Carbohydrate accumulation and sucrose metabolism responses in tomato seedling leaves when subjected to different light qualities

Light qualities are thought to affect many plant physiological processes during growth and development. To investigate how light qualities influence tomato (Solanum lycopersicum) seedlings, the present study evaluated the effects of different light qualities generated by light emitting diodes (LEDs) with the same photosynthetic photon flux density (PPFD), including monochromic light red (657 nm, R), blue (457 nm, B), purple (417 nm, P) or white (W), combination of R and B lights (R:B = 1:1, 1R1B and R:B = 3:1, 3R1B) on the net photosynthetic rate (Pn), growth rate, carbohydrate content and sucrose-metabolizing enzyme activities. The results showed that relative to W, the seedling plant height and stem diameter were significantly promoted by combination of R and B lights and monochromic R light. However, the level of root growth was lower under R and P light, and the seedling growth and Pn were significantly suppressed under the latter. Additionally, R light significantly increased the contents of fructose and glucose, and combination of R and B lights significantly enhanced total carbohydrate, starch and sucrose accumulation, especially for 3R1B treatment. Activity of sucrose synthase (SS) was promoted under the different treatments and reached its highest value under 3R1B, which appeared to be a major contributor to the significantly higher content of starch under this treatment. Furthermore, R, B and P light reduced activity of sucrose phosphate synthase (SPS). Activities of acid invertase (AI) and neutral invertase (NI) were significantly increased by R light, but were markedly reduced under P. The results presented here indicated that monochromic R and combination lights 3R1B, could regulate the plant morphology and photosynthesis by the effects on the metabolism of carbohydrate into fructose, glucose, sucrose and starch, mainly through the enhanced activities of AI and NI under the former treatment, while SS and SPS in the latter treatment, respectively. They also improved the end-product output in tomato leaves, and may ultimately improve the yield and quality of tomato fruit.