Role Of Stomata Research Articles

Integrated physiological, biochemical, and transcriptomic analyses of Bruguiera gymnorhiza leaves under long-term copper stress: Stomatal size, wax crystals and composition

Copper (Cu) is a necessary mineral nutrient for plant growth and development and is involved in several morphological, physiological, and biochemical processes; however, high concentrations of Cu can negatively impact these processes. The role of stomata in responding to various biotic and abiotic stimuli has not been studied in Bruguiera gymnorhiza, particularly in terms of their coordinated interactions at the molecular, physiological, and biochemical levels. Moreover, numerous plants employ strategies such as the presence of thick waxy cuticles on their leaf epidermis and the closing of stomata to reduce water loss. Thus, this study investigates the accumulation of Cu in B. gymnorhiza and its effect on leaf morphology and the molecular response under different Cu treatments (0, 200, and 400 mg L⁻¹, Cu0, Cu200, and Cu400, respectively) during a two years stress period. The results show that Cu stress affected accumulation and transport, increased the activities of peroxidase and ascorbate peroxidase, concentrations of soluble sugar, proline, and H2O2, and decreased the activity of catalase and content of malondialdehyde. Also, Cu-induced stress decreased the uptake of phosphorus and nitrogen and inhibited plant photosynthesis, which consequently led to reduced plant growth. Scanning electron microscopy combined with gas chromatography-mass spectrometry showed that B. gymnorhiza leaves had higher wax crystals and compositions under increased Cu stress, which forced the leaf’s stomata to be closed. Also, the contents of alkanes, alcohols, primary alcohol levels (C26:0, C28:0, C30:0, and C32:0), n-Alkanes (C29 and C30), and other wax loads were significantly higher, while fatty acid (C12, C16, and C18) was lower in Cu200 and Cu400 compared to Cu0. Furthermore, the transcriptomic analyses revealed 1240 (771 up- and 469 downregulated), 1000 (723 up- and 277 down-regulated), and 1476 (808 up- and 668 downregulated) differentially expressed genes in Cu0 vs Cu200, Cu0 vs Cu400, and Cu200 vs Cu400, respectively. RNA-seq analyses showed that Cu mainly affected eight pathways, including photosynthesis, cutin, suberin, and wax biosynthesis. This study provides a reference for understanding mangrove response to heavy metal stress and developing novel management practices.

Water Loss: A Postharvest Quality Marker in Apple Storage

AbstractApple fruit can be stored for long periods of time, especially with the use of controlled atmosphere storage, but like many fruits and vegetables are susceptible to water loss. Water loss can result in compromised appearance such as skin shriveling, as well as loss of firmness, and reduced saleable weight, which in turn affect the income of growers and other industry stakeholders. Preharvest factors that can influence water loss in apples during the postharvest period include climate, cultivar, fruit size, tree age, orchard practices, and harvest maturity. Postharvest factors such as the storage temperature, relative humidity, storage type, and duration can also affect water loss in apple fruit during storage. The mechanisms of cuticle biosynthesis in water permeance, the role of stomata and lenticels, microcracking, crosstalk with mechanical injuries, storage disorders, and decay incidence during the storage of apples are reviewed. Additionally, the review summarizes: preharvest and postharvest factors influencing water loss; recent management strategies including pre-cooling, cold storage, controlled atmospheres, packaging, and anti-senescence chemicals; the use of edible coatings, as well as other non-chemical approaches for modulating water loss and maintaining storage quality. The review also provides direction for the industry to manage this destructive problem in the postharvest supply chain of apple fruit.

Photosynthesis - beyond the leaf.

Although leaves are considered the main site for photosynthesis, other green nonfoliar tissues can carry out considerable amounts of photosynthetic carbon assimilation. With photosynthesis, a potential target for improving crop productivity, physiology and contribution of nonfoliar tissues to overall plant carbon acquisition is gaining increasing attention. This review will provide an overview of nonfoliar photosynthesis, the role of stomata in these tissues and methodologies for quantification and the contribution to overall carbon gain.

FORCAsT‐gs: Importance of Stomatal Conductance Parameterization to Estimated Ozone Deposition Velocity

AbstractThe role of stomata in regulating photosynthesis and transpiration, and hence governing global biogeochemical cycles and climate, is well‐known. Less well‐understood, however, is the importance of stomatal control to the exchange of other trace gases between terrestrial vegetation and the atmosphere. Yet these gases determine atmospheric composition, and hence air quality and climate, on scales ranging from local to global, and seconds to decades. Vegetation is a major sink for ground‐level ozone via the process of dry deposition and the primary source of many biogenic volatile organic compounds (BVOCs). The rate of dry deposition is largely controlled by the rate of diffusion of a gas through the stomata, and this also governs the emission rate of some key BVOCs. It is critical therefore that canopy‐atmosphere exchange models capture the physiological processes controlling stomatal conductance and the transfer of trace gases other than carbon dioxide and water vapor. We incorporate three of the most widely used coupled stomatal conductance‐photosynthesis models into the one‐dimensional multi‐layer FORest Canopy‐Atmosphere Transfer (FORCAsT1.0) model to assess the importance of choice of parameterization on simulated ozone deposition rates. Modeled GPP and stomatal conductance across a broad range of ecosystems differ by up to a factor of two between the best and worst performing model configurations. This leads to divergences in seasonal and diel profiles of ozone deposition velocity of up to 30% and deposition rate of up to 13%, demonstrating that the choice of stomatal conductance parameterization is critical in accurate quantification of ozone deposition.

Ultrasound irradiation effect on photosynthesis and transpiration of aquatic lirium plants

Purpose To find possible causes of the photobaric response decrease in photoacoustic measurements on Aquatic Lirium plants after ultrasonic irradiations reported elsewhere contributing to understanding the effect of ultrasonic irradiation on them and searching for environmentally friendly methodologies to control this weed. Materials and Methods The plants were extracted from their natural habitat in Xochimilco water canals, Mexico City. The irradiations on the plants were carried out to 2 hours exposure time, 17 kHz frequency, and 30 W x 4 output power. We used the photoacoustic spectroscopy technique at room temperature in the range of 400–750 nm to analyze the optical absorption response of photosynthetic pigments before and after ultrasonic irradiations. To monitor the leave transpiration rate, we used an LI-COR 6400XT portable system, expressed in units of mols H2O per second per unit area of the leaf surface. Results We obtained a significant decrease of the chlorophylls bands amplitude in the photoacoustic spectroscopy spectra and a drastic reduction in the leaves transpiration rate of irradiated plants that depends on the time elapsed after irradiation. Remarkable physical changes in the leaves and petioles of the irradiated plants were observed with the naked eye. Conclusions A significantly decreasing photosynthesis and transpiration in the leaves of the irradiated lirium plants were obtained. Together with the observed physical changes in the leaves and petioles, these results suggest an alteration in the water transport and the overall gas exchange mechanisms affecting the irradiated leaves' transpiration and photosynthesis processes that agree with the photobaric response decrease reported elsewhere. Due to the fundamental role of stomata in these mechanisms, it is suggested, as a possible cause, that the ultrasonic-induced disruption of stomata's mechanical operation by guard cells prevents them from performing their function normally. A hypothesis to be confirmed in future studies, for which a line of action is proposed.

Legacy effects of historical grazing alter leaf stomatal characteristics in progeny plants.

Grazing, one of the primary utilization modes of grassland, is the main cause of grassland degradation. Historical overgrazing results in dwarf phenotype and decreased photosynthesis of perennial plants. However, it remains unknown what the mechanism underlying of this legacy effect is, and the role of stomata in the resulting decreased photosynthesis also remains unclear. To address these questions, differences in stomatal density, length and width on both adaxial and abaxial epidermis were compared between overgrazing and ungrazed Leymus chinensis offspring by using rhizome buds cultivated in a greenhouse, and the correlation between photosynthetic capacity and stomatal behavior was also investigated. Our results showed that historical grazing significantly impacted phenotype, photosynthesis and stomatal traits of L. chinensis. The offspring plants taken from overgrazed parents were dwarfed compared to those taken from ungrazed parents, and the photosynthesis and stomatal conductance of plants with a grazing history decreased by 28.6% and 21.3%, respectively. In addition, stomatal density and length on adaxial and abaxial leaf surfaces were significantly increased; however, stomatal width on abaxial leaf surfaces of overgrazed L. chinensis was significantly decreased compared with ungrazed individuals. Moreover, the expression patterns of eight genes related to stomatal regulation were tested: seven were down-regulated (2–18 times) and one was up-regulated (three times). Genes, involved in ABC transporter and receptor-like serine/threonine protein kinase were down-regulated. These results suggest that legacy effects of historical grazing affect the stomatal conductance by decreasing the stomatal width in progeny plants, which thus results in lower photosynthesis. Furthermore, changes of stomatal traits and function were regulated by the inhibition of ABC transporter and serine/threonine protein kinase. These findings are helpful for future exploration of the possible mechanisms underlying the response of grassland plants to long-term overgrazing.

Role of Stomata in Adaptation of Plantago maritima L. Plants to Tidal Dynamics on the White Sea Coast

Structural and functional traits in leaves of sea plantain (Plantago maritima L.) inhabiting intertidal and supratidal zones of the White Sea coast were studied. At the intertidal, the plants were found to have smaller and thicker leaves with more frequent stomata and thinner cover tissues in comparison with the plants collected from the supratidal. Leaf fluorescence and stomatal conductance obeyed the circadian rhythm in plants residing in the supratidal. The tested parameters considerably changed according to tidal dynamics in the intertidal plants. Two stable states (high and low tides) and two transition states (while water level ascends or descends) were defined. In both stable states, the plants demonstrated high functional parameters of fluorescence (Fv/Fm, Y(II), and ETR) and open stomata. In the transitional states, the stomata were partly closed, and the functional activity was inhibited. Leaf structural peculiarities, leaf gas films, the inducible CO2-concentrating mechanism, and the determining role of the stomata are considered in terms of the hypotheses explaining the responses of sea plantain to tidal dynamics on the White Sea coast.

Rice plants overexpressing OsEPF1 show reduced stomatal density and increased root cortical aerenchyma formation

Stomata are adjustable pores in the aerial epidermis of plants. The role of stomata is usually described in terms of the trade-off between CO2 uptake and water loss. Little consideration has been given to their interaction with below-ground development or diffusion of other gases. We overexpressed the rice EPIDERMAL PATTERNING FACTOR1 (OsEPF1) to produce rice plants with reduced stomatal densities, resulting in lowered leaf stomatal conductance and enhanced water use efficiency. Surprisingly, we found that root cortical aerenchyma (RCA) is formed constitutively in OsEPF1OE lines regardless of tissue age and position. Aerenchyma is tissue containing air-spaces that can develop in the plant root during stressful conditions, e.g. oxygen deficiency when it functions to increase O2 diffusion from shoot to root. The relationship with stomata is unknown. We conclude that RCA development and stomatal development are linked by two possible mechanisms: first that reduced stomatal conductance inhibits the diffusion of oxygen to the root, creating an oxygen deficit and stimulating the formation of RCA, second that an unknown EPF signalling pathway may be involved. Our observations have fundamental implications for the understanding of whole plant gas diffusion and root-to-shoot signalling events.

Postinvasive Bacterial Resistance Conferred by Open Stomata in Rice.

Stomata are leaf pores that regulate gas exchange and water transpiration in response to environmental cues. They also function in innate immunity by limiting pathogen entry through actively closing in so-called stomatal defense. However, roles of stomata in plant disease resistance are not fully elucidated, especially in monocots. Here, we report that non-race specific resistance of the rice abscisic acid-deficient mutant Osaba1 to Xanthomonas oryzae pv. oryzae is due to increased stomatal conductance. Reducing stomatal conductance in the Osaba1 mutant increases its susceptibility to X. oryzae pv. oryzae. Artificial opening of stomata in wild-type plants leads to enhanced resistance to X. oryzae pv. oryzae. The rice mutant es1-1 with constitutively higher stomatal conductance exhibits strong resistance to X. oryzae pv. oryzae. Additionally, Osaba1 and es1-1 are resistant to X. oryzae pv. oryzicola. The data support that open stomata confer postinvasive resistance against bacterial pathogens in rice, and such resistance probably results from decreased leaf water potential. Our findings reveal a novel role of stomata in plant immunity through modulation of leaf water status, which provides physiological insight into the interactions between plant, pathogen, and environment.

Plant virus infections control stomatal development.

Stomata are important regulators of carbon dioxide uptake and transpirational water loss. They also represent points of vulnerability as bacterial and fungal pathogens utilise this natural opening as an entry portal, and thus have an increasingly complex relationship. Unlike the situation with bacterial and fungal pathogens, we know very little about the role of stomata in viral infection. Here we report findings showing that viral infection influences stomatal development in two susceptible host systems (Nicotiana tabacum with TMV (Tobacco mosaic virus), and Arabidopsis thaliana with TVCV (Turnip vein-clearing virus)), but not in resistant host systems (Nicotiana glutinosa and Chenopodium quinoa with TMV). Virus infected plants had significantly lower stomatal indices in systemic leaves of susceptible systems; N. tabacum 9.8% reduction and A. thaliana 12.3% reduction, but not in the resistant hosts. Stomatal density in systemic leaves was also significantly reduced in virus infected A. thaliana by 19.6% but not in N. tabacum or the resistant systems. In addition, transpiration rate was significantly reduced in TMV infected N. tabacum.

Tomato-Pseudomonas syringae interactions under elevated CO₂ concentration: the role of stomata.

Increasing atmospheric CO₂ concentrations ([CO₂]) in agricultural and natural ecosystems is known to reduce plant stomatal opening, but it is unclear whether these CO₂-induced stomatal alterations are associated with foliar pathogen infections. In this study, tomato plants were grown under ambient and elevated [CO₂] and inoculated with Pseudomonas syringae pv. tomato strain DC3000, a strain that is virulent on tomato plants. We found that elevated [CO₂] enhanced tomato defence against P. syringae. Scanning electron microscopy analysis revealed that stomatal aperture of elevated [CO₂] plants was considerably smaller than their ambient counterparts, which affected the behaviour of P. syringae bacteria on the upper surface of epidermal peels. Pharmacological experiments revealed that nitric oxide (NO) played a role in elevated [CO₂]-induced stomatal closure. Silencing key genes involved in NO generation and stomatal closing, nitrate reductase (NR) and guard cell slow-type anion channel 1 (SLAC1), blocked elevated [CO₂]-induced stomatal closure and resulted in significant increases in P. syringae infection. However, the SLAC1-silenced plants, but not the NR-silenced plants, still had significantly higher defence under elevated [CO₂] compared with plants treated with ambient [CO₂]. Similar results were obtained when the stomata-limiting factor for P. syringae entry was excluded by syringe infiltration inoculation. These results indicate that elevated [CO₂] induces defence against P. syringae in tomato plants, not only by reducing the stomata-mediated entry of P. syringae but also by invoking a stomata-independent pathway to counteract P. syringae. This information is valuable for designing proper strategies against bacterial pathogens under changing agricultural and natural ecosystems.

A PRELIMINARY STUDY INTO PROTOCOLS FOR THE LONG-TERM COLD STORAGE OF LEUCOSPERMUM POTTED PLANTS

The potted plant industry based on Proteaceae species in South Africa is limited to the domestic markets as protocols for long-term cold storage as required for sea freight to European export markets have not been established. A preliminary investigation was carried out where potted plants of 3 Leucospermum cultivars 'High Gold', 'Raziya' and 'Scarlett Ribbon' were evaluated for chilling injury when stored at 0.5, 4 and 7°C respectively for a 21-day period. 'Scarlett Ribbon' showed the least chilling injury over the entire 10 day evaluation period, irrespective of storage temperature. However, no temperature regime could be recommended as an optimum storage temperature as all plants, irrespective of temperature treatment were considered unmarketable by day 5 of evaluation. In a following trial, potted plants of another 3 Leucospermum cultivars 'Jelena', 'Soleil' and 'Scarlett Ribbon' were evaluated for chilling injury when stored for a 21 day period at 6°C, either in darkness at 100% relative humidity (%RH) as untreated control plants or sprayed with the anti-transpirant Vapor Guard or kept under low light conditions at 60-70% RH. Plants exposed to reduced relative humidity and low light level throughout the storage period displayed no or little chilling injury symptoms and were rated as marketable at the end of the 10 day evaluation period, whereas both the dark-stored control and Vapor Guard- treated plants were considered unmarketable before or on day 5 of evaluation. Significantly lower stomatal conductance was measured within 24 h after removal from storage in potted plants stored in light. Similarly, 'Jelena', the cultivar which had the lowest chilling injury in this trial, also displayed significantly lower stomatal conductance in comparison to 'Scarlett Ribbon' and 'Soleil'. The role of stomata in the incidence of chilling injury during long-term cold storage of potted plants and stem of the members of Proteaceae requires further investigation.

High temperature exposure increases plant cooling capacity

Summary Plants inhabit different environments and have evolved mechanisms to optimise growth within defined temperature ranges. In Arabidopsis thaliana , growth at high temperature (28°C) results in striking elongation of stems and increased leaf elevation from the soil surface [1–3]. Despite insights into the molecular control of these responses [1–5], their physiological significance remains unknown. Here, we analysed the impact of high temperature-mediated development on plant water use strategy. We present the surprising finding that Arabidopsis plants developed at high temperature (28°C) show increased water loss and enhanced leaf cooling capacity in these conditions, despite producing fewer leaf surface pores (stomata). Our data suggest that plant architectural adaptations to high temperature may enhance evaporative leaf cooling in well-watered environments.

Stomatal control as a driver of plant evolution

Stomata are the pores on a leaf surface through which plants regulate the uptake of carbon dioxide (CO2) for photosynthesis against the loss of water via transpiration. Turgor changes in the guard cells determine the area of stomatal pore through which gaseous diffusion can occur, thus maintaining a constant internal environment within the leaf (Gregory et al., 1950). Stomata first occurred in the fossil record ;400 million years ago (Ma), and are largely identical in form to the stomatal complexes of many extant plants, illustrating their effectiveness and importance to terrestrial plants (Edwards et al., 1998). Stomatal control is critical to a plant’s adaptation to its environment; it is this fundamental importance that has led to a wealth of stomatal research ranging in scale from biomolecular analysis to landscape processes (e.g. Gedney et al., 2006; Hu et al., 2010). The first issue of Journal of Experimental Botany, published 60 years ago, contained four papers relating to stomatal function. These included an analysis by Heath of the effects of atmospheric CO2 concentration ([CO2]) on stomatal aperture and conductance; an area of research that is increasingly relevant to our understanding of the past and prediction of future vegetation responses to atmospheric composition. Heath (1950) was the first to observe that reductions in [CO2] below ambient levels induced stomatal opening, an ecophysiological response of great interest, and that the site of CO2 sensing was most probably in the substomatal cavity and not the guard cells. Stomatal research has become vastly important to crop production, biodiversity responses, and hydrology (particularly in terms of ‘run-off’) with respect to rising atmospheric [CO2], changing water regimes, and growing populations. As our understanding of stomatal physiology develops, the role of stomata in the evolution of terrestrial vegetation and development of the terrestrial landscape and atmospheric composition is becoming increasingly evident, alongside the use of fossil stomata as palaeo-proxies of past atmospheres (e.g. McElwain et al., 2004; Berry et al., 2010; Smith et al., 2010). The stomatal control responses of plants consist of ‘shortterm’ stomatal aperture changes in response to availability of water, light, temperature, wind speed, and carbon dioxide, and also ‘longer term’ changes in stomatal density that set the limits for maximum stomatal conductance in response to atmospheric [CO2], light intensity/quality, and root-to-shoot signals of water availability (Schoch et al., 1980, 1984; Davies et al., 2000; Casson et al., 2009). Stomatal control determines the water use efficiency (WUE) of a plant by optimizing water lost against carbon gained. Additionally, the stomatal control mechanisms employed by a plant species will determine: the risk of xylem embolism by reducing the probability of cavitation through stomatal closure during episodes of high transpirative demand (Brodribb and Jordan, 2008; Meinzer et al., 2009); leaf temperature and resistance to heat stress (Srivastava et al., 1995; Jones et al., 2002); tolerance of toxic atmospheric gases (Mansfield and Majernik, 1970); nutrient uptake via promotion of root mass flow (Van Vuuren et al., 1997); and the maximum rate of photosynthesis (Korner et al., 1979). Those plant species with more effective stomatal control will be expected to be more successful than those with less effective stomatal control. However, not all plant species, or individuals within a species, possess equally effective stomatal control, in the setting of either stomatal numbers or the regulation of stomatal aperture (i.e. speed and ‘tightness’ of closure). Given that any trait that confers a selective advantage is likely to become universal within a population (McNeilly, 1968), it may be reasonable to assume that stomatal control incurs certain ‘costs’, and that these costs have played a significant role in plant evolution over the last 400 million years. The origination of major plant groups, and morphological advances such as the development of planate leaves, coincide with periods of ‘low’ atmospheric [CO2] (Fig. 1) (Woodward, 1998; Beerling et al., 2001). The reduced availability of the substrate for photosynthesis is predicted to be compensated by increases in the carboxylation efficiency of RubisCO and enhanced stomatal conductance to maintain CO2 uptake during periods of low [CO2] (Woodward, 1998; Franks and Beerling, 2009). This elevated stomatal conductance incurs higher rates of water loss and associated risks of desiccation and xylem embolism, in addition to the metabolic costs of enhanced construction of stomatal complexes. It is these costs during periods of low [CO2] that may serve as evolutionary tipping points, where species with more efficient and effective stomata and hydraulic systems are favoured (Robinson, 1994; Brodribb

Effects of height on treetop transpiration and stomatal conductance in coast redwood (Sequoia sempervirens)

Treetops become increasingly constrained by gravity-induced water stress as they approach maximum height. Here we examine the effects of height on seasonal and diurnal sap flow dynamics at the tops of 12 unsuppressed Sequoia sempervirens (D. Don) Endl. (coast redwood) trees 68-113 m tall during one growing season. Average treetop sap velocity (V(S)), transpiration per unit leaf area (E(L)) and stomatal conductance per unit leaf area (G(S)) significantly decreased with increasing height. These differences in sap flow were associated with an unexpected decrease in treetop sapwood area-to-leaf area ratios (A(S):A(L)) in the tallest trees. Both E(L) and G(S) declined as soil moisture decreased and vapor pressure deficit (D) increased throughout the growing season with a greater decline in shorter trees. Under high soil moisture and light conditions, reference G(S) (G(Sref); G(S) at D = 1 kPa) and sensitivity of G(S) to D (-δ; dG(S)/dlnD) significantly decreased with increasing height. The close relationship we observed between G(Sref) and -δ is consistent with the role of stomata in regulating E(L) and leaf water potential (Ψ(L)). Our results confirm that increasing tree height reduces gas exchange of treetop foliage and thereby contributes to lower carbon assimilation and height growth rates as S. sempervirens approaches maximum height.

Plant Development: Three Steps for Stomata

Three basic helix-loop-helix proteins regulate sequential steps in the formation of stomata: SPEECHLESS initiates entry into the stomatal lineage; MUTE controls asymmetric divisions of stomatal precursor cells; and FAMA promotes guard cell differentiation.

Guard Cells: Transcription Factors Regulate Stomatal Movements

Recent work shows that transcription factors are necessary for stomatal movements in plants. Different members of the plant-specific R2R3-MYB transcription factor family are required for mediating stomatal opening in response to light and stomatal closure in response to darkness.

Lowered stomata conductance promotes the oxidative burst, an essential factor in the promotion of programmed cell death

The role of stomata in photosynthesis and their importance in plant productivity is well established and a number of studies have shown their importance for the plant's interaction with the abiotic environment. However an increasing amount of evidence is pointing toward an important role for stomata also in biotic defense responses. Using programmed cell death mutants and pathogen infection we set out to investigate the role of stomata in biotic stress. Physiological characterization of the lsd1 mutant of Arabidopsis thaliana revealed a correlation between stomata conductance, H2O2 production and the spreading of cell death. When challenging wild type plants with the bacterial pathogen Pseudomonas syringae, stomata closed and limiting gas exchange strongly enhanced the spreading of cell death both in infected plants and in the lsd1 mutant. Further studies showed that limiting gas exchange enhances the production of reactive oxygen species that lead to the formation of focused hypersensitive response like lesions in ws-0. Our results clearly indicate the importance of stomata regulation in the spreading of cell death.

The role of stomata in sensing and driving environmental change.

Stomata, the small pores on the surfaces of leaves and stalks, regulate the flow of gases in and out of leaves and thus plants as a whole. They adapt to local and global changes on all timescales from minutes to millennia. Recent data from diverse fields are establishing their central importance to plant physiology, evolution and global ecology. Stomatal morphology, distribution and behaviour respond to a spectrum of signals, from intracellular signalling to global climatic change. Such concerted adaptation results from a web of control systems, reminiscent of a 'scale-free' network, whose untangling requires integrated approaches beyond those currently used.

Investigation into the importance of the stomatal pathway in the exchange of PCBs between air and plants.

The transfer of persistent organic pollutants (POPs) from air to vegetation is an important air-surface exchange process that affects global cycling and can result in human and wildlife exposure via the terrestrial food chain. To improve understanding of this process, the role of stomata in uptake of gas-phase polychlorinated biphenyls (PCBs) was investigated using Hemerocallis x hybrida "Black Eyed Stella", a plant with a high stomatal density. Uptake of PCBs was monitored over a 72-h period in the presence and absence of light. Uptake rates were significantly greater in illuminated (stomata open) plants than unilluminated (stomata closed) plants for 18 of the 28 measured PCB congeners (p < 0.05). Depuration of PCBs was monitored in a subsequent experiment over a period of 3 weeks. Levels after 3 weeks of depuration time were still much higher than the concentration prior to contamination. Tri- and tetrachlorinated PCBs showed the greatest depuration, with less than 20% and 50% of accumulated PCBs respectively remaining, while approximately 70% of higher chlorinated PCB congeners remained in the plants at the end of the experiment. Treatments with/without light (to control stomatal opening during uptake) and with/without abscisic acid (ABA) application (to control stomatal opening during depuration) were compared. After contamination indoors for 3 days, there was a significantly higher concentration of PCBs (p < 0.05) in the light contaminated plants than the dark-contaminated plants for 13 of the 28 measured PCB congeners. The ABA treatment affected depuration of PCB-18 only. "Light/ABA-treated" plants had a significantly slower depuration rate for PCB-18 than "light/untreated", "dark/ABA-treated", and "dark/untreated" plants (p < 0.05). The results of the study indicate that there is a stomatal effect on the rate of exchange of PCBs between Hemerocallis leaves and air.