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Abstract The study on raising of strong tomato seedlings in high-temperature season is of great practical significance， because high-temperature condition in summer and autumn causes excessive growth of tomato seedlings easily. ‘Fenniya’ variety was selected as an experimental material， an orthogonal experiment was designed with rhizospheric temperature （A）， light intensity （B） and nutrient salt concentration （C） as three factors which were designed with three levels， so as to study the effects of different treatments on root activity， strong seedling index， hypocotyl length and average internode length. The results showed that the most important factor for tomato seedlings root activity， hypocotyl length and internode length was nutrient salt concentration. In terms of effect on seedling index， substrate temperature should be reduced properly， to avoid excessive shading. It was primarily determined that the effects of three factors on the raising of strong tomato seedlings ranked as factor C， factor B and factor A from high to low； and the screened optimal seedling raising conditions were the rhizospheric temperature at （3+1） ℃， the shading rate of （50+5）% and the EC value of （5.0+0.5） mS/cm.
Key words Tomato； Hole tray seedling matrix； Orthogonal experiment design； Environmental conditions
Tomato （Solanum lycopersicon） is a main facility crop in the world， as well as one of the vegetables with the largest facility cultivation area in China[1]. Vegetable plug seedlings overcome the disadvantages of traditional seedling raising such as serious soil-borne diseases， low survival rate， long recovery time and slow growth and development[2]. However， in the process of raising tomato plug seedlings， due to large seeding density and small seedling spacing， environmental stresses including high temperature， high humidity and weak light would cause excessive elongation of seedling stem and hypocotyledonary axis， easily， thereby giving rise to formation of leggy seedlings[3]， while the raising of strong seedlings is crucial to early maturation and high yield of tomato[4]. At present， there have been many studies at home and abroad on the control of excessive growth of plug seedlings， and many measures have been put forward： chemical regulation， physical stimulation， and other regulation measures such as temperature regulation， illumination regulation and nutrient regulation. Chemical regulation adopts plant growth regulators to control excessive growth of seedlings， while this method is susceptible to environment， and misoperation would result in difficult recovery of seedling growth after field planting[5-7]. Physical stimulation controls excessive growth of seedlings through mechanical stimulation， friction and blowing， while this method not only causes mechanical injury to seedlings easily， but also has the need for instrument and manpower， so seedling raising cost is improved to a certain degree[8-11]. There are also many progresses in temperature regulation， illumination regulation and nutrient regulation. Researches show that in the coordination process of plants to high air temperature and high rhizospheric temperature， the root system plays a key role， and a proper stable rhizospheric temperature is an important guarantee for the growth and metabolism of plant root system[12-14]. In recent years， the regulation of rhizospheric temperature have attracted more and more attention[15-17]. Previous studies[18-20] used shading net as an effective measure for alleviating high temperature and strong light， and there are also studies[21] showing that high-concentration nutrient solution could effectively inhibit excessive growth of young seedlings. However， the inhibition on excessive growth of young seedlings is a combined action of multiple factors， and it is necessary to further study simple methods with low cost and stable effect for inhibiting excessive growth of young seedlings. Raising strong seedlings through environmental regulation is a preferable measure for raising strong seedlings， while existing reports are mostly limited to single factor study， and fewer studies have been conducted on concentration of nutrient solution， illumination strength and rhizospheric cooling. Therefore， with tomato "Fenniya" as an experimental material， different combined treatments of three factors were designed to study the optimal compound treatment， so as to achieve the purpose of raising strong seedlings in high-temperature strong-illumination season. Materials and Methods
Experimental materials
Tomato variety ‘Fenniya’ produced by Yuyou Seed Co.， Ltd. was selected as an experimental material. Other materials including shading net： three-needle （shading net 50%+5%）， four-needle （shading net 70% +5%）， six-needle （shading net 90%+5%）， 25-hole hole trays （transformed from 50-hole hole trays， specification 27 cm×27 cm）， tape， vernier caliper， measuring cylinder and measuring glass.
Experimental methods
The field experiment was carried out from July 13 to August 8， 2016， in the sunshade of the experimental base of the third campus of Henan Agricultural University. The indoor experiment was carried out from August 9 to August 10， 2016， in the laboratory of College of Horticulture， Henan Agricultural University. A three-factor three-level orthogonal experiment was designed according to L9 （34） orthogonal table. Various factors were represented by A， B and C， the three levels were represented by 1， 2 and 3， and the specific design is shown in Table 1.
The seedbed was in the specification of 255 cm×40 cm×3 （length×width×number）， and the bottom of the seedbed was laid with a foam board with a thickness of 2 cm.
The rhizospheric cooling was regulated according to different proportions of cooling area， using micro-spraying hoses. Under each hole tray， 2， 3 and 4 micro-spraying hoses were laid， respectively， corresponding to three cooling levels， with the proportions of cooling areas at 22%， 33% and 44%， respectively. At the substrate temperature≥28 ℃， the rhizospheric cooling ranges were （1±1）， （3±1） and （6±1） ℃， respectively. The micro-spraying hoses were flatly laid on the seedbed， and covered with fine sand having a thickness of 2 cm， which was then covered with a layer of polyethylene film. The hole trays were placed on seedbed. Foam water boxes were used as water storage tanks （60 cm×45 cm×40 cm）， and the water in the tanks were controlled at （18±2） ℃ by adding ice. An HQB-3000 micro sinking pump （with the maximum flow rate at 2 500 L/m and the maximum lift of 3 m） was used to perform circulating cooling， and the circulating time was set as： 07：00-19：00 on sunny day， no circulation on rainy day. The light intensity was controlled by shading net， and three-needle （shading rate at （50±5）%）， four-needle （shading rate at （70±5）%） and six-needle （shading rate at （90±5）%） shading nets were used as 3 treatments. Shading was performed in the period of 10：00-18：00， and not performed on rainy day. The EC value of nutrient solution was regulated with Hoagland formula at different dosages， and set with two stages. The first stage （the first 15 d， from the emergence of white buds in hole trays） was designed with three EC values， i.e.， 0， 7.5 and 10.0 mS/cm （error within ±0.5）； and the second stage had the EC value of 0.75 mS/cm. There were nine treatments in total： A1B1C1， A1B2C2， A1B3C3， A2B1C2， A2B2C3， A2B3C1， A3B1C3， A3B2C1 and A3B3C2. Seedlings were raised in hole trays with substrate， and there were 25 plants in one treatment， which was designed with three replicates. The substrate was commercial nursery substrate. According to the operation of raising of seedlings in hole trays， the moisture content in substrate was regulated to 60%-70%， and then the substrate was filled in trays and pressed （the holes had a depth of about 1 cm）. The seeds were pre-germinated at varying temperatures and then sown and covered with substrate. The hole trays sown with substrate were finally placed in prepared on ridges and irrigated. After emergence of seedlings， they were irrigated with the nutrient solutions with different EC values， respectively， once per day， at a rate of 10 ml/hole； and at the second stage， irrigation was performed with the solution with EC value of 0.75 mS/cm， with the lower limit of moisture in the substrate controlled at about 75% and other two factors kept constant. Three plants were randomly selected from each replicate 26 d after seeding， for the determination of related indices， including plant height， stem diameter of the base part， dry matter weights of the aboveground and underground parts， hypocotyl length， average length of the first， second and third internode， root activity and strong seedling index.
Determination of items
Three plants were selected from each treatment and replicate for determination of indices. Plant height was determined with a scale， and the stem diameter of the base part was measured with a vernier caliper （plant height： from the rhizome part to the growth point of the plant； stem diameter of the base part： the middle point of the part above the substrate under the cotyledon）. Hypocotyl length and internode length were determined with a ruler （hypocotyl length： from the base part of the stem to the cotyledon； internode length： from the cotyledon to the main leaf， and the main leaf to next main leaf）. Dry weight： The correspodning part was subjected to deactivation of enzymes at 105 ℃ for 15 min， oven-dried at 75 ℃ to constant weight， and weighed with an electronic scale. Strong seedling index was calculated according to following formula： Strong seedling index=Dry weight of whole plant×（Stem diameter/Plant height+Dry weight of the underground part/Dry weight of the aboveground part）. Root activity was determined by TTC method. Data analysis
The experimental data were subjected to analysis of variance with software DPS 7.05. Statistical analysis and plotting were performed with software WPS Office 2016. Multiple comparisons were performed by LSD method （P<0.05）.
Results and Analysis
Intuitive analysis of effects of different combined treatments on different indices of tomato seedlings
It could be seen from Table 2 that the strong seedling index exhibited a trend of increasing at first and decreasing then with the rhizospheric cooling range and EC value of nutrient solution increasing， and decreasing gradually with the increase of shading rate. The effects of various factors on strong seedling index ranked as B>A>C， and the optimal combination was A2B3C2. The root activity showed a trend of increasing at first and decreasing then with the increase of rhizospheric cooling range， decreasing at first and increasing then with the increase of shading rate， and decreasing with the EC value of the nutrient solution increasing. The effects of various factors on root activity ranked as C>A>B， and the optimal combination was A2B3C1. The hypocotyl length of tomato seedlings increased with the increase of rhizospheric cooling range， and decreased with the EC value of the nutrient solution increasing. The effects of various factors on hypocotyl length of tomato ranked as C>B>A. The average internode length of the first， second and third internode exhibited a trend of increasing at first and decreasing then with the increase of rhizospheric cooling range， increasing with the shading rate increasing， and decreasing with the EC value of the nutrient solution increasing. And effects of various factors on the average internode length of the first， second and third internode ranked as C>A>B.
H1， H2， H3 and H4 denote the indicators of tomato seedlings in strong seedling index， root activity， hypocotyl length， average internode， respectively. Ti is the sum of the results of its level. ti=Ti/3， reflects the average value of its level.
Agricultural Biotechnology2018
Variance analysis on effects of different treatments on measured indices
As shown in Table 3， the effects of different combined treatments on different indices of tomato seedlings were subjected to variance analysis. For strong seedling index， it was not affected by the three factors greatly， and among them， factor B had the highest effect on the strong seedling index of tomato seedlings. For root activity of tomato seedlings， factor C very significantly affected root activity of tomato seedlings， the effects of factors A and B on root activity also reached the significant level， while the effect of factor B was the lowest. As to hypocotyl length， the effects of factors B and C also reached the significant level， factor A exhibited no significant effect， and factor C exhibited the highest effect. In the case of the average internode length of tomato seedlings， factor C had the highest as well as significant effect， while factors A and B exhibited non-significant effects， among which factor B showed the lowest effect. The orders of the effects of various factors on different indices of tomato seedlings were in accordance with the range analysis. It could be seen from the significance of the effects of various factors on various indices of tomato seedlings that in the orthogonal experiment， factor C had the highest effect on tomato seedlings， the effect of the factor B was the second， and factor A had the lowest effect. Comparison of effects of different treatments on observation indices
It could be seen from Fig. 1A and Fig. 1B that the different combinations of the three environmental factors exhibited accordant impact trend of hypocotyl length and average internode length. Among different combinations of factors A and B， the two indices exhibited the trends of decreasing， decreasing at first and increasing then， and increasing at first and decreasing then under A1， A2 and A3， respectively， with the EC value of the nutrient solution increasing， which was not in accordance with the intuitive analysis of the effects of factor C on the two indices. It could be seen from Fig. 1C that among the 9 treatments， treatment A2B3C1 exhibited the highest root activity， which was far higher than those of other treatments， reaching the significant level， which accorded with the optimal combination obtained by the intuitive analysis of root activity. It could be seen from Fig. 1D that treatment 6 had the highest strong seedling index which was significantly different from treatments 1， 3， 4 and 7， so it was the optimal combination among all the treatments.
Discussion and Conclusions
It could be seen comprehensively from the experiment results and the significance of the effects of various factors on various indices of tomato seedlings that in summer with high temperature and strong light， the optimal seedling raising conditions were the rhizospheric temperature at （3+1） ℃， the shading rate of （50+5）% and the EC value of （5.0+0.5） mS/cm. For raising strong seedlings， factor C had the highest effect， factor B was next to it， and factor C had the lowest effect. As to the effects on root activity， hypocotyl length and average internode length， salt concentration of the nutrient solution was the most important factor； and properly reducing substrate temperature and avoiding excessive shading were beneficial to the raising of strong seedlings.
It could be seen from the analysis of the strong seedling index of tomato seedlings that A2B3C2 which did not appear but was the optimal combination for raising strong tomato seedlings in this orthogonal experiment. Besides， among the most proximate treatments 4， 6 and 9， only treatment 6 had the factor C， which had the lowest effect on strong seedling index， not located at the best level， and located at the lowest level at the first stage. Therefore， treatment 6 was the best combination which could reduce cost for seedling raising. It could be seen from variance analysis that the EC value of the nutrient solution had the lowest effect on the strong seedling index of tomato seedlings， which was related to the change of the EC value at the second stage， during which the difference in the EC value of the substrate subjected to the treatment at the first stage decreased gradually， so the effect of factor C on the strong seedling index was also lowered. The inhibition of the excessive growth of tomato seedlings is a process suffering from the action of multiple factors， and the analysis showed that factor C significantly affected hypocotyl length and average internode length. With the increase of factor C， the two indices were not on the decrease， which disaccorded with the intuitive analysis of single factor. It could be seen the three factors act with each other， and the specific interaction needs further validation. References
[1] LIU AR， CHEN SC， WANG MB， et al. Effects of heat stress on photosynthesis and chlorophyll fluorescence parameters in tomato seedlings[J]. Acta Agriculturae Boreali-Occidentalis Sinica， 2010， 19（5）： 145-148. （in Chinese）
[2] CHANG YJ， WANG DS， CHEN H， et al. Effect of different seedling substrates on growth of cucumber seedlings[J]. Mod Agric Sci Technol， 2011（1）： 129-131. （in Chinese）
[3] GAO XX， ZHANG ZG， DUAN Y， et al. Inhibition effect of high strength nutrient solution on hypocotyl stretch of cucumber and tomato seedlings[J]. Journal of Plant Nutrition and Fertilizer， 2014（5）： 1234-1242. （in Chinese）
[4] WANG LC. Effects of plug size， water supply and plant growth regulator on the growth of tomato （Lycopersicum Esculentum Mill.） plug seedlings[D]. Taian： Shandong agricultural university， 2006. （in Chinese）
[5] YIN JF， CHEN FY， LI JQ， et al. Effects of seed soaking with uniconazole on the growth and physiological characters of tomato seedlings[J]. Journal of China Agricultural University， 2004， 9（2）： 8-11. （in Chinese）
[6] GARNER LC， BJORKMAN T. Using impedance for mechanical conditioning of tomato transplants to control excessive stem elongation[J]. Hort Science， 1997， 32（22）： 227-229.
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[8] GARNER LC， BJORKMAN T. Mechanical conditioning for controlling excessive elongation in tomato transplants：Sensitivity to does， frequency， and timing of brushing[J]. Journal of American Society for Horticultural Science， 1996， 121（5）： 894-900.
[9] DUMAN I， DUZYAMAN E. Growth control in processing tomato seedlings[J]. Acta Horticulture， 2005， 613： 95-102.
[10] GARNER LC， BJORKMAN T. Mechanical conditioning of tomato seedlings improves transplant quality without deleterious effects on field performance[J]. Hort Science， 1999， 34： 848-851.
[11] HEUCHERT JC， MITCHELL CA. Inhibition of shoot growth in greenhouse grown tomato by periodic gyrator shaking[J]. Journal of the American Society for Horticultural Science， 1983， 108： 795-800.
[12] NKANSAH GO， ITO T. Effect of air and root zone temperatures on physiological characteristics and yield of heat-tolerant and non heat-tolerant tomato cultivars[J]. Journal of the American Society for Horticultural Science， 1995， 64（3）： 315-320. [13] DUMAN I， DUZYAMAN E. Growth control in processing tomato seedlings[J]. Acta Horticulture， 2005， 613： 95-102.
[14] SONG ML， WEN XZ， LI YL. Effects of high rhizosphere temperature on plant growth and metabolism： A review[J]. Chinese Journal of Ecology， 2010， 29（11）： 2258-2264. （in Chinese）
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Key words Tomato； Hole tray seedling matrix； Orthogonal experiment design； Environmental conditions
Tomato （Solanum lycopersicon） is a main facility crop in the world， as well as one of the vegetables with the largest facility cultivation area in China[1]. Vegetable plug seedlings overcome the disadvantages of traditional seedling raising such as serious soil-borne diseases， low survival rate， long recovery time and slow growth and development[2]. However， in the process of raising tomato plug seedlings， due to large seeding density and small seedling spacing， environmental stresses including high temperature， high humidity and weak light would cause excessive elongation of seedling stem and hypocotyledonary axis， easily， thereby giving rise to formation of leggy seedlings[3]， while the raising of strong seedlings is crucial to early maturation and high yield of tomato[4]. At present， there have been many studies at home and abroad on the control of excessive growth of plug seedlings， and many measures have been put forward： chemical regulation， physical stimulation， and other regulation measures such as temperature regulation， illumination regulation and nutrient regulation. Chemical regulation adopts plant growth regulators to control excessive growth of seedlings， while this method is susceptible to environment， and misoperation would result in difficult recovery of seedling growth after field planting[5-7]. Physical stimulation controls excessive growth of seedlings through mechanical stimulation， friction and blowing， while this method not only causes mechanical injury to seedlings easily， but also has the need for instrument and manpower， so seedling raising cost is improved to a certain degree[8-11]. There are also many progresses in temperature regulation， illumination regulation and nutrient regulation. Researches show that in the coordination process of plants to high air temperature and high rhizospheric temperature， the root system plays a key role， and a proper stable rhizospheric temperature is an important guarantee for the growth and metabolism of plant root system[12-14]. In recent years， the regulation of rhizospheric temperature have attracted more and more attention[15-17]. Previous studies[18-20] used shading net as an effective measure for alleviating high temperature and strong light， and there are also studies[21] showing that high-concentration nutrient solution could effectively inhibit excessive growth of young seedlings. However， the inhibition on excessive growth of young seedlings is a combined action of multiple factors， and it is necessary to further study simple methods with low cost and stable effect for inhibiting excessive growth of young seedlings. Raising strong seedlings through environmental regulation is a preferable measure for raising strong seedlings， while existing reports are mostly limited to single factor study， and fewer studies have been conducted on concentration of nutrient solution， illumination strength and rhizospheric cooling. Therefore， with tomato "Fenniya" as an experimental material， different combined treatments of three factors were designed to study the optimal compound treatment， so as to achieve the purpose of raising strong seedlings in high-temperature strong-illumination season. Materials and Methods
Experimental materials
Tomato variety ‘Fenniya’ produced by Yuyou Seed Co.， Ltd. was selected as an experimental material. Other materials including shading net： three-needle （shading net 50%+5%）， four-needle （shading net 70% +5%）， six-needle （shading net 90%+5%）， 25-hole hole trays （transformed from 50-hole hole trays， specification 27 cm×27 cm）， tape， vernier caliper， measuring cylinder and measuring glass.
Experimental methods
The field experiment was carried out from July 13 to August 8， 2016， in the sunshade of the experimental base of the third campus of Henan Agricultural University. The indoor experiment was carried out from August 9 to August 10， 2016， in the laboratory of College of Horticulture， Henan Agricultural University. A three-factor three-level orthogonal experiment was designed according to L9 （34） orthogonal table. Various factors were represented by A， B and C， the three levels were represented by 1， 2 and 3， and the specific design is shown in Table 1.
The seedbed was in the specification of 255 cm×40 cm×3 （length×width×number）， and the bottom of the seedbed was laid with a foam board with a thickness of 2 cm.
The rhizospheric cooling was regulated according to different proportions of cooling area， using micro-spraying hoses. Under each hole tray， 2， 3 and 4 micro-spraying hoses were laid， respectively， corresponding to three cooling levels， with the proportions of cooling areas at 22%， 33% and 44%， respectively. At the substrate temperature≥28 ℃， the rhizospheric cooling ranges were （1±1）， （3±1） and （6±1） ℃， respectively. The micro-spraying hoses were flatly laid on the seedbed， and covered with fine sand having a thickness of 2 cm， which was then covered with a layer of polyethylene film. The hole trays were placed on seedbed. Foam water boxes were used as water storage tanks （60 cm×45 cm×40 cm）， and the water in the tanks were controlled at （18±2） ℃ by adding ice. An HQB-3000 micro sinking pump （with the maximum flow rate at 2 500 L/m and the maximum lift of 3 m） was used to perform circulating cooling， and the circulating time was set as： 07：00-19：00 on sunny day， no circulation on rainy day. The light intensity was controlled by shading net， and three-needle （shading rate at （50±5）%）， four-needle （shading rate at （70±5）%） and six-needle （shading rate at （90±5）%） shading nets were used as 3 treatments. Shading was performed in the period of 10：00-18：00， and not performed on rainy day. The EC value of nutrient solution was regulated with Hoagland formula at different dosages， and set with two stages. The first stage （the first 15 d， from the emergence of white buds in hole trays） was designed with three EC values， i.e.， 0， 7.5 and 10.0 mS/cm （error within ±0.5）； and the second stage had the EC value of 0.75 mS/cm. There were nine treatments in total： A1B1C1， A1B2C2， A1B3C3， A2B1C2， A2B2C3， A2B3C1， A3B1C3， A3B2C1 and A3B3C2. Seedlings were raised in hole trays with substrate， and there were 25 plants in one treatment， which was designed with three replicates. The substrate was commercial nursery substrate. According to the operation of raising of seedlings in hole trays， the moisture content in substrate was regulated to 60%-70%， and then the substrate was filled in trays and pressed （the holes had a depth of about 1 cm）. The seeds were pre-germinated at varying temperatures and then sown and covered with substrate. The hole trays sown with substrate were finally placed in prepared on ridges and irrigated. After emergence of seedlings， they were irrigated with the nutrient solutions with different EC values， respectively， once per day， at a rate of 10 ml/hole； and at the second stage， irrigation was performed with the solution with EC value of 0.75 mS/cm， with the lower limit of moisture in the substrate controlled at about 75% and other two factors kept constant. Three plants were randomly selected from each replicate 26 d after seeding， for the determination of related indices， including plant height， stem diameter of the base part， dry matter weights of the aboveground and underground parts， hypocotyl length， average length of the first， second and third internode， root activity and strong seedling index.
Determination of items
Three plants were selected from each treatment and replicate for determination of indices. Plant height was determined with a scale， and the stem diameter of the base part was measured with a vernier caliper （plant height： from the rhizome part to the growth point of the plant； stem diameter of the base part： the middle point of the part above the substrate under the cotyledon）. Hypocotyl length and internode length were determined with a ruler （hypocotyl length： from the base part of the stem to the cotyledon； internode length： from the cotyledon to the main leaf， and the main leaf to next main leaf）. Dry weight： The correspodning part was subjected to deactivation of enzymes at 105 ℃ for 15 min， oven-dried at 75 ℃ to constant weight， and weighed with an electronic scale. Strong seedling index was calculated according to following formula： Strong seedling index=Dry weight of whole plant×（Stem diameter/Plant height+Dry weight of the underground part/Dry weight of the aboveground part）. Root activity was determined by TTC method. Data analysis
The experimental data were subjected to analysis of variance with software DPS 7.05. Statistical analysis and plotting were performed with software WPS Office 2016. Multiple comparisons were performed by LSD method （P<0.05）.
Results and Analysis
Intuitive analysis of effects of different combined treatments on different indices of tomato seedlings
It could be seen from Table 2 that the strong seedling index exhibited a trend of increasing at first and decreasing then with the rhizospheric cooling range and EC value of nutrient solution increasing， and decreasing gradually with the increase of shading rate. The effects of various factors on strong seedling index ranked as B>A>C， and the optimal combination was A2B3C2. The root activity showed a trend of increasing at first and decreasing then with the increase of rhizospheric cooling range， decreasing at first and increasing then with the increase of shading rate， and decreasing with the EC value of the nutrient solution increasing. The effects of various factors on root activity ranked as C>A>B， and the optimal combination was A2B3C1. The hypocotyl length of tomato seedlings increased with the increase of rhizospheric cooling range， and decreased with the EC value of the nutrient solution increasing. The effects of various factors on hypocotyl length of tomato ranked as C>B>A. The average internode length of the first， second and third internode exhibited a trend of increasing at first and decreasing then with the increase of rhizospheric cooling range， increasing with the shading rate increasing， and decreasing with the EC value of the nutrient solution increasing. And effects of various factors on the average internode length of the first， second and third internode ranked as C>A>B.
H1， H2， H3 and H4 denote the indicators of tomato seedlings in strong seedling index， root activity， hypocotyl length， average internode， respectively. Ti is the sum of the results of its level. ti=Ti/3， reflects the average value of its level.
Agricultural Biotechnology2018
Variance analysis on effects of different treatments on measured indices
As shown in Table 3， the effects of different combined treatments on different indices of tomato seedlings were subjected to variance analysis. For strong seedling index， it was not affected by the three factors greatly， and among them， factor B had the highest effect on the strong seedling index of tomato seedlings. For root activity of tomato seedlings， factor C very significantly affected root activity of tomato seedlings， the effects of factors A and B on root activity also reached the significant level， while the effect of factor B was the lowest. As to hypocotyl length， the effects of factors B and C also reached the significant level， factor A exhibited no significant effect， and factor C exhibited the highest effect. In the case of the average internode length of tomato seedlings， factor C had the highest as well as significant effect， while factors A and B exhibited non-significant effects， among which factor B showed the lowest effect. The orders of the effects of various factors on different indices of tomato seedlings were in accordance with the range analysis. It could be seen from the significance of the effects of various factors on various indices of tomato seedlings that in the orthogonal experiment， factor C had the highest effect on tomato seedlings， the effect of the factor B was the second， and factor A had the lowest effect. Comparison of effects of different treatments on observation indices
It could be seen from Fig. 1A and Fig. 1B that the different combinations of the three environmental factors exhibited accordant impact trend of hypocotyl length and average internode length. Among different combinations of factors A and B， the two indices exhibited the trends of decreasing， decreasing at first and increasing then， and increasing at first and decreasing then under A1， A2 and A3， respectively， with the EC value of the nutrient solution increasing， which was not in accordance with the intuitive analysis of the effects of factor C on the two indices. It could be seen from Fig. 1C that among the 9 treatments， treatment A2B3C1 exhibited the highest root activity， which was far higher than those of other treatments， reaching the significant level， which accorded with the optimal combination obtained by the intuitive analysis of root activity. It could be seen from Fig. 1D that treatment 6 had the highest strong seedling index which was significantly different from treatments 1， 3， 4 and 7， so it was the optimal combination among all the treatments.
Discussion and Conclusions
It could be seen comprehensively from the experiment results and the significance of the effects of various factors on various indices of tomato seedlings that in summer with high temperature and strong light， the optimal seedling raising conditions were the rhizospheric temperature at （3+1） ℃， the shading rate of （50+5）% and the EC value of （5.0+0.5） mS/cm. For raising strong seedlings， factor C had the highest effect， factor B was next to it， and factor C had the lowest effect. As to the effects on root activity， hypocotyl length and average internode length， salt concentration of the nutrient solution was the most important factor； and properly reducing substrate temperature and avoiding excessive shading were beneficial to the raising of strong seedlings.
It could be seen from the analysis of the strong seedling index of tomato seedlings that A2B3C2 which did not appear but was the optimal combination for raising strong tomato seedlings in this orthogonal experiment. Besides， among the most proximate treatments 4， 6 and 9， only treatment 6 had the factor C， which had the lowest effect on strong seedling index， not located at the best level， and located at the lowest level at the first stage. Therefore， treatment 6 was the best combination which could reduce cost for seedling raising. It could be seen from variance analysis that the EC value of the nutrient solution had the lowest effect on the strong seedling index of tomato seedlings， which was related to the change of the EC value at the second stage， during which the difference in the EC value of the substrate subjected to the treatment at the first stage decreased gradually， so the effect of factor C on the strong seedling index was also lowered. The inhibition of the excessive growth of tomato seedlings is a process suffering from the action of multiple factors， and the analysis showed that factor C significantly affected hypocotyl length and average internode length. With the increase of factor C， the two indices were not on the decrease， which disaccorded with the intuitive analysis of single factor. It could be seen the three factors act with each other， and the specific interaction needs further validation. References
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