Ecophysiological consequences of differences in plant size: in situ carbon gain and water relations of the epiphytic bromeliad, Vriesea sanguinolenta

This field study with the C₃ bromeliad Vriesea sanguinolenta (Cogn. & Marchal 1874) was initiated to explore the importance of size‐related ecophysiological changes in vascular epiphytes in a natural tropical setting. In this species, a step change from atmospheric to tank‐forming life form occurs during early ontogeny, followed by a continuous size increase of individuals with water‐impounding tanks. Although our study focused on the water‐impounding phase, this growth pattern also allowed us to compare ecophysiological consequences of a step change in life form with those associated with size increments among plants of identical life form. The shift in life form was accompanied by relatively minor changes, for example in leaf morphology (decrease in leaf thickness and trichome density) and leaf physiology (decrease in photosynthetic capacity), while there were more substantial changes during the tank‐forming phase. A major trend was a decreasing dependence of larger plants on internally stored water due to a more efficient tank. We suggest that the resulting, more reliable water supply in larger plants may be the proximate cause for the observed size‐related differences in leaf anatomy (relative reduction of water storage tissue, and relative and absolute increase in chlorenchyma thickness), leaf morphology (increase in stomatal density, decrease in trichome density), and leaf physiology (increase in net rates of CO₂ uptake, more conservative stomatal behaviour, higher residual transpiration). The results are compared with previous studies on heteroblasty in bromeliads, but are also discussed in the context of a gradual shift from a drought‐tolerance to a drought‐avoidance strategy.     

Ecophysiology of the Soft Tree Fern,Dicksonia antarctica Labill

Abstract Environmental constraints on gas exchange, stomatal conductance and water relations were investigated in the Soft Tree Fern, Dicksonia antarctica, at sites across its natural distribution and in the glasshouse. Dicksonia antarctica exhibited strong stomatal response down to a vapour pressure deficit (VPD) of 0.25 kPa, an unusual characteristic when compared with other ground fern species. Net photosynthetic rate may be a response of the microenvironment prevalent during frond development, reflecting acclimatory capacity. Both these ecophysiological characteristics are consistent with the ecological niche of D. antarctica, a long‐lived, fire‐resistant species that, during its lifetime, may be exposed to: (i) a humid environment beneath a rainforest canopy; and (ii) an exposed environment following wildfire. Maximum net photosynthesis and quantum yield of photosynthesis correlated strongly with VPD and the maximum net photosynthetic rate of 10.8 µmol m^?2 s^?1 was the highest yet recorded for a fern. These observations are consistent with the relatively low growth typically observed in D. antarctica on sunny, exposed sites and vice versa on cool, humid sites exposed to sunflecks. Favourable water relations maintained under conditions of moderate VPD (2.03 kPa) were probably due to stomatal control. However, inadequate rainfall or high VPD (4.98 kPa) caused water stress, recovery of which was limited by slow water transport through fronds. These observations are consistent with the limitation of D. antarctica distribution to sites sheltered from hot winds and with reliable water supply. The funnel‐shaped rosette of fronds of D. antarctica may harvest rainfall and make it accessible to aerial roots situated at the base of fronds. This process may maintain favourable water relations independently of a subterranean root system. This proposed strategy of water acquisition is unique for a fern species and may eliminate a need for soil moisture competition with surrounding plant species. It is suggested that the ecophysiological characteristics observed in D. antarctica in this study may contribute to the ecological niche it occupies, which is characterized by a variable environment.     

Coordination and trade‐offs between leaf and stem hydraulic traits and stomatal regulation along a spectrum of isohydry to anisohydry

The degree of plant iso/anisohydry, a widely used framework for classifying species‐specific hydraulic strategies, integrates multiple components of the whole‐plant hydraulic pathway. However, little is known about how it associates with coordination of functional and structural traits within and across different organs. We examined stem and leaf hydraulic capacitance and conductivity/conductance, stem xylem anatomical features, stomatal regulation of daily minimum leaf and stem water potential (Ψ), and the kinetics of stomatal responses to vapour pressure deficit (VPD) in six diverse woody species differing markedly in their degree of iso/anisohydry. At the stem level, more anisohydric species had higher wood density and lower native capacitance and conductivity. Like stems, leaves of more anisohydric species had lower hydraulic conductance; however, unlike stems, their leaves had higher native capacitance at their daily minimum values of leaf Ψ. Moreover, rates of VPD‐induced stomatal closure were related to intrinsic rather than native leaf capacitance and were not associated with species' degree of iso/anisohydry. Our results suggest a trade‐off between hydraulic storage and efficiency in the leaf, but a coordination between hydraulic storage and efficiency in the stem along a spectrum of plant iso/anisohydry.     

Photosynthesis is limited at high leaf to air vapor pressure deficit in a mutant of Arabidopsis thaliana that lacks trienoic fatty acids

We have investigated the role of polyunsaturated fatty acids in photosynthesis using a triple mutant of Arabidopsis thaliana that lacks trienoic fatty acids (fad 3-2 fad 7-2 fad 8). Though this mutant is male sterile, vegetative growth and development under normal conditions are largely unaffected (McConn and Browse, 1996 Plant Cell 8: 403–416). At 0.2–1.0 kPa vapor pressure deficit (low VPD), maximum photosynthetic rates of wild-type and mutant plants were similar while stomatal conductance rates were up to 2 times higher in mutant plants. However, light-saturated rates of carbon assimilation and stomatal conductance in the mutant were lower than in wild-type plants when measured at ambient (35 Pa) CO₂ and 2.0–2.8 kPa vapor pressure deficit (high VPD). The limitation to photosynthesis in the mutant plants at high VPD was overcome by saturating partial pressures of CO₂ suggesting a stomatal limitation. Chlorophyll fluorescence measurements indicate that differences observed in maximum assimilation rates were not due to limitations within the photochemical reactions of photosynthesis. Stomatal response to VPD and intrinsic water use efficiency was drastically different in mutant versus wild-type plants. The results of this investigation indicate that for Arabidopsis, polyunsaturated fatty acids may be an important determinant of responses of photosynthesis and stomatal conductance to environmental stresses such as high VPD. This revised version was published online in June 2006 with corrections to the Cover Date.     

Stomatal responses to light and humidity in sugarcane: prediction of daily time courses and identification of potential selection criteria

Abstract Stomatal sensitivities to light and VPD have potential as quantitative selection criteria in programs designed to enhance water-use efficiency of sugarcane and other crops. These responses were characterized using gas exchange techniques and then simulated by a mathematical relationship describing conductance as a function of photon fluence rates and VPD values. The same form of relationship simulated stomatal responses of well-watered greenhouse- and field-grown plants. A comparison between simulated and measured conductance values showed a close correlation, indicating that light and VPD responses of stomata are dominant input signals modulating stomatal conductance in sugarcane. Observed conductance of Hawaiian sugarcane in a commerical production area appeared larger than required to support prevailing rates of carbon assimilation, since predicted intercellular CO₂ was greater than required to saturate its C₄ photosynthesis. Manipulation of the relationship describing stomatal conductance allowed us to simulate the responses of plants with hypothetically altered stomatal sensitivities to VPD or to light, using micrometeorological data collected in the field. Further simulation indicated that selection for clones with altered stomatal sensitivity to either light or VPD could improve the water-use efficiency of sugarcane without inhibiting current high levels of productivity.     

Acclimation to humidity modifies the link between leaf size and the density of veins and stomata

The coordination of veins and stomata during leaf acclimation to sun and shade can be facilitated by differential epidermal cell expansion so large leaves with low vein and stomatal densities grow in shade, effectively balancing liquid‐ and vapour‐phase conductances. As the difference in vapour pressure between leaf and atmosphere (VPD) determines transpiration at any given stomatal density, we predict that plants grown under high VPD will modify the balance between veins and stomata to accommodate greater maximum transpiration. Thus, we examined the developmental responses of these traits to contrasting VPD in a woody angiosperm (Toona ciliata M. Roem.) and tested whether the relationship between them was altered. High VPD leaves were one‐third the size of low VPD leaves with only marginally greater vein and stomatal density. Transpirational homeostasis was thus maintained by reducing stomatal conductance. VPD acclimation changed leaf size by modifying cell number. Hence, plasticity in vein and stomatal density appears to be generated by plasticity in cell size rather than cell number. Thus, VPD affects cell number and leaf size without changing the relationship between liquid‐ and vapour‐phase conductances. This results in inefficient acclimation to VPD as stomata remain partially closed under high VPD.     

Stomatal responses to long-term high vapor pressure deficits mediated most limitation of photosynthesis in tomatoes

Plants grown at high vapor pressure deficit (VPD) usually present decreased photosynthesis, but stomatal and mesophyll limitation to photosynthesis remain poorly quantified. To better understand the regulation of high VPD on photosynthesis and plant growth in tomatoes, we investigated the limitation of stomatal conductance and mesophyll conductance to photosynthesis and relative importance of stomatal morphology and function in stomatal conductance. Both the net photosynthesis rate and total biomass were significantly limited by high VPD. Meanwhile, stomatal conductance and mesophyll conductance were decreased under high VPD. The stomatal conductance limitation was responsible for 60% of the total photosynthetic limitation. Moreover, a reduction in stomatal density and stomatal size occurred under high VPD, which was significantly correlated with the down-regulation of stomatal conductance. The stomatal morphology contributed to more than half the change in stomatal conductance. Nevertheless, stomatal movement was also an important factor in regulating stomatal conductance. The decrease of hydraulic conductance and transpiration rate with no significant difference in relative water content, leaf water potential, and/or osmotic potential suggested passive hydraulic regulation in the feedforward responses of stomata to high VPD.     

Stomatal acclimation to vapour pressure deficit doubles transpiration of small tree seedlings with warming

Future climate change is expected to increase temperature (T) and atmospheric vapour pressure deficit (VPD) in many regions, but the effect of persistent warming on plant stomatal behaviour is highly uncertain. We investigated the effect of experimental warming of 1.9–5.1 °C and increased VPD of 0.5–1.3 kPa on transpiration and stomatal conductance (g_s) of tree seedlings in the temperate forest understory (Duke Forest, North Carolina, USA). We observed peaked responses of transpiration to VPD in all seedlings, and the optimum VPD for transpiration (D_opt) shifted proportionally with increasing chamber VPD. Warming increased mean water use of Carya by 140% and Quercus by 150%, but had no significant effect on water use of Acer. Increased water use of ring‐porous species was attributed to (1) higher air T and (2) stomatal acclimation to VPD resulting in higher g_s and more sensitive stomata, and thereby less efficient water use. Stomatal acclimation maintained homeostasis of leaf T and carbon gain despite increased VPD, revealing that short‐term stomatal responses to VPD may not be representative of long‐term exposure. Acclimation responses differ from expectations of decreasing g_s with increasing VPD and may necessitate revision of current models based on this assumption.     

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。     

Effect of boundary layer conductance on the response of stomata to humidity

Abstract. Leaf conductance responses to leaf to air water vapour partial pressure difference (VPD) have been measured at air speeds of 0.5 and 3.0 ms⁻¹ in single attached leaves of three species in order to test the hypothesis that leaf conductance response to VPD is controlled by evaporation from the outer surface of the epidermis, rather than by evaporation through stomata. Total conductance decreased linearly with increassing VPD at both air speeds, but was decreased 1.6 3.0 times as much as by a given incrase in VPD at high than at low air speed. depending on species. In all species the relationship between leaf conductance and the gradient for evaporation from the epidermis was the same at both values of boundary layer conductance, supporting the hypothesis that direct epidermal evaporation controls stomatal guard cell behaviour in responses of stomata to VPD in these species.     

Stomatal response to air humidity and its relation to stomatal density in a wide range of warm climate species

The gas exchange of 19 widely different warm climate species was observed at different leaf to air vapour pressure deficits (VPD). In all species stomata tended to close as VPD increased resulting in a decrease in net photosynthesis. The absolute reduction in leaf conductance per unit increase in VPD was greatest in those species which had a large leaf conductance at low VPDs. This would be expected even if stomata of all species were equally sensitive. However the percentage reduction in net photosynthesis (used as a measure of the relative sensitivity of stomata of the different species) was also closely related to the maximal conductance at low VPD. Similarily the relative sensitivity of stomata to changes in VPD was closely related to the weighted stomatal density or crowding index.The hypothesis is presented that stomatal closure at different VPDs is related to peristomatal evaporation coupled with a high resistance between the epidermis and the mesophyll and low resistance between the stomatal apparatus and the epidermal cells. This hypothesis is consistent with the greater relative sensitivity of stomata on leaves with a high crowding index.The results and the hypothesis are discussed in the light of selection, for optimal productivity under differing conditions of relative humidity and soil water availablility, by observation of stomatal density and distribution on the two sides of the leaf.Visiting scientist, plant physiologist and research assitant of the Cassava Program     

Stomatal sensitivity to carbon dioxide and humidity: a comparison of two c(3) and two c(4) grass species

The sensitivity of stomatal conductance to changes of CO₂ concentration and leaf-air vapor pressure difference (VPD) was compared between two C₃ and two C₄ grass species. There was no evidence that stomata of the C₄ species were more sensitive to CO₂ than stomata of the C₃ species. The sensitivity of stomatal conductance to CO₂ change was linearly proportional to the magnitude of stomatal conductance, as determined by the VPD, the same slope fitting the data for all four species. Similarly, the sensitivity of stomatal conductance to VPD was linearly proportional to the magnitude of stomatal conductance. At small VPD, the ratio of intercellular to ambient CO₂ concentration, C_i/C_a, was similar in all species (0.8-0.9) but declined with increasing VPD, so that, at large VPD, C_i/C_a was 0.7 and 0.5 (approximately) in C₃ and C₄ species, respectively. Transpiration efficiency (net CO₂ assimilation rate/transpiration rate) was larger in the C₄ species than in the C₃ species at current atmospheric CO₂ concentrations, but the relative increase due to high CO₂ was larger in the C₃ than in the C₄ species.     

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.     

Ozone‐induced stomatal sluggishness changes stomatal parameters of Jarvis‐type model in white birch and deciduous oak

• Stomatal ozone flux is closely related to ozone injury to plants. Jarvis‐type multiplicative model has been recommended for estimating stomatal ozone flux in forest trees. Ozone can change stomatal conductance by both stomatal closure and less efficient stomatal control (stomatal sluggishness). However, current Jarvis‐type models do not account for these ozone effects on stomatal conductance in forest trees.
• We examined seasonal course of stomatal conductance in two common deciduous tree species native to northern Japan (white birch: Betula platyphylla var. japonica ; deciduous oak: Quercus mongolica var. crispula ) grown under free‐air ozone exposure. We innovatively considered stomatal sluggishness in the Jarvis‐type model using a simple parameter, s , relating to cumulative ozone uptake (defined as POD : phytotoxic ozone dose).
• We found that ozone decreased stomatal conductance of white birch leaves after full expansion (?28%). However, such a reduction of stomatal conductance by ozone fell in late summer (?10%). At the same time, ozone reduced stomatal sensitivity of white birch to VPD and increased stomatal conductance under low light conditions. In contrast, in deciduous oak, ozone did not clearly change the model parameters.
• The consideration of both ozone‐induced stomatal closure and stomatal sluggishness improved the model performance to estimate stomatal conductance and to explain the dose–response relationship on ozone‐induced decline of photosynthesis of white birch. Our results indicate that ozone effects on stomatal conductance (i.e . stomatal closure and stomatal sluggishness) are crucial for modelling studies to determine stomatal response in deciduous trees, especially in species sensitive to ozone.

     

冬小麦叶片气孔导度模型水分响应函数的参数化

植物气孔导度模型的水分响应函数用来模拟水分胁迫对气孔导度的影响过程, 是模拟缺水环境下植物与大气间水、碳交换过程的关键算法。水分响应函数包括空气湿度响应函数和土壤湿度(或植物水势)响应函数, 该研究基于田间实验观测, 分析了冬小麦(Triticum aestivum)叶片气孔导度对不同空气饱和差和不同土壤体积含水量或叶水势的响应规律。一个土壤水分梯度的田间处理在中国科学院禹城综合试验站实施, 不同水分胁迫下的冬小麦叶片气体交换过程和气孔导度以及其他的温湿度数据被观测, 同时观测了土壤含水量和叶水势。实验数据表明, 冬小麦叶片气孔导度对空气饱和差的响应呈现双曲线规律, 变化趋势显示大约1 kPa空气饱和差是一个有用的阈值, 在小于1 kPa时, 冬小麦气孔导度对空气饱和差变化反应敏感, 而大于1 kPa后则反应缓慢; 分析土壤体积含水量与中午叶片气孔导度的关系发现, 中午叶片气孔导度随土壤含水量增加大致呈现线性增加趋势, 但在平均土壤体积含水量大于大约25%以后, 气孔导度不再明显增加, 而是维持在较高导度值上下波动; 冬小麦中午叶片水势与相应的气孔导度之间, 随着叶水势的增加, 气孔导度呈现增加趋势。根据冬小麦气孔导度对空气湿度、土壤湿度和叶水势的响应规律, 研究分别采用双曲线和幂指数形式拟合了水汽响应函数, 用三段线性方程拟合了土壤湿度响应函数和植物水势响应函数, 得到的参数可以为模型模拟冬小麦的各类水、热、碳交换过程采用。     

The photosynthetic response of tobacco plants overexpressing ice plant aquaporin McMIPB to a soil water deficit and high vapor pressure deficit

We investigated the photosynthetic capacity and plant growth of tobacco plants overexpressing ice plant (Mesembryanthemum crystallinum L.) aquaporin McMIPB under (1) a well-watered growth condition, (2) a well-watered and temporal higher vapor pressure deficit (VPD) condition, and (3) a soil water deficit growth condition to investigate the effect of McMIPB on photosynthetic responses under moderate soil and atmospheric humidity and water deficit conditions. Transgenic plants showed a significantly higher photosynthesis rate (by 48 %), higher mesophyll conductance (by 52 %), and enhanced growth under the well-watered growth condition than those of control plants. Decreases in the photosynthesis rate and stomatal conductance from ambient to higher VPD were slightly higher in transgenic plants than those in control plants. When plants were grown under the soil water deficit condition, decreases in the photosynthesis rate and stomatal conductance were less significant in transgenic plants than those in control plants. McMIPB is likely to work as a CO₂ transporter, as well as control the regulation of stomata to water deficits.     

Soil water deficits decrease the internal conductance to CO2 transfer but atmospheric water deficits do not

The internal conductance to CO₂ supply from substomatal cavitiesto sites of carboxylation poses a large limitation to photosynthesis.It is known that internal conductance is decreased by soil waterdeficits, but it is not known if it is affected by atmosphericwater deficits (i.e. leaf to air vapour pressure deficit, VPD).The aim of this paper was to examine the responses of internalconductance to atmospheric and soil water deficits in seedlingsof the evergreen perennial Eucalyptus regnans F. Muell and theherbaceous plants Solanum lycopersicum (formerly Lycopersiconesculentum) Mill. and Phaseolus vulgaris L. Internal conductancewas estimated with the variable J method from concurrent measurementsof gas exchange and fluorescence. In all three species steady-statestomatal conductance decreased by 30% as VPD increased from1 kPa to 2 kPa. In no species was internal conductance affectedby VPD despite large effects on stomatal conductance. In contrast,soil water deficits decreased stomatal conductance and internalconductance of all three species. Decreases in stomatal andinternal conductance under water deficit were proportional,but this proportionality differed among species, and thus therelationship between stomatal and internal conductance differedamong species. These findings indicate that soil water deficitsaffect internal conductance while atmospheric water deficitsdo not. The reasons for this distinction are unknown but areconsistent with soil and atmospheric water deficits having differingeffects on leaf physiology and/or root–shoot communication. Key words: Carbon dioxide, drought, internal conductance, mesophyll conductance, photosynthesis, stomatal conductance, transfer conductance, vapour pressure deficit, water deficit Received 11 October 2007; Revised 9 November 2007 Accepted 15 November 2007     

Stomatal function,density and pattern,and CO2 assimilation in Arabidopsis thaliana tmm1 and sdd1‐1 mutants

• Stomata modulate the exchange of water and CO₂ between plant and atmosphere. Although stomatal density is known to affect CO₂ diffusion into the leaf and thus photosynthetic rate, the effect of stomatal density and patterning on CO₂ assimilation is not fully understood.
• We used wild types Col‐0 and C24 and stomatal mutants sdd1‐1 and tmm1 of Arabidopsis thaliana, differing in stomatal density and pattern, to study the effects of these variations on both stomatal and mesophyll conductance and CO₂ assimilation rate. Anatomical parameters of stomata, leaf temperature and carbon isotope discrimination were also assessed.
• Our results indicate that increased stomatal density enhanced stomatal conductance in sdd1‐1 plants, with no effect on photosynthesis, due to both unchanged photosynthetic capacity and decreased mesophyll conductance. Clustering (abnormal patterning formed by clusters of two or more stomata) and a highly unequal distribution of stomata between the adaxial and abaxial leaf sides in tmm1 mutants also had no effect on photosynthesis.
• Except at very high stomatal densities, stomatal conductance and water loss were proportional to stomatal density. Stomatal formation in clusters reduced stomatal dynamics and their operational range as well as the efficiency of CO₂ transport.

     

Differential response of stomata to air humidity in the parasitic mistletoe (Phthirusa pyrifolia) and its host,mandarin orange (Citrus resitulata)

Measurements of CO₂ and H₂O exchange rate and the calculated leaf conductance of attached leaves were conducted over a range of leaf-to-air vapour pressure difference (VPD) (1.5 to 5.5 kPa) to compare the response of the parasitic mistletoe, Phthirusa pyrifolia, with that of its host, the mandarin orange, Citrus reticulata. Seedlings of the host infected with the parasite were grown in well-watered and adequately fertilized large pots outdoors at the CIAT headquarters, Palmira, Colombia, South America. Observations of leaf anatomy of the parasite and nutrient analysis of young tissues of both the parasite and host were made. The photosynthetic rate of the host decreased linearly with increased VPD, whereas the parasite showed a constant rate. This trend coincided with similar responses in leaf conductance. Due to the insensitivity of the parasite stomata, the transpiration rate increased linearly with VPD as compared with an initial increase and then a decrease in the host transpiration rate. The higher photosynthetic rate and the closure of stomata of the host resulted in high water use efficiency as compared with that of the parasite. The parasite accumulated in its leaves more N, P, K and less Ca and Mg than the host. The significance of the host-parasite differential response to air humidity is discussed in relation to mechanism underlying stomatal sensitivity and in the context of host-parasite association.Visiting Scientist, Coordinator, and Research Assistant of the Cassava Physiology Program.     

Stomatal dynamics are limited by leaf hydraulics in ferns and conifers: results from simultaneous measurements of liquid and vapour fluxes in leaves

Stomatal responsiveness to vapour pressure deficit (VPD) results in continuous regulation of daytime gas‐exchange directly influencing leaf water status and carbon gain. Current models can reasonably predict steady‐state stomatal conductance (g_s) to changes in VPD but the g_s dynamics between steady‐states are poorly known. Here, we used a diverse sample of conifers and ferns to show that leaf hydraulic architecture, in particular leaf capacitance, has a major role in determining the g_s response time to perturbations in VPD. By using simultaneous measurements of liquid and vapour fluxes into and out of leaves, the in situ fluctuations in leaf water balance were calculated and appeared to be closely tracked by changes in g_s thus supporting a passive model of stomatal control. Indeed, good agreement was found between observed and predicted g_s when using a hydropassive model based on hydraulic traits. We contend that a simple passive hydraulic control of stomata in response to changes in leaf water status provides for efficient stomatal responses to VPD in ferns and conifers, leading to closure rates as fast or faster than those seen in most angiosperms.