Abscisic Acid Content, Transpiration, and Stomatal Conductance As Related to Leaf Age in Plants of Xanthium strumarium L

Among the four uppermost leaves of greenhouse-grown plants of Xanthium strumarium L. the content of abscisic acid per unit fresh or dry weight was highest in the youngest leaf and decreased gradually with increasing age of the leaves. Expressed per leaf, the second youngest leaf was richest in ABA; the amount of ABA per leaf declined only slightly as the leaves expanded. Transpiration and stomatal conductance were negatively correlated with the ABA concentration in the leaves; the youngest leaf lost the least amount of water. This correlation was always very good if the youngest leaf was compared with the older leaves but not always good among the older leaves. Since stomatal sensitivity to exogenous (±)-ABA was the same in leaves of all four age groups ABA may be in at least two compartments in the leaf, one of which is isolated from the guard cells.     

Abscisic Acid Content and Stomatal Sensitivity to CO(2) in Leaves of Xanthium strumarium L. after Pretreatments in Warm and Cold Growth Chambers

The degree of stomatal sensitivity to CO₂ was positively correlated with the content of abscisic acid of leaves of Xanthium strumarium grown in a greenhouse and then transferred for 24 hours or more to a cold (5/10 C, night/day) or a warm growth chamber (20/23 C). This correlation did not exist in plants kept in the greehouse continuously (high abscisic acid, no CO₂ sensitivity), nor in plants transferred from the cold to the warm chamber (low abscisic acid, high CO₂ sensitivity). The abscisic acid content of leaves was correlated with water content only within narrow limits, if at all. At equal water contents, prechilled leaves contained more abscisic acid than leaves of plants pretreated in the warm chamber. There appear to be at least two compartments for abscisic acid in the leaf.     

Photosynthesis and Ribulose 1,5-Bisphosphate Concentrations in Intact Leaves of Xanthium strumarium L

The interacting effects of the rate of ribulose 1,5-bisphosphate (RuBP) regeneration and the rate of RuBP utilization as influenced by the amount and activation of RuBP carboxylase on photosynthesis and RuBP concentrations were resolved in experiments which examined the kinetics of the response of photosynthesis and RuBP concentrations after step changes from a rate-saturating to a rate-limiting light intensity in Xanthium strumarium. Because RuBP carboxylase requires several minutes to deactivate in vivo, it was possible to observe the effect of reducing the rate of RuBP regeneration on the RuBP concentration at constant enzyme activation state by sampling very soon after reducing the light intensity. Samples taken over longer time periods showed the effect of changes in enzyme activation at constant RuBP regeneration rate on RuBP concentration and photosynthetic rate. Within 15 s of lowering the light intensity from 1500 to 600 microEinsteins per square meter per second the RuBP concentration in the leaves dropped below the enzyme active site concentration, indicating that RuBP regeneration rate was limiting for photosynthesis. After longer intervals of time, the RuBP concentration in the leaf increased as the RuBP carboxylase assumed a new steady state activation level. No change in the rate of photosynthesis was observed during the interval that RuBP concentration increased. It is concluded that the rate of photosynthesis at the lower light intensity was limited by the rate of RuBP regeneration and that parallel changes in the activation of RuBP carboxylase occurred such that concentrations of RuBP at steady state were not altered by changes in light intensity.     

Response of Photosynthesis, Stomatal Conductance and Water Use Efficiency to Elevated CO2 and Nutrient Supply in Acclimated Seedlings of Phaseolus vulgaris L.

Plants of Phaseolus vulgaris L were grown from seed in open-topgrowth chambers at present day (350 µmol mol^–1)and double the present day (700 µmol mol^–1) atmosphericCO₂ concentration with either low (L, without additional nutrientsolution) or relatively high (H, with additional nutrient solution)nutrient supply Measurements of assimilation rate, stomatalconductance and water use efficiency were started 17 d aftersowing on each fully expanded, primary leaf of three plantsper treatment Measurements were made in external CO₂ concentrations(C₂) of 200, 350, 450, 550 and 700 µmol mol^–1 andrelated to both C_a and to C₁, the mean intercellular space CO₂concentration Fully adjusted, steady state measurements weremade after approx 2 h equilibration at each CO₂ concentration The rate of CO₂ assimilation by leaves increased and stomatalconductance decreased similarly over the range of C_a or C₁ inall four CO₂ and nutrient supply treatments but both assimilationrate and stomatal conductance were higher in the high nutrientsupply treatment than in the low nutrient treatment The relationbetween assimilation rate or stomatal conductance and C₁ wasnot significantly different amongst plants grown in present-dayor elevated CO₂ concentration in either nutrient supply treatment,i e there was no evidence of down regulation of photosynthesisor stomatal response Increase in CO₂ concentration from 350to 700 µmol mol^–1 doubled water use efficiency ofindividual leaves in the high nutrient supply treatment andtripled water use efficiency in the low nutrient supply treatment The results support the hypothesis that acclimation phenomenaresult from unbalanced growth that occurs after the seed reservesare exhausted, when the supply of resources becomes growth limiting CO₂ enrichment, Phaseolus vulgaris L., net CO₂ assimilation rate, stomatal conductance, water use efficiency     

Atmospheric CO2 enrichment alters energy assimilation, investment and allocation in Xanthium strumarium

Energy-use efficiency and energy assimilation, investment and allocation patterns are likely to influence plant growth responses to increasing atmospheric CO2 concentration ([CO2]). Here, we describe the influence of elevated [CO2] on energetic properties as a mechanism of growth responses in Xanthium strumarium. Individuals of X. strumarium were grown at ambient or elevated [CO2] and harvested. Total biomass and energetic construction costs (CC) of leaves, stems, roots and fruits and percentage of total biomass and energy allocated to these components were determined. Photosynthetic energy-use efficiency (PEUE) was calculated as the ratio of total energy gained via photosynthetic activity (Atotal) to leaf CC. Elevated [CO2] increased leaf Atotal, but decreased CC per unit mass of leaves and roots. Consequently, X. strumarium individuals produced more leaf and root biomass at elevated [CO2] without increasing total energy investment in these structures (CCtotal). Whole-plant biomass was associated positively with PEUE. Whole-plant construction required 16.1% less energy than modeled whole-plant energy investment had CC not responded to increased [CO2]. As a physiological mechanism affecting growth, altered energetic properties could positively influence productivity of X. strumarium, and potentially other species, at elevated [CO2].     

Control of Photosynthesis and Stomatal Conductance in Ricinus communis L. (Castor Bean) by Leaf to Air Vapor Pressure Deficit

Castor bean (Ricinus communis L.) has a high photosynthetic capacity under high humidity and a pronounced sensitivity of photosynthesis to high water vapor pressure deficit (VPD). The sensitivity of photosynthesis to varying VPD was analyzed by measuring CO₂ assimilation, stomatal conductance (g_s), quantum yield of photosystem II (_II), and nonphotochemical quenching of chlorophyll fluorescence (q_N) under different VPD. Under both medium (1000) and high (1800 micromoles quanta per square meter per second) light intensities, CO₂ assimilation decreased as the VPD between the leaf and the air around the leaf increased. The g_s initially dropped rapidly with increasing VPD and then showed a slower decrease above a VPD of 10 to 20 millibars. Over a temperature range from 20 to 40°C, CO₂ assimilation and g_s were inhibited by high VPD (20 millibars). However, the rate of transpiration increased with increasing temperature at either low or high VPD due to an increase in g_s. The relative inhibition of photosynthesis under photorespiring (atmospheric levels of CO₂ and O₂) versus nonphotorespiring (700 microbars CO₂ and 2% O₂) conditions was greater under high VPD (30 millibars) than under low VPD (3 millibars). Also, with increasing light intensity the relative inhibition of photosynthesis by O₂ increased under high VPD, but decreased under low VPD. The effect of high VPD on photosynthesis under various conditions could not be totally accounted for by the decrease in the intercellular CO₂ in the leaf (C_i) where C_i was estimated from gas exchange measurements. However, estimates of C_i from measurements of _II and q_N suggest that the decrease in photosynthesis and increase in photorespiration under high VPD can be totally accounted for by stomatal closure and a decrease in C_i. The results also suggest that nonuniform closure of stomata may occur in well-watered plants under high VPD, causing overestimates in the calculation of C_i from gas exchange measurements. Under low VPD, 30°C, high light, and saturating CO₂, castor bean (C₃ tropical shrub) has a rate of photosynthesis (61 micromoles CO₂ per square meter per second) that is about 50% higher than that of tobacco (C₃) or maize (C₄) under the same conditions. The chlorophyll content, total soluble protein, and ribulose-1,5-bisphosphate carboxylase/oxygenase level on a leaf area basis were much higher in castor bean than in maize or tobacco, which accounts for its high rates of photosynthesis under low VPD.     

Ethylene Release from Leaves of Xanthium strumarium L. and Zea mays L.

The release of ethylene into sealed Erlenmeyer flasks by intactleaves and leaf discs of Xanthium strumarium L. a C₃ plant andZea mays L. a C₄ plant were compared both in white light andin darkness. The effects of the presence or absence of addedCO₂ (in the form of sodium bicarbonate) the photosynthetic inhibitor3-[3,4-dichlorophenyl]-l, l-dimethyl urea (DCMU) and 1-aminocyclopropane-1-carboxylicacid (ACC), the precursor of ethylene in higher plants, werealso investigated. The rate of ethylene release from leaf tissue of Xanthium inthe absence of added CO₂ was markedly reduced in the light (i.e.at the CO₂ compensation point). Treatments that would enhancethe CO₂ availability to the tissue (i.e. added bicarbonate,darkness, treatment with DCMU) allowed higher levels of ethylenerelease. Incubation of the tissue with ACC considerably enhancedthe release of ethylene compared to that from the correspondingcontrol tissue without ACC. However, the pattern of ethylenerelease induced by the various treatments was similar with orwithout added ACC. When tissue, in the absence of added CO₂, was transferred fromlight to darkness, and back to light for 90 min periods, theethylene release rates Increased during the interposed darkperiod but resumed the lower rate during the final light period.The addition of CO₂ in the light resulted in a similar rateof ethylene release to that found in the dark. The overall pattern of ethylene release from Zea leaf tissuesubjected to light and dark in the presence or absence of addedCO₂ was similar to that of Xanthium. However, two or three timesmore ethylene was released from maize leaves in the light whenCO² was added compared to that generated in the dark. This isin marked contrast to Xanthium, where, under the light conditionsused, the ethylene release rate in the dark equalled or exceededthat occurring in the light, even in the presence of high levelsof CO₂. A very low rate of ethylene release was observed atthe CO₂ compensation point of maize. A speculative model is presented to explain how photosyntheticactivity might act as a key factor in regulating ethylene evolutionfrom leaf tissue in these experiments. It invokes the conceptof an inhibition by CO₂ of ethylene retention or breakdown thuspermitting more ethylene to be released from the leaves.     

Theoretical Considerations when Estimating the Mesophyll Conductance to CO(2) Flux by Analysis of the Response of Photosynthesis to CO(2)

The conductance for CO₂ diffusion in the mesophyll of leaves can limit photosynthesis. We have studied two methods for determining the mesophyll conductance to CO₂ diffusion in leaves. We generated an ideal set of photosynthesis rates over a range of partial pressures of CO₂ in the stroma and studied the effect of altering the mesophyll diffusion conductance on the measured response of photosynthesis to intercellular CO₂ partial pressure. We used the ideal data set to test the sensitivity of the two methods to small errors in the parameters used to determine mesophyll conductance. The two methods were also used to determine mesophyll conductance of several leaves using measured rather than ideal data sets. It is concluded that both methods can be used to determine mesophyll conductance and each method has particular strengths. We believe both methods will prove useful in the future.     

Enhanced Photosynthesis and Stomatal Conductance of Pima Cotton (Gossypium barbadense L.) Bred for Increased Yield

Yield of Pima cotton (Gossypium barbadense L.) has tripled over the last 40 years with the development of new cultivars. Six genetic lines representing successive stages in the breeding process (one primitive noncultivated accession, four cultivars with release dates from 1949 to 1983, and one unreleased breeding line) were grown in a greenhouse, and their gas exchange properties were compared. Among the cultivated types, genetic advances were closely associated with increasing single-leaf photosynthetic rate (A) and stomatal conductance (g_s), especially in the morning. The A and g_s of the primitive line approached those of the cultivated types early in the morning, but were much lower for the rest of the day. In both morning and afternoon, A was correlated with g_s across genotypes but was not correlated with leaf thickness, concentrations of chlorophyll or starch, or intercellular CO₂ concentration (c_i). In the oldest cultivar, the relationship of A to c_i did not change between morning and afternoon. In the two most recent lines, the slopes of the A:c_i curves at limiting c_i exceeded that of the oldest cultivar by 25 to 50% in the morning, but the differences were much smaller in the afternoon. The maximum A of the newer lines at high c_i exceeded that of the oldest cultivar only in the morning. Breeding for increasing yield has enhanced the photosynthetic capacity and stomatal conductance of Pima cotton and altered the diurnal regulation of photosynthesis.     

Different effects of variation in Xanthium strumarium L. (Compositae) on two insect seed predators

Summary To determine the relative importance of variation in several plant characters on susceptibility to herbivores, we examined patterns of seed predation by two monophagous insect species and patterns of variation in ten populations of the cocklebur, Xanthium strumarium. Multiple regression analysis disclosed that one seed predator was most influenced by plant chemical variation, the other was significantly influenced by both chemical and morphological variation, but variation in yet another character, general burr size, was most important in conferring resistance to both insects simultaneously. The plant populations differed most in this character. Although many of the plant characters were correlated with each other, those important in determining susceptibility to each insect species were uncorrelated and independent of those conferring resistance to both insects simultaneously.These results imply that ecological similar herbivores may be influenced by different aspects of plant variation, and that predictions of evolutionary responses of local plant populations to herbivory may require knowledge of the structure of local herbivore communities and the dynamics of their establishment.     

Differentiating Day from Night Effects of High Ambient [CO2] on the Gas Exchange and Growth of Xanthium strumarium L. Exposed to Salinity Stress

Sodium chloride, at a concentration of 88 mol m^-3in half strengthHoagland nutrient solution, increased dry weight per unit areaofXanthium strumarium L. leaves by 19%, and chlorophyll by 45%compared to plants grown without added NaCl at ambient (350µmol mol^-1) CO₂concentration. Photosynthesis, per unitleaf area, was almost unaffected. Even so, over a 4-week period,growth (dry weight increment) was reduced in the salt treatmentby 50%. This could be ascribed to a large reduction in leafarea (>60%) and to an approx. 20% increase in the rate ofdark respiration (Rd). Raising ambient [CO₂] from zero to 2000 µmol mol^-1decreasedRd in both control and salinized plants (by 20% at 1000, andby 50% at 2000 µmol mol^-1CO₂concentration) compared toRd in the absence of ambient CO₂. High night-time [CO₂] hadno significant effect on growth of non-salinized plants, irrespectiveof day-time ambient [CO₂]. Growth reduction caused by salt wasreduced from 51% in plants grown in 350 µmol mol^-1throughoutthe day, to 31% in those grown continuously in 900 µmolmol^-1[CO₂]. The effect of [CO₂] at night on salinized plants depended onthe daytime CO₂concentration. Under 350 µmol mol^-1day-time[CO₂], 900 µmol mol^-1at night reduced growth over a 4-weekperiod by 9% (P <0.05) and 1700 µmol mol^-1reduced itby 14% (P <0.01). However, under 900 µmol mol^-1day-time[CO₂], 900vs . 350 µmol mol^-1[CO₂] at night increasedgrowth by 17% (P <0.01). It is concluded that there is both a functional and an otiose(functionless) component to Rd, which is increased by salt.Under conditions of low photosynthesis (such as here, in thelow day-time [CO₂] regime) the otiose component is small andhigh night-time [CO₂] partly suppresses functional Rd, therebyreducing salt tolerance. In plants growing under conditionswhich stimulate photosynthesis (e.g. with increased daytime[CO₂]), elevated [CO₂] at night suppresses mainly the otiosecomponent of respiration, thus increasing growth. Consequently,in regions of adequate water and sunlight, the predicted furtherelevation of the world atmospheric [CO₂] may increase plantsalinity tolerance. Xanthium strumarium ; respiration; photosynthesis; salt stress; sodium chloride; carbon dioxide; atmosphere     

Calcium Effects on Stomatal Movement in Commelina communis L. : Use of EGTA to Modulate Stomatal Response to Light, KCl and CO(2)

Stomatal movements depend on both ion influx and efflux; attainment of steady state apertures reflects modulation of either or both processes. The role of Ca²⁺ in those two processes was investigated in isolated epidermal strips of Commelina communis, using the Ca²⁺ chelator EGTA to reduce apoplastic [Ca²⁺]. The results suggest that a certain concentration of Ca²⁺ is an absolute requirement for salt efflux and stomatal closure. EGTA (2 millimolar) increased KCl-dependent stomatal opening in darkness and completely inhibited the dark-induced closure of initially open stomata. Closure was inhibited even in a KCl-free medium. Thus, maintenance of stomata in the open state does not necessarily depend on continued K⁺ influx but on the inhibition of salt efflux. Opening in the dark was stimulated by IAA in a concentration-dependent manner, up to 15.4 micrometer without reaching saturation, while the response to EGTA leveled off at 9.2 micrometer. IAA did not inhibit stomatal closure to the extent it stimulated opening. The response to IAA is thus consistent with a primary stimulation of opening, while EGTA can be considered a specific inhibitor of stomatal closing since it inhibits closure to a much larger degree than it stimulates opening. CO₂ causes concentration-dependent reduction in the steady state stomatal aperture. EGTA completely reversed CO₂-induced closing of open stomata but only partially prevented the inhibition of opening.     

In vitro cultured cocklebur (Xanthium strumarium L.) responses to dimercaptopropanesulfonic acid and monosodium methanearsonate

Monosodium methanearsonate (MSMA) is an effective herbicide used for the control of Xanthium strumarium (cocklebur), a serious weed problem in cotton production. Recently, MSMA-resistant cocklebur was reported, which reduced MSMA effectiveness in the cocklebur control strategy. The mode of action of MSMA may involve the reduction of the pentavalent arsenic in MSMA to a trivalent form, which may then form a complex with sulfhydryl-containing enzymes. Dimercaptopro-panesulfonic acid (DMPS) chelates trivalent arsenic, resulting in a nontoxic complex. The objective of this study was to determine the effect of DMPS and MSMA on the growth of in vitro grown cocklebur shoot and callus tissues. Lateral and terminal shoots were grown on basal Murashige and Skoog medium containing 6.25 mg L^–1 (0.0428 mm) MSMA alone and with various concentrations of DMPS. Callus was cultured on basal callus medium also containing 6.25 mg L^–1 MSMA alone and an equimolar concentration of DMPS. Shoot injury symptoms, dry root weight, and fresh weight of callus were recorded. Susceptible cocklebur shoots showed severe discoloration and death when treated with MSMA. Resistant shoots and cultured shoot tips were not adversely affected. DMPS alone did not inhibit growth of these tissues. Callus induced from susceptible tissue was inhibited by MSMA and DMPS alone. In all combinations of DMPS and MSMA tested, DMPS did not counteract the MSMA response. Thus, the pentavalent form of arsenic in MSMA apparently is not reduced to the trivalent form by the plant and hence is not involved in the mechanism of actionAbbreviations MSMA monosodium methanearsonate - DMPS dimercaptopropanesulfonic acid - SM shoot medium - CM callus medium - ANOVA analysis of variance     

Xanthatin and xanthinosin from the burs of Xanthium strumarium L. as potential anticancer agents

Xanthatin and xanthinosin, 2 sesquiterpene lactones isolated from the burs of Xanthiun strumarium L. (cocklebur), showed moderate to high in vitro cytotoxic activity in the human cancer cell lines WiDr ATCC (colon), MDA-MB-231 ATCC (breast), and NCI-417 (lung). Xanthatin and xanthinosin were purified as the result of a multi-screening bioassay-guided study of wild plant species of the family Asteraceae, collected from various sites in Saskatchewan, Canada. Seventy-five extracts at a single concentration of 100 microg/mL were evaluated for in vitro cytotoxicity to the human cancer cell lines used. The chloroform extract of Carduus nutans L. (nodding thistle) aerial parts (IC50, 9.3 microg/mL) and the hexane extract of Echinacea angustifolia DC. (narrow-leaved purple coneflower) root (IC50, 4.0 microg/mL) were moderately to highly cytotoxic to the lung cancer cell line. The chloroform extracts of X. strumarium L. burs and Tanacetum vulgare L. (tansy) aerial parts exhibited the highest cytotoxicity for all cell lines tested; their IC50 values, obtained from multidose testing, ranged from 0.1 to 6.2 microg/mL (X. strumarium) and from 2.4 to 9.1 microg/mL (T. vulgare). Further purification of the chloroform fraction of X. strumarium yielded xanthatin and xanthinosin in high yields. This is the first time that these compounds have been reported in the burs of X. strumarium. Their IC50 values are also reported herein.     

Responses of Soybean Leaf Angle, Photosynthesis and Stomatal Conductance to Leaf and Soil Water Potential

The hypothesis that soil water potential (_s) is better correlatedto heliotropic leaf orientation, photosaturated photosyntheticCO₂ assimilation and stomatal conductance during periods oflimited water availability than is bulk leaf water potential(₁) was examined in greenhouse-grown soybean (Glycine max) plants,submitted to a progressive drought. Paired plants were exposedto either 1000 or 100 µmol m^–2 s^–1 photonflux densities (PFD) for 45–60 mins. The higher irradianceinduced short-term decreases in ₁, due to increased transpiration,while _l in the plant exposed to low PFD did not decrease. Thesechanges in ₁ occurred independently of changes in soil waterstatus. Concurrent to the light treatments, a single attachedleaf from each of the two plants was isolated from the restof the plant by shading, and the pulvinus of its terminal leafletwas exposed to a perpendicular PFD of 500 µmol m–²S^–1. Leaf movement of this leaflet was recorded in responseto this light, until a stable leaflet angle was achieved. Valuesof _s and _l (before and after light treatment), and photosaturatedrates of photosynthesis and stomatal conductance, were thenmeasured on these leaves. Leaflet angle and gas exchange werebetter correlated with _s (r² = 0.50, 0.50 and 0.57 for angle,photosynthesis and conductance, respectively) than with _l especiallywhen _l was the result of short-term, high-light induced changesin leaf water status (r² = 0.36, 0.32 and 0.49, for the sameparameters). Leaflet angle was also correlated with stomatalconductance (r² = 0.61) and photosynthetic rate (r² = 0.60),suggesting a close association between leaf orientation, leafmetabolism and soil water availability. Glycine max (L.) Merr. cv. Essex, soybean, heliotropism, water potential, photosynthesis, stomatal conductance, solar tracking     

Variation in fruit size and susceptibility to seed predation among and within populations of the cocklebur,Xanthium strumarium L.

Summary Burr size is the major factor affecting variation in the intensity of predation by two species of insect on the seeds of the cocklebur, Xanthium strumarium. Mean burr size varied among 10 adjacent local populations studied over three years, as did intensity of seed predation. Seed predation was more intense in populations with low mean burr length and declined linearly with increasing burr length under field and experimental conditions. Seed predation thus is a selective factor influencing the evolution of both burr size and correlated protective characteristics such as burr spine length and wall thickness. As in some other plants, morphological rather than chemical features appear to pose the major barrier to attack by host-specific seed predators. The advantage of more highly developed tissues protecting seeds may occur at the expense of total seed production.     

Interactive Effects of Salinity and Ozone Pollution on Photosynthesis,Stomatal Conductance,Growth, and Assimilate Partitioning of Wheat (Triticum aestivum L.)

Plants of an Egyptian cultivar of wheat (Triticum aestivum L. cv. Giza 63) were exposed in open-top chambers (OTCs) for 8 h d^–1 for up to 75 d to a factorial combination of two levels of salinity (0 and 50 mM NaCl) and two levels of O₃ (filtered air and 50 mm³ m^–3). Exposure to 50 mm³ m^–3 O₃ significantly decreased stomatal conductance (g _s), net photosynthetic rate (P _N), and chlorophyll (Chl) content by 20, 25, and 21 %, respectively. This reduction resulted in a change in assimilate allocation in favour of shoot growth leading to a decrease in root to shoot ratio and eventually to a decrease in relative growth rate (RGR) of both root and shoot. There was a very large reduction in yield parameters, especially in the number of ears/plant and 1 000-grain mass. Soil salinity significantly reduced P _N and g _s by 17 and 15 %, respectively, while Chl content was increased by 17 %. Root growth was decreased leading to an increase in root/shoot ratio. Yield parameters were decreased due to salt stress. There was antagonistic interaction between salinity (50 mM NaCl) and O₃ (50 mm³ m^–3) showing that salinity effectively protects against the adverse effects of O₃ by increasing g _s during O₃ fumigation.     

Osmotic Stress Temporarily Reverses the Inhibitions of Photosynthesis and Stomatal Conductance by Abscisic Acid--Evidence that Abscisic Acid Induces a Localized Closure of Stomata in Intact, Detached Leaves

Ward, D. A. and Drake, B. G. 1988. Osmotic stress temporarilyreverses the inhibitions of photosynthesis and stomatal conductanceby abscisic acid—evidence that abscisic acid induces alocalized closure of stomata in intact, detached leaves.—J.exp. Bot 39: 147–155. The influence of osmotic stress on whole leaf gas exchange wasmonitored in detached leaves of Glycine max supplied with anexogenous concentration (10^–5 mol dm^–3) of ±abscisicacid (ABA) sufficient to inhibit net photosynthesis and stomatalconductance by 60% and 70%, respectively, under a saturatingirradiance and normal air. Raising the osmotic (sorbitol) concentrationof the ABA solutions feeding leaves elicited rapid and synchronousreversals of the ABA-dependent inhibitions of net photosynthesisand conductance. These reversals reached a peak simultaneously,after which photosynthesis and conductance declined. The magnitudeof the transient stimulations at peak height was dependent uponthe sorbitol concentration of the ABA feeding solution, althoughthe time-course of the transients (half time, 4–6 min)was similar for the different osmotic concentrations applied.Irrespective of transient size the relative changes of photosynthesisand conductance were comparable; consequently the calculatedpartial pressure of CO₂ in the substomatal space (Ci) remainedrelatively constant during the transient phase. In contrastto the ABA-treated leaves, elevating the osmotic concentrationof the distilled water supply feeding control leaves stimulatedconductance to a much greater relative extent than photosynthesis.The co-stimulations of photosynthesis and conductance inducedin ABA-treated leaves by osmotic shock were not due to a restrictionin the transpirational uptake of ABA and occurred irrespectiveof the source osmoticum applied. These data are consistent with the hypothesis that the ABA-dependentinhibition of photosynthesis at constant Ci is an artifact causedby the spatially heterogeneous closure of stomata in responseto ABA. Alternative explanations for the responses are, however,considered. Key words: Abscisic acid, photosynthesis, osmotic stress, Glycine max, stomatal conductance     

Cytokinins in Xanthium strumarium L.: Distribution in the Plant and Production in the Root System

Cytokinins from leaf laminae, buds, petioles, stems, roots,and root exudate of mature vegetative plants of Xanthium strumariumL. were extracted, fractionated, and partially characterizedby means of column chromatography with Sephadex LH20. Two peaksof cytokinin activity with elution volumes corresponding tozeatin and zeatin riboside were detected, in varying concentrations,in all plant parts. A third cytokinin, detected only in petiolesand in expanding and mature leaves, eluted off the Sephadexcolumn before zeatin riboside. This cytokinin (peak ‘a’)was converted to zeatin or to a zeatin-like cytokinin followingboth acid hydrolysis and treatment with ß-glucosidase.Peak ‘a’ was not detected in buds or in the youngestdeveloping leaves but was the predominant cytokinin presentin half-expanded and more mature leaves. By contrast, the zeatinriboside-like peak (peak ‘b’) constituted the majorcytokinin in root exudate, apical buds, and the youngest developingleaves, while not greatly contributing to the cytokinin contentof mature leaves. The detopped root system was shown to be capable of cytokininproduction. The distribution of cyrtokinins in the plant isdiscussed in relation to their probable origin in the root system.