Osmotic Stress and Ion-Specific Effects on Xylem Abscisic Acid and the Relevance to Salinity Tolerance in Poplar

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 salt-sensitive 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.     

Growth, Gas Exchange, Abscisic Acid, and Calmodulin Response to Salt Stress in Three Poplars

In the present study, we Investigated the effects of Increaslng sallnlty on growth, gas exchange, absclslc acld （ABA）, calmodulln （CAM）, and the relevance to salt tolerance In seedllngs of Populus euphratlca Ollv. and cutUngs of P. ＂pupularls 35-44＂ （P. popularls） and P. x euramerlcana cv. 1-214 （P. cv. Itallca）. The relatlve growth rates of shoot helght （RGR,） for P. cv. Itallca and P. popularls were severely reduced by Increaslng salt stress, whereas the growth reductlon was relatlvely less in P. euphratica. Slmllarly, P. euphratlca malntalned hlgher net photosynthetlc rates （Pn） and unlt transplration rotes （TRN） than P. cv. Itallca and P. popularls under condltlons of hlgher sallnlty. Sallnity caused a slgnlficant increase In leaf ABA and CaM In the three genotypes after the onset of stress, but NaCl-induced ABA and CaM accumulatlon was more pronounced In P. euphratlca, suggeeUng that P. euphratlca plants are more sensitlve in sensing soil salinlty than the other two poplars. Furthermore, P. euphratica maintained relatively higher ABA and CaM concentrations under conditions of high salinity. The higher capacity to synthesize stress signals, namely ABA and CaM, In P. euphratica and the contrlbuUon of this to the salt resistance of P. euphratica are discussed.     

盐胁迫条件下杨树盐分与甜菜碱及糖类物质变化

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Cl-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1*!899.8 μmol/L,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。     

胡杨木质部水分传导对盐胁迫的响应与适应北大核心CSCD

解析植物木质部导水率对逆境的响应和适应对促进植物抗逆性机理研究和受损植被恢复具有重要意义。该文以荒漠河岸林建群种胡杨(Populus euphratica)为研究对象,系统分析了胡杨幼株根、茎、叶水分传输通道对不同浓度盐胁迫的响应和适应。结果表明:(1)胡杨幼株根系对盐胁迫的敏感性高于茎和叶,盐胁迫下根系生长和根尖数显著受到抑制,根木质部易于发生栓塞,导水率明显降低。(2)胡杨幼株茎木质部导水率对盐胁迫的响应依盐浓度而定,轻度(0.05 mol·L–1 Na Cl)和中度(0.15 mol·L–1 Na Cl)盐胁迫下,胡杨可以通过协调导管输水的有效性和安全性来调节木质部的导水率,维持植物正常生长;重度(0.30 mol·L–1 Na Cl)盐胁迫下,胡杨茎木质部导管输水有效性和安全性均明显降低,木质部导水率显著下降,并伴随叶片气孔导度的显著降低,从而严重抑制了胡杨的光合和生长。     

Drought tolerance, abscisic acid and electrolyte leakage in fast-and slow-growing black spruce (Picea mariana) progenies

To investigate the role that drought tolerance plays in growth, abscisic acid (ABA) accumulation and electrolyte leakage during water stress were compared in fast- and slow-growing black spruce ( Picea mariana [Mill.] B. S. P.) progenies. Changes in the ABA content of the needles were quantified using an indirect enzyme-linked immuno-sorbent assay validated by gas chromatography electron capture detection. Needle electrolyte leakage was estimated using a conductivity bridge. Seedlings were stressed using (1) osmotic stress, induced by a stepwise increase in concentrations of polyethylene glycol 3 350 (PEG) for ABA study and (2) air drying for electrolyte leakage study. Progenies did not differ in ABA levels under unstressed conditions, but progeny differences were observed under osmotic stress. Needle ABA content increased up to 500% under osmotic stress. Slow-growing black spruce progenies (25 and 46) accumulated more ABA under moderate (18% PEG), but not severe (25% PEG), osmotic stress. The slow-growing progenies also leaked more electrolytes under moderate to severe water stress and lost 50% electrolytes at a higher xylem tension, suggesting they suffered more injury and were less dehydration tolerant. Our previously-published results showed that slow-growing progenies lost their photosynthesis and stomatal conductance more quickly during osmotic stress and recovered more slowly after rehydration. Therefore, tolerance of dehydration leading to a maintenance of physiological integrity during drought stress could explain the fast growth rates of more vigorous black spruce progenies.     

Salt, nutrient uptake and transport, and ABA of Populus euphratica; a hybrid in response to increasing soil NaCl

The uptake and transport of salt ions (Na⁺, Cl^-), 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 2 (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 206 mM, whereas P. euphratica exhibited leaf damage after day 21 when soil NaCl exceeded 354 mM. 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 Cl^-. 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.     

Adaptation to high salinity in poplar involves changes in xylem anatomy and auxin physiology

To investigate the physiological basis of salt adaptation in poplar, we compared the effect of salt stress on wood anatomy and auxin physiology of the salt-resistant Populus euphratica and salt-sensitive Populus x canescens. Both poplar species showed decreases in vessel lumina associated with increases in wall strength in response to salt, however, in P. euphratica at three-fold higher salt concentrations than in P. x canescens. The predicted hydraulic conductivity of the wood formed under salt stress decreased in P. x canescens, while in P. euphratica, no significant effects of salt on conductivity and transpiration were observed. The concentration of free indole-3-acetic acid (IAA) decreased under salt stress in the xylem of both poplar species, but to a larger extent in P. x canescens than in P. euphratica. Only salt-treated P. euphratica exhibited an increase in IAA-conjugates in the xylem. Genes homologous to the auxin-amidohydrolase ILL3 were isolated from the xylems of P. euphratica and P. x canescens. For functional analysis, the auxin-amidohydrolase from P. x canescens was overexpressed in Arabidopsis. Transgenic Arabidopsis plants were more resistant to salt stress than the wild-type plants. Increased sensitivity of the transgenic Arabidopsis to IAA-Leu showed that the encoded hydrolase used IAA-Leu as a substrate. These results suggest that poplar can use IAA-amidoconjugates in the stem as a source of auxin to balance the effects of salt stress on auxin physiology.     

Na~ and Water Uptake in Relation to the Radial Reflection Coefficient of Root in Arrowleaf Saltbush Under Salt Stress

The response of halophyte arrowleaf saltbush(Atriplex triangularis Willd)plants to a gradient of salt stress were investigatedwith hydroponically cultured seedlings.Under salt stress,both the Na~ uptake into root xylem and negative pressures inxylem vessels increased with the elevation of salinity(up to 500 mol/m~3)in the root environment.However,the increment innegative pressures in root xylem far from matches the decrease in the osmotic potential of the root bathing solutions,evenwhen the osmotic potential of xylem sap is taken into consideration.The total water potential of xylem sap in arrowleafsaltbush roots was close to the osmotic potential of root bathing solutions when the salt stress was low,but a progressivelyincreased gap between the water potential of xylem sap and the osmotic potential of root bathing solutions was observedwhen the salinity in the root environment was enhanced.The maximum gap was 1.4 MPa at a salinity level of 500 mol/m~3without apparent dehydration of the tested plants.This discrepancy could not be explained with the current theories inplant physiology.The radial reflection coefficient of root in arrowleaf saltbush decreased with the enhanced salt stress wasand accompanied by an increase in the Na~ uptake into xylem sap.However,the relative Na~ in xylem exudates based onthe corresponding NaCl concentration in the root bathing solutions showed a tendency of decrease.The results showedthat the reduction in the radial reflection coefficient of roots in the arrowleaf saltbush did not lead to a mass influx of NaClinto xylem when the radial reflection coefficient of the root was considerably small;and that arrowleaf saltbush could usesmall xylem pressures to counterbalance the salt stresses,either with the uptake of large amounts of salt,or with thedevelopment of xylem pressures dangerously negative.This strategy could be one of the mechanisms behind the highresistance of arrowleaf saltbush plants to salt stress.     

胡杨细胞和组织结构与其耐盐性关系的研究

The characteristics of the cell and organ structures of Populus euphratica Oliv. in relation to salt and osmotic tolerance were compared with those of P. tomentosa Cart. in vitro under the electron and light microscopic observation. P. euphratica exhibited characteristic structure which was associated with salt stress. It had well-developed epidermis and exodermis in the root tip and poorly developed conducting tissue in leaf. Root hairs were formed closer to the root tips. AsP. euphratica were stressed with salt and PEG, more abundance of chondriosomes and plastids in the cytoplasm and more containing substance in the plastid were observed and the osmophilic substance was obviously displayed in the cytoplasm and in the posterior margin of the vacuole. The filamentous structure, bigger nucleus and nucleolus were visualized in the stressed suspension-cultured cells of P. euphratica. The meristemic cells in the root tip of P. euphratica could maintain their structure when the plant was subjected to 8 g/L NaCl stress. It was also demonstrated that the cell wall and plasmalemma of P. euphratica were tightly combined as a dentate form, explaining why the cell could endure severe salt or osmotic stress and resist to plasmolysis indicating that P. euphratica possesses a solid structure base as a defense to salt stress.     

Na^＋ and Water Uptake in Relation to the Radial Reflection Coefficient of Root in Arrowleaf Saltbush Under Salt Stress

The response of halophyte arrowleaf saltbush (Atriplex triangularis Willd) plants to a gradient of salt stress were investigated with hydroponically cultured seedlings. Under salt stress, both the Na+ uptake into root xylem and negative pressures in xylem vessels increased with the elevation of salinity (up to 500 mol/m3) in the root environment. However, the increment in negative pressures in root xylem far from matches the decrease in the osmotic potential of the root bathing solutions, even when the osmotic potential of xylem sap is taken into consideration. The total water potential of xylem sap in arrowleaf saltbush roots was close to the osmotic potential of root bathing solutions when the salt stress was low, but a progressively increased gap between the water potential of xylem sap and the osmotic potential of root bathing solutions was observed when the salinity in the root environment was enhanced. The maximum gap was 1.4 MPa at a salinity level of 500 mol/m3 without apparent dehydration of the tested plants. This discrepancy could not be explained with the current theories in plant physiology. The radial reflection coefficient of root in arrowleaf saltbush decreased with the enhanced salt stress was and accompanied by an increase in the Na+ uptake into xylem sap. However, the relative Na+ in xylem exudates based on the corresponding NaCl concentration in the root bathing solutions showed a tendency of decrease. The results showed that the reduction in the radial reflection coefficient of roots in the arrowleaf saltbush did not lead to a mass influx of NaCl into xylem when the radial reflection coefficient of the root was considerably small; and that arrowleaf saltbush could use small xylem pressures to counterbalance the salt stresses, either with the uptake of large amounts of salt, or with the development of xylem pressures dangerously negative. This strategy could be one of the mechanisms behind the high resistance of arrowleaf saltbush plants to salt stress.     

胡杨细胞和组织结构与其耐盐性关系的研究

利用电子显微镜和光学显微镜中相差和微分干涉等技术对胡杨（ＰｏｐｕｌｕｓｅｕｐｈｒａｔｉｃａＯｌｉｖ．）悬浮培养的细胞和试管苗的组织和细胞结构以及盐胁迫对其结构影响进行了研究。与毛白杨（Ｐ．ｔｏｍｅｎｔｏｓａＣａｒ．）相比,胡杨存在与耐盐相关的结构,如根尖具有较发达的表皮和外皮层,根毛着生靠近根端;叶片输导组织不发达;细胞中线粒体和质体较丰富。盐胁迫和渗透胁迫可使胡杨细胞质中的线粒体和质体变得更为丰富,质体中内含体增多,在细胞质中和液泡内缘出现明显的嗜锇物质,在悬浮培养的细胞内表现出明显的丝状结构,细胞核变大,核仁明显。受到８ｇ／ＬＮａＣｌ胁迫的胡杨根尖分生细胞仍可维持正常的结构。在超微结构研究中还发现,胡杨细胞膜与细胞壁之间呈齿状结合,说明了膜与壁之间结合的牢固性和稳定性,解释了胡杨细胞在胁迫中不易发生质壁分离的原因。     

Implication of ABA and proline on cell membrane injury of water deficit stressed barley seedlings

The aim of this work was to examine the ability of ABA and proline to counteract the deleterious effect of water deficit stress on cell membrane injuries. Six-day-old seedlings of two barley genotypes (cv. Aramir, line R567) were treated with ABA (2·10⁻⁴ M) or proline (0.1 M) for 24 h, and then subjected to osmotic stress for 24h, by immersing their roots in polyethylene glycol (PEG 6000) solution of osmotic potential of −1.0 MPa and −1.5 MPa or by submerging the leaf pieces in PEG solution of osmotic potential of −1.6 MPa. Pretreatment of plants with ABA and proline caused an increase of free proline level in the leaves. Plants treated with ABA exhibited a lower membrane injury index under water stress conditions than those untreated even when no effect of this hormone on RWC in the leaves of stressed plants was observed. Pretreatment of plants with proline prevented to some extent membrane damage in leaves of the stressed seedlings, but only in the case when stress was imposed to roots. Improvement in water status of leaves was also observed in seedlings pretreatment with proline. The protective effect of both ABA and proline was more pronounced in line R567 that exhibited higher membrane injury under water deficit stress conditions.     

Stress induction of abscisic acid in maize roots

Moderate water stresses in the range 0 to −0.6 MPa applied with PEG 6000 to excised roots of Zea mays L. var. LG 11 induced increases of up to four-fold in the amount of abscisic acid (ABA) determined in the tissue after a 12 h period of xylem exudation. The ABA concentration of xylem exudate collected after a 2 h water stress also increased by up to four-fold. Salt stresses, induced with NaCl solutions, resulted in similar increases in the ABA concentrations. ABA concentrations in both root tissue and xylem exudate were highest 4 h after removal of the stress and then declined over a subsequent 8 h period. These results are interpreted in support of the concept that root-produced ABA may have a role in the fine control of the plant's water balance.     

等渗的盐分和水分胁迫对杠柳种子萌发的影响

通过等渗的NaCl和PEG溶液模拟盐分和水分胁迫,设置渗透梯度,在控制条件下对杠柳种子的萌发过程中总萌发率、幼苗鲜重、胚根生长、种子活力、发芽值等指标系统研究,对种子萌发率与渗透势之间关系进行回归分析,主要结果包括：（1）杠柳种子萌发过程中总萌发率、幼苗鲜重、活力指数和发芽值四项指标均随NaCl和PEG溶液的渗透势降低逐渐下降,综合研究活力指数和发芽值表明渗透势≥-0.5 MPa和≤-1.4 MPa时,PEG对种子萌发抑制作用大于NaCl,其他情况相反;（2）杠柳种子逐日萌发率和胚根日变化研究表明,与NaCl相比,PEG推迟杠柳种子萌发,并且对胚根增长抑制作用较大;（3）建立盐分和水分胁迫条件下种子萌发率与渗透势回归方程,发现杠柳种子在PEG胁迫下的萌发临界值和极限值为-1.0和-1.4 MPa,在NaCl胁迫下是-0.9和-1.3 MPa,解除胁迫条件,不同处理的杠柳种子复水萌发率均达到100%。说明杠柳具有良好的耐盐抗旱的特性。     

Effect of water and salt stresses on growth, chlorophyll, mineral ions and organic solutes contents, and enzymes activity in mung bean seedlings

The experiment was made by using different concentrations of polyethylene glycol (PEG) or salt solutions to decrease the osmotic potential of the growth medium to reveal the response of mung bean (Vigna radiata) to water and salt stresses. No germination (emergence of the seedling) occurred at medium osmotic potential lower than -1.0 MPa in all treatments. It was found that the activity of α-amylase and protease, and contents of proline, saccharides and the soluble proteins decreased in the germinating seeds during 3-d stress. However, after 10-d stress, the contents of organic solutes and the activity of the hydrolytic enzymes increased. Growth, chlorophyll content and mineral uptake were also significantly reduced under stress. The seedlings under water stress induced by PEG were affected much more than under salinity. This may be due to the maintenance of a higher succulence under salt stress than under PEG-induced water stress. This revised version was published online in August 2006 with corrections to the Cover Date.     

Genetic Variability in Recovery Growth and Synthesis of Stress Proteins in Response to Polyethylene Glycol and Salt Stress in Finger Millet

Marked differences were found among 28 finger millet genotypes(Eleusine coracana Gaertn.) in acquired tolerance to osmoticstress as assessed by the recovery of root growth from severestress [-1·2 MPa polyethylene glycol, (PEG) or 400 mMNaCl]. However, these differences in tolerance were observedonly when the seedlings were subjected to a preceding mild inductionstress (-0·6 MPa PEG or 200 mM NaCl). In two contrastinggenotypes, synthesis of stress-induced proteins was studied.Proteins with apparent molecular weight of 70-72, 52, 37, 34and 23 kDa were synthesized in the highly responsive genotype(GE 415) and poorly responsive (VL 481) genotype following amild induction stress (200 mM NaCl). However, GE-415 synthesizeda 54 kDa protein that was not observed in VL-481. Addition ofabscisic acid (ABA) to the induction medium containing 200 mMNaCl enhanced the acquired tolerance of finger millet seedlingsover those without ABA in association with the appearance ofseveral ABA-responsive proteins. GE-415 required much less ABAthan VL-481 to obtain the same response. With 10 µM ABA+ 200 mM, A NaCl induction stress, GE-415 had significantlyhigher endogenous ABA. In association with higher levels ofABA, GE-415 had greater recovery root growth following severestress from 600 mM NaCl. Pretreatment with 10 µM ABA +200 mM NaCl induced several proteins with apparent molecularweights of 70-72, 54, 45, 36, 29 and 21 kDa in both genotypes.Qualitatively, GE-415 synthesized a unique 23-24 kDa proteinand quantitatively there was significantly more of the 21 kDaprotein in GE-415 compared to VL-481. The results indicate thatthe synthesis of stress proteins is correlated with the observedvariation in acquired tolerance of the two genotypes.Copyright1995, 1999 Academic Press Eleusine coracana Gaertn., salinity, polyethylene glycol, stress proteins, ABA, ABA-responsive proteins, finger millet seedlings     

Transport, synthesis and catabolism of abscisic acid (ABA) in intact plants of castor bean (Ricinus communis L.) under phosphate deficiency and moderate salinity

Flows of abscisic acid (ABA) were investigated in whole plantsof castor bean (Ricinus communis) grown in sand culture undereither phosphate deficiency or moderate salinity. Xylem transportof ABA in P-deficient plants was stimulated by a factor of 6whereas phloem transport was affected only very slightly. ABAdeposition into leaves of P-deficient plants was not appreciablydifferent from the controls because of strong net degradationin leaves. Since conjugation of ABA was strongly reduced inall organs of P-deficient plants ABA was presumably metabolizedmainly to phaseic acid and dihydrophaseic acid. The increasedimport of ABA occurred predominantly into fully differentiatedbut not senescent leaves and showed a good correlation withthe inhibition of leaf conductance under P deficiency. As with low-P-plants salt stress increased ABA synthesis inroots and associated transport in the xylem. However, salinitycaused a distinctly greater accumulation of ABA in the leaves,stem segments and the apex than in P-deficient plants. As opposedto P deficiency, ABA export in the phloem from the leaves wasstimulated by salinity. Modelling of ABA flows within an individualleaf over its life cycle showed that young growing leaves importedABA from both phloem and xylem, whereas the adult non-senescentleaves were a source of ABA and thus provided a potential shoot-to-rootstress signal as well as an acceptor for reciprocal signalsfrom root to shoot. In senescing leaves ABA flows and accumulationwere somewhat retarded and ABA was lost in net terms by exportfrom the leaf. Key words: Abscisic acid, phosphorus deficiency, salt stress, phloem and xylem transport     

The Method of ABA Application Affects Salt Stress Responses in Resistant and Sensitive Potato Lines

The phytohormone abscisic acid (ABA) has been proposed to act as a mediator in plant responses to a range of stresses, including salt stress. Most studies of ABA response apply ABA as a single dose. This may not resemble the prolonged increasing endogenous ABA levels that can occur in association with slowly increasing salinity stresses in nature or field situations. Salt stress response based on method of ABA application was examined in four potato genotypes of varying salt stress resistance: the sensitive ABA-deficient mutant and its normal sibling, a resistant genotype line 9506, and commercial cultivar ‘Norland’ of moderate resistance. ABA was applied by root drench at 0, 50, 75, or 100 μM concentrations through a single dose, or by slowly increasing multiple ABA doses in a sand-based growing system under greenhouse conditions. Salt tolerance was then evaluated after 2 weeks of exposure to 150–180 mM NaCl stress. The method of ABA application had a marked effect on the responses to salt stress. Plant responses to the method of ABA application were differentiated according to (1) growth rate, (2) root water content, and (3) apparent shoot growth response. Under a single dose, growth rate increased in all genotypes under salt stress, whereas slowly increasing multiple ABA applications generally maintained stable growth rates except in the ABA-deficient mutant where there was an upward growth trend. Percent root water content was elevated only under slowly increasing multiple ABA doses in two genotypes, whereas none of the single-dose treatments induced any change. The single ABA dose enhanced vertical growth, whereas the slowly increasing multiple ABA dose applications enhanced lateral shoot growth. Because exogenous application is still an artificial system, endogenous ABA was supplied through grafting of ABA-deficient mutant scions onto rootstocks with known elevated ABA levels. Multiple exogenous ABA applications as low as 50 μM elicited similar shoot water content responses as grafting treatments without ABA application in the mutant genotype but had no effect on the ABA normal sibling. Shoot dry weight was significantly increased through grafting over all exogenous ABA treatments. Our study further indicates that the method of ABA application regime in itself can alter plant responses under salt stress and that certain application regimes may reflect responses to elevated endogenous levels of ABA.