The Effects Of LED Light Qualities On The Growth And Photosynthesis Of Cucumber Seedlings Under Low Light

Light acts on plants not only as the sole source of energy, but also as a source of external information, affecting growth, development and shade avoidance. Plant development and physiology are strongly influenced by the light spectrum of the growth environment. In the late autumn, spring and winter, the shortening of illumination time and the larger irradiance fluctuation are serious problems for plant development, thus hampering crop establishment in the growth facility. What’s more, this seriously affects both yield and quality of the vegetable. So it is important to study the effects of light quality on the growth and development of cucumbers especially on photosynthesis under low light in order to provide theoretical and technical support for fill light quality in the field of cultivation technology.This study uses light qualities with high purity, a combination of proportional modulation convenient, low heat, light intensity control, small size, long life and other important features of the new semiconductor light-emitting diodes (light emitting diode, LED) to investigate gas exchange, chlorophyll fluorescence kinetics, chloroplast ultrastructure and other physiological indicators for exploring the influence of monochromatic LED on plant morphogenesis and the development of photosynthetic apparatus in cucumber (Cucumis sativus) leaves from emergence to full expansion under weak light. We found that blue light (B) could significantly increase the leaf area of cucumbers, shoot elongation, chlorophyll a/b, net photosynthetic rate and stomatal conductance. In addition, the comparisons of Fv/Fm (maximal PSII photochemistry efficiency) and PIABS (photosynthetic performance index) between B-, W (white light)-and R (red light)-grown leaves suggested that blue light was essential for the development of photosynthetic apparatus under weak light. B-grown leaves had the lowest chlorophyll content under weak light, however, they had well-developed chloroplasts with the highest stacked degree of lamellae and the lowest starch accumulates, this probably explained to a considerable extent the highest PNC (net photosynthetic rate per unit chlorophyll). These results demonstrated that blue light optimized photosynthetic performance by improving the photosynthetic rate, increasing leaf area, and prolonging active photosynthesis duration under low irradiance (50 μmol m-2s-1’). Therefore, compared with red light, blue light plays a vital role on the normal development of chloroplast and efficient operation of photosynthetic function of cucumber seedlings under low light stress.In order to explore the influence of chloroplasts and related changes in photosynthetic characteristics for cucumber(Cucumis sativus) functional leaves grown under different combinations of red and blue light, control white light (W), red light (R) with dominant wavelengths of 635 nm, blue light (B) with dominant wavelengths of 455 nm, R:B=7:1 and R:B=1:1 provided by light-emitting diodes were applied to study the influences of different combinations of red and blue light on cucumber functional leaves under weak light environment. Results showed that the stack of thylakoid membranes was severely squeezed, grana and stroma lamellae had no rules with the lowest maximum photochemical efficiency (Fv/Fm), net photosynthetic rate (PN) and stomatal conductance (gs) of R-grown leaves under low light. Under B treatment, the SPAD value and deviation of excitation energy from full balance between PS1 and PS2 (p/a-1) were the lowest, the leaf area was followed W. While the specific leaf weight (SLW), PN, gs, photosynthetic performance index (PIcsm) were the largest under R:B=1:1. And the number of lamellar was the most along with its regular and order. Although PN under W and R:B=7:1 were greater than B, there was no significantly difference in gs each other. Further analysis indicated that cucumber leaves grown under R alone (0%blue) demonstrated dysfunctional photosynthetic operation, blue light was necessary for the efficient operation of cucumber photosynthetic apparatus under weak light. Compared with other light qualities, when the red-blue ratio of 1:1, it was most beneficial for the development of chloroplast and high photosynthetic rate under low light.