Relative Stomatal Conductance Research Articles

FOLIAR APPLICATION OF SUGARCANE MOLASSES CAN ALLEVIATE STRESSFUL CONDITION IN GLYCINE MAX (L.) MERR. CROP UNDER FIELD CONDITIONS

The objective of the present study was to investigate the effects of sugarcane molasses application at 2 days (D2) and 7 days (D7) after precipitation after a period of drought stress on some physiological, growth, and yield parameters of soybean under field conditions. The experiment was set to obtain 14 plants per meter of row, and considered two treatments: molasses application at 2 days (D2) and 7 days (D7) after precipitation, after ca. 1 month without rainfall. Twenty days after the second application, thermographic and chlorophyll a fluorescence measurements were taken at the reproductive stage (R5 - early seed filling). Measurements were conducted between 12:00 and 14:00, representing the hottest hours of the day, on fully expanded leaves. Additionally, plant growth and yield measurements were taken from the same plants at the reproductive stage R7. The plot treated with sugarcane molasses application at D2 showed increased productivity (ca. 58%) due to higher stomatal conductance as reflected by water stress and relative stomatal conductance thermal index conditions.

Quantifying the role of ozone-caused damage to vegetation in the Earth system: a new parameterization scheme for photosynthetic and stomatal responses

Abstract. Surface ozone (O3) is the primary air pollutant threatening global vegetation. It typically reduces the photosynthetic rate and stomatal conductance, leading to changes in carbon, water, and energy cycles; vegetation structure and composition; and climate. Several parameterization schemes have been developed to integrate the photosynthetic and stomatal responses to O3 exposure in regional and global process-based models to simulate time- and space-varying O3 plant damage and its cascading dynamic influence. However, these schemes are calibrated based on limited observations and often fail to reproduce the response relationships in observations, impeding accurate assessments of the role of O3 plant damage in the Earth system. This study proposes a new parameterization scheme to utilize the extensive observations from O3 fumigation experiments to inform large-scale modeling. It is built on 4210 paired data points of photosynthetic and stomatal responses compiled from the peer-reviewed literature, more than 6 times larger than those employed in earlier schemes. Functions of phytotoxic O3 dose (POD) are found to accurately reproduce the statistically significant linear or nonlinear relationships observed between POD and either relative leaf photosynthetic rate or relative stomatal conductance for needleleaf trees, broadleaf trees, shrubs, grasses, and crops. These eliminate the practice in earlier schemes of setting response functions as constants and applying the response function from one vegetation type to another. It outperforms the old scheme in the Community Land Model (CLM), which skillfully reproduces the observed response for crop photosynthetic rate only. The nonlinear response functions we developed depict decreasing plant sensitivity with increases in POD, enabling models to implicitly capture the variability in plant ozone tolerance and the shift among plant species for both intra- and inter-PFTs (plant functional types) within a vegetation type observed in the real world. Then, the new scheme is incorporated into the Community Earth System Model version 2.2 (CESM2.2), specifically its land component CLM5, to quantify the global impacts of present-day O3 plant damage by comparing the simulations with and without O3 plant damage. Results show that O3 exposure reduces the global leaf photosynthetic rate by 8.5 % and stomatal conductance by 7.4 %, around half the estimates using the old scheme. Furthermore, the new scheme improves global gross primary productivity (GPP) simulations, decreasing RMSE by 11.1 % relative to simulations without O3 plant damage and by 11.7 % compared to the old scheme. These results underscore the importance of including O3 plant damage in large-scale process-based models and the effectiveness of the new scheme in assessing and projecting globally the role of O3 plant damage in the Earth system.

GsNAC2 gene enhances saline-alkali stress tolerance by promoting plant growth and regulating glutathione metabolism in Sorghum bicolor.

The quality and yields of Sorghum bicolor plants are seriously affected by saline-alkali conditions. NAC (NAM, ATAF, and CUC) transcription factors are plant specific and have various functions in plant development and response to various stresses. To investigate how GsNAC2 functions in sorghum responses to saline-alkali treatment, the characteristics of GsNAC2 were analysed by bioinformatics methods, and NaHCO3:Na2CO3 (5:1, 75mM, pH 9.63) saline-alkali stress solution was applied when sorghum plants were 2weeks old. The research results show that GsNAC2 belongs to the NAC gene family. GsNAC2 was significantly induced by saline-alkali treatment and strongly expressed in sorghum leaves. GsNAC2-overexpressing sorghum plants had increased plant height, dry weight, moisture content, root activity, leaf length, chlorophyll content, stomatal conductance, relative root activity, relative chlorophyll content, relative stomatal conductance, and relative transpiration rate after saline-alkali treatment. Lower H2O2 and O2- levels, relative permeability of the plasma membrane, and malondialdehyde (MDA) content were found in GsNAC2-overexpressing sorghum. In transcriptome analysis, clusters of orthologous groups (COG) analysis showed that a high proportion of differentially-expressed genes (DEGs) participated in defence mechanisms at each processing time, and 18 DEGs related to synthetic glutathione were obtained. Gene expression analysis revealed that key genes in glutathione biosynthesis pathways were upregulated. GR and GSH-Px activities were increased, and GSH accumulated more with the overexpression of GsNAC2 after saline-alkali treatment. Furthermore, these results suggest that GsNAC2 acts as a potentially important regulator in response to saline-alkali stress and may be used in molecular breeding to improve crop yields under adverse environmental conditions.

Kaolin particle film mitigates supra-optimal temperature stress effects at leaf scale and increases bean size and productivity of Coffea canephora

Summary In young plants of Coffea canephora, fine particle film based on calcined kaolin (KF) causes a decrease in leaf temperature (Tleaf), minimizing the damages to the photochemical apparatus, especially in summer season, but no report about the ecophysiological responses to KF over phenology is available on this species. We hypothesized that greater ecophysiological effects of KF would occur during the summer phenophase of leaf area and berry expansion (BE) compared with autumn berry maturation phase (BR), and that those benefits will have impact on bean productivity and physical quality. In this sense, the present study aimed to analyze the effects of KF on some ecophysiological parameters in the last phenophases of C. canephora biennial cycle, bean productivity, and bean size classification. During the complete phenological cycle, eight applications of KF were performed, totaling 80 kg of KF ha−1 in two years. KF applications and ecophysiological measurements occurred during the BE and BR phenophases. Firstly, we documented temporal dynamics in responses of field-grown C. canephora adult plants to KF application. KF reduced Tleaf and crop water stress index and increased the thermal index of relative stomatal conductance and relative chlorophyll content (SPAD index) observed in the autumn BR (opposite to young plants). The positive impact of KF on chlorophyll a fluorescence was proportionally similar among the observed phenophases. Secondly, KF increased bean size 16 by 50% and increased total productivity. The plants treated with KF increased productivity by 1.7 t ha− commercially useful bean mass compared to the control. The kaolin dose of 40 kg ha−1 distributed four times per year was highly effective as a protection strategy against high-light and elevated Tair.

Kaolin Film Increases Gas Exchange Parameters of Coffee Seedlings During Transference From Nursery to Full Sunlight.

Increases in water use efficiency (WUE) and the reduction of negative impacts of high temperatures associated with high solar radiation are being achieved with the application of fine particle film of calcined and purified kaolin (KF) on the leaves and fruits of various plant species. KF was applied on young Coffea arabica and Coffea canephora plants before their transition from nursery to full sunlight during autumn and summer. The effects of KF were evaluated through the responses of leaf temperature (Tleaf), net CO2 assimilation rate (A), stomatal conductance (gs), transpiration (E), WUE, crop water stress index (CWSI), index of relative stomatal conductance (Ig), initial fluorescence (F0), and photosynthetic index (PI) in the first 2–3 weeks after the plant transitions to the full sun. All measurements were performed at midday. In Coffea plants, KF decreased the Tleaf up to 6.7°C/5.6°C and reduced the CWSI. The plants that were not protected with KF showed lower A, gs, E, and Ig than those protected with KF. C. canephora plants protected with KF achieved higher WUE compared with those not protected by 11.23% in autumn and 95.58% in summer. In both Coffea sp., KF application reduced F0, indicating reduced physical dissociation of the PSII reaction centers from the light-harvesting system, which was supported with increased PI. The use of KF can be recommended as a management strategy in the transition of Coffea seedlings from the nursery shade to the full sunlight, to protect leaves against the excessive solar radiation and high temperatures, especially in C. canephora during the summer.

Revisiting integral water capacity on the basis of stomatal conductance at various soil and root length densities in sunflower plant

Computation of integral water capacity (IWC), as an alternative criterion for soil water availability to plant, requires developing of weighting function (ω(h)) for each limiting factor may prevail within a particular water or suction range (e. g., hd and hw) in the root medium and diminish water availability to plant. So far defining hd and hw limits for a particular limiting factor has been primarily soil property based. We investigated the possibility of plant response based ω (h) as a proxy. For this purpose, a sandy clay loam soil was evenly compacted to three bulk densities (Db) of 1.35, 1.55 and 1.75 Mg m-3 in large (30 cm diameter and 70 cm depth) PVC tubes (called pots hereafter). Sunflower (Helianthus Annuus L) seedlings were planted in the pots and after their full establishment (at late vegetative growth stage) root pruning was implemented by cutting off approximately 75%, 50%, 25% and 0% of the root branches in the 0-25 cm section along the root crown. Two periods of wetting and drying cycles were subsequently imposed and soil water contents and midday leaf stomatal conductance were routinely measured. At the end of the experiment total root length in the samples separately taken from three sections (5-25, 25-50 and 50-70 cm) of each pot was measured using GSA image analyzer software and converted to root length density (RLD). Relative stomatal conductance (g/gc), presumably varying between 0 to 1 and comprising impact of all water availability limiting factors in the root medium, as function of h were developed and substituted for ω(h) and the plant response based integral water capacity (IWCP) was then computed and compared between various imposed treatments and with least limiting water range (LLWR) and plant available water (PAW). For the treatments without root pruning (L4), increase in soil compaction (from D1 to D2 and from D1 to D3) reduced IWCP by about 17.5% and 20.5%, respectively. The corresponding decreases for LLWR were, 22% and 66%, respectively, and negligible for PAW. These results showed that PAW is almost irresponsive and LLWR is over responsive to soil compaction or bulk density while IWCP seems reflect more realistic influence of soil compaction on soil water availability to plants. Decrease in IWCP from L4 to L1 was by 43%. In contrary to LLWR and PAW, our proposed method showed the effect of root length density on plant available water.

Sensitivity of Grapevine Soil–Water Balance to Rainfall Spatial Variability at Local Scale Level

In rainfed agriculture systems, rainfall water management (harvesting, storage and efficient use) is a key issue. At local scale (i.e. from 100m to 50 km), the impact of rainfall space and time variability on crop water availability is seldom addressed. In order to accurately depict the space and time variations of rainfall at local scale, a dense rain-gauges network composed of 45 rain-gauges has been deployed over 28 km² area, in Burgundy vineyards (North-East France). Rainfall data collected by each rain-gauge from 2014 to 2016 was used as input variables in the grapevine water balance model proposed by Lebon et al. All other climate variables, vineyard and soil parameters were kept strictly identical for each simulation in order to capture the consequences of the sole spatial variability of rainfall on vineyard water status. As rainfall dynamics impact on the vineyard depends on the soil water content, water balance was modelled considering soils with low (50 mm) and medium (150 mm) soil water holding capacities, representative of the soils of the area. The impact of modelled soil water availability for grapevine was assessed using the water deficit stress index (WDSI), i.e. the relative stomatal conductance. Local rainfall variability throughout the vine vegetative period lead to large variations in WDSI: it varied up to 0.3 within the study area solely because of rainfall spatial variability. Using a set of 34 weather stations at mesoscale level over Burgundy (186 km from North to South), we showed that local rainfall might contribute to change in grapevine water status as large as 50% of the simulated regional water balance spatial variability. Our results indicates that local rainfall and its impacts on agricultural production are probably not sufficiently considered in farming systems, potentially leading to in-accurate water management (cover-crop, irrigation) due to sparse rainfall network.

Corrigendum

Functional EcologyVolume 33, Issue 9 p. 1811-1812 CORRIGENDUMFree Access Corrigendum This article corrects the following: More than iso/anisohydry: Hydroscapes integrate plant water use and drought tolerance traits in 10 eucalypt species from contrasting climates Ximeng Li, Chris J. Blackman, Jennifer M. R. Peters, Brendan Choat, Paul D. Rymer, Belinda E. Medlyn, David T. Tissue, Rafael Oliveira, Volume 33Issue 6Functional Ecology pages: 1035-1049 First Published online: April 7, 2019 First published: 22 July 2019 https://doi.org/10.1111/1365-2435.13412AboutSectionsPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinkedInRedditWechat Li, X, Blackman, C. J., Peters, J. M. R., Choat, B., Rymer, P. D., Medlyn, B. E., Tissue, D. T. (2019). More than iso/anisohydry: Hydroscapes integrate plant water use and drought tolerance traits in 10 eucalypt species from contrasting climates. Functional Ecology, 2019; 33: 1035–1049. https://doi.org/10.1111/1365-2435.13320 In the paper by Li et al. (2019), Figures 3 and 4 were reversed. The figures are reproduced correctly below. Figure 3Open in figure viewerPowerPoint Bi-plot showing the results of principal component analysis (PCA) among metrics of isohydricity and plant traits. Traits used for PCA are turgor loss point (TLP), stomatal closure point (Pgs90), xylem water potential inducing 50% loss of conductivity in stem (Px50) and leaf (PL50), maximum stomatal conductance (gsmax), maximum stem conductivity (Kmax) and Huber value (HV). Different vegetation types are abbreviated using initials and are coloured differently (DSF, dry sclerophyll forest; GW, grassy woodland; SAW, semi-arid woodland; WSF, wet sclerophyll forest Figure 4Open in figure viewerPowerPoint Percentage loss of xylem conductivity of leaf (solid line, light grey band), stem (open triangle, light blue band) and relative stomatal conductance (closed circle, pink band) responds to water potential in two species (Figure 2a,b) with contrasting Hydroscape area (Figure 2c,d). Shaded bands represent confidence interval for stem and stomatal response to water potential and denote standard error for leaf response. Inset images show the propagation of embolism within leaf vein network during dehydration. Veins are coloured differently according to the water potential at which embolism occurs. Numbers below the scale bar indicate the water potential at which first and last embolism events were observed. Colours of data points in panel c and panel d represent different individuals. R2 of linear regression is also shown. Asterisk indicates significant level (***p < .001; **p < .01; *p < .05) The authors apologize for any confusion. Reference Li, X., Blackman, C. J., Peters, J. M. R., Choat, B., Rymer, P. D., Medlyn, B. E., & Tissue, D. T. (2019). More than iso/anisohydry: Hydroscapes integrate plant water use and drought tolerance traits in 10 eucalypt species from contrasting climates. Functional Ecology, 33, 1035– 1049. https://doi.org/10.1111/1365-2435.13320 Volume33, Issue9September 2019Pages 1811-1812 FiguresReferencesRelatedInformation

Factors determining stomatal and non-stomatal (residual) transpiration and their contribution towards salinity tolerance in contrasting barley genotypes

Eighty barley (Hordeum vulgare L.) genotypes of different geographical origin and contrasting in salinity stress tolerance were grown under glasshouse conditions and exposed to high salinity stress (300 mM NaCl) for four weeks to investigate the relationship between leaf gas exchange, tissue ionic relations, and plant salinity tolerance. Four weeks after the treatment commenced, stomatal conductance, stomatal density, residual transpiration, chlorophyll content, leaf sap Na, K, Cl and leaf sap osmolality were measured. Responses to salinity stress differed greatly among the genotypes. The overall salinity tolerance significantly correlated with leaf Na+ content, osmolality and the residual transpiration. At the same time, no significant correlation between salinity tolerance and stomatal conductance was found. The residual transpiration in stressed plants correlated negatively with the leaf sap osmolality of control plants. A significant correlation was found between changes in the residual transpiration and changes in leaf Cl− content but no such correlation was found for leaf Na+. Higher stomatal density was correlated with higher osmolality under both salinity stress and control conditions. The stomatal density correlated negatively with the residual transpiration under salinity stress conditions but positively with K+ accumulation in the shoot under both control and salinity stress conditions. Higher relative stomatal conductance correlated with higher residual transpiration and lower stomatal density under salinity stress. Interestingly, stomatal conductance correlated very strongly with Cl− accumulation in the shoot under stress conditions but negatively under control conditions. Taking together, these results suggest that increasing stomatal density as well as minimization of the residual transpiration may be a promising way of improving water use efficiency and increase salinity tolerance in barley. Our data also show that residual transpiration is strongly affected by the number of stomatal pores on the leaf surface.

Thermal data to monitor crop-water status in irrigated Mediterranean viticulture

Canopy temperature (TC) can be a robust indicator of grapevine water status. However, the assessment of TC by thermography in field conditions requires optimization, especially under variable environmental conditions (daily and seasonal) just like it occurs in Mediterranean areas. Besides, simplicity and robustness should be the basis of a wider use of thermography in field conditions. Therefore the comparison of simpler and more complex approaches in terms of thermal indicators is wise to test. Our major aims were: i) to assess the performance of four common thermal indicators (canopy temperature − TC, Crop Water Stress Index − CWSI, index of relative stomatal conductance − IG, and the difference between TC and the surrounding air − ΔTcanopy-air) to support irrigation decisions ii) to optimize the timing of thermal measurements for different genotypes and iii) to obtain mathematical functions to estimate leaf gas exchange parameters on the basis of thermal data. The trial was conducted in the summer of 2013, in south Portugal. Two V. vinifera red varieties, Touriga Nacional and Aragonez (syn. Tempranillo) were tested. Vines were subjected to two irrigation regimes: i) sustained-deficit irrigation (based on the farm’s schedule − control) and ii) regulated-deficit irrigation (∼50% of the control). We found that measurements done between 11:00 and 17:00h provide the most significant correlations between TC, CWSI and IG and leaf stomatal conductance and net photosynthesis for both genotypes. Different linear mathematical functions were obtained to estimate leaf gas exchange based on the best performing thermal indicators under field conditions. Our results emphasize the value of TC as a relevant explanatory variable of vine’s physiological status, in spite of being a simpler and non-normalized thermal indicator. The potential relevance of TC for grapevine modelling and phenotyping is also discussed.

Sensitivity of Two Quinoa (ChenopodiumquinoaWilld.) Varieties to Progressive Drought Stress

AbstractQuinoa (ChenopodiumquinoaWilld.) is a highly nutritious Andean seed crop which shows great potential to grow under a range of hostile environments. The objective of this study was to investigate the differences of drought tolerance of a Bolivian (Achachino) and a Danish (Titicaca) variety, and especially drought‐related adaption strategies. Soil water status was expressed as the fraction of transpirable soil water (FTSW). Relative stomatal conductance (RSC), relative transpiration (RT) and relative leaf water potential (RLW) were calculated by determining stomatal conductance, transpiration rate and leaf water potential of the drought‐treated plants relative to those of fully irrigated plants. The responses of RSC, RT and RLW to decreasing FTSW were described by a linear‐plateau model. The critical value of FTSW was the threshold of FTSW where the parameters studied decreased. The thresholds increased CS for stomatal conductance, CT for transpiration and CLfor leaf water potential. Achachino showed significantly lower CT and CL when compared with Titicaca, implying that transpiration and leaf water potential were less affected under mild drought conditions in the Bolivian variety. CS in Achachino was significantly higher than CL and CT, which indicated that stomatal conductance declined before transpiration and leaf water potential were reduced. Such difference was found in Titicaca where reduction of leaf area had more effect on transpiration than stomatal closure. Slower growth rate and smaller leaf area in combination with a lower stomatal conductance was found to contribute to drought resistance in Achachino. ABA concentration in the xylem sap tended to increase in both varieties after 2 days onset of drought, prior to decline in leaf water potential. Titicaca showed significantly (P &lt; 0.05) higher ABA concentration when compared with Achachino under both fully irrigated and drought conditions. Titicaca had higher xylem nutrient concentration in comparison with Achachino in both fully‐watered and drought plants at day 2 after onset of soil drying. It was concluded that Titicaca was more sensitive to progressive drought than Achachino which avoided water loss by means of lower growth rate and smaller leaf area.

Isolation and characterization of an arabidopsis drought-resistant mutant vrm1

Drought is the major abiotic stress restricting plant growth. To gain insight into the regulatory mechanisms for drought stress tolerance, we performed a genetic screen for identifying Arabidopsis (Arabidopsis thaliana) drought-resistant mutants. A drought-resistant mutant vrm1 was isolated because of its increased root growth and fresh weight under drought stress. Compared with the wild type (WT), the vrm1 mutant was more resistant to drought stress. The vrm1 plants showed the higher levels of free proline and soluble sugars and the lower relative stomatal conductance as well as the lower water loss rate than the WT under drought stress. Moreover, the higher transcription levels of drought stress-responsive genes NCED3, RD22, RD29A, and COR15A were detected in vrm1 plants subjected to drought and mannitol stresses as compared with WT plants. These results suggest that increased drought resistance of the vrm1 mutant was associated with, at least in part, the higher expression of these drought stress-responsive genes.

Isolation and Characterization of an Arabidopsis Drought-Resistant Mutantvrm1

Drought is the major abiotic stress restricting plant growth. To gain insight into the regulatory mechanisms for drought stress tolerance, we performed a genetic screen for identifying Arabidopsis (Arabi� dopsis thaliana) droughtresistant mutants. A droughtresistant mutant vrm1 was isolated because of its increased root growth and fresh weight under drought stress. Compared with the wild type (WT), the vrm1 mutant was more resistant to drought stress. The vrm1 plants showed the higher levels of free proline and sol� uble sugars and the lower relative stomatal conductance as well as the lower water loss rate than the WT under drought stress. Moreover, the higher transcription levels of drought stressresponsive genes NCED3, RD22, RD29A, and COR15A were detected in vrm1 plants subjected to drought and mannitol stresses as compared with WT plants. These results suggest that increased drought resistance of the vrm1 mutant was associated with, at least in part, the higher expression of these drought stressresponsive genes.

Drought Responses of Two Bambara Groundnut (Vigna subterranea L. Verdc.) Landraces Collected from a Dry and a Humid Area of Africa

AbstractDrought adaptation strategies of two bambara groundnut landraces, Uniswa Red and S19‐3, collected from contrasting environments in Africa, were compared. Our objectives were to investigate the relative significance of effective stomatal control induced by the abscisic acid (ABA) signalling and osmotic adjustment in regulating plant water relations in general for this legume species. The ABA concentration [ABA] in the leaf increased linearly with declining relative leaf water content, and there were significantly higher [ABA] in Uniswa Red compared with S19‐3 at the final harvest in the drought‐stressed plants. Estimated by a linear‐plateau model, S19‐3 initiated the reduction in transpiration at a significantly lower soil water threshold (FTSW = 0.50 ± 0.024) than Uniswa Red (FTSW = 0.69 ± 0.023) indicating that the latter was more sensitive in reducing plant water use in response to soil drying. A similar trend was found for stomatal closure during soil drying, although the soil water thresholds at which relative stomatal conductance (gs) started to decline were not significantly different between the two landraces. By an early closure of stomata and hence an early reduction in transpiration rate during soil drying, Uniswa Red could be defined as a ‘water‐saver’ such that it maintains leaf water status to a great extent of soil water deficit. This strategy is important for survival during intermittent drought. While S19‐3 could be defined as a ‘water‐spender’ with a late closure of stomata, hence a late declining of transpiration rate during soil drying allowed the landrace to maximize its water use despite giving up its leaf water relations. Such drought response together with a fast phenological development of S19‐3 indicates that the landrace is capable of escaping from terminal drought while maximizing its water use and productivity when soil water is available.

Hydraulic and chemical signals in the control of leaf expansion and stomatal conductance in soybean exposed to drought stress.

Both hydraulic and chemical signals are probably important in regulating leaf growth and stomatal conductance of soybean (Glycine max L. Merr.) under drought stress. However, until now they have not been investigated concomitantly in this species. To explore this, a pot experiment in a temperature-regulated greenhouse was conducted, in which plants were subjected to progressive drought during early reproductive stages. Biophysical parameters, viz. relative leaf expansion rate, stomatal conductance, leaf turgor, leaf [ABA], xylem pH and xylem [ABA] were followed in control and stressed plants. Drought stress decreased relative leaf expansion rate, stomatal conductance and leaf turgor, whereas it increased leaf [ABA], xylem pH and xylem [ABA]. As soil dried, significant differences between water treatments for relative leaf expansion rate, stomatal conductance, leaf turgor, leaf [ABA], xylem pH and xylem [ABA] were observed at 14, 9, 14, 14, 14 and 9 d after imposition of stress, respectively. The relationships of relative values for relative leaf expansion rate, stomatal conductance, leaf turgor, leaf [ABA] and xylem pH to the fraction of transpirable soil water (FTSW) were well described by linear-plateau functions that allowed calculation of the soil-water thresholds at which processes in stressed plants began to diverge from well-watered controls. The soil-water threshold for stomatal conductance (0.64) was significantly higher than that for relative leaf expansion rate (0.29), xylem pH (0.28), leaf [ABA] (0.27) and leaf turgor (0.25). Relative xylem [ABA] increased, first linearly (when FTSW > 0.5) and then exponentially (when FTSW < 0.5) with decreasing FTSW. Relative stomatal conductance decreased exponentially with increasing relative xylem [ABA] (r2=0.98). Decreased stomatal conductance coincided with an increase in xylem [ABA] and occurred before any significant change of leaf turgor could be detected, indicating that chemical signals (seemingly root-originated ABA) control stomatal behaviour at moderate soil water deficits. Relative relative leaf expansion rate was linearly correlated with relative leaf turgor (r2=0.93), relative xylem pH (r2=0.97) and relative leaf [ABA] (r2=0.98), implying that both hydraulic and chemical signals were probably involved in regulation of leaf expansion at severe soil water deficits.

Photosynthetic Induction in Seedlings of Six Tropical Rainforest Tree Species

In order to investigate the differences in capacity of sunfleck utilization between species with different ecological characteristics, the photosynthetic characteristics and photosynthetic induction responses were examined with LI-6400 portable photosynthesis system in seedlings of six tropical rainforest species acclimated in simulated understory light environment for more than one and a half years. The species studied were Mallotus macrostachys , a pioneer species, Pometia tomentosa , Shorea chinensis , Barringtonia pendala , Linociera in-signis , Lasianthus hookeri, four canopy species and an extremely shade-tolerant understory shrub. After leaves were shaded in darkness for 3 h, photosynthetic induction was measured under optimum temperature ((26±1.2) ℃) and humidity (75% ±℃5%) and normal ambient CO2((400±0.5) μmol CO2·mol-1 air) in the morning. The time courses of induction varied from a sigmoidal to a hyperbolic increase in net photosynthesis . The time required to reach 90% of maximum net photosynthetic rate differed among species, and was short (4.4 - 12.5 min) , which was consistent with induction times reported in other tropical shade-tolerant species. Seedlings of M. macrostachys and L. hookeri were induced two to three times more quickly than the other four canopy species. The response of attaining maximal stomatal conductance significantly lagged behind the increase in net photosynthetic rate during the induction course. When fully induced leaves were shaded in darkness for 20 min, loss of induction was moderate in all species. Among these species, seedlings of M. macrostachys had a relatively rapid induction loss, which was due to rapid loss of stomatal conductance and biochemical factors. The loss of B. pendala was mainly due to biochemical limitation as stomatal conductance decreased only slowly. Seedlings of L. hookeri showed the slowest loss of induction as a result of maintaining higher relative stomatal conductance and maximum carboxylation capacity. Thus this effective utilization of sun-flecks is likely to be a critical determinant of growth and generation for understory species.

Leaf Expansion, Stomatal Conductance, and Transpiration of Vegetable Amaranth (Amaranthus sp.) in Response to Soil Drying

This study was designed to quantify the responses of leaf expansion, stomatal conductance, and transpiration of four genotypes of vegetable amaranth [Amaranthus tricolor L. (Hin Choi), A. tricolor L. (Co. 2), A. blitum L. (WS80-192), and A. cruentus L. (RRC 1027)] to soil drying. Two greenhouse experiments were conducted during 1999 and 2000. Soil water status was expressed as the fraction of transpirable soil water (FTSW). Leaf expansion rates, stomatal conductances, and transpiration rates of the stressed plants were determined relative to those of nonstressed plants, and expressed as relative leaf expansion (RLE), relative stomatal conductance (RSC), and relative transpiration (RT), respectively. The rate of soil water extraction differed among genotypes, with RRC 1027 depleting soil water fastest and Hin Choi slowest. Whereas in 1999 all genotypes were equally efficient in soil water use, RRC 1027 extracted a greater volume of transpirable soil water than the other genotypes in 2000. The responses of RLE, RSC, and RT to FTSW were well described by linear-plateau models which allowed calculation of soil-water thresholds for leaf expansion (CL), stomatal conductance (CS), and transpiration (CT). Values for CL were higher than for CS and CT. CL was similar for the four genotypes in each year, whereas, CS and CT differed among genotypes. CS and CT was lowest for Hin Choi and highest for WS80-192. Differences of CL, CS, and CT between the two experiments might have been due to the different soils used in the experiments and the different evaporative demands during the drought cycles. Under drought stress, the reduction of transpiration of vegetable amaranth was due mainly to reduction of stomatal conductance, not to reduction of leaf expansion. The relative reduction of dry weight caused by drought stress was positively correlated with CS or CT across the four genotypes. Variation in CS and CT among amaranth genotypes revealed different responses to drought stress, which could make them suitable for different drought situations.

Correlation between Ozone Resistance and Relative Chlorophyll Fluorescence or Relative Stomatal Conductance of Bedding Plants

Eight species of bedding plants were exposed to 400 ppb ozone (O3) for 4 h. On the basis of the resulting foliar injury, Madagascar periwinkle (Catharanthus roseus) and impatiens (Impatiens walleriana) were the most resistant to O3, and wax begonia (Begonia xsemperflorens-cultorum) was the most sensitive to O3. Clorophyll fluorescence (Fv/Fm) and stomatal conductance were measured before and after the O3 fumigation. There was a significant regression between, the degree of foliage injury by O3 and relative chlorophyll fluorescence (RCF, ratio of post-fumigation Fv/Fm: pre-fumigation Fv/Fm) or relative stomatal conductance (RSC, ratio of post-fumigation stomatal conductance: pre-fumigation stomatal conductance) (r=-0.84, P＜0.001 and r=0.64, P＜0.05 respectively). That is, species of tested bedding plants that had stronger O3 resistance generally had higher RCF and lower RSC values. It is suggested that RCF and RSC measurements could serve as indicators to screen plants for O3 resistance.

Estimation of maize evapotranspiration under water deficits in a semiarid region

A field study was conducted to investigate the response of leaf water potentials ( ψ l) and stomatal conductance ( C s) of maize crop to soil water availability, and to test and compare the soil water adjustment coefficient ( K s) functions for estimation of actual evapotranspiration (ET) under water deficits. The results showed that correlation coefficients of K s to C s and ψ l peaked at 09:30 hours, and then decreased, indicating that ψ l and C s at 09:30 hours were better predictors of plant water status. The correlations of K s to relative leaf water potential ( ψ l/ ψ lm) and relative leaf stomatal conductance ( C s/ C sm) were better than that of K s to ψ l and C s directly. K swas also significantly related to soil water availability ( A w). Correlation with K s was reduced in the following order: C s/ C sm > A w > ψ l/ ψ lm. The procedure was used that reference crop evapotranspiration (ET 0) was estimated by the modified Penman formula and with a crop coefficient ( K c) and different K s functions. The results showed that it was the best estimation with K s function based on the relative stomatal conductance, and at least in the case of maize that the soil water adjustment coefficient K s based on relative stomatal conductance C s/ C sm provided a means of predicting required adjustments in ET estimation for different soil water status.

Plant water relations ofTamarix ramosissimain response to the imposition and alleviation of soil moisture stress

The effect of an extended drydown on the water relations ofTamarix ramosissima(saltcedar), a desert phreatophyte, was investigated along the floodplain of the Virgin River (southern Nevada) during a hot dry summer period. Seedlings ofTamarixwere grown in lysimeters (120 cm depth, 51 cm radius) positioned along a gradient from the desert's edge to the interior of the stand and monitored for growth and evapo-transpiration over a 2-year period prior to this study. Water tables were maintained at approximately 60 cm during the evapo-transpiration study by applying weekly irrigation directly to the water table via piezometers. At the end of the 2-year period, three lysimeters were selected for a drydown experiment (desert's edge, river's edge, open stand). On 7 July 1995 all irrigation was terminated for a 29-day period. After the drydown period was over, irrigation was applied for an 18-day period to re-establish water table depths. Sapflow (transpiration) was monitored during the entire 47-day period (drydown, wetup) with stem flow gauges, soil moisture with time domain reflectometry, and plant water relations with a steady state porometer and pressure chamber. Results indicated that sapflow decreased significantly as water tables and stored soil water declined. Daily sapflow totals on a leaf area basis were higher for the plant growing along the river's edge, with midday hourly values significantly higher when a water table was present. On day 26, sapflow reached zero in the open stand lysimeter. As sapflow approached zero, a significant decrease in the time in which daily sapflow stopped occurred, rather than a delayed sapflow start time. Upon application of water on day 29, all three plants growing in the lysimeters responded by increasing sapflow within a 24-h period. Relative stomatal conductance (p= 0·001), leaf xylem water potential (p= 0·001) and sapflow (p= 0·001) measured during both the drydown and wetup stages were all linearly correlated with relative soil water in storage, indicating that relative soil water in storage could decline by as much as 90% and that upon alleviation of the soil moisture stress no hysteresis would be observed within these parameters. Such results indicate thatTamarixcan be subjected to significant soil water deficits and still respond rapidly to the presence of water. We conclude that significantly longer drydown periods without plant contact with a water table would be required to cause catastrophic cavitation inTamarix.