Zeaxanthin Levels Increase In Mustard (Brassica juncea L. ‘Florida Broadleaf') Microgreens Following High Light Treatment

The xanthophyll cycle pigments of zeaxanthin, antheraxanthin, and violaxanthin are vital for energy dissipation of excess absorbed light.  Environmental stress which acts to lower a plant’s photosynthetic rate will increase the need for xanthophyll carotenoids.  Carotenoids are some of the most effective antioxidants in nature; therefore, it would be beneficial to increase zeaxanthin in vegetable crops for human consumption.  Microgreens are a specialty leafy green harvested shortly after the first true leaves have emerged.  They are harvested just above the roots and consumed fresh, as opposed to sprouts which are eaten whole.  The objective of this study was to increase zeaxanthin concentrations in mustard (Brassica juncea L. ‘Florida Broadleaf’) microgreens through exposure to high light treatments just prior to tissue harvest.  Mustard microgreens were grown in a modified environment using Sure to Grow® pads of polyethylene teraphthalate (PET) fibers.  Seeds were germinated on the pads and grown under a 14 h photoperiod under normal light treatment (NLT) conditions (275 µmol·m²·sec¹) in controlled environments.  Upon emergence of the first true leaf, a nutrient solution of 40 mg Nitrogen per L (from 20N-8.8P-16.6K) was used to submerge the growing pads and light treatments were initiated.  Microgreens were grown under either a NLT at 275 µmol·m²·sec¹, or a high light treatment (HLT) at 463 µmol·m²·sec¹.  Microgreens were harvested after both light treatments had accumulated 36 h during the photoperiod.  Leaf pigments were extracted from freeze-dried tissues and measured for chlorophylls and carotenoids using HPLC separation.  Significant decreases in chlorophyll a (P ≤ 0.001) and b (P ≤ 0.01) concentrations in the HLT indicated high light stress had occurred.  There were significant decreases in β-carotene (P ≤ 0.01) and neoxanthin (P ≤ 0.001) in the HLT; however, lutein concentrations were unchanged.  A 133% increase in zeaxanthin (P ≤ 0.001) and a 50% increase in antheraxanthin (P ≤ 0.01) concentrations occurred under the HLT.  Doubling the zeaxanthin level in microgreens with a simple change in light management could be a way for producers to increase the nutritional value of this increasing popular specialty crop.