Leaf Abscisic Acid Concentration Research Articles

Changes in abscisic acid and its glucose ester in Phaseolus vulgaris L. during chilling and water stress

Levels of free and conjugated abscisic acid (ABA) were determined in leaves and roots of intact bean (Phaseolus vulgaris L., cv. Mondragone) seedlings under chilling (3ΔC) and drought as well as during recovery from stress. Abscisic acid-glucose ester (ABAGE) was the only conjugate releasing free ABA after alkaline hydrolysis of the crude aqueous extracts. During the first 20–30 h chilled plants rapidly dehydrated and wilted without any change in ABA and ABAGE levels. Subsequently, leaf and root ABA levels increased and plants regained turgor. ABAGE concentration showed a slight increase in leaves but not in roots. Upon recovery from chilling a transient, but significant, rise in leaf ABA content was observed, while no appreciable change in ABAGE was found. Drought triggered ABA accumulation in leaves and roots, while a rise in ABAGE content was detected only in leaf tissues. Recovery from stress caused a drop in ABA levels without a correspondent increase in ABAGE concentration. We conclude that ABAGE is not a source of free ABA during either chilling or water stress and that only a small proportion of the ABA produced under stress is metabolised to ABAGE during recovery.

Changes in cell ultrastructure and endogenous abscisic acid and indole-3-actic acid concentrations in Fatsia japonica leaves under polyethylene glycol-induced water stress

Ultrastructural alterations in epidermal and mesophyll cells as well as variations in bulk leaf endogenous ABA and IAA concentrations were studied in PEG-treated plants of Fatsia japonica Decne & Plank. Under stress induced by PEG vesicles containing fibrous material and electron-dense bodies associated with plasma membranes were observed. Cytochemical examination indicated that electron-dense bodies corresponded to lipids and the fibrous material of the vesicles were polysaccharides. Chloroplasts, mitochondria, nuclei and Golgi apparatus also showed modifications. A strong relationship was found between increasing PEG-induced water stress, increasing endogenous ABA and ultrastructural changes. In relation with leaf ontogeny and ABA concentration a higher ABA level was observed in younger than in older leaves. The differences in the endogenous concentrations of indole-3-acetic acid are unclear, except after 7 days of PEG-treatment. The increase in the endogenous abscisic acid concentration could be related with the ultrastructural changes.

Effects of soil freezing and drought stress on abscisic acid content of sugar maple sap and leaves.

In 1991 and 1992, mature maple trees (Acer saccharum Marsh.) were freeze-stressed or drought-stressed by preventing precipitation (snow or rain) from reaching the forest floor under selected trees. Lack of snow cover caused a decrease in soil temperature to well below 0 degrees C from December to April and a lowering of the soil water content to 10%. The abscisic acid (ABA) concentration in the spring sap of deep-soil frost-stressed trees was significantly higher than in control or drought-stressed trees. The increase in ABA concentration in the xylem sap in the spring of 1991 and 1992 preceded symptoms of canopy decline and a decrease in leaf area that were observed during the summers of 1991 and 1992. These results suggest a role for ABA in root-to-shoot communication in response to environmental stress. The largest differences in ABA concentration induced by the treatments was found in sap collected at the end of sap flow. The increase in ABA concentration in spring sap at the end of the sap flow could be used as an early indicator of stress suffered by trees during the winter. Not only did the increase in ABA concentration occur before any visible symptoms of tree decline appeared, but the trees that showed the most evident decline had the highest ABA concentrations in the spring sap. Leaf ABA concentration was not a good indicator of induced stress.

Mechanisms of Different Responses of Leaf Photosynthesis in African Rice(Oryza glaberrima Steud.) and Rice(Oryza sativa L.) to Low Leaf Water Potential.

This research was conducted to elucidate the mechanisms in water stress tolerance of Oryza glaberrima Steud. with special reference to stomatal conductance. Leaf photosynthesis (LPS) of O. glaberrima Steud. with developing soil water deficiency, was still maintained above zero when the leaf water potential decreased to as low as -2.0 MPa, while in O. sativa L. LPS became zero at -1.6 MPa. Stomatal conductances also changed in parallel with LPS in both species, suggesting that the different response of LPS to reduced leaf water potential is due to the different stomatal response between two species. In the relationship between the stomatal conductance and abscisic acid (ABA) content in the leaves, Saka, a line of O. glaberrima Steud., showed a slower decrease of stomatal conductance with increase of ABA than Nipponbare, a cultivar of O. sativa L. This attenuated response of stomatal conductance to leaf ABA content in O. glaberrima Steud. was supposed to be the cause of maintenance of higher LPS than in O. sativa L. under low leaf water potential conditions. The Rubisco content showed no significant difference in the response to developing low leaf water potential between the two species.

Abscisic acid accumulation in leaves of two contrasting hybrid poplar clones affected by nitrogen fertilization plus cyclic flooding and soil drying.

Cuttings of hybrid Populus clones Tristis and Eugenei growing in pots in a greenhouse were treated with nitrogen fertilizer at two rates and subjected to repeated soil flooding or drying. Periodically, gas exchange measurements and radioimmunoassays, to determine abscisic acid (ABA) concentrations, were made on recently mature leaves.In both clones, photosynthesis and stomatal conductance were depressed five days after flooding, but leaf ABA concentrations remained relatively constant. In contrast, an initial, 9-day period of soil drying resulted in substantial ABA accumulation in leaves, which closely correlated with declines in photosynthesis and conductance. A second soil drying cycle of up to 9 days was less effective in modifying gas exchange and leaf ABA concentrations. High-N supply stimulated leaf ABA production as the soil dried. On the resumption of watering, gas exchange in Tristis recovered fully and rapidly and leaf ABA concentrations quickly returned to control values, whereas gas exchange in Eugenei recovered slowly and leaf ABA concentrations remained high for longer.Gas exchange in Eugenei was unaffected by soil drying until leaf ABA concentrations exceeded 100 ng g(dw) (-1), whereas Tristis showed a reduction in stomatal conductance and photosynthesis at leaf ABA concentrations of only 10 ng g(dw) (-1). A rise in internal CO(2) concentrations was associated with increased leaf ABA concentrations in Tristis, but not in Eugenei. Clonal differences in the relationship between gas exchange and leaf ABA concentration suggest contrasting physiological strategies for survival under prolonged drying conditions.

ABA AND STOMATAL RESPONSES IN TEPARY AND COMMON BEAN

Tepary beans (Phaseolus acutifolius Gray) are more drought tolerant and have stomata that are more sensitive to low leaf water potentials (ψ w) than common beans (P. vulgaris L.). This study was designed to examine the role of ABA in controlling stomatal behaviour in these species. Comparison of the bulk leaf ABA content does not explain why tepary stomata are more sensitive to low leaf ψ w compared to common bean (at -1.4 MPa ABA content increased 40-fold in common bean and 25-fold in tepary). We hypothesize that the greater sensitivity of tepary stomata to low leaf ψ w is related to a higher concentration of ABA in the xylem sap, and/or to a greater sensitivity of tepary stomata to ABA. Xylem sap of well-watered and water stressed plants is analyzed to determine the concentration of ABA, and whether ABA is a putative candidate serving as a chemical root signal in response to water stress in Phaseolus. To test stomatal sensitivity to ABA, epidermal strips and detached leaves are exposed to a range of ABA concentrations. The relationship between stomatal aperture and different ABA concentrations is discussed.

Xylem ABA controls the stomatal conductance of field‐grown maize subjected to soil compaction or soil drying

ABSTRACTStomatal conductance of individual leaves was measured in a maize field, together with leaf water potential, leaf turgor, xylem ABA concentration and leaf ABA concentration in the same leaves. Stomatal conductance showed a tight relationship with xylem ABA, but not with the current leaf water status or with the concentration of ABA in the bulk leaf. The relationship between stomatal conductance and xylem [ABA] was common for variations in xylem [ABA] linked to the decline with time of the soil water reserve, to simultaneous differences between plants grown on compacted, non‐compacted and irrigated soil, and to plant‐to‐plant variability. Therefore, this relationship is unlikely to be fortuitous or due to synchronous variations. These results suggest that increased concentration of ABA in the xylem sap in response to stress can control the gas exchange of plants under field conditions.

Responses of seven diverse rice cultivars to water deficits. III. Accumulation of abscisic acid and proline in relation to leaf water-potential and osmotic adjustment

Foliar content of free abscisic acid (ABA) and proline, leaf water potential ( ψ 1), and osmotic potential ( ψ 0) were determined in seven diverse rice ( Oryza saliva L.) cultivars after these were subjected to a 10-day period of withholding irrigation or to well-watered conditions. For three of the cultivars, periodic determination of leaf ABA was performed during differential irrigation and recovery. Diurnal kinetics of leaf ABA content, along with observations on leaf bulk turgor ( ψ t) and gas exchange, were established in two cultivars under well-watered conditions. Water-stress-induced proline and ABA accumulation differed among cultivars. Across cultivars, stress-induced proline accumulation was correlated negatively with midday ψ 1 and positively with osmotic adjustment, but ABA accumulation was not. Proline accumulation therefore gave a similar discrimination among cultivars as did predawn turgid osmotic potential and ψ 1. Diurnal oscillations of ψ 0 differed among cultivars but were not related to ABA accumulation or ψ 1. Interpretation of stress-induced ABA accumulation kinetics was difficult due to large diurnal oscillations. The relative amplitude of diurnal ABA oscillation reflected diurnal changes in ψ 1 and ψ t across cultivars. Diurnal ABA response to ψ t of a drought-resistant cultivar was less sensitive than that of a susceptible cultivar under well-watered conditions. Highest ABA concentrations were observed at 16:00 h. Diurnal ABA accumulation lagged 4 hours behind changes in leaf bulk turgor. The results suggest that high- and low-ABA-accumulating rice cultivars exist, but no clear linkage of this trait to behavior under water stress was observed.

Seasonal changes in endogenous ABA and IAA and the influence of applied ABA and auxin in relation to shoot growth and abscission in Valencia Orange (Citrus sinensis (L.) Osbeck)

Seasonal changes in endogenous IAA and ABA were measured by gas chromatography/mass-spectrometry. Highest concentrations of ABA occurred in leaves. There was a major ABA peak in early spring (up to 1360 ng g−1 dw). Levels were low in summer (90 ng g−1 dw). There was a minor ABA peak in autumn. Endogenous IAA in leaves was highest in winter/spring (up to 76 ng g−1 dw). Applied ABA promoted abscission of leaves and shoots while applied NAA delayed abscission. The main peak in leaf-ABA content was followed by extensive shoot abscission. The involvement of ABA and IAA in regulation of flush growth was not clear.

Rapid Adjustment of Guard-Cell Abscisic Acid Levels to Current Leaf-Water Status

Detached broad bean (Vicia faba L.) leaflets were water stressed; within 15 minutes, guard-cell abscisic acid (ABA) concentration increased ninefold. This result eliminates the apparent discrepancy raised by reports of no correlation between initial water-stress effects on stomata and leaf ABA concentration. Six hours after stress relief, guard-cell ABA concentration was near the prestress value, which would seem to implicate other factors in stress aftereffects on stomata.

Drought‐stress‐induced polypeptide accumulation in tomato leaves

Abstract. Tomato (Lycopersicon esculentum Mill ev. Ailsa Craig) leaf polypeptides, radiolabelled with L‐[36S] methionine during drought stress, were separated by two‐dimensional polyacrylamide gel electrophoresis. Three sets of polypeptides were distinguished: those that decreased, increased or transiently increased in response to drought stress. Three sets of polypeptides also could be distinguished during recovery from drought stress: those that increased, decreased rapidly or decreased slowly and could still be detected after 6 h of recovery. The set of polypeptides that decreased during drought stress was the same set that increased during recovery and those that increased during stress, decreased during recovery. The leaf ABA concentration increased rapidly in response to drought stress and decreased rapidly during recovery from drought stress, returning to the nonstress level after 6 h of rehydration. There was a correlation between the accumulation of some of the drought‐induced polypeptides and the pattern of ABA accumulation during stress and recovery from stress.

Changes in the concentration of ABA in xylem sap as a function of changing soil water status can account for changes in leaf conductance and growth

Abstract. Maize seedlings (Zea mays L. John Innes F1 hybrid) were grown in a greenhouse in l‐m‐long tubes of soil. When the plants were well established, water was withheld from half of the tubes. Control plants were watered every day during the 20‐d experimental period. The soil drying treatment resulted in a substantial restriction of stomatal conductance and a limitation in shoot growth, even though there was no detectable difference in the water relations of watered and unwatered plants. From day 7 of the soil drying treatment, xylem ABA concentrations (measured using the sap exuded from detopped plants) were substantially increased in unwatered plants compared to values recorded with sap from plants watered every day. Measurements of water potential through the profile of unwatered soil suggest that xylem ABA concentrations reflects the extent of soil drying. Leaf ABA content was a much less sensitive indicator of the effect of soil drying and during the whole of experimental period there was no significant difference between ABA concentration in leaves of well watered and unwatered plants. In a second set of experiments, ABA was fed to part of the roots of potted maize plants to manipulate xylem ABA concentration. These manipulations suggested that the increases in ABA concentration in xylem sap, which resulted from soil drying, were adequate to explain the observed variation in stomatal conductance and might also explain the restriction in leaf growth rate. These results are discussed in the light of recent work which suggests that stomatal responses to soil drying are partly attributable to an as‐yet unidentified inhibitor of stomatal opening.

Sequential response of whole plant water relations to prolonged soil drying and the involvement of xylem sap ABA in the regulation of stomatal behaviour of sunflower plants

SUMMARYSunflower plants (Helianihus animus cv. Tall Single Yellow} were grown in the greenhouse in drain pipes (100 mm inside diameter and 1 m long) rilled with John Innes No. 2 compost. When the fifth leaf had emerged, half of the plants were left unwatered for 6 days, rewatered for 2 days and then not watered for another 12 days. Measurements of water relations and abaxial stomatal conductance were made at each leaf position at regular intervals during the experimental period. Estimates were also made of soil water potentials along the soil profile and of ABA concentrations in xylem sap and leaves.Soil drying led to some reduction in stomatal conductance alter only 3 days but leaf turgors were not reduced until day 13 (6 days after rewatering). When the water relations of leaves did change, older leases became substantially dehydrated while high turgors were recorded in younger leaves. Leaf ABA content measured on the third youngest leaf hardly changed over the first 13 days of the experiment, despite substantial soil drying, while xylem ABA concentrations changed very significantly and dynamically as soil water status varied, even when there was no effect of soil drying on leaf water relations. We argue that the highest ABA concentrations in the xylem, found as a result of substantial soil drying, arise from synthesis in both the roots and the older leaves, and act to delay the development of water deficit in younger leases.In other experiments ABA solutions were watered on to the root systems of sunflower plants to increase ABA concentrations in xylem sap. The stomatal response to applied ABA was quantitatively very similar to that to ABA generated as a result of soil drying. There was a log‐linear relationship between the reduction of leaf conductance and the increase of ABA concentration m xylem sap.

Influence of Cadmium on Water Relations, Stomatal Resistance, and Abscisic Acid Content in Expanding Bean Leaves

Ten day old bush bean plants (Phaseolus vulgaris L. cv Contender) were used to analyze the effects of 3 micromolar Cd on the time courses of expansion growth, dry weight, leaf water relations, stomatal resistance, and abscisic acid (ABA) levels in roots and leaves. Control and Cd-treated plants were grown for 144 hours in nutrient solution. Samples were taken at 24 hour intervals. At the 96 and 144 hour harvests, additional measurements were made on excised leaves which were allowed to dry for 2 hours. From the 48 hour harvest, Cd-treated plants showed lower leaf relative water contents and higher stomatal resistances than controls. At the same time, root and leaf expansion growth, but not dry weight, was significantly reduced. The turgor potentials of leaves from Cd-treated plants were nonsignificantly higher than those of control leaves. A significant increase (almost 400%) of the leaf ABA concentration was detected after 120 hours exposure to Cd. But Cd was found to inhibit ABA accumulation during drying of excised leaves. It is concluded that Cd-induced decrease of expansion growth is not due to turgor decrease. The possible mechanisms of Cd-induced stomatal closure are discussed.

Leaf Gas Exchange and Water Relations of Lupins and Wheat. III. Abscisic Acid and Drought-Induced Stomatal Closure

Changes in the content of endogenous abscisic acid (ABA) were followed in glasshouse experiments during stomatal closure induced by drought in leaves of lupin (Lupinus cosentinii Guss. cv. Eregulla) and wheat (Triticum aestivum L. cvv. Gamenya and Warigal), species which differ in stomatal sensitivity to changes in leaf water potential. Increases in bulk leaf ABA concentration were closely correlated with decreases in leaf conductance in both species. In lupin, substantial increases in ABA and decreases in conductance occurred over a very narrow range of leaf water potential. ABA concentrations in wheat leaves were highly negatively correlated with bulk leaf turgor, but there was no significant relationship between ABA and turgor in lupin. However, ABA accumulated progressively in the leaves of both species as soil water content decreased. Stomatal closure in lupin could be induced by supplying exogenous ABA to detached leaves via the transpiration stream at concentrations of 10-4 to 10-2 mol m-3 of (+)-ABA. Abaxial stomata closed more readily than those on the adaxial surface in response to both drought and applied ABA. Stomatal response to ABA was not affected by the presence of the cytokinin zeatin, and zeatin by itself had no effect on conductance. When treatments designed to reduce endogenous cytokinin concentrations were imposed (prolonged leaf detachment or prior drought), stomatal response to low concentrations of ABA was enhanced. However, such treatments did not significantly change the stomatal response to high ABA concentrations, nor affect the stomatal conductance of leaves supplied with water alone. It is concluded that drought-induced stomatal closure could be mediated by ABA in both wheat and lupin, despite the initially small change in leaf water status in the latter species.

Effects of mechanical stress or abscisic acid on growth, water status and leaf abscisic acid content of eggplant seedlings

Container-grown eggplant ( Solanum melongena L. var. esculentum Nees. ‘Burpee's Black Beauty’) seedlings were conditioned with brief, periodic mechanical stress or abscisic acid (ABA) in a greenhouse prior to outdoor exposure. Mechanical stress consisted of seismic (shaking) or thigmic (stem flexing) treatment. Exogenous ABA (10 −3 or 10 −4 M) was applied as a soil drench 3 days prior to outdoor transfer. During conditioning, only thigmic stress reduced stem elongation and only 10 −3 M ABA reduced relative growth rate (RGR). Both conditioning treatments increased leaf specific chlorophyll content, but mechanical stress did not affect leaf ABA content. Outdoor exposure of unconditioned eggplant seedlings decreased RGR and leaf-specific chlorophyll content, but tended to increase leaf ABA content relative to that of plants maintained in the greenhouse. Conditioning did not affect RGR of plants subsequently transferred outdoors, but did reduce stem growth. Seismic stress applied in the greenhouse reduced dry weight gain by plants subsequently transferred outdoors. Mechanical stress treatments increased leaf water potential by 18–25% relative to that of untreated plants.

Water-stress-induced changes in the abscisic acid content of guard cells and other cells of Vicia faba L. leaves as determined by enzyme-amplified immunoassay

A highly sensitive, solid-phase, enzyme-amplified immunoassay for the plant growth regulator (+)-abscisic acid (ABA) was developed. The assay sensitivity (0.2-10 fmol) was sufficient for analyzing free ABA in homogeneous tissue samples dissected from Vicia faba L. leaves. Eight hours after detached leaves had been desiccated to 10% loss of fresh weight, the bulk leaf ABA content increased from </=0.2 to 6.2 ng.(mg dry weight)(-1). Epidermal tissue, spongy parenchyma cells, and palisade parenchyma cells from this water-stressed leaf had the following ABA contents, respectively: 4.8, 9.4, and 9.0 ng.(mg dry weight)(-1). Guard cells, which respond to exogenous ABA by losing solutes and volume, were also assayed. When they were dissected from control (fully hydrated) leaves, their ABA content was approximately 0.7 fg.(cell pair)(-1) [[unk]0.2 ng.(mg dry weight)(-1)]. In contrast, the ABA content of guard cells of water-stressed leaves was approximately 17.7 fg.(cell pair)(-1). These results indicate that ABA accumulation in a highly stressed V. faba leaflet is generalized; guard cells contain only 0.15% of bulk leaf ABA. The time course for loss of ABA from guard cells of a floating epidermal peel was studied. There was little loss within 30 min, but after 4 hr, the ABA content was only 17% of the original value. These results indicate that the bulk of guard cell ABA is not readily diffusible (i.e., probably not apoplastic). The results also indicate that common laboratory procedures results in lowered guard cell ABA content.

Growth response of barley and tomato to nitrogen stress and its control by abscisic acid, water relations and photosynthesis

Barley (Hordeum vulgare L.) and tomato Lycopersicon esculentum Mill.) were grown hydroponically and examined 2, 5, and 10 d after being deprived of nitrogen (N) supply. Leaf elongation rate declined in both species in response to N stress before there was any reduction in rate of dryweight accumulation. Changes in water transport to the shoot could not explain reduced leaf elongation in tomato because leaf water content and water potential were unaffected by N stress at the time leaf elongation began to decline. Tomato maintained its shoot water status in N-stressed plants, despite reduced water absorption per gram root, because the decline in root hydraulic conductance with N stress was matched by a decline in stomatal conductance. In barley the decline in leaf elongation coincided with a small (8%) decline in water content per unit area of young leaves; this decline occurred because root hydraulic conductance was reduced more strongly by N stress than was stomatal conductance. Nitrogen stress caused a rapid decline in tissue NO 3 (-) pools and in NO 3 (-) flux to the xylem, particularly in tomato which had smaller tissue NO 3 (-) reserves. Even in barley, tissue NO 3 (-) reserves were too small and were mobilized too slowly (60% in 2 d) to support maximal growth for more than a few hours. Organic N mobilized from old leaves provided an additional N source to support continued growth of N-stressed plants. Abscisic acid (ABA) levels increased in leaves of both species within 2 d in response to N stress. Addition of ABA to roots caused an increase in volume of xylem exudate but had no effect upon NO 3 (-) flux to the xylem. After leaf-elongation rate had been reduced by N stress, photosynthesis declined in both barley and tomato. This decline was associated with increased leaf ABA content, reduced stomatal conductance and a decrease in organic N content. We suggest that N stress reduces growth by several mechanisms operating on different time scales: (1) increased leaf ABA content causing reduced cell-wall extensibility and leaf elongation and (2) a more gradual decline in photosynthesis caused by ABA-induced stomatal closure and by a decrease in leaf organic N.

Changing Responses of Stomata to Abscisic Acid and CO2as Leaves and Plants Age

Willmer, C. M., Wilson, A. B. and Jones, H. G. 1988. Changing responses of stomata to abscisic acid and C02 as leaves and plants age.—J. exp. Bot. 39: 401-410. Stomatal conductances were measured in ageing leaves of Commelina communis L. as plants developed; stomatal responses to C02 and abscisic acid (ABA) in epidermal strips of C. communis taken from ageing leaves of developing plants and in epidermal strips from the same-aged leaves (the first fully-expanded leaf) of developing plants were also monitored. Stomatal behaviour was correlated with parallel measurements of photosynthesis and leaf ABA concentrations. Stomatal conductance in intact leaves decreased from a maximum of 0-9 cm s~1 at full leaf expansion to zero about 30 d later when leaves were very senescent. Conductances declined more slowly with age in unshaded leaves. Photosynthesis of leaf slices also declined with age from a maximum at full leaf expansion until about 30 d later when no 02 exchange was detectable. Exogenously applied ABA (0-1 mol m~3) did not affect respiration or photosynthesis. In epidermal strips taken from ageing leaves the widest stomatal apertures occurred about 10 d after full leaf expansion (just before flowering began) and then decreased with age; this decrease was less dramatic in unshaded leaves. The inhibitory effects of ABA on stomatal opening in epidermal strips decreased as leaves aged and was greater in the presence of C02 than in its absence. When leaves were almost fully-senescent stomata were still able to open. At this stage, guard cells remained healthy-looking with green chloroplasts while mesophyll cells were senescing and their chloroplasts were yellow. Similar data were obtained for stomata in epidermal strips taken from the same-aged leaves of ageing plants. The inhibitory effects of ABA on stomatal opening also decreased with plant age. In ageing leaves both free and conjugated ABA concentrations remained low before increasing dramatically about 30 d after full leaf expansion when senescence was well advanced. Concentrations of free and conjugated ABA remained similar to each other at all times. It is concluded that the restriction of stomatal movements in intact leaves as the leaves and plants age is due mainly to a fall in photosynthetic capacity of the leaves which affects intracellular C02 levels rather than to an inherent inability of the stomata to function normally. Since stomatal aperture in epidermal strips declines with plant and leaf age and stomata become less responsive to ABA (while endogenous leaf ABA levels remain fairly constant until leaf senescence) it is suggested that some signal, other than ABA, is transmitted from the leaf or other parts of the plant to the stomata and influences their behaviour.

Control of Stomatal Behaviour by Abscisic Acid which Apparently Originates in the Roots

Zhang, J., Schurr, U. and Davies, W. J. 1987. Control of stomatal behaviour by abscisic acid which apparently originates in the roots.—J. exp. Bot. 38: 1174-1181. Two experiments indicate that abscisic acid (ABA) may influence stomatal behaviour of Commelina communis L. Stomatal conductance could not be correlated with bulk leaf ABA content but when the abaxial epidermis was assayed for ABA, small increases in ABA content correlated well with limitations of leaf conductance. Restricted conductance of the abaxial surface of leaves was associated with an increase of approximately 40 amole ABA per stomatal complex. This agrees with previously published figures. When roots of individual plants were split between two containers, drying the soil around one part of the root system restricted leaf conductance, even though leaf water relations were not affected. Increased ABA content of the epidermis coincided with increased ABA content of the roots in drying soil. Other roots of the same plant in moist soil did not show increased ABA content. These results suggest that in drying soil, ABA can move from the roots to the epidermis and restrict stomatal aperture even when leaf water potentials and turgors remain constant. The importance of this mechanism in providing a sensitive foliar response to decreasing soil moisture is discussed. Key words—Soil drying, ABA, roots, stomata, water relations. Correspondence to: Department of Biological Sciences, University of Lancaster, Bailrigg, Lancaster LAI 4YQ, U.K.