Residual nutational activity of the sunflower hypocotyl in simulated weightlessness

Does gravity drive circumnutation? One model ascribes hypocotylcircumnutation to a continuing series of geotropic responseseach of which overshoots thereby maintaining a sustained oscillation.However, some features of the observed movements are not easilyreconciled with the model. The critical feature of this modelis the requirement that circumnutation must exhibit an absolutedependence on a g-force. Experiments with Helianthns annum onhorizontal clinostats demonstrated an 80% reduction in the amplitudeof hypocotyl circumnutation (compared with upright plants atone g) although the oscillations continued at simulated "zerog". It is not certain that the clinostat environment adequatelysimulates the weightless environment of space but, if it does,we may expect a space experiment to demonstrate that hypocotylnutation in Helianthus annuus is not fully dependent on gravity. (Received October 13, 1978; )     

Do the polyunsaturated aldehydes produced by Phaeocystis pouchetii (Hariot) Lagerheim influence diatom growth during the spring bloom in Northern Norway?

The marine bloom-forming phytoplankter Phaeocystis pouchetii(Prymnesiophyceae) is known to produce and release the cytotoxic,ß,,-unsaturated aldehyde 2-trans-4-trans-decadienal(DD), known to inhibit mitotic cell divisions in several differentcell types. The possible allelopathic effects of DD on monoculturesof three common diatoms from the coastal waters of northernNorway were assessed. The results showed that division ratesfor all three diatom species (Skeletonema costatum, Chaetocerossocialis and Thalassiosira antarctica) decreased as concentrationof DD increased. Furthermore field data from the spring bloomin Vestfjorden (2000 and 2001) were analysed to examine whetherthe presence of P. pouchetii influences other species adversely.Our data revealed no significant adverse effect of P. pouchetiion diatom presence since diatom diversity generally was positivelycorrelated to P. pouchetii biomass. The year with the lowestamounts of P. pouchetii had the lowest diversities, and thediatom species composition and abundance was comparable to situationswhere P. pouchetii was either absent or present in minute amounts.At some instances low diversity co-occurred with large fractionsof P. pouchetii, but since this were at the strongest verticalmixing we believe this to be a result of physical control. P.pouchetii and S. costatum were the most frequently co-occurringspecies, and since they are both known producers of polyunsaturatedaldehydes we cannot exclude the possibility that the presenceof DD released from P. pouchetii induced by heavy grazing mightinfluence the growth of other phytoplankton species.     

Immunological Studies on the Tuber-bearing Solanums III. Variability within S. bulbocastanum and its hybrids with species in series Pinnatisecta

It was shown in the previous paper of this series that the Mexicanspecies Solanum bulbocastanum, S. cardiophyllum subsp. cardiophyllumand subsp. ehrenbergii, S. michoacanum and S. pinnatisectummay be differentiated to some extent by the double diffusiontechnique, and more fully by immuno-electrophoretic analysis.Above all, the relative electrophoretic mobilities of certaincomponents, as expressed by the index have been found to be very constant for each species, with the apparent exception of S. bulbocastanum. A detailed investigation of S. bulbocastanum has now shown thatalthough the many clones of this species appeared similar whenexamined by double diffusion, differences were exposed by immuno-electrophoreticanalysis. In particular, the values fell into two discrete groups, 0.15 to 0.23 and 0.30 to 0.46. Theplants in these groups had been collected from Oaxaca and Guatemala,and from Central Mexico, respectively. Most of the former grouphad been identified as S. bulbocastanum subsp. partitum, andall the latter group as subsp. bulbocastanum and subsp. dolichophyllum.Not only was the wide variation in values of index resolved into two meaningful groups, but it was also found thatall lines within any one collection had almost identical values. The index value for S. cardiophyllum subsp.cardiophyllum is 1.0, and for subsp. ehrenbergii is 0.7. The F₁ hybrids between either ofthese subspecies and S. bulbocastanum had fairly uniform valuesclose to the calculated means of the parental values. The shapesof the leaves of these hybrids were also uniform and more orless intermediate between those of the parents in each case. The F₂ hybrids between S. bulbocastanum and S. pinnatisectum,and also the triple hybrids between these two species and eitherS. cardiophyllum or S. michoacanum, showed a wide range of valuesof index and of leaf shapes, with partial segregation towards the parental types. The serological andmorphological characters segregated independently. S. X sambucinum was found to have a wide variation in serologicaland morphological characters. This variation was consistentwith the theory that S. X sambucinum is a natural hybrid betweenS. pinnatisectum and S. cardiophyllum subsp. ehrenbergii. Within the groups of species investigated, the relative electrophoreticmobilities of certain antigens as displayed by immuno-electrophoreticanalysis were found to vary significantly in different species,in different subspecies of the same species, and in specieshybrids.     

Ecophysiology of Niche Occupation by Two Giant Rosette Plants, Lobelia gibberoa Hemsl and Solanecio gigas(Vatke) C. Jeffrey, in an Afromontane Forest Valley

The niche occupation of two giant rosette plants, Lobelia gibberoaHemsl and Solanecio gigas(Vatke) C. Jeffrey, was investigatedin a small mountain valley in an afromontane forest, centralEthiopia. Plant distribution, density, life-form, morphologyand microsite conditions were related to transpiration, chlorophylla fluorescence and ¹³C analysis to explain how ecophysiologicaltraits and morphology determine the niche differentiation ofplants of similar life-forms. L. gibberoa was more abundantat the humid northern flank of the valley while S. gigas wasequally distributed in the northern and drier southern flanks;both species occurred in the valley bottom. Photosynthetic capacity,as determined by chlorophyll a fluorescence, was similar forboth species. In dry locations, S. gigas transpired somewhatless and had lower leaf conductance for water vapour, g, than in wet locations. Both specieshad the highest gand ¹³C-isotopediscrimination in the wettest locations. Overall for the ecophysiologicaltraits, site differences were larger than differences betweenthe two species. L. gibberoa had a well developed vascular cylinder,maintained a large number of rosettes and had an average leafarea index (LAI) of 2.8, which may restrict it to the more humidlocations. S. gigas, with a poorly developed vascular bundle,had many fewer rosettes, an average LAI of 1.5, and reactedmore to the water status of its habitat. It was thus also capableof colonizing drier locations. The role of morphological andanatomical traits and ecophysiological features in niche occupationas revealed by assessment of vegetation is discussed. Copyright2001 Annals of Botany Company Life forms, Lobelia gibberoa, afromontane forest, niche occupation, Solanecio gigas     

Respiration and germination rates of tomato seeds at suboptimal temperatures and reduced water potentials

Suboptimal temperature (T) affects germination rates (reciprocalof time to radicle emergence) on a thermal time basis; thatis, the T in excess of a base or minimum temperature multipliedby the time to a given per cent germination [t_g) is a constant.Respiration rates are also sensitive to T, and proportionalrelationships are often found between respiration rates andgermination rates. Reduced water potential () delays seed germinationon a hydrotime basis (i.e. the in excess of a base water potentialmultiplied by t_g is a constant). It was tested whether respirationrates prior to radicle emergence vary in proportion to T and as expected from the thermal and hydrotime models. Respirationrates (C0₂ evolution) of cold-tolerant, rapidly germinating(PI 341988) and cold-sensitive, more slowly germinating (T5)tomato [Lycopersicon esculentum Mill.) seeds were evaluatedover a range of T and conditions. For both genotypes, respirationrates until the beginning of radicle emergence were relatedto T on a thermal time basis and increased approximately linearlywith above -2.0 MPa, consistent with the hydrotime model. Respirationrates were uniquely related to germination rates, regardlessof whether germination timing was affected by T, , or genotype.However, germination timing was unaffected when respirationrates were manipulated by varying 0₂ partial pressure. Thus,while both germination and respiration rates vary with T and consistent with thermal and hydrotime models of biologicaltime, respiration rates per se were not the limiting factorin germination timing of tomato seeds. Key words: Lycopersicon esculentum Mill., tomato, germination, respiration, temperature, water potential     

Effects of Priming and Endosperm Integrity on Seed Germination Rates of Tomato Genotypes: I. GERMINATION AT SUBOPTIMAL TEMPERATURE

The bases of differences in germination rates (GR_g = inverseof time to germination [t_g] of percentage g) among three cold/salt-toleranttomato (Lycopersicon esculentum Mill.) accessions (PI 341988,PI 120256, and PI 174263) and one cold/salt-sensitive tomatocultivar (T5) were investigated. The effects of seed priming(6 d imbibition in aerated –1.2 MPa polyethylene glycolsolution at 20 ?C followed by redrying) and of removing theendosperm/testa cap covering the radicle on the temperaturesensitivity of GR_g, and the interaction of these treatmentswith genotypes, were also examined. GR_g decreased linearly withdecreasing temperature for all genotypes and seed treatments.The minimum or base temperatures for germination (T_b) variedby 1 ?C among the tomato lines, so genotypic differences inGR_g were due to differing thermal time requirements for germination.The mean thermal time requirement for germination of T5 seeds was 22% and 19% greater than that of PI 341988 andPI 120256 seeds, respectively, but only 9% greater than thatof PI 174263 seeds. Seed priming did not lower Tb of any genotype,but significantly reduced by 24, 49, 41, and 49% in T5, PI 341988, PI 120256, and PI 174263, respectively,indicating that priming increased the rate at which the seedsprogressed towards germination when T>T_b, but did not lowerthe minimum temperature at which germination could occur. Primingincreased the GR_g of T5 seeds to equal or exceed those of control(non-primed) seeds of the cold/salt-tolerant genotypes at anyT>T_b, but the PI lines exhibited an even greater responseto priming. Times to germination within each seed lot were normallydistributed on a logarithmic scale. Priming increased the variancein tg within a seed lot when compared to control seeds. However,the variation in thermal time for germination between the 10thand 90th percentiles of the seed population (_T(10–90))was relatively unaffected by priming due to the reduction in in primed seeds. Removing the endosperm cap and testa opposite the radicle tip decreased almost 6-fold and and reduced T_b by 5 ?C in T5 and PI 341988,implicating processes in the endosperm/testa as the limitingfactors in germination at suboptimal temperatures. Key words: Lycopersicon esculentum Mill., tomato, genetic variation, seed priming, thermal time, germination rate     

Effects of Priming and Endosperm Integrity on Seed Germination Rates of Tomato Genotypes: II. GERMINATION AT REDUCED WATER POTENTIAL

Seed germination rates (GR =inverse of time to germination)are sensitive to genetic, environmental, and physiological factors.We have compared the GR of tomato (Lycopersicon esculentum Mill.)seeds of cultivar T5 to those of rapidly germinating L. esculentumgenotypes PI 341988 and PI 120256 over a range of water potential(). The influence of seed priming treatments and removal ofthe endosperm/testa cap enclosing the radicle tip on germinationat reduced were also assessed. Germination time-courses atdifferent 's were analysed according to a model that identifieda base, or minimum, allowing germination of a specific percentage(g) of the seed population (_b(g)), and a ‘hydrotime constant’(_H) indicating the rate of progress toward germination per MPa.h.The distribution of _b(g) determined by probit analysis was characterizedby a mean base (_b) and the standard deviation in _b among seeds(_b). The three derived parameters, _b, _b) and _H, were sufficientto predict the time-courses of germination of intact seeds atany . A normalized time-scale for comparing germination responsesto reduced is introduced. The time to germination at any (t_g())can be normalized to be equivalent to that observed in water(t_g(0)) according to the equation t_g(0)=[l–(/_b(g))]t_g().PI 341988 seeds were more tolerant of reduced and had a morerapid GR than T5 seeds due to both a lower _b and a smaller _H.The rapid germination of PI 120256, on the other hand, couldbe attributed entirely to a smaller _H. Seed priming (6 d in–1.2 MPa polyethylene glycol 8000 solution at 20 ?C followedby drying) increased GR at all >_b(g), but did not lower theminimum allowing germination; i.e. priming reduced _H withoutlowering _b. Removing the endosperm/testa cap (cut seeds) markedlyincreased GR and lowered the mean required to inhibit germinationby 0.7 to 0.9 MPa. However, this resulted primarily from downwardadjustment in _b during the incubation of cut seeds at low inthe test solutions. The difference in _b between intact and cutseeds incubated at high was much less (0.l MPa), indicatingthat at the time of radicle protrusion, the endosperm had weakenedto the point where it constituted only a small mechanical barrier.In the intact seed, endosperm weakening and the downward adjustmentin embryo _b ceased at < –0.6 MPa, while the reductionin _H associated with priming proceeded down to at least –1.2MPa. Based on these data and on the pressure required to pushthe embryos from the seeds at various times after imbibition,it appears that the primary effect of priming was to shortenthe time required for final endosperm weakening to occur. However,as priming increased GR even in cut seeds, priming effects onthe embryo may control the rate of endosperm weakening. Key words: tomato, Lycopersicon esculentum Mill., water potential, germination rate, seed priming, genetic variation     

Mycorrhizal influence on hydraulic and hormonal factors implicated in the control of stomatal conductance during drought

During drying, mycorrhizal plants often maintain higher stomatalconductance (g_s) than similarly-sized and -nourished non-mycorrhizalplants, but the mechanism of mycorrhizal influence remains unclear.Several hydraulic and non-hydraulic factors previously implicatedin control of stomatal behavior during drought were measured,to learn which are affected when roots of cowpea (Vigna unguiculata[L.] Walp. cv. White Acre) are extensively colonized by Glomusintraradices Schenck and Smith isolate UT143. At low soil watercontents (), mycorrhizal plants maintained higher g_s, transpirationand shoot water potential () than non-mycorrhizal plants. Thesehigher foliar water status characters were associated with lowerxylemsap abscisic acid concentrations ([ABA]) and lower ABAfluxes to leaves in mycorrhizal plants at low soil . Stomatalconductance was most closely correlated with xylem-sap [ABA],ABA flux to leaves and shoot . Stomatal conductance was notcorrelated with xylemsap concentrations of calcium or zeatinriboside equivalents, or with xylem-sap pH, nor were these xylem-sapconstituents affected by mycorrhizal symbiosis. Stomata of mycorrhizaland non-mycorhizal leaves showed similar sensitivities to ABA,whether leaves were intact or detached. It is concluded thatmycorrhizal fungi probably increased the capability of rootsystems to scavenge water in drier soil, resulting in less strainto foliage and hence higher g_s, and shoot at particular soil. Key words: Abscisic acid, cytokinins, Glomus intraradices     

The Osmotic Pressure of Concentrated Solutions of Polyethylene Glycol 6000, and its Variation with Temperature

The vapour pressures of aqueous solutions of polyethylene glycol6000 have been measured (by equilibration with sucrose solutions)up to the saturation point at 25 °C (1.45 g g^–1 water).The reduced-osmotic-pressure (/c), when plotted versus concentration(c), rapidly and linearly increased up to a concentration ofabout 0.8 g g^–1 (crossing the similar plot for sucrose).Above this concentration, the reduced-osmotic-pressure rosemore slowly, but still more rapidly than sucrose. The maximumosmotic pressure achieved at saturation was nearly 18 MPa. Usingthe virial equation: /c= RT/M + RTA₂c, the calculated secondvirial coefficient (A₂) for the linear part is 4.5 x 10^–3mol g^–1, a value slightly greater than most literaturevalues at 25 °C. Data are cited showing that A₂ varies linearlyfrom 5–6 x 10^x3 at 0 °C, to zero at 80–90 °C     

Isozyme Variation in Wild and Cultivated Species of the Genus Trifolium L. (Leguminosae)

Isozyme patterns for PGI (phosphoglucoisomerase), MDH (malatedehydrogenase), EST (esterase) and SOD (superoxide dismutase)were evaluated in 36 accessions of eight Trifolium species,namely T. incarnatum, T. polymorphum, T. pratense, T. repens,T. resupinatum, T. riograndense, T. subterraneum and T. vesiculosum.Similarity between species and accessions was estimated by Jaccard’ssimilarity index based on presence or absence of bands. TheUPGMA method was utilized for the groupings and dendrogram constructionwith the aid of the NTSYS-PC program. Interspecific similaritywas low (J 0.351). The dendrogram presented eight groups, eachone corresponding to a taxonomic species. Most of the accessionsof the same species grouped together at J > 0.50, exceptT. riograndense, T. repens and T. pratense. These three speciesshowed the lowest similarity between their accessions, reflectinghigher intraspecific variation. Some accession-specific bandswere identified. The species groupings are consistent with traditionaltaxonomic species delimitation. Therefore isozyme patterns,especially when several systems are employed, are useful andreliable biochemical markers for the taxonomic delimitationand characterization of Trifolium germplasm. Copyright 2000Annals of Botany Company Genetic diversity, germplasm characterization, isoenzymatic variability, biochemical markers, isozymes,Trifolium     

In vitro Regeneration from Excised Leaf Discs of Three Brassica Species

Excised leaf discs of three Brassica species, B. oleracea, B.napus, and B. campestris were induced to produce adventitiousbuds and subsequently entire plants by culture on media withspecific combinations of 6-benzylaminopurine (BAP) and -naphthylaceticacid (NAA). Each species required a particular hormone concentrationfor optimum growth and differentiation: B. oleracea, BAP 10mg^–1 and NAA 1 mg 1^–1; B. napus, BAP 10 mg 1^–1and NAA 10 mg 1^–1; B. campestris, BAP 1 mg 1^–1 andNAA 10 mg 1^–1. In a more detailed study on one of these species, namely B.oleracea, the relative influence of other media components suchas amino acids and other organic additives was examined. Itwas also found that the source and size of the explant greatlyaffected the growth response, as did the size of the culturevessel. The regenerated plants dislayed a range of ploidy as well asphenotypic abnormalities. Findings are discussed in relation to results from other tissueculture systems.     

Switched single-electrode voltage-clamp amplifiers allow precise measurement of gap junction conductance

Measurement of gapjunction conductance(g_j) with patch-clampamplifiers can, due to series resistance problems, be subject toconsiderable errors when large currents are measured. Formulas developed to correct for these errors unfortunately depend on exactestimates of series resistance, which are not always easy to obtain.Discontinuous single-electrode voltage-clamp amplifiers (DSEVCs) wereshown to overcome series resistance problems in single whole cellrecording. With the use of two synchronized DSEVCs, the simulatedg_j in a modelcircuit can be measured with a maximum error of <5% in all recordingsituations investigated (series resistance, 5-47 M; membraneresistance, 20-1,000 M; g_j, 1-100nS). At a very lowg_j of 100 pS, theerror sometimes exceeded 5% (maximum of 15%), but the error wasalways <5% when membrane resistance was >100 M. The precisionof the measurements is independent of series resistance, membraneresistance, and g_j. Consequently,it is possible to calculateg_j directly from Ohm's law, i.e., without using correction formulas. Our results suggest that DSEVCs should be used to measureg_j if largecurrents must be recorded, i.e., if cells are well coupled or ifmembrane resistance is low.     

Genetic diversity and geographic differentiation in endangered Ammopiptanthus (Leguminosae) populations in desert regions of northwest China as revealed by ISSR analysis

• Background and Aims The desert legume genus Ammopiptanthuscomprises two currently endangered species, A. mongolicus andA. nanus. Genetic variability and genetic differentiation betweenthe two species and within each species were examined. • Methods Inter-simple sequence repeat (ISSR) marker datawere obtained and analysed with respect to genetic diversity,structure and gene flow. • Key Results Despite the morphological similarity betweenA. mongolicus and A. nanus, the two species are geneticallydistinct from each other, indicated by 63 % species-specificbands. Low genetic variability was detected for both populationlevel (Shannon indices of diversity H_pop = 0·106, percentageof polymorphic loci P = 18·55 % for A. mongolicus; H_pop= 0·070, P = 12·24 % for A. nanus) and specieslevel (H_sp = 0·1832, P = 39·39 % for A. mongolicus;H_sp = 0·1026, P = 25·89 % for A. nanus). Moderategenetic differentiation was found based on different measures(AMOVA _ST and Hickory ^B) in both A. mongolicus (0·3743–0·3744)and A. nanus (0·2162–0·2369). • Conclusions The significant genetic difference betweenthe two species might be due to a possible vicariant evolutionaryevent from a single common ancestor through the fragmentationof their common ancestor's range. Conservation strategies forthese two endangered species are proposed.     

DISTRIBUTION AND POPULATION DYNAMICS OF THREE POPULATIONS OF SIPHONARIA ON ROCKY INTERTIDAL SHORES IN HONG KONG

The pulmonate limpets Siphonaria japonica and Siphonana siriusoccur over a wide range of local habitat types in terms of exposureto wave action and salinity. This is a study of these two specieson three different shore types in Hong Kong, ranging from anextremely exposed, high salinity (32–35) shore at Caped'Aguilar to a sheltered, low salinity (16–33) shore atTai Lam Chung. Both species are restricted to the low shore,year round. S. japonica is a winter breeder and recruitmentoccurred between October and January. The recruitment of S.sirius could not be recognised from the size-frequency histograms.The algal standing crop at Tai Lam Chung was higher than atWu Kwai Sha during the winter period, i.e., between Octoberand April. Seasonal fluctuations in growth rate were recordedfor both Siphonaria species with the time of fastest growthoccurring in winter. S. japonica grew faster at Tai Lam Chungthan at Wu Kwai Sha. Food availability is thought to be an importantfactor affecting growth. (Received 17 September 1993; accepted 16 June 1994)     

Studies on the Physiology of Nodule Formation: VIII. The Influence of the Size of the Rhizosphere Population of Nodule Bacteria on Root Hair Infection in Clover

With an adequate inoculum the number of infected root hairsin three species of clover (Trifolium parviflorum, T. patensand T. glomeratum) increased exponentially with time in twophases; the increase was rapid during the first 8-10 days beforenodulation begins, but slower afterwards. T. parviflorum hadmost infections and T. glomeratum the fewest. Experiments on varying inoculum size, using an avirulent mutanstrain of Rhizobhtm trifolii as diluent, showed that root-hairinfection was differentially limited by inoculum size duringthe two phases. Infection in all three species was about doubledby doubling the density of the virulent bacteria in the rhizospherebefore nodulation begins. After nodulation bacterial densityhad to be increased much more than twice to double the numberof infections. This increase in the infecting population wasinversely related to the numbers of infections formed on thethree host species. Early infection and nodulation were promoted by high bacterialdensity in the rhizosphere.     

Effects of External Potassium Supply on Compartmentation and Flux Characteristics of Radiocaesium in Intact Spring Wheat Roots

Originally published in Annals of Botany84: 639–644 1999.For technical reasons beyond our control the flux symbol wasomitted from this paper. The paper is reprinted here in itsentirety. Short term experiments investigated the effects of potassiumsupply on radiocaesium influx/efflux and the radiocaesium compartmentationin intact spring wheat roots. Short term (24–72 h) influxanalysis showed that net influxes of radiocaesium to both rootand xylem were reduced approximately ten-times by increasingexternal potassium concentration from 50 µM to 200 µM.Efflux analysis distinguished three components for radiocaesium(namely cell wall+free space, cytoplasm and vacuole) and showedthat the rates of Cs⁺efflux at an external potassium concentrationof 100 µM (19.16 and 1.70 Bq g^-1min^-1for _coand _vo, respectively)were about three-times faster than those at 50 µM (7.24and to 0.41 Bq g^-1min^-1for _coand _vo, respectively). The resultsalso showed that external potassium concentration did not havea significant effect on the distribution of¹³⁷Cs between cytoplasmand vacuole, as indicated by the ratio of¹³⁷Cs in the two compartments.Results obtained in this study suggested that the inhibitoryeffect of potassium on the net uptake of radiocaesium by theplant root may be partially ascribed to the fact that at higherexternal potassium concentrations Cs⁺efflux rates were muchhigher. The mechanisms involved are discussed. Copyright 2000Annals of Botany Company Compartmentation, efflux analysis, potassium, radiocaesium, Triticum aestivum, wheat.     

Three Interpretations of the Self-thinning Rule

The self-thinning rule states that w, the mean biomass per plantin a dense monospecific stand, is related to p, the number ofplants in a unit area of that stand, by the power law relationshipw = Kp^–, where is approximately three-halves. The supportfor this rule, theoretical as well as experimental, has thusfar been largely empirical. In an effort to provide a firmertheoretical basis for it, three different and somewhat independenttheoretical models are propounded and shown to lead to powerlaw relationships with characteristic exponents in the vicinityof three-halves. theoretical models, mathematical model, three-halves law, density-effect, self-thinning     

Water Transport in Isolated Maize Roots

A simple model has been devised which predicts the concentration,C^x_s, of salt (e.g. KCl) in the exudate from isolated roots asa function of the salt concentration, C⁰_s, in the medium. Thechief assumption, made in deriving the relationship betweenC^x_s and C⁰₅, is that the exudation of water, J, from the rootsconsists of two components (one being osmotic, Ø, inorigin and the other, Ø⁰, flowing in the absence of anosmotic gradient). The exudation of salt, J_s, calculated asJ C^x_s, was found to be dependent on C⁰_s. Our investigationson maize roots were concerned with estimations of L_p and Ø^vand determinations of C^x_s as a function of C⁰_s. Satisfactoryagreement between prediction and experiment was found in thesepreliminary studies. It is considered that water movement inisolated roots can be explained by a simple osmotic model withthe additional possibility that a relatively small non-osmoticwater flow occurs.     

Modelling environmental variation in Young's modulus for Pinus radiata and implications for determination of critical buckling height

• Background and Aims Although density-specific stiffness,E/, (where E is Young's modulus and is wood density) is oftenassumed constant by the elastic similarity model, and in determinationof critical buckling height (H_crit), few studies have testedthis assumption within species. Here this assumption is testedfor Pinus radiata growing across an environmental gradient,and theory is combined with data to develop a model of Young'smodulus. • Methods Analyses use an extensive series of environmentalplots covering the range of climatic and edaphic conditionsover which P. radiata is grown in New Zealand. Reduced majoraxis regression was used to determine scaling exponents betweenlog–log plots of H_crit vs. groundline diameter (D), andE/ vs. D. Path analysis was used to identify significant directand indirect (through stem slenderness) edaphic and climaticinfluences on E. • Key Results Density-specific stiffness exhibited 3-foldvariation. As E/ scaled positively with D, the exponent of 0·95between H_crit and D exceeded the assumed value of 0·67under constant E/. The final path analysis model included meanair temperature in early autumn (T_aut) and slenderness as significant(P < 0·05) positive direct influences on E. Tree leafarea index and T_aut were indirectly associated with E throughtheir significant (P < 0·05) positive direct relationshipwith stem slenderness. Young's modulus was most sensitive toT_aut, followed by stem slenderness then leaf area index, andthe final model explained 76 % of the variance in E. • Conclusions The findings suggest that within speciesE/ variation may influence H_crit and the scaling exponent betweenD and H_crit so important in assumptions regarding allometricrelationships. The model presented may provide a useful meansof determining variation in E, E/ and H_crit across environmentalgradients.     

Horizontal distribution and dominant species of heteropods in the East China Sea

This article discusses the horizontal distribution and dominantspecies of heteropods in the East China Sea. The ecologicalcharacteristics of heteropods and their adaptability to theenvironments were also considered. Oceanographic census wascarried out in the East China Sea (23°30 '33° N and118°30'128° E) in four seasons from 1997 to 2000. Itwas found that the total abundance showed obvious seasonal variations.It peaked in autumn with a mean value of 21.03 ind. (100 m³)^–1,followed by summer (4.89 ind. (100 m³)^–1). The lowestabundance occurred in winter and spring. As to the horizontaldistribution, abundance in summer and autumn was higher in thenearshore than in the offshore of the East China Sea. In winterand spring, heteropods were barely found in the northern nearshore.Three dominant species were observed in four investigated seasons,in which only Atlanta rosea dominated in winter, spring andsummer while Atlanta peroni and Atlanta lesueuri mainly dominatedin autumn. These two dominant species observed in autumn exhibiteda rather higher occurrence frequency than A. rosea. Temperaturewas found to be a major influencing factor whereas salinitywas a minor one. Comparing their adaptability, A. rosea cansurvive in a wider temperature range (1628°C), which enablesit to dominate in four seasons, while A. peroni survives ina relatively narrow temperature range (1928°CC) and therange for A. lesueuri was even more narrow (2128°CC). Moreover,A. rosea was also adapted to a wider range of salinity. However,the abundance of A. rosea in autumn was lower than those ofA. peroni and A. lesueuri. It can be thereby inferred that themultiplication speed of A. rosea was lower than the other twospecies within the same favorable temperature range. Due totheir adaptability to high salinity, the distribution of heteropodswas closely related to the domain of the Taiwan Warm Currentand Kuroshio. Especially for A. peroni and A. lesueuri, theirhigh abundance areas are always indicative of the lasting existenceof strong warm currents. The negative values of aggregationindices indicated relatively even distribution of heteropodsin the East China Sea. The high abundance area (31°00' N,126°00' E) of heteropods in autumn is on the migration pathwayof the mackerel (Scomber japonicus Houttuyn). Thus, there isalso a fishing ground of the mackerel. This suggested that thehigh abundance area of heteropods in autumn is important tothe fisheries in the East China Sea. Comparing with the historicalrecords, the abundance of heteropods appeared to increase inthe past 40 years. This may be a result of strengthened warmcurrents due to global warming.