Hydraulic conductance and water potential gradients in squash leaves showing mycorrhiza-induced increases in stomatal conductance

Stomatal conductance (g _s) and transpiration rates vary widely across plant species. Leaf hydraulic conductance (k _leaf) tends to change with g _s, to maintain hydraulic homeostasis and prevent wide and potentially harmful fluctuations in transpiration-induced water potential gradients across the leaf (ΔΨ _leaf). Because arbuscular mycorrhizal (AM) symbiosis often increases g _s in the plant host, we tested whether the symbiosis affects leaf hydraulic homeostasis. Specifically, we tested whether k _leaf changes with g _s to maintain ΔΨ _leaf or whether ΔΨ _leaf differs when g _s differs in AM and non-AM plants. Colonization of squash plants with Glomus intraradices resulted in increased g _s relative to non-AM controls, by an average of 27% under amply watered, unstressed conditions. Stomatal conductance was similar in AM and non-AM plants with exposure to NaCl stress. Across all AM and NaCl treatments, k _leaf did change in synchrony with g _s (positive correlation of g _s and k _leaf), corroborating leaf tendency toward hydraulic homeostasis under varying rates of transpirational water loss. However, k _leaf did not increase in AM plants to compensate for the higher g _s of unstressed AM plants relative to non-AM plants. Consequently, ΔΨ _leaf did tend to be higher in AM leaves. A trend toward slightly higher ΔΨ _leaf has been observed recently in more highly evolved plant taxa having higher productivity. Higher ΔΨ _leaf in leaves of mycorrhizal plants would therefore be consistent with the higher rates of gas exchange that often accompany mycorrhizal symbiosis and that are presumed to be necessary to supply the carbon needs of the fungal symbiont.     

Different photosynthetic responses of wild and cultivated plants to high irradiance

In addition to other factors, high altitude (HA) environment is characterized by high photosynthetic photon flux density (PPFD). Photosynthetic characteristics of wild and cultivated plants were studied at different irradiances at Losar, India (altitude 4 200 m). Wild plants were tolerant to high PPFDs. Slopes of curve between net photosynthetic rate (P _N) and intercellular CO₂ concentration (C _i) or stomatal conductance (g _s) increased with increase in irradiance suggesting insensitivity or tolerance of these plants to higher PPFD. Cultivated plants, however, were sensitive to higher PPFD, their slopes of curves between P _N and C _i or g _s decreased with increased PPFD. Tolerance or insensitivity to higher PPFD was an important parameter affecting plant performance at HA.     

Aquaporin expression in response to different water stress intensities and recovery in Richter-110 (<Emphasis Type="Italic">Vitis</Emphasis> sp.): relationship with ecophysiological status

Aquaporins seem essential for the regulation of plant water status and expenses. Richter-110 is a Vitis hybrid (Vitis berlandieri × rupestris) reputed to be strongly drought-tolerant. Three irrigation treatments were established in Richter-110 plants growing outdoors defined by the resulting maximum stomatal conductance (g _s), and ensuring water stress situations not severe enough as to stop photosynthesis and growth: well-watered plants (g _s about 250 mmol H₂O m⁻² s⁻¹), moderate water stress (g _s about 150 mmol H₂O m⁻² s⁻¹) and severe water stress (g _s about 50 mmol H₂O m⁻² s⁻¹). Plants under water stress were kept at constant water availability for 7 days to check for possible acclimation. Finally, plants were re-watered, and allowed to recover, for 3 days. Stomatal conductance, leaf water potential, xylem abscisic acid (ABA) content and root and stem hydraulic conductivity were determined. The relative amounts of expression of mRNA encoding seven putative aquaporins were determined in roots and leaves by RT-PCR. The decrease in stomatal conductance with moderate and severe water stress was associated with increasing ABA contents, but not with the leaf water potential and hydraulic conductivities, which remained unchanged during the entire experiment. Aquaporin gene expression varied depending on which aquaporin, water stress level and the plant organ. We suggest that aquaporin expression was responsive to water stress as part of the homeostasis, which resulted in constant leaf water potential and hydraulic conductivity.     

Drought acclimation of two deciduous tree species of different layers in a temperate forest canopy

Water-use strategies of Populus tremula and Tilia cordata, and the role of abscisic acid in these strategies, were analysed. P. tremula dominated in the overstorey and T. cordata in the lower layer of the tree canopy of the temperate deciduous forest canopy. Shoot water potential (), bulk-leaf abscisic acid concentration ([ABA]_leaf), abscisic acid concentration in xylem sap ([ABA]_xyl), and rate of stomatal closure following the supply of exogenous ABA (v) decreased acropetally through the whole tree canopy, and foliar water content per area (w), concentration of the leaf osmoticum (c), maximum leaf-specific hydraulic conductance of shoot (L), stomatal conductance (g_s), and the threshold dose per leaf area of the exogenous ABA (d_a) required to reduce stomatal conductance increased acropetally through the tree canopy (from the base of the foliage of T. cordata to the top of the foliage of P. tremula) in non-stressed trees. The threshold dose per leaf dry mass of the exogenous ABA (d_w) required to reduce stomatal conductance, was similar through the tree canopy. After a drought period (3 weeks), the , w, L, g_s, d_a and d_w had decreased, and c and v had increased in both species. Yet, the effect of the drought period was more pronounced on L, g_s, d_a, d_w and v in T. cordata, and on , w and c in P. tremula. It was concluded that the water use of the species of the lower canopy layer—T. cordata, is more conservative than that of the species of the overstorey, P. tremula. [ABA]_leaf had not been significantly changed in these trees, and [ABA]_xyl had increased during the drought period only in P. tremula. The relations between [ABA]_leaf, [ABA]_xyl and the stomatal conductance, the osmotic adjustment and the shoot hydraulic conductance are also discussed.     

Leaf water and carbohydrate status of VA mycorrhizal rose exposed to drought stress

Summary Shoot water relations and carbohydrate levels were compared for droughted nonmycorrhizal and vesicular-arbuscular (VA) mycorrhizalRosa hybrida L. cv ‘Samantha’ plants grown with high and low phosphorus fertilization. Leaf diffusive conductance (g _i ) of plants colonized byGlomus intraradices Schenk and Smith andGlomus deserticola Trappe, Bloss and Menge were 2 × and 1.5× greater, respectively, than in nonmycorrhizal plants. Regardless of P fertilization, leaf osmotic and bulk water potentials were 0.5 to 1.1 MPa higher in mycorrhizal than in nonmycorrhizal plants. Leaf starch, chlorophyll and water contents while fructose, glucose and total soluble carbohydrates were lower. Level of P fertilization had no effect on water relations or soluble carbohydrate content of nonmycorrhizal roses. The water status of droughted rose was impoved more byG. intraradices than byG. deserticola. Washington State University College of Agriculture and Home Economics Research Center Scientific Paper No. 7375.     

Effect of irradiance during growth of Glycine max on photosynthetic capacity and percent activation of ribulose 1,5-bisphosphate carboxylase

The initial (in vivo) and total (activity present after preincubation with CO₂ and Mg²⁺) activities of ribulose bisphosphate carboxylase were both assayed in extracts of leaves of soybean (Glycine max) plants which had been grown under 4 different irradiance levels. The total carboxylase activity per unit leaf area decreased with decreased irradiance during growth but was not different on a dry weight basis. The initial activity as a percentage of the total activity was unchanged (approximately 95%) except in leaves of plants grown at the lowest irradiance (74%). When the plants grown at the lowest irradiance were exposed to high irradiance, the initial activity was increased to 93% of the total. Light saturated rates of photosynthesis per unit leaf area were lower and saturated at lower irradiance for plants grown at lower irradiances. Initial carboxylase activity was correlated closely (r²=0.84) with leaf photosynthesis rate on a dry weight basis.     

Abscisic acid and indole-3-acetic acid contents in orange trees infected by Xylella fastidiosa and submitted to cycles of water stress

Pêra sweet orange plants (Citrus sinensis L. Osbeck) grafted on Rangpur lime rootstock (1 year-old) (Citrus limonia Osbeck) were inoculated with Xylella fastidiosa, a xylem-limited bacterium pathogen, which causes Citrus Variegated Chlorosis (CVC). Since it was known that water deficiency in the field enhances CVC-effects on the plant, the trees were submitted to three cycles of water stress during a one year period (March and October, 1998; and April, 1999) and divided in four treatments: healthy plants (HP); water-stressed healthy plants (WSHP); diseased plants (DP) and water-stressed diseased plants (WSDP). Stomatal conductance (g _s) of water-stressed diseased plants decreased in the first and second cycles of water deficiency, as the stress was increasing. The low stomatal conductance verified may be due to the high concentrations of abscisic acid (ABA) found in these plants. In the third cycle, values of g _s in diseased plants were, usually, lower than in the healthy ones. In healthy plants, g _s was reduced when these plants were submitted to water deficiency, independently of the cycle. The drop in leaf water potential in healthy plants was faster after irrigation was withheld, because healthy plants transpired more and, therefore, the water content of the substrate decreased more quickly. When the irrigation of WSDP was withheld in the third cycle, it was not possible to detect increases in ABA contents, suggesting that other factors could be acting to diminish the stomatal conductance in these plants. The presence of Xylella fastidiosa did not induce an increase in indole-3-acetic acid content in the leaves. After three cycles of water deficiency, the concentrations of indole-3-acetic acid in WSHP and WSDP were lower than those concentrations in the irrigated controls on the day water stress was more severe.     

A coupled model of stomatal conductance and photosynthesis for winter wheat

The model couples stomatal conductance (g _s) and net photosynthetic rate (P _N) describing not only part of the curve up to and including saturation irradiance (I _max), but also the range above the saturation irradiance. Maximum stomatal conductance (g _smax) and I _max can be calculated by the coupled model. For winter wheat (Triticum aestivum) the fitted results showed that maximum P _N (P _max) at 600 μmol mol⁻¹ was more than at 350 μmol mol⁻¹ under the same leaf temperature, which can not be explained by the stomatal closure at high CO₂ concentration because g _smax at 600 μmol mol⁻¹ was less than at 350 μmol mol⁻¹. The irradiance-response curves for winter wheat had similar tendency, e.g. at 25 °C and 350 μmol mol⁻¹ both P _N and g _s almost synchronously reached the maximum values at about 1 600 μmol m⁻² s⁻¹. At 25 °C and 600 μmol mol⁻¹ the I _max corresponding to P _max and g _smax was 2 080 and 1 575 μmol m⁻² s⁻¹, respectively.     

Water relations and stomatal characteristics of Mediterranean plants with different growth forms and leaf habits: responses to water stress and recovery

The aim of this study was to extent the range of knowledge about water relations and stomatal responses to water stress to ten Mediterranean plants with different growth forms and leaf habits. Plants were subjected to different levels of water stress and a treatment of recovery. Stomatal attributes (stomatal density, StoD), stomatal conductance (g _s), stomatal responsiveness to water stress (SR), leaf water relations (pre-dawn and midday leaf water potential and relative water content), soil to leaf apparent hydraulic conductance (K _L) and bulk modulus of elasticity (ε) were determined. The observed wide range of water relations and stomatal characteristics was found to be partially depended on the growth form. Maximum g _s was related to StoD and the stomatal area index (SAI), while g _s evolution after water stress and recovery was highly correlated with K _L. Relationships between SR to water deficit and other morphological leaf traits, such as StoD, LMA or ε, provided no general correlations when including all species. It is concluded that a high variability is present among Mediterranean plants reflecting a continuum of leaf water relations and stomatal behaviour in response to water stress.     

Control of transpiration in three coffee cultivars: the role of hydraulic and crown architecture

Water use and hydraulic architecture were studied in the coffee (Coffea arabica) cultivars San Ramon, Yellow Caturra and Typica growing in the field under similar environmental conditions. The cultivars differed in growth habit, crown architecture, basal sapwood area and total leaf surface area. Transpiration per unit leaf area (E), stomatal conductance (g _s), crown conductance (g _c), total hydraulic conductance of the soil/leaf pathway (G _t) and the stomatal decoupling coefficient, omega (Ω) (Jarvis and McNaughton 1986) were assessed over a range of soil moisture and during partial defoliation treatments. The relationship between sap flow and sapwood area was linear and appeared to be similar for the three cultivars. Variation in g _c, E, and G _t of intact plants and leaf area-specific hydraulic conductivity (k _l) of excised lateral branches was negatively correlated with variation in the ratio of leaf area to sapwood area. Transpiration, g _c, and g _s were positively correlated with G _t. Transpiration and G _t varied with total leaf area and were greatest at intermediate values (10 m²) of leaf area. Omega was greatest in Yellow Caturra, the cultivar with the greatest leaf area and a dense crown, and was smallest in Typica, the cultivar with an open crown. Differences in omega were attributable primarily to differences in leaf boundary layer conductance among the cultivars. Plants of each cultivar that were 40% defoliated maintained sap flows comparable to pretreatment plants, but expected compensatory increases in g _s were not consistently observed. Despite their contrasting crown morphologies and hydraulic architecture, the three cultivars shared common relationships between water use and hydraulic architectural traits. Received: 17 February 1999 / Accepted: 28 July 1999     

Effects of elevated CO2 on leaf gas exchange in beech and oak at two levels of nutrient supply: consequences for sensitivity to drought in beech

Beech (Fagus sylvatica L.) and pedunculate oak (Quercus robur L.) were grown from seed for two whole seasons at two CO₂ concentrations (ambient and ambient + 250 μmol mol^?1) with two levels of soil nutrient supply. Measurements of net leaf photosynthetic rate (A) and stomatal conductance (g_s) of well-watered plants were taken over both seasons; a drought treatment was applied in the middle of the second growing season to a separate sample of beech drawn from the same population. The net leaf photosynthetic rate of well-watered plants was stimulated in elevated CO₂ by an average of 75% in beech and 33% in oak; the effect continued through both growing seasons at both nutrient levels. There were no interactive effects of CO₂ concentration and nutrient level on A or g_s in beech or oak. Stomatal conductance was reduced in elevated CO₂ by an average of 34% in oak, but in beech there were no significant reductions in g_s except under cloudy conditions (–22% in elevated CO₂). During drought, there was no effect of CO₂ concentration on g_s in beech grown with high nutrients, but for beech grown with low nutrients, g_s was significantly higher in elevated CO₂, causing more rapid soil drying. With high nutrient supply, soil drying was more rapid at elevated CO₂ due to increased leaf area. It appears that beech may substantially increase whole-plant water consumption in elevated CO₂, especially under conditions of high temperature and irradiance when damage due to high evaporative demand is most likely to occur, thereby putting itself at risk during periods of drought.     

Leaf characteristics and gas exchange of the mangrove <Emphasis Type="Italic">Laguncularia racemosa</Emphasis> as affected by salinity

Under constant salinity we analysed the leaf characteristics of Laguncularia racemosa (L.) Gaertn. in combination with gas exchange and carbon isotopic composition to estimate leaf water-use efficiency (WUE) and potential nitrogen-use efficiency (NUE). NaCl was not added to the control plants and the others were maintained at salinities of 15 and 30 ‰ (S₀, S₁₅, and S₃₀, respectively). Leaf succulence, sodium (Na), nitrogen (N), and chlorophyll (Chl) contents increased under salinity. Salinity had a negative impact on net photosynthetic rate (P _N) and stomatal conductance (g _s) at high and moderated irradiances. Potential NUE declined significantly (p<0.05) with salinity by 37 and 58 % at S₁₅ and S₃₀, respectively, compared to S₀ plants. Conversely, compared to S₀ plants, P _N/g _s increased under saline conditions by 12 % (S₁₅) and 50 % (S₃₀). Thus, WUE inferred from P _N/g _s was consistent with salinity improved short-term WUE. Long-term leaf WUE was also enhanced by salinity as suggested by significantly increased leaf δ¹³C with salinity. Improved WUE under salinity explains the eco-physiological success of mangrove species under increasing salinity. Conversely, decline in NUE may pose a problem for L. racemosa under hyper-saline environments regardless of N availability.     

Excess irradiance causes early symptoms of senescence during leaf expansion in photoautotrophically <Emphasis Type="Italic">in vitro</Emphasis> grown tobacco plants

Photosynthetic parameters, growth, and pigment contents were determined during expansion of the fourth leaf of in vitro photoautotrophically cultured Nicotiana tabacum L. plants at three irradiances [photosynthetically active radiation (400–700 nm): low, LI 60 μmol m⁻² s⁻¹; middle, MI 180 μmol m⁻² s⁻¹; and high, HI 270 μmol m⁻² s⁻¹]. During leaf expansion, several symptoms usually accompanying leaf senescence appeared very early in HI and then in MI plants. Symptoms of senescence in developing leaves were: decreasing chlorophyll (Chl) a+b content and Chl a/b ratio, decreasing both maximum (F_V/F_M) and actual (Φ_PS2) photochemical efficiency of photosystem 2, and increasing non-photochemical quenching. Nevertheless, net photosynthetic oxygen evolution rate (P _N) did not decrease consistently with decrease in Chl content, but exhibited a typical ontogenetic course with gradual increase. P _N reached its maximum before full leaf expansion and then tended to decline. Thus excess irradiance during in vitro cultivation did not cause early start of leaf senescence, but impaired photosynthetic performance and Chl content in leaves and changed their typical ontogenetic course.     

Differential Photosynthetic Acclimation Pattern to Limiting Growth-Irradiance in Two Types of C4 Plants

Photosynthetic acclimation to reduced growth irradiances (650 and 200 µmol m^–2 s^–1) in Eleusine coracana (L.) Garten, a nicotinamide adenine dinucleotide-malic enzyme (NAD-ME) C₄ species and Gomphrena globosa L., a nicotinamide adenine dinucleotide phosphate-malic enzyme (NADP-ME) C₄ species were investigated. E. coracana plants acclimated in 4 and 8 d to 650 and 200 µmol m^–2 s^–1, respectively, whereas G. globosa plants took 8 and 10 d, respectively, to acclimate to the same irradiances. The acclimation to reduced irradiance was achieved in both species by greater partitioning of chlorophyll towards the light-harvesting antennae at the expense of functional components. However, magnitude of increase in the light-harvesting antenna was higher in E. coracana as compared to G. globosa. Superior photosynthetic acclimation to reduced irradiance in G. globosa was due to the smaller change in functions of the cytochrome b ₆/f complex, photosystem (PS) 1 and PS2 leading to the higher carbon fixation rates compared to E. coracana.     

Changes in stomatal frequency, stomatal conductance and cuticle thickness during leaf expansion in the broad-leaved evergreen species, Eucalyptus regnans

Aspects of leaf anatomical and physiological development were investigated in the broad-leaved evergreen species, Eucalyptus regnans F.Muell. Newly emergent leaves were tagged in the field and measured for stomatal conductance while a subset was collected every 14 days for the measurement of stomata and cuticle over a 113-day period. Cuticle thickness increased during leaf expansion, the increase following a sigmoid curve. Stomatal frequency (no. mm⁻²) decreased from 56 to 113 days after leaf emergence. The frequency of both immature and intermediate developmental stages of stomata also decreased over this time, but the total number of stomata per leaf remained relatively constant. Stomatal conductance (g _s) of young expanding leaves increased during expansion, and was significantly linearly correlated with stomatal frequency (excluding immature stomata), and with cuticle thickness. The progressive increase in g _s in young developing leaves was contrary to the observed changes in structural characteristics (increased cuticle thickness and decreased stomatal frequency). This increase in g _s with development may be related to the progressive increase in number of mature stomata with larger apertures and, therefore, a higher total pore area in fully expanded leaves.     

Responses of Ribulose-1,5-Bisphosphate Carboxylase,Protein Content,and Stomatal Conductance to Water Deficit in Maize,Tomato, and Bean

We compared responses of maize, tomato, and bean plants to water stress. Maize reached a severe water deficit (leaf water potential –1.90 MPa) in a longer period of time as compared with tomato and bean plants. Maize stomatal conductance (g _s) decreased at mild water deficit. g _s of tomato and bean decreased gradually and did not reach values as low as in maize. The protein content was maintained in maize and decreased at low water potential (_w); in tomato it fluctuated and also decreased at low _w; in bean it gradually decreased. Ribulose-1,5-bisphosphate carboxylase/oxygenase activity remained high at mild and moderate stress in maize and tomato plants; in bean it remained high only at mild stress.     

Limitation of coffee leaf photosynthesis by stomatal conductance and light availability under different shade levels

In agroforestry systems, the effect of shade trees on coffee net photosynthesis (A _n) has been the object of debates among coffee scientists. In this study, we undertook over 600 coffee A _n “spot” measurements under four different artificial shade levels (100, 72, 45 and 19% of full solar irradiance) and analyzed limitations to A _n by low light availability (photon flux density, PFD) and stomatal conductance (g _s). These gas exchange measurements were carried out during two consecutive coffee growing seasons in a commercial plantation in the Orosi valley of Costa Rica. Levels of A _n were related to PFD and g _s in order to calculate envelope functions which were used to establish PFD or g _s limitations to A _n. Under the growing conditions of the present trial, mean leaf A _n remained stable for growth irradiance (GI) as low as 45% of full sun and decreased by ~20% at 19% GI. Limitation to A _n due to g _s was strong in full sun and decreasing with increasing shade levels. On the other hand, limitation due to PFD remained at a similar level for all shade treatments. These different evolutions of limitations of A _n by PFD and g _s in response to shade explain the absence of a decrease in coffee leaf A _n with a shade level up to 55%. Consequently, these results confirm that Arabica coffee is a shade-adapted plant with leaves that can maintain a high photosynthetic performance under low light availability.     

Gas exchange response of barley and pea cultivars to altitude variation in Himalaya

Leaf stomatal density (SD), net photosynthetic rates (P _N), and stomatal conductance (g _s) of Hordeum vulgare and Pisum sativum cultivars in Himalaya increased with altitude. Higher P _N and leaf temperature under low CO₂ partial pressure at high altitudes could evoke a higher g _s and SD to allow sufficient influx of CO₂ as well as more efficient leaf cooling through transpiration.     

Net CO2 assimilation of taro and cocoyam as affected by shading and leaf age

Taro and cocoyam were grown outdoors in either full sun or under 40% shade. Leaves were tagged as they emerged and the effect of leaf age on net CO₂ assimilation rate (A) was determined. The effects of shading on A, transpiration (E), stomatal conductance for CO₂ (g_c) and H₂O (g_s), and water use efficiency (WUE) were also determined for leaves of a single age for each species. The effect of leaf age on A was similar for both species. Net CO₂ assimilation rates increased as leaf age increased up to 28 days with the exception of a sharp decline in A for 21 day-old leaves which corresponded to unusually low temperatures during the period of leaf expansion. A generally decreased as leaves aged beyond 28 days. Cocoyam had higher A rates than taro. Leaves of shade-grown plants had higher rates of A and E for both species at photosynthetic photon flux densities (PPFD) up to 1600 mol s^–1 m^–2. Shade-grown leaves of cocoyam had greater leaf dry weights per area (LW/A) and a trend toward higher g_c and g_s than sun-grown leaves. Shade leaves of taro had greater g_c and g₃ rates than sun-grown leaves. The data suggest that taro and cocoyam are highly adapted to moderate shade conditions.     

Effects of Angular Leaf Spot and Rust on Leaf Gas Exchange and Yield of Common Bean (Phaseolus Vulgaris)

Isolated and interactive effects of angular leaf spot (caused by Phaeoisariopsis griseola) and rust (caused by Uromyces appendiculatus) on leaf gas exchange and yield was studied in common bean (Phaseolus vulgaris L. cv. Carioca) plants. Gas exchange was measured on 37, 44, 51, and 58 d after planting using a portable photosynthesis system. The inoculation of plants with P. griseola (P), U. appendiculatus (U), and the combination of both pathogens (P+U) caused a significant reduction of net photosynthetic rate (P _N) and yield. The reduction of stomatal conductance (g _s), P _N, and yield was higher under P and combination of P+U than under U treatment. By effect of U, the reduction on yield was higher than the reductions on gas exchange parameters. On the treatment P+U, a reduction of 23 % in P _N and a correspondent reduction of 32 % in yield was observed. The interactive effects of the pathogens on yield could be explained in part by the decreases in g _s and in P _N of diseased bean leaves. The combined effect of both diseases on yield and gas exchange parameters suggests an antagonistic interaction.