Patchy Stomatal Closure Research Articles

Temporal fluctuation of patchy stomatal closure in leaves of <i>Dipterocarpus sublamellatus</i> at upper canopy in Peninsular Malaysia over the last decade

Temporal fluctuation of patchy stomatal closure in leaves of <i>Dipterocarpus sublamellatus</i> at upper canopy in Peninsular Malaysia over the last decade

Occurrence of stomatal patchiness and its spatial scale in leaves from various sizes of trees distributed in a South-east Asian tropical rainforest in Peninsular Malaysia.

In this study, we demonstrated the occurrence of stomatal patchiness and its spatial scale in leaves from various sizes of trees grown in a lowland dipterocarp forest in Peninsular Malaysia. To evaluate the patterns of stomatal behavior, we used three techniques simultaneously to analyze heterobaric or homobaric leaves from five tree species ranging from 0.6 to 31 m in height: (i) diurnal changes in chlorophyll fluorescence imaging, (ii) observation and simulation of leaf gas-exchange rates and (iii) a pressure-infiltration method. Measurements were performed in situ with 1000 or 500 μmol m(-2) s(-1) photosynthetic photon flux density. Diurnal patterns in the spatial distribution of photosynthetic electron transport rate (J) mapped from chlorophyll fluorescence images, a comparison of observed and simulated leaf gas-exchange rates, and the spatial distribution of stomatal apertures obtained from the acid-fuchsin-infiltrated area showed that patchy stomatal closure coupled with severe midday depression of photosynthesis occurred in Neobalanocarpus heimii (King) Ashton, a higher canopy tree with heterobaric leaves due to the higher leaf temperature and vapor pressure deficit. However, subcanopy or understory trees showed uniform stomatal behavior throughout the day, although they also have heterobaric leaves. These results suggest that the occurrence of stomatal patchiness is determined by tree size and/or environmental conditions. The analysis of spatial scale by chlorophyll fluorescence imaging showed that several adjacent anatomical patches (lamina areas bounded by bundle-sheath extensions within the lamina) may co-operate for the distributed patterns of J and stomatal apertures.

Effects of pulses of elevated carbon dioxide concentration on stomatal conductance and photosynthesis in wheat and rice

Systems for exposing plants to elevated concentrations of CO₂ may expose plants to pulses of CO₂ concentrations considerably above the control set point. The importance of such pulses to plant function is unknown. Single leaves of wheat (Triticum aestivum cv Choptank) and rice (Oryza sativa cv Akitakomachi) were exposed for 30 minutes to pulses of elevated CO₂ similar to the frequency, duration and magnitude of pulses observed in free-air CO₂ enrichment systems. Stomatal conductance decreased within a few minutes of exposure to once per minute pulses of high CO₂ of all the durations tested, in both species. Both species had 20-35% lower stomatal conductance for at least 30 min after the termination of the pulses. After the pulses had stopped, in all cases photosynthesis was below the values expected for the observed substomatal CO₂ concentration, which suggests that either patchy stomatal closure occurred or that photosynthesis was directly inhibited. It was also found that a single, 2 s pulse of elevated CO₂ concentration reaching a maximum of 1000 µmol mol⁻¹ decreased stomatal conductance in both species. On the basis of these results, it is probable that plants in many CO₂ enrichment systems have lower photosynthesis and stomatal conductance than would plants exposed to the same mean CO₂ concentration but without pulses of higher concentration.

Observation of the scale of patchy stomatal behavior in leaves of Quercus crispula using an Imaging-PAM chlorophyll fluorometer

Patchy stomatal closure occurs in plants with heterobaric leaves, in which vertical extensions of bundle sheath cells delimit the mesophyll and restrict the diffusion of CO(2). The scale of patchy stomatal behavior was investigated in this study. The distribution of PSII quantum yield (Φ(II)) obtained from chlorophyll fluorescence images was used to evaluate the scale of stomatal patchiness and its relationship with leaf photosynthesis in the sun leaves of 2-year-old saplings of Quercus crispula Blume. Fluorescent patches were observed only during the day with low stomatal conductance. Comparison of numerical simulation of leaf gas exchange and chlorophyll fluorescence images showed that heterogeneous distribution of electron transport rate through PSII (J) was observed following stomatal closure with a bimodal manner under both natural and saturated photosynthetic photon flux densities. Thus, fluorescence patterns can be interpreted in terms of patchy stomatal closure. The mapping of J from chlorophyll fluorescence images showed that the scale of stomatal patchiness was approximately 2.5-fold larger than that of anatomical patches (lamina areas bounded by bundle sheath extensions within lamina). Our results suggest the spatial scale of stomatal patches in Q. crispula leaves.

Comparison of physiological and antioxidant responses of Anoda cristata and cotton to progressive drought

Ratnayaka HH, Molin WT & Sterling TM (2012). Comparison of physiological and antioxidant responses of Anoda cristata and cotton to progressive drought. Weed Research52, 358–366. Summary Knowledge about physiological responses of weed and crop to drought is needed to minimise crop losses caused by weed interference and to improve cropping systems and crop cultivars. In this study, gas exchange, photochemistry and antioxidant defences of two Anoda cristata (spurred anoda) accessions from Mississippi and New Mexico and two cotton types (Gossypium hirsutum and Gossypium barbadense) were evaluated under unstressed conditions and progressive drought. Under no stress, net photosynthesis (Pnet) and quantum yield (ΦPSII) were similar, but stomatal conductance (gs) was higher in A. cristata than cotton. Glutathione reductase activity was highest in A. cristata from Mississippi. Cotton had at least 92% greater α-tocopherol concentration than A. cristata. Progressive drought that suppressed gas exchange to near zero decreased biomass and ΦPSII, but increased xanthophyll cycle conversion state in both A. cristata and cotton. Anoda cristata had prolonged gas exchange at lower leaf water contents than cotton. Air-to-leaf temperature difference was at least 66% greater in A. cristata, while α-tocopherol concentration increased only in cotton under progressive drought. Anoda cristata appears to use patchy stomatal closure and alternative electron sinks to minimise light damage during severe drought. Furthermore, sustained water-holding capacity and assimilation helped by prolonged stomatal activity, larger pigment pools and lower leaf temperature in A. cristata may confer stress tolerance and consequent success in cotton –A. cristata interference.

Stomatal patchiness in the Mediterranean holm oak (Quercus ilex L.) under water stress in the nursery and in the forest

The evergreen holm oak Quercus ilex L. is the most representative tree in Mediterranean forests. Accurate estimation of the limiting factors of photosynthesis for Q. ilex and the prediction of ecosystem water-use efficiency by mechanistic models can be achieved only by establishing whether this species shows heterogenic stomatal aperture, and, if so, the circumstances in which this occurs. Here, we collected gas-exchange and chlorophyll fluorescence data in Q. ilex leaves from a nursery to measure the effects of stomatal oscillations on PSII quantum yield (Φ(PSII)) under water stress. Stomatal conductance (g(s)) was used as an integrative indicator of the degree of water stress. Images of chlorophyll fluorescence showed heterogeneous Φ(PSII) when g(s) was <50 mmol H(2)O m(-2) s(-1), representative of severe drought and corresponding to a container capacity <45%. Stomatal patchiness was related to a coefficient of variation (CV) of Φ(PSII) values >2.5%. A parallel study in the forest confirmed heterogeneous Φ(PSII) values in leaves in response to declining water availability. Three kinds of Q. ilex individuals were distinguished: those resprouting after a clear-cut (resprouts, R); intact individuals growing in the same clear-cut area as resprouts (controls, C); and intact individuals in a nearby, undisturbed area (forest controls, CF). Patchiness increased in C and CF in response to increasing drought from early May to late July, whereas in R, Φ(PSII) values were maintained as a result of their improved water relations since the pre-existing roots were associated with a smaller aerial biomass. Patchiness was related to a % CV of Φ(PSII) values >4 and associated in the summer with mean g(s) values of 30 mmol H(2)O m(-2) s(-1). Under milder drought in spring, Φ(PSII) patchiness was less strictly related to g(s) variations, pointing to biochemical limitants of photosynthesis. The occurrence of heterogenic photosynthesis caused by patchy stomatal closure in Q. ilex during severe drought should be taken into account in ecosystem modelling in which harsher water stress conditions associated with climate change are predicted.

Patchy stomatal behavior during midday depression of leaf CO2 exchange in tropical trees

We investigated effects of heterogeneous stomatal behavior on diurnal patterns of leaf gas exchange in 10 tree species. Observations were made in middle and upper canopy layers of potted tropical rainforest trees in a nursery at the Forest Research Institute Malaysia. Measurements were taken from 29 January to 3 February 2010. We measured in situ diurnal changes in net photosynthetic rate and stomatal conductance in three leaves of each species under natural light. In both top-canopy and sub-canopy species, midday depression of net assimilation rate occurred in late morning. Numerical analysis showed that patchy bimodal stomatal behavior occurred only during midday depression, suggesting that the distribution pattern of stomatal apertures (either uniform or non-uniform stomatal behavior) varies flexibly within single days. Direct observation of stomatal aperture using Suzuki's Universal Micro-Printing (SUMP) method demonstrated midday patchy stomatal closure that fits a bimodal pattern in Shorea leprosula Miq., Shorea macrantha Brandis. and Dipterocarpus tempehes V.Sl. Inhibition of net assimilation rate and stomatal conductance appears to be a response to changes in vapor pressure deficit (VPD). Variable stomatal closure with increasing VPD is a mechanism used by a range of species to prevent excess water loss from leaves through evapotranspiration (viz., inhibition of midday leaf gas exchange). Bimodal stomatal closure may occur among adjacent stomata within a single patch, rather than among patches on a single leaf. Our results suggest theoccurrence of patches at several scales within single leaves. Further analysis should consider variable spatial scales in heterogeneous stomatal behavior between and within patches and within single leaves.

Midday depression of leaf CO2 exchange within the crown of Dipterocarpus sublamellatus in a lowland dipterocarp forest in Peninsular Malaysia

We observed diurnal and seasonal patterns of leaf-scale gas exchange within the crown of a Dipterocarpus sublamellatus Foxw. tree growing in a lowland dipterocarp forest at Pasoh, Peninsular Malaysia. Observations were carried out nine times over 6 years, from September 2002 to December 2007. Observation periods included both wet and mild-dry periods, and natural and saturated photosynthetic photon flux density (PPFD) light conditions. In situ measurements of the diurnal change in net photosynthetic rate and in stomatal conductance were carried out on canopy leaves of a 40-m-tall D. sublamellatus tree, which was accessed from a canopy corridor. A diurnal change in electron transport rate was observed under saturated PPFD conditions. The maximum net assimilation rate was approximately 10 micromol m(-2) s(-1). There was a clear inhibition of the net assimilation rate coupled with stomatal closure after late morning and this inhibition occurred year-round. Although the electron transport rate decreased alongside this inhibition, it sometimes followed on. Numerical analysis showed that the main factor in the inhibition of the net assimilation rate was patchy bimodal stomatal closure, which occurred in both mild-dry and wet periods. The midday depression occurred year-round, though there are fluctuations in soil moisture during the mild-dry and wet periods. The magnitude of the inhibition was not related to soil water content but was related to vapor pressure deficit (VPD): that is, whether the days were sunny and hot or cloudy and cool. On cloudy, cool days in the wet period, the net photosynthesis was only moderately inhibited, but it still decreased in the afternoon and was coupled with patchy stomatal closure, even in quite moderate VPD, leaf temperature and PPFD conditions. Our results suggest that patchy stomatal closure signaled by the increase in VPD, in transpiration and by circadian rhythms, was the key factor in constraining midday leaf gas exchange of the D. sublamellatus canopy leaves.

Diffusive and Metabolic Limitations to Photosynthesis under Drought and Salinity in C3 Plants

Drought and salinity are two widespread environmental conditions leading to low water availability for plants. Low water availability is considered the main environmental factor limiting photosynthesis and, consequently, plant growth and yield worldwide. There has been a long-standing controversy as to whether drought and salt stresses mainly limit photosynthesis through diffusive resistances or by metabolic impairment. Reviewing in vitro and in vivo measurements, it is concluded that salt and drought stress predominantly affect diffusion of CO(2) in the leaves through a decrease of stomatal and mesophyll conductances, but not the biochemical capacity to assimilate CO(2), at mild to rather severe stress levels. The general failure of metabolism observed at more severe stress suggests the occurrence of secondary oxidative stresses, particularly under high-light conditions. Estimates of photosynthetic limitations based on the photosynthetic response to intercellular CO(2) may lead to artefactual conclusions, even if patchy stomatal closure and the relative increase of cuticular conductance are taken into account, as decreasing mesophyll conductance can cause the CO(2) concentration in chloroplasts of stressed leaves to be considerably lower than the intercellular CO(2) concentration. Measurements based on the photosynthetic response to chloroplast CO(2) often confirm that the photosynthetic capacity is preserved but photosynthesis is limited by diffusive resistances in drought and salt-stressed leaves.

Inverse modeling of seasonal drought effects on canopy CO2/H2O exchange in three Mediterranean ecosystems

We present a two‐criteria inverse modeling approach to analyze the effects of seasonal drought on ecosystem gas exchange at three Mediterranean sites. The three sites include two nearly monospecific Quercus ilex L. forests, one on karstic limestone (Puéchabon), the other on fluvial sand with access to groundwater (Castelporziano), and a typical multispecies shrubland on limestone (Arca di Noè). A canopy gas exchange model Process Pixel Net Ecosystem Exchange (PROXELNEE), which contains the Farquhar photosynthesis model coupled to stomatal conductance via the Ball‐Berry model, was inverted in order to estimate the seasonal time course of canopy parameters from hourly values of ecosystem gross carbon uptake and transpiration. It was shown that an inverse estimation of leaf‐level parameters was impossible when optimizing against ecosystem H2O or CO2 fluxes alone (unidentifiable parameters). In contrast, a criterion that constrained the optimization against both H2O and CO2 fluxes yielded stable estimates of leaf‐level parameters. Two separate model inversions were implemented to test two alternative hypotheses about the response to drought: a reduction in active leaf area as a result of patchy stomatal closure or a change in photosynthetic capacities. In contrast to a previously tested hypothesis of classical (uniform) stomatal control, both hypotheses were equally able to describe the seasonality of carbon uptake and transpiration on all three sites, with a decline during the drought and recovery after autumn rainfall. Large reductions of up to 80%, in either active leaf area or photosynthetic capacities, were necessary to describe the observed carbon and water fluxes at the end of the drought period. With a threshold‐type relationship, soil water content was an excellent predictor of these changes. With the drought‐dependent parameter changes included, the canopy model explains 80–90% of the variance of hourly gross CO2 uptake (root mean squared error (RMSE): 1.1–2.6 μmol m−2 s−1) and 70–80% of the variance of hourly transpiration (RMSE: 0.02–0.03 mm h−1) at all sites. In addition to drought effects, changes in leaf photosynthetic activity not related to water availability, i.e., high spring activity, were detected through the inverse modeling approach. Moreover, our study exemplifies a kind of multiconstraint inverse modeling that can be profitably used for calibrating ecosystem models that are meant for global applications with ecosystem flux data.

Severe drought effects on ecosystem CO2 and H2O fluxes at three Mediterranean evergreen sites: revision of current hypotheses?

AbstractEddy covariance and sapflow data from three Mediterranean ecosystems were analysed via top‐down approaches in conjunction with a mechanistic ecosystem gas‐exchange model to test current assumptions about drought effects on ecosystem respiration and canopy CO2/H2O exchange. The three sites include two nearly monospecific Quercus ilex L. forests – one on karstic limestone (Puéchabon), the other on fluvial sand with access to ground water (Castelporziano) – and a typical mixed macchia on limestone (Arca di Noè). Estimates of ecosystem respiration were derived from light response curves of net ecosystem CO2 exchange. Subsequently, values of ecosystem gross carbon uptake were computed from eddy covariance CO2 fluxes and estimates of ecosystem respiration as a function of soil temperature and moisture. Bulk canopy conductance was calculated by inversion of the Penman‐Monteith equation. In a top‐down analysis, it was shown that all three sites exhibit similar behaviour in terms of their overall response to drought. In contrast to common assumptions, at all sites ecosystem respiration revealed a decreasing temperature sensitivity (Q10) in response to drought. Soil temperature and soil water content explained 70–80% of the seasonal variability of ecosystem respiration. During the drought, light‐saturated ecosystem gross carbon uptake and day‐time averaged canopy conductance declined by up to 90%. These changes were closely related to soil water content. Ecosystem water‐use efficiency of gross carbon uptake decreased during the drought, regardless whether evapotranspiration from eddy covariance or transpiration from sapflow had been used for the calculation. We evidence that this clearly contrasts current models of canopy function which predict increasing ecosystem water‐use efficiency (WUE) during the drought. Four potential explanations to those results were identified (patchy stomatal closure, changes in physiological capacities of photosynthesis, decreases in mesophyll conductance for CO2, and photoinhibition), which will be tested in a forthcoming paper. It is suggested to incorporate the new findings into current biogeochemical models after further testing as this will improve estimates of climate change effects on (semi)arid ecosystems' carbon balances.

Quantifying stomatal and non-stomatal limitations to carbon assimilation resulting from leaf aging and drought in mature deciduous tree species.

Gas exchange techniques were used to investigate light-saturated carbon assimilation and its stomatal and non-stomatal limitations over two seasons in mature trees of five species in a closed deciduous forest. Stomatal and non-stomatal contributions to decreases in assimilation resulting from leaf age and drought were quantified relative to the maximum rates obtained early in the season at optimal soil water contents. Although carbon assimilation, stomatal conductance and photosynthetic capacity (V(cmax)) decreased with leaf age, decreases in V(cmax) accounted for about 75% of the leaf-age related reduction in light-saturated assimilation rates, with a secondary role for stomatal conductance (around 25%). However, when considered independently from leaf age, the drought response was dominated by stomatal limitations, accounting for about 75% of the total limitation. Some of the analytical difficulties associated with computing limitation partitioning are discussed, including path dependence, patchy stomatal closure and diffusion in the mesophyll. Although these considerations may introduce errors in our estimates, our analysis establishes some reasonable boundaries on relative limitations and shows differences between drought and non-drought years. Estimating seasonal limitations under natural conditions, as shown in this study, provides a useful basis for comparing limitation processes between years and species.

Spatial and seasonal variability of photosynthetic parameters and their relationship to leaf nitrogen in a deciduous forest

We used gas exchange techniques to estimate maximum rate of carboxylation (V(cmax)), a measure of photosynthetic capacity, in the understory and upper crown of a closed deciduous forest over two seasons. There was extensive variability in photosynthetic capacity as a result of vertical canopy position, species type, leaf age and drought. Photosynthetic capacity was greater in oaks than in maples and greater in the overstory than in the understory. Parameter V(cmax) was maximal early in the season but declined slowly throughout most of the summer, and then more rapidly during senescence. There was also an apparent decline during drought in some trees. Variability in V(cmax) as a result of species or vertical canopy gradients was described well by changes in leaf nitrogen per unit area (N(a)). However, temporal changes in V(cmax) were often poorly correlated with leaf nitrogen, especially in spring and summer and during drought. This poor correlation may be the result of a seasonally dependent fractional allocation of leaf nitrogen to Rubisco; however, we could not discount Rubisco inactivation, patchy stomatal closure or changes in mesophyll resistance. Consequently, when a single annual regression equation of V(cmax) versus N(a) was used for this site, there were substantial errors in the temporal patterns in V(cmax) that will inevitably result in modeling errors.

Abscisic acid is not necessarily required for the induction of patchy stomatal closure

application of the plant hormone abscisic acid (ABA) to the leaf ( Terashima et al., 1988; Daley et al., 1989; Mott, Patchy stomatal closure was observed in leaves of 1995), Mansfield et al. (1990) discussed heterogeneous transgenic plants of Nicotiana plumbaginifolia produ- concentrations of abscisic acid on the leaf blade or a cing antibodies that block the action of abscisic acid. heterogeneous sensitivity of stomata to ABA as possible Stomatal patchiness was induced by leaf detachment explanations for the induction of stomatal patchiness. and subsequent water loss. Stomatal closure was fol- Schroeder and Hagiwara (1990) concluded from their lowed by an irreversible reduction of maximal chloro- results that the sensitivity of stomata to ABA can vary phyll fluorescence. The degree of deviation from the to a large degree. A/c i -curve is correlated with steady-state diffusion Recently, transgenic tobacco plants were produced conductance before leaf detachment. It is concluded which express large amounts of single chain antibodies that a heterogeneous sensitivity of stomata to abscisic that block ABA and are hypersensitive to water stress acid is not directly involved in the induction of patchy (Artsaenko et al., 1995). Such plants were regarded as stomatal closure. being of interest in order to test whether ABA is functionally involved in the induction of stomatal patchiness. In

Short communication. Abscisic acid is not necessarily required for the induction of patchy stomatal closure

Short communication. Abscisic acid is not necessarily required for the induction of patchy stomatal closure

Interactions among stomata in response to perturbations in humidity

ABSTRACTThe existence of patchy stomatal closure suggests interactions among neighbouring stomata that synchronize stomatal movements in small areas of a leaf. To test for such interactions, water vapour partial pressure (ewv) for a small group of stomata was controlled independently of that for the surrounding stomata using gas flow from a small needle. The ewv for the surrounding stomata was controlled with a larger gas flow, termed the primary flow. The spatial pattern of ewv isobars caused by the needle flow was assessed experimentally and theoretically. Stomatal apertures were monitored following perturbations in ewv of the primary flow and the needle flow. When ewv of the primary flow was perturbed and that of the needle flow held constant, stomata for which there was little or no perturbation in ewv responded similarly to stomata experiencing the perturbation. When the ewv of the needle flow was perturbed and that of the primary flow held constant, many stomata experiencing little or no perturbation responded similarly to those experiencing a large perturbation. The results are discussed in relation to a mechanism for stomatal interactions that has been proposed in a previous study [Haefner, Buckley &amp; Mott (1997) Plant, Cell and Environment 20, 1087–1097, this issue].

Dynamics of Changing Intercellular CO2 Concentration (ci) during Drought and Determination of Minimum Functional ci.

Nine conifer species with narrow (<5 mm), single-veined leaves were selected for the purpose of examining changes in intercellular CO2 concentration (ci) during drought. Due to the leaf morphology of the study plants, the confounding effects of nonhomogenous photosynthesis common to most reticulate-veined angiosperms were largely avoided, giving a clear picture of ci dynamics under increasing drought. A characteristic biphasic response was observed in all species, with an initial stomatal control phase resulting in a substantial reduction in ci as stomatal conductance (gs) decreased. As gs reached low levels, a strong nonstomatal limitation phase was observed, causing ci to increase as gs approached a minimum. This nonstomatal phase was linked to a concomitant rapid decrease in the fluorescence parameter quantum efficiency, indicating the onset of nonreversible photoinhibition. The ratio of internal to atmospheric CO2 concentration (ci/ca) decreased from values of between 0.68 and 0.57 in undroughted plants to a minimum, (ci/ca)min, which was well defined in each species, ranging from 0.10 in Actinostrobus acuminatus to 0.36 in Acmopyle pancheri. A high correlation was found to exist between (ci/ca)min and leaf water potential measured at (ci/ca)min. Species developing high maximum intrinsic water use efficiencies (low [ci/ca]min), such as A. acuminatus, did so at lower leaf water potentials (-4.5 MPa) than more mesic species (-1.75 MPa for A. pancheri). It is concluded that in the absence of patchy stomatal closure, (ci/ca)min gives a good representation of the drought tolerance of foliage.

Effects of patchy stomatal closure on gas exchange measurements following abscisic acid treatment

ABSTRACTGas exchange data and images of leaf fluorescence were collected concurrently as stomata responded to abscisic acid (ABA) application. When 10−5kmolm−3 ABA was applied to the transpiration stream in a short pulse, stomatal conductance (gs), photosynthesis (A) and intercellular CO2 concentration (Ci) decreased rapidly after a short lag period and became approximately constant after 2h. There was an apparent reduction in the A versus c1 relationship as stomata closed, but the data returned to the A versus C1 curve while stomatal conductance was constant or slowly rising during the second hour after ABA treatment. Larger amounts of ABA administered during the pulse caused larger deviations from the A versus c1 relationship. When 10−7kmolm−3 ABA was applied continuously through the transpiration stream, gs, A and Ci decreased, but there was no substantial deviation from the A versus c{ curve. Fluorescence images were patchy as stomata closed for all experiments, but became slowly more uniform during the time that gas exchange was returning to the A versus Cj curve. The distribution of con‐ductance among patches was not bimodal, and larger devi‐ations from the A versus ct curve had greater ranges of pixel values and more pixel values representing low values of Cj during stomatal closure than did experiments show‐ing small or no deviation. Estimates of A and gs from fluo‐rescence images compared favourably with measured val‐ues in most cases, suggesting that the patchy distributions of fluorescence were caused by patchy distributions of stomatal conductance and that apparent reductions in the A versus ct relationship were the result of these patchy stomatai distributions and not direct effects of ABA on mesophyll functioning. The data show that stomatal patches can be temporary and that patchiness may not be reflected in gas exchange data if the range of stomatal con‐ductances is not large. These observations may explain some of the discrepancies among previous studies concerning the effect of ABA on the A versus Ci relationship.

Morphological and Physiological Consequences of a Positive Plant Interaction

It is known that plants can create acceptable environments for other plant species that may be unable to physiologically adjust to harsh physical conditions in isolation. In this study, we explore the morphological and physiological responses of a positive interaction on a common New England salt marsh shrub, Iva frutescens L. In previous experiments, we have shown that Iva benefits from the presence of the black rush, Juncus gerardi Loisel. Juncus removal in the lower marsh caused increases in soil salinity and anoxia, which resulted in lower Iva biomass, growth, and survival. We hypothesize that the positive effect of Juncus on Iva is a consequence of (1) the turf morphology of Juncus shading soil surfaces, thereby minimizing salt accumulation, and (2) the increased soil oxygen content surrounding Juncus caused by radial oxygen loss from its rhizopshere. However, the specific morphological and physiological benefits conferred on Iva are unknown. We conducted experiments in the salt marsh to isolate the effects of Juncus neighbors, waterlogging, and salt water stress on Iva. We found that Iva, when exposed to salty, oxygen—deprived soil conditions created in the absence of Juncus, responded more negatively to salinity than waterlogging, although the interaction between salinity and waterlogging was often significant. Iva without neighbors showed decreases in plant height, leaf density, leaf area, and flower density compared to controls or plants with fresh—water additions. Leaves of Iva without neighbors responded to salt stress and waterlogging by increasing leaf thickness and succulence. The increase in soil salinity of plants without neighbors caused plants to have correspondingly low water potentials, indicating that they were severely water stressed. Physiologically, Iva plants had a low mean photosynthetic rate, transpiration rate, water—use efficiency, and stomatal conductance compared to controls or Iva with fresh—water additions. Low water—use efficiency in Iva without neighbors was correlated with stomatal closure but also with high intercellular CO2 concentrations. The mechanisms behind this apparent anomaly may include both nonstomatal inhibition of photosynthesis as well as patchy stomatal closure, although it is uncertain which predominates from our data. Our results show that the presence of Juncus, with its superior ability to withstand waterlogging and salt stress, can create a hospitable environment for Iva, extending its distribution to lower intertidal habitats in which it does not have the morphological or physiological plasticity to normally cope.

Stomatal patchiness

Different behaviour of small groups of stomata on a single leaf blade (stomatal patchiness) is reviewed. The occurrence of stomatal patchiness depends on plant species, age, leaf position, environmental conditions,etc. The possibility of errors in conventional evaluation of stomatal and non-stomatal (biochemical) limitations of photosynthesis resulting from patchy stomatal closure is analysed. The consequences of stomatal patchiness for leaf and plant photosynthesis and water economy are discussed. A brief survey of the techniques currently used for detection and quantification of stomatal patchiness is presented.