End-of-production Cooling Alters Foliage Color, Yield, and Nutrition of Red Leaf Lettuce

Abstract

In controlled environments (CE), temperature can be adjusted to increase anthocyanin concentration and thus improve foliage color of crops before harvest. Our objective was to quantify how end-of-production (EOP) cooling influences yield, growth, development, quality parameters, and foliage color of red leaf lettuce (Lactuca sativa). Seeds of red leaf lettuce ‘Barlach’, ‘Rouxai’, and ‘Thurinus’ were sown in a growth chamber with a mean daily temperature (MDT) set point of 22 °C, carbon dioxide (CO2) concentration of 500 µmol·mol−1, and a photosynthetic photon flux density (PPFD) of 180 µmol·m−2·s−1 provided by light-emitting diodes (LEDs). After 11 days, seedlings were transplanted into deep-flow hydroponic tanks in the same growth chamber with a CO2 concentration of 800 µmol·mol−1, day/night temperature set point of 28/21 °C (MDT of 26 °C) and under LEDs that provided a PPFD of 300 µmol·m−2·s−1 for 17 h·d−1. During the last 6 to 8 days of production, plants were either left in the same conditions or transferred to growth chambers with a constant MDT of 8, 14, 20, or 26 °C. In addition, spectrum was adjusted to a ratio (%) of 75:25 blue (400 to 500 nm):red (600 to 700 nm) (B:R) light and a PPFD of 150 µmol·m−2·s−1. EOP cooling negatively influenced shoot fresh mass (SFM) and dry mass of ‘Barlach’, ‘Rouxai’, and ‘Thurinus’. Compared with uncooled plants, the SFM and shoot dry mass (SDM) in the 14 °C EOP cooling treatment were 27% and 17% (‘Barlach’), 25% and 20% (‘Rouxai’), and 51% and 52% (‘Thurinus’) smaller, respectively. The chromametric a* value of each cultivar increased, indicating a change in color from green to red, under all EOP cooling treatments. By day 2, a* of ‘Rouxai’ and ‘Thurinus’ at 14 °C increased from −1.7 to 0.06 and from −0.99 to 1.08, respectively. By day 6 of EOP treatment, a* of ‘Barlach’ under the EOP 14 °C treatment increased from −4.18 to −1.66, whereas the a* of uncooled plants decreased from −5.06 to −6.97. Plants exposed to EOP cooling also had greater anthocyanin concentrations. At 14 °C ‘Barlach’, ‘Rouxai’, and ‘Thurinus’ possessed 62%, 53%, and 59% greater anthocyanin than the control. Foliar concentrations of magnesium, manganese, and zinc increased by 23%, 20%, and 21% in ‘Barlach’, and by 26%, 21%, and 13% in ‘Rouxai’ at 14 °C. We observed the highest concentration of violaxanthin, lutein, and total carotenoids in ‘Barlach’ at 14 °C and at 20 °C in ‘Thurinus’. Total water-soluble vitamin (WSV) concentration of ‘Rouxai’ and ‘Thurinus’ was reduced under EOP cooling. In addition, we observed altered concentrations among the WSV, such as reduced vitamin B1, but the highest concentration of vitamin B6 for each cultivar in plants exposed to EOP. Vitamin C was 24%, 29%, and 37% greater in ‘Barlach’; 19%, 23%, and 24% greater in ‘Rouxai’; and 34%, 29%, and 45% greater in ‘Thurinus’ control plants than plants exposed to 20, 14, or 8 °C, respectively.

Keywords

• anthocyanin
• carotenoid
• coloration
• controlled environment agriculture
• lactuca sativa
• leafy greens
• vertical farming
• water-soluble vitamins