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Abstract Blue and red light are spectral wavelengths more effective for plants. The effects of different ratios of red and blue light （R/B=2， R/B=4， R/B=8， R/B=12） provided by LEDs on morphology and photosynthetic characteristics of tomato seedlings were studied. The results showed that plant height， stem diameter， fresh weight， dry weight， seedling index and G value increased with the increase of R/B ratio until 8. On the contrary， SPDA value decreased with the increase of R/B ratio. Photosynthetic characteristics were measured by CO2 assimilation （Pn）， stomatal conductance （gs） and intracellular CO2 concentration （Ci）. Pn and gs decreased with the increase of R/B ratio. Furthermore， similar trend was investigated in photochemical quenching （qP） and electron transport rate （ETR）. Results of this study suggest that compared with white LED， appropriate combination of red and blue light can enhance plant growth and photosynthetic characteristics， and the optimal blue/red ratio for tomato growth was R/B=8.
Key words Red and blue light； Tomato； Morphology； Photosynthetic characteristics
Light is a particularly important environmental factor influencing plant development. Plants sense the intensity， duration and quality of light to regulate plant growth pattern[1]， such as germination， photosynthesis， vegetative growth and flowering induction. Compared with others， light quality are more complex on plant morphology and physiology， because different light quality has different effects on plant growth and development[2]. Among them， blue （B， 400-500 nm） and red （R， 600-700 nm） light are spectral wavelengths more effective for plants because photosynthetic pigments have high light absorption at 400-500 nm and 600-700 nm[3]. A number of studies have shown that blue light in combination with red light can meet the requirement of plant growth[4-5]. Yorio et al.[6] showed that compared with red light alone， combination of red and blue light can increase the yield of radish crops， lettuce and spinach. Hogewoning et al.[7] also showed that growth of cucumber under 100R?? 0B% caused the a loss of photochemical efficiency of photosystem II （PSII）， the maximum photosynthetic capacity per leaf area and the Chl content， and just 7% blue light could avoid any overt dysfunction in photosynthesis. Whereas the suitable proportion of red and blue light for fresh and dry weight were species specifically. For example， the optimal red/blue light ratios for fresh and dry weight were 7/3 in strawberry plantlets[8] and 9 in cucumber seedlings[4]. Tomato is known to be one of the most economically important plant species in greenhouse production in China， but there are different conclusions in the study of optimal red/blue light ratios in tomato seedlings. For example， Liu et al.[9] found that the dry weight of cherry tomato seedlings was greater in the monochromatic B treatment than in 1B?? 1R and 3B?? 1G?? 3R treatments. However Wollaeger and Runkle[10] found that tomato seedlings grown under 100R have greater dry mass than those grown under 50G?? 50R， 25B?? 25G?? 50R， 50B?? 50G， 50B?? 50R. Therefore， this study was conducted to evaluate the effects of different ratios of red and blue light on growth and photosynthetic characteristics， so as to optimize the spectrum for tomato seedling production systems.
Materials and Methods
Plant material and light treatments
Tomato （.cv. Shenfen No. 8） seeds were germinated in a growth medium filled with a mixture of coconut husk， vermiculite and peat （1?? 1?? 1， V?? V?? V） in trays in an indoor growth chamber. When the first true leaf fully expanded， seedlings were treated under different ratios of red and blue light. The seedlings were then cultured with a 12 h photoperiod of light and exposed to an equal photosynthetic photon flux density （PPFD） of 200 ??mol/（m2?¤s） under each of the four red/blue LED ratios which were named R/B=2， R/B=4， R/B=8 and R/B=12， respectively， and a multiwavelength white LED was set as control （CK）. Temperature and relative humidity in the growth chamber were maintained at 25 ??/20 ?? （day/night） and 60%-70% （day/night）， respectively. The samples were harvested after planting for 25 d. All the treatments were repeated for three times.
Measurement of plant morphology
Plant height was measured using a ruler. Stem diameter was measured below the cotyledons using a digital caliper. Fresh and dry weights were measured with an electronic balance. Relative chlorophyll content was measured with TYS??A. Seedling index was calculated by Dry weight ?? stem diameter/Plant height and G value was calculated by Dry weight/25 （planting days）.
Gas exchange and chlorophyll fluorescence measurements
Leaf gas exchange in response to different ratios of red and blue light were measured simultaneously with the portable photosynthesis system （LI??6400XT； Li??COR Lincoln NE， USA）. Leaf CO2 assimilation rate （Pn）， stomatal conductance （gs） and intercellular CO2 concentration （Ci） were done at 25 ??， with 200 ??mol/（m2?¤s） photosynthetic photon flux density （PPFD） and a reference CO2 concentration of 450 ??mol/mol， using ambient humidity （60%-80%， relative humidity）. Chlorophyll fluorescence was measured using an Imaging??PAM Chlorophyll Fluorometer comprising a computer??operated PAM??control unit （Walz， Effeltrich， Germany）. Chlorophyll fluorescence parameters， photochemical quenching （qP）， electron transport rare （ETR） and non??photochemical quenching （NPQ） were calculated according to the methods outlined by Van and Snel[11].
Statistical analysis
The differences between treatments were established using ANOVA （analysis of variance）. Means separation was performed by the Duncan??s multiple range test （P??0.05）.Results and Analyses
Effects of different combinations of red and blue light on plant morphology
All seedlings grew well under different combinations of red and blue light， but the morphology in seedling showed significant differences between different treatments. In our present study， plant height， stem diameter， fresh weight and dry weight increased with the increase of red light up to the R/B=8 treatment （Fig. 1）. For example， plant weight， stem diameter， fresh weight and dry weight were 145.30%， 18.54%， 111.60%， and 16.33% higher than those grown under CK treatment. Similar trends were observed in seedling index and G value. However， seedling index in all R/B treatments decreased compared with CK expect for R/B=8 treatment. Furthermore， the relative Chl content （SPAD value） decreased with increasing R/B ratio （Fig. 2）. SPAD value decreased by 5.06%， 9.27%， 16.06% and 23.92% in R/B=2， R/B=4， R/B=8 and R/B=12 treatments compared with CK， respectively.
Effects of different combinations of red and blue light on photosynthetic characteristics
To investigate whether different combinations of red and blue light can affect photosynthesis， CO2 assimilation （Pn）， stomatal conductance （gs） and intracellular CO2 concentration （Ci） were measured （Fig. 3）. Increased R/B ratio resulted in the decreased Pn and gs. Pn and gs were highest in the treatment of R/B=2， which were 71.05% and 63.90% higher than those of R/B=12 treatment， respectively. In addition， no significant differences in Ci among different light treatments expect R/B=2 and R/B=4 treatments.
The values of qP， NPQ， and ETR were measured at a PPFD of 200 ??mol/（m2?¤s）， which was close to their growth PPFD （Fig. 4）. qP value deceased with the increase of R/B ratio， whereas the highest qP was obtained from plants grown under CK treatment. Similar trend was investigated in ETR in the change of R/B ratio. There were no significant differences in NPQ values among plants grown under different light treatments. Discussions
It has been reported that mixture of R and B are spectrum wavelengths more effective for plant growth compared with monochromatic light[12-13]. So in the present study， we studied the effects of different ratios of red and blue light （R/B=2， R/B=4， R/B=8， R/B=12） on morphology and photosynthetic characteristics of tomato seedlings. The results showed that plant height， stem diameter， fresh weight， dry weight， seedling index and G value increased with the increase of R/B ratio until 8. These results correspond to most studies [7， 12]. For example， Wang et al.[12] demonstrated the leaf morphology with different red/blue light ratios of 12， 8， 4， and 1. They reported that shoot DW increased with the red/blue light ratio increasing， caused by increasing leaf number and leaf area under the higher red/blue light ratio. Furthermore， red light was important in shoot elongation and plant morphology through phytochrome[14]. And this is the same as our results that plant height increased with the increase of R/B ration except R/B=12. In the present study， the optimal red / blue ratio for tomato growth was R/B=8. However， Nanya et al.[14] showed that tomato dry mass was greater under the 10B?? 90R treatment. Liu et al.[9] found that the dry weight of cherry tomato seedlings was greater in the monochromatic B treatment. Whereas Wollaeger and Runkle[10] found that tomato seedlings grown under 100R have greater dry mass than those grown under 50G?? 50R， 25B?? 25G?? 50R， 50B?? 50G and 50B?? 50R. These different results may depend on different varieties and experiments of tomato. Furthermore， it may have other suitable R/B ratios between 8-12 for tomato growth and this will need our further studies. It is worth noting that plant height， stem diameter， fresh weight and dry weight were higher in R/B=8 treatment， comparing with the CK. These results suggest that compared with white LED， appropriate combination of red and blue light can enhance plant growth and photosynthetic characteristics.
It was reported that red light was more effective to increase Pn compared with blue light because red wavelengths （600-700 nm） are efficiently absorbed by photosynthetic pigments[15-16]. However， it was observed that Pn and gs decreased with the increase of R/B ratio， accompanied by the decrease of SPDA value in present study. Similar result was also found by Hogewoning et al.[7]， who found that in cucumber seedlings， B??deficiency led to leaf photosynthetic machinery dysfunction， leading to lower Pn and Amax. They concluded that increasing B light was capable of stimulating "high irradiance leaf characteristics" even under constant irradiance [7]. Furthermore， similar trend was investigated in photochemical quenching （qP） and electron transport rate （ETR）. qP and ETR values of tomato grown under high level of red LED treatment were lower than those of plants under other lower level of red LED， indicating that plants under higher blue light treatments utilized more light energy absorbed by Chl for photochemistry[17-18]. Apparently， the increases in plant height， stem diameter， fresh weight， and dry weight with increasing R/B ratio was observed， while opposite trend was shown in Pn. Similar results have been reported in previous studies in cucumber seedlings[7] and lettuces[12]. This could be attributed that shoot dry weight accumulation of tomato plant was determined not only by Pn， but also other related factors， such as leaf area. Furthermore， we just measured single leaf that cannot represent the Pn of whole plant[19].
To sum up， plant height， stem diameter， fresh weight， dry weight， seedling index and G value increased with the increase of R/B ratio until 8. On the contrary， SPDA value decreased with the increase of R/B ratio. Pn and gs decreased with the increase of R/B ratio. Furthermore， similar trend was investigated in photochemical quenching （qP） and electron transport rate （ETR）. Results of this study suggest that compared with white LED， appropriate combination of red and blue light can enhance plant growth and photosynthetic characteristics， and the optimal blue/red ratio for tomato growth was R/B=8.
References
[1] JIAO XL， LAU OS， DENG XW. Light??regulated transcriptional networks in higher plants[J].Nature Reviews Genetics， 2007， 8：217-230.
[2] WANG H， GU M， CUI JX， et al. Effects of light quality on CO2 assimilation， chlorophyll??fluorescence quenching， expression of Calvin cycle genes and carbohydrate accumulation in Cucumis sativus[J]. Journal of Photochemistry and Photobiology B： Biology， 2009， 96： 30-37.
[3] PFUNDEL E， BAAKE E. A quantitative description of fluorescence excitation spectra in intact bean leaves greened under intermittent light[J]. Photosynthesis Research， 1990， 26， 19-28.
[4] HERNANDEZ R， KUBOTA C. Physiological responses of cucumber seedlings under different blue and red photon flux ratios using LEDs[J]. Environmental and experimental botany， 2016， 121：66-74.
[5] WANG J， LU W， TONG Y， et al. Leaf morphology， photosynthetic performance， chlorophyll fluorescence， stomatal development of lettuce （Lactuca sativa L.） exposed to different ratios of red light to blue light[J]. Frontiers in plant science， 2016， 7： 250.
[6] YOROIO NC， GOINS GD， KAGIE H R， et al. Improving spinach， radish， and lettuce growth under red light??emitting diodes （LEDs） with blue light supplementation[J]. HortScience， 2001， 36： 380-383.
[7] HOGEWONING SW， TROUWBORST G， MALJAARS H， et al. Blue light dose??responses of leaf photosynthesis， morphology， and chemical composition of Cucumis sativus grown under different combinations of red and blue light[J]. Journal of experimental botany， 2010b， 61： 3107-3117. [8] NHUT D T， TAKAMURA T， WATANABE H， et al. Responses of strawberry plantlets cultured in vitro under superbright red and blue light??emitting diodes（LEDs）[J]. Plant Cell Tissue and Organ Culture， 2003， 73： 43-52.
[9] LIU XY， CHANF TT， GUO SR， et al. Effect of different light quality of LED on growth and photosynthetic character in cherry tomato seedling[J]. Acta Horticulturae， 2011， 907： 325-330.
[10] WOLLAEGER HM， RUNKLE ES. Growth of impatiens， petunia， salvia， and tomato seedlings under blue， green， and red light??emitting diodes[J]. Hortscience， 2014， 49：734-740.
[11] VAN KO， SNEL JFH. The use of chlorophyll fuorescence nomenclature in plant stress physiology[J]. Photosynthesis research， 1990， 25： 147-150.
[12] WANG J， LU W， TONG Y， et al. Leaf morphology， photosynthetic performance， chlorophyll fluorescence， stomatal development of lettuce （Lactuca sativa L.） exposed to different ratios of red light to blue light[J]. Frontiers in plant science， 2016， 7： 250.
[13] MUNEER S， KIM EJ， PARK JS， et al. Influence of green， red and blue light emitting diodes on multiprotein complex proteins and photosynthetic activity under different light intensities in lettuce leaves （Lactuca sativa L.）[J]. International journal of molecular science， 2014， 15： 4657-4670.
[14] NANYA K， ISHIGAMI Y， HIKOSAKA S， et al. Effects of blue and red light on stem elongation and flowering of tomato seedlings[J]. Acta Horticulturae， 2012， 956：264-266.
[15] CHANG SX， LI CX， YAO XY， et al. Morphological， photosynthetic， and physiological responses of rapeseed leaf to different combinations of red and blue lights at the rosette stage[J]. Frontiers in plant science， 2016， 08： 03.
[16] MCCREE KJ. The action spectrum， absorbance and quantum yield of photosynthesis in crop plants[J]. Agricultural Meteorology， 1972， 9： 191-216.
[17] HE J， QIN L， LIU Y， et al. Photosynthetic capacities and productivity of indoor hydroponically grown Brassica alboglabra Bailey under different light sources[J].American Journal of Plant Sciences， 2015， 6： 554-563.
[18] HE J， QIN L， CHONG ELC， et al. Plant growth and photosynthetic characteristics of Mesembryanthemum crystallinum grown aeroponically under different blue?? and red??LEDs[J]. Frontiers in plant science， 2017， 03： 17.
[19] YOROIO NC， GOINS GD， KAGIE HR， et al. Improving spinach， radish， and lettuce growth under red light??emitting diodes （LEDs） with blue light supplementation[J]. HortScience， 2001， 36： 380-383.
Key words Red and blue light； Tomato； Morphology； Photosynthetic characteristics
Light is a particularly important environmental factor influencing plant development. Plants sense the intensity， duration and quality of light to regulate plant growth pattern[1]， such as germination， photosynthesis， vegetative growth and flowering induction. Compared with others， light quality are more complex on plant morphology and physiology， because different light quality has different effects on plant growth and development[2]. Among them， blue （B， 400-500 nm） and red （R， 600-700 nm） light are spectral wavelengths more effective for plants because photosynthetic pigments have high light absorption at 400-500 nm and 600-700 nm[3]. A number of studies have shown that blue light in combination with red light can meet the requirement of plant growth[4-5]. Yorio et al.[6] showed that compared with red light alone， combination of red and blue light can increase the yield of radish crops， lettuce and spinach. Hogewoning et al.[7] also showed that growth of cucumber under 100R?? 0B% caused the a loss of photochemical efficiency of photosystem II （PSII）， the maximum photosynthetic capacity per leaf area and the Chl content， and just 7% blue light could avoid any overt dysfunction in photosynthesis. Whereas the suitable proportion of red and blue light for fresh and dry weight were species specifically. For example， the optimal red/blue light ratios for fresh and dry weight were 7/3 in strawberry plantlets[8] and 9 in cucumber seedlings[4]. Tomato is known to be one of the most economically important plant species in greenhouse production in China， but there are different conclusions in the study of optimal red/blue light ratios in tomato seedlings. For example， Liu et al.[9] found that the dry weight of cherry tomato seedlings was greater in the monochromatic B treatment than in 1B?? 1R and 3B?? 1G?? 3R treatments. However Wollaeger and Runkle[10] found that tomato seedlings grown under 100R have greater dry mass than those grown under 50G?? 50R， 25B?? 25G?? 50R， 50B?? 50G， 50B?? 50R. Therefore， this study was conducted to evaluate the effects of different ratios of red and blue light on growth and photosynthetic characteristics， so as to optimize the spectrum for tomato seedling production systems.
Materials and Methods
Plant material and light treatments
Tomato （.cv. Shenfen No. 8） seeds were germinated in a growth medium filled with a mixture of coconut husk， vermiculite and peat （1?? 1?? 1， V?? V?? V） in trays in an indoor growth chamber. When the first true leaf fully expanded， seedlings were treated under different ratios of red and blue light. The seedlings were then cultured with a 12 h photoperiod of light and exposed to an equal photosynthetic photon flux density （PPFD） of 200 ??mol/（m2?¤s） under each of the four red/blue LED ratios which were named R/B=2， R/B=4， R/B=8 and R/B=12， respectively， and a multiwavelength white LED was set as control （CK）. Temperature and relative humidity in the growth chamber were maintained at 25 ??/20 ?? （day/night） and 60%-70% （day/night）， respectively. The samples were harvested after planting for 25 d. All the treatments were repeated for three times.
Measurement of plant morphology
Plant height was measured using a ruler. Stem diameter was measured below the cotyledons using a digital caliper. Fresh and dry weights were measured with an electronic balance. Relative chlorophyll content was measured with TYS??A. Seedling index was calculated by Dry weight ?? stem diameter/Plant height and G value was calculated by Dry weight/25 （planting days）.
Gas exchange and chlorophyll fluorescence measurements
Leaf gas exchange in response to different ratios of red and blue light were measured simultaneously with the portable photosynthesis system （LI??6400XT； Li??COR Lincoln NE， USA）. Leaf CO2 assimilation rate （Pn）， stomatal conductance （gs） and intercellular CO2 concentration （Ci） were done at 25 ??， with 200 ??mol/（m2?¤s） photosynthetic photon flux density （PPFD） and a reference CO2 concentration of 450 ??mol/mol， using ambient humidity （60%-80%， relative humidity）. Chlorophyll fluorescence was measured using an Imaging??PAM Chlorophyll Fluorometer comprising a computer??operated PAM??control unit （Walz， Effeltrich， Germany）. Chlorophyll fluorescence parameters， photochemical quenching （qP）， electron transport rare （ETR） and non??photochemical quenching （NPQ） were calculated according to the methods outlined by Van and Snel[11].
Statistical analysis
The differences between treatments were established using ANOVA （analysis of variance）. Means separation was performed by the Duncan??s multiple range test （P??0.05）.Results and Analyses
Effects of different combinations of red and blue light on plant morphology
All seedlings grew well under different combinations of red and blue light， but the morphology in seedling showed significant differences between different treatments. In our present study， plant height， stem diameter， fresh weight and dry weight increased with the increase of red light up to the R/B=8 treatment （Fig. 1）. For example， plant weight， stem diameter， fresh weight and dry weight were 145.30%， 18.54%， 111.60%， and 16.33% higher than those grown under CK treatment. Similar trends were observed in seedling index and G value. However， seedling index in all R/B treatments decreased compared with CK expect for R/B=8 treatment. Furthermore， the relative Chl content （SPAD value） decreased with increasing R/B ratio （Fig. 2）. SPAD value decreased by 5.06%， 9.27%， 16.06% and 23.92% in R/B=2， R/B=4， R/B=8 and R/B=12 treatments compared with CK， respectively.
Effects of different combinations of red and blue light on photosynthetic characteristics
To investigate whether different combinations of red and blue light can affect photosynthesis， CO2 assimilation （Pn）， stomatal conductance （gs） and intracellular CO2 concentration （Ci） were measured （Fig. 3）. Increased R/B ratio resulted in the decreased Pn and gs. Pn and gs were highest in the treatment of R/B=2， which were 71.05% and 63.90% higher than those of R/B=12 treatment， respectively. In addition， no significant differences in Ci among different light treatments expect R/B=2 and R/B=4 treatments.
The values of qP， NPQ， and ETR were measured at a PPFD of 200 ??mol/（m2?¤s）， which was close to their growth PPFD （Fig. 4）. qP value deceased with the increase of R/B ratio， whereas the highest qP was obtained from plants grown under CK treatment. Similar trend was investigated in ETR in the change of R/B ratio. There were no significant differences in NPQ values among plants grown under different light treatments. Discussions
It has been reported that mixture of R and B are spectrum wavelengths more effective for plant growth compared with monochromatic light[12-13]. So in the present study， we studied the effects of different ratios of red and blue light （R/B=2， R/B=4， R/B=8， R/B=12） on morphology and photosynthetic characteristics of tomato seedlings. The results showed that plant height， stem diameter， fresh weight， dry weight， seedling index and G value increased with the increase of R/B ratio until 8. These results correspond to most studies [7， 12]. For example， Wang et al.[12] demonstrated the leaf morphology with different red/blue light ratios of 12， 8， 4， and 1. They reported that shoot DW increased with the red/blue light ratio increasing， caused by increasing leaf number and leaf area under the higher red/blue light ratio. Furthermore， red light was important in shoot elongation and plant morphology through phytochrome[14]. And this is the same as our results that plant height increased with the increase of R/B ration except R/B=12. In the present study， the optimal red / blue ratio for tomato growth was R/B=8. However， Nanya et al.[14] showed that tomato dry mass was greater under the 10B?? 90R treatment. Liu et al.[9] found that the dry weight of cherry tomato seedlings was greater in the monochromatic B treatment. Whereas Wollaeger and Runkle[10] found that tomato seedlings grown under 100R have greater dry mass than those grown under 50G?? 50R， 25B?? 25G?? 50R， 50B?? 50G and 50B?? 50R. These different results may depend on different varieties and experiments of tomato. Furthermore， it may have other suitable R/B ratios between 8-12 for tomato growth and this will need our further studies. It is worth noting that plant height， stem diameter， fresh weight and dry weight were higher in R/B=8 treatment， comparing with the CK. These results suggest that compared with white LED， appropriate combination of red and blue light can enhance plant growth and photosynthetic characteristics.
It was reported that red light was more effective to increase Pn compared with blue light because red wavelengths （600-700 nm） are efficiently absorbed by photosynthetic pigments[15-16]. However， it was observed that Pn and gs decreased with the increase of R/B ratio， accompanied by the decrease of SPDA value in present study. Similar result was also found by Hogewoning et al.[7]， who found that in cucumber seedlings， B??deficiency led to leaf photosynthetic machinery dysfunction， leading to lower Pn and Amax. They concluded that increasing B light was capable of stimulating "high irradiance leaf characteristics" even under constant irradiance [7]. Furthermore， similar trend was investigated in photochemical quenching （qP） and electron transport rate （ETR）. qP and ETR values of tomato grown under high level of red LED treatment were lower than those of plants under other lower level of red LED， indicating that plants under higher blue light treatments utilized more light energy absorbed by Chl for photochemistry[17-18]. Apparently， the increases in plant height， stem diameter， fresh weight， and dry weight with increasing R/B ratio was observed， while opposite trend was shown in Pn. Similar results have been reported in previous studies in cucumber seedlings[7] and lettuces[12]. This could be attributed that shoot dry weight accumulation of tomato plant was determined not only by Pn， but also other related factors， such as leaf area. Furthermore， we just measured single leaf that cannot represent the Pn of whole plant[19].
To sum up， plant height， stem diameter， fresh weight， dry weight， seedling index and G value increased with the increase of R/B ratio until 8. On the contrary， SPDA value decreased with the increase of R/B ratio. Pn and gs decreased with the increase of R/B ratio. Furthermore， similar trend was investigated in photochemical quenching （qP） and electron transport rate （ETR）. Results of this study suggest that compared with white LED， appropriate combination of red and blue light can enhance plant growth and photosynthetic characteristics， and the optimal blue/red ratio for tomato growth was R/B=8.
References
[1] JIAO XL， LAU OS， DENG XW. Light??regulated transcriptional networks in higher plants[J].Nature Reviews Genetics， 2007， 8：217-230.
[2] WANG H， GU M， CUI JX， et al. Effects of light quality on CO2 assimilation， chlorophyll??fluorescence quenching， expression of Calvin cycle genes and carbohydrate accumulation in Cucumis sativus[J]. Journal of Photochemistry and Photobiology B： Biology， 2009， 96： 30-37.
[3] PFUNDEL E， BAAKE E. A quantitative description of fluorescence excitation spectra in intact bean leaves greened under intermittent light[J]. Photosynthesis Research， 1990， 26， 19-28.
[4] HERNANDEZ R， KUBOTA C. Physiological responses of cucumber seedlings under different blue and red photon flux ratios using LEDs[J]. Environmental and experimental botany， 2016， 121：66-74.
[5] WANG J， LU W， TONG Y， et al. Leaf morphology， photosynthetic performance， chlorophyll fluorescence， stomatal development of lettuce （Lactuca sativa L.） exposed to different ratios of red light to blue light[J]. Frontiers in plant science， 2016， 7： 250.
[6] YOROIO NC， GOINS GD， KAGIE H R， et al. Improving spinach， radish， and lettuce growth under red light??emitting diodes （LEDs） with blue light supplementation[J]. HortScience， 2001， 36： 380-383.
[7] HOGEWONING SW， TROUWBORST G， MALJAARS H， et al. Blue light dose??responses of leaf photosynthesis， morphology， and chemical composition of Cucumis sativus grown under different combinations of red and blue light[J]. Journal of experimental botany， 2010b， 61： 3107-3117. [8] NHUT D T， TAKAMURA T， WATANABE H， et al. Responses of strawberry plantlets cultured in vitro under superbright red and blue light??emitting diodes（LEDs）[J]. Plant Cell Tissue and Organ Culture， 2003， 73： 43-52.
[9] LIU XY， CHANF TT， GUO SR， et al. Effect of different light quality of LED on growth and photosynthetic character in cherry tomato seedling[J]. Acta Horticulturae， 2011， 907： 325-330.
[10] WOLLAEGER HM， RUNKLE ES. Growth of impatiens， petunia， salvia， and tomato seedlings under blue， green， and red light??emitting diodes[J]. Hortscience， 2014， 49：734-740.
[11] VAN KO， SNEL JFH. The use of chlorophyll fuorescence nomenclature in plant stress physiology[J]. Photosynthesis research， 1990， 25： 147-150.
[12] WANG J， LU W， TONG Y， et al. Leaf morphology， photosynthetic performance， chlorophyll fluorescence， stomatal development of lettuce （Lactuca sativa L.） exposed to different ratios of red light to blue light[J]. Frontiers in plant science， 2016， 7： 250.
[13] MUNEER S， KIM EJ， PARK JS， et al. Influence of green， red and blue light emitting diodes on multiprotein complex proteins and photosynthetic activity under different light intensities in lettuce leaves （Lactuca sativa L.）[J]. International journal of molecular science， 2014， 15： 4657-4670.
[14] NANYA K， ISHIGAMI Y， HIKOSAKA S， et al. Effects of blue and red light on stem elongation and flowering of tomato seedlings[J]. Acta Horticulturae， 2012， 956：264-266.
[15] CHANG SX， LI CX， YAO XY， et al. Morphological， photosynthetic， and physiological responses of rapeseed leaf to different combinations of red and blue lights at the rosette stage[J]. Frontiers in plant science， 2016， 08： 03.
[16] MCCREE KJ. The action spectrum， absorbance and quantum yield of photosynthesis in crop plants[J]. Agricultural Meteorology， 1972， 9： 191-216.
[17] HE J， QIN L， LIU Y， et al. Photosynthetic capacities and productivity of indoor hydroponically grown Brassica alboglabra Bailey under different light sources[J].American Journal of Plant Sciences， 2015， 6： 554-563.
[18] HE J， QIN L， CHONG ELC， et al. Plant growth and photosynthetic characteristics of Mesembryanthemum crystallinum grown aeroponically under different blue?? and red??LEDs[J]. Frontiers in plant science， 2017， 03： 17.
[19] YOROIO NC， GOINS GD， KAGIE HR， et al. Improving spinach， radish， and lettuce growth under red light??emitting diodes （LEDs） with blue light supplementation[J]. HortScience， 2001， 36： 380-383.