论文部分内容阅读
Abstract An Asparagus officinalis L. cultivar NJ978 was used to study the growing rate， salt ion content and mineral ion uptake and distribution in plants under salt stress. The results showed that salt stress significantly inhibited seedling growth and there was a negative relationship between seedling growth and salt concentration. The seedlings growth showed no significant decrease under low salt stress （NaCl≤50 mmol/L）， while high concentrations of NaCl led to detrimental effects on the growth of A. officinalis L.. With the increase of NaCl concentration， the Na+ content increased gradually in A. officinalis L. roots， stems and leaves while the K+ and Ca2+ contents had slow decreases. Under low salt stress， the seedlings of A. officinalis L. could prevent Na+ from transporting to aerial part by withholding Na+ in root， which could keep the balance of ions in aerial part， but under high salt stress （NaCl > 100 mmol/L）， the aerial part of seedlings accumulated superabundance of Na+， which limited the uptake of K+ and Ca2+， so seedlings were damaged heavily， which manifested as the gradual decrease of K+/Na+ and Ca2+/Na+ in the roots and aerial part of the seedlings. The selective absorption of K+， Ca2+ and Na+ （ASK，Na and ASCa，Na） was significantly increased with increased salinity， while the selective transportation （TSK，Na and TSCa，Na） increased at first and then decreased with the increased salinity. Therefore， the strong ability of salt exclusion and the regionalized distribution of ions in roots， stems and leaves could be one of the salt tolerance mechanisms of A. officinalis L.
Key words Asparagus officinalis L.； Salt stress； Growth； Ion distribution.
Soil salinization is one of the main stress factors restricting plant growth. According to statistics， the area of ？？various types of salinealkali soil in China is about 9.91×107 hm2. Therefore， the full development and utilization of salinealkali soil is an important issue in current agricultural development[1-2].
After plants are exposed to salt stress， growth is mainly impeded. The reasons include osmotic stress[3]， ion toxicity[4]， electrolyte imbalance[5-6]， and nutrient deficit[7]. The plant absorbs and accumulates inorganic salt ions in the lowsalt environment for osmotic adjustment， thereby increasing the cell concentration， decreasing the osmotic potential of the cells， and preventing cell dehydration[8-9]. Under high salt stress， a large amount of Na+ influxes into cells， which not only destroys ion balance in cells， but also affects the distribution of intracellular inorganic salts[9-11]. The reconstruction of ionic homeostasis is an important mechanism for plants to combat high salt stress. Membrane transporters involved in transmembrane ion transport （such as H+ATPase， Ca2+ATPase， transmembrane proteins， various ion channels） play an important role in reconstructing the ionic homeostasis in cells under salt stress[9，12-14]. Salt tolerance of plants is closely related to the selective absorption and exchange of Na+ and K+. Maintaining normal ion content and distribution in cytoplasm under salt stress is essential for plant survival[15]. Asparagus officinalis L. is a herbaceous， perennial plant of the genus Liliaceae， which is characterized by drought and salinity resistance and is an important economic and medicinal plant. At present， studies on A. officinalis at home and abroad mainly focus on cultivation techniques， composition analysis and tissue culture[16-17]， while there are relatively few reports on its metabolic reactions under salt stress. In this study， pot experiment was conducted to study the growth of A. officinalis and the distribution of Na+， K+， and Ca2+ ions in the body under simulated salt stress to explore the damage mechanism of salt on the growth of A. officinalis and provide basis for the full development of A. officinalis resources.
Materials and Methods
Material cultivation
The test was conducted in the sunlight greenhouse of the Institute of Cash Crops， Hebei Academy of Agriculture and Forestry Sciences in 2011. The A. officinalis variety NJ978 was used as material. In February 2011， seedlings were grown in solar greenhouses and the seedlings were transplanted 60 d after emergence to the flowerpots with the size of 40 cm × 40 cm.
At 10 d after transplanting， the seedlings with similar growth conditions were selected for salt treatment. There were 5 salt （NaCl） levels set for the test， namely， 50， 100， 150， 200， and 300 mmol/L， respectively， and fresh water treatment was set as the control. In order to avoid the effect of salt impact， the concentration of NaCl solution was gradually increased by 50 mmol/L per day. The various indicators of seedlings were determined through sampling after 20 d of salt stress treatment.
Test indexes and methods
Determination of plant growth and moisture content For each treatment， 3 plants were taken. After rinsed with distilled water and dried the surface， the aerial parts and roots were separated， and the fresh weight （Wf） was weighed. After deactivation at 105 ℃ for 10 min， the seedlings were dried at 80 ℃ to constant weight to weigh the dry weight （Wd）.
Moisture content∥% = （Wf - Wd）/Wf × 100%
Determination of Na+， K+ and Ca2+ contents Ion extraction was performed according to the method of Wang et al.[18]. The samples were divided into three parts： root， stem， and leaf. After drying and crushing， the samples were sieved through a 40mesh sieve. And then 0.1 g of the samples were taken and added with 20 ml of deionized water， and the mixture was boiled for 2 h. After cooling， the mixture was centrifuged at 5 000 r/min for 15 min， and the supernatant was brought to the constant volume of 50 ml. The contents of K+， Na+， and Ca2+ were measured using a GGX600 atomic absorption spectrophotometer. ASX， Na and TSX， Na calculation Ion selective absorption coefficient （ASX， Na） = Whole {[X+]/[Na+]}/medium {[X+]/[Na+]}；
Ion transport coefficient （TSX， Na） = Sink organ {[X+]/[Na+]}/Source organ {[X+]/[Na+]}.
Data processing
Test data were analyzed using SAS statistical analysis software， and Duncan’s multiple comparison method was used for statistical analysis， P<0.05.
Results and Analysis
Effect of salt stress on growth and water content of A. officinalis seedlings
In terms of plant morphology， when NaCl ≤ 100 mmol/L， the A. officinalis seedlings showed no obvious salty damage symptoms. When the NaCl concentration increased to 200 mmol/L， the leaves at the top of seedlings showed the symptoms of chlorosis and yellowing， and when NaCl increased to 300 mmol/L， the leaves at the top of the seedlings were withered. As shown in Table 1， the fresh weights of the roots （r=-0.988， P<0.01） and aerial parts （r=-0.985， P<0.01） of A. officinalis seedlings were negatively correlated with NaCl concentration under NaCl stress， and the difference reached the significant level with the control at NaCl ≥ 100 mmol/L （P<0.05）. There was no significant difference from the control in plant heights， root dry weight and aerial part dry weight at NaCl ≤ 100 mmol/L， but with the increase of salt concentration， plant heights， dry weight of roots and dry weights of the aerial parts gradually decreased and showed significant difference with the control （P<0.05）. The water content in the root and aerial part decreased with the increase of salt stress. The water content of the aerial parts showed significant difference from the control at NaCl ≥200 mmol/L （P<0.05）， but the water content in roots did not reach a significant level in each treatment.
Contents of Na+， K+ and Ca2+ in A. officinalis seedlings under NaCl stress
Content of Na+ As shown in Fig. 1， the Na+ content in A. officinalis seedlings increased with the increase of NaCl concentration. Under low NaCl concentration （≤ 100 mmol/L）， the increase of Na+ content in roots was greater than that in stems and leaves， and in the treatment with 100 mmol/L NaCl， the content of Na+ in roots， stems and leaves increased by 94.6%， 34.9% and 13.1%， respectively， compared with the control. With the increase of salt stress， the Na+ content in stems and leaves increased rapidly， while the Na+ content in roots increased slowly. The NaCl concentration of 100-150 mmol/L was the point when the Na+ of aerial parts changed from slow increase into quick increase. The results indicated that the roots of A. officinalis had a certain ability to retain Na+， and under low salt stress， A. officinalis seedlings could retain Na+ in the roots， which can inhibit their transport to the shoots， thus reducing the damage of salt to the plants. Content of K+， Ca2+ The contents of K+ in roots， stems and leaves of A. officinalis seedlings under NaCl stress were lower than those of the control， and negatively correlated with salt concentration （Fig. 2）， with roots of （r=-0.989， P<0.01）， stems of （r=-0.952， P<0.01）， leaves of （r=-0.964， P<0.01）. At NaCl ≥100 mmol/L， the content of K+ in roots showed significant difference from the control （P<0.05， while the contents of K+ content in stems and leaves showed significant differences from the control at NaCl ≥ 150 mmol/L （P < 0.05）. Aimilar to the changes of K+， the content of Ca2+ decreased with the increase of salt stress. At NaCl ≤ 50 mmol/L， there was no significant difference from the control in Ca2+ content in roots， stems， and leaves， but the differences reached significant level from the control in all other treatments （P<0.05）.
With the increase of NaCl concentration in soil， the contents of K+ and Ca2+ in roots， stems and leaves of A. officinalis seedlings gradually decreased， but they did not show a significant decrease in lowsalt conditions， and the relatively stable ion balance could be maintained. Under high salt stress， the absorption and transport of K+ and Ca2+ were severely inhibited， thus affecting normal plant growth.
Effect of NaCl stress on the selective absorption and transportation of Na+， K+ and Ca2+ in A. officinalis seedlings
As shown in Table 2， under NaCl stress， the selective absorption coefficients ASK，Na and ASCa，Na of Na+， K+ and Ca2+ in the roots of A. officinalis seedlings gradually increased with the increase of salt stress intensity. Under low salt stress， the transport coefficients TSK，Na and TSCa，Na of K+ and Ca2+ from the roots to the aerial parts were significantly higher than those of the control， and both decreased gradually under high salt stress.
Discussion
Salt stress damages plants in many ways， such as ion poisoning， osmotic stress， and nutrient deficiency， thereby impairing the normal metabolism of cells， eventually leading to the obstruction of plants’ growth and even death[2]. The decline of biomass is the most intuitive performance of plants under salt stress[19-20]. In this study， with the increase of salt concentration， the growth of A. officinalis seedlings gradually declines， the plant height decreases， the fresh weight and dry weight drop， and the inhibitory effect of salt stress on the growth of the aerial parts of A. officinalis is greater than that of the root system， which is consistent with the research results of Zhu et al.[9]. Under salt stress， the excessive Na+ in soil affects the absorption， transportation， and distribution of Na+， K+， Ca2+， and Mg2+ ions in soil， and pseudohalophytes generally improve their adaptability to saline and alkaline environments through salt rejection[20]. Sun et al. believe that the interception of Na+ by salttolerant mulberry cultivars in roots limits the transport of Na+ to the aerial parts[21]. In this study， salt stress leads to the accumulation of Na+ in A. officinalis seedlings， and in the lowsalt environment， the root Na+ content increases rapidly， while the accumulation of Na+ in the aerial parts show no great increase， indicating that the interception of Na+ in the roots of A. officinalis limits the transport of Na+ to the aerial parts， thus avoiding the ion poisoning caused by the accumulation of Na+ ions in the aerial parts.
Under salt stress， the competition between Na+， K+ and Ca2+ leads to the decrease of K+ and Ca2+ contents in plants， which has been verified in this study. The contents of K+ and Ca2+ in the roots， stems， and leaves of A. officinalis seedlings all decreases with the increase of NaCl concentration. Under low salt stress， the contents of K+ and Ca2+ in stems and leaves show little decrease， indicating that A. officinalis has a certain adaptability in low salt environment. Ca2+ plays an important role in maintaining the integrity and stability of plant cell membranes[22]； Ron Mittler et al. has demonstrated that Ca2+ together with ABA， ROS， is involved in signal transmission and expression of stress resistance in plants[23]； Heather Knight has also confirmed that endogenous Ca2+ in plants is closely related to salt tolerance， and under salt stress， Ca2+ on the plasma membrane may be replaced by Na+， which reduces the selectivity and stability of the cell membrane， leading to the influx of salt ions and leakage of nutrients.[23].
The ionselective absorption coefficient and transport coefficient can reflect the selectivity of the plant for ion absorption and transport to aerial parts[21， 25]. The results of this study show that with the increase of external salt concentration， K+/Na+ and Ca2+/Na+ in A. officinalis body decreases gradually， which is caused by the increase of Na+ and the decrease of K+ and Ca2+. The absorption capacity of the roots of A. officinalis seedlings to K+ and Ca2+ increases with the increase of external salt concentration， while the transport capacity of K+ and Ca2+ to the aerial parts of plants increases rapidly under low salt stress （NaCl ≤100 mmol/L）， but increases with the increase of external salt concentration. However， TSK，Na， TSCa，Na decreases significantly under high salt stress， which is due to the fact that with the increase of salt ions in the external environment， a large amount of Na+ enters the plant and causes ion toxicity， and the energy consumption of cells for the selective absorption and transportation of ions is greatly increased， leading to metabolic disorders in the body. References
[1]WANG ZQ， ZHU SQ， YU RP. Chinese saline soil[M]. Beijing： Science Press， 1993.
[2]ZHAO KF， FAN H. Halophytes and their adaptation to salty habitats[M]. Beijing： Science Press， 2005.
[3]ZHENG QS， LIU L， LIU YL， et al. Effect of salt and water stresses on osmotic adjustment and osmotica accumulation in Aloe vera seedlings[J]. Journal of Plant Physiology and Molecular Biology， 2003，29（6）： 585-588.
[4]YANG CW， LI CY， ZHANG ML， et al. pH and ion balance in wheat under salt or alkali stress[J]. Chinese Journal of Applied Ecology， 2008，19（5）： 1000-1005.
[5]MUNNS R. Comparative physiology of salt and water stress[J]. Plant， Cell & Environment， 2002，25（2）： 239-250.
[6]YEO A. Molecular biology of salt tolerance in the context of wholeplant physiology[J]. Journal of Experimental Botany， 1998， 49（323）： 915-929.
[7]CARET CT， GRIEVE CM. Mineral nutrition， growth and germination of Antirrhinum majus L. （Snapdragon） when produced under increasingly saline conditions[J]. HortScience， 2008，43（3）： 710-718.
[8]YANG SH， JI J， WNAG G. Effects of salt stress on plants and the mechanism of salt tolerance[J]. World Scitech R & D， 2006，28（4）： 70-76.
[9]ZHU Y， TAN GE， HE CQ， et al. Effect of salinization on growth and ion homeostasis in seedlings of Festuca arundinacea[J]. Acta Ecologica Sinica， 2007，27（12）： 5447-5454.
[10]RAMOLIYA PJ， PATEL HM， PANDEY AN. Effect of salinization of soil on growth and macro and micronutrient accumulation in seedlings of Salvadora persica （Salvadoraceae）[J]. Forest Ecology and Management， 2004， 202（1） ：181-193.
[11]NIU X， BRESSAN RA， HASEGAWA PM， et al. Ion homeostasis in NaCl stress environments[J]. Plant physiology， 1995，109（3）： 735.
[12]ZHU JK. Regulation of ion homeostasis under salt stress[J]. Current opinion in plant biology， 2003，6（5）： 441-445.
[13]WANG JY， ZHANG GH， SU Q， et al. Research advances about the relation between membrane spanned ion transporter and salt tolerance in plants[J]. Acta Botanica BorealiOccidentalia Sinica， 2006，26（3）： 635-640.
[14]SERRANO R， RODRIGUEZNAVARRO A. Ion homeostasis during salt stress in plants[J]. Current opinion in cell biology， 2001，13（4）： 399-404.
[15]NING JF， ZHENG QS， YANG SH， et al. Impact of high salt stress on Apocynum venetum growth and ionic homeostasis[J]. Chinese Journal of Applied Ecology， 2010，21（2）： 325-330.
[16]MIE LC， LI BH， HUANG RH. Advance in green asparagus cultivation in China[J]. Chinese Agricultural Science Bulletin， 2006，22（12）： 204-208. [17]JU YD. Chemical components in Asparagus officinalis and their medical effects[J]. China Horticultural Abstracts， 2011， 2： 125-126.
[18]WANG BS， ZHAO KF. Comparison of extractive methods of Na and K in wheat leaves[J]. Plant Physiology Communication， 1995，31（1）： 50-52.
[19]CHENG YJ， GUO SR， ZHANG RH， et al. Mitigative effect of exogenous Ca（NO3）2 in cucumber seedlings under salt stress[J]. Acta Botanica BorealiOccidetanlia Sinica， 2009，29（9）： 1853-1859.
[20]XIE YZ， HE P， WANG CY， et al. The effects of exogenous CaCl2， SA， and SNP on physiological traits of Cassia obtusifolia L. seedlings under NaCl stress[J]. Journal of Southwest University （Natural Science Edition）， 2013，35（5）： 36-43.
[21]SUN JB， SUN GY， LIU XD， et al. Effects of salt stress on mulberry seedlings growth， leaf water status， and ion distribution in various organs[J]. Chinese Journal of Applied Ecology， 2009，20（3）： 543-548.
[22]LV ZX， WANG ZG. The influence of calcium on distribution of inorganic ions and membrance fatty acids in roots of Triticum aestivum seedlings under salt stress[J]. Acta Phytophysiologica Sinica， 1993，19（4）： 325-332.
[23]MITTLER R. Oxidative stress， antioxidants and stress tolerance[J]. Trends in plant science， 2002，7（9）： 405-410.
[24]KNIGHT H， KNIGHT MR. Abiotic stress signalling pathways： specificity and crosstalk[J]. Trends in plant science， 200，6（6）： 262-267.
[25]GUO CX， WANG WL， ZHENG CS， et al. Effects of exogenous salicylic acid on ions contents and net photosynthetic rate in chrysanthemum under salt stress[J]. Scientia Agricultura Sinica， 2011，44（15）： 3185-3192.
Editor： Na LI Proofreader： Xinxiu ZHU
Key words Asparagus officinalis L.； Salt stress； Growth； Ion distribution.
Soil salinization is one of the main stress factors restricting plant growth. According to statistics， the area of ？？various types of salinealkali soil in China is about 9.91×107 hm2. Therefore， the full development and utilization of salinealkali soil is an important issue in current agricultural development[1-2].
After plants are exposed to salt stress， growth is mainly impeded. The reasons include osmotic stress[3]， ion toxicity[4]， electrolyte imbalance[5-6]， and nutrient deficit[7]. The plant absorbs and accumulates inorganic salt ions in the lowsalt environment for osmotic adjustment， thereby increasing the cell concentration， decreasing the osmotic potential of the cells， and preventing cell dehydration[8-9]. Under high salt stress， a large amount of Na+ influxes into cells， which not only destroys ion balance in cells， but also affects the distribution of intracellular inorganic salts[9-11]. The reconstruction of ionic homeostasis is an important mechanism for plants to combat high salt stress. Membrane transporters involved in transmembrane ion transport （such as H+ATPase， Ca2+ATPase， transmembrane proteins， various ion channels） play an important role in reconstructing the ionic homeostasis in cells under salt stress[9，12-14]. Salt tolerance of plants is closely related to the selective absorption and exchange of Na+ and K+. Maintaining normal ion content and distribution in cytoplasm under salt stress is essential for plant survival[15]. Asparagus officinalis L. is a herbaceous， perennial plant of the genus Liliaceae， which is characterized by drought and salinity resistance and is an important economic and medicinal plant. At present， studies on A. officinalis at home and abroad mainly focus on cultivation techniques， composition analysis and tissue culture[16-17]， while there are relatively few reports on its metabolic reactions under salt stress. In this study， pot experiment was conducted to study the growth of A. officinalis and the distribution of Na+， K+， and Ca2+ ions in the body under simulated salt stress to explore the damage mechanism of salt on the growth of A. officinalis and provide basis for the full development of A. officinalis resources.
Materials and Methods
Material cultivation
The test was conducted in the sunlight greenhouse of the Institute of Cash Crops， Hebei Academy of Agriculture and Forestry Sciences in 2011. The A. officinalis variety NJ978 was used as material. In February 2011， seedlings were grown in solar greenhouses and the seedlings were transplanted 60 d after emergence to the flowerpots with the size of 40 cm × 40 cm.
At 10 d after transplanting， the seedlings with similar growth conditions were selected for salt treatment. There were 5 salt （NaCl） levels set for the test， namely， 50， 100， 150， 200， and 300 mmol/L， respectively， and fresh water treatment was set as the control. In order to avoid the effect of salt impact， the concentration of NaCl solution was gradually increased by 50 mmol/L per day. The various indicators of seedlings were determined through sampling after 20 d of salt stress treatment.
Test indexes and methods
Determination of plant growth and moisture content For each treatment， 3 plants were taken. After rinsed with distilled water and dried the surface， the aerial parts and roots were separated， and the fresh weight （Wf） was weighed. After deactivation at 105 ℃ for 10 min， the seedlings were dried at 80 ℃ to constant weight to weigh the dry weight （Wd）.
Moisture content∥% = （Wf - Wd）/Wf × 100%
Determination of Na+， K+ and Ca2+ contents Ion extraction was performed according to the method of Wang et al.[18]. The samples were divided into three parts： root， stem， and leaf. After drying and crushing， the samples were sieved through a 40mesh sieve. And then 0.1 g of the samples were taken and added with 20 ml of deionized water， and the mixture was boiled for 2 h. After cooling， the mixture was centrifuged at 5 000 r/min for 15 min， and the supernatant was brought to the constant volume of 50 ml. The contents of K+， Na+， and Ca2+ were measured using a GGX600 atomic absorption spectrophotometer. ASX， Na and TSX， Na calculation Ion selective absorption coefficient （ASX， Na） = Whole {[X+]/[Na+]}/medium {[X+]/[Na+]}；
Ion transport coefficient （TSX， Na） = Sink organ {[X+]/[Na+]}/Source organ {[X+]/[Na+]}.
Data processing
Test data were analyzed using SAS statistical analysis software， and Duncan’s multiple comparison method was used for statistical analysis， P<0.05.
Results and Analysis
Effect of salt stress on growth and water content of A. officinalis seedlings
In terms of plant morphology， when NaCl ≤ 100 mmol/L， the A. officinalis seedlings showed no obvious salty damage symptoms. When the NaCl concentration increased to 200 mmol/L， the leaves at the top of seedlings showed the symptoms of chlorosis and yellowing， and when NaCl increased to 300 mmol/L， the leaves at the top of the seedlings were withered. As shown in Table 1， the fresh weights of the roots （r=-0.988， P<0.01） and aerial parts （r=-0.985， P<0.01） of A. officinalis seedlings were negatively correlated with NaCl concentration under NaCl stress， and the difference reached the significant level with the control at NaCl ≥ 100 mmol/L （P<0.05）. There was no significant difference from the control in plant heights， root dry weight and aerial part dry weight at NaCl ≤ 100 mmol/L， but with the increase of salt concentration， plant heights， dry weight of roots and dry weights of the aerial parts gradually decreased and showed significant difference with the control （P<0.05）. The water content in the root and aerial part decreased with the increase of salt stress. The water content of the aerial parts showed significant difference from the control at NaCl ≥200 mmol/L （P<0.05）， but the water content in roots did not reach a significant level in each treatment.
Contents of Na+， K+ and Ca2+ in A. officinalis seedlings under NaCl stress
Content of Na+ As shown in Fig. 1， the Na+ content in A. officinalis seedlings increased with the increase of NaCl concentration. Under low NaCl concentration （≤ 100 mmol/L）， the increase of Na+ content in roots was greater than that in stems and leaves， and in the treatment with 100 mmol/L NaCl， the content of Na+ in roots， stems and leaves increased by 94.6%， 34.9% and 13.1%， respectively， compared with the control. With the increase of salt stress， the Na+ content in stems and leaves increased rapidly， while the Na+ content in roots increased slowly. The NaCl concentration of 100-150 mmol/L was the point when the Na+ of aerial parts changed from slow increase into quick increase. The results indicated that the roots of A. officinalis had a certain ability to retain Na+， and under low salt stress， A. officinalis seedlings could retain Na+ in the roots， which can inhibit their transport to the shoots， thus reducing the damage of salt to the plants. Content of K+， Ca2+ The contents of K+ in roots， stems and leaves of A. officinalis seedlings under NaCl stress were lower than those of the control， and negatively correlated with salt concentration （Fig. 2）， with roots of （r=-0.989， P<0.01）， stems of （r=-0.952， P<0.01）， leaves of （r=-0.964， P<0.01）. At NaCl ≥100 mmol/L， the content of K+ in roots showed significant difference from the control （P<0.05， while the contents of K+ content in stems and leaves showed significant differences from the control at NaCl ≥ 150 mmol/L （P < 0.05）. Aimilar to the changes of K+， the content of Ca2+ decreased with the increase of salt stress. At NaCl ≤ 50 mmol/L， there was no significant difference from the control in Ca2+ content in roots， stems， and leaves， but the differences reached significant level from the control in all other treatments （P<0.05）.
With the increase of NaCl concentration in soil， the contents of K+ and Ca2+ in roots， stems and leaves of A. officinalis seedlings gradually decreased， but they did not show a significant decrease in lowsalt conditions， and the relatively stable ion balance could be maintained. Under high salt stress， the absorption and transport of K+ and Ca2+ were severely inhibited， thus affecting normal plant growth.
Effect of NaCl stress on the selective absorption and transportation of Na+， K+ and Ca2+ in A. officinalis seedlings
As shown in Table 2， under NaCl stress， the selective absorption coefficients ASK，Na and ASCa，Na of Na+， K+ and Ca2+ in the roots of A. officinalis seedlings gradually increased with the increase of salt stress intensity. Under low salt stress， the transport coefficients TSK，Na and TSCa，Na of K+ and Ca2+ from the roots to the aerial parts were significantly higher than those of the control， and both decreased gradually under high salt stress.
Discussion
Salt stress damages plants in many ways， such as ion poisoning， osmotic stress， and nutrient deficiency， thereby impairing the normal metabolism of cells， eventually leading to the obstruction of plants’ growth and even death[2]. The decline of biomass is the most intuitive performance of plants under salt stress[19-20]. In this study， with the increase of salt concentration， the growth of A. officinalis seedlings gradually declines， the plant height decreases， the fresh weight and dry weight drop， and the inhibitory effect of salt stress on the growth of the aerial parts of A. officinalis is greater than that of the root system， which is consistent with the research results of Zhu et al.[9]. Under salt stress， the excessive Na+ in soil affects the absorption， transportation， and distribution of Na+， K+， Ca2+， and Mg2+ ions in soil， and pseudohalophytes generally improve their adaptability to saline and alkaline environments through salt rejection[20]. Sun et al. believe that the interception of Na+ by salttolerant mulberry cultivars in roots limits the transport of Na+ to the aerial parts[21]. In this study， salt stress leads to the accumulation of Na+ in A. officinalis seedlings， and in the lowsalt environment， the root Na+ content increases rapidly， while the accumulation of Na+ in the aerial parts show no great increase， indicating that the interception of Na+ in the roots of A. officinalis limits the transport of Na+ to the aerial parts， thus avoiding the ion poisoning caused by the accumulation of Na+ ions in the aerial parts.
Under salt stress， the competition between Na+， K+ and Ca2+ leads to the decrease of K+ and Ca2+ contents in plants， which has been verified in this study. The contents of K+ and Ca2+ in the roots， stems， and leaves of A. officinalis seedlings all decreases with the increase of NaCl concentration. Under low salt stress， the contents of K+ and Ca2+ in stems and leaves show little decrease， indicating that A. officinalis has a certain adaptability in low salt environment. Ca2+ plays an important role in maintaining the integrity and stability of plant cell membranes[22]； Ron Mittler et al. has demonstrated that Ca2+ together with ABA， ROS， is involved in signal transmission and expression of stress resistance in plants[23]； Heather Knight has also confirmed that endogenous Ca2+ in plants is closely related to salt tolerance， and under salt stress， Ca2+ on the plasma membrane may be replaced by Na+， which reduces the selectivity and stability of the cell membrane， leading to the influx of salt ions and leakage of nutrients.[23].
The ionselective absorption coefficient and transport coefficient can reflect the selectivity of the plant for ion absorption and transport to aerial parts[21， 25]. The results of this study show that with the increase of external salt concentration， K+/Na+ and Ca2+/Na+ in A. officinalis body decreases gradually， which is caused by the increase of Na+ and the decrease of K+ and Ca2+. The absorption capacity of the roots of A. officinalis seedlings to K+ and Ca2+ increases with the increase of external salt concentration， while the transport capacity of K+ and Ca2+ to the aerial parts of plants increases rapidly under low salt stress （NaCl ≤100 mmol/L）， but increases with the increase of external salt concentration. However， TSK，Na， TSCa，Na decreases significantly under high salt stress， which is due to the fact that with the increase of salt ions in the external environment， a large amount of Na+ enters the plant and causes ion toxicity， and the energy consumption of cells for the selective absorption and transportation of ions is greatly increased， leading to metabolic disorders in the body. References
[1]WANG ZQ， ZHU SQ， YU RP. Chinese saline soil[M]. Beijing： Science Press， 1993.
[2]ZHAO KF， FAN H. Halophytes and their adaptation to salty habitats[M]. Beijing： Science Press， 2005.
[3]ZHENG QS， LIU L， LIU YL， et al. Effect of salt and water stresses on osmotic adjustment and osmotica accumulation in Aloe vera seedlings[J]. Journal of Plant Physiology and Molecular Biology， 2003，29（6）： 585-588.
[4]YANG CW， LI CY， ZHANG ML， et al. pH and ion balance in wheat under salt or alkali stress[J]. Chinese Journal of Applied Ecology， 2008，19（5）： 1000-1005.
[5]MUNNS R. Comparative physiology of salt and water stress[J]. Plant， Cell & Environment， 2002，25（2）： 239-250.
[6]YEO A. Molecular biology of salt tolerance in the context of wholeplant physiology[J]. Journal of Experimental Botany， 1998， 49（323）： 915-929.
[7]CARET CT， GRIEVE CM. Mineral nutrition， growth and germination of Antirrhinum majus L. （Snapdragon） when produced under increasingly saline conditions[J]. HortScience， 2008，43（3）： 710-718.
[8]YANG SH， JI J， WNAG G. Effects of salt stress on plants and the mechanism of salt tolerance[J]. World Scitech R & D， 2006，28（4）： 70-76.
[9]ZHU Y， TAN GE， HE CQ， et al. Effect of salinization on growth and ion homeostasis in seedlings of Festuca arundinacea[J]. Acta Ecologica Sinica， 2007，27（12）： 5447-5454.
[10]RAMOLIYA PJ， PATEL HM， PANDEY AN. Effect of salinization of soil on growth and macro and micronutrient accumulation in seedlings of Salvadora persica （Salvadoraceae）[J]. Forest Ecology and Management， 2004， 202（1） ：181-193.
[11]NIU X， BRESSAN RA， HASEGAWA PM， et al. Ion homeostasis in NaCl stress environments[J]. Plant physiology， 1995，109（3）： 735.
[12]ZHU JK. Regulation of ion homeostasis under salt stress[J]. Current opinion in plant biology， 2003，6（5）： 441-445.
[13]WANG JY， ZHANG GH， SU Q， et al. Research advances about the relation between membrane spanned ion transporter and salt tolerance in plants[J]. Acta Botanica BorealiOccidentalia Sinica， 2006，26（3）： 635-640.
[14]SERRANO R， RODRIGUEZNAVARRO A. Ion homeostasis during salt stress in plants[J]. Current opinion in cell biology， 2001，13（4）： 399-404.
[15]NING JF， ZHENG QS， YANG SH， et al. Impact of high salt stress on Apocynum venetum growth and ionic homeostasis[J]. Chinese Journal of Applied Ecology， 2010，21（2）： 325-330.
[16]MIE LC， LI BH， HUANG RH. Advance in green asparagus cultivation in China[J]. Chinese Agricultural Science Bulletin， 2006，22（12）： 204-208. [17]JU YD. Chemical components in Asparagus officinalis and their medical effects[J]. China Horticultural Abstracts， 2011， 2： 125-126.
[18]WANG BS， ZHAO KF. Comparison of extractive methods of Na and K in wheat leaves[J]. Plant Physiology Communication， 1995，31（1）： 50-52.
[19]CHENG YJ， GUO SR， ZHANG RH， et al. Mitigative effect of exogenous Ca（NO3）2 in cucumber seedlings under salt stress[J]. Acta Botanica BorealiOccidetanlia Sinica， 2009，29（9）： 1853-1859.
[20]XIE YZ， HE P， WANG CY， et al. The effects of exogenous CaCl2， SA， and SNP on physiological traits of Cassia obtusifolia L. seedlings under NaCl stress[J]. Journal of Southwest University （Natural Science Edition）， 2013，35（5）： 36-43.
[21]SUN JB， SUN GY， LIU XD， et al. Effects of salt stress on mulberry seedlings growth， leaf water status， and ion distribution in various organs[J]. Chinese Journal of Applied Ecology， 2009，20（3）： 543-548.
[22]LV ZX， WANG ZG. The influence of calcium on distribution of inorganic ions and membrance fatty acids in roots of Triticum aestivum seedlings under salt stress[J]. Acta Phytophysiologica Sinica， 1993，19（4）： 325-332.
[23]MITTLER R. Oxidative stress， antioxidants and stress tolerance[J]. Trends in plant science， 2002，7（9）： 405-410.
[24]KNIGHT H， KNIGHT MR. Abiotic stress signalling pathways： specificity and crosstalk[J]. Trends in plant science， 200，6（6）： 262-267.
[25]GUO CX， WANG WL， ZHENG CS， et al. Effects of exogenous salicylic acid on ions contents and net photosynthetic rate in chrysanthemum under salt stress[J]. Scientia Agricultura Sinica， 2011，44（15）： 3185-3192.
Editor： Na LI Proofreader： Xinxiu ZHU