JXB Advance Access originally published online on September 18, 2006
Journal of Experimental Botany 2006 57(14):3669-3678; doi:10.1093/jxb/erl114
RESEARCH PAPER |
Dynamics of spatial heterogeneity of stomatal closure in Tradescantia virginiana altered by growth at high relative air humidity
1Horticultural Production Chains Group, Plant Sciences, Wageningen University, Marijkeweg 22, 6709 PG, Wageningen, The Netherlands
2Department of Horticultural Sciences, Faculty of Agriculture, Lorestan University, PO Box 465, Korramabad, Iran
* To whom correspondence should be addressed. E-mail: Hossein.Rezaeinejad{at}wur.nl
Received 6 March 2006; Accepted 6 July 2006
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Abstract |
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The spatial heterogeneity of stomatal closure in response to rapid desiccation of excised well-watered Tradescantia virginiana leaves grown at moderate (55%) or high (90%) relative air humidity (RH) was studied using a chlorophyll fluorescence imaging system under non-photorespiratory conditions. Following rapid desiccation, excised leaves grown at high RH had both a greater heterogeneity and a higher average value of PSII efficiency (
PSII) compared with leaves grown at moderate RH. Larger decreases in relative water content resulted in smaller decreases in water potential and PSII of high RH-grown leaves compared with moderate RH-grown leaves. Moreover, the PSII of excised high RH-grown leaves decreased less with decreasing water potential, implying that the stomata of high RH-grown leaves are less sensitive to decreases in leaf water potential compared with moderate RH-grown leaves. After desiccation, some non-closing stomata were distributed around the main vein in high RH-grown leaves. Direct measurements of stomatal aperture showed 77% stomatal closure in the margins after 2 h desiccation compared with 40% closure of stomata in the main-vein areas in high RH-grown leaves. Faster closure of stomata in leaf margins compared with main-vein areas of leaves grown at high RH was related to substantially lower relative water content in these areas of the leaves. Key words: Desiccation, patchiness, PSII efficiency, relative water content, stomata, water potential
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Introduction |
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Stomata play a dominant role in the control of plant water relations and photosynthesis. Stomatal behaviour is the result of interactions between physiological factors and environmental conditions (Assmann, 1993; Kearns and Assmann, 1993; Hetherington and Woodward, 2003). Moreover, stomatal response characteristics depend on the growing conditions in which the stomata developed. One of the most important growing conditions affecting stomatal response is relative air humidity (RH). For example, stomatal malfunctioning has been reported in roses grown at an RH above 85% (Torre and Fjeld, 2001; Torre et al., 2003). Furthermore, a failure of stomata to close in response to desiccation or abscisic acid (ABA) has been shown in leafy cuttings rooted at high RH (Fordham et al., 2001) and plants propagated in vitro (Ziv et al., 1987; Santamaria et al., 1993). Recent research has shown that whether grown at moderate (55%) or high (90%) RH, the stomata in the leaves of Tradescantia virginiana decreased their aperture in response to desiccation, ABA application, and exposure to darkness (Rezaei Nejad and van Meeteren, 2005). However, transpiration rate and stomatal conductance and aperture in the high RH-grown plants remained higher than in the moderate RH-grown plants (Rezaei Nejad and van Meeteren, 2005), indicating a quantitative effect of RH during growth on stomatal functioning. This difference was because some of the stomata that developed in high RH closed only partially, or not at all. The distribution over a leaf surface of these less responsive stomata is unknown. Several authors have shown that stomatal aperture in distinct areas of a leaf can be well below the mean stomatal aperture of the leaf. This phenomenon has been called ‘patchy’ stomatal closure, and it can be induced by changes in a range of environmental factors, such as water and salt stress, changes in light intensity, changes in ambient CO2 partial pressure and low air humidity (Downton et al., 1988; Beyschlag and Pfanz, 1990; During, 1992; Mott et al., 1993; Eckstien et al., 1996; Beyschlag and Eckstien, 2001). However, the effect of long-term development in high RH on the heterogeneity of stomatal closure over a leaf surface is unknown. It is also unknown to what extent the developmentally determined differences in stomatal responses correlate with changes in other leaf hydraulic properties. Such correlations could point to either compensatory mechanisms or co-adaptation of hydraulic properties.
In recent years, chlorophyll fluorescence has been widely used to study the photosynthetic performance of many plant species in response to stress (Lu and Zhang, 1998; Meyer and Genty, 1999; Lichtenthaler and Babani, 2000; Maxwell and Johnson, 2000). Stomatal closure causes lower availability of CO2 inside the leaf and thus a decrease in the rate of carboxylation (Cornic and Massacci, 1996; Cornic, 2000), but its effect on PSII efficiency (PSII) is not proportional to the decrease in carboxylation because of photorespiration in normal air. Photorespiration can be almost completely eliminated by placing the leaf in an atmosphere with an O2 concentration of 
2% (Genty et al., 1990). Moreover, if the loss of PSII is due solely to stomatal closure, exposing desiccated leaves to a CO2 concentration sufficiently high to overcome the stomatal closure should restore PSII to the control value, as shown by Meyer and Genty (1999). Thus, a measurement of PSII under low O2 concentration may be used to detect the closure of stomata, and non-destructive chlorophyll fluorescence imaging techniques allow the spatial and temporal dynamics of this phenomenon to be studied (Daley et al., 1989; Meyer and Genty, 1998, 1999; Harbinson et al., 2005). In previous studies, attention was directed to the measurements of stomata that closed rapidly following water stress. In this study, the focus is more on how the closure in response to water stress of some stomata is delayed or entirely prevented. Moreover, as far as is known, there has been no report where the estimates of stomatal closure obtained by means of PSII measurements has been correlated with direct measurements of stomatal closure and water relation parameters in plants subjected to water stress.
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Materials and methods |
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Plant material and growth conditions
Young Tradescantia virginiana L. plants were grown in plastic pots (15 cm diameter) filled with commercial potting compost (Potgrond 4, Hortimea, Lent, The Netherlands) in two growth chambers, each with a different RH (moderate, 55±5%; high, 90±5%) at Wageningen University. The temperature was 21±0.5 °C resulting in vapour pressure deficits of 1.12 and 0.25 kPa for moderate-and high-RH conditions, respectively. The light intensity was 120±10 µmol m–2 s–1 (measured with an LI-250 light meter; Li-Cor, Lincoln, NE, USA) produced by fluorescent tubes (TLD 33; Philips) with a photoperiod of 16 h d–1. Although this light intensity is low, T. virginiana is a shade plant and measurements of its CO2 fixation irradiance response showed that 120 µmol m–2 s–1 is about 40% saturating for CO2 fixation. The plants were kept well watered and given a nutrient solution weekly (concentration: 2 g l–1; KristalonTM, Yara, Rotterdam, The Netherlands). The CO2 concentration in the growth chambers was 360±30 µmol mol–1 (measured with a CO2 analyser ADC225; MK3 Analytical Development Co. Ltd, Hoddesdon, UK). Young fully expanded leaves were used in the experiments.
Mapping of PSII photochemical yield using chlorophyll fluorescence imaging
One intact leaf (attached to the plant) from a high RH-grown plant and another leaf from a moderate RH-grown plant were put side by side inside a gas-tight cuvette that was placed in the imaging area of a commercial chlorophyll fluorescence imaging system (FluorCam 700MF; Photon Systems Instruments, Brno, Czech Republic). In the imaging system, an 8-bit, 512x512 pixel, black and white CCD camera equipped with an F1.2/2.8–6 mm objective was used to record fluorescence images. Images were recorded during short measuring flashes superimposed upon a continuous actinic irradiance of 100 µmol m–2 s–1. These flashes and the continuous actinic irradiance were provided by two panels each containing 345 orange light-emitting diodes. Saturating white light pulses with 2500 µmol m–2 s–1 intensity were generated with a 250 W halogen lamp. While this is a relatively low irradiance for a saturating irradiance, it was sufficient to saturate QA reduction in these leaves as the 
F'/F'm obtained with 2500 µmol m–2 s–1 irradiance was 97% of that obtained with a saturating irradiance of 6000 µmol m–2 s–1. Once the leaves had reached a steady state, two successive series of fluorescence images were digitized and averaged, one just before (It), and the other (I'm) during the saturating light flash that causes a transitory saturation of photochemistry. The averaged fluorescence intensities obtained for each related pixel in images It and I'm were used directly as relative measurements of fluorescence yield and used to calculate values for PSII (Genty et al., 1989). The frequency distribution histogram, average value, and standard deviation of PSII per image were calculated using the FluorCam software (version 5.0). The fluorescence measurements were conducted under an atmosphere of 20 mmol mol–1 O2, 350 µmol mol–1 CO2, with the remainder N2 (normal CO2; a non-photorespiratory condition) in the cuvette. The relative humidity in the air flowing through the cuvette was 40±2% and was produced by passing the air through a temperature-controlled column of iron (II)-sulphate heptahydrate (Fluka). The cuvette temperature was 22±1 °C. After a steady state was reached, the first image that was taken from the leaves (which were still attached) served as a control. The desiccation process was begun by excising both leaves from their plants, and images were then taken every 30 min for 150 min. At the end of the desiccation period an image was made after 5 min exposure to 20 mmol mol–1 O2, 5000 µmol mol–1 CO2, with the remainder N2 (high CO2) to test for the recovery of PSII under high-CO2 conditions. The experiment was repeated with seven leaves from seven randomly selected plants in each RH treatment.
Stomatal aperture measurements
Based on many preliminary observations, it was established that margins always had lower PSII than main-vein areas. Measurements of stomatal aperture were made from these two areas on leaves that had not themselves been imaged. Using a silicon rubber impression technique (Smith et al., 1989), stomatal aperture was determined from young fully expanded leaves at approximately two-thirds of the distance from the base to the tip. One leaf per plant and one impression (5 mmx10 mm) per location on the leaf were used. Impressions were made on attached leaves and on detached leaves 2 h after their separation from the plants. All plants and leaves were kept in a test room (40% RH, 20 °C, 1.40 kPa vapour pressure deficit, and 100 µmol m–2 s–1 irradiance) for the duration of this treatment. Stomatal aperture was determined with digitized video images (x800 magnification) of abaxial stomata (10 stomata per impression) using a microscope (Leica; Aristoplan) connected to a Nikon digital imaging DXM-1200 camera. Image processing was done using the free UTHSCSA ImageTool program (University of Texas Health Science Centre at San Antonio, TX, USA).
Measurements of water potential and relative water content (RWC)
Guided by the images obtained from the imaging system, leaf discs or segments were cut from the different regions of a leaf and were used for the measurements of either water potential or RWC. Leaf discs were cut with a 5 mm diameter cork-borer and the edges of the cut discs were swabbed with tissue to remove the liquid released by localized cell crushing. Leaf water potential was measured using a Wescor Vapro (model 5520; Wescor, Logan, UT, USA) vapour pressure osmometer. Leaf discs were sealed inside the chamber for at least 2 h to ensure water vapour equilibrium. To obtain RWC, leaf discs or segments were first weighed, and then floated in distilled water for 4 h, then reweighed, after which their dry weight was determined.
Measurements of stomatal size and density
Stomatal size and density were measured on epidermal strips from young fully expanded leaves at approximately two-thirds of the distance from the base to the tip. The epidermal strips were removed from the margins and main-vein areas of abaxial surface of leaves and cut into 5 mmx10 mm pieces using the technique of Weyers and Meidner (1990). One leaf per plant and one strip per location of the leaf were used. The strips were preincubated for 2 h in a stomata-opening medium (10 mM MES-KOH, pH 6.15, 50 mM KCl) in the test room. The measurement of guard cell length was done on 10 strips in each location and 10 randomly selected stomata in each strip from digitized video images of stomata. The stomatal density was calculated from the counts of the number of stomata in 20 strips in each location of the leaf.
Statistical analysis
For PSII, stomatal aperture, and stomatal size and density, data were subjected to analysis of variance. Data in Fig. 4 were analysed using repeated-measures analysis of variance. Student's t-test was used for mean separation (P=0.05). The relationships between PSII and RWC, water potential and RWC, PSII and water potential were fitted using linear regressions. The parameters of fitted lines were compared statistically with an F-test. GraphPad Prism 4 for Windows (GraphPad Software, San Diego, CA, USA) was used for all statistical analyses and curve fitting.
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Results |
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Figure 1 shows the mean
PSII (i.e. the mean of the average PSII values that were obtained from individual images) of leaves from moderate and high RH-grown plants (i) before excision (control) under normal CO2, (ii) 150 min after excision under normal CO2, and (iii) 150 min after excision under high CO2. There was a significant interaction between RH during growth and rapid desiccation after excision of the leaves (P=0.003). In moderate RH-grown plants, there was a significant difference in the PSII of leaves in different treatments (P <0.001) and leaves before excision had a high PSII. With rapid desiccation after excision, PSII decreased significantly, and with the transition to high CO2, PSII almost completely recovered to the value of the control images. Qualitatively the same results were found in leaves grown at high RH. The average recovery of PSII in both groups of the plants in high CO2 treatments was around 95%. Moreover, there was a significant difference between the PSII of leaves grown at moderate and high RH after desiccation under normal CO2 (P=0.01).
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Images of
PSII (Fig. 2) and corresponding frequency distributions (Fig. 3) show different trends in the distribution of photosynthetic activity of T. virginiana leaves grown at moderate or high RH. Before excision and in an atmosphere of normal CO2, PSII was both high and homogeneously distributed over the leaves, irrespective of whether they had been grown at a moderate or a high RH (Fig. 2A). The frequency distributions of both leaves had an almost identical shape: a narrow distribution around 0.7, negatively skewed (Fig. 3A). During desiccation after excision, leaves grown at moderate RH showed a rapid decrease of PSII, especially during the second hour of desiccation (Fig. 2B–E). Although leaves grown at moderate RH showed some heterogeneity after 90 min of desiccation (Fig. 2C), the frequency distribution of their PSII was unimodal and symmetrical (Fig. 3C). With increasing duration of desiccation, the PSII of the moderate RH leaves remained homogeneously distributed over the leaf surface (Fig. 2D–E) and the frequency distributions shifted progressively towards the lower PSII values (Fig. 3D–E). In leaves grown at high RH, PSII decreased less during desiccation (Fig. 2B–E). The PSII distributions initially became more negatively skewed and after 90 min of desiccation the distribution became rather broad (Fig. 3B–E). After 2 h of desiccation the distribution of PSII in leaves grown at high RH was much more heterogeneous than in leaves grown at moderate RH, with clear differences between different areas of a leaf. In marginal parts of the high RH-grown leaves, PSII decreased during desiccation, while around the main vein, PSII always remained high. The average PSII in the desiccated leaves of both leaf types recovered almost completely after 5 min exposure to high CO2 (Fig. 2F), and the frequency distributions were negatively skewed (Fig. 3F). The tips of the leaves were located outside the cuvette and Fig. 2 showed that PSII in normal air with photorespiration was high.
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Figure 4 shows how the mean
PSII of images of leaves grown at moderate and high RH changes with time of desiccation. Before excision (time 0), PSII in both groups of leaves was high and there was no significant difference between them. With desiccation after excision, PSII of leaves from moderate RH-grown plants decreased earlier than it did for leaves from high RH-grown plants. The interaction between RH during growth and time of desiccation after excision was significant (P <0.0001).
Figure 5 shows in more detail the effect of rapid desiccation after excision on PSII and its frequency distribution in two different regions (leaf margin and main-vein area) of a leaf grown at high RH. The skewed frequency distribution of PSII from the entire leaf image is the result of a clear difference in the frequency distributions of the two regions. The RWC in the margin area which has a low PSII, was much lower than in the main-vein area where the PSII was higher.
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In leaves grown at high RH, the interaction between the location of stomata and the effect of rapid desiccation after excision on stomatal aperture was significant (P <0.0001) (Fig. 6A). The average stomatal aperture at the margins of non-desiccated attached leaves grown at high RH was higher than around the main veins (P=0.02). After excision, stomatal aperture decreased with desiccation (P <0.0001) in both parts of the leaf, but after 2 h of desiccation the average stomatal aperture was significantly wider around the main veins (P=0.0003). After 2 h of desiccation, stomatal aperture at the leaf margins decreased by 77%, while around the main veins the decrease was only 40%. Before excision, the stomatal aperture distribution at the leaf margins was more in the wider-aperture classes compared with the areas around the main vein (Fig. 6B). Two hours after excision, >50% of stomata in the margins were in the 0–2 µm size class and the distribution was positively skewed. In the main-vein areas, although the stomata on average decreased their aperture, they still remained slightly or completely open (Fig. 6C).
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There were decreases in
PSII (Fig. 7A) and leaf water potential (Fig. 7B) as RWC decreased in moderate or high RH-grown leaves. However, at the same values of RWC, the PSII and water potential in high RH-grown leaves were higher than in the moderate RH-grown leaves. Considering either the moderate or the high RH-grown plants, there were no significant differences between the slopes of the regression lines calculated from data obtained from the leaf margins and main-vein areas. However, when the moderate and high RH-grown plants were compared, there were significant differences in both the relationships of PSII and RWC, and leaf water potential and RWC (P < 0.0001). Furthermore, PSII decreased as leaf water potential decreased in both groups of plants (Fig. 8). However, compared with moderate RH-grown leaves, the PSII of high RH-grown leaves decreased less with decreasing water potential, as revealed in the significant difference (P=0.0003) between the slopes of regression lines (0.35±0.05 and 0.12±0.02 for moderate and high RH-grown plants, respectively).
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Regardless of the growth conditions, there was no significant difference between the length of guard cells (Table 1) in the margins and main-vein areas of leaves. However, stomatal density in the leaf margins was significantly higher than in main-vein areas in both groups of leaves grown at moderate and high RH (P <0.0001).
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Discussion |
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The similarity of the mean
PSII of leaves before desiccation under normal CO2 and after desiccation under high CO2 suggests that the closure of stomata is the main factor causing lower PSII via a decrease in the CO2 concentration inside the leaf, as has been shown in Rosa rubiginosa (Meyer and Genty, 1998, 1999). However, as high CO2 did not completely restore the PSII distributions to the pre-desiccation pattern this also suggests that desiccation has other direct effects on photosynthesis. An explanation would be a higher mesophyll resistance to CO2 uptake in desiccated leaves due to water loss (Harbinson et al., 2005). In both leaf types, stomata responded to rapid desiccation after excision. However, the homogeneity, speed, and degree of stomatal closure were less in high RH-grown plants (Figs 2, 3).
The result of PSII measurements in this study is in accordance with the results of a previous study which showed higher leaf conductance, leaf transpiration rate, and average stomatal aperture in leaves grown at high RH in response to 2 h desiccation compared with leaves grown at moderate RH (Rezaei Nejad and van Meeteren, 2005). The higher PSII and stomatal aperture around the main vein in leaves grown at high humidity revealed that the stomata in this area did not close in response to desiccation, while in the area near leaf margins the stomata closed and thus PSII was reduced.
Some studies have shown that the heterogeneity of photosynthesis induced by dehydration (Meyer and Genty, 1999) or ABA treatment (Meyer and Genty, 1998) was due to heterogeneous distribution of stomatal closure over the leaflet of Rosa rubiginosa which is heterobaric. Although heterogeneity of stomatal closure was observed in high RH-grown Tradescantia plants, which are monocotyledons, the pattern is different from that observed in the leaves of dicotyledons, for example, Rosa and Xanthium. There are differences between the venation (especially minor veins) and stomatal arrangement in homobaric (Tradescantia) and heterobaric (Rosa) leaves. In Rosa, stomata are distributed and oriented randomly over the leaf surface, and the minor veins subdivide the leaf into heterobaric zones. Stomata of Tradescantia are distributed in intercostal areas and in linear arrays, with the orientation of stomatal pores generally parallel to the long axis of the leaf. Nonetheless, it is clear that a heterogeneous stomatal response can be induced in monocotyledonous homobaric leaves, resulting in an uneven distribution of PSII. It is also noteworthy that the large differences in PSII can be established over relatively small lateral distances. The width of the leaves used in these experiments was about 1 cm, which implies that gradients of PSII can be maintained over lateral distances of millimetres, indicating that lateral diffusion of CO2 through the mesophyll air spaces is not efficient over these distances (Morison et al., 2005).
The present results suggest that the heterogeneity of stomatal resistance, and consequently of photosynthesis, induced by dehydration is affected both by the history of growth conditions in which stomata developed and the duration of desiccation. In main-vein areas of leaves grown at high RH, stomatal closure is much delayed or even absent. Moreover, it is not clear to what extent the mechanism proposed for stomatal patchiness induced by, for example, drought stress (Mott and Buckley, 2000) will be useful in explaining the behaviour of less-responsive stomata found in leaves grown at high RH.
The present results show that the relationships of PSII and RWC, and water potential and RWC have been affected by the high RH during growth (Fig. 7). Although PSII and water potential decreased as RWC decreased in both groups of plants, they both decreased less with decreasing RWC in the high RH-grown plants than in the moderate RH-grown plants. Regardless of the growth conditions, the leaf margins had the same responses of PSII and water potential to a decrease in RWC as the main-vein areas, in spite of the fact that growth at high RH resulted in different responses between the leaf margins and main-vein areas. Moreover, PSII of high RH-grown leaves decreased less with decreasing water potential compared with moderate RH-grown leaves (Fig. 8). From these observations the following can be concluded. First, growth at high RH greatly and uniformly modifies the leaf tissue properties such that leaf water potential becomes less sensitive to RWC. Secondly, growth at high RH greatly and uniformly modifies the response of stomatal aperture such that it also becomes less sensitive to a decrease in RWC and leaf water potential. Thirdly, the differences in stomatal aperture and PSII that developed between the leaf margins and main-vein areas in the high RH-grown leaves during desiccation implies that gradients of RWC, and thus water potential, can be established and maintained between main vein and leaf margin zones, a distance of around 5 mm.
The changes in the relationship between water potential and RWC during growth at high RH could be due to changes in the osmotic potential and/or pressure potential. It has been shown that cell size has a major effect on changes in cellular osmotic potential due to water loss. The bigger the cell size, the higher (less negative) the osmotic potential can be with the same water loss (Cutler et al., 1977). Bigger stomata and epidermal cells have been reported in Tradescantia virginiana grown at high RH (Rezaei Nejad and van Meeteren, 2005). Therefore, leaves grown at high RH would need to have a lower water content to reach the same value of osmotic potential as leaves grown at moderate RH.
Although the bigger guard cells found in leaves grown at high RH may have an effect on the osmotic potential in stomata, it does not completely explain why stomata of leaves grown at high RH do not close in response to prolonged desiccation. Stomatal movement is the result of changes in guard cell turgor. These are regulated in response to the complex integration of numerous signals that ultimately act upon a network of ion channels in the plasma and vacuolar membranes of guard cells. The movement of water in response to these fluxes results in changes of the turgor pressure of the guard cells. ABA is a key component of the signal-transduction pathway for stomatal closure (Leung and Giraudat, 1998). Many studies have suggested that the short-term effects of elevated ABA concentrations on stomatal functioning are reversible (Ackerson, 1980; Trejo et al., 1995; Tardieu et al., 1996), but its long-term effects on developmental changes and functioning of stomata are permanent (Brown et al., 1976; Cutler et al., 1977; Franks and Farquhar, 2001). All of these studies were concerned with increased ABA levels, resulting in lower conductance and opening with lower stomatal conductance at any given guard cell turgor. It is proposed that a very low ABA concentration in well-watered plants during growth at high RH might also cause structural or physiological changes in stomata, reducing stomatal responsiveness to a lowered hydration state. However, further research is needed to support this suggestion.
The PSII of the leaf margins in high RH-grown leaves were lower than those found in main-vein areas after the same duration of desiccation (Fig. 5). A lower PSII was also correlated with a lower RWC (Figs 5, 7A). There were no differences in the relationships between RWC and PSII or water potential in the leaf margins and the main-vein areas in leaves grown at either moderate or high RH. So, it can be concluded that the lower PSII that develops in the leaf margins after desiccation is not due to a faster response of stomata. Rather it is a consequence of the lower water content of this tissue resulting in lower turgor of the guard cells. The difference between the RWCs of the two areas of leaf might be the result of differences in transpiration rates of these two areas. The stomatal density (Table 1) and the initial stomatal aperture (Fig. 6A) are higher in leaf margins which would result in a higher transpiration rate. When the leaf is detached from the plant, the water flux from the leaf would therefore be faster in the leaf margin compared with the main-vein area. This greater rate of water loss would only result in a lower water potential if the diffusive resistance for lateral water movement from the main vein to margins were sufficiently high, thus preventing equilibration of the two tissue zones (Fig. 7). It was surprising that such a large lateral diffusive resistance was present in these leaves.
In conclusion, the results of this research have provided further support for the effectiveness of chlorophyll fluorescence imaging systems in the study of stomatal behaviour, and the heterogeneity of this behaviour. Direct measurements of stomatal aperture and its distribution in this research confirmed the indication of stomatal closure obtained from the chlorophyll fluorescence images under low O2 concentration. Different trends of stomatal closure and heterogeneity in response to rapid desiccation after excision in leaves grown at moderate and high RH suggest that high RH during the development of stomata affected stomatal behaviour in response to rapid water stress. The higher average values of PSII and stomatal aperture in response to desiccation in the main-vein areas compared with leaf margins of leaves grown at high RH confirmed the non-uniform closure of stomata and the location of non-closing stomata mostly around the main vein of the leaves.
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Acknowledgements |
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This research was financed by the Ministry of Science, Research, and Technology of I.R. Iran. The authors also thank the anonymous referees and handling editor for their helpful comments and suggestions.
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Abbreviations |
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ABA, abscisic acid; Ft, the steady-state value of fluorescence at a certain light level; Fm', maximum fluorescence at a certain light level;
F'/Fm', the ratio of the difference between (Fm'–Ft) and Fm' is equal to PSII if during the saturating light pulse the QA pool is completely reduced; PSII, relative quantum yield or efficiency for electron transport by photosystem II; PSII, photosystem II; QA, primary quinone acceptor of photosystem II; RH, relative air humidity; RWC, relative water content. |
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