The Germination Characteristics of Tithonia rotundifolia

The achenes of the plant Tithonia rotundifolia show an embryodormancy, requiring a 3-month period of after-ripening to stimulategermination. Mechanical scarification did not enhance imbibitionof the achenes, indicating that the thick bi-layered achenewall does not impede water uptake. The light conditions necessaryfor optimal germination changed with achene age and the achenesbecame less sensitive to temperature. About 30–40% ofrecently harvested achenes germinated in the dark at 25 °C.Following a period of dry storage some of the achenes developeda light requirement and germination increased at 20, 25 and30 °C. Gibberellic acid stimulated the germination of achenestested after 12 weeks dry storage, and could substitute to someextent for the light and temperature requirements of the storedachenes. Tithonia rotundifolia, seed germination, gibberellic acid     

Waterfowl feces as a source of nutrients to a prairie wetland: responses of microinvertebrates to experimental additions

We tested the hypothesis that inorganic nutrients released fromwaterfowl feces would stimulate primary production, therebyaffecting microinvertebrate grazers, by making controlledadditions of waterfowl feces to fishless wetland enclosures andmeasuring the response of planktonic and phytophilouscladocerans, copepods, and rotifers. Feces were added in twopulses, four weeks apart, to duplicate enclosures at a ‘high’level (115 g m⁻² wet feces), simulating the total P load(1.6 g m⁻²) applied in an earlier fertilizationexperiment, and a ‘low’ level (11.5 g m⁻²). Density ofmicrocrustacean grazers in the water column increased inresponse to both feces additions, although the response wasmore noticeable after the second feces addition. After eachaddition, cladocerans (predominantly Ceriodaphnia dubia)and copepodites in the water column (and associated withperiphyton on acrylic rods in the water column) were mostabundant in enclosures with high loading. In contrast, densityof microcrustacean grazers associated with macrophytes(predominantly Chydorus spp. and copepodites) increasedin response to the second feces addition only.Microinvertebrate density increased only slightly with lowfeces loading. Community composition showed similar changesover the season in all enclosures, and differences in relativeabundance were not attributable to treatment effects. Given thesmall effects produced by nutrient additions that greatlyexceed natural loadings, nutrients leaching from waterfowlfeces do not appear to have a significant impact on the foodwebof this wetland. This revised version was published online in July 2006 with corrections to the Cover Date.     

Effects of Temperature on Cold Acclimation and Deacclimation in Flower Buds of Evergreen Azaleas

The effects of various storage temperature/duration combinations(5, 10 and 17°/4, 8, 12 and 16 weeks) on cold acclimationand deacclimation of flower buds were studied in four speciesof evergreen azaleas having different natural distribution andcold hardiness. The freezing process and the exotherm temperaturedistribution of florets in excised whole buds determined bydifferential thermal analysis were used as the diagnostics todetermine the degree of bud acclimation and deacclimation. Theacclimation in buds lasted for as long as 12 to 16 weeks at5°C storage, and from 8 to 12 weeks at 10°C, and itappeared to be maintained after the chilling requirement forbreaking bud dormancy had been satisfied. Therefore, bud acclimationseems to be maintained independently from bud dormancy. Thedehardening effect on acclimated buds occurred as a result ofshort exposures to higher temperatures or long exposures tolower temperatures, and there was no relation between the rateof deacclimation and the degree of hardiness in each species.Among three storage temperatures examined, 5°C was the mosteffective for the maintenance of cold acclimation in flowerbuds and the small difference of floret water contents at 5and 10°C storage is not significant. (Received August 28, 1982; Accepted February 4, 1983)     

Base water potential of Picea abies as a characteristic of the soil water status

Variation in base water potential (Ψ_b, a daily maximum level of plant water potential, which is presumed to correspond to the condition of equilibrium between the soil and plant water potentials) was examined in shoots of Norway spruce trees growing in well-drained and waterlogged soils. The influence of soil water content, air temperature, and vapour pressure deficit of the atmosphere on Ψ_b was studied using the pressure chamber technique. Maximum daily water potentials were not always observable before dawn; some were registered up to two hours later. This tendency being characteristic of trees growing under stress (shade, waterlogging) conditions, increased with declining soil water availability. In trees growing in well-drained soil, Ψ_b depended asymptotically on the available soil water storage (R²=0.73), while the values were slightly influenced by vapour pressure deficit of the atmosphere as well. In trees growing in waterlogged soil, Ψ_b was independent of the soil water storage, but sensitive to the vapour pressure deficit.     

Development of white spruce (Picea glauca (Moench.) Voss) somatic embryos during culture with abscisic acid and osmoticum, and their tolerance to drying and frozen storage

The limit of permeability of white spruce (Picea glauca [Moench.]Voss) somatic embryo cell walls to molecules was in the orderof 30 . Polyethylene glycols (PEGs) and dextrans of molecularweights greater than 1000 and 6000, respectively, produced anonpermeating (non-plasmolysing) water stress which improvedembryo development. Somatic embryos converted to plantlets atfrequencies of 76–84% following slow drying and storageat –20 C for 1 year, which was similar to the 77% recordedfor control somatic embryos slowly dried then germinated withoutfreezing or storage. Culture for 7–8 weeks with mediumcontaining abscisic acid, 3% sucrose, and 7.5% PEG 4000 yieldedsomatic embryos with five times the embryo storage lipid contentrecorded for zygotic embryos. During culture with PEG the moisturecontent of the somatic embryos decreased from 96% for immaturesuspension-cultured somatic embryos, to 47% for mature embryos.Somatic embryos cultured for 7–8 weeks survived rapiddrying to 5% moisture content, and converted to plantlets atfrequencies of 60–70%, but no somatic embryos survivedrapid drying when cultured for only 4 weeks; however, slow dryingdid induce desiccation tolerance in 3-week cultured somaticembryos. Abscisic acid was important to maintain embryos ina developmental state, but ABA alone did not induce desiccationtolerance. In order to induce desiccation tolerance a waterstress treatment was required. Tolerance of rapid drying coincidedwith moisture contents below 55%, which occurred after 5 weeksof culture in the presence of PEG 4000 and abscisic acid. Key words: Dextran, molecular weight, polyethylene glycol, triacylglycerol, water stress     

Osmoconditioning and Ageing of Pepper Seeds During Storage

The effects of osmoconditioning on the germination at 15 and25 °C of pepper (Capsicum annuum L.) seeds were studiedover a 3-year period with respect to temperature of storage.Untreated seeds stored at 5 °C showed high germinabilitythroughout the entire storage period, whereas untreated seedsstored at 25 °C showed a progressive decline in germinability,especially when assayed at 15 °C. Seeds that had been osmoconditionedprior to storage retained a high level of germinability irrespectiveof either storage or germination temperatures. When seeds thathad been stored at 25 °C were osmoconditioned after storage,there was a significantly higher germinability (assayed at 15 °C) in comparison with the corresponding untreated seeds.Seeds that were osmoconditioned twice (prior to and after storage)germinated in a similar way to those that had been osmoconditionedonce only Lactuca saliva L., lettuce, Hordeum oulgare L., barley, seed storage, moisture content, relative humidity, water potential, temperature, oxygen     

A note on the effect of storage on the chemical resistance of spores of Bacillus subtilis SA22 and Bacillus subtilis globigii B17

Spores of Bacillus subtilis SA22, harvested from nutrient agar after 9 d at 30°C and stored in distilled water at 4°C, were unaltered in their resistance to 17.7% hydrogen peroxide or 0.04% peracetic acid after storage for up to 134 weeks. Three spore crops of B. subtilis globigii were unaffected by storage for up to 134 weeks with respect to 17.7% hydrogen peroxide resistance but were significantly more resistant to 0.04% peracetic acid following storage.     

Simple measures of channel habitat complexity predict transient hydraulic storage in streams

Stream thalweg depth profiles (along path of greatest channel depth) and woody debris tallies have recently become components of routine field procedures for quantifying physical habitat in national stream monitoring efforts. Mean residual depth, standard deviation of thalweg depth, and large woody debris (LWD) volumes are potential metrics of habitat complexity calculated from these survey data. We used 42 intensive dye-transit studies to demonstrate the relevance of these easily measured channel habitat complexity metrics to transient hydraulic (“dead zone”) storage, a channel process important for biotic habitat as well as retention and “spiraling” of dissolved and particulate nutrients. We examined transient storage and channel morphology in small gravel and cobble-bedded upland streams (wetted width 2–5 m; slopes 2.6–8.3%) representing a wide range of flow stages, LWD loading, and channel complexity, including measurements before and after LWD was added to enhance fish habitat. While transient storage volume fraction decreased as flow stage increased in simple channels, those with complex morphology and well-developed riparian vegetation maintained high transient storage fractions even during storm flows. LWD additions increased transient storage and channel complexity over the 2 years of post-treatment measurements. We predict with considerable precision two different formulations of transient hydraulic storage fraction using single-variable linear regressions on residual depth (R ² = 0.61–0.89), thalweg depth variance (R ² = 0.64–0.91), or large woody debris volume (R ² = 0.48–0.74). Demonstration of these likely causal associations contributes to understanding the process of transient storage and redefines the use of thalweg profile metrics as a new approach to quantifying morphologic and hydraulic complexity in streams.     

Effects of storage and processing on the ascorbic acid content of concentrates prepared from Chaetoceros calcitrans

Microalgae concentrates, prepared by centrifuging axenic (bacteria-free) cultures of Chaetoceros calcitrans (Paulsen) Takano, were processed and stored under different experimental conditions. The content of ascorbic acid was examined in the concentrates, to assess potential changes in their nutritional properties. In algae pastes stored at 4 °C, it reduced by 29% after 4 weeks storage. As most of the ascorbic acid was retained intracellularly (92%) after resuspension, most of the cells had remained intact. In frozen and dried paste preparations, the losses of ascorbic acid ranged from minor (11% after liquid nitrogen storage for 4 weeks) to major (≥94% after drying at 100 °C for 2 h or at 60 °C overnight). However, most of the remaining ascorbic acid (>85%) in these preparations was rapidly leached from cells upon resuspension. Therefore, pastes stored at 4 °C may have the best potential as an ‘off-the-shelf’ microalgal food product for mariculture. Pastes should now be assessed in animal feeding trials, before being recommended for widespread use in the industry.     

Water and Seed Survival

Between about –350 and –14 MPa the rate of lossof viability in orthodox seeds is a positive function of waterpotential. The relative effect of water potential has been analysedin an oily seed (lettuce) and a non-oily seed (barley) and foundto be more or less identical. The lower limit for the relationin various species coincides with a seed moisture content (wetbasis) between about 2 and 6%. Below this level there is littleor no improvement in longevity with reduction in moisture content.The upper limit coincides with moisture contents of between15 and 28%, depending on whether the seeds are oily or non-oily.A water potential of about –14 MPa is the threshold forrespiration which increases more-or-less linearly with increasein water potential above this level. Above this threshold, andproviding oxygen is available to sustain respiration, seed longevityincreases with increase in water potential except that, unlessthe seeds are dormant, germination may be initiated at a waterpotential of about –1·5 to –0·5 MPa.In the absence of oxygen there may be a slight further declinein longevity with increase in water potential above –14MPa before longevity reaches a minimum value Since they cannot be dried very much without immediate lossof viability, recalcitrant seeds survive longest in the presenceof oxygen at maximum water potential commensurate with preventinggermination. The threshold water potential for immediate lossof viability has not been determined for most species but itis probable that it is close to the water potential typicalof the permanent wilting point in these plants, say –2MPa Lactuca saliva L., lettuce, Hordeum oulgare L., barley, seed storage, moisture content, relative humidity, water potential, temperature, oxygen     

Tillering, Leaf Expansion and Growth of Plants of Two Cultivars of Perennial Ryegrass Grown using Hydroponics at Two Water Potentials

Tillering and growth parameters of perennial ryegrass cultivarsWendy (diploid) and Condesa (tetraploid) were determined ina glasshouse experiment using hydroponics at low (–1·3MPa) and normal water potential (0 MPa). At –1·3MPa, leaf extension rate was reduced by 36%. Final plant tillernumber was 20% lower at –1·3 MPa because of a 12%reduction in the leaf appearance rate in the first weeks afterthe start of the treatments. Site filling, the relative increasein tiller number per leaf appearance interval, was high (0.61)-butstill lower than theoretically possible-and was only slightlyaffected by water potential. Site filling was shown to be strictlyrelated to the number of inhibited plus unemerged tiller buds.Dry matter production was 64% lower at –1·3 MPa.Relative growth rate (RGR) was, on average, 17% lower at –1·3MPa, but the reduction was greater just after the treatmentsstarted. Also, net assimilation rate (NAR) was reduced moreby low water potential just after the start of the treatments.Specific leaf area (SLA) was 13 % lower at –1·3MPa for Wendy, but not significantly reduced for Condesa. Contraryto expectations based on the theory of the functional balancebetween root and shoot, leaf weight ratio was slightly higherat –1·3 MPa. From comparison of the results ofthis study with published data, it is concluded that effectsof drought in the field on tillering cannot be attributed onlyto low water potential. Lolium perenne L., perennial ryegrass, tillering, site filling, leaf appearance, leaf extension, growth analysis, water potential     

Effects of low concentrations of heavy metals on plankton community dynamics in a small, shallow, fertile lake

Significant differences in phytoplankton abundance developedamong identical, untreated 2.3-m³ enclosures, maintained ina shallow, fertile lake in northern Italy for 14 days in June1981. Differences in soluble reactive phosphorus concentrationwere significant after 7 days and developed more rapidly ifthe water columns were isolated from the sediment. Three replicateenclosures were used per treatment in experiments carried outbetween November 1981 and June 1983 to investigate heavy metaltoxicity. This enabled between-treatment variation caused bythe heavy metal additions to be distinguished from within-treatmentvariation caused by containing the water column within enclosures.For plankton abundance and physico-chemical variables, the within-treatmentcoefficient of variation exceeded ±50% if experimentswere >3 weeks. Containment within enclosures affected theecosystem differently in different seasons but, in general,planktonic populations resembled those in the lake. Cadmium,copper and mercury additions of between 10 and 100 µgl^–1 had little immediate effect upon phytoplankton biomassin summer. In winter, marked reductions occurred in the first2 days of the experiment. Zooplankton was more sensitive thanphytoplankton to the heavy metals in summer, cladoceran androtifer populations declining precipitously, shortly after heavymetal addition in each experiment. Copepods were the most heavy-metal-tolerantzooplankters. As the direct toxicity of the heavy metals subsided,there was disruption of zooplankton predation and nutrient regeneration.In summer, reduced grazing by heavy-metal-sensitive zooplanktoncaused algae to proliferate, whereas in winter, inorganic nutrients,which accumulated in the treatments due to heavy-metal-induceddecreases in plankton biomass, later supported larger populationsof algae than would otherwise have occurred. Low nutrient concentrationsin summer probably reflected rapid uptake in productive conditions.Mercury persisted in the water for a shorter period than dideither cadmium or copper, and accumulated more rapidly in periphyton.Copper remained longest in the water column and was largelyassociated with suspended particulate material. Cadmium alsopersisted in the water column but remained largely in solution.Heavy metal accumulation in sediment was not evident, but mayhave been masked by high background concentrations.     

Phytochrome-induced Germination, Endosperm Softening and Embryo Growth Potential in Datura ferox Seeds: Sensitivity to Low Water Potential and Time to Escape to FR Reversal

In Datura ferox seeds, the far-red absorbing form of phytochrome(Pfr) induces endosperm softening, larger embryo growthpotential,and germination. We investigated the effect of exposing theseeds to a range of water potentials in the presence of Pfronits induction of these responses. In addition, the escape timeto far-red-light (FR) reversal of the three responses wasdetermined. Low water potential inhibited Pfr action on endosperm softeningand germination in a similar way. In both cases, a 50% reductionin the response to a saturating red-light (R) irradiation wasobserved at a water potential of c. —0·5 MPa andtherewas very good correlation between the percentage numberof seeds with softened endosperm at 45 h after R and germinationat 72 h after R (R2=0·95). In contrast, the effect ofdecreasing the external water potential on Pfr induction ofa larger embryogrowth potential was more complex. Moderate decreasesin water potential (—0·3 to —0·5 MPa)enhanced Pfr action and thegrowth potential of the embryos waslarger (20—25%) than the water controls; water potentialsbelow —0·7 MPa inhibited the Pfr stimulus. The escape time to FR reversal of the R effect was shorter forthe increase in embryo growth potential than for endospermsoftening.Twenty-four h after R, the embryo response had escaped in morethan 80% of the population whereas endospermsoftening and germinationwere susceptible to FR inhibition in 100% of the seeds. These results indicate that in D. ferox seeds the increase inembryo growth potential is not sufficient for germination andthatendosperm softening is a necessary condition. Key words: Germination, dormancy, phytochrome, endosperm softening, water potential     

Novel bioreactor technology for mass propagation of potato microtubers

Potato (Solanum tuberosum L. ssp. tuberosum) microtubers were produced in vitro with Liquid Lab™ Rocker system. A thin-layer liquid culture was applied together with a regular pitch in autoclavable simple plastic vessels. All cultures were carried out at room temperature without contamination problems. Each cultivar tested (Asterix, Timo, Van Gogh, Velox) formed microtubers in the Liquid Lab system. The mean number of microtubers per vessel (50 explants) varied between 30 (cv. Asterix in 8 weeks tuber induction) and 75 microtubers (cv. Velox in 11 weeks tuber induction). Majority (63%) of the microtubers was sufficient by size and weight (above 200 mg) for further storage at dormancy (4°C). The cv. Velox yielded the highest number of microtubers with cultivation capacity. As a result of prolonged microtuber induction of 2–3 weeks, more microtubers with competence for cultivation were obtained per cultivar, except for cv. Van Gogh. Still, the mean weight of Van Gogh microtubers was significantly higher after prolonged microtuber induction (0.67 g) than after short induction (0.51 g). In conclusion, Liquid Lab™ Rocker system is a novel, efficient and rapid system for mass propagation of potato.     

Magnesium deficiency of kiwifruit (Actinidia deliciosa)

Magnesium deficiency was associated with large yield reductions in a five-year-old commercial kiwifruit (Actinidia deliciosa) orchard. The effect on yield resulted primarily from a reduction in fruit numbers, there being no difference in mean fruit weight between fruit harvested from affected and unaffected vines. Magnesium deficiency had no deleterious effect on postharvest storage characteristics of fruit stored at 0.5–1°C for 18 weeks; fruit from deficient vines were firmer but had slightly lower soluble solids than fruit from control vines. Although deficiency symptoms were first observed on the basal leaves of the non-fruiting shoots mid season, indications of the impending deficiency could be established very early in the season using foliar analysis. Magnesium concentrations in youngest fully expanded leaves (YFEL) on the affected vines were less than 2.0 g kg⁻¹ DM four weeks after budbreak and remained below this value for the rest of the season; concentrations in YFEL on unaffected vines did not decrease below this value and gradually increased after fruitset to 4.5 g kg⁻¹ DM at harvest. To avert potential production losses, it is suggested that soluble magnesium fertilizers (containing at least 200 kg ha⁻¹ Mg) should be broadcast early in the season if foliar magnesium concentrations less than 2.0 gkg⁻¹ DM are measured four–six weeks after budbreak.     

Role of an N-Terminal Splice Segment in the Activation of the Cation Channel TRPM2 by ADP-Ribose and Hydrogen Peroxide

In the dysfunctional splice variant TRPM2-ΔN, a stretch of 20 amino acids (aa 537–556) is missing within the N-terminal cytosolic tail of the cation channel TRPM2. The ΔN-stretch overlaps with two IQ-like calmodulin-binding domains. Moreover, it contains two PxxP motifs implicated in protein–protein interactions. Here, we constructed variants to test whether any of these motifs may explain why TRPM2-ΔN does not respond to stimulation with either ADP ribose or hydrogen peroxide. Each of the two IQ-motifs could be removed without loss of channel function. Similarly, deletion of either one or both PxxP motifs had no effect. Moreover, the single point mutation D543E associated with bipolar disorder does not change the activation of TRPM2. We conclude that no functional role can be attributed to any of the structural motifs within the ΔN-stretch that may be a spacer segment for other functional sites in the N terminus.     

The effect of selenium on the localization of autometallographic mercury in dorsal root ganglia of rats

The autometallographic technique was used to demonstrate the localization of mercury in dorsal root ganglia of adult Wistar rats. The animals were either exposed to mercury vapour, 100 μg Hg m⁻³, 6 h day⁻¹, 5 days per week, or treated with organic mercury in the drinking water, 20 mg CH₃HgCl per litre, for 4 weeks. The effect of orally administered sodium selenite on the pattern of intracellular distribution of mercury in these two situations was investigated. In rats exposed to mercury vapour alone, faint staining was present in ganglion cells. The selenite induced a conspicuous increase in the number of stained cells and in the intracellular staining intensity. In rats treated with organic mercury, mercury deposits were detected within ganglion cells and macrophages. The number of mercury-containing cells was increased by co- administration of selenite. In addition, satellite cells, the capsule and vessel walls were faintly stained. Twenty weeks after cessation of the organic mercury treatment, mercury staining was reduced. Again, selenite treatment enhanced staining intensity. When studied using the electron microscope, mercury was restricted to lysosomes, irrespective of treatments. The present study shows that the deposition of autometallographic mercury in the dorsal root ganglia depends on the chemical type of mercury, the co-administration of selenite and the length of the survival period. This revised version was published online in November 2006 with corrections to the Cover Date.     

Water-storage capacity ofThuja,Tsuga andAcer stems measured by dehydration isotherms

Water-storage capacity was measured inThuja occidentalis L.,Tsuga canadensis (L.) Carr., andAcer saccharum Marsh. during the dehydration of stem segments 1.5–2.5 cm in diameter. Stem water potential was measured with a temperature-corrected stem hygrometer and cavitations were detected acoustically. Water loss was measured by weight change. Dehydration isotherms consistently displayed three phases. The first phase, from water potential (Ψ) 0 to about −0.2 MPa, had a high capacitance (C>0.4kg water lost· (1 of tissue)⁻¹· MPa⁻¹) and we have attributed this high C to capillary water as defined by Zimmermann (1983, Xylem structure and the ascent of sap, Springer-Verlag). The second phase from Ψ=−0.5 to about −2.0 had the lowest C values (<0.02 kg·l⁻¹·MPa⁻¹) and was accompanied by a few cavitation events. This phase may have been a transition zone between capillary storage and water released by cavitation events as well as water drawn from living cells of the bark. The third phase also had a high C (about 0.07–0.22kg·l⁻¹·MPa⁻¹) and was associated with many cavitation events while Ψ declined below about −2.5 MPa; we presume the high capacitance was the consequence of water released by cavitation events. We discuss the ecological adaptive advantage of these three phases of water-storage in trees. In moist environments, water withdrawn from capillary storage may be an important fraction of transpiration, but may be of little adaptive advantage. For most of the growth season trees draw mainly on elastic storage, but stem elastic storage is less than leaf elastic storage and therefore unlikely to be important. In very dry environments, water relased by cavitation events might be important to the short-term survival of trees.     

Do tree-ring traits reflect different water deficit responses in young poplar clones (Populus × canadensis Mönch ‘I-214’ and P. deltoides ‘Dvina’)?

Poplar clones are known to display a wide range of tolerance to drought and water-use efficiency, but the effects of water deficit on stem growth and tree-ring characteristics are rarely taken into account. This study was conducted in order to investigate whether the main tree-ring traits correlate with irrigation regimes during the growing season in ‘I-214’ and ‘Dvina’ 4-year-old poplar clone saplings grown in concrete tanks, during three consecutive years. Total carbon, stable carbon isotope, Klason lignin and α-cellulose contents were analyzed to characterize wood biochemistry; ring width, wood density, mean vessel density and mean vessel lumen area were analyzed to characterize wood anatomy to assess the influence of irrigation regime. In both clones, wood formed in 2005 was more enriched in ¹³C, suggesting drought-induced stomatal closure. Wood formed in 2006 was less variable in δ¹³C in relation to irrigation regimes. ‘Dvina’ showed higher Klason lignin content and wood density than ‘I-214’, whatever the irrigation regime, despite the larger ring widths. ‘Dvina’ has the potential to recover promptly after drought stress, but at the expense of poor wood technological properties, while ‘I-214’ could continue to grow more uniformly under limited water availability, though at a lower rate.     

Water source partitioning among trees growing on shallow karst soils in a seasonally dry tropical climate

The sources of water used by woody vegetation growing on karst soils in seasonally dry tropical regions are little known. In northern Yucatan (Mexico), trees withstand 4–6 months of annual drought in spite of the small water storage capacity of the shallow karst soil. We hypothesized that adult evergreen trees in Yucatan tap the aquifer for a reliable supply of water during the prolonged dry season. The naturally occurring concentration gradients in oxygen and hydrogen stable isotopes in soil, bedrock, groundwater and plant stem water were used to determine the sources of water used by native evergreen and drought-deciduous tree species. While the trees studied grew over a permanent water table (9–20 m depth), pit excavation showed that roots were largely restricted to the upper 2 m of the soil/bedrock profile. At the peak of the dry season, the δ¹⁸O signatures of potential water sources for the vegetation ranged from 4.1 ± 1.1‰ in topsoil to −4.3 ± 0.1‰ in groundwater. The δ¹⁸O values of tree stem water ranged from −2.8 ± 0.3‰ in Talisia olivaeformis to 0.8 ± 1‰ in Ficus cotinifolia, demonstrating vertical partitioning of soil/bedrock water among tree species. Stem water δ¹⁸O values were significantly different from that of groundwater for all the tree species investigated. Stem water samples plotted to the right of the meteoric water line, indicating utilization of water sources subject to evaporative isotopic enrichment. Foliar δ¹³C in adult trees varied widely among species, ranging from −25.3 ± 0.3‰ in Enterolobium cyclocarpum to −28.7 ± 0.4‰ in T. olivaeformis. Contrary to initial expectations, data indicate that native trees growing on shallow karst soils in northern Yucatan use little or no groundwater and depend mostly on water stored within the upper 2–3 m of the soil/bedrock profile. Water storage in subsurface soil-filled cavities and in the porous limestone bedrock is apparently sufficient to sustain adult evergreen trees throughout the pronounced dry season.