The Diatom Genus Tabellaria: I. Taxonomy and Morphology

A morphological study of natural populations of Tabellaria infresh water suggested that there were two fundamentally differenttypes of growth (and therefore frustule structure) in the genus,and this has been confirmed by means of unialgal clone cultures.The existence of these different types of growth forms the basisfor a revision of the genus. A diagnosis of T. quadriseptatanov. spec, is given, the specific limits of T. fenestrata andT. flocculosa are redefined, and the occurrence of T. binalisin Britain is reported for the first time.     

Involvement of tRNA Bound Cytokinin on the Gall Formation in Zizania

To understand the role of the conjugated forms of cytokininin gall formation, tRNA was extracted and purified from vegetativeand flowering shoots of Zizania, then hydrolysed by RNase T,or acid to release the conjugated forms of cytokinin. The resultsindicated that the level of cw-zeatin riboside fromZizania galltissue was 3 to 5 times higher than that of flowering Zizania,and the cytokinin activity in terms of kinetin equivalence was1.6 times higher in Zizania gall tissue than that of floweringZizania. It was found that higher amounts of cytokinin alsopromote protein synthesis. Due to the difference in levels ofthe conjugated forms of cytokinin between vegetative and floweringZizaniashoots, it is implied that the conjugated form of cytokininmay play an important role during the process of gall formationin Zizania latifolia. Key words: Gall formation, cytokinin, Zizania latifolia, RNA, smut fungus, water oat     

Immunological Studies on the Tuber-bearing Solanums: II. Relationships of the North American Species

Investigations using the Ouchtlerlony double-diffusion techniqueand immuno-electrophoretic analysis showed a much greater numberand diversity of antigens amongst the North American speciesof Solomon than had been found previously amongst the SouthAmerican ones. Double diffusion against antisera to S. tuberosum,S. acaule, S. iopetalum, S. bulbocastanum, and S. cardiophyllumsubsp. ehrenbergii, showed a difference between the speciesin series Bulbocastana and Pinnati-secta, and those in seriesLongipedicellata and Demissa. This technique did not providemuch information on the similarities or differences betweenthe species within these two groups. However, immuno-electrophoreticanalysis of the same species with the antisera to S. bulbocastanumand to S. iopetalum gave results which were easier to analyse,and provided much greater resolution of the individual species. At least one identical antigen was found in all the speciesstudied. Some antigens were consistently present in all speciesin certain series, while others had no apparent significancein their occurrence. Three fast-moving antigens were found tohave different, but characteristic, relative mobilities in differentspecies in the series Bulbocastana and Pinnatisecta. The relativemobilities of these three components were similar in the twosubspecies of S. cardiopkyllum and in S. sam-budnum which isconsidered to be a hybrid of subsp. ehrenbergii with S. pinnatisectum. A separate study on S. moreUiforme and S. clarum emphasizedthe distinctness of S. moreUiforme from all other potatoes,but indicated that S. clarum was more closely related to S.bulbocastanum and subsp. ehrenbergii than to S. tuberosum. With antisera to S. tuberosum and S. iopetalum on the one hand,and to S. bulbocastanum on the other, extracts of S. polyadeniumwere found to react more strongly than the species in seriesBulbocastana and Pinnatisecta with the first two antisera, andmore strongly than those in Longipedicellata and Demissa withthe other antiserum. This indicated that S. polyadenium hadsome of the group-specific antigens from both groups, and therefore,that it may be in some intermediate position between these twomajor groups of species. Problems of evolutionary relationship between Mexican speciesand series are discussed in the light of serological, morphological,arid cytogenetical evidence.     

Spatial heterogeneity and dynamics of vernal phytoplankton species in the Baltic Sea in April-May 1986

The mesoscale phytoplankton distribution was studied as partof an international joint Baltic Sea Patchiness Experiment (PEX'86)during April–May 1986 in the open Baltic Proper. The studyperiod covered the peak phase of the phytoplankton spring bloom.The spatio-temporal dynamics of four dominating phytoplanktonspecies, Achnanthes taeniata, Chaetoceros spp., Skeletonemacostatum and Thalassiosira levanderi were studied within anarea of 20x40 nmi with grids of 2 and 4 nmi spatial resolution.The results showed highly varying spatial distributions forall species, and the variability was accentuated on the synopticspace scale before the development of the seasonal thermocline.The maxima of Chaetoceros spp. and Thalassiosira levanderi coincidedwith the anticyclonic and cyclonic eddies prevailing in thearea. Skeletonema costatum was found in high abundances onlywithin a warmer water mass of higher salinity advecting intothe area. The results pointed out that different successionalstages can simultaneously be found even in adjacent water massesand both the phytoplankton growth and composition during thebloom peak phase in the Central Baltic depend on complex factors,mainly those connected with mesoscale hydrodynamic features(eddies, frontal zones, jet currents).     

Small-scale distribution of macroplankton and micronekton in the Ligurian Sea (Mediterranean Sea) as observed from the manned submersible Cyana

A series of eight submersible dives (the MIGRAGEL I cruise)was made during late April 1986 using the French submersibleCyana to investigate macrozooptankton in the upper 400–700m of the water column. Paired day and night dives were madeat stations 3, 6, 13 and 23 nautical miles off Cape Ferrat,near Villefranche-sur-Mer, France; the distances represent differentareas in the frontal system of the Ligurian Sea. Detailed day/nightvertical distribution data are shown for the most abundant species;these include the narcomedusa Solmissus albescens, teleost fishCyclothone spp., small appendiculanans (primarily Oikopleuraalbicans), large appendicularians (an undescribed oikopleurid),diphyid siphonophores (mostly Chelophyes appendiculata) andan abundant lobate ctenophore. Salps, pyrosomes, amphipods (Phroniumasedentaria), pteropods (Cavolinia inflexa), macroscopic ‘star-like’protozoa and marine snow are also briefly discussed. The coastalzone was dominated by small appendicularians in the upper layers,with other filter feeders including large appendicularians indeeper water—these just above a non-migratory populationof carnivorous Cyclothone. The carnivorous medusa Solmissusalbescens moved throughout the upper 600 m in the course ofits diel vertical migration. Offshore, carnivores were dominantthroughout the water column, with numerous diphyid siphonophoresin the upper layers, and Cyclothone, lobate ctenophores andmacroprotozoa abundant in deeper water. Solmissus was also present,and was more numerous offshore than in the coastal zone.     

Molecular evolution of ribulose-1, 5-bisphosphate carboxylase

The structural homology of the two constituent subunits (A andB) of ribulose- 1,5-bisphosphate carboxylase from various originswas determined using the statistical method of Marchalonis andWeltman [Comp. Biochem. Biophys. 38B, 609–625 (1971)].It was found that the large catalytic subunit (A) is structurallyhomologous among the enzymes of divergent origins, from primitivephotosynthetic bacteria (Bacteriophyta) through the green algae(Chlorophyta) to higher plants (Tracheophyta). In contrast,the small regulatory subunit (B) was found to be structurallyquite different among the different species. The genetic conservationof subunit A during the phylogenetic evolution of the ribulose-1,5-bisphosphatecarboxylase molecule indicates its origin from a common ancestralgene. ¹ This is paper XXXIII in the series "Structure and Functionof Chloroplast Proteins". (Received July 23, 1975; )     

Influence of Endophyte and Water Regime Upon Tall Fescue Accessions. I. Growth Characteristics

Superior growth and persistence has been reported in endophyte-infectedgrasses; however, findings may have been confounded by experimentconditions including plant genotype. A controUed-environmentstudy was designed to address some growth characteristics offour tall fescue (Festuca arundinacea Schreb.) accessions asinfluenced by endophyte (Acremonium coenophiahim Morgan-Joneset Gams) and water regime. Endophyte-infected plants were collected,vegetatively propagated and some treated with propiconazole(11 kg a.i. ha^–1) to develop non-infected isolines ofeach accession. The phenotypically diverse accessions, eachrepresented by infected and non-infected isolines, were grownwith adequate (–0–03 MPa), or a series of deficit(<–1·5 MPa) and recovery water regimes, replicatedthree times. Plant growth characteristics were measured during(leaf elongation and tillering) and upon conclusion (phytomassproduction, tillering, and leaf area) of the study. Leaf elongation,as a function of leaf length, was significantly different amongaccessions, and generally decreased with water deficit althoughsome non-infected isolines were not affected. Water deficitdepressed tiller production in virtually all accessions whileendophyte effects depended upon accession. Leaf blade yieldwas not significantly influenced by endophyte status or interactionof endophyte, with water regime and accession; however, pseudostem(stem base and leaf sheath), root and dead leaf yields wereaffected in some cases. Non-structural carbohydrate concentrationin all plant parts except roots, was decreased by water deficit,whereas root non-structural carbohydrate concentration tendedto increase with water deficit. Non-structural carbohydratesof all plant parts was not influenced by endophyte status. Tallfescue-endophyte association responses vary due to genotype,therefore a simple generalization of endophyte impact upon tallfescue productivity and persistence is not possible based uponthe results of this study Festuca arundinacea Schreb., Acremonium coenophiahim Morgan-Jones et Gams, leaf elongation, phytomass production, tillering, water deficit, non-structural carbohydrate     

The Effect of Salinity Stress upon Protein Synthesis of Germinating Wheat Embryos

Two differently salt-sensitive wheat genotypes were imbibedin 0·4 M NaCl for 72 h or, alternatively, for 48 h andthen transferred to water. Seed germination, fresh weight andprotein synthesis in embryos were determined. The followingdifferences were found in the synthesis of in vivo [³⁵S]methionine-labelledproteins during salt imbibition: (a) a general decrease or disappearanceof polypeptides specific to the radicle emergence phase in thesalt-sensitive genotype; (b) a new synthesis of polypeptideswhich are not found during water imbibition and are common toboth genotypes; (c) a differential synthesis of polypeptidesthat are unique to each cultivar. Upon return to water, salt-inducedproteins ceased to be synthesized while proteins associatedwith an advanced germination phase were actively produced. Theseresults suggest that the expression of 'salt stress' proteinsis related to the adaptation process of seeds to salinity aswell as to the genetic constitution of a selected salt-tolerantgenotype.Copyright 1993, 1999 Academic Press Triticum durum, wheat, embryo, salt stress, protein synthesis     

Metabolism and elemental composition of aggregate and solitary forms of Salpa thompsoni (Tunicata: Thaliacea) in waters off the Antarctic Peninsula during austral summer 1999

Oxygen consumption, ammonia excretion and phosphate excretionrates were measured on the aggregate form (aggregated sexualblastozooid generation) and solitary form (solitary asexualoozooid generation) of Salpa thompsoni sampled from waters offthe Antarctic Peninsula from December 1999 to February 2000,in conjunction with body composition analysis (water, ash, carbon,nitrogen and phosphorus). The specific metabolic rates of S.thompsoni were weight-independent. No significant differencesbetween the aggregate and solitary forms were observed for theweight-specific rates with the exception of the oxygen consumptionrate on phosphorus weight. Metabolic loss estimated for theaggregate and solitary forms was 3.0 and 3.5% day^–1 forbody carbon, 1.6 and 1.8% for body nitrogen, 3.9 and 10.6% forbody phosphorus, respectively. Contents of carbon (6.0% of dryweight for the aggregate form and 5.1% for the solitary form),nitrogen (1.5 and 1.3%) and phosphorus (0.15 and 0.11%) tendedto decrease with the increase of dry weight. All mean valuesof each body constituent (water, ash, C, N and P) for the aggregateand solitary forms were not significantly different in the samedry weight ranges.     

Diurnal change in the relationship between stomatal conductance and abscisic acid in the xylem sap of field-grown peach trees

To evaluate whether abscisic acid (ABA) in the xylem sap playsan important role in controlling stomatal aperture of field-grownPrunus persica trees under drought conditions, stomatal conductance(g) and xylem ABA concentrations were monitored both in irrigatedand non-irrigated trees, on two consecutive summer days (threetimes a day). Stomata1 conductance of non-irrigated trees hada morning maximum and declined afterwards. The changes in gduring the day, rather than resulting from variations in theconcentrations of ABA in the xylem sap or the delivery rateof this compound to the leaves, were associated with changesin the relationship between g and xylem ABA. The stomata ofwater-stressed trees opened during the first hours of the day,despite the occurrence of a high concentration of ABA in thexylem sap. However, stomatal responsiveness to ABA in the xylemwas enhanced throughout the day. As a result, a tight inverserelationship between g and the logarithm of xylem ABA concentrationwas found both at midday and in the afternoon. A similar relationshipbetween g and ABA was found when exogenous ABA was fed to leavesdetached from well-watered trees. These results indicate thatABA derived from the xylem may account for the differences ing observed between field-grown peach trees growing with differentsoil water availabilities. Several possible explanations forthe apparent low stomatal sensitivity to xylem ABA in the morning,are discussed, such as high leaf water potential, low temperatureand high cytokinin activity. Key words: Prunus persica L., stomata, xylem ABA, water deficits, root-to-shoot communication     

Interrelations between the vertical distribution of Daphnia and chlorophyll a in two large limnetic enclosures

This paper summarises the major factors influencing the daytimevertical distribution of Daphnia hyalina var. laciutris (Sars)in two large experimental enclosures (Lund Tubes). In both tubes,the depth at which the Daphnia aggregated during the day wasclosely related to sub-surface ir-radiance. On a few occasionsaggregations of Daphnia were found in ‘dark’ waterassociated with a deep chlorophyll maximum. On many occasions,however, the animals' light response per se ensured their aggregationat depths of maximum phytoplankton abundance. The most importantfactor influencing the dispersion of animals in the water columnwas the steepness of the light gradient. In turbid water verticalaggregations were well defined, whereas in clear water the animalswere widely dispersed around their depth of maximum abundance.Daphnia also tended to disperse in the water column when theirpopulation density was high or when food was scarce. A simplemodel based on surface irradiance, water turbidity and populationdensity explained the basic pattern of vertical distributionthroughout the season. The implications of these findings arediscussed in relation to the effects of zooplankton grazingand speculations on the adaptive significance of depth regulation.     

Potential Dry Matter and Water Import Rates in the Tomato Fruit in Relationship to Fruit Size

The tomato fruit was compared to a sphere with a radius R. Radialgrowth rates in the fruit (F_IW and F_ID) due to water importor to dry matter import, respectively, which are also the waterimport rate or dry matter import rate per unit surface areaof fruit, were calculated from two sets of published results.This data referred to fruits which swelled in such a way thatthe availability of assimilates had little effect on growth.Two varieties differentiated the two series of results and inone series, three trials were differentiated by the salinityof the nutrient solution. In all trials, it was found that F_IW and F_ID decreased whenR increased. Two phases were observed for F_IW: after a firstphase, F_IW decreased more quickly and almost linearly when Rincreased. F_ID was constant or decreased with respect to R.Except at the beginning of growth at the greatest salinity,there were clearly linear regressions between F_IW and F_ID suchas F_ID = aF_IW-b; where b was lower with higher salinity. Thechanges of the concentration of imported dry matter (F_ID/F_IWwere examined in terms of R and F_IW/R. The mechanisms controllingthe changes in F_IW and F_ID were discussed. The results suggestedfruit radius was an important parameter of these mechanisms.Thus, water import rate and dry matter import rate could eachbe considered to be the product of two factors: fruit surfacearea, which is directly dependent on fruit radius, and waterimport rate or dry matter import rate per unit of fruit surfacearea.Copyright 1993, 1999 Academic Press Dry matter, fruit growth, logistic model, sink size, tomato, water transfer     

Seasonal Production of Secondary Phloem in the Twigs of Certain Tropical Timber Trees

Secondary phloem production in four deciduous (Albizzia lebbeck,Dalbergia sissoo, Tectona grandis and Terminalia crenulata)and three evergreen plants (Calophyllum inophyllum, Mangiferaindica and Morinda tinctoria) is briefly described. The totalduration of phloem production for each year was worked out forall these plants. In three of the four deciduous trees therewere two instalments of phloem production in correspondencewith the presence of two flushes of cambial activity while inTectona grandis and in all the three evergreen trees there wasonly one instalment. The time of initiation and cessation ofphloem tissue production was found to be variable in the differentplants studied. Periodicity in the production of different componentsof phloem tissue as well as the difference in the dimensionsof the different phloic elements produced during each flushof cambial activity resulted in detectable growth increments(or ‘rings’) within the phloem. There was a distinctshortening of the different phloem elements during the approachof dormancy/least activity. Conspicuous changes were found inthe ergastic contents of phloem parenchyma and ray cells adjacentto the cambial zone during the initiation of cambial activityand during the approach of dormancy/least activity. Seasonal growth, secondary phloem, deciduous and evergreen trees, cambial activity     

Ecological studies on the phytoplankton of Korsfjorden, western Norway. The dynamics of a spring bloom seen in relation to hydrographical conditions and light regime

The investigation was carried out at one station in Korsfjorden,a typical deep silled fjord of western Norway. During 14 cruisesfrom 4 February to 30 June 1977 ATP, chlorophll a, phaeopigmenta, and in situ ¹⁴C-assimilation were measured in the net (>30µm), nano and ultraplankton (<5 µm). Sampleswere collected from five light depths within euphotic zone.The impact of hydrographical conditions and light regime onthe bloom dynamics was also studied. In the periods 4 February-7 March and 13 April-30 June, ultraplankton contributed >60%to the total primary production while net and nanoplankton dominatedfrom 7 March to 13 April. The diatoms Skeletonema costatum,Chaetoceros compressits and C. debilis, and Rhizosolenia hebetatavar. semispina made up the main part of the biomass on 21 March,28 March and 4 April respectively. A shade adapted diatom societywas located at the top of the nutricline in late June with S.costatum, Chaetoceros spp., and Thalassiosira spp. as the dominantspecies. The highest assimilation number of eight for the netplankton and four for the ultraplankton were found at the depthof 32% light intensity on 28 March and 24 May respectively.Linear relationships were found between chlorophyll a and ATPfor the different size fractions with regression slopes rangingfrom 4.3 to 5.8. The total primary production for the periodof investigation was calculated to 74 g C m^–2. Light regimeand water column stability were decisive factors for the outburstof the first diatom bloom in late March. Grazing on net planktondiatoms increased during late March-early April. Changes inthe longshore wind-stress component were found to be essentialfor the understanding of the bloom dynamics.     

Genome Comparison In Silico in Neisseria Suggests Integration of Filamentous Bacteriophages by their Own Transposase

We have identified filamentous prophages, Nf (Neisserial filamentousphages), during an in silico genome comparison in Neisseria.Comparison of three genomes of Neisseria meningitidis and oneof Neisseria gonorrhoeae revealed four subtypes of Nf. Elevenintact copies are located at different loci in the four genomes.Each intact copy of Nf is flanked by duplication of 5'-CT and,at its right end, carries a transposase homologue (pivNM/irg)of RNaseH/Retroviral integrase superfamily. The phylogeny ofthese putative transposases and that of phage-related proteinson Nfs are congruent. Following circularization of Nfs, a promoter-likesequence forms. The sequence at the junction of these predictedcircular forms (5'-atCTtatat) was found in a related plasmid(pMU1) at a corresponding locus. Several structural variantsof Nfs—partially inverted, internally deleted and truncated—werealso identified. The partial inversion seems to be a productof site-specific recombination between two 5'-CTtat sequencesthat are in inverse orientation, one at its end and the otherupstream of pivNM/irg. Formation of internally deleted variantsprobably proceeded through replicative transposition that alsoinvolved two 5'-CTtat sequences. We concluded that the PivNM/Irgtransposase on Nfs integrated their circular forms into thechromosomal 5'-CT-containing sequences and probably mediatedthe above rearrangements.     

Photoinhibition and the diurnal variation of phytoplankton photosynthesis--I. Development of a photosynthesis--irradiance model from studies of in situ responses

Diurnal series of fluorescence and photosynthesis assays wereconducted in high altitude (3803 m), tropical (16°), LakeTiticaca (Peru/Bolivia). Near-surface diurnal thermoclines formedon typical days of high photon flux density (PFD, {small tilde}2000 µE m^–2 s^–1). In the depth range of diurnalstratification profiles of in vivo fluorescence, both without(F_a and with (F_b DCMU, exhibited a mean decrease of 64% frommorning to mid-day, but little change (mean increase of 1.5%)through the afternoon. Three times during the day surface, mid-depth(3–5 m) and deep (15–20 m) phytoplankton sampleswere incubated with H¹⁴CO₃^– under short (<2 h) exposuresto a range of in situ PFDs. Comparison of phytoplankton in differentsamples (ANOVA) showed identical photosynthetic response insunrise (isothermal) samples but a significant drop in surfaceand mid-depth photosynthesis at all PFDs during times of diurnalstratification. Similarly, both low-light () and light-saturated(P² _max photosynthetic parameters were lower in mid-day surfacesamples compared to deep samples. In addition, previously photoinhibitedsamples had a higher threshold intensity for photoinhibition,I_T. These results, together with diurnal time series of fluorescencefrom in situ incubations, demonstrate that recovery from extendedepisodes of photoinhibition during diurnal stratification isslower than suggested by previous observations in vitro. Photosynthesisby near-surface phytoplankton is different in light increasingup to I_T than light decreasing from I_T. This effect can be modeledby reducing and P_max as a function of the maximum photoinhibitingPFD in the diurnal light history. ¹Present address: Division of Molecular Plant Biology, Universityof California, Berkeley, Berkeley, CA 94720, USA     

Ecophysiological Aspects of the Woody Root HemiparasiteSantalum acuminatum(R. Br.) A. DC and its Common Hosts in South Western Australia

The water and nitrogen relationships of the xylem-tapping roothemiparasite, quandong (Santalum acuminatum) and its principalhosts were examined at a series of sites in native coastal heathlandsof south west Australia. Assessments based on densities of above-groundbiomass, ground cover and frequencies of haustoria on host rootsindicated that woody N₂fixers (legumes andAllocasuarina) wereprincipal hosts ofSantalum. ¹⁵N values for shoot dry matterof component species suggested these N₂fixers were strongly(70% or more) dependent on atmospheric N and thatSantalumderivedN principally from these species. Structural studies of haustoriashowed the interface with host xylem to be almost entirely comprisedof parenchymatous tissue. No luminal continuities were observedbetween xylem conducting elements of the partners. Formationand functional life of haustoria were closely coordinated withseasonal growth of hosts, with some haustoria surviving summerand overlapping functionally with new ones establishing in thenext autumn. Transpiration and photosynthetic rates of the parasitewere consistently less and water use efficiencies very similarto those of the principal hostAcacia rostellifera. ¹³C valuesof foliage ofSantalumand this host were similar, but large variationsin ¹³C values for above-ground dry matter of parasite and hostsbetween study sites prevented evaluations of water stress orwater-use-efficiency based on carbon isotope discrimination.Specific hydraulic conductivities of roots ofSantalumwere consistentlylower than those ofAcacia, a finding consistent with more conservativewater use by the parasite than the host. Santalum acuminatum; root hemiparasite; ¹⁵N discrimination; water relations; haustorial structure; root conductivity     

Gas Exchange and Water Relations of the Root Hemi-parasiteSantalum albumL. in Association with Legume and Non-legume Hosts

This paper examines foliar nitrogen (N) levels, photosynthesis,transpiration, water use efficiency and tissue water relationsof the xylem-tapping root hemi-parasiteSantalum albumin potculture with various N₂-fixing woody hosts, a non-fixing host(a eucalypt), or in the absence of a host. Foliar N concentrationsofSantalumwere significantly greater than corresponding hostsand higher when on N₂-fixing hosts than on the eucalypt, orwithout a host. Strong positive relationships were evident inSantalumbetweenfoliar N concentration, rates of net photosynthesis and instantaneouswater use efficiencies. Photosynthesis rate and water use efficiencyofSantalumwere generally lower than in corresponding hosts,but transpiration rates were not noticeably different betweenassociations.     

A TYROSINASE-LIKE ENZYME IN CEPAEA NEMORALIS AND ITS POSSIBLE RELATION WITH SHELL BANDING POLYMORPHISM

Shell banding polymorphism in Cepaea nemoralis may be relatedto tyrosinase activity in this gastropod, as the hyalozonatephenotype shows bands with no pigmentation. We aimed to establishif there is tyrosinase activity in Cepaea, and this activitywas analyzed for different organs on electrophoresis gel slabsand oxygen electrode. We have found two isocnzymatic forms,one of them polymorphic, with high activity in the foot andmantle, organs both involved with the pigmentation of body andshell. We propose that several isoenzymatic forms are functional inthe melanosome. These may be activated during the cell lysis,as hyalozonate samples display tyrosinase activity. (Received 22 June 1988; accepted 5 September 1988)     

Aspects of the Water Relations of Ileostylus micranthus (Hook. f.) Tieghem, a New Zealand Mistletoe

Leaf water potentials in the mistletoe, Ileostylus micranthusgrowing outdoors decreased rapidly during the early part ofthe day but remained relatively steady in the early afternoondespite increases in atmospheric vapour pressure deficit (vpd).Minimum water potentials of the mistletoe were relatively constant.They were held at values lower than those of hosts when thelatter maintained high water potentials but approached or evenexceeded those of hosts when they developed low water potentials.In contrast, cut shoots of Ileostylus usually maintained higherwater contents and leaf water potentials than those of its hostswhen both were desiccated separately in the laboratory. Pressure-volumeanalyses indicated that Ileostylus had lower water potentialat full turgor, a lower water potential but higher relativewater content at turgor loss, and a higher bulk modulus of elasticitythan the following four hosts: the native Kunzea ericoides andCoprosmapropinqua, and the introduced Ribes sanguineum and Teline monspessulana.Water potential at turgor loss (_tlp) was strongly correlatedwith the minimum field water potential of both mistletoes andhosts. When _tlpof mistletoe and host is similar (as on Kunzeaand Ribes) field water potentials are also similar, but when_tlpis lower in the mistletoe (as on Coprosma and Teline), thefield water potential of the mistletoe is lower than that ofits host. Consequently, I. micranthus is likely to be more frequenton hosts that maintain high field water potentials than on hoststhat develop low water potentials. Copyright 1999 Annals ofBotany Company Water relations, water potential, osmotic potential, pressure-volume, Ileostylus micranthus , mistletoe, New Zealand.