Fig 1.
Spectral distribution of the white background light (peaks at 451, 605, and 659 nm) and end-of-production (EOP) supplemental ultraviolet-B (UVB; peak at 309 nm), ultraviolet-A (UVA; peak at 368 nm), violet (V; peak at 404 nm), blue (B; peak at 444nm) and red (R; peak at 659 nm) light.
The white background light was applied at 200 µmol m-2 s-1 for 16 h/day throughout the 28-d production period (W200; control). The EOP supplemental UVB radiation was applied at 3 µmol m-2 s-1 for 16 hours per day during the last seven days of the production period, while the EOP UVA, V, B and R lights were applied at a higher intensity of 60 µmol m-2 s-1. The subscript number after each waveband indicates its photon flux density in μmol m-2 s-1.
Fig 2.
Image analysis of normalized difference anthocyanin index (NDAI) following the method described by Kim and van Iersel [54].
Graphs show representative RGB images of lettuce ‘Rouxai’ with low (a) and high (d) anthocyanin content, and their corresponding NDAI images (b and e) and histograms (c and f).
Fig 3.
Representative plants of lettuce ‘Red Salad Bowl’ and ‘Rouxai’ grown under different light spectral treatments.
EOP stands for end-of-production (the last 7 days of a 28-d production period). The photoperiod in all six treatment was 16 hours light and 8 hours dark, with EOP treatments applied throughout the 16-hour light period.
Fig 4.
Shoot fresh weight (a and b), shoot dry weight (c and d), and total leaf area (e and f) of lettuce ‘Red Salad Bowl’ (a, c, and e) and ‘Rouxai’ (b, d, and f) under different spectral treatments.
Within each lettuce cultivar, different letters indicate significance at P < 0.05 among the light treatments. Error bars represent SE (n = 12; 6 plants per treatment x 2 blocks). See Fig 1 legend for detailed information on the spectral treatments.
Fig 5.
Anthocyanin index (pigment extraction analysis) (a and b) and normalized difference anthocyanin index (NDAI; image-based analysis) (c and d) in lettuce ‘Red Salad Bowl’ (a and c) and ‘Rouxai’ (b and d) grown under different spectral treatments.
Within each lettuce cultivar, different letters indicate significance at P < 0.05 among the spectral treatments. Error bars represent SE (n = 12; 6 plants per treatment x 2 blocks). The image-based NDAI showed strong linear correlation with the extraction-based anthocyanin index in both ‘Red Salad Bowl’ (e) and ‘Rouxai’ (f). See Fig 1 legend for detailed information on the spectral treatments.
Fig 6.
Total phenolic contents of lettuce ‘Red Salad Bowl’ (a) and ‘Rouxai’ (b) in units of gallic acid equivalents (mg g-1 fresh weight).
Within each lettuce cultivar, different letters indicate significance at P < 0.05 among the spectral treatments. Error bars represent SE (n = 12; 6 plants per treatment x 2 blocks). See Fig 1 legend for detailed information on the spectral treatments.
Fig 7.
Total carotenoids (a and b) and chlorophyll contents (c and d) in lettuce ‘Red Salad Bowl’ (a and c) and ‘Rouxai’ (b and d) grown under different spectral treatments.
Total chlorophyll content was calculated as the sum of chlorophyll a and b contents. Within each lettuce cultivar, different letters indicate significance at P < 0.05 among the light treatments. Error bars represent SE (n = 12). See Fig 1 legend for detailed information on the spectral treatments.
Fig 8.
Ascorbic acid, dehydroascorbic acid, and total ascorbic acid levels of lettuce ‘Red Salad Bowl’ (a) and ‘Rouxai’ (b) grown under different light spectral treatments.
Within each lettuce cultivar, different letters indicate significance at P < 0.05 among the spectral treatments. Error bars represent SE (n = 6). See Fig 1 legend for detailed information on the spectral treatments.