The effects of hydrogel on growth and ion relationships of a salt resistant woody species, Populus euphratica, were investigated under saline conditions. The hydrogel used was Stockosorb K410, a highly crosslinked polyacrylamide with about 40% of the amide group hydrolysed to carboxylic groups. Amendment of saline soil (potassium mine refuse) with 0.6% hydrogel improved seedling growth (2.7-fold higher biomass) over a period of 2 years, even though plant growth was reduced by salinity. Hydrogel-treated plants had approximately 3.5-fold higher root length and root surface area than those grown in unamended saline soil. In addition, over 6% of total roots were aggregated in gel fragments. Tissue and cellular ion analysis showed that growth improvement appeared to be the result of increased capacity for salt exclusion and enhancement of Ca2+ uptake. X-ray microanalysis of root compartments indicated that the presence of polymer restricted apoplastic Na+ in both young and old roots, and limited apoplastic and cytoplastic Clˉin old roots while increasing Clˉ compartmentation in cortical vacuoles of both young and old roots. Collectively, radical transport of salt ions (Na+ and Clˉ) through the cortex into the xylem was lowered and subsequent axial transport was limited. Hydrogel treatment enhanced uptake of Ca2+ and microanalysis showed that enrichment of Ca2+ in root tissue mainly occurred in the apoplast. In conclusion, enhanced Ca2+ uptake and the increased capacity of P. euphratica to exclude salt were the result of improved Ca2+/Na+ concentration of soil solution available to the plant. Hydrogel amendment improves the quality of soil solutions by lowering salt level as a result of its salt-buffering capacity and enriching Ca2+ uptake, because of the polymer's cationexchange character. Accordingly, root aggregation allows good contact of roots with a Ca2+ source and reduces contact with Na+ and Clˉ, which presumably plays a major role in enhancing salt tolerance of P. euphratica.

The uptake and transport of salt ions (Na+, Clˉ), macronutrients (K+, Ca2+, Mg2+) and abscisic acid (ABA) response to increasing soil salinity were examined in 2-year-old seedlings of Populus euphratica and a hybrid, P. talassica Kom×(P. euphratica+Salix alba L.). Leaf burn symptoms appeared in the hybrid after 8 days of exposure to salinity when soil NaCl concentration increased to 206mM, whereas P. euphratica exhibited leaf damage after day 21 when soil NaCl exceeded 354mM. Leaf necrosis was the result of excess salt accumulation since the injury followed an abrupt increase of endogenous salt levels. Compared with the hybrid, P. euphratica exhibited a greater capacity to exclude salt ions from leaves under increasing salinity, especially Clˉ. Salt treatment altered nutrient balance of the hybrid, leaf K+, Ca2+ and Mg2+ concentrations significantly declined and the same trends were observed in roots with the exception of K+. Although K+ levels decreased in salinised P. euphratica, increasing salinity did not affect the levels of Ca2+ and Mg2+ in leaves, but did increase the uptake of these nutrients when salt stress was initiated. NaCl-induced increase of ABA concentration in xylem sap [ABA] was observed in the two tested genotypes, however xylem [ABA] increased more rapidly in P. euphratica and a fivefold increase of xylem [ABA] was recorded after the first day of exposure to salt stress. Therefore, we conclude that the increase of Ca2+ uptake may be associated with the rise of ABA, and thus contributes to membrane integrity maintenance, which enables P. euphratica to regulate uptake and transport of salt ions under high levels of external salinity in the longer term.

以抗旱耐盐性强的胡杨（Populus euphratica）和非抗盐的群众杨（P. popularis '35-44'）为实验材料，研究了盐胁迫条件下盐分与甜菜碱、还原糖、蔗糖以及水溶性糖等细胞相容溶质的动态变化。两种杨树在盐处理期间表现出明显差异：群众杨下部叶片首先表现出盐害症状，处理后两周苗木上部叶片也出现盐害并脱落。而胡杨在试验期间仅下部叶片发黄脱落，盐处理15天后落叶量仅为16%。群众杨盐害症状的出现主要是由叶片中盐离子的大量累积所致。与之比较，胡杨拒吸Na+的能力及控制Clˉ 转运的能力均优于群众杨。另外，胡杨的耐盐性强于群众杨也与其有机溶质的变化有关。受到盐胁迫后胡杨根叶中甜菜碱浓度显著提高，在处理后15天达到最高值，特别是叶片中甜菜碱的浓度提高了243倍，达到1899.8μ，根中甜菜碱含量也增加了9倍。此外，盐处理后胡杨叶和根中的还原糖、水溶性糖和蔗糖含量均呈明显上升趋势，分别在第4天和第15天达到峰值。与胡杨相反，耐盐性弱的群众杨 在盐胁迫期间，叶中甜菜碱和糖含量并无显著提高，根中糖分水平还明显降低。由此可以得出结论，胡杨渗透调节能力高于群众杨，是其耐盐性强的重要生理基础之一。

研究了渗透胁迫和盐胁迫下一年生胡杨（Populus euphratica Oliv.）幼苗的木质部汁液脱落（ABA）、离子浓度及叶片气体交换的变化。PEG6000（溶液渗透势-0.24MPa）、50mmol/L含钠离子的盐溶液（NaNO3：NaHCO3：NaH2PO4=5：4：1，pH6.8，渗透势-.024MPa）和50mmol/L含氯离子的盐溶液（KCl：NH4Cl=1：1，渗透势-0.24MPa）3种处理都显著降低了苗木的净光合速率（Pn）和蒸腾速率（TRN），但盐处理植株的TRN高于PEG处理的苗木。木质部汁液ABA的浓度在PEG处理后1h达到峰值，之后开始下降，降到对照水平后又逐渐回升。盐处理苗木的ABA也是在处理开始后就迅速升高，但之后ABA水平明显高于PEG处理的植株。结果显示，渗透胁迫和离子胁迫都能提高胡杨木质部汁液ABA的浓度：盐处理开始后ABA的迅速升高主要是渗透胁迫的作用，而此后离子胁迫（Na+和Clˉ）对ABA水平的提高具有重要作用。钠盐处理对胡杨净光合速率和蒸腾速率的抑制作用高于氯盐处理，其木质部汁液中较高水平的ABA和盐离子（Na+和Clˉ）是可能的原因。钠盐处理苗木的盐离子（Na+和Clˉ）水平高于氯盐处理，主要是由以下两方面的原因所致：（1）细胞膜上的Ca2被Na+所取代，增加了膜的透性；（2）胡杨根细胞液泡对Na+的区隔化能力较弱（与区隔Clˉ相比）。另外，盐胁迫下胡杨能保持对营养元素K+、Ca2+和Mg2的吸收，这也是其抗盐性强的重要原因。

Abstract: The effect of a 20-day NaCl treatment on shoot growth, transpiration, ion uptake and transport, and intracellular ion compartmentation was investigated in regenerated plants of Populus euphratica Oliv. and Populus tomentosa Carr. Plants watered with 100 mM NaCl for 8 days and then 200 mM NaCl for 12 days exhibited soil NaCl concentrations of 60 and 95 mM, respectively. Unit transpiration rates and relative growth rates of P. tomentosa were restricted more by salinity as compared with P. euphratica. Salinized P. tomentosa exhibited leaf necrosis whereas no damage was seen in stressed P. euphratica. Compared with P. tomentosa, P. euphratica had considerably lower rates of net root uptake and transport of salt ions (Na+ and Clˉ) to the shoots under salinity. The relatively lower unit transpiration rates of P. euphratica and the lower salt concentrations in the xylem of salinized P. euphratica contribute to its greater capacity for salt exclusion. X-ray microanalysis showed that P. euphratica had a greater ability to restrict radial salt transport in roots by blocking apoplasmic salt transport and sequestering more Clˉin cortical vacuoles. In addition, P. euphratica maintained higher K+ uptake and transport than P. tomentosa in the presence of high external Na+ concentrations. Résumé: L'effet d'un traitement au NaCl appliqué pendant 20 jours sur la croissance des pousses, la transpiration, l'absorption et le transport des ions et le compartimentage intracellulaire des ions a été étudié sur des plants régénérés de Populus euphratica Oliv. et Populus tomentosa Carr. Les plants arrosés avec 100 mM de NaCl pendant 8 jours et par la suite avec 200 mM de NaCl pendant 12 jours avaient des concentrations de NaCl dans le sol de respectivement 60 et 95 mM. Le taux de transpiration unitaire et le taux de croissance relative de P. tomentosa étaient davantage inhibés par la salinité comparativement à P. euphratica. L'exposition au sel a provoqué des nécroses foliaires chez P. tomentosa mais non chez P. euphratica. Comparativement à P. tomentosa, P. euphratica avait des taux d'absorption racinaire nette et de transport des ions de sel (Na+ et Clˉ) vers les pousses considérablement plus faibles en conditions de salinité. Le taux de transpiration unitaire et la concentration de sel dans le xylème relativement plus faibles chez P. euphratica exposé au sel contribuent à sa plus grande capacité d'exclure le sel. Des analyses microscopiques aux rayons X montrent que P. euphratica peut mieux restreindre le transport radial du sel dans les racines en bloquant le transport du sel dans l'apoplasme et en séquestrant plus de Clˉdans les vacuoles des cellules corticales. De plus, P. euphratica absorbe et transporte plus de K+ que P. tomentosa en présence de concentrations élevées de Na+ dans le milieu externe.

We designed two experiments to investigate the osmotic stress and ion-specific effects on xylem abscisic acid (ABA) and the relevance to salinity tolerance in one-year-old seedlings of Populus euphratica Oliv. (a salt-resistant genotype) and one-year-old rooted cuttings of P. 'popularis 35-44' (P. popularis) (a saltsensitive genotype). Net photosynthetic rates (Pn) and unit transpiration rates (TRN) of the two genotypes were significantly decreased upon osmotic shock caused by PEG 6000 (osmotic potential=-0.24 MPa) or iso-NaCl (50 mM). Shoot xylem ABA concentrations in both genotypes increased rapidly after the onset of PEG stress, resulting from a decreased water flow. NaCl-treated trees of P. euphratica maintained considerably greater concentrations of ABA than PEG-treated plants in a longer term, whereas salinized P. popularis exhibited a transient accumulation of ABA in the shoot. TRN was greatly enhanced in both genotypes when pressure (0.24 MPa) was applied to counteract the osmotic suction of 50 mM NaCl. Pressurizing of root systems diluted solutes in the root xylem, but the dilution effect was more pronounced in P. popularis. Root xylem ABA concentrations in P. euphratica steadily increased with salt stress although pressurization lowered its levels. In contrast, there were no observed changes in ABA response to salinity in pressured P. popularis. Therefore, we concluded that the salt-tolerant P. euphratica had a greater capacity to synthesize ABA under saline conditions, which may partially result from specific salt effects. In addition, P. euphratica exhibited a higher capacity for salt (Na+ and Cl-) transport control under salt stress, compared with P. popularis. The possible association between ABA and salt transport limitation, and the relevance to salinity tolerance were discussed.

We investigated the effects of endogenous and exogenous abscisic acid (ABA) on polyamines (PAs) and ethylene synthesis and their relevance to leaf senescence in 1-year-old intact cuttings of a drought-sensitive poplar genotype Populus×euramericana cv. I-214 (cv. Italica) and a drought-tolerant genotype P. popularis 35-44 (P. popularis). P. popularis exhibited a transitory and moderate increase in xylem ABA concentrations in response to water stress and no leaf abscission during the period of drought (30% of field capacity; the soil water potential was-2.108 MPa). In contrast, leaf shedding occurred in stressed cv. Italica following a sustained increase in xylem ABA concentrations (up to 1.73 μM). Application of ABA to the transpiration stream accelerated leaf abscission in both genotypes with a threshold of approximately 1.7-1.8μM. Initial elevation of xylem ABA concentrations reduced PA levels but enhanced ethylene synthesis simultaneously at the onset of water stress or the ABA treatment. Moreover, sharp increases in ABA concentrations, i. e. over 1.7µM, severely restricted PA synthesis in aged leaves in the longer term (within 3 days). Leaves abscised following a progressive decline of PA levels, when putrescine decreased below approximately 0.5 µmol g-1 fresh weight, and spermidine and spermine became almost undetectable by HPLC. Therefore, we concluded that a drastic reduction in the level of PAs might increase the sensitivity of the leaf to ethylene, thus accelerating defoliation, even though the ethylene emission of stressed plants returned to prestress values by day 3. Young leaves were not shed during the period corresponding to the increase in xylem ABA, which appeared to result from the lesser reductions of PA. Compared with P. popularis, the inhibitory effect of ABA on PA synthesis was more pronounced in the drought-sensitive genotype cv. Italica. On the other hand, cv. Italica plants typically produced more ethylene than P. popularis. Taken together, these observations might explain our previous finding that ABA-treated plants of cv. Italica experienced more leaf loss than P. popularis (Chen et al. 1997).