Cuticular Conductance and the Humidity Response of Stomata

Meidner, H. 1986. Cuticular conductance and the humidity responseof stomata.—J. exp. Bot. 37: 517–525. Detailed measurements of cuticular vapour loss from leaves ofseveral species showed that cuticular conductance declined froman early morning maximum of 0?02 cm s^–1 to between 0?004and 0.005 cm scm s^–1 even in the absence of stomatal transpiration.Re-establishment of the maximum conductance occurred only ina humid atmosphere and when the xylem system was under pressure(simulated mild root pressure) Cuticular vapour loss alone is,therefore, unlikely to be the underlying mechanism of the humidityresponse of Stomata. Evidence for the existence of a humidity-sensing feed-forwardmechanism is discussed and it is shown that when detailed measurementsare made the humidity response is found to have two phases.This indicates a perturbation of the fine turgor balance betweenepidermat and guard cells that exists in a transpiring leaf.It is argued that the humidity response can be accounted forby reference to hydropassive movements which initiate a metabolicadjustment of the guard cells to altered evaporative demand. Key words: Cuticle, conductance, humidity, stomata, transpiration     

Stomatal responses to humidity in air and helox

Abstract. Stomatal responses to humidity were studied in several species using normal air and a helium: oxygen mixture (79:21 v/v, with CO₂ and water vapour added), which we termed 'helox'. Since water vapour diffuses 2.33 times faster in helox than in air, it was possible to vary the water-vapour concentration difference between the leaf and the air at the leaf surface independently of the transpiration rate and vice versa. The CO₂ concentration at the evaporating surfaces ( c_i ), leaf temperature and photon flux density were kept constant throughout the experiments. The results of these experiments were consistent with a mechanism for Stomatal responses to humidity that is based on the rate of water loss from the leaf. Stomata apparently did not directly sense and respond to either the water vapour concentration at the leaf surface or the difference in water vapour concentration between the leaf interior and the leaf surface. In addition, stomatal responses that caused reductions in transpiration rate at low humidities were accompanied by decreases in photosynthesis at constant c_i , suggesting heterogeneous (patchy) stomatal closure.     

A Method for Separating Cuticular and Stomatal Components of Gas Exchange by Amphistomatous Leaves: II. EXPERIMENTAL SOLUTION

The apparent cuticular component of transpiration of stomatabearing leaf epidermis was estimated by restricting stomataldiffusion by mass flow of air in the opposite direction. Thiswas achieved by applying an air pressure gradient across theamphistomatous leaf. Some assumptions of the previously suggestedmethod (antrcek and Slav?k, 1990) were experimentally verifiedusing maize leaves. The technique makes possible a quantitativeestimation of cuticular water loss including that of the externalperistomatal (i.e. vapour not passing through the pores) andthe respective conductance when the stomata are partially open. In addition to the fact that the cuticular portion of the totalleaf vapour loss (i.e. relative cuticular transpiration) dependson stomatal opening, even the absolute value of apparent cuticulartranspiration was (1) increased by lower vapour pressure deficitand (2) decreased with closing stomata. These changes, inducedby variations in a vapour pressure deficit of 2.45?0.35 kPa,ranged between 0.66?0.14µg cm ^–2 s^–1. Theabsolute value of apparent cuticular transpiration changed onaverage by a factor of 2.3 due to stomata opening or closingwhich was induced by turning the light on or by exogenous ABAapplication. Possible interference by residual vapour diffusingthrough the stomatal pore was evaluated by the model application.An attempt was also made to assess the cuticular component ofCO₂-uptake rate. Experimental results are discussed in contextwith the feedforward response of stomata to air humidity. Key words: Cuticular transpiration, cuticular CO₂-uptake, feedforward response, maize     

Ecophysiological relevance of cuticular transpiration of deciduous and evergreen plants in relation to stomatal closure and leaf water potential

The water permeability of the leaves of three deciduous plants (Acer campestre, Fagus sylvatica, Quercus petraea) and two evergreen plants (Hedera helix, Ilex aquifolium) was analysed in order to assess its role as a mechanism of drought resistance. Cuticular permeances were determined by measurement of the water loss through adaxial, astomatous leaf surfaces. Minimum conductances after complete stomatal closure were obtained by leaf drying curves. The comparison of the water permeabilities determined with these two experimental systems revealed good agreement in the case of Acer, Fagus, Quercus, and Ilex. For Hedera the minimum conductance was 3-fold higher than the cuticular permeance indicating a significant contribution of residual stomatal transpiration. The leaf water potential was measured as a function of water content and analysed by pressure-volume curves. The influence of water potential as a component of the driving force for transpirational water loss was assessed in order to identify modifications of the cuticular barrier by the leaf water content. The ecophysiological meaning of the water relations parameters describing transpiration under drought conditions (cuticular transpiration, minimum transpiration, residual stomatal transpiration, effect of leaf water content on transpiration) and the water relations parameters derived from pressure-volume curves (osmotic potential at full saturation, turgor loss point, bulk modulus of elasticity) are discussed with regard to adaptations for drought resistance.     

Stomatal response to humidity in sugarcane and soybean: effect of vapour pressure difference on the kinetics of the blue light response

Abstract. The effect of atmospheric humidity on the kinetics of stomatal responses was quantified in gas exchange experiments using sugarcane ( Saccharum spp. hybrid) and soybean ( Glycine max ). Pulses of blue light were used to elicit pulses of stomatal conductance that were mediated by the specific blue light response of guard cells. Kinetic parameters of the conductance response were more closely related to leaf-air vapour pressure difference (VPD) than to relative humidity or transpiration. Increasing VPD significantly accelerated stomatal opening in both sugarcane and soybean, despite an approximately five-fold faster response in sugarcane. In contrast, the kinetics of stomatal recovery (closure) following the pulse were similar in the two species. Acceleration of opening by high VPD was observed even under conditions where soybean exhibited a feedforward response of decreasing transpiration (E) with increasing evaporative demand (VPD). This result suggests that epidermal, rather than bulk leaf, water status mediates the VPD effect on stomatal kinetics. The data are consistent with the hypothesis that increased cpidermal water loss at high VPD decreases the backpressure exerted by neighbouring cells on guard cells. allowing more rapid stomatal opening per unit of guard cell metabolic response to blue light.     

The kinetics of stomatal responses to VPD in Vicia faba: electrophysiological and water relations models

Abstract. An Ohm's law analogy is frequently employed to calculate parameters of leaf gas exchange. For example, resistance to water vapour loss is calculated as the quotient of vapour pressure difference (VPD) and vapour loss by transpiration. In the present research, this electrical analogy was extended. Steady-state transpiration as a function of VPD, assayed in leaflets of Vicia faba using gas exchange techniques, was compared with steady-state K⁺ current magnitude as a function of voltage in isolated guard cell protoplasts of Vicia faba, assayed using the patch clamping technique in the whole cell configuration. An electrophysiological model originally developed to explain the kinetics of current changes following step changes in voltage across a cell membrane was used to fit the kinetics of transpiration changes following step changes in VPD applied to leaflets of Vicia faba. Following step increases in VPD, transpiration exhibited an initial increase, reflecting the increased driving force for water loss and, for large step increases in VPD, a transient decrease in stomatal resistance. Transpiration subsequently declined, reflecting stomatal closure. By analogy to electrophysiological responses, it is hypothesized that the humidity parameter that is sensed by guard cells is VPD. Two models based on epidermal water relations were also applied to transpiration kinetics. In the first model, the transient increase in transpiration following a step increase in VPD was attributed partially to an increase in the Physical driving force (VPD) and partially to a transient decrease in stomatal resistance resulting from reduced epidermal backpressure. In the second model, the transient decrease in stomatal resistance was attributed to a direct response of the guard cells to VPD. Both models based on water relations gave good fits of the data, emphasizing the need for further study regarding the metabolic nature of the guard cell response to humidity.     

ABA-deficient (aba1) and ABA-insensitive (abi1-1, abi2-1) mutants of Arabidopsis have a wild-type stomatal response to humidity

In most plant species, a decrease in atmospheric humidity at the leaf surface triggers a decrease in stomatal conductance. While guard cells appear to respond to humidity‐induced changes in transpiration rate, as opposed to relative humidity or vapour pressure difference, the underlying cellular mechanisms for this response remain unknown. In the present set of experiments, abscisic acid (ABA)‐deficient (aba1) and ABA‐insensitive (abi1‐1 and abi2‐1) mutants of Arabidopsis thaliana were used to test the hypothesis that the humidity signal is transduced by changes in the flux or concentration of ABA delivered to the stomatal complex in the transpiration stream. In gas exchange experiments, stomatal conductance was as sensitive to changes in vapour pressure difference in aba1, abi1‐1 and abi2‐1 mutant plants as in wild‐type plants. These experiments appear to rule out an obligate role for either the concentration or flux of ABA or ABA conjugates as mediators of the guard cell response to atmospheric water potential. The results stand in contrast to the well‐established role of ABA in mediating guard cell responses to decreases in soil water potential.     

空气湿度与土壤水分胁迫对紫花苜蓿叶表皮蜡质特性的影响

选用2个抗旱性不同的紫花苜蓿品种,敖汉(强抗旱)和三得利(弱抗旱),设置空气湿度(45%-55%和75%-85%)和土壤水分胁迫(75%和35%田间持水量)处理,分析紫花苜蓿叶表皮蜡质含量、组分及晶体结构、气体交换参数、水势及脯氨酸含量的变化规律。结果表明,单独土壤水分胁迫时,紫花苜蓿叶表皮蜡质晶体结构及蜡质总量无显著变化;敖汉蜡质组分中烷类、酯类含量增加,醇类含量下降;三得利醇类含量下降,烷类、酯类含量变化不显著。低空气湿度胁迫时,两品种蜡质总量无显著变化,烷类和酯类含量显著增加,醇类含量显著下降,叶表皮片状蜡质晶体结构熔融呈弥漫性,扩大了对叶表面积的覆盖,其蒸腾速率显著低于正常湿度。复合胁迫处理时,叶表皮片状蜡质晶体结构继续呈弥漫性,烷类、酯类、未知蜡质组分含量均高于单独胁迫处理,醇类含量最低,而蜡质总量除三得利显著高于对照外,其余均无显著差异。紫花苜蓿叶表皮蜡质各组分含量(除醇类)及蜡质总量与光合速率呈显著负相关,与蒸腾速率无显著相关关系。蜡质总量与叶水势呈显著正相关。总体上,敖汉蜡质总量显著高于三得利,蜡质组分中烷类物质的增加有助于提高植株的抗旱性。在复合胁迫下,强抗旱品种主要通过气孔因素控制水分散失,而弱抗旱品种通过气孔和非气孔因素共同控制植物水分散失。     

Co-ordination of vapour and liquid phase water transport properties in plants

The pathway for water movement from the soil through plants to the atmosphere can be represented by a series of liquid and vapour phase resistances. Stomatal regulation of vapour phase resistance balances transpiration with the efficiency of water supply to the leaves, avoiding leaf desiccation at one extreme, and unnecessary restriction of carbon dioxide uptake at the other. In addition to maintaining a long-term balance between vapour and liquid phase water transport resistances in plants, stomata are exquisitely sensitive to short-term, dynamic perturbations of liquid water transport. In balancing vapour and liquid phase water transport, stomata do not seem to distinguish among potential sources of variation in the apparent efficiency of delivery of water per guard cell complex. Therefore, an apparent soil-to-leaf hydraulic conductance based on relationships between liquid water fluxes and driving forces in situ seems to be the most versatile for interpretation of stomatal regulatory behaviour that achieves relative homeostasis of leaf water status in intact plants. Components of dynamic variation in apparent hydraulic conductance in intact plants include, exchange of water between the transpiration stream and internal storage compartments via capacitive discharge and recharge, cavitation and its reversal, temperature-induced changes in the viscosity of water, direct effects of xylem sap composition on xylem hydraulic properties, and endogenous and environmentally induced variation in the activity of membrane water channels in the hydraulic pathway. Stomatal responses to humidity must also be considered in interpreting co-ordination of vapour and liquid phase water transport because homeostasis of bulk leaf water status can only be achieved through regulation of the actual transpirational flux. Results of studies conducted with multiple species point to considerable convergence with regard to co-ordination of stomatal and hydraulic properties. Because stomata apparently sense and respond to integrated and dynamic soil-to-leaf water transport properties, studies involving intact plants under both natural and controlled conditions are likely to yield the most useful new insights concerning stomatal co-ordination of transpiration with soil and plant hydraulic properties.     

Transpiration in potato plants infected with Verticillium spp

Potato plants (cv. King Edward) infected with Verticillium albo-atrum and with V. dahliae transpired more slowly than healthy plants; this difference increased as the disease progressed. Diurnal fluctuations in transpiration were smaller in infected plants than in controls because infection markedly reduced water loss during the normal daytime peak period. Transpiration at night was unaffected by infection.
Both stomatal and cuticular transpiration of single, detached leaves were reduced by infection. A linear correlation was obtained between 'water saturation deficit' and transpiration rate in both diseased and healthy plants until the leaves wilted, suggesting that reductions in the stomatal rate are a consequence of the greater water deficits found in diseased plants, the differences in cuticular rates probably being due to anatomical differences between healthy and diseased leaves.
Close parallels between transpiration and water deficit indicate that in diseased plants water loss is largely determined by leaf water content. Thus wilting, commonly seen as a symptom of infection, is not the result of excessive water loss but follows a reduction in the supply of water to the leaves.
The author thanks Professor I. Isaac of this department and Dr G. C. Evans of the Botany School, Cambridge for their advice. The research was sponsored by the Potato Marketing Board.     

Does transpiration control stomatal responses to water vapour pressure deficit?

Three types of observations were used to test the hypothesis that the response of stomatal conductance to a change in vapour pressure deficit is controlled by whole-leaf transpiration rate or by feedback from leaf water potential. Varying the leaf water potential of a measured leaf by controlling the transpiration rate of other leaves on the plant did not affect the response of stomatal conductance to vapour pressure deficit in Glycine max. In three species, stomatal sensitivity to vapour pressure deficit was eliminated when measurements were made at near-zero carbon dioxide concentrations, despite the much higher transpiration rates of leaves at low carbon dioxide. In Abutilon theophrasti, increasing vapour pressure deficit sometimes resulted in both decreased stomatal conductance and a lower transpiration rate even though the response of assimilation rate to the calculated substomatal carbon dioxide concentration indicated that there was no ‘patchy’ stomatal closure at high vapour pressure deficit in this case. These results are not consistent with stomatal closure at high vapour pressure deficit caused by increased whole-leaf transpiration rate or by lower leaf water potential. The lack of response of conductance to vapour pressure deficit in carbon dioxide-free air suggests that abscisic acid may mediate the response.     

Comparing model predictions and experimental data for the response of stomatal conductance and guard cell turgor to manipulations of cuticular conductance, leaf-to-air vapour pressure difference and temperature: feedback mechanisms are able to account for all observations

Stomata respond to increasing leaf-to-air vapour pressure difference (LAVPD) ( D ) by closing. The mechanism by which this occurs is debated. A role for feedback and peristomatal transpiration has been proposed. In this paper, we apply a recent mechanistic model of stomatal behaviour, and compare model and experimental data for the influence of increasing D on stomatal conductance.
We manipulated cuticular conductance ( g _c) by three independent methods. First, we increased g _c by using a solvent mixture applied to both leaf surfaces prior to determining stomatal responses to D ; second, we increased g _c by increasing leaf temperature at constant D ; and third, we coated a small area of leaf with a light oil to decrease g _c. In all three experiments, experimental data and model outputs showed very close agreement.
We conclude, from the close agreement between model and experimental data and the fact that manipulations of g _c, and hence cuticular transpiration, influenced g _s in ways consistent with a feedback mechanism, that feedback is central in determining stomatal responses to D .     

Stomatal and environmental control of transpiration in a lowland tropical forest tree

Stomatal control of crown transpiration was studied in Anacardium excelsum, a large-leaved, emergent canopy species common in the moist forests of Central and northern South America. A construction crane equipped with a gondola was used to gain access to the uppermost level in the crown of a 35-m-tall individual. Stomatal conductance at the single leaf scale, and transpiration and total vapour phase conductance (stomatal and boundary layer) at the branch scale were measured simultaneously using the independent techniques of porometry and stem heat balance, respectively. This permitted the sensitivity of transpiration to a marginal change in stomatal conductance to be evaluated using a dimensionless coupling coefficient (1-ω) ranging from zero to 1, with 1 representing maximal stomatal control of transpiration. Average stomatal conductance varied from 0.09 mol m^?2 s^?1 during the dry season to 0.3 mol m^?2 s^?1 during the wet season. Since boundary layer conductance was relatively low (0.4 mol m^?2 s^?1), 1-ω ranged from 0.46 during the dry season to only 0.25 during the wet season. A pronounced stomatal response to humidity was observed, which strongly limited transpiration as evaporative demand increased. The stomatal response to humidity was apparent only when the leaf surface was used as the reference point for measurement of external vapour pressure. Average transpiration was predicted to be nearly the same during the dry and wet seasons despite a 1 kPa difference in the prevailing leaf-to-air vapour pressure difference. The patterns of stomatal behaviour and transpiration observed were consistent with recent proposals that stomatal responses to humidity are based on sensing the transpiration rate itself.     

Epidermal transpiration and stomatal responses to humidity: Some hypotheses explored

Abstract Stomatal responses to humidity as affected by both evaporation from the epidermis and the hydraulic conductance of the transpiration stream to evaporation sites on the epidermis are discussed.
Recent estimates of evaporation from the inner walls of the epidermis are too high because the cell wall surfaces were assumed completely wet, and leaves have usually been considered isothermal.
It is suggested that a fall in humidity increases evaporation from the epidermis, and that stomata respond to the consequent fall in water potential. Cuticular transiration is inversely related to stomatal conductance. Thus, evaporation from the epidermis is dependent on the stomatal, boundary layer, and cuticular conductances, and on evaporation from the inner walls of the epidermis. Stomatal responses to humidity will change as the boundary layer conductance changes.
The conductance of the transpiration stream is a determinant of the water potential of the epidermis. Water potentials of adjacent cells will be more similar if flow is symplastic than if it is apoplastic. It is concluded that flow in living tissues is primarily symplastic over long distances, but over shorter distances it is increasingly apoplastic, and that stomatal responses to humidity are mediated by the water potential of the whole epidermis.     

几个气孔模型在自然条件下的适用性

在自然条件下,用气孔下腔与叶面间的水汽压差（ＶＰＤｓ）取代原有气孔模型中的大气湿度因子,可以明显提高气孔模型在自然条件下的适用性。理论分析指出,在气孔模型中,用ＶＰＤｓ表达气孔导度对湿度的响应与用蒸腾速率表达气孔导度对蒸腾失水的响应是等价的。     

Plant response to drought stress simulated by ABA application: Changes in chemical composition of cuticular waxes

Plant cuticles form the interface between epidermal plant cells and the atmosphere. The cuticle creates an effective barrier against water loss, bacterial and fungal infection and also protects plant tissue from UV radiation. It is composed of the cutin matrix and embedded soluble lipids also called waxes. Chemical composition of cuticular waxes and physiological properties of cuticles are affected by internal regulatory mechanisms and environmental conditions (e.g. drought, light, and humidity). Here, we tested the effect of drought stress simulation by the exogenous application of abscisic acid (ABA) on cuticular wax amount and composition. ABA-treated plants and control plants differed in total aboveground biomass, leaf area, stomatal density and aperture, and carbon isotope composition. They did not differ in total wax amount per area but there were peculiar differences in the abundance of particular components. ABA-treated plants contained significantly higher proportions of aliphatic components characterized by chain length larger than C₂₆, compared to control plants. This trend was consistent both between and within different functional groups of wax components. This can lead to a higher hydrophobicity of the cuticular transpiration barrier and thus decrease cuticular water loss in ABA-treated plants. At both ABA-treated and control plants alcohols with chain length C₂₄ and C₂₆ were predominant. Such a shift towards wax compounds having a higher average chain length under drought conditions can be interpreted as an adaptive response of plants towards drought stress.     

Stomatal and cuticular transpiration of beans (Phaseolus vulgaris M.) attacked by rust (Uromyces appendiculatus [Pers.] Link)

The changes of stomatal and cuticular transpiration of bean plants were investigated by graphical transpiration curves method (Slavík 1958). Bean leaves were infected by fungusUromyces appendiculatus (Pers.) Link. After the infection the intensity of stomatal transpiration had a decreasing tendency. Beginning with the sixth day after infection, the proportion of stomatal and cuticular transpirations becomes more expressive, i.e. the leaves transpire more by cuticles than by stomata. The higher share of cuticular transpiration brings extensive water relations to the diseased plants.     

Stomatal response to environment and a possible interrelation between stomatal effects on transpiration and CO2 assimilation

Abstract It had been hypothesized that if daily CO₂ assimilation is to be maximized at a given level of daily transpiration, stomatal apertures should change during the day so that the gain ratio (?A/?g)/(?E/?g) remains constant. These partial differentials describe the sensitivity of assimilation rate (A) and transpiration rate (E) to changes in stomatal conductance (g). Experiments were conducted to determine whether stomata respond to environment in a manner which results in constant gain ratios. Gas–exchange measurements were made of the stomatal and photosynthetic responses of Vigna unguiculata L. Walp. in controlled environments. Leaf conductance to water vapour responded to step changes in temperature and humidity so that for different steady-state conditions the gain ratio remained constant on all but one day. Depletion of water in the root zone resulted in day-to-day increases in gain ratio which were correlated with decreases in maximum leaf conductance to water vapour. The significance of the results for plant adaptation and stomatal mechanisms, and methods for measuring the gain ratio, are discussed.     

Stomatal Response to Humidity and Lanthanum

Lanthanum fed to the base of excised leaves of Sesamum indicum L. and Helianthus annuus L. was used as a tracer to investigate by electron microscopy the path of water in the apoplast of leaves. The generally random distribution of lanthanum in cell walls provided no support for the hypothesis that cuticular transpiration may be greater for guard cells than for adjacent epidermal cells. Occasionally, accumulations of lanthanum were observed in anticlinal walls of epidermal cells and at the outer surface of the plasma membrane but lanthanum was not observed in the symplast. The influx of ⁸⁶Rb to excised roots of sesame and sunflower was inhibited during incubation with 0.5 mM lanthanum or calcium for 15 or for 180 min. Stomata of sunflower partially closed when 2.5 mM lanthanum was supplied to the base of excised shoots in a potometer, whereas this treatment had little effect on stomatal conductance of sesame shoots maintained in a constant environment. Supplying 2.5 mM lanthanum to the base of sesame shoots strongly inhibited stomatal opening response to increase in ambient humidity but had little effect on stomatal opening response to light. It was concluded that stomatal opening response to increased humidity may be dependent upon some process, such as ion influx, that is inhibited by lanthanum, and that opening response to humidity may differ in mechanism from stomatal opening response to increased irradiance.