盐胁迫对大豆根系木质部压力和Na+吸收的影响

取栽培大豆的水培幼苗为材料,用木质部压力探针和原子吸收分光光度计测定了盐胁迫条件下其根木质部压力和伤流液中Na~+含量的变化,以分析大豆抗盐吸水的机制.结果表明:在25～150 mmol/L NaCl的浓度范围内,随着盐胁迫强度的增加,大豆根木质部负压力的绝对值逐渐增大,但相对负压力和根的径向反射系数则逐渐减小;木质部伤流液中Na~+含量逐渐增加,但Na~+的相对含量则逐渐降低.同时,虽然根系吸水所需的木质部负压力(压力势)及根木质部伤流液的渗透势随着盐胁迫强度的增加都有所下降,但两者共同作用使木质部水势下降的幅度远远小于根外溶液水势(渗透势)下降的幅度,即随着根外溶液盐浓度的升高,根木质部溶液的总水势逐渐高出根外溶液的水势.上述结果说明,在盐胁迫下大豆可以利用相对小的木质部负压力逆水势梯度吸水,且通过避免对Na~+的过量吸收来适应盐胁迫环境.     

土壤盐分对三角叶滨藜抗旱性能的影响

本实验用不同浓度NaCl溶液浇灌后进行干旱处理,测定了不同处理条件下三角叶滨藜幼苗在干旱持续过程中植株生长状况、叶片光合作用、抗氧化酶活性以及植株水分状况等的变化,以分析土壤盐分对三角叶滨藜耐旱性的影响。结果显示,用0.1—0.4 mol/L NaCl溶液浇灌后进行干旱处理（“干旱 NaCl”）的三角叶滨藜幼苗在干旱持续期间植株生长量、叶片净光合速率均明显高于用水浇灌后进行干旱处理（“干旱”）的三角叶滨藜幼苗,且后者在干旱处理的后期导致了叶片光合机构的明显损伤,前者则无。同时,“干旱 NaCl”处理的植株叶片超氧化物歧化酶、过氧化物酶和过氧化氢酶活性的变化幅度明显小于“干旱”处理的,且前者的丙二醛含量显著低于后者。进一步分析各处理土壤和植株水分状况发现,与“干旱”处理相比,“干旱 NaCl”处理显著提高了土壤的保水能力,增加了植株对Na 的吸收和积累,降低了叶片渗透势。由此可见,土壤中适量盐分存在能够增加三角叶滨藜对Na 的吸收和积累、降低组织渗透势、维持较强的吸水和保水力,从而减弱因干旱脱水导致的过量活性氧自由基产生对细胞膜系统的损伤和对光合机构的破坏,有利于维持相对较高的物质生产能力和细胞的持续生长,最终表现为耐旱性能的增强。     

NaCl对渗透胁迫下三角叶滨藜光合作用和水分状况的调节

以溶液培养的三角叶滨藜(Atriplex triangularis)为材料, 测定分析了在PEG诱导的渗透胁迫条件下, 适量的NaCl对其光合作用和水分吸收的影响, 以探讨环境溶液中NaCl对植物适应干旱的影响。结果表明, PEG诱导的渗透胁迫导致三角叶滨藜植株吸水困难、叶绿素含量降低、光合系统受损、生长受抑制、生物量减少; 而在PEG渗透胁迫的处理液中添加10–40 mmol·L^–1NaCl可以明显降低植株水势和叶片渗透势, 维持较高的细胞膨压, 减缓PEG渗透胁迫对光合系统的破坏作用, 保证相对较高的光合速率和生长速度, 从而有效增强了三角叶滨藜对渗透胁迫的适应能力。     

盐胁迫对三角叶滨藜根选择透性和反射系数的影响

以Hoagland溶液培养的三角叶滨藜幼苗为材料,分别采用电导率法、原子吸收分光光度计法和压力室法测定了不同浓度盐胁迫下三角叶滨藜根的质膜透性、离子吸收和根系反射系数,分析其抗盐特点和机制.结果表明:随着盐胁迫强度的增加,三角叶滨藜根细胞质膜透性增大、根系反射系数减小;盐胁迫导致三角叶滨藜根系对K+的总吸收量减少、对Na+的总吸收量增多,但对Na+的相对吸收量减少、对K+的相对吸收量增加.盐胁迫条件下,三角叶滨藜根系对离子吸收有较强的调节能力;而根系反射系数的减小有利于根系用较小的负压力吸收水分,减小木质部空化的危险.说明三角叶滨藜具有较高的抗盐能力.     

等渗盐分与水分胁迫对三角叶滨藜和玉米光合作用的影响

以溶液培养的三角叶滨藜(Atriplex triangularis)和玉米(Zeamays)为材料,测定了等渗的盐分和水分胁迫对2种植物光合作用的短期影响。结果表明:等渗的水分和盐分胁迫均会造成三角叶滨藜和玉米净光合速率(Pn)的降低,而且随着胁迫程度的增强,水分胁迫引起Pn下降的幅度要明显高于等渗的盐分胁迫;在较低渗透胁迫强度下,2种胁迫导致光合速率下降的主要原因是气孔限制;但在环境溶液渗透势为-1.0MPa时,水分胁迫对光合作用的影响逐渐转化成非气孔限制,而盐胁迫仍然是气孔限制起主要作用;由此可见,等渗透势的水分胁迫对2种植物光合系统的影响要明显大于盐分胁迫。     

NaCl胁迫对滨藜生长及其根和叶中无机离子含量的影响

用不同浓度的NaCl溶液处理滨藜（Atriplex undulata）植株后发现：100mmol/L NaCl促进滨藜地上部及根部生物量积累,高于100mmol/l NaCl,抑制其生长。各部位的Na^ 和Cl^-含量随着NaCl浓度的升高明显增加。NaCl处理下,叶片的Na^ 和Cl^-含量高于根部,幼叶高于老叶,且叶片的K^ （除500mmol/L NaCl处理的老叶外）,根部的K^ 、Ca^2 和NO3^-无显著变化。叶片的Na/K比随NaCl浓度的升高而增加,但较高浓度时下降（在老叶中）或维持稳定（在幼叶中）。随着处理浓度的增加,各部位对K^ 的选择性（相对于Na^ ）增加,渗透调节能力增强,渗透势降低。另外,还讨论了滨藜对盐渍生境的适应特征。     

盐胁迫下盐芥渗透调节物质的积累及其渗透调节作用

用含有NaCl0、50、100、200、300、400mmol/L的Hoagland培养液处理盐芥幼苗一定时间后,分别测定其根和叶含水量、渗透势、几种无机和有机渗透调节物质含量,并计算了渗透调节物质在不同条件下的计算渗透势值(COP).结果表明:随盐处理浓度的增加,盐芥根和叶的含水量和渗透势逐渐降低;Na 和Cl-是根和叶积累的无机渗透调节物质;SS、OA和FAA是根积累的有机渗透调节物质,Pro是叶和根积累的有机渗透调节物质.Na X-ray微区分析表明液泡是积累Na 的主要部位.     

等渗的盐分和水分胁迫对芦荟幼苗生长和离子分布的效应

 研究了等渗透势（-0.44、-0.88 MPa）NaCl和PEG 6000处理对六叶龄芦荟(Aloe vera)幼苗叶片生长速率、干物质积累、电解质渗漏和离子吸收、分配的效应。结果表明: -0.44、-0.88 MPa NaCl和PEG处理10 d均明显抑制芦荟幼苗叶片伸长生长,植株干物质积累速率显著降低, 叶片含水量降低,叶片细胞电解质渗漏率上升。NaCl对芦荟幼苗生长的抑制作用显著大于PEG处理的。不同器官离子含量、根系和叶片横切面X-射线微区分析结果表明, NaCl胁迫导致芦荟体内Na+、Cl－含量显著上升,根中增幅明显高于叶片,其中Cl－尤为显著。NaCl胁迫严重抑制芦荟对K+ 和Ca2+ 的吸收及其向叶片的运输,根、叶K+/Na+、Ca2+/Na+ 比率显著下降,而PEG胁迫对离子平衡的干扰较轻,是芦荟对水分胁迫的适应能力高于盐胁迫的主要原因之一。但芦荟对 -0.44～-0.88 MPa NaCl胁迫仍有一定的适应能力,主要原因是：1) 根系对离子的选择性吸收和运输较强,并随着盐胁迫强度增加其选择性增强; 2) 芦荟叶片中的盐分在贮水组织中显著积累,明显高于其它组织细胞。同时,芦荟是CAM（景天酸代谢）途径植物,蒸腾极小,盐分随蒸腾流进入地上部的机会小。     

不同光照强度下三角叶滨藜光合作用对盐激胁迫的响应

以溶液培养的三角叶滨藜植株为材料研究了不同光照条件下其叶片光合作用对盐(NaCl)激胁迫的即刻反应及变化规律.结果表明,三角叶滨藜光合作用对盐激胁迫的响应有8 min左右的滞后期.在光照强度为100umol·m-2·-1和100 mmol·L-1浓度NaCl共同作用下,三角叶滨藜叶片净光合速率略有上升;但随NaCl浓度和光照强度进一步增加,其净光合速率呈下降趋势,且光照越强,盐胁迫导致的净光合速率下降幅度越大.同时,弱光下或强光低浓度NaCl胁迫下,盐激胁迫导致的净光合速率下降主要是气孔限制引起的;而强光下,高浓度的NaCl胁迫导致的净光合速率下降在盐激胁迫处理的前30-40 min主要由气孔限制引起.40 min后则主要由非气孔限制引起.可见,不同光照强度和NaCl浓度胁迫下三角叶滨藜叶片光合作用响应规律不同,引起净光合速率下降机制各异.     

盐胁迫下沙棘的渗透调节效应

分别用含有0、100、200和300mmol LNaCl的Hoagland培养液处理1年生沙棘(HippophaerhamnoidesL.)苗30d后,测定其鲜重,干重,含水量,可溶性糖、脯氨酸和无机离子(Na 、Cl-)的含量及叶片渗透势和渗透调节能力。结果表明:100mmol LNaCl处理的沙棘地上部和根的鲜重和干重最大,其含水量也最大;NaCl浓度超过100mmol L时,沙棘地上部分和根的鲜重和干重随盐浓度增加而逐步下降,其下降的趋势为地上部大于根部。随NaCl浓度不断升高,沙棘体内Na 和Cl-浓度随之升高,茎叶和根系中Cl-含量明显高于Na ,对Na 的相对吸收量多于Cl-。沙棘对盐胁迫有一定的适应能力,随NaCl浓度的升高,沙棘叶内脯氨酸含量升高,可溶性糖含量增加,渗透势降低,渗透调节能力增强。本结果可为盐碱地营造沙棘林提供依据。     

Evaluation of water stress control with polyethylene glycols by analysis of guttation

The water relations of pepper plants (Capsicum frutescens L.) under conditions conducive to guttation were studied to evaluate the control of plant water stress with polyethylene glycols. The addition of polyethylene glycol 6000 to the nutrient solution resulted in water relations similar to those expected in soil at the same water potentials. Specifically, xylem pressure potential in the root and leaf became more negative during a 24-hour treatment period, while osmotic potential of the root xylem sap remained constant. The decrease in pressure potential was closely correlated with the decrease in osmotic potential of the nutrient solution. In contrast, the addition of polyethylene glycol 400 to the nutrient medium resulted in a reduction of osmotic potential in the root xylem sap; this osmotic adjustment in the xylem was large enough to establish an osmotic gradient for entry of water and cause guttation at a nutrient solution osmotic potential of −4.8 bars. Pressure potential in the root and leaf xylem became negative only at nutrient solution osmotic potentials lower than −4.8 bars. About half of the xylem osmotic adjustment in the presence of polyethylene glycol 400 was caused by increased accumulation of K⁺, Na⁺, Ca²⁺, and Mg²⁺ in the root xylem. These studies indicate that larger polyethylene glycol molecules such as polyethylene glycol 6000 are more useful for simulating soil water stress than smaller molecules such as polyethylene glycol 400.     

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.     

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.     

Water relations in silver birch during springtime: How is sap pressurised?

• Positive sap pressures are produced in the xylem of birch trees in boreal conditions during the time between the thawing of the soil and bud break. During this period, xylem embolisms accumulated during wintertime are refilled with water. The mechanism for xylem sap pressurization and its environmental drivers are not well known.
• We measured xylem sap flow, xylem sap pressure, xylem sap osmotic concentration, xylem and whole stem diameter changes, and stem and root non‐structural carbohydrate concentrations, along with meteorological conditions at two sites in Finland during and after the sap pressurisation period.
• The diurnal dynamics of xylem sap pressure and sap flow during the sap pressurisation period varied, but were more often opposite to the diurnal pattern after bud burst, i.e. sap pressure increased and sap flow rate mostly decreased when temperature increased. Net conversion of soluble sugars to starch in the stem and roots occurred during the sap pressurisation period. Xylem sap osmotic pressure was small in comparison to total sap pressure, and it did not follow changes in environmental conditions or tree water relations.
• Based on these findings, we suggest that xylem sap pressurisation and embolism refilling occur gradually over a few weeks through water transfer from parenchyma cells to xylem vessels during daytime, and then the parenchyma are refilled mostly during nighttime by water uptake from soil. Possible drivers for water transfer from parenchyma cells to vessels are discussed. Also the functioning of thermal dissipation probes in conditions of changing stem water content is discussed.

     

The effects of water and multi-nutrient stress on xylem sap chemistry,photosynthesis and transpiration of seedlings of two Eucalypts

Summary Ten seedlings each of Eucalyptus kitsoniana Maiden and Eucalyptus globulus Labill. were subjected to two levels of water stress and two levels of nutrient stress (macro and micro-nutrients) in a greenhouse for 3 weeks. The objectives were to determine the degree to which seedlings show differences in sap chemistry, photosynthesis and transpiration that relate to the environments in which these two species live. Whole plants were then extracted for xylem sap using a pressure chamber and the sap was analyzed for 14 elements using an inductively coupled plasma spectrometer and a nitrometer. For E. kitsoniana water and nutrient stress, applied separately or in combination, significantly reduced leaf conductance, transpiration, photosynthesis and midday water potential. Nutrient stress alone had less effect than water stress on most functions measured. Water stress alone reduced the root/shoot ratio; the combination of water and nutrient stress increased the root/shoot ratio, primarily because of reduced shoot weight. In E. kitsoniana, water stress alone or in combination with nutrient stress increased the xylem sap concentrations of B and Si. Multi-nutrient stress alone, or in combination with water stress, significantly decreased sap Zn and K. For this species, sap N was decreased by nutrient stress, but increased by water stress. E. globulus had significantly lower transpiration rates and less root mass than E. kitsoniana. Slightly lower leaf conductance and photosynthesis were not significant in E. globulus compared to E. kitsoniana. Water and nutrient stress reduced conductance, transpiration (except for nutrient stress) and photosynthesis, and the effects of water stress on E. globulus were greater than the effects of nutrient stress. Midday water potential was reduced by water stress. Water or nutrient stress alone did not alter seedling root/shoot ratio, but the combination of water and nutrient stress significantly increased the root/shoot ratio for both species. For E. globulus, sap concentrations of Mn, Na, Si and K were increased by water stress (alone or in combination with nutrient stress). Sap N increased with water stress or combined stresses, but decreased under nutrient stress alone. When the two species were compared, E. globulus generally had lower or similar nutrient concentrations in the sap, with Ca, Mg, Mn and P significantly lower than in E. kitsoniana. Seedlings of these two species show strong site adaptations to water and nutrient availability.     

Bicarbonate-induced alkalinization of the xylem sap in intact maize seedlings as measured in situ with a novel xylem pH probe

In higher plants the pH of the xylem sap plays an important role in drought signaling, growth regulation, and plant nutrition. However, the interpretation of the data is very controversial. The main reason for this is that the xylem pH in intact plants was not directly accessible hitherto. We present here a novel, minimally-invasive probe based on the xylem pressure-potential probe (used for measuring directly xylem pressure and the electrical potential between root xylem sap and medium). Single-tipped, double-barreled capillaries were used, one barrel served as H(+)-selective electrode, whereas pressure and electrical potential were recorded by the other one. Upon insertion of the probe into the root xylem of maize (Zea mays) seedlings, pH values ranging between about 4.2 and 4.9 were monitored when the roots were immersed in standard nutrient solution. The pH did not respond to changes in light irradiation (up to 300 micromol m(-2) s(-1)), but increased upon exposure of the root to 5 or 20 mm bicarbonate in the bath solution. Changes in pH could also be recorded in transpiring plants when the root was cut below the insertion point of the probe and placed in media with different pH. The data support the hypothesis of Mengel ([1994] Plant Soil 165: 275-283) that upon external supply with bicarbonate Fe is immobilized in the leaf apoplast via changes in xylem pH.     

Hormonal Control of the Shoot-to-Root Ratio Is Not Related to Water Deficiency in Wheat Plants under Mineral Deficiency

We measured the content of hormones, the rate of growth, and some parameters of water regime (water content, transpiration, and stomatal and hydraulic conductivities) one and two days after wheat plant transfer from 10 to 1% Hoagland-Arnon nutrient medium. It was shown that, a day after dilution of nutrient solution, the content of various cytokinin forms decreased in the xylem sap, shoots, and roots. This decrease was most pronounced in the case of zeatin in the xylem sap and zeatin riboside in the mature zone of the first leaf. ABA was found to accumulate in shoots. A day after dilution of nutrient solution, we observed root elongation evidently induced by mineral nutrient deficiency, and this accelerated root growth was maintained later. Two days after dilution of nutrient solution, we observed the slowing of shoot weight accumulation, whereas root weight remained unchanged. Plant growth response could be related to ABA accumulation in shoots and cytokinin depletion in the whole plant. A reduced hydraulic conductivity and water content in the growing leaf zone was detected only two days after dilution of nutrient solution. Thus, changes in the growth rates and hormone contents could not result from disturbances in water regime induced by mineral nutrient deficiency.     

Ionic control of the lateral exchange of water between vascular bundles in tomato

Ions can enhance water flow through the xylem via changes in the hydraulic resistance at border pit membranes. Because flow between adjacent xylem vessels occurs primarily via bordered pit fields, it is hypothesized that xylem sap ion concentrations would affect lateral movement of water more than longitudinal flow. Using tomato as a model system, evidence is presented for ion-mediated changes in xylem hydraulic resistance and the lateral transport of water. Water flow between adjacent xylem bundles increased by approximately 50% in the presence of ions while longitudinal flow only increased by approximately 20%. However, the enhancement of lateral exchange due to ions was magnified by the presence of a pressure difference between vascular bundles. These results indicate that the degree of nutrient-sharing among sectors of a plant may depend on both nutrient concentration and the availability of water in the root zone.     

Stomatal Behaviours of Aspen (<Emphasis Type="Italic">Populus tremuloides</Emphasis>) Plants in Response to Low Root Temperature in Hydroponics

Hydroponic-grown seedlings of aspen (Populus tremuloides Michx.) were used to investigate how low root temperatures (5°C) affect stomatal conductance and water relations. An isohydric manner of the stomatal behaviour was found with the seedlings when their roots were subjected to the low temperature. Stomatal conductance rapidly and dramatically reduced in response to the low root temperature, while the xylem water potential did not significantly alter. Under the low root temperature, pH value of the xylem sap increased from 6.15 to 6.72 within the initial 4 h, while abscisic acid (ABA) concentration increased by the eighth hour of treatment. K⁺ concentration of the xylem sap significantly decreased within the 8th h and then reversed by the 24th h. The ion change was accompanied by a decrease and then an increase in the electrical conductivity, and an increase and then a decrease in the osmotic potential. The tempo of physiological responses to the low root temperature suggests that the rapid pH change of the xylem sap was the initial factor which triggered stomatal closure in low temperature-treated seedlings, and that the role of the more slowly accumulating ABA was likely to reinforce the stomatal closure. Xylem sap from the seedlings subjected low root temperature affected stomatal aperture on leaf discs when they were floated on the sap solution. The stomatal aperture correlated (P = 0.006) with the changed pattern of [K⁺] in the sap while the range of pH or ABA found in the xylem sap did not influence stomatal aperture of leaf discs in solution. The effect of xylem sap on stomatal aperture on leaf discs was different from on stomatal conductance in the intact seedlings. Comparison was made with previous study with the soil-grown seedlings.