Red and blue wavelengths affect the morphology, energy use efficiency and nutritional content of lettuce (Lactuca sativa L.)

Abstract

Since red (R) and blue (B) LED light has different quantum efficiency and photoelectric conversion efficiency, mixed RB with different proportions of R and B results in varied energy consumption. In order to improve the energy use efficiency of the closed-type plant production systems, the effects of R and B proportions on the electric use efficiency (EUE), light use efficiency (LUE) as well as the quality of butter leaf lettuce were evaluated in this study. Lettuce seedlings were cultivated in a plant factory with artificial lighting (PFAL) and subjected to eleven combinations of R and B (100%R, 90%R, 80%R, 70%R, 60%R, 50%R, 40%R, 30%R, 20%R, 10%R, 0%R; the rest of the photons in each treatment were B) with the same total photosynthetic photon flux density (PPFD) and photoperiod (200 ± 3 μmol·m−2·s−1, 16 h) for 35 days. The results showed that palpable petiole distortion appeared when R proportion was more than 70% and the distortion was aggravated with the increase of R proportion. The highest EUE and LUE were both detected in lettuce under 90%R treatment, which were respectively 3.64% and 1.20%. The least number of photons and the least electricity amount required to produce 1 g dry weight of lettuce was respectively 2.92 mol and 1.67 MJ, which were both detected in lettuce treated with 90%R. The sucrose content in lettuce treated with more than 50%R was significantly higher than those treated with less than 50%R (50%R included). Lettuce treated with 80%R possessed the highest soluble sugar content as well as the lowest crude fiber and nitrate content (not significantly different with the minimum values). R proportion exceeding 50% in mixed RB light was beneficial to the accumulation of hexose and sucrose, as well as the decomposition of nitrate in lettuce. The vitamin C content in lettuce treated with 100%R was significantly higher than that in lettuce under other treatments in the study. On the whole, the study indicated that the proportions of R and B affected the energy use efficiency and quality of lettuce in closed plant factory, however the responses of plants to the proportions of R and B varied according to different indexes. Thus, some indexes of top priority should be determined before choosing the optimal proportions of R and B.

Ratio of Intensities of Blue and Red Light at Cultivation Influences Photosynthetic Light Reactions, Respiration, Growth, and Reflectance Indices in Lettuce

“…First, using a red illumination variant during cultivation increases lettuce growth ( Figure 9 and Figure 10 ) and also increases the dry weight, indicating an increase in the plant productivity. This result is in a good accordance with the number of works that also show the stimulation of growth and productivity in lettuce being treated with red light and the decrease in these processes when the plants are being treated with blue light [ 41 , 50 , 51 , 52 , 78 ]. However, we were unable to a reveal an increase in the photosynthetic CO 2 assimilation when using the red illumination variant ( Figure 2 ).…”

“…As a result, investigating the influence of illumination spectra on the physiological processes in lettuce during cultivation is an important task. In recent years, numerous studies (e.g., [ 50 , 51 , 52 , 75 , 76 ]) that have investigated the influence of LED illumination with different intensity ratios of red and blue light on lettuce growth, productivity, biochemical composition, and many other parameters, have been published. In particular, there are works that directly analyze the photosynthetic parameters [ 41 , 46 , 75 , 77 ] and leaf optical properties [ 44 ] in lettuce plants that have been cultivated under different light spectra.…”

“…However, some questions require further investigation. (i) Previous studies have shown [ 41 , 50 , 51 , 52 , 78 ] that increasing the portion of blue light during lettuce cultivation decreases the dry weight and productivity per unit of the light intensity in plants. In contrast, this increase in blue light can stimulate A hv [ 41 , 47 ]; this effect could be related the increased stomata opening and the increased chlorophyll content.…”

“…(ii) Data about the of influence of the portion of the blue light illumination on chlorophyll content are contradictory. Some works [ 41 , 52 ] have shown that increasing this portion increases the chlorophyll content (mainly, total chlorophylls and chlorophyll a); other works [ 50 ] do not show any significant changes in the chlorophyll content. (iii) There are works shown the influence of the illumination spectra during lettuce cultivation on LEF [ 75 , 77 ]; however, investigations into the influence of these spectra on CEF are practically absent.…”

“…The aim of the present work was the analysis of influence of two variants of illumination spectra during cultivation on the physiological parameters of lettuce, including the parameters of photosynthetic dark and light reactions, dark respiration rate, growth parameters, and the main reflectance indices of the leaves. Green leaf lettuce ( Lactuca sativa L.) was used in this investigation because it is an important agricultural plant that is widely cultivated under artificial illumination (including illumination by LEDs with narrow spectral bands [ 49 , 50 , 51 , 52 ]). The first variant of illumination (the “red” variant) included red and white light with a small portion of the blue light (<10% from the total light intensity and 13% from the red light intensity) because this cultivation variant causes relatively high rates of growth and photosynthesis in lettuce [ 41 , 51 ].…”