Effects of Alternating Light Spectrum on the Mineral Element Level of Lettuce

Lettuce (Lactuca sativa) was cultured in a completely enclosed plant factory and irradiated with an intelligent LED light system with accurate regulation of spectral space-time distribution. Absorption and content of eight mineral elements such as K, P, Ca, Mg, Fe, Mn, Zn and Cu in Lactuca sativa exposed to alternating red and blue light spectrum with different alternating intervals were studied by ICP-AES technology. The results showed that; (1) Compared with concurrent red and blue light spectrum, all the alternating spectrum treatments increased shoot biomass of lettuce, the increasing range of fresh weight and dry weight was respectively 18.6%similar to 67.4% and 5.1%similar to 88.0%. All the alternating spectrum treatments significantly increased the accumulation of Mg, Fe and Zn in the lettuce plant (p<0.05); all the alternating spectrum treatments decreased the content of Ca in lettuce to different extents. (2) Fe content in lettuce subjected to R/B(5 m) was significantly higher than any other treatment, increasing by 38.87%similar to 85.37% compared with the other treatments. Alternating red and blue light spectrum with high alternating frequency seemed to stimulate Fe uptake by lettuce plants. (3) Alternating red and blue light spectrum enhanced the use efficiency of light photons and electricity by lettuce. Compared with RB, all alternating treatments significantly improved the light and electric use efficiency of leaf lettuce by about 34.3%similar to 87.5% and 34.6%similar to 87.9%; Among them, the light and electric use efficiency of leaf lettuce were both the largest under the alternating treatment with an interval of 4 h, which were 6.13% and 2.01% respectively. Except for the alternating treatments with the intervals of 5 m and 10 m, no significant difference existed in the light and electric use efficiency of plants among the alternating spectrum treatments. (4) The absorption of K and Mg by lettuce showed antagonism under the alternating intervals of 10m, 15 m, 60 m and 4 h. (5) The contents of P, Ca, Fe and Mn in lettuce treated with R/B(30 m) showed the lowest level among treatments, and the contents of P and Ca were significantly lower than the control.