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Abstract Idesia polycarpa is a kind of woody oil plant with high quality and yield. It is suitable to be planted in mountainous area. In this study， polyethylene glycol （PEG） stress environment was used to evaluate the growth response of I. polycarpa under drought conditions. The results showed that the seed germination parameters were inhibited to different degrees， and the growth and respiration rate of the seedlings increased slightly under the low concentration of PEG， and obviously decreased when the concentration of PEG was over 15%.
Key words Idesia polycarpa； Seed； PEG drought stress； Drought resistance
Idesia polycarpa also known as Shuidonggua， Shuidongtong and Youputao， belonging to Idesia of Flacourtiaceae[1-2]. It is a wooly oil plant species with high quality and yield， and its fruit has an oil extraction rate up to 40%， and the content of linoleic acid accounts for 78.6%. The fruit yield， oil extraction rate and linoleic acid content of I. polycarpa are remarkably higher than those of popularized woody oil plants， and it thus has good development and application prospect. I. polycarpa is not strict in climatic condition， with wide adaptability and high resistance to low temperature and high temperature （-14-40 ℃）， and could normally grow under the annual precipitation of 800-2 000 mm. I. polycarpa has strong adaptability to soil， with good tolerance to drought and poor soil. It is adaptive to neutral， slightly acidic and slightly alkali soil， and distributed widely in North China， Central China， and southwestern and northwestern hilly areas in China. At present， I. polycarpa is suitable to be developed and popularized on barren mountains， to green mountainous regions and improve ecological environment[3-4].
The development of I. polycarpa would often face the problem of water deficiency in mountainous regions and sloping fields. Mountainous regions and sloping fields have poor soil physicochemical properties， and could not well keep moisture and easily cause drought stress， which seriously influences growth and development of I. polycarpa， and greatly restricts the large-area popularization and plantation of I. polycarpa. Drought seriously inhibits growth and development， photosynthesis， respiration and nitrogen metabolism of plant， and the maintaining of available moisture in plant under drought environment is crucial to the survival of plant[5]. The methods for studying drought resistance in plant are mainly soil drought and simulated osmotic stress， but due to complex soil system composition and difficult differentiation of own stress and osmotic effect， the research on soil drought is perplexed to certain degree. Polyethylene glycol 6000 （PEG-6000， PEG for short） is a kind of high polymer with very strong hydrophilcity， which is often used as an osmotic regulator applied to simulate drought stress[6]. In this study， different concentrations of PEG solutions were used for simulating different degrees of stress environment， and through the determination of indices related to seed germination， the overall effect of drought stress on germination of I. polycarpa seeds was investigated， so as to discuss the drought resistance and adaptability of I. polycarpa seeds during germination. Furthermore， indices of root respiration pathway and the activity of key enzymes in respiration were determined and analyzed， so as to study the response characteristics of related enzymes in I. polycarpa to different degrees of drought stress. This study will provide a basis for the water management of I. polycarpa.
Materials and Methods
Experimental materials
The experimental materials were three I. polycarpa varieties： ‘Exuan 1’， ‘Exuan 2’ and ‘Chuantong 32’， respectively. The seeds were provided by Enshi Woody Oil Plants Institute in Hubei Province.
Experimental methods
PEG treatment of seeds
According the method in reference[7]， full seeds of I. polycarpa with uniform size free of diseases and pests were surface-sterilized with 75% ethanol for 1 min， soaked in 1.0% sodium hypochlorite for 3-5 min， and flushed with distilled water to odorless， followed by air-drying. Glass culture dishes （120 mm） and filter paper were sterilized in an autoclave （121℃， 20-30 min）.
Two layers of filter paper was laid in each sterilized culture dish， and 50 seeds were uniformly placed in each culture dish. Distilled water （CK） and 10%， 15%， 20% and 25% PEG solutions （A0-A4） were added into culture dishes， respectively. The seeds were then placed in an illumination incubator at 25 ℃ to allow germination. Each treatment had three replicates.
The seeds in various treatments were observed continuously for 10 d， and the number of germinated seeds was recorded. After germination， five seedlings were selected from each replicate， for the determination of radicle length and hypocotyl length of seedlings， the average values of which were used as radicle and germ lengths， respectively. The fresh weight of seedling was determined.
PEG treatments of seedlings
Normally germinated seedlings were transplanted into nursery pots containing 1 kg of vermiculite according to 15 plants/pot. After 40 d of culture， grouping was performed， healthy I. polycarpa plants growing uniformly at the stage of two leaves and one heart were selected and subjected to drought treatment， i.e.， applying Hoagland nutrient solutions containing different concentrations of PEG into vermiculite continuously for 10 d. The PEG-simulated drought stress was also set with above five treatments A0-A4， each of which included three replicates. Seedlings were collected on the 1st， 3rd， 5th， 7th and 10th day after the application of PEG， respectively， for the determination of seedling growth， root vigor and root respiratory rate indices. Data determination and statistics
Seed germination indices
Various indices were determined according to methods in references[8-9]：
（1） Germination rate：
Germination rate （%）=Number of germinated seeds/Number of tested seeds×100% （10 d， the percentage of germinated seeds in tested seeds）.
（2） Germination index：
Germination index GI=∑（Gt/Dt）（Gt is the number of germinated seeds on the tth day， and Dt is the corresponding number of days for germination）.
（3） Seed vigor index：
Vigor index VI=∑（Gt/Dt）×S （wherein S is average length of buds on the 10th day）.
Physiological indices of seedlings
（1） Plant height： Plant height was measured with straight scale from the level of ground to the height position of plant stem.
（2） Biomass： The whole plant was excavated， cleaned and divided to the aboveground and underground parts. The two parts were subjected to deactivation of enzymes at 105 ℃ in an oven for 1 h and then oven-dried at 55 ℃ for 48 h， and the weights were measured after cooling.
（3） Determination of root activity and respiratory rate.
Root activity was determined by TTC method. Respiratory rate was determined with reference to the methods of Bouma[10] and Mao et al.[11]. A brown woody root with basically uniform diameter was selected， and cut rapidly， obtaining a section of about 0.1 g， which was then cut into segments with a length of about 2 mm with a two-edged knife （the same below）. The root segments were added into a reaction cup， which was covered， and the determination program was initiated. The liquid in the reaction cup was controlled at 25 ℃， and the index was determined with an Oxy-Lab oxygen electrode produced by HANSATECH company in Britain.
Statistical analysis
The statistical analysis was performed with Excel and SPSS17.0.
Results and Analysis
Effect of PEG stress on germination rate of I. polycarpa seeds
A higher germination rate of seeds indicates that there are more seeds with vitality， and there will be more seedlings after seeding[12]. It could be seen from Fig. 1 that the germination rates of the three I. polycarpa materials were lower in the different PEG concentration treatments than in the CK， indicating that the various varieties were affected by PEG stress. Under the stress of 0%-10% PEG， the germination rates of various seeds were on the decrease remarkably， but the ranges were smaller， and the decreasing trends were slower， indicating that the drought stress simulated with low concentration of PEG slightly affected the germination rate of I. polycarpa seeds. When the PEG concentration was higher than 10%， the decreasing trends of the germination rates of the various varieties were more remarkable， and under the stress of 25% PEG， except ‘Exuan 1’， the germination rates of other two kinds of I. polycarpa seeds were both lower than 20%， indicating that with the increase of PEG concentration， the germination rate of I. polycarpa seeds was remarkably inhibited. Without stress， the germination rates of the three varieties all exceeded 90%， without significant differences. And the germination rates ranked as ‘Chuantong 32’>‘Exuan 1’>‘Exuan 2’. Furthermore， it could be seen from Fig. 1 that in the 10%-25% PEG treatments， ‘Exuan 1’ exhibited the highest germination rate， which was remarkably higher than those of ‘Exuan 2’ and ‘Chuantong 32’， indicating that ‘Exuan 1’ seeds had better tolerance to the simulated PEG drought than ‘Exuan 2’ and ‘Chuantong 32’. ‘Exuan 2’ and ‘Chuantong 32’ exhibited no remarkable differences under various PEG gradient treatments， indicating that the two have more accordant tolerance to simulated PEG drought， and their drought resistance is relatively weaker.
Effect of PEG stress on germination index and vigor index of I. polycarpa seeds
Germination index could more comprehensively reflect the germination speed and uniformity of seed under practical condition than simple germination rate[13-14]. Vital index represents the potential of rapid uniform germination of seeds， and the growth and production potential of seeds. Under drought stress， the higher the vital index is， the stronger the drought resistance it has[15].
The effects of different concentrations of PEG stress on the germination index and vigor index of the three I. polycarpa varieties are shown in Fig. 2. It could be seen that with the increase of PEG concentration， the three I. polycarpa varieties all exhibited remarkable decreasing trends of germination index and vigor index， and when the concentration reached 25%， the indices were the lowest， indicating that a high concentration of PEG has stronger inhibitory effect on the germination activity of seeds.
Under the same concentration， the germination index and vigor index of ‘Exuan 1’ seeds were remarkably higher than those of ‘Exuan 2’ and ‘Chuantong 32’， while there were smaller differences between ‘Exuan 2’ and ‘Chuantong 32’， indicating that the seeds of ‘Exuan 1’ have stronger drought resistance and adaptability under stress than ‘Exuan 2’ and ‘Chuantong 32’ during germination.
Effects of PEG stress on radicle， germ and fresh weight of I. polycarpa seedlings
After seed germination， radicle length， germ length and fresh weight of seedling were determined， to investigate the effect of PEG stress on seedlings. It could be seen from Table 1 that when the PEG concentration was 10%， radicle length， germ length and fresh weight of seedling of ‘Exuan 1’ all slightly increased compared with the CK， while ‘Exuan 2’ and ‘Chuantong 32’ exhibited the values lower than the CK， without significant differences. With the PEG concentration increasing from 10%， the determined indices of the three varieties were all on the decrease， and when the concentration reached 25%， the seedlings of ‘Exuan 2’ and ‘Chuantong 32’ died seriously， leading to the condition that the statistical analysis could not be performed. The overall results showed that PEG at a low concentration could promote the growth of radicle， germ and fresh weight of seedling to certain degrees of ‘Exuan 1’， while PEG at a high concentration had a significant inhibitory effect on the growth of embryos. Kaiming SHI et al. Growth Response of Idesia polycarpa to Polyethylene Glycol
Effect of PEG stress on biomass of I. polycarpa seedlings
The potted seedlings treated with different concentrations of PEG continuously withered physiologically from the 3rd day， partial seedling leaves were blacked and withered until the 7th day， and to the 10th day， the seedlings were lying peacefully on the edge of dying. The effects of PEG stress on the growth characteristics of I. polycarpa seedlings were studied through the determination of PEG stress on the dry matter contents in the aboveground and underground parts， root shoot ratio and dead seedling rate of I. polycarpa seedlings on the 7th day of PEG stress.
It could be seen from Table 2 that with the increase of PEG concentration， the dry matter contents in the stem and leaf and the root were on the decrease， and under the same PEG concentration， and the stem and leaf dry matter content ranked as ‘Exuan 1’>‘Exuan 2’>‘Chuantong 32’， and the root dry matter content ranked as ‘Exuan 1’>‘Chuantong 32’>‘Exuan 2’， among which ‘Exuan 1’ had prominent superiority， ‘Exuan 2’ and ‘Chuantong 32’ were not significantly different. It could be seen from the root shoot ratio data that ‘Exuan 1’ showed the highest value of 1.32 at the PEG concentration of 15%， ‘Exuan 2’ exhibited the highest value of 1.59 at the PEG concentration of 20%， and ‘Chuantong 32’ had the highest value of 1.67 at the PEG concentration of 20%. When the PEG concentration was over 20%， the root shoot ratios of the three varieties decreased remarkably. With continuous PEG treatment on the three varieties， the dead seedling proportion increased with the PEG concentration increasing， and when the PEG concentration reached 25%， ‘Exuan 2’ and ‘Chuantong 32’ both had dead seedling proportions over 90%， while the dead seedling proportion of ‘Exuan 1’ was 83.69%. Comprehensively， continuous drought stress simulated with PEG on I. polycarpa seedlings had a remarkable inhibitory effect on the growth indices of the seedlings of various varieties. It could be seen from the determined indices of the three varieties that the seedlings of ‘Exuan 1’ has stronger resistance to the PEG stress than other two varieties.
Effects of PEG stress on root vigor and respiratory rate of I. polycarpa seedlings
Root vigor reflects the metabolic capability of root system， and directly influences plant growth and stress resistance. Table 3 shows the determination results of root vigor and root respiratory rate of seedlings on the 7th day of drought treatment. It could be seen from Table 3 that with the PEG concentration increasing， the root vigor of various varieties decreased linearly. Among them， ‘Exuan 2’ showed the highest root vigor. At the PEG concentration of 15%， the three varieties differed remarkably in determined root vigor. In other treatments， ‘Exuan 2’ had higher root vigor， while there was no remarkable difference between other two varieties. Root respiratory rate is an important indicator of root metabolism， which reflects the activity state of root absorption and renewal. It could be seen from Table 3 that the root respiratory rates of the three varieties all decreased with the PEG concentration increasing； and among the various PEG treatments， ‘Exuan 1’ showed the highest determined respiratory rate， and the values of ‘Exuan 2’ and ‘Chuantong 32’ decreased sequentially. When the PEG concentration reached 25%， the root respiration of the three varieties was very weak， and among them， ‘Chuantong 32’ had a respiratory rate of 0.06 μmol/（min·g）， i.e.， its root respiration was nearly stopped.
Discussion
Drought is one of the most important environmental factors inhibiting plant photosynthesis[16]， and drought causes plant water stress， thereby adversely affecting plant growth， photosynthesis， respiration and nutrition metabolism[17].
In this study， five different concentrations of PEG solutions were used to simulate drought stress on I. polycarpa seedlings， and it could be seen from germination rate， germination index and seed vigor that whether for low concentration treatments or high concentration treatments， the various PEG treatments all inhibited the germination of seeds， which accords with the result of the study conducted by Yang et al.[18] on PEG-simulated drought stress on sunflower seed germination. However， Yao et al.[19] studied the effect of PEG-simulated drought stress on Quercus variabilis， and found that the germination rate， germination index and vital index values were all higher than those of the CK under lower concentrations （5% and 10%） of PEG， but lower than those of the CK under higher concentrations of PEG （20% and 30%）， which accords with the research result of Mai et al.[20] on Cercidiphyllum japonicum seeds. Moreover， the studies of many scholars show that PEG could promote seed germination and improve seed vigor in a lower concentration range， and has certain effect on seedling resistance to stress[21-22]. Different scholars observed different responses of seeds germination under the stress of low concentrations of PEG， which might be caused by the differences in the sensitivity of different seeds to drought stress. The results of this study show that I. polycarpa seedlings have weaker drought tolerance. Overall， the drought stress simulated with PEG with a concentration higher than 15% all remarkably inhibits the germination of various seeds. It also could be seen from the results of this study that under the stress of PEG at the lower concentration （10%）， the seed germination vigor indices of the three varieties exhibited gentle decreasing trends， while when the PEG concentration exceeded 15%， the values decreased sharply. Simply from seed germination vigor indices， it could be seen that the drought resistance characteristics of ‘Exuan 1’ were remarkably higher than ‘Exuan 2’ and ‘Chuantong 32’ during germination， while the germination of I. polycarpa seeds is rather difficult under natural condition. This result could provide reference for seed selection and seedling raising of I. polycarpa in relatively arid regions in future. The root of plant is an important organ for the absorption of water and nutrients and the synthesis of various physiological activators. When the root system encounters drought stress， plant growth would be affected. The results of this study showed that after the germination of I. polycarpa seeds， radicle length and germ length of seedlings were remarkably stressed by PEG， and with the PEG concentration increasing， hypocotyl growth was seriously aggravated， and under the stress of 25% PEG， the germ and radicle of ‘Exuan 2’ and ‘Chuantong 32’ hardly grew. The fresh weight data of seedlings also reflected this trend. The stem and leaf dry matter content of potted seedlings decreased remarkably after PEG drought treatment， and when the PEG concentration was improved to 25%， the dead seedling rate of ‘Exuan 1’ exceeded 83%， and the values of ‘Exuan 2’ and ‘Chuantong 32’ exceeded 90%， indicating that continuous PEG drought treatment aggravated the adverse effect on seedling growth.
It is acknowledged conventionally that certain degree of water stress would enhance plant respiration[23-24]. The results of this study showed that under PEG-simulated drought stress， root vigor and respiratory rate of I. polycarpa seedlings both showed obvious decreasing trends with the PEG concentration increasing， indicating that drought stress might enhance the respiratory metabolism of I. polycarpa within a narrow adaptive range， or only the PEG at a concentration lower than 5%， or lower degree of slight stress would give rise to the enhancement of respiratory metabolism. Above results further show that I. polycarpa might be more sensitive to drought stress than many plant species.
References
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[2] JIANG XB， LI DW， GONG BC， et al. Primary studies on growth regulation of seedling of Idesia polycarpa Maxim. from different populations[J]. Journal of Henan Agricultural Sciences， 2012， 41（5）： 133-136. （in Chinese）
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[4] ZHOU GX， WU FQ， LI CY， et al. Advance in studies of the resource exploration of Idesia polycarpa in Chaotian District of Guangyuan City[J]. Journal of Sichuan Forestry Science and Technology， 2009， 30（3）： 70-73. （in Chinese） [5] WANG DQ， ZHOU YF， LU ZL， et al. Root morphology and activity of stay green sorghum under water stress[J]. Agricultural Research in the Arid Areas， 2012， 30（2）： 73-76， 130. （in Chinese）
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Key words Idesia polycarpa； Seed； PEG drought stress； Drought resistance
Idesia polycarpa also known as Shuidonggua， Shuidongtong and Youputao， belonging to Idesia of Flacourtiaceae[1-2]. It is a wooly oil plant species with high quality and yield， and its fruit has an oil extraction rate up to 40%， and the content of linoleic acid accounts for 78.6%. The fruit yield， oil extraction rate and linoleic acid content of I. polycarpa are remarkably higher than those of popularized woody oil plants， and it thus has good development and application prospect. I. polycarpa is not strict in climatic condition， with wide adaptability and high resistance to low temperature and high temperature （-14-40 ℃）， and could normally grow under the annual precipitation of 800-2 000 mm. I. polycarpa has strong adaptability to soil， with good tolerance to drought and poor soil. It is adaptive to neutral， slightly acidic and slightly alkali soil， and distributed widely in North China， Central China， and southwestern and northwestern hilly areas in China. At present， I. polycarpa is suitable to be developed and popularized on barren mountains， to green mountainous regions and improve ecological environment[3-4].
The development of I. polycarpa would often face the problem of water deficiency in mountainous regions and sloping fields. Mountainous regions and sloping fields have poor soil physicochemical properties， and could not well keep moisture and easily cause drought stress， which seriously influences growth and development of I. polycarpa， and greatly restricts the large-area popularization and plantation of I. polycarpa. Drought seriously inhibits growth and development， photosynthesis， respiration and nitrogen metabolism of plant， and the maintaining of available moisture in plant under drought environment is crucial to the survival of plant[5]. The methods for studying drought resistance in plant are mainly soil drought and simulated osmotic stress， but due to complex soil system composition and difficult differentiation of own stress and osmotic effect， the research on soil drought is perplexed to certain degree. Polyethylene glycol 6000 （PEG-6000， PEG for short） is a kind of high polymer with very strong hydrophilcity， which is often used as an osmotic regulator applied to simulate drought stress[6]. In this study， different concentrations of PEG solutions were used for simulating different degrees of stress environment， and through the determination of indices related to seed germination， the overall effect of drought stress on germination of I. polycarpa seeds was investigated， so as to discuss the drought resistance and adaptability of I. polycarpa seeds during germination. Furthermore， indices of root respiration pathway and the activity of key enzymes in respiration were determined and analyzed， so as to study the response characteristics of related enzymes in I. polycarpa to different degrees of drought stress. This study will provide a basis for the water management of I. polycarpa.
Materials and Methods
Experimental materials
The experimental materials were three I. polycarpa varieties： ‘Exuan 1’， ‘Exuan 2’ and ‘Chuantong 32’， respectively. The seeds were provided by Enshi Woody Oil Plants Institute in Hubei Province.
Experimental methods
PEG treatment of seeds
According the method in reference[7]， full seeds of I. polycarpa with uniform size free of diseases and pests were surface-sterilized with 75% ethanol for 1 min， soaked in 1.0% sodium hypochlorite for 3-5 min， and flushed with distilled water to odorless， followed by air-drying. Glass culture dishes （120 mm） and filter paper were sterilized in an autoclave （121℃， 20-30 min）.
Two layers of filter paper was laid in each sterilized culture dish， and 50 seeds were uniformly placed in each culture dish. Distilled water （CK） and 10%， 15%， 20% and 25% PEG solutions （A0-A4） were added into culture dishes， respectively. The seeds were then placed in an illumination incubator at 25 ℃ to allow germination. Each treatment had three replicates.
The seeds in various treatments were observed continuously for 10 d， and the number of germinated seeds was recorded. After germination， five seedlings were selected from each replicate， for the determination of radicle length and hypocotyl length of seedlings， the average values of which were used as radicle and germ lengths， respectively. The fresh weight of seedling was determined.
PEG treatments of seedlings
Normally germinated seedlings were transplanted into nursery pots containing 1 kg of vermiculite according to 15 plants/pot. After 40 d of culture， grouping was performed， healthy I. polycarpa plants growing uniformly at the stage of two leaves and one heart were selected and subjected to drought treatment， i.e.， applying Hoagland nutrient solutions containing different concentrations of PEG into vermiculite continuously for 10 d. The PEG-simulated drought stress was also set with above five treatments A0-A4， each of which included three replicates. Seedlings were collected on the 1st， 3rd， 5th， 7th and 10th day after the application of PEG， respectively， for the determination of seedling growth， root vigor and root respiratory rate indices. Data determination and statistics
Seed germination indices
Various indices were determined according to methods in references[8-9]：
（1） Germination rate：
Germination rate （%）=Number of germinated seeds/Number of tested seeds×100% （10 d， the percentage of germinated seeds in tested seeds）.
（2） Germination index：
Germination index GI=∑（Gt/Dt）（Gt is the number of germinated seeds on the tth day， and Dt is the corresponding number of days for germination）.
（3） Seed vigor index：
Vigor index VI=∑（Gt/Dt）×S （wherein S is average length of buds on the 10th day）.
Physiological indices of seedlings
（1） Plant height： Plant height was measured with straight scale from the level of ground to the height position of plant stem.
（2） Biomass： The whole plant was excavated， cleaned and divided to the aboveground and underground parts. The two parts were subjected to deactivation of enzymes at 105 ℃ in an oven for 1 h and then oven-dried at 55 ℃ for 48 h， and the weights were measured after cooling.
（3） Determination of root activity and respiratory rate.
Root activity was determined by TTC method. Respiratory rate was determined with reference to the methods of Bouma[10] and Mao et al.[11]. A brown woody root with basically uniform diameter was selected， and cut rapidly， obtaining a section of about 0.1 g， which was then cut into segments with a length of about 2 mm with a two-edged knife （the same below）. The root segments were added into a reaction cup， which was covered， and the determination program was initiated. The liquid in the reaction cup was controlled at 25 ℃， and the index was determined with an Oxy-Lab oxygen electrode produced by HANSATECH company in Britain.
Statistical analysis
The statistical analysis was performed with Excel and SPSS17.0.
Results and Analysis
Effect of PEG stress on germination rate of I. polycarpa seeds
A higher germination rate of seeds indicates that there are more seeds with vitality， and there will be more seedlings after seeding[12]. It could be seen from Fig. 1 that the germination rates of the three I. polycarpa materials were lower in the different PEG concentration treatments than in the CK， indicating that the various varieties were affected by PEG stress. Under the stress of 0%-10% PEG， the germination rates of various seeds were on the decrease remarkably， but the ranges were smaller， and the decreasing trends were slower， indicating that the drought stress simulated with low concentration of PEG slightly affected the germination rate of I. polycarpa seeds. When the PEG concentration was higher than 10%， the decreasing trends of the germination rates of the various varieties were more remarkable， and under the stress of 25% PEG， except ‘Exuan 1’， the germination rates of other two kinds of I. polycarpa seeds were both lower than 20%， indicating that with the increase of PEG concentration， the germination rate of I. polycarpa seeds was remarkably inhibited. Without stress， the germination rates of the three varieties all exceeded 90%， without significant differences. And the germination rates ranked as ‘Chuantong 32’>‘Exuan 1’>‘Exuan 2’. Furthermore， it could be seen from Fig. 1 that in the 10%-25% PEG treatments， ‘Exuan 1’ exhibited the highest germination rate， which was remarkably higher than those of ‘Exuan 2’ and ‘Chuantong 32’， indicating that ‘Exuan 1’ seeds had better tolerance to the simulated PEG drought than ‘Exuan 2’ and ‘Chuantong 32’. ‘Exuan 2’ and ‘Chuantong 32’ exhibited no remarkable differences under various PEG gradient treatments， indicating that the two have more accordant tolerance to simulated PEG drought， and their drought resistance is relatively weaker.
Effect of PEG stress on germination index and vigor index of I. polycarpa seeds
Germination index could more comprehensively reflect the germination speed and uniformity of seed under practical condition than simple germination rate[13-14]. Vital index represents the potential of rapid uniform germination of seeds， and the growth and production potential of seeds. Under drought stress， the higher the vital index is， the stronger the drought resistance it has[15].
The effects of different concentrations of PEG stress on the germination index and vigor index of the three I. polycarpa varieties are shown in Fig. 2. It could be seen that with the increase of PEG concentration， the three I. polycarpa varieties all exhibited remarkable decreasing trends of germination index and vigor index， and when the concentration reached 25%， the indices were the lowest， indicating that a high concentration of PEG has stronger inhibitory effect on the germination activity of seeds.
Under the same concentration， the germination index and vigor index of ‘Exuan 1’ seeds were remarkably higher than those of ‘Exuan 2’ and ‘Chuantong 32’， while there were smaller differences between ‘Exuan 2’ and ‘Chuantong 32’， indicating that the seeds of ‘Exuan 1’ have stronger drought resistance and adaptability under stress than ‘Exuan 2’ and ‘Chuantong 32’ during germination.
Effects of PEG stress on radicle， germ and fresh weight of I. polycarpa seedlings
After seed germination， radicle length， germ length and fresh weight of seedling were determined， to investigate the effect of PEG stress on seedlings. It could be seen from Table 1 that when the PEG concentration was 10%， radicle length， germ length and fresh weight of seedling of ‘Exuan 1’ all slightly increased compared with the CK， while ‘Exuan 2’ and ‘Chuantong 32’ exhibited the values lower than the CK， without significant differences. With the PEG concentration increasing from 10%， the determined indices of the three varieties were all on the decrease， and when the concentration reached 25%， the seedlings of ‘Exuan 2’ and ‘Chuantong 32’ died seriously， leading to the condition that the statistical analysis could not be performed. The overall results showed that PEG at a low concentration could promote the growth of radicle， germ and fresh weight of seedling to certain degrees of ‘Exuan 1’， while PEG at a high concentration had a significant inhibitory effect on the growth of embryos. Kaiming SHI et al. Growth Response of Idesia polycarpa to Polyethylene Glycol
Effect of PEG stress on biomass of I. polycarpa seedlings
The potted seedlings treated with different concentrations of PEG continuously withered physiologically from the 3rd day， partial seedling leaves were blacked and withered until the 7th day， and to the 10th day， the seedlings were lying peacefully on the edge of dying. The effects of PEG stress on the growth characteristics of I. polycarpa seedlings were studied through the determination of PEG stress on the dry matter contents in the aboveground and underground parts， root shoot ratio and dead seedling rate of I. polycarpa seedlings on the 7th day of PEG stress.
It could be seen from Table 2 that with the increase of PEG concentration， the dry matter contents in the stem and leaf and the root were on the decrease， and under the same PEG concentration， and the stem and leaf dry matter content ranked as ‘Exuan 1’>‘Exuan 2’>‘Chuantong 32’， and the root dry matter content ranked as ‘Exuan 1’>‘Chuantong 32’>‘Exuan 2’， among which ‘Exuan 1’ had prominent superiority， ‘Exuan 2’ and ‘Chuantong 32’ were not significantly different. It could be seen from the root shoot ratio data that ‘Exuan 1’ showed the highest value of 1.32 at the PEG concentration of 15%， ‘Exuan 2’ exhibited the highest value of 1.59 at the PEG concentration of 20%， and ‘Chuantong 32’ had the highest value of 1.67 at the PEG concentration of 20%. When the PEG concentration was over 20%， the root shoot ratios of the three varieties decreased remarkably. With continuous PEG treatment on the three varieties， the dead seedling proportion increased with the PEG concentration increasing， and when the PEG concentration reached 25%， ‘Exuan 2’ and ‘Chuantong 32’ both had dead seedling proportions over 90%， while the dead seedling proportion of ‘Exuan 1’ was 83.69%. Comprehensively， continuous drought stress simulated with PEG on I. polycarpa seedlings had a remarkable inhibitory effect on the growth indices of the seedlings of various varieties. It could be seen from the determined indices of the three varieties that the seedlings of ‘Exuan 1’ has stronger resistance to the PEG stress than other two varieties.
Effects of PEG stress on root vigor and respiratory rate of I. polycarpa seedlings
Root vigor reflects the metabolic capability of root system， and directly influences plant growth and stress resistance. Table 3 shows the determination results of root vigor and root respiratory rate of seedlings on the 7th day of drought treatment. It could be seen from Table 3 that with the PEG concentration increasing， the root vigor of various varieties decreased linearly. Among them， ‘Exuan 2’ showed the highest root vigor. At the PEG concentration of 15%， the three varieties differed remarkably in determined root vigor. In other treatments， ‘Exuan 2’ had higher root vigor， while there was no remarkable difference between other two varieties. Root respiratory rate is an important indicator of root metabolism， which reflects the activity state of root absorption and renewal. It could be seen from Table 3 that the root respiratory rates of the three varieties all decreased with the PEG concentration increasing； and among the various PEG treatments， ‘Exuan 1’ showed the highest determined respiratory rate， and the values of ‘Exuan 2’ and ‘Chuantong 32’ decreased sequentially. When the PEG concentration reached 25%， the root respiration of the three varieties was very weak， and among them， ‘Chuantong 32’ had a respiratory rate of 0.06 μmol/（min·g）， i.e.， its root respiration was nearly stopped.
Discussion
Drought is one of the most important environmental factors inhibiting plant photosynthesis[16]， and drought causes plant water stress， thereby adversely affecting plant growth， photosynthesis， respiration and nutrition metabolism[17].
In this study， five different concentrations of PEG solutions were used to simulate drought stress on I. polycarpa seedlings， and it could be seen from germination rate， germination index and seed vigor that whether for low concentration treatments or high concentration treatments， the various PEG treatments all inhibited the germination of seeds， which accords with the result of the study conducted by Yang et al.[18] on PEG-simulated drought stress on sunflower seed germination. However， Yao et al.[19] studied the effect of PEG-simulated drought stress on Quercus variabilis， and found that the germination rate， germination index and vital index values were all higher than those of the CK under lower concentrations （5% and 10%） of PEG， but lower than those of the CK under higher concentrations of PEG （20% and 30%）， which accords with the research result of Mai et al.[20] on Cercidiphyllum japonicum seeds. Moreover， the studies of many scholars show that PEG could promote seed germination and improve seed vigor in a lower concentration range， and has certain effect on seedling resistance to stress[21-22]. Different scholars observed different responses of seeds germination under the stress of low concentrations of PEG， which might be caused by the differences in the sensitivity of different seeds to drought stress. The results of this study show that I. polycarpa seedlings have weaker drought tolerance. Overall， the drought stress simulated with PEG with a concentration higher than 15% all remarkably inhibits the germination of various seeds. It also could be seen from the results of this study that under the stress of PEG at the lower concentration （10%）， the seed germination vigor indices of the three varieties exhibited gentle decreasing trends， while when the PEG concentration exceeded 15%， the values decreased sharply. Simply from seed germination vigor indices， it could be seen that the drought resistance characteristics of ‘Exuan 1’ were remarkably higher than ‘Exuan 2’ and ‘Chuantong 32’ during germination， while the germination of I. polycarpa seeds is rather difficult under natural condition. This result could provide reference for seed selection and seedling raising of I. polycarpa in relatively arid regions in future. The root of plant is an important organ for the absorption of water and nutrients and the synthesis of various physiological activators. When the root system encounters drought stress， plant growth would be affected. The results of this study showed that after the germination of I. polycarpa seeds， radicle length and germ length of seedlings were remarkably stressed by PEG， and with the PEG concentration increasing， hypocotyl growth was seriously aggravated， and under the stress of 25% PEG， the germ and radicle of ‘Exuan 2’ and ‘Chuantong 32’ hardly grew. The fresh weight data of seedlings also reflected this trend. The stem and leaf dry matter content of potted seedlings decreased remarkably after PEG drought treatment， and when the PEG concentration was improved to 25%， the dead seedling rate of ‘Exuan 1’ exceeded 83%， and the values of ‘Exuan 2’ and ‘Chuantong 32’ exceeded 90%， indicating that continuous PEG drought treatment aggravated the adverse effect on seedling growth.
It is acknowledged conventionally that certain degree of water stress would enhance plant respiration[23-24]. The results of this study showed that under PEG-simulated drought stress， root vigor and respiratory rate of I. polycarpa seedlings both showed obvious decreasing trends with the PEG concentration increasing， indicating that drought stress might enhance the respiratory metabolism of I. polycarpa within a narrow adaptive range， or only the PEG at a concentration lower than 5%， or lower degree of slight stress would give rise to the enhancement of respiratory metabolism. Above results further show that I. polycarpa might be more sensitive to drought stress than many plant species.
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