Effect Of Spectra On Photosynthesis In Tomato

Greenhouse horticulture is a modern agricultural method, which can guarantee a balanced supply of vegetables in the whole year. But the lower photosynthetic capacity caused by insufficient light has become an important limiting factor for high and stable yield of vegetables. Thus, the measures of improving the energy efficiency and their regulated mechanisms are not only the basic content and important research direction of photosynthesis, but also the key to improve photosynthetic productivity of vegetable crop.In this study, the tomato, one of the world’s most valuable vegetable crops, was used as material. To solve the problems of the differences of light absorption between chlorophyll and leaf, light utilization characteristics inside the leaf at off-peak spectral of chlorophyll absorption and the scarcity of photosynthetic products caused by weak intensity under red or blue light, spectral energy and signal-regulation were studied firstly. The effects of light quality on light absorption inside the leaf, energy utilization system of photosynthetic system, the kinetics of photosynthetic enzyme, photosynthetic pigment content, anatomical structure, photosynthesis-related genes’ expression, carbon metabolites in tomato were further analyzed to elucidate the regulated mechanisms and the adaptation of photosynthetic apparatus to light quality. The main results were as follows:1. The absorption of tomato leaf was measured under single-color LED light, and then light environment inside the leaf was studied by energy flow distribution and Kubelka-Munk theory. When the adaxial epidermis of tomato leaf was directly radiated, the absorptivity, reflectance and transmittance for photosynthetically active radiation(PAR) of different LEDs varied from 79.08 to 88.18 %, 9.82 % to 12.01% and 0.79 % to 5.08 %, respectively. However, leaf absorptivity was reduced by 2.07% ~4.39%(520 nm) after irradiation on abaxial epidermis, compared with those of adaxial epidermis. Furthermore, the predicted absorptivity through K-M model for different tissues to LEDs was found in the following order: blue light(86.32% at 445 nm and 83.79% at 470 nm) > red light(83.38% at 625 nm and 82.13% at 660 nm) > green light(77.53% at 520 nm) for palisade tissue, green light(13.15% at 520 nm) > red light(11.64% at 625 nm and 13.06% at 660 nm) > blue light(6.99% at 445 nm and 7.83% at 470 nm) for spongy tissue, respectively. It is concluded that the cross section of tomato leaf is qualitatively and quantitatively different, possibly a photosynthetic function to accommodate to the ecological environment.2. Energy utilization of tomato leaf under single-color LED light was studied. The results showed that as the intensity of light quality rises(200~1 500 μmol·m-2·s-1), the maximal photochemical efficiency(Fv/Fm), the effective PSII quantum yield(Y(II)), the apparent electron transport rate(ETR) and the open PSII reaction centers(Fv’/Fm’) of tomato leaves which was under red light and blue light treatments all decreased. Meanwhile, Fv/Fm, Y(II), ETR and Fv’/Fm’ under blue lights decrease more than under red and green lights, and the light intensity causing decrease under blue light is obviously lower than under red and green light. It shows that blue light is more sensitive in the photoinhibition of potato leaves. Moreover, the intensity of light quality photoinhibition of Fv’/Fm’ is lower than ETR, green light is stronger than blue light in the Y(NPQ) of PSⅡ, and blue light is stronger than green light in the Y(NO) of PSⅡ, and red light is in the middle in both situations. It indicates that in the protection mechanisms of light quality photoinhibition, the antenna protein complexes is the first place to reduce and control light absorption and capture of PS II; then, the protection of regulatory pathways of energy dissipation of PSⅡfor red and green light photoinhibition is stronger, and the protection of the non-regulatory pathways of energy dissipation of PSⅡfor blue light photoinhibition is stronger. The lower net photosynthetic rate(Pn), light conversion efficiency(LCE) and apparent quantum yield(AQY) were recorded in blue light, compared with those of red and green light. The Pn of tomato leaf illuminated with red light(625 nm) was higher than that illuminated with green light(520 nm) among 600 μmol·m-2·s-1 and 2000 μmol·m-2·s-1 PAR.3. Effects of light quality on plant growth and leaf photosynthetic activity in tomato were studied. The blue light can improve stem diameter, dry matter content, palisade/spongy ratio, cell tense ratio. The red light(625 nm) increased the content of chlorophyll a, chlorophyll b and carotenoid, but chla/chlb was the lowest; Light absorption of leaf under the blue light(460 nm) was the largest at red and the blue region, followed by blue 445 nm, while the green light droped to a minimum. Fv/Fm and Fv’/ Fm’ of leaf under the red light(660 nm) were lower than control, but no significant differences between the other treatments; Y(II)and ETR of leaf under the red light(625 nm) was higher than control, 7.03% and 10.09%, respectively. Pn and LCE under the blue light were the highest, comparison of Pn under the green and red light related to light intensity. The blue light can significantly increase Vcmax、Jmax and TPU, Then, is 460 nm, 625 nm, 660 nm, 520 nm in turn. The red light(625nm) promoted the expression of rbc S, RCA and GADPH gene, the green light(520nm) promoted the expression of Cab gene, but PGK gene expression differences among the light quality processing is not significant.4. Effects of light quality and heat stress on the photosystem activity of tomato fruit surface were studied. The results showed that as the intensity of light quality rises(200~2500 umol·m-2·s-1), the maximal photochemical efficiency(Fv/Fm), the effective PS II quantum yield(Y(II)), the apparent electron transport rate(ETR) and the open PSII reaction centers(Fv’/Fm’) of tomato fruit surface which was under red light and blue light treatments all decreased in the shape of “S”, while the relative deviation(β/α-1) from full balance between two photosystems rised gradually, indicating that excitation energy high intensity has broken the balance of excitation energy distribution between PSII and PSI, thus leading to the poorer cooperation of two photosystems. Chlorophyll fluorescence of tomato fruit surface which was under green light treatments was proved to be feasible, showing that green light may reduce the chlorophyll to absorb excessive of light energy and decrease photoinhibition. Fv/Fm、Y(Ⅱ)、ETR and Fv’/Fm’ of tomato fruit surface which was under blue light was smaller than red light and blue light, these gaps will widen with an increase in intensity, which indicated blue light was sensitive to photoinhibition. In case of lower light intensity(200~1000 μmol·m-2·s-1), Fv/Fm、Y(Ⅱ) and Fv’/Fm’ of tomato fruit surface which was under red light was higher than green light, while over 1000 μmol·m-2·s-1 leading to reverse results, which showed that the capacity of adapting to high light intensity of tomato fruit surface which was under green light was strong.The effect of heat stress on the photochemical activity and chlorophyll fluorescence spectra in tomato fruit epidermis was studied The results showed that the maximal photochemical efficiency(Fv/Fm) decreased steadliy under lower temperatue of heat shock stress(36~43 ℃), which indicated that the PSII could be ihhibited partly under temperature stress, and at this point, the increased Y(NPQ) dissipated the surplus energy so as to alleviate further damage to photosynthetic apparatus. When the temperature exceeded 43℃, the quantum yield of nonregulated energy dissipation(Y(NO)) increased significantly, Fv/Fm, the open PSII reaction centers(Fv’/Fm’) and the apparent electron transport rate(ETR) decreased sharply, and Y(NPQ) began to decrease, indicating that antenna pigment dissipation mechanism of PSII reaction center might have been damaged, and consequently the self-adjustment function of high-temperature stress began to decrease. Meantime, PSII reaction center turned inactive and the photoinhibition was aggravated. When the temperature exceeded the enaturalization midpoint temperature(Tm(Fv/Fm), 51.4 ℃), the relative deviation(β/α-1) from full balance between two photosystems increased, while the fluorescence decline ratio(Rfd) decreased sharply, showing that the excitation energy distribution was seriously imbalanced and the assimilation capacity of CO2 was extremely weak. Tm(Fv/Fm) calculated from Gibbs free energy( GD) was higher than Tm[Y(Ⅱ)], indicating that the heat resistance of PS II was a little stronger than that of the whole photosynthesis.