Influence of Cultivar and Growth Stage on Phytochemicals in Pac Choi

Plant polyphenols are plant secondary metabolites that have been widely researched for their health-promoting properties, which are mostly related to their antioxidant activity. Phenolic compounds, along with ascorbic acid, are recognized for their potent antioxidant activity in the plant response to abiotic stress. As an important group of antioxidant phenolic compounds, anthocyanins are pigments responsible for the red, blue, purple, and orange color of all plants. Pac choi (Brassica rapa var. chinensis) is a popular leafy green produced by many growers in Florida for local markets and the Asian vegetable market. While pac choi is known for green leaves with white or light green petioles, the red pac choi cultivar is also becoming available in most recent years. This study was performed to examine the impacts of cultivar and growth stage on phenolic and ascorbic acid contents in organically produced pac choi. Three pac choi cultivars Joi Choi, Feng Qing Choi, and Red Choi were analyzed for total phenolics, ascorbic acid, and anthocyanins at two weeks after transplanting (baby leaf stage) and four weeks after transplanting (mature leaf stage), respectively. Folin-Ciocalteu method was used to determine the total phenolic content (expressed as mg gallic acid equivalents/g FW), while total monomeric anthocyanin pigment content (expressed as μg cyanidin 3,5-diglucoside equivalents/g FW) was measured using the pH differential method. A randomized complete block design with 3 replications was used with duplicate samples for each cultivar at each growth stage. As predicted, ‘Red Choi’ had significantly greater anthocyanin content when compared to ‘Joi Choi’ and ‘Feng Qing Choi’. For total phenolic concentration, ‘Red Choi’ was significantly higher than Feng Qing Choi, which was significantly higher than Joi Choi. Feng Qing Choi and Red Choi had similar levels of ascorbic acid, while both had significantly greater levels than Joi Choi. Interestingly, all 3 cultivars had significantly higher levels of total phenolics and ascorbic acid at the baby leaf stage in comparison with the mature leaf stage, while there was not a significant difference in anthocyanin content between growth stages. Moreover, ‘Feng Qing’ showed greater differences between the growth stages in total phenolic and ascorbic acid concentrations than the other two cultivars. The results demonstrated that levels of polyphenols and ascorbic acid in pac choi can be impacted by not only the varietal difference but also different growth stages of plants.