樟子松幼苗生长及光合特性对强风沙流吹袭的响应

为了解樟子松幼苗对不同时间强风沙流吹袭的生理生态响应,2013年春季在内蒙古科尔沁沙地研究了8级大风风沙流(风速18m·s-1,风沙流强度173g·cm-1·min-1)吹袭10、20和30min下樟子松幼苗生长与光合特性的变化。结果显示:(1)随着风吹时间的增加,樟子松的株高生长量减少、茎粗生长加快,落叶数量增加,其中30min处理与CK相比的株高生长量下降52.63%,茎粗生长量增加233.30%,落叶指数增加466.70%。(2)风沙流吹袭没有改变樟子松幼苗的净光合速率、蒸腾速率、气孔导度和胞间CO2浓度的日变化规律,但日光合峰值下降,日最大蒸腾速率增加;与CK相比,30min处理的日最大光合速率下降22.69%,日最大蒸腾速率增加11.89%。(3)随风吹时间增加,其叶片温度、叶片相对含水量、日均光合速率、水分利用效率下降,30min处理较CK依次下降0.60%、4.37%、28.57%和31.58%,且日均蒸腾速率、气孔导度和胞间CO2浓度增加,30min处理较CK依次增加6.25%、6.67%和12.60%。研究表明,在持续风沙流胁迫下,樟子松幼苗光合作用受到抑制,蒸腾耗水增加,水分利用效率降低;樟子松幼苗生长速率降低主要源于光合面积减少和光合作用受到抑制,而其光合速率下降主要因幼苗叶片叶温和叶片含水量下降所致,蒸腾速率的增加主要源于气孔导度的增加;为了适应风沙流持续吹袭的胁迫,樟子松幼苗采取了降低株高生长速率,加快茎粗生长速率的适应策略。     

The relationship between the methyl-erythritol phosphate pathway leading to emission of volatile isoprenoids and abscisic acid content in leaves

It was investigated whether the methyl-erythritol phosphate (MEP) pathway that generates volatile isoprenoids and carotenoids also produces foliar abscisic acid (ABA) and controls stomatal opening. When the MEP pathway was blocked by fosmidomycin and volatile isoprenoid emission was largely suppressed, leaf ABA content decreased to about 50% and leaf stomatal conductance increased significantly. No effect of fosmidomycin was seen in leaves with constitutively high rates of stomatal conductance and in plant species with low foliar ABA concentration. In all other cases, isoprene emission was directly associated with foliar ABA, but ABA reduction upon MEP pathway inhibition was also observed in plant species that do not emit isoprenoids. Stomatal closure causing a midday depression of photosynthesis was also associated with a concurrent increase of isoprene emission and ABA content. It is suggested that the MEP pathway generates a labile pool of ABA that responds rapidly to environmental changes. This pool also regulates stomatal conductance, possibly when coping with frequent changes of water availability. MEP pathway inhibition by leaf darkening, and its down-regulation by exposure to elevated CO2, was also associated with a reduction of foliar ABA content. However, stomatal conductance was reduced, indicating that stomatal aperture is not regulated by the MEP-dependent foliar ABA pool, under these specific cases.     

喀斯特石漠化地区土壤养分对泡核桃功能性状的影响

为了探究喀斯特石漠化地区植物叶片功能性状及影响因素,以及揭示其对石漠化环境的适应机理,该文以中国南方喀斯特高原峡谷地区的泡核桃(Juglans sigillata)为对象,揭示土壤养分对叶片结构和光合性状的影响效应。结果表明：(1)泡核桃叶功能性状随石漠化等级增加,叶面积减小,比叶面积增大,叶干物质含量和叶组织密度先降后升,蒸腾速率、胞间CO₂浓度、气孔导度和光能利用率先下降后升高,其他性状变化趋势不显著。(2)冗余分析表明土壤养分能够解释37.4%的光合性状变异与53.4%的结构性状变异,其中全磷和溶解性有机碳对光合性状影响最大,而对结构性状影响最显著的是碱解氮和速效磷。(3)比叶面积分别与叶干物质含量极显著负相关,与净光合速率极显著正相关,叶厚度与叶组织密度极显著负相关,蒸腾速率与胞间CO₂浓度、气孔导度极显著正相关,水分利用速率与蒸腾速率、胞间CO₂浓度、气孔导度极显著负相关,光能利用率与净光合速率显著正相关。研究结果表明,泡核桃为适应喀斯特石漠化的特殊生境采取增强生长功能性状,同时提高资源获取能力的开拓型生长策略...     

采煤塌陷影响下土壤含水量变化对柠条气孔导度、蒸腾与光合作用速率的影响

采煤塌陷引起的土壤环境因子的变化对矿区植物生长的影响越来越受到人们的关注,气孔导度、蒸腾与光合作用作为环境变化响应的敏感因子,研究植物气孔导度、蒸腾与光合作用的变化是揭示荒漠矿区自然环境变化及其规律的重要手段之一。研究采煤塌陷条件下植物光合生理的变化是探究煤炭开采对植物叶片水分蒸腾散失和CO_2同化速率影响的关键环节,是探讨采煤塌陷影响下植物能量与水分交换动态的基础,而采煤矿区植物叶片气孔导度、蒸腾与光合作用速率对采煤塌陷影响下土壤含水量变化的响应如何尚不清楚。选取神东煤田大柳塔矿区52302工作面为实验场地,以生态修复物种柠条为研究对象,对采煤塌陷区和对照区柠条叶片气孔导度、蒸腾和光合作用速率以及土壤体积含水量进行监测,分析了采煤塌陷条件下土壤含水量的变化以及其对柠条叶片气孔导度、蒸腾与光合作用速率的影响。结果显示:(1)煤炭井工开采在地表形成大量裂缝,破坏了土体结构,潜水位埋深降低,土壤含水量均低于沉陷初期,相对于对照区,硬梁和风沙塌陷区土壤含水量分别降低了18.61%、21.12%;(2)柠条叶片气孔导度、蒸腾和光合作用速率均与土壤含水量呈正相关关系;煤炭开采沉陷增加了地表水分散失,加剧了土壤水分胁迫程度,为了减少蒸腾导致的水分散失,柠条叶片气孔阻力增加,从而气孔导度降低,阻碍了光合作用CO_2的供应,从而导致柠条叶片光合作用速率的降低,蒸腾速率也显著降低。     

Xylem transport and shoot accumulation of lumichrome, a newly recognized rhizobial signal, alters root respiration, stomatal conductance, leaf transpiration and photosynthetic rates in legumes and cereals

* Root respiration, stomatal conductance, leaf transpiration and photosynthetic rates were measured in phytotron and field-grown plants following the application of 5 or 10 nM lumichrome, 10 nM ABA (abscisic acid) and 10 ml of 0.2 OD600 infective rhizobial cells. * Providing soybean and cowpea roots with their respective homologous rhizobia and/or purified lumichrome increased the concentration of this molecule in xylem sap and leaf extracts. Relative to control, rhizobial inoculation and lumichrome application significantly increased root respiration in maize, decreased it in lupin, but had no effect on the other test species. * Applying either lumichrome (10 nM), infective rhizobial cells or ABA to roots of plants for 44 h in growth chambers altered leaf stomatal conductance and transpiration in cowpea, lupin, soybean, Bambara groundnut and maize, but not in pea or sorghum. Where stomatal conductance was increased by lumichrome application or rhizobial inoculation, it resulted in increased leaf transpiration relative to control plants. Treating roots of field plants of cowpea with this metabolite up to 63 d after planting showed decreased stomatal conductance, which affected CO2 intake and reduction by Rubisco. * The effect of rhizobial inoculation closely mirrored that of lumichrome application to roots, indicating that rhizobial effects on these physiological activities were most likely due to lumichrome released into the rhizosphere.     

干旱胁迫对降香黄檀幼苗光合生理特性的影响

采用温室盆栽方法,设置对照(CK)、轻度(LS)、中度(MS)和重度(HS)干旱胁迫4个水分条件,研究不同水分条件对降香黄檀幼苗光合和生理特性的影响。结果表明:(1)随着干旱胁迫程度增加,降香黄檀幼苗叶片叶绿素总含量总体呈现出下降趋势。(2)降香黄檀幼苗叶片净光合速率、气孔导度、胞间CO2浓度和蒸腾速率随着干旱胁迫强度增加均呈现出先增加后降低趋势,且MS和HS处理下的气孔导度和胞间CO2浓度同时降低,此时幼苗光合能力的下降主要受气孔因素限制。(3)随着干旱胁迫强度的增加,降香黄檀幼苗叶片细胞膜相对透性、丙二醛含量、游离脯氨酸含量和POD活性均呈现出增加趋势,而同期SOD和CAT活性呈现出先升高后降低趋势。可见,降香黄檀幼苗在轻度干旱胁迫下可通过增加叶片保护酶活性来清除活性氧对其组织造成的伤害,但胁迫超过一定程度后保护酶活性下降,表明降香黄檀幼苗的耐旱能力有限。     

The efficiency of water use in water stressed plants is increased due to ABA induced stomatal closure

Gas exchange and abscisic acid content of Digitalis lanata EHRH. have been examined at different levels of plant water stress. Net photosynthesis, transpiration and conductance of attached leaves declined rapidly at first, then more slowly following the withholding of irrigation. The intercellular partial pressure of CO₂ decreased slightly. The concentration of 2-cis(S)ABA increased about eight-fold in the leaves of non-irrigated plants as compared with well-watered controls. A close linear correlation was found between the ABA content of the leaves and their conductance on a leaf area basis. In contrast, the plot of net assimilation versus ABA concentration was curvilinear, leading to an increased efficiency of water use during stress. After rewatering, photosynthesis reached control values earlier than transpiration, leaf conductance and ABA content. From these data it is concluded that transpiration through the stomata is directly controlled by the ABA content, whereas net photosynthesis is influenced additionally by other factors.Possible reasons for the responses of photosynthesis and water use efficiency to different stress and ABA levels are discussed.Abbreviations A net CO₂ assimilation - ABA abscisic acid - C_i intercellular CO₂ concentration - g stomatal conductance - T transpiration - WUE water use efficiency     

Stomatal control of transpiration from a developing sugarcane canopy

Abstract. Stomatal conductance of single leaves and transpiration from an entire sugarcane (Saccharum spp. hybrid) canopy were measured simultaneously using independent techniques. Stomatal and environmental controls of transpiration were assessed at three stages of canopy development, corresponding to leaf area indices (L) of 2.2, 3.6 and 5.6. Leaf and canopy boundary layers impeded transport of transpired water vapour away from the canopy, causing humidity around the leaves to find its own value through local equilibration rather than a value determined by the humidity of the bulk air mass above the canopy. This tended to uncouple transpiration from direct stomatal control, so that transpiration predicted from measurement of stomatal conductance and leaf-to-air vapour pressure differences was increasingly overestimated as the reference point for ambient vapour pressure measurement was moved farther from the leaf and into the bulk air. The partitioning of control between net radiation and stomata was expressed as a dimensionless decoupling coefficent ranging from zero to 1.0. When the stomatal aperture was near its maximum this coefficient was approximately 0.9, indicating that small reductions in stomatal aperture would have had little effect on canopy transpiration. Maximum rates of transpiration were, however, limited by large adjustments in maximum stomatal conductance during canopy development. The product of maximum stomatal conductance and L. a potential total canopy conductance in the absence of boundary layer effects, remained constant as L increased. Similarly, maximum canopy conductance, derived from independent micrometeorological measurements, also remained constant over this period. Calculations indicated that combined leaf and canopy boundary layer conductance decreased with increasing L such that the ratio of boundary layer conductance to maximum stomatal conductance remained nearly constant at approximately 0.5. These observations indicated that stomata adjusted to maintain both transpiration and the degree of stomatal control of transpiration constant as canopy development proceeded.     

Salicylic acid induces stomatal closure by modulating endogenous hormone levels in cucumber cotyledons

Salicylic acid (SA) is one of the most important signaling molecules in plant growth and defense responses to biotic and abiotic stresses. Here, the effect of exogenous SA on the stomatal movements was investigated in cotyledons of cucumber (Cucumis sativus L.) seedlings. Application of different SA concentrations could induce the reduction in stomatal aperture and conductance, especially at a concentration of 0.5 mM. Using the isolated epidermal strips, stomata were found to close notably in response to exogenous SA, even at a concentration as low as 0.001 mM. Further study showed that a SA-induced decrease in the stomatal aperture was intensified by the higher SA concentrations, longer exposure, and lower pH of the medium. In addition, to understand the relationship between stomatal closure and endogenous hormone contents, the levels of ABA, IAA, and gibberellin (GA₃) were assayed under SA treatment. SA significantly increased endogenous ABA but not IAA and GA₃ content. A significant negative correlation (p ≤ 0.01) was observed between stomatal conductance and the ratio of ABA to (GA₃ + IAA) during SA application. It was suggested that exogenous SA could change the balance of endogenous hormones and thereby induce stomatal closure in cotyledons of cucumber seedlings.     

Senescence progression in a single darkened cotyledon depends on the light status of the other cotyledon in Cucurbita pepo (zucchini) seedlings: potential involvement of cytokinins and cytokinin oxidase/dehydrogenase activity

Darkness mediates different senescence-related responses depending on the targeting of dark treatment (whole plants or individual leaves) and on the organs that perceive the signal (leaves or cotyledons). As no data are available on the potential role of darkness to promote senescence when applied to individual cotyledons, we have investigated how darkness affects the progression of senescence in either a single or both individually darkened cotyledons of young 10-day-old Cucurbita pepo (zucchini) seedlings. Strong acceleration of senescence was observed when both cotyledons were darkened as judged by the damage in their anatomical structure, deterioration of chloroplast ultrastructure in parallel with decreased photosynthetic rate and photochemical quantum efficiency of PSII. In addition, the endogenous levels of cytokinins (CKs) and IAA were strongly reduced. In a single individually darkened cotyledon, the structure and function of the photosynthetic apparatus as well as the contents of endogenous CKs and IAA were much less affected by darkness, thus suggesting inhibitory effect of the illuminated cotyledon on the senescence of the darkened one. Apparently, the effect of darkness to accelerate/delay senescence in a single darkened cotyledon depends on the light status of the other cotyledon from the pair. The close positive correlation between CK content and the activity of CK oxidase/dehydrogenase (CKX; EC 1.4.3.18/1.5.99.12) suggested that CKX was essentially involved in the mechanisms of downregulation of endogenous CK levels. Our results indicated that CKX-regulated CK signaling could be a possible regulatory mechanism controlling senescence in individually darkened cotyledons.     

Stomatal action directly feeds back on leaf turgor: new insights into the regulation of the plant water status from non‐invasive pressure probe measurements

Uptake of CO₂ by the leaf is associated with loss of water. Control of stomatal aperture by volume changes of guard cell pairs optimizes the efficiency of water use. Under water stress, the protein kinase OPEN STOMATA 1 (OST1) activates the guard‐cell anion release channel SLOW ANION CHANNEL‐ASSOCIATED 1 (SLAC1), and thereby triggers stomatal closure. Plants with mutated OST1 and SLAC1 are defective in guard‐cell turgor regulation. To study the effect of stomatal movement on leaf turgor using intact leaves of Arabidopsis, we used a new pressure probe to monitor transpiration and turgor pressure simultaneously and non‐invasively. This probe permits routine easy access to parameters related to water status and stomatal conductance under physiological conditions using the model plant Arabidopsis thaliana. Long‐term leaf turgor pressure recordings over several weeks showed a drop in turgor during the day and recovery at night. Thus pressure changes directly correlated with the degree of plant transpiration. Leaf turgor of wild‐type plants responded to CO₂, light, humidity, ozone and abscisic acid (ABA) in a guard cell‐specific manner. Pressure probe measurements of mutants lacking OST1 and SLAC1 function indicated impairment in stomatal responses to light and humidity. In contrast to wild‐type plants, leaves from well‐watered ost1 plants exposed to a dry atmosphere wilted after light‐induced stomatal opening. Experiments with open stomata mutants indicated that the hydraulic conductance of leaf stomata is higher than that of the root–shoot continuum. Thus leaf turgor appears to rely to a large extent on the anion channel activity of autonomously regulated stomatal guard cells.     

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.     

Contrasting physiological effects of partial root zone drying in field-grown grapevine (Vitis vinifera L. cv. Monastrell) according to total soil water availability

Different spatial distributions of soil moisture were imposed on field-grown grapevines by applying the same irrigation volumes to the entire (DI; deficit irrigation) or part of the (PRD; partial root zone drying) root zone. Five treatments were applied: controls irrigated at 60% ETc (crop evapotranspiration) for the whole season (308 mm year(-1)); DI-1 and PRD-1 that received the same irrigation as controls before fruit set, 30% ETc from fruit set to harvest and 45% ETc post-harvest (192 mm year(-1)); and DI-2 and PRD-2 that were the same, except that 15% ETc was applied from fruit set to harvest (142 mm year(-1)). Compared with DI-1, PRD-1 maintained higher leaf area post-veraison and increased root water uptake, whole-plant hydraulic conductance, leaf transpiration, stomatal conductance, and photosynthesis, but decreased intrinsic gas exchange efficiency without causing differences in leaf xylem abscisic acid (ABA) concentration. Compared with DI-2, PRD-2 increased leaf xylem ABA concentration and decreased root water uptake, whole-plant hydraulic conductance, leaf transpiration, stomatal conductance, and photosynthesis, mainly at the beginning of PRD cycles. Distinctive PRD effects (e.g. greater stomatal closure) depended on the volumetric soil water content of the wet root zone, as predicted from a model of root-to-shoot ABA signalling.     

盐胁迫对沙拐枣光合生理特性的影响

以采自甘肃民勤一年生的沙拐枣幼苗为试材,对不同NaCl浓度(0、50、100、200、300mmol·L~(-1))处理下沙拐枣光合生理特性进行分析,并对各生理指标与地上生物量进行灰色关联度分析,以探讨荒漠植物沙拐枣的抗盐机理,为沙拐枣的保护及其恢复荒漠生态系统稳定提供理论依据。结果显示:随着NaCl浓度的升高,沙拐枣同化枝内脯氨酸含量逐渐增大,而其可溶性糖含量逐渐减小;在低浓度NaCl(50mmol·L~(-1) NaCl)处理下,同化枝光合参数均增加,且净光合速率(Pn)、气孔导度(Gs)、蒸腾速率(Tr)均达到最大值,比对照分别显著增加了33.3%、68.0%、60.8%;与50mmol·L~(-1) NaCl相比,处理浓度超过50mmol·L~(-1) NaCl时,Pn、Gs、Tr均降低;同化枝叶绿素b含量随着NaCl浓度的增加而降低,而叶绿素a和总叶绿素含量均呈先增加后降低的趋势。灰色关联度分析发现,同化枝的Tr、Gs、Ci以及叶绿素b与地上生物量的关联度较大。研究表明,低盐浓度NaCl激活了沙拐枣的某些生理机制,有利于植株的光合作用和生长,而植物在高盐浓度胁迫时能通过调节脯氨酸和可溶性糖的含量,减少叶绿素含量、Pn和Tr等维持自身的生长。     

Involvement of root ABA and hydraulic conductivity in the control of water relations in wheat plants exposed to increased evaporative demand

We studied the possible involvement of ABA in the control of water relations under conditions of increased evaporative demand. Warming the air by 3°C increased stomatal conductance and raised transpiration rates of hydroponically grown Triticum durum plants while bringing about a temporary loss of relative water content (RWC) and immediate cessation of leaf extension. However, both RWC and extension growth recovered within 30 min although transpiration remained high. The restoration of leaf hydration and growth were enabled by increased root hydraulic conductivity after increasing the air temperature. The use of mercuric chloride (an inhibitor of water channels) to interfere with the rise on root hydraulic conductivity hindered the restoration of extension growth. Air warming increased ABA content in roots and decreased it in shoots. We propose this redistribution of ABA in favour of the roots which increased the root hydraulic conductivity sufficiently to permit rapid recovery of shoot hydration and leaf elongation rates without the involvement of stomatal closure. This proposal is based on known ability of ABA to increase hydraulic conductivity confirmed in these experiments by measuring the effect of exogenous ABA on osmotically driven flow of xylem sap from the roots. Accumulation of root ABA was mainly the outcome of increased export from the shoots. When phloem transport in air-warmed plants was inhibited by cooling the shoot base this prevented ABA enrichment of the roots and favoured an accumulation of ABA in the shoot. As a consequence, stomata closed.     

Intraspecific variation in sensitivity to ultraviolet-B radiation in endogenous hormones and photosynthetic characteristics of 10 wheat cultivars grown under field conditions

Field studies were conducted to determine the potential of altering endogenous hormones and photosynthetic characteristics and intraspecific variation in sensitivity of 10 wheat (Triticum aestivum) cultivars (four tolerant, two middle sensitive and four sensitive) to enhanced ultraviolet-B (UV-B, 280–315 nm) radiation under field conditions. The supplemental UV-B radiation was 5.00 kJ m⁻², simulating a depletion of 20% stratospheric ozone. Responses were cultivar-specific. Out of the 10 tested wheat cultivars, six showed significant decrease in IAA content. UV-B radiation significantly increased ZR content in two wheat cultivars and significantly decreased in five cultivars. ABA content of three wheat cultivars was increased significantly, while that of five cultivars was decreased significantly. UV-B radiation significantly increased the stomatal conductance of three cultivars, and significantly decreased that of four cultivars. Intercellular CO₂ concentrations were significantly increased in five cultivars and significantly decreased in one cultivar (Mianyang 20). Transpiration rate of three cultivars significantly increased, while that of three cultivars significantly decreased. UV-B radiation significantly decreased the net photosynthetic rate of six cultivars. Intraspecific differences were found for the different measured parameters. For seven measured parameters, UV-B radiation had significant effects on five wheat cultivars, while no effect on the others. Significant correlations were observed between net photosynthetic rate and stomatal conductance, intercellular CO₂ concentrations and transpiration rate in eight cultivars. UV-B radiation might change stomatal conductance, intercellular CO₂ concentrations and transpiration rate, thus resulting in changes in net photosynthetic rate.     

Asymmetric responses of adaxial and abaxial stomata to elevated CO2: impacts on the control of gas exchange by leaves

The response of adaxial and abaxial stomatal conductance in Rumex obtusifolius to growth at elevated atmospheric concentrations of CO₂ (250 μmol mol^?1 above ambient) was investigated over two growing seasons. The conductance of both the adaxial and abaxial leaf surfaces was found to be reduced by elevated concentrations of CO₂. Elevated CO₂ caused a much greater reduction in conductance for the adaxial surface than for the abaxial surface. The absence of effects upon stomatal density indicated that the reductions were probably the result of changes in stomatal aperture. Partitioning of gas exchange between the leaf surfaces revealed that increased concentrations of CO₂ caused increased rates of photosynthesis only via the abaxial surface. Additionally, leaf thickness was found to increase during growth at elevated concentrations of CO₂. The tendency for these amphistomatous leaves to develop a distribution of conductance approaching that of hypostomatous leaves clearly reduced their maximum photosynthetic potential. This conclusion was supported by measurements of stomatal limitation, which showed greater values for the adaxial surfaces, and greater values at elevated CO₂. This reduction in photosynthesis may in part be caused by higher diffusive limitations imposed because of increased leaf thickness. In an uncoupled canopy, asymmetrical stomatal responses of the kind identified here may appreciably reduce transpiration. Species which show symmetrical responses are less likely to show reduced transpirational rates, and a redistribution of water loss between species may occur. The implications of asymmetrical stomatal responses for photosynthesis and canopy transpiration are discussed.     

Abscisic acid and transpiration rate are involved in the response to boron toxicity in Arabidopsis plants

Boron (B) is an essential microelement for vascular plant development, but its toxicity is a major problem affecting crop yields in arid and semi‐arid areas of the world. In the literature, several genes involved in abscisic acid (ABA) signalling and responses are upregulated in Arabidopsis roots after treatment with excess B. It is known that the AtNCED3 gene, which encodes a crucial enzyme for ABA biosynthesis, plays a key role in the plant response to drought stress. In this study, root AtNCED3 expression and shoot ABA content were rapidly increased in wild‐type plants upon B‐toxicity treatment. The Arabidopsis ABA‐deficient nced3‐2 mutant had higher transpiration rate, stomatal conductance and accumulated more B in their shoots than wild‐type plants, facts that were associated with the lower levels of ABA in this mutant. However, in wild‐type plants, B toxicity caused a significant reduction in stomatal conductance, resulting in a decreased transpiration rate. This response could be a mechanism to limit the transport of excess B from the roots to the leaves under B toxicity. In agreement with the higher transpiration rate of the nced3‐2 mutant, this genotype showed an increased leaf B concentration and damage upon exposure to 5 mM B. Under B toxicity, ABA application decreased B accumulation in wild‐type and nced3‐2 plants. In summary, this work shows that excess B applied to the roots leads to rapid changes in AtNCED3 expression and gas exchange parameters that would contribute to restrain the B entry into the leaves, this effect being mediated by ABA.     

Early stomatal closure in waterlogged pea plants is mediated by abscisic acid in the absence of foliar water deficits

Abstract Soil waterlogging decreased leaf conductance (interpreted as stomatal closure) of vegetative pea plants (Pisuin sativum L. cv. ‘Sprite’) approximately 24 h after the start of flooding, i.e. from the beginning of the second 16 h-long photo-period. Both adaxial and abaxial surfaces of leaves of various ages and the stipules were affected. Stomatal closure was sustained for at least 3 d with no decrease in foliar hydration measured as water content per unit area, leaf water potential or leaf water saturation deficit. Instead, leaves became increasingly hydrated in association with slower transpiration. These changes in the waterlogged plants over 3 d were accompanied by up to 10-fold increases in the concentration of endogenous abscisic acid (ABA). Waterlogging also increased foliar hydration and ABA concentrations in the dark. Leaves detached from non-waterlogged plants and maintained in vials of water for up to 3 d behaved in a similar way to leaves on flooded plants, i.e. stomata closed in the absence of a water deficit but in association with increased ABA content. Applying ABA through the transpiration stream to freshly detached leaflets partially closed stomata within 15 min. The extractable concentrations of ABA associated with this closure were similar to those found in flooded plants. When an ABA-deficient ‘wilty’ mutant of pea was waterlogged, the extent of stomatal closure was less pronounced than that in ordinary non-mutant plants, and the associated increase in foliar ABA was correspondingly smaller. Similarly, waterlogging closed stomata of tomato plants within 24 h, but no such closure was seen in ‘flacca’, a corresponding ABA-deficient mutant. The results provide an example of stomatal closure brought about by stress in the root environment in the absence of water deficiency. The correlative factor operating between the roots and shoots appeared to be an inhibition of ABA transport out of the shoots of flooded plants, causing the hormone to accumulate in the leaves.     

Control of Stomatal Behaviour by Abscisic Acid which Apparently Originates in the Roots

Two experiments indicate that abscisic acid (ABA) may influencestomatal behaviour of Commelina communis L. Stomatal conductancecould not be correlated with bulk leaf ABA content but whenthe abaxial epidermis was assayed for ABA, small increases inABA content correlated well with limitations of leaf conductance.Restricted conductance of the abaxial surface of leaves wasassociated with an increase of approximately 40 amole ABA perstomatal complex. This agrees with previously published figures. When roots of individual plants were split between two containers,drying the soil around one part of the root system restrictedleaf conductance, even though leaf water relations were notaffected. Increased ABA content of the epidermis coincided withincreased ABA content of the roots in drying soil. Other rootsof the same plant in moist soil did not show increased ABA content.These results suggest that in drying soil, ABA can move fromthe roots to the epidermis and restrict stomatal aperture evenwhen leaf water potentials and turgors remain constant. Theimportance of this mechanism in providing a sensitive foliarresponse to decreasing soil moisture is discussed. Key words: Soil drying, ABA, roots, stomata, water relations