Search

Light quality can affect the growth of plants, regulate the accumulation and metabolism of substances, thereby improving the growth and quality of plants. Alfalfa is a vital forage with wide distribution and high value for feeding. However, few studies have investigated the effects of light quality on growth, quality, and nitrogen (N) metabolism regulation of alfalfa under artificial light control environment. In this study, the light intensity was 400 μmol·m−2·s−1 and photoperiods was 16/8 h. Five light quality treatments were used for alfalfa production, red-blue ratio of 3:1, 4:1 and 5:1, respectively, and on the basis of red-blue ratio of 4:1, 5% and 10% red light were replaced by green light were also used (R3B1, R4B1, R5B1, R19B5G1 and R18B5G2). The results showed that although the stomatal conductance and lysine content of the R3B1 treatment were significantly higher than other treatments, but the leaf area, fresh yield, contents of soluble sugar, sucrose, nitrate and ammonium, as well as the enzyme activities of NR, NiR, GOGAT and GS were the lowest among all treatments. The R3B1 treatment resulted in growth inhibition and reduced N metabolism levels. Strikingly, compared with the R4B1 treatment, the synthesis of Chl a+b, Chl a, Chl b, and free amino acid, as well as the accumulation of nitrate and crude protein, and the activity of GS also increased significantly in the two treatments which green light replaced partial of red light. However, the contents of soluble sugar, glutamate and lysine, as well as the activities of GOGAT and GDH, were significantly reduced in the R19B5G1 and R18B5G2 treatments. The carotenoids and sucrose contents of R19B5G1 treatment were significantly higher than R4B1 treatment, and the fresh weight and NR activity of R18B5G2 were significantly higher than R4B1 treatment. In addition, the content of starch and soluble protein and AK activity were not affected by light quality. In general, green light instead of partial red light increased the content of photosynthetic pigment and promoted the growth of alfalfa. Although the levels of glutamate, cysteine and lysine were reduced, it was beneficial to the accumulation of nitrate, free amino acid and crude protein, maintained the stability of N metabolism and improved the feeding value. These findings provide valuable insight into the effects of light quality regulation on value of feeding and N metabolism of alfalfa. • The treatment of red, blue and green light expansion of alfalfa leaves and the increase of yield. • The red, blue and green light quality significantly affected the nitrogen metabolism of alfalfa. • Replacing the red light with green light improves accumulates more crude protein content. • The use of green light reduces the accumulation of soluble sugar, glutamate, cysteine and lysine. [ABSTRACT FROM AUTHOR]

There are interspecies differences in lettuce CL tolerance to LED red and blue light. It is important to clarify the response mechanism of tolerant and sensitive lettuce to CL to achieve efficient application of CL in artificial light plant factories. In the experiment, red-blue LEDs were used to provide a conventional light (360μmol·m−2·s−1, red:blue ratio 4:1 and 18/6 h photoperiod) and three continuous light (CL) treatments with various red:blue ratios (270μmol·m−2·s−1, red:blue ratio 4:1, 2:1 and 1:1) for CL-sensitive and tolerant lettuce cultivars (Yidali and Zishan) with the same daily light integral. The effects of CL quality on the growth, diurnal starch and sucrose accumulation and metabolism of two lettuces were studied by sampling at 6:00, 12:00, 18:00 and 24:00 on the 12 days after light treatment. The results showed that red-blue CL up-regulated shoot biomass of two cultivars, which might be attributed to root and leafy morphology, and chlorophyll content responses. CL with the 80% red light produced the greatest yield, but minor and medium leafy injury was found only on Yidali. CL improved starch and sucrose contents diurnally in two lettuce cultivars, and they totally decreased with blue light proportion. Sucrose synthase (SS), sucrose phosphate synthase (SPS), acid invertase (AI), neutral invertase (NI), adenosine diphosphate glucose pyrophosphorylas (AGP), α-amylase and β-amylase activities under CL were improved in two lettuces compared with the conventional light. Starch and sucrose contents and SS, SPS and AGP activities basically decreased with blue light proportion for two cultivars, while AI, NI, α-amylase and β-amylase activities increased inversely totally. CL irrespectively of its quality disordered the circadian rhythms of carbohydrate accumulation and metabolic enzyme activities in two lettuces. Starch content related closely positively with AGP, α-amylase and β-amylase activities for Zishan, but positively AGP and β-amylase activities for Yidali. Sucrose content correlated negatively with AI and NI activities for Zishan, while positively SS and SPS activities for Yidali. To sum up, red-blue CL with low-dose blue light could improve Yidali and Zishan lettuce yield, starch and sucrose accumulation, and metabolic enzyme activity, but CL disordered the circadian rhythms of carbohydrate accumulation and metabolic enzyme activity irrespectively of spectral quality. Yidali and Zishan lettuces are sensitive and tolerant cultivars to red-blue CL, which is not up to their abilities to maintain circadian rhythms of carbohydrate accumulation and metabolism. Furthermore, starch accumulation in two cultivars was controlled jointly by synthesis and decomposition enzymes, but sucrose accumulation in Yidali lettuce depended on decomposition enzymes rather than synthesis enzyme for purple lettuce. • Sensitive and tolerant lettuce cultivars were grown under red-blue CL of various qualities for 12 days. • CL improved lettuce yield, starch and sucrose contents, and metabolic enzyme activity. • CL disordered the circadian rhythms of carbohydrate accumulation and metabolic enzyme activity. • Spectral quality affected diurnal carbohydrate accumulation and metabolic enzyme activity. • Lettuce tolerance to CL did not due to circadian rhythms of carbohydrate accumulation and metabolism. [ABSTRACT FROM AUTHOR]

• This is the first study focused on the dynamic responses of ascorbate metabolism to long-term continuous light. • The increase of lettuce shoot biomass under continuous light dependent on specific leaf weight rather than leaf area. • Increased AsA and DHA level under continuous light was mainly attributed to greater GalLDH and APX activity respectively. • Ascorbate-glutathione cycle pays little contribution to ascorbate accumulation under continuous light. To understand the dynamic changes of lettuce growth, ascorbate pool and metabolism under long-term red and blue continuous light (CL, 24/0 h, 200 μmol·m−2·s−1), growth parameters, ascorbate pool size, as well as activities and gene expression levels of key enzymes involved in the ascorbate metabolism in lettuce leaves were investigated every 3 days during 15 days. In comparison with normal photoperiod (Control, 16/8 h, 200 μmol·m−2·s−1), lettuce plants gained greater shoot fresh weight and dry matter under CL, which depended more on specific leaf weight than leaf area. Both reduced ascorbate (AsA) and dehydroascorbate (DHA) contents were remarkably enhanced by CL and keep a relatively stable level during the experiment. The consistent greater contents of AsA and DHA were respectively attributed to the up-regulation of L-galactono-1,4-lactone dehydrogenase and ascorbate peroxidase activity rather than their gene expression. Although glutathione contents and glutathione reductase activities were elevated by CL, their contribution to AsA regeneration was limited on account of similar activity and expression level of dehydroascorbate reductase with control. In addition, H 2 O 2 content was also raised obviously under CL compared with control, but it was maintained at a relative safety and stable level from day 6. Fluorescence imaging results also showed that CL did not induce oxidative damage on lettuce leaves compared with control. It is concluded that lettuce plants grown under red and blue continuous light for 15 days obtained greater shoot biomass and ascorbate pool size without leaf injury, and 9 days was the most cost-effective duration for yield improvement from an energy standpoint. Ascorbate accumulation under continuous light was primarily and rapidly regulated by the enzymes involved in AsA biosynthesis and oxidation at enzymatic activity level rather than gene expression level. [ABSTRACT FROM AUTHOR]

• EOP high light irradiation increased the ascorbate content of lettuce. • EOP high light intensity regulated the expression of genes and the activity of enzymes involved in ascorbate synthesis and cycling. • EOP high light intensity induced the production of ROS, and the content of ascorbate, as an important antioxidant, increased accordingly. • EOP high light intensity increased the carbohydrate content of lettuce. Increasing the L-ascorbate (AsA) content of vegetables is very important for improving plant quality in greenhouses and plant factories because AsA is an antioxidant that is essential for the proper function of plants and the human body. End-of-production (EOP) high light irradiation is an economical preharvest practice for improving product quality. Here, we studied the dynamic responses of the AsA pool and AsA metabolism in lettuce exposed to EOP high light irradiation (300, 400, and 500 μmol m−2·s−1) from red and blue LEDs over a 16-h time course. Compared to the control (200 μmol m−2·s−1), the content of AsA and total AsA (T-AsA) in lettuce leaves significantly increased after 8 h of high light irradiation, and the contents of AsA and T-AsA increased by 42.4 % and 36.9 %, respectively, under 500 μmol m−2·s−1 irradiation. Transcript levels of the AsA metabolism-related genes GME , GGP , GPP , GLDH , APX , MDHAR1 , MDHAR2 , DHAR1 , DHAR2 , and GR were upregulated under higher light. Likewise, the activities of AsA-related enzymes (GLDH, APX, DHAR, and GR) increased under 500 μmol m−2·s−1. The expression of genes (GGP , GPP , GLDH , MDHAR1 , MDHAR2 , DHAR1 , DHAR2 , and GR) was more sensitive to high light irradiation in AsA metabolism. Both high light intensity and long irradiation time increased the soluble sugar content. Moreover, high light intensity (500 μmol m−2·s−1) increased the content of H 2 O 2 , O 2 *−, and MDA in lettuce leaves, especially at 16 h. In conclusion, EOP high light increased AsA levels by upregulating the expression of genes and the activity of enzymes in AsA metabolism. The increase in soluble sugar availability and the signaling of reactive oxygen species also contributed to AsA accumulation. We therefore recommend 8 h of high light (500 μmol m−2·s−1) irradiation before harvest to increase the AsA content of lettuce. [ABSTRACT FROM AUTHOR]