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Abstract Taking Cucurbita maxima and Cucurbita moschata as root stocks， and ‘Jinyou No 3’ cucumber as scion， the effects of different root zone temperature conditions optimal temperature （CK） （18-20 ℃）， suboptimal temperature （13-15 ℃） and low temperature （8-10 ℃） on the growth and photosynthesis indexes were studied. The results showed that， compared with optimal temperature （CK）， suboptimal temperature and low temperature produced a significant inhibition of growth on cucumbers. The plant height， stem diameter， leaf area， number of leaves and dry weight of aboveground part were all reduced， dry weight of underground part and root shoot ratio all increased， while the inhibition was more significant at low temperature. Low and suboptimal temperature conditions also reduced SPAD value， net photosynthetic rate， transpiration rate， intercellular CO2 concentration and stomatal conductance of the grafted cucumber. And there were differences between different grafted seedlings， and seedlings with‘black seeds’as stock performed better than those with ‘white seeds’ as stock at low temperature.
Key words Root zone temperature； Grafted cucumber； Growth characteristics； Photosynthetic parameters
Cucumber is an important economic crop in facility cultivation in northern areas of China， which has higher nutritional value， and is favored by people. However， in cultivation of cucumber in winter in northern areas of China， the growth of cucumber is always influenced by temperature. Environmental factors in facility environment such as low temperature greatly influenced early maturation， high yield and quality of melon crop[1]， resulting in great economic loss to farmers. Directing at this problem， grafting is considered to be a method for improving the resistance of cucumber to low-temperature stress， and has been successfully applied to the production process of cucumber in winter. Li et al.[2] showed that under a certain period of low temperature treatment， the root system of grafted cucumber had higher resistance to low-temperature stress than that of self-rooted cucumber. Hu et al.[3] also found that under long-term low temperature， grafted cucumber seedlings and self-rooted cucumber seedlings both suffered from stomatal inhibition， but grafted seedlings maintained relatively higher stomatal conductance and photosynthetic assimilation efficiency than self-rooted cucumber and produced less excessive excitation energy， their extent of injury of photosynthetic organs was reduced， and their adaptability to low temperature was thus stronger. As to the selection of stock， Cucurbita ficifolia and Cucurbita moschata could serve as stock. A large number of previous studies have demonstrated that two kinds of grafted cucumbers has better performance in resisting low-temperature stress[4-6]， but most studies of low-temperature stress focused on air temperature， and there were few studies on the effects of ground temperature on growth and development of different grafted cucumber. Ground temperature not only affects water absorption and mineral metabolism of cucumber， but also restricts normal photosynthesis and respiration. Walker[7] reported that the change of 1 ℃ at root zone could cause remarkable change of plant growth. Sun et al.[8] showed that a suitable root zone temperature could accelerate the development process of poinsettia and significantly improve quality of potted flower， and specifically， branch length， plant height， flower diameter， crown breadth， bract area and commodity rate could be improved significantly. Liang et al.[9] found through the investigation of the effect of different root zone temperature on growth and photosynthetic parameter of pepper seedlings that compared with the optimal temperature treatment （CK）， suboptimal temperature and low-temperature treatments reduced leaf SPAD value， photosynthetic rate， transpiration rate and intercellular CO2 concentration of young pepper seedlings. Therefore， this experiment simulated different ground temperatures to investigate the effects of ground temperature on two kinds of grafted cucumbers， so as to find a stock variety suitable for cultivation of grafted cucumber in northern areas. This study will provide theoretical and practical basis for ground temperature control of cucumber cultivation in solar greenhouse.
Materials and Methods
Experimental materials
The stocks were C. ficifolia （Linghai Nongguang Seed Co.， Ltd.） and C. moschata （Beijing Shuoyuan Seed Co.， Ltd.） frequently used in production practice， and the scion was "Jinyou 35". The grafted cucumber with C. ficifolia as stock was called "black seed"， and that with C. moschata as stock was called "white seed".
Experimental methods
This experiment was carried out in solar greenhouse of the teaching base of Inner Mongolia Agricultural University. Raising of seedlings was started on April 3， and on April 10， when the first true leaf of pumpkin just spread， and the cotyledon of scion fully spread， grating was performed by cleft grafting. On May 6， temperature treatment was performed after recovery of the grafted seedlings. There were three ground temperature treatments： optimal temperature （CK： 18-20 ℃）， suboptimal temperature （13-15 ℃） and low temperature （8-10 ℃）， each of which had three replications. After 50 d of continuous treatment， various indexes were measured in fruiting period of cucumber. The low-temperature treatment： on sealed foam boxes （60 cm×35 cm）， the tops were cut， giving circles with diameter slightly smaller than flowerpots， flowerpots containing cucumber were put on the circles of the foam boxes， on which wet hay was laid， and frozen ice bags （Biyuntian， 250 ml） were placed in the foam boxes. Ice bags in each box were exchanged three times daily after moving away the flowerpots at 8：00， 12：00 and 17：00. Trial tests in early period showed that 20 ice bags should be placed to achieve the low-temperature treatment， 12 ice bags were needed for the suboptimal temperature treatment， and 4 ice bags should be provided for the optimal temperature treatment. However， with the weather getting warm， the number of ice bags should be adjusted every 5 d to satisfy the requirement for temperature. On June 25， the low temperature treatment needed 30 ice bags， the suboptimal temperature required 20 ice bags， and the optimal temperature treatment required 12 ice bags.
Determination of items Determination of morphological indexes
Ground diameter should be determined with a ZDR-20 temperature and humidity recorder. After the temperature treatment， plant height （from grafting position to growing point） and leaf length and width （the third leaf from top to bottom） was measured with meter ruler， and leaf area was calculated according to following equation： leaf area=14.61-5L+0.94L2+0.47W+0.63W2-0.62LW （L： leaf length， W： leaf width）； stem diameter was measured with a vernier caliper， and leaf number was also recorded； and dry weight was obtained by performing deactivation of enzymes at 105 ℃ for 15 min， and oven-drying at 75 ℃. For each index， three plants were measured.
Determination of photosynthetic indexes
After the temperature treatment， a health functional leaf （the third leaf from top to bottom） spreading completely was selected from each treatment， and determined with an LI-6400 photosynthesis system on a sunny day during 9：30-11：30 for net photosynthetic rate， transpiration rate， intracellular CO2 concentration and stomatal conductance， and each leaf was determined for five times， obtaining values which were averaged. Also， the SPAD values of cucumber leaves were also determined with a SPAD-502 chlorophyll meter （Knoica Minolta， Japan）， and each leaf was determined for five times， obtaining values which were averaged.
Data analysis
Data was organized and used for plotting with WPS office software， and variance analysis was performed according to ANOVA in SAS software.
Results and Analysis
Changes of air temperature and ground temperature during experimental period
It could be seen from Fig. 1 that during the experimental period， the average temperatures of various ground temperature treatments all fluctuated within set temperature range. Among them， the optimal temperature （CK） treatment had the ground temperature the lowest at 18.1 ℃， the highest at 19.6 ℃； the suboptimal temperature treatment had the ground temperature the lowest at 13.2 ℃， the highest at 14.5 ℃； and the ground temperature of the low-temperature treatment ranged from 8.1 to 9.6 ℃. The air temperature increased continuously， the lowest at 19.8 ℃， the highest at 26.7 ℃.
Effect of different root zone temperatures on growth morphology of cucumber
It could be seen from Table 1 that with the root zone temperature ranging from high to low， whether for the "black seed" or "white seed" grafted seedlings， under the low temperature and the suboptimal temperature conditions， the plant height， stem diameter， leaf area and leaf number of seedlings were different from those under the optimal temperature （CK） condition at significant or very significant level. It was indicated that with the decrease of the root zone temperature， the growth of the aboveground part of seedling was affected. There were also differences between the two different kinds of grafted seedlings. Under the optimal， suboptimal and low temperature conditions， there were very significant differences in plant height between "black seed" and "white seed"， while under the optimal and suboptimal temperature conditions， there were no remarkable differences in leaf area and leaf number between them， but very significant differences existed in leaf area and leaf number compared with the low-temperature condition. Under the three temperature conditions， there were significant differences in stem diameter of "black seed"， while "white seed" exhibited a significant difference in stem diameter between the optimal and low temperature conditions. Different capital letters in the same column mean very significant difference （P<0.01） and different lowercase letters mean significant difference （P<0.5） . The same below.
Shijun SUN et al. Effects of Root Zone Temperature on Growth and Photosynthetic Parameters of Grafted Cucumber
Effect of different root zone temperatures on dry matter accumulation of cucumber
It could be seen from Table 2 that under different root zone temperature conditions， there were differences in dry weight of aboveground part， underground part and whole plant and root shoot ratio between the two kinds of grafted seedlings. With the decrease of root zone temperature， the dry weights of aboveground part and whole plant decreased gradually， while the dry weight of underground part and root shoot ratio increased gradually， indicating that root zone temperature affected dry matter accumulation of cucumber， the cooling treatment of root zone stimulated the cold resistance of roots， the improvement of energy metabolism level facilitated the distribution of dry matter to root system at higher ratio， and the growth of the root system thus increased. The comparison of the optimal temperature and suboptimal treatments showed that "black seed" and "white seed" had no significant differences in dry weights of aboveground part， underground part and whole plant and root shoot ratio. The comparison between the suboptimal temperature and low temperature treatments， "black seed" and "white seed" had very significant differences in dry weight of aboveground part， and significant differences in dry weights of underground part and whole plant and root shoot ratio. Under the same temperature， the dry matter accumulation of "black seed" was higher than that of "white seed". It was indicated that "black seed" was better than "white seed" in low-temperature resistance， more dry matter began to be stored in underground part， to alleviate low temperature stress at root zone and maintain certain physiological activity of the root part. Therefore， "black seed" exhibited stronger low temperature tolerance.
Effect of different root zone temperatures on SPAD of cucumber
It could be seen from Table 3 that with the reduction of root zone temperature， leaf SPAD of cucumber seedlings decreased gradually， there was a significant difference in SPAD of "black seed" between the optimal and suboptimal temperature treatments， and there were no significant differences in SPAD of "white seed". However， under suboptimal and low temperature conditions， there was a significant difference in SPAD value of "black seed"， and there was a very significant difference in SPAD value of "white seed". It could been seen that low temperature stress has a more remarkable effect on chlorophyll content of "white seed" than that on "black seed". Low temperature stress restricts the synthesis of chlorophyll， and aggravates the degradation of chlorophyll， resulting in the reduction of total content of chlorophyll， photosynthesis is inhibited， and photosynthate is reduced， which also affects morphological indexes of plant. Effects of different root zone temperatures on photosynthetic parameters of cucumber leaves
It could be seen from Table 4 that with the reduction of root zone temperature， the net photosynthetic rate， transpiration rate， intracellular CO2 concentration and stomatal conductance of leaves of grafted cucumber were all on the decrease. For both the two kinds of cucumbers， there were very significant differences in net photosynthetic rate， intracellular CO2 concentration， stomatal conductance and transpiration rate between low temperature and optimal temperature （CK） treatments. It is indicated that lower root zone temperature reduces the transpiration rate of seedlings， and the reduction of transpiration pull directly inhibits the absorption and upward transport of water and mineral elements in root zone. And root zone temperature also reduces intracellular CO2 concentration， resulting in insufficient supply of photosynthetic raw materials， and the reduction of stomatal conductance also weakens the conduction capacity of photosynthetic substrates. These influences caused the reduction of net photosynthetic rate， the accumulation of assimilate decreases， plant growth vigor is weakened， and the accumulation of dry matter decreases. Furthermore， the photosynthetic parameters of "black seed" were relatively higher than that of "white seed"， indicating that "black seed" has stronger low temperature tolerance， and could perform certain photosynthetic activity under lower temperature.
Discussion and Conclusion
Plant is a unified whole consisting of various organs， which are different in morphological structure and function， but mutually restrict and depend on each other in growth[10]. The aboveground part and underground part as two parts of plant are also correlated. The underground part， i.e.， the root system is the main organ of plant for absorbing water and nutrients， while the aboveground part provides saccharides and vitamins for growth of the root system. The successful application of grafted seedlings in production is also benefited from deep root system of stock with strong absorption capacity， which could provide more water and nutrients for the growth of the aboveground part[11]. Furthermore， grafted seedlings also could sustain certain absorption capacity under the condition of temperature stress. Masuda et al.[12] reported that the grafted cucumber seedlings with C. maxima as stock had stronger nutrient and water absorption capacities than self-rooted seedlings at low temperature. When plant suffers from temperature stress， its root system is much more sensitive than its aboveground part[13]. Root resists stress by adjusting its growth on one hand， and produces stress signal and conducts it upwards on the other hand， thereby playing a role of remotely control the growth and development of aboveground part. Wang et al.[14] showed that the synthesis of GA3 and IAA in cucumber leaves is promoted under low temperature condition， and it thus could be seen that root system of plant is a key organ and also play an important role in plant growth and resistance to low temperature stress， which is also the entry point of this study. This study showed that after a certain period of ground temperature treatment， the two kinds of grafted seedlings exhibited remarkably reduced growth vigor of aboveground part， dry weight of underground part， dry weight of whole plant and root shoot ratio， and under the same temperature condition， various growth indexes of "black seed" were all higher than those of "white seed". Yan et al.[15] reported that under the same aboveground conditions， the low soil temperature at 10 ℃ greatly affected the growth of cucumber， which had growth vigor and dry weights of root， stem and leaf remarkably lower than those of cucumber grown in heated soil. Awal et al.[16] reported that with the root zone temperature increasing from 12 to 25 ℃， the root function of supplying water and nutrients was improved significantly， and meanwhile， root shoot ratio was reduced. Yu et al.[17] cucumber seedlings with C. maxima as stock exhibited better growth and cold resistance than Xinshizuo. Therefore， the result of this study basically accords with that of Yan et al. The results of this study also showed that the SPAD value， net photosynthetic rate， transpiration rate， intracellular CO2 concentration and stomatal conductance of leaves of cucumber seedlings were in positive correlation with root zone temperature. With the reduction of root zone temperature， these photosynthetic indexes were also on the decrease， and meanwhile， the photosynthetic indexes of "black seed" were also higher than those of "white seed". Li et al.[18] reported that after a certain period of low temperature treatment on soil， chlorophyll content decreased， photosynthetic rate， transpiration rate and stomatal conductance also decreased， but intracellular CO2 concentration increased. This is inconsistent with the decrease of intracellular CO2 concentration in this study， which might be due to that the experimental materials were at different growth stages. Yang et al. [19] showed that at different growth stages， the same soil temperature had different effects on the same physiological index， and intracellular CO2 concentration was in quadratic curve relation with soil temperature.
This study showed that different root zone temperatures significantly affected the growth and photosynthetic parameters of grafted cucumber. When the temperature was in the optimal temperature range （CK： 18-20 ℃）， cucumber could exhibit normal growth and development. When the temperature was in the suboptimal temperature range （13-15 ℃）， normal growth of cucumber was significantly inhibited， and SPAD value， net photosynthetic rate， transpiration rate， intracellular CO2 concentration and stomatal conductance of cucumber were also reduced. When the root zone was at low temperature （8-10 ℃）， the effect on growth of cucumber was more remarkable. Therefore， proper improvement of ground diameter plays an important role in the production of early spring and overwintering cucumber.
In addition， whether under optimal temperature condition or suboptimal or low temperature conditions， various indexes of "black seed" were all higher than those of "white seed". Especially under low temperature stress， "black seed" could better resist low temperature stress.
References
[1] ROSENTHAL RN， WOODBRIDGE CG. Root temperature and nutrient levels of chrysanthemum shoots[J]. Hort Science， 1973， 8（1）： 26-27
[2] LI ZX. Physiological responses to temperature stress in grafted cucumber and cucumber in solar greenhouse[D]. Hohhot： Inner Mongolia Agricultural University， 2010. [3] HU CM， ZHU YL， YANG LF， et al. Comparison of photosynthetic characteristics of grafted and own-root seedlings of cucumber under low temperature circumstances[J]. Acta Botanica Boreali-Occidentalia Sinica， 2006（2）： 247-253.
[4] TIAN XM. Effect of different stocks on the low temperature and salt tolerance of Grafted Cucumber[D]. Jinan： Shandong Agricultural University， 2011.
[5] QIE LJ， QI TQ， SU JP， et al. Effects of low temperature and poor light on physiological characters of grafted cucumber plants on different stocks[J]. Journal of Southwest University： Natural Science Edition， 2008（10）： 68-72.
[6] LI X， LIU FC， WANG J， et al. Compared on protective enzymes activity in leaves of different grafted cucumber seedling under low temperature stress[J]. Journal of Anhui Agricultural Sciences， 2011（4）： 1956-1958.
[7] WALKER JM. One degree increments in soil temperature affect maize seedling behavior[J]. Pro Soc Soil Sci Amer， 1969， 33： 729-736.
[8] SUN ZF， LI M， ZAI XL， et al. Effect of root-zone heating on growth， development and visual quality of poinsettia[J]. Acta Agriculturae Boreali-occidentalis Sinica， 2006（6）： 179-182.
[9] LIAN Y， CUI SM， BAO XX， et al. Effect of different root zone temperature on growth and photosynthetic parameters of pepper seedlings[J]. Northern Horticulture， 2014， 15： 43-45.
[10] LI HS. Modern plant physiology[M]. Beijing： Higher Education Press， 2012.
[11] ZENG YA. Studies on the physiological mechanism of grafting advantage of Cucumber[D]. Nanjing： Nanjing Agricultural University， 2004.
[12] MASUDA M，GOMI K. Mineral absorption and oxygen consumption in grafted and No grafted cucumber[J]. J Jpn Soc Sci， 1984， 52（4）： 414-419.
[13]YU JQ， ZHOU YH， HUANG LF， et al. Chill-induced inhibition of photosynthesis： genotypic variation within Cucumis sativus[J].Plant and Cell Physiology， 2002， 43： 1182-1188.
[14] WANG LL， YU XH. Effect of low soil temperature on contents of cucumber seedings endogenesis GA3 and IAA[J]. Northern Horticulture， 2004（3）： 44-45.
[15] YAN QY， DUAN ZQ， LI X， et al. Effect of root zone temperature on growth of cucumber and nutrient utilization in soils[J]. Acta Pedologica Sinica， 2013（4）： 752-760.
[16] AWAL MA， IKEDA T， ITOH R. The effect of soil temperature on source/sink economy in peanut （Arachis hypogaea）[J]. Environmental and Experimental Botany， 2003， 50（1）： 41-50.
[17] YU XC， XING YX， MA H， et al. Effect of different rootstocks and scions on chilling tolerance in grafted cucumber seedlings[J]. Scientia Agricultura Sinica， 1998， 31（2）： 41-47.
[18] LI ZY， LU YH， XU L. Effect of low temperature of soil on physiological-biochemical characteristics of grafted cucumber[J]. Acta Horticulturae Sinica，1998（3）： 51-56.
[19] YANG ZS， ZHANG Q， HAO XC， et al. The Response of stomatal conductance and intercellular CO2 of spring wheat to climatic factors in semi-arid rain-fed region of loess plateau[J]. Science Technology and Engineering， 2014， 33： 20-27.
Key words Root zone temperature； Grafted cucumber； Growth characteristics； Photosynthetic parameters
Cucumber is an important economic crop in facility cultivation in northern areas of China， which has higher nutritional value， and is favored by people. However， in cultivation of cucumber in winter in northern areas of China， the growth of cucumber is always influenced by temperature. Environmental factors in facility environment such as low temperature greatly influenced early maturation， high yield and quality of melon crop[1]， resulting in great economic loss to farmers. Directing at this problem， grafting is considered to be a method for improving the resistance of cucumber to low-temperature stress， and has been successfully applied to the production process of cucumber in winter. Li et al.[2] showed that under a certain period of low temperature treatment， the root system of grafted cucumber had higher resistance to low-temperature stress than that of self-rooted cucumber. Hu et al.[3] also found that under long-term low temperature， grafted cucumber seedlings and self-rooted cucumber seedlings both suffered from stomatal inhibition， but grafted seedlings maintained relatively higher stomatal conductance and photosynthetic assimilation efficiency than self-rooted cucumber and produced less excessive excitation energy， their extent of injury of photosynthetic organs was reduced， and their adaptability to low temperature was thus stronger. As to the selection of stock， Cucurbita ficifolia and Cucurbita moschata could serve as stock. A large number of previous studies have demonstrated that two kinds of grafted cucumbers has better performance in resisting low-temperature stress[4-6]， but most studies of low-temperature stress focused on air temperature， and there were few studies on the effects of ground temperature on growth and development of different grafted cucumber. Ground temperature not only affects water absorption and mineral metabolism of cucumber， but also restricts normal photosynthesis and respiration. Walker[7] reported that the change of 1 ℃ at root zone could cause remarkable change of plant growth. Sun et al.[8] showed that a suitable root zone temperature could accelerate the development process of poinsettia and significantly improve quality of potted flower， and specifically， branch length， plant height， flower diameter， crown breadth， bract area and commodity rate could be improved significantly. Liang et al.[9] found through the investigation of the effect of different root zone temperature on growth and photosynthetic parameter of pepper seedlings that compared with the optimal temperature treatment （CK）， suboptimal temperature and low-temperature treatments reduced leaf SPAD value， photosynthetic rate， transpiration rate and intercellular CO2 concentration of young pepper seedlings. Therefore， this experiment simulated different ground temperatures to investigate the effects of ground temperature on two kinds of grafted cucumbers， so as to find a stock variety suitable for cultivation of grafted cucumber in northern areas. This study will provide theoretical and practical basis for ground temperature control of cucumber cultivation in solar greenhouse.
Materials and Methods
Experimental materials
The stocks were C. ficifolia （Linghai Nongguang Seed Co.， Ltd.） and C. moschata （Beijing Shuoyuan Seed Co.， Ltd.） frequently used in production practice， and the scion was "Jinyou 35". The grafted cucumber with C. ficifolia as stock was called "black seed"， and that with C. moschata as stock was called "white seed".
Experimental methods
This experiment was carried out in solar greenhouse of the teaching base of Inner Mongolia Agricultural University. Raising of seedlings was started on April 3， and on April 10， when the first true leaf of pumpkin just spread， and the cotyledon of scion fully spread， grating was performed by cleft grafting. On May 6， temperature treatment was performed after recovery of the grafted seedlings. There were three ground temperature treatments： optimal temperature （CK： 18-20 ℃）， suboptimal temperature （13-15 ℃） and low temperature （8-10 ℃）， each of which had three replications. After 50 d of continuous treatment， various indexes were measured in fruiting period of cucumber. The low-temperature treatment： on sealed foam boxes （60 cm×35 cm）， the tops were cut， giving circles with diameter slightly smaller than flowerpots， flowerpots containing cucumber were put on the circles of the foam boxes， on which wet hay was laid， and frozen ice bags （Biyuntian， 250 ml） were placed in the foam boxes. Ice bags in each box were exchanged three times daily after moving away the flowerpots at 8：00， 12：00 and 17：00. Trial tests in early period showed that 20 ice bags should be placed to achieve the low-temperature treatment， 12 ice bags were needed for the suboptimal temperature treatment， and 4 ice bags should be provided for the optimal temperature treatment. However， with the weather getting warm， the number of ice bags should be adjusted every 5 d to satisfy the requirement for temperature. On June 25， the low temperature treatment needed 30 ice bags， the suboptimal temperature required 20 ice bags， and the optimal temperature treatment required 12 ice bags.
Determination of items Determination of morphological indexes
Ground diameter should be determined with a ZDR-20 temperature and humidity recorder. After the temperature treatment， plant height （from grafting position to growing point） and leaf length and width （the third leaf from top to bottom） was measured with meter ruler， and leaf area was calculated according to following equation： leaf area=14.61-5L+0.94L2+0.47W+0.63W2-0.62LW （L： leaf length， W： leaf width）； stem diameter was measured with a vernier caliper， and leaf number was also recorded； and dry weight was obtained by performing deactivation of enzymes at 105 ℃ for 15 min， and oven-drying at 75 ℃. For each index， three plants were measured.
Determination of photosynthetic indexes
After the temperature treatment， a health functional leaf （the third leaf from top to bottom） spreading completely was selected from each treatment， and determined with an LI-6400 photosynthesis system on a sunny day during 9：30-11：30 for net photosynthetic rate， transpiration rate， intracellular CO2 concentration and stomatal conductance， and each leaf was determined for five times， obtaining values which were averaged. Also， the SPAD values of cucumber leaves were also determined with a SPAD-502 chlorophyll meter （Knoica Minolta， Japan）， and each leaf was determined for five times， obtaining values which were averaged.
Data analysis
Data was organized and used for plotting with WPS office software， and variance analysis was performed according to ANOVA in SAS software.
Results and Analysis
Changes of air temperature and ground temperature during experimental period
It could be seen from Fig. 1 that during the experimental period， the average temperatures of various ground temperature treatments all fluctuated within set temperature range. Among them， the optimal temperature （CK） treatment had the ground temperature the lowest at 18.1 ℃， the highest at 19.6 ℃； the suboptimal temperature treatment had the ground temperature the lowest at 13.2 ℃， the highest at 14.5 ℃； and the ground temperature of the low-temperature treatment ranged from 8.1 to 9.6 ℃. The air temperature increased continuously， the lowest at 19.8 ℃， the highest at 26.7 ℃.
Effect of different root zone temperatures on growth morphology of cucumber
It could be seen from Table 1 that with the root zone temperature ranging from high to low， whether for the "black seed" or "white seed" grafted seedlings， under the low temperature and the suboptimal temperature conditions， the plant height， stem diameter， leaf area and leaf number of seedlings were different from those under the optimal temperature （CK） condition at significant or very significant level. It was indicated that with the decrease of the root zone temperature， the growth of the aboveground part of seedling was affected. There were also differences between the two different kinds of grafted seedlings. Under the optimal， suboptimal and low temperature conditions， there were very significant differences in plant height between "black seed" and "white seed"， while under the optimal and suboptimal temperature conditions， there were no remarkable differences in leaf area and leaf number between them， but very significant differences existed in leaf area and leaf number compared with the low-temperature condition. Under the three temperature conditions， there were significant differences in stem diameter of "black seed"， while "white seed" exhibited a significant difference in stem diameter between the optimal and low temperature conditions. Different capital letters in the same column mean very significant difference （P<0.01） and different lowercase letters mean significant difference （P<0.5） . The same below.
Shijun SUN et al. Effects of Root Zone Temperature on Growth and Photosynthetic Parameters of Grafted Cucumber
Effect of different root zone temperatures on dry matter accumulation of cucumber
It could be seen from Table 2 that under different root zone temperature conditions， there were differences in dry weight of aboveground part， underground part and whole plant and root shoot ratio between the two kinds of grafted seedlings. With the decrease of root zone temperature， the dry weights of aboveground part and whole plant decreased gradually， while the dry weight of underground part and root shoot ratio increased gradually， indicating that root zone temperature affected dry matter accumulation of cucumber， the cooling treatment of root zone stimulated the cold resistance of roots， the improvement of energy metabolism level facilitated the distribution of dry matter to root system at higher ratio， and the growth of the root system thus increased. The comparison of the optimal temperature and suboptimal treatments showed that "black seed" and "white seed" had no significant differences in dry weights of aboveground part， underground part and whole plant and root shoot ratio. The comparison between the suboptimal temperature and low temperature treatments， "black seed" and "white seed" had very significant differences in dry weight of aboveground part， and significant differences in dry weights of underground part and whole plant and root shoot ratio. Under the same temperature， the dry matter accumulation of "black seed" was higher than that of "white seed". It was indicated that "black seed" was better than "white seed" in low-temperature resistance， more dry matter began to be stored in underground part， to alleviate low temperature stress at root zone and maintain certain physiological activity of the root part. Therefore， "black seed" exhibited stronger low temperature tolerance.
Effect of different root zone temperatures on SPAD of cucumber
It could be seen from Table 3 that with the reduction of root zone temperature， leaf SPAD of cucumber seedlings decreased gradually， there was a significant difference in SPAD of "black seed" between the optimal and suboptimal temperature treatments， and there were no significant differences in SPAD of "white seed". However， under suboptimal and low temperature conditions， there was a significant difference in SPAD value of "black seed"， and there was a very significant difference in SPAD value of "white seed". It could been seen that low temperature stress has a more remarkable effect on chlorophyll content of "white seed" than that on "black seed". Low temperature stress restricts the synthesis of chlorophyll， and aggravates the degradation of chlorophyll， resulting in the reduction of total content of chlorophyll， photosynthesis is inhibited， and photosynthate is reduced， which also affects morphological indexes of plant. Effects of different root zone temperatures on photosynthetic parameters of cucumber leaves
It could be seen from Table 4 that with the reduction of root zone temperature， the net photosynthetic rate， transpiration rate， intracellular CO2 concentration and stomatal conductance of leaves of grafted cucumber were all on the decrease. For both the two kinds of cucumbers， there were very significant differences in net photosynthetic rate， intracellular CO2 concentration， stomatal conductance and transpiration rate between low temperature and optimal temperature （CK） treatments. It is indicated that lower root zone temperature reduces the transpiration rate of seedlings， and the reduction of transpiration pull directly inhibits the absorption and upward transport of water and mineral elements in root zone. And root zone temperature also reduces intracellular CO2 concentration， resulting in insufficient supply of photosynthetic raw materials， and the reduction of stomatal conductance also weakens the conduction capacity of photosynthetic substrates. These influences caused the reduction of net photosynthetic rate， the accumulation of assimilate decreases， plant growth vigor is weakened， and the accumulation of dry matter decreases. Furthermore， the photosynthetic parameters of "black seed" were relatively higher than that of "white seed"， indicating that "black seed" has stronger low temperature tolerance， and could perform certain photosynthetic activity under lower temperature.
Discussion and Conclusion
Plant is a unified whole consisting of various organs， which are different in morphological structure and function， but mutually restrict and depend on each other in growth[10]. The aboveground part and underground part as two parts of plant are also correlated. The underground part， i.e.， the root system is the main organ of plant for absorbing water and nutrients， while the aboveground part provides saccharides and vitamins for growth of the root system. The successful application of grafted seedlings in production is also benefited from deep root system of stock with strong absorption capacity， which could provide more water and nutrients for the growth of the aboveground part[11]. Furthermore， grafted seedlings also could sustain certain absorption capacity under the condition of temperature stress. Masuda et al.[12] reported that the grafted cucumber seedlings with C. maxima as stock had stronger nutrient and water absorption capacities than self-rooted seedlings at low temperature. When plant suffers from temperature stress， its root system is much more sensitive than its aboveground part[13]. Root resists stress by adjusting its growth on one hand， and produces stress signal and conducts it upwards on the other hand， thereby playing a role of remotely control the growth and development of aboveground part. Wang et al.[14] showed that the synthesis of GA3 and IAA in cucumber leaves is promoted under low temperature condition， and it thus could be seen that root system of plant is a key organ and also play an important role in plant growth and resistance to low temperature stress， which is also the entry point of this study. This study showed that after a certain period of ground temperature treatment， the two kinds of grafted seedlings exhibited remarkably reduced growth vigor of aboveground part， dry weight of underground part， dry weight of whole plant and root shoot ratio， and under the same temperature condition， various growth indexes of "black seed" were all higher than those of "white seed". Yan et al.[15] reported that under the same aboveground conditions， the low soil temperature at 10 ℃ greatly affected the growth of cucumber， which had growth vigor and dry weights of root， stem and leaf remarkably lower than those of cucumber grown in heated soil. Awal et al.[16] reported that with the root zone temperature increasing from 12 to 25 ℃， the root function of supplying water and nutrients was improved significantly， and meanwhile， root shoot ratio was reduced. Yu et al.[17] cucumber seedlings with C. maxima as stock exhibited better growth and cold resistance than Xinshizuo. Therefore， the result of this study basically accords with that of Yan et al. The results of this study also showed that the SPAD value， net photosynthetic rate， transpiration rate， intracellular CO2 concentration and stomatal conductance of leaves of cucumber seedlings were in positive correlation with root zone temperature. With the reduction of root zone temperature， these photosynthetic indexes were also on the decrease， and meanwhile， the photosynthetic indexes of "black seed" were also higher than those of "white seed". Li et al.[18] reported that after a certain period of low temperature treatment on soil， chlorophyll content decreased， photosynthetic rate， transpiration rate and stomatal conductance also decreased， but intracellular CO2 concentration increased. This is inconsistent with the decrease of intracellular CO2 concentration in this study， which might be due to that the experimental materials were at different growth stages. Yang et al. [19] showed that at different growth stages， the same soil temperature had different effects on the same physiological index， and intracellular CO2 concentration was in quadratic curve relation with soil temperature.
This study showed that different root zone temperatures significantly affected the growth and photosynthetic parameters of grafted cucumber. When the temperature was in the optimal temperature range （CK： 18-20 ℃）， cucumber could exhibit normal growth and development. When the temperature was in the suboptimal temperature range （13-15 ℃）， normal growth of cucumber was significantly inhibited， and SPAD value， net photosynthetic rate， transpiration rate， intracellular CO2 concentration and stomatal conductance of cucumber were also reduced. When the root zone was at low temperature （8-10 ℃）， the effect on growth of cucumber was more remarkable. Therefore， proper improvement of ground diameter plays an important role in the production of early spring and overwintering cucumber.
In addition， whether under optimal temperature condition or suboptimal or low temperature conditions， various indexes of "black seed" were all higher than those of "white seed". Especially under low temperature stress， "black seed" could better resist low temperature stress.
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