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.     

不同光强下单叶蔓荆的光合蒸腾与离子累积的关系

同步分析了生长在滨海滩涂沙地的灌木单叶蔓荆钠和钾离子的累积量与植物的水分累积消耗量与光合产物累积量的日变化特点和累积的关系,并比较了单叶蔓荆在晴天和阴天的木质部溶液离子浓度与植物水势的关系和水分利用效率的差别,结果表明:单叶蔓荆在光强度较高的晴天的水分利用效率高于阴天,在晴天的光合产物累积达到了阴天的约4倍,却只消耗了约为阴天3倍的水。随着植物体水势的降低,单叶蔓荆木质部溶液的钠离子的浓度和钾离子浓度呈降低趋势但不明显。木质部溶液的钾离子浓度和钾离子累积量无论在晴天和阴天都明显低于钠离子的浓度和累积量。单叶蔓荆的高浓度的钠离子吸收有可能在液泡累积并降低细胞的渗透势,增加细胞的吸水能力和植物的抗旱抗盐能力。     

Environmental Regulation of CO2 Exchange Pattern in Facultative CAM Plants

Three facultative CAM plants, Sedum spectabile, S. aizoon and Mesembryanthemum cordifolium, could take up CO2 throughout the night and daytime, and no phase Ill was observed during cloudy weather. The CO2 exchange patterns during cloudy day differed obviously from that during sunny day. But in the obligate CAM plants, Kalanchoe daigremontiana, Orostachys fimbriatus and Bryophyllum pinnatum, there were phase Ⅲ during cloudy day. These results showed that the COs exchange patterns with uptake of CO2 throughout the night and daytime were universal in facultative CAM plants during cloudy day, but not in obligate CAM plants, of which the CO2 exchange patterns were very stable. In the three facultative CAM plants, the difference of exchange patterns between cloudy and sunny days depended mainly on temperature change. The effect of the temperature on CO2 exchange patterns was mediated by the decarboxylation rate. At high temperature, the decarboxylation rate could be enhanced. It was found that the accumulation of malic acid at night in the three obligate CAM plants was much more than that in the three facultative C AM plants. So during cloudy day, the decarboxytion rate in the three obligate CAM plants was also higher. This might be an important cause that obligate CAM plants need not to take up CO2 during the daytime.     

In Situ Measurement of Epidermal Cell Turgor, Leaf Water Potential, and Gas Exchange in Tradescantia virginiana L

A combined system has been developed in which epidermal cell turgor, leaf water potential, and gas exchange were determined for transpiring leaves of Tradescantia virginiana L. Uniform and stable values of turgor were observed in epidermal cells (stomatal complex cells were not studied) under stable environmental conditions for both upper and lower epidermises. The changes in epidermal cell turgor that were associated with changes in leaf transpiration were larger than the changes in leaf water potential, indicating the presence of transpirationally induced within-leaf water potential gradients. Estimates of 3 to 5 millimoles per square meter per second per megapascal were obtained for the value of within-leaf hydraulic conductivity. Step changes in atmospheric humidity caused rapid changes in epidermal cell turgor with little or no initial change in stomatal conductance, indicating little direct relation between stomatal humidity response and epidermal water status. The significance of within-leaf water potential gradients to measurements of plant water potential and to current hypotheses regarding stomatal response to humidity is discussed.     

Recovery of turgor by wilted, excised cabbage leaves in the absence of water uptake : a new factor in drought acclimation

Cabbage leaves excised from a fully turgid plant wilt within 20 minutes to 2 hours (depending on plant age) with a loss of about 10% relative water content (RWC). If droughted for 2 to 4 days in a high relative humidity leaf chamber, they may acclimate, recovering their turgor without the absorption of water, in fact at a loss of 15 to 25% RWC. This turgor recovery in the absence of water uptake occurs only if (a) the rate of water loss is slow enough (about 1-5% RWC per day after the first 24 hours drought loss of about 15% RWC), (b) if the leaves are no longer growing actively. Osmotic adjustment accompanies the turgor adjustment, but cannot be the cause in the absence of water uptake. The recovery of turgor by wilted cabbage leaves in the absence of water uptake cannot be explained by (a) transfer of reserve water from apoplast to symplast either from the cell walls or from the vessel lumens by cavitation or (b) metabolic loss of dry matter and gain of water. It can be explained by a contraction of the cell walls around the partially dehydrated protoplasts, until they regain their elastic extensibility. These proposed cell wall changes during drought acclimation are therefore the opposite of those occurring during growth. This hypothesis therefore explains the long recognized inverse relation between growth and acclimation. Two predictions of this hypothesis were tested and substantiated.     

Transpiration Induces Radial Turgor Pressure Gradients in Wheat and Maize Roots

Previous studies have shown both the presence and the absence of radial turgor and osmotic pressure gradients across the cortex of roots. In this work, gradients were sought in the roots of wheat (Triticum aestivum) and maize (Zea mays) under conditions in which transpiration flux across the root was varied This was done by altering the relative humidity above the plant, by excising the root, or by using plants in which the leaves were too young to transpire. Roots of different ages (4-65 d) were studied and radial profiles at different distances from the tip (5-30 mm) were measured. In both species, gradients of turgor and osmotic pressure (increasing inward) were found under transpiring conditions but not when transpiration was inhibited. The presence of radial turgor and osmotic pressure gradients, and the behavior of the gradient when transpiration is interrupted, indicate that active membrane transport or radial solvent drag may play an important role in the distribution of solutes across the root cortex in transpiring plants. Contrary to the conventional view, the flow of water and solutes across the symplastic pathway through the plasmodesmata cannot be inwardly directed under transpiring conditions.     

Leaf patch clamp pressure probe measurements on olive leaves in a nearly turgorless state

The non‐invasive leaf patch clamp pressure (LPCP) probe measures the attenuated pressure of a leaf patch, P_p, in response to an externally applied magnetic force. P_p is inversely coupled with leaf turgor pressure, P_c, i.e. at high P_c values the P_p values are small and at low P_c values the P_p values are high. This relationship between P_c and P_p could also be verified for 2‐m tall olive trees under laboratory conditions using the cell turgor pressure probe. When the laboratory plants were subjected to severe water stress (P_c dropped below ca. 50 kPa), P_p curves show reverse diurnal changes, i.e. during the light regime (high transpiration) a minimum P_p value, and during darkness a peak P_p value is recorded. This reversal of the P_p curves was completely reversible. Upon watering, the original diurnal P_p changes were re‐established within 2–3 days. Olive trees in the field showed a similar turnover of the shape of the P_p curves upon drought, despite pronounced fluctuations in microclimate. The reversal of the P_p curves is most likely due to accumulation of air in the leaves. This assumption was supported with cross‐sections through leaves subjected to prolonged drought. In contrast to well‐watered leaves, microscopic inspection of leaves exhibiting inverse diurnal P_p curves revealed large air‐filled areas in parenchyma tissue. Significantly larger amounts of air could also be extracted from water‐stressed leaves than from well‐watered leaves using the cell turgor pressure probe. Furthermore, theoretical analysis of the experimental P_p curves shows that the propagation of pressure through the nearly turgorless leaf must be exclusively dictated by air. Equations are derived that provide valuable information about the water status of olive leaves close to zero P_c.     

Mineral nutrition and the water relations of plants

Summary When young tomato plants were transferred from nutrient solution to mineral-free water, reductions in transpiration, water content of the shoots and stomatal aperture were not accompanied by a reduction in the relative water content or an increase in the suction pressure of the leaves. The relative water content of the leaves was increased and the suction pressure was little affected.Following transfer of the plants to mineral-free water, the mineral content of the shoots and the osmotic pressure of expressed leaf sap were reduced. It was concluded that mineral salts were necessary for maintaining the osmotic pressure of the leaf cell sap and that this was achieved, at least in part, by maintaining the mineral concentration of the sap. The amount of water that could be taken up by leaves and their turgor pressure were related to the osmotic pressure of the sap and calculations of turgor pressure showed that it was less in the leaves of plants with their roots in mineral-free water than in the leaves of plants in nutrient solution.Evidence was obtained that in leaflets detached from plants with their roots in mineral-free water, stomatal closure could occur at a higher water content than in leaflets detached from plants in nutrient solution, indicating a further role of minerals in leaf water relations. It is suggested that this role may be related to the properties of the cell walls.     

Day-night changes in leaf water relations associated with the rhythm of crassulacean acid metabolism in Kalanchoë daigremontiana

A study was made of the day-night changes under controlled environmental conditions in the bulk-leaf water relations of Kalanchoë daigremontiana, a plant showing Crassulacean acid metabolism. In addition to nocturnal stomatal opening and net CO₂ uptake, the leaves of well-watered plants showed high rates of gas exchange during the whole of the second part of the light period. Measurements with the pressure chamber showed that xylem tension increased during the night and then decreased towards a minimum at about midday; a significant increase in xylem tension was also seen in the late afternoon. Cell-sap osmotic pressure paralleled leaf malate content and was maximum at dawn and minimum at dusk. The relationship between these two variables indicated that the nocturnally synthesized malate was apparently behaving as an ideal osmoticum. To estimate bulk-leaf turgor pressure, values for water potential were derived by correcting the pressurechamber readings for the osmotic pressure of the xylem sap. This itself was found to depend on the malate content of the leaves. Bulk-leaf turgor pressure changed rhythmically during the day-night cycle; turgor was low during the late afternoon and for most of the night, but increased quickly to a maximum of 0.20 MPa around midday. In water-stressed plants, where net CO₂ uptake was restricted to the dark period, there was also an increase in bulk-leaf turgor pressure at the start of the light period, but of reduced magnitude. Such changes in turgor pressure are likely to be of considerable ecological importance for the water economy of crassulacean-acid-metabolism plants growing in their natural habitats.Abbreviation and symbols CAM Crassulacean acid metabolism - P turgor pressure - osmotic pressure - water potential Dedicated to Professor Dr. H. Ziegler on the occasion of his 60th birthday     

Day/night variations in turgor pressure in individual cells of Mesembryanthemum crystallinum L.

Summary Using a pressure probe, turgor pressure was directly determined in leaf-mesophyll cells and the giant epidermal bladder cells of stems, petioles and leaves of the halophilic plant Mesembryanthemum crystallinum. Experimental plants were grown under non-saline conditions. They displayed the photosynthetic characteristics typical of C₃-plants when 10 weeks old and performed weak CAM when 16 weeks old. In 10 week old plants, the turgor pressure (P) of bladder cells of stems was 0.30 MPa; of bladder cells of petioles 0.19 MPa, and of bladder cells of leaves 0.04 MPa. In bladder cells from leaves of 16 week old plants, marked changes in turgor pressure were observed during day/night cycles. Maximum turgor occurred at noon and was paralleled by a decrease in the osmotic pressure of the bladder cell sap. Similar changes in the cell water relations were observed in plants in which traspirational water loss was prevented by high ambient relative humidity. Turgor pressure of mesophyll cells also increased during day-time showing macimum values in the early morning. No such changes in turgor pressure and osmotic pressure were observed in bladder and mesophyll cells of the 10 week old plants not showing the diurnal acid fluctuation typical of CAMAbbreviations CAM crassulacean acid metabolism - V volume of the cells (mm³) - P turgor pressure (MPa) - volumetric elastic modulus (MPa) - ⁱ osmotic pressure of the cell sap (MPa) - T _1/2 half-time of water exchange (s) - Lp hydraulic conductivity of the cell membrane (m·s^-1·MPa^-1) - A surface area of cells (mm²) - P pressure changes (MPa) - V volume changes (mm³) - nocturanal nighttime - diurnal daytime     

Acclimation of leaf growth to low water potentials in sunflower

Abstract Leaf growth is one of the most sensitive of plant processes to water deficits and is frequently inhibited in field crops. Plants were acclimated for 2 weeks under a moderate soil water deficit to determine whether the sensitivity of leaf growth could be altered by sustained exposure to low water potentials. Leaf growth under these conditions was less than in the controls because expansion occurred more slowly and for less of the day than in control leaves. However, acclimated leaves were able to grow at leaf water potentials (Ψ₁) low enough to inhibit growth completely in control plants. This ability was associated with osmotic adjustment and maintenance of turgor in the acclimated leaves. Upon rewatering, the growth of acclimated leaves increased but was less than the growth of controls, despite higher concentrations of cell solute and greater turgor in the acclimated leaves than in controls. Therefore, factors other than turgor and osmotic adjustment limited the growth of acclimated leaves at high ψ₁ Four potentially controlling factors were investigated and the results showed that acclimated leaves were less extensible and required more turgor to initiate growth than control leaves. The slow growth of acclimated leaves was not due to a decrease in the water potential gradient for water uptake, although changes in the apparent hydraulic conductivity for water transport could have occurred. It was concluded that leaf growth acclimated to low ψ₁, by adjusting osmotically, and the concomitant maintenance of turgor permitted growth where none otherwise would occur. However, changes in the extensibility of the tissue and the turgor necessary to initiate growth caused generally slow growth in the acclimated leaves.     

Tissue water relations in elfin cloud forest tree species of Serrania de Macuira,Guajira, Colombia

Summary Diurnal courses of stomatal conductance, leaf water potential, and the components of tissue water potential were measured in six canopy species in an elfin cloud forest. High values of stomatal conductance were measured on cloudy days and during early morning and late afternoon of sunny days. Decreases in stomatal conductance with increases in vapour pressure deficit may have been a response to avoid further water deficits and suggested a stomatal response to changes in relative humidity. Daily transpiration varied between 470 and 1014 g m^-2 day^-1 during cloudy days and between 532 and 944 g m^-2 day^-1 during clear days. Stomatal conductance may have also responded to changes in leaf water potential, which was minimum at noon. The minimum tissue water potential measured in the field was -1.8 MPa in Myrcianthes fragrans, and the minimum turgor pressure was 0.49 MPa also in M. fragrans. There was a correlation between the osmotic potential and the minimum tissue water potential, suggesting that osmotic potential plays a major role in the maintenance of turgor in these species, in spite of the great variability in the elastic properties of leaf tissues. Turgor pressure decreased during the day following the course of water potential but never approached the turgor loss point, as it has been measured in some lowland rain forest trees. This is a strong indication that elfin cloud forest trees do not suffer severe water deficits, and that small tree stature is not directly related to water shortage.     

Daily Changes in CO(2) and Water Vapor Exchange, Chlorophyll Fluorescence, and Leaf Water Relations in the Halophyte Mesembryanthemum crystallinum during the Induction of Crassulacean Acid Metabolism in Response to High NaCl Salinity

Simultaneous measurements of net CO₂ exchange, water vapor exchange, and leaf water relations were performed in Mesembryanthemum crystallinum during the development of crassulacean acid metabolism (CAM) in response to high NaCl salinity in the rooting medium. Determinations of chlorophyll a fluorescence were used to estimate relative changes in electron transport rate. Alterations in leaf mass per unit area, which—on a short-term basis—largely reflect changes in water content, were recorded continuously with a beta-gauge. Turgor pressure of mesophyll cells was determined with a pressure probe. As reported previously (K Winter, DJ von Willert [1972] Z Pflanzenphysiol 67: 166-170), recently expanded leaves of plants grown under nonsaline conditions showed gas-exchange characteristics of a C₃ plant. Although these plants were not exposed to any particular stress treatment, water content and turgor pressure regularly decreased toward the end of the 12 hour light periods and recovered during the following 12 hours of darkness. When the NaCl concentration of the rooting medium was raised to 400 millimolar, in increments of 100 millimolar given at the onset of the photoperiods for 4 consecutive days, leaf water content and turgor pressure decreased by as much as 30 and 60%, respectively, during the course of the photoperiods. These transient decreases probably triggered the induction of the biochemical machinery which is required for CAM to operate. After several days at 400 millimolar NaCl, when leaves showed features typical of CAM, overall turgor pressure and leaf mass per unit area had increased above the levels before onset of the salt treatment, and diurnal alterations in leaf water content were reduced. Net carbon gain during photoperiods and average intercellular CO₂ partial pressures at which net CO₂ uptake occurred, progressively decreased upon salinization. Reversible diurnal depressions in leaf conductance and net CO₂ uptake, with minima recorded in the middle of the photoperiods, preceded the occurrence of nocturnal net CO₂ uptake. During these reductions, intercellular CO₂ partial pressure and rates of photosynthetic electron transport decreased. With advancing age, leaves of plants grown under nonsaline conditions exhibited progressively greater diurnal reductions in turgor pressure and developed a low degree of CAM activity.     

铁皮石斛叶片光合作用的碳代谢途径

 利用LI-6400光合测定系统测定了不同天气条件下铁皮石斛（Dendrobium officinale）叶片24h CO2吸收的动态以及CO2吸收对光强和温度的响应。晴天的白天和夜间铁皮石斛都能吸收CO2,中午CO2吸收速率为负值, CO2的交换方式具景天酸代谢途径(CAM)的特点。阴雨天,只有白天吸收CO2,夜间表现为暗呼吸,光合作用碳代谢的途径为C3途径。在多云的天气条件下,白天吸收CO2,并持续至日落后。夜间21∶00仍有CO2吸收,23∶00以后至次日凌晨处于暗呼吸状态。在500 μmol·m-2·s-1光照件下,20℃出现最大CO2吸收值。在夜间,25℃时CO2的吸收速率最高。有光和无光条件下,低温或高温引起CO2吸收速率下降均为非气孔因素所致。晴天上午,铁皮石斛叶片的表观量子产额为0.035,光合补偿点为2.9μmol·m-2·s-1,饱和光强为500μmol·m-2·s-1,强光下出现光抑制现象。叶片受到强光预先照射后,即使光照减弱光抑制效应仍保持一段时间,致使光合补偿点升高,表观量子产额下降,相同光强下的CO2吸收效率降低。结果表明：铁皮石斛为兼性CAM植物,随着环境条件的变化,其光合作用在景天酸代谢途径(CAM)与C3途径间变化。     

Photosynthesis in the fleeting shadows: an overlooked opportunity for increasing crop productivity?

Photosynthesis measurements are traditionally taken under steady‐state conditions; however, leaves in crop fields experience frequent fluctuations in light and take time to respond. This slow response reduces the efficiency of carbon assimilation. Transitions from low to high light require photosynthetic induction, including the activation of Rubisco and the opening of stomata, whereas transitions from high to low light require the relaxation of dissipative energy processes, collectively known as non‐photochemical quenching (NPQ). Previous attempts to assess the impact of these delays on net carbon assimilation have used simplified models of crop canopies, limiting the accuracy of predictions. Here, we use ray tracing to predict the spatial and temporal dynamics of lighting for a rendered mature Glycine max (soybean) canopy to review the relative importance of these delays on net cumulative assimilation over the course of both a sunny and a cloudy summer day. Combined limitations result in a 13% reduction in crop carbon assimilation on both sunny and cloudy days, with induction being more important on cloudy than on sunny days. Genetic variation in NPQ relaxation rates and photosynthetic induction in parental lines of a soybean nested association mapping (NAM) population was assessed. Short‐term NPQ relaxation (<30 min) showed little variation across the NAM lines, but substantial variation was found in the speeds of photosynthetic induction, attributable to Rubisco activation. Over the course of a sunny and an intermittently cloudy day these would translate to substantial differences in total crop carbon assimilation. These findings suggest an unexplored potential for breeding improved photosynthetic potential in our major crops.     

不同径级的西伯利亚红松树干液流及蒸腾耗水特征的差异

基于热扩散技术,采用TDP法连续监测了新疆喀纳斯国家自然保护区内不同径级西伯利亚红松的树干液流,分析其在生长季内(6~9月)的液流变化及蒸腾耗水特性,为阐明喀纳斯保护区优势树种水分循环机理,以及理解区域尺度上森林生态系统水分循环及其过程应对未来气候变化的响应机制提供依据。结果显示:(1)不同径级西伯利亚红松在晴、阴、雨3种天气条件下的树干液流日动态变化均呈昼高夜低的多峰型曲线,但变化频率和变化幅度差异明显,日最大液流值的排序为晴天阴天雨天。(2)树干液流的发生较光合有效辐射的变化存在明显的滞后效应,不同径级西伯利亚红松的最大液流峰值滞后时间在30~207min。(3)西伯利亚红松的月平均树干液流的大小顺序为7月8月9月6月,且相同径级树干阳生面的液流速率均大于阴生面。(4)西伯利亚红松全株的蒸腾耗水量为7月份的最大,其值占整个生长季的61.8%;且大径级阳生面的蒸腾耗水总量(6 716.79g)和阴生面蒸腾耗水总量(4 649.08g)分别是相应小径级阳生面和阴生面的2.00和2.45倍。(5)气温、空气相对湿度和光合有效辐射是影响西伯利亚树干液流的主要因素,同时0~5cm和20~30cm土壤温度对其影响也较大。研究表明,西伯利亚红松在生长过程中,大径级树干的液流和蒸腾耗水量大于小径级,主要发生部位为树干的阳生面,且在7月份的变化最明显。     

Xylem Flow and its Driving Forces in a Tropical Liana: Concomitant Flow-Sensitive NMR Imaging and Pressure Probe Measurements

Abstract: Flow-sensitive NMR imaging and pressure probe techniques were used for measuring xylem water flow and its driving forces (i.e., xylem pressure as well as cell turgor and osmotic pressure gradients) in a tropical liana, Epipremnum aureum. Selection of tall specimens allowed continuous and simultaneous measurements of all parameters at various distances from the root under diurnally changing environmental conditions. Well hydrated plants exhibited exactly linearly correlated dynamic changes in xylem tension and flow velocity. Concomitant multiple-probe insertions along the plant shoot revealed xylem and turgor pressure gradients with changing magnitudes due to environmental changes and plant orientation (upright, apex-down, or horizontal). The data suggest that in upright and - to a lesser extent - in horizontal plants the transpirational water loss by the cells towards the apex during the day is not fully compensated by water uptake through the night. Thus, longitudinal cellular osmotic pressure gradients exist. Due to the tight hydraulic coupling of the xylem and the tissue cells these gradients represent (besides the transpiration-induced tension in the xylem) an additional tension component for anti-gravitational water movement from the roots through the vessels to the apex.     

Temporal Dynamics of Powdery Mildew (Oidium neolycopersici) and its Effects on the Host Growth Dynamics of Tomato

Controlled glasshouse experiments were conducted to investigate the temporal progress of powdery mildew and its effects on host dynamics of tomato, without and with one fungicide application. Healthy tomato transplants (5‐ to 6‐week old) were artificially inoculated with powdery mildew, and disease progress as well as host growth were monitored in both fungicide sprayed and unsprayed treatments and compared with non‐inoculated plants. Actual disease severity on a plant basis increased in unsprayed plants reaching maximum severity in the proportionate range of 0.53–0.83. One fungicide spray significantly reduced the maximum disease severity by two‐ to fourfolds. Despite adjustments for defoliation, declines in the proportion of disease severity between successive assessments were evident. Whereas the estimated growth rates of diseased plants were significantly lower than that of healthy plants, no significant differences were observed in the maximum leaf area formed of inoculated and non‐inoculated plants. A considerable effect of the powdery mildew epidemics was manifested through hastened shrivelling and defoliation of diseased leaves within the tomato canopy. An average of 18–29% and 40–52% of leaves had abscised from the plant canopy at the last date of assessment in sprayed and non‐sprayed plants, respectively. Accordingly, defoliation accounted for 14–33.3% and 58.3–63.1% losses in leaf area of sprayed and non‐sprayed plants, respectively. Duration of healthy leaf area and yield of inoculated plants were also significantly reduced by powdery mildew epidemics.     

Effects of environmental parameters and irrigation on the turgor pressure of banana plants measured using the non‐invasive,online monitoring leaf patch clamp pressure probe

Turgor pressure provides a sensitive indicator for irrigation scheduling. Leaf turgor pressure of Musa acuminate was measured by using the so‐called leaf patch clamp pressure probe, i.e. by application of an external, magnetically generated and constantly retained clamp pressure to a leaf patch and determination of the attenuated output pressure P_p that is highly correlated with the turgor pressure. Real‐time recording of P_p values was made using wireless telemetric transmitters, which send the data to a receiver base station where data are logged and transferred to a GPRS modem linked to an Internet server. Probes functioned over several months under field and laboratory conditions without damage to the leaf patch. Measurements showed that the magnetic‐based probe could monitor very sensitively changes in turgor pressure induced by changes in microclimate (temperature, relative humidity, irradiation and wind) and irrigation. Irrigation effects could clearly be distinguished from environmental effects. Interestingly, oscillations in stomatal aperture, which occurred frequently below turgor pressures of 100 kPa towards noon at high transpiration or at high wind speed, were reflected in the P_p values. The period of pressure oscillations was comparable with the period of oscillations in transpiration and photosynthesis. Multiple probe readings on individual leaves and/or on several leaves over the entire height of the plants further emphasised the great impact of this non‐invasive turgor pressure sensor system for elucidating the dynamics of short‐ and long‐distance water transport in higher plants.     

Root growth and water relations of oak and birch seedlings

Summary First year seedlings of English oak (Quercus Cobur) and silver birch (Betula pendula) were subjected to pressure-volume analysis to investigate the water potential components and cell wall properties of single leaves. It was hoped that this rapid-drying technique would differentiate between reductions in plant solute potential resulting from dehydration and the effects of solute accumulation.Comparison of results from these experiments with those of slow drying treatments (over a number of days) with plants growing in tubes of soil, indicated that some solute accumulation may have occurred in drying oak leaves. High leaf turgor and leaf conductance were maintained for a significant period of the drying cycle. Roots of well-watered oak plants extended deep into the soil profile, and possibly as a result of solute regulation and therefore turgor maintenance, root growth of unwatered plants was greater than that of their well-watered counterparts. This was particularly the case deep in the profile. As a result of deep root penetration, water deep in the soil core was used by oak plants to maintain plant turgor, and quite low soil water potentials were recorded in the lower soil segments.Root growth of well-watered birch seedlings was prolific but roots of both well-watered and unwatered plants were restricted to the upper part of the profile. Root growth of unwatered plants was reduced despite the existence of high soil water potentials deep in the profile. Shallow rooting birch seedlings were unable to use this water.Pressure-volume analysis indicated that significant reductions of water potential, which are required for water uptake from drying soil, would occur in oak with only a small reduction in plant water content compared to the situation in birch. This was a result of the low solute potential in oak leaves combined with a high modulus of elasticity of cell walls. Deep rooting of oak seedlings, combined with these characteristics, which will be particularly important when soil deep in the profile begins to dry, mean that this species may be comparatively successful when growing on dry sites.