Supplementary red-LED lighting affects phytochemicals and nitrate of baby leaf lettuce

Lettuce (Lactuca saliva L.) varieties (red leaf 'Multired 4', green leaf 'Multigreen 3' and light green leaf 'Multiblond 2') were grown under high pressure sodium lamps (16-h) at industrial greenhouse and irradiated with supplemental solid-state lighting (LED) (November, 2010). Three days before harvesting, supplementary lighting from red 638-nm LEDs was applied within a 16-h photoperiod and the net photosynthetically active flux density was maintained at similar to 300 mu mol m (2)s (1). Such a pre-harvest treatment remarkably reduced content of nitrate in red and green leaf lettuce to 56.2% and 20.0%, respectively, but after LED treatment in 'Multiblond 2' the content of nitrate increased 6 times. The LED treatment led to an increased contents of total phenolic compounds (52.7% and 14.5%) and increased free-radical scavenging activity (2.7% and 16.4%) in 'Multired 4' and 'Multiblond 2', respectively. However, 'Multigreen 3' showed statistically unreliable decrease of total phenols and free-radical scavenging activity. The significant increase of vitamin C after LED treatment was observed only in 'Multired 4' (63.3%). The variation of total anthocyanin content was statistically unreliable. Lettuce cultured with short-term supplemental 638-red light radiation appeared to be potent in antioxidant capacity. As many of natural antioxidants (vitamin C, anthocyanins, phenolic compounds) are consumed together in plants, the antioxidant property or heir combination and synergistic interaction leads to the enhancement of antioxidant effectiveness of natural antioxidants. We demonstrated that red light affects the expression of antioxidant ability of lettuce, revealing that light quality affects the physiological metabolism, antioxidant activity and reduction of nitrate. Besides it was found that the effect of supplementary red light was variety dependent. The sensitivity of a variety to lighting conditions was determined by natural level of antioxidant compounds accumulated in lettuce leaves.