phenylalanine N-hydroxylase (CYP79A2). Overexpression of these genes was in concordance with the enhancement of total glucosinolates titer in florets in response to both the doses of hydrogen peroxide and the higher dose of UV-B. The most effective stress to induce glucosinolates in broccoli was hydrogen peroxide, where glucoraphanin, glucobrassicin and its metabolites were all enhanced. Low doses of UV-B, UV-C and heat enhanced titers of neoglucobrassicin. Ozone on the other hand, enhanced titers of 4-methoxyglucobrassicin and 4-hydroxyglucobrassicin. Further, the indole pathway is favorably affected by the oxidative stresses compared with the aliphatic glucosinolate pathway. Results of this investigation suggest that abiotic stresses may enhance the content of glucosinolates in broccoli florets during the postharvest storage.