Are Roots a Source of Abscisic Acid for the Shoots of Flooded Pea Plants?

Flooding the soil for 2–5 d decreased stomatal conductancesof pea plants (Pisum sativum L., cv. Sprite) with six or sevenleaves. This coincided with slower transpiration, increasedleaf water potentials and increased concentrations of abscisicacid (ABA) in the leaves. No increase in ABA was found in theterminal 20 mm of roots of flooded plants over the same timeperiod. Small stomatal conductances associated with increases in foliarABA were also found in plants grown in nutrient solution whenaeration was halted, causing the equilibrium partial pressuresof dissolved oxygen to fall below 05 It Pa. No increase in ABAconcentration in young secondary roots of the non-aerated plantswas detected after 24, 48 or 72 h, even when the shoot, thepresumed site of deposition for any ABA from the roots, wasremoved 5–6 h before analysis. Similarly, ABA concentrations in roots were not increased whenthe nutrient solution was de-oxygenated by continuous purgingwith nitrogen gas. The abscisic acid concentration in leaf epidermis,the tissue most likely to be the recipient of any ABA movingin the transpiration stream from oxygen-deficient roots, waslower than in the remaining parts of the leaf when examinedin the mutant Argenteum which possesses easily removable epidermallayers. It is concluded that the leaves of plants subjectedto flooding of the soil or oxygen shortage in the root environmentare not enriched substantially with ABA from the roots. A moreprobable source of this growth regulator is the leaf itself. Key words: Pisum sativum, flooding, roots, hormones, aeration stress, abscisic acid, Argenteum mutant     

Apioceridae (Insecta: Diptera): cladistic reappraisal and biogeography

Twenty members of the fly families Apioceridae and Mydidae are compared in terms of 77 adult characters. Cladistic analysis of 91 synapomorphies provides a well corroborated reconstruction of lineage relationships, and reveals that the Apioceridae is paraphyletic with respect to the Mydidae. In order to render the Apioceridae monophyletic, the genus Rhaphiomidas and the subfamily Megascelinae of the Apioceridae are transferred to the Mydidae. The subfamily Rhaphiomidinae is reinstated to accommodate Rhaphiomidas , comprising the most plesiomorphic mydids. The relationships of the remaining subfamilies of Mydidae are discussed in the context of these findings. The genus Apiocera is divided into four subgenera, the type subgenus Apiocera Westwood in Australia, Pyrocera subgen. nov. in North America, and the subgenera Ripidosyrma Hermann and Anypenus Philippi are applied to the South African and South American species, respectively. A key to the four subgenera of Apiocera is provided. The biogeographic relationships of the subgenera and genera of the Apioceridae and the Megascelinae are discussed. Although considered an example of a transantarctic or Gondwanan group, we argue that the distribution of the Apioceridae predates the Mesozoic supercontinent Gondwanaland and extends onto sections of Pangaea, and should be termed 'Pangaean'. The cladistic relationships between the genera of Apioceridae and Megascelinae are consistent with the geological vicariance of the fragments of Pangaea on which they now occur.     

Seasonal changes of osmotic pressure, symplasmic water content and tissue elasticity in the blades of dune grasses growing in situ along the coast of Oregon

Abstract Midday water potentials of blades of the dune grasses Ammophila arenaria (L.) Link and Elymus mollis Trin. ex Spreng. growing in situ declined over the summer growing period, indicating a trend of increasing water stress. An analysis of the water relations characteristics of these blades using pressure-volume techniques demonstrated that both species increased bulk osmotic pressure at full hydration () and, therefore, bulk turgor as an acclimation response. In A. arenaria, however, the increase of osmotic pressure (+ 0.35 MPa) was entirely the result of decreasing symplasmic water content. The increase of osmotic pressure (+ 0.54 MPa) observed in E. mollis blades was due to solute accumulation (72% of Δ) and to a lesser degree, decreased symplasmic water content (28% of Δ). Osmotic adjustment in E. mollis blades was accompanied by a significant decrease in tissue elasticity (_max went from 12 to 19 MPa). The elastic properties of A. arenaria blades remained constant over the same period and had a maximum modulus (10 MPa) that was always less than that of E. mollis, As estimated from Höfler plots, these seasonal adjustments of osmotic pressure and differences in tissue elasticity enabled plants in situ to maintain turgor pressure in the range of 0.5–0.6 MPa at the lowest water potentials of mid-August. Laboratorygrown plants exhibited the species-specific differences in osmotic pressure, turgor pressure, and tissue elasticity observed in field plants. Although certain alterations of leaf structure were expected to coincide with the observed changes and species-specific differences in symplasmic water content and tissue elasticity, these could not be detected by measurements of specific leaf weight or the ratio of dry matter to saturated water content.     

Response of carbon fluxes to drought in a coastal plain loblolly pine forest

Full accounting of ecosystem carbon (C) pools and fluxes in coastal plain ecosystems remains less studied compared with upland systems, even though the C stocks in these systems may be up to an order of magnitude higher, making them a potentially important component in regional C cycle. Here, we report C pools and CO₂ exchange rates during three hydrologically contrasting years (i.e. 2005–2007) in a coastal plain loblolly pine plantation in North Carolina, USA. The daily temperatures were similar among the study years and to the long‐term (1971–2000) average, whereas the amount and timing of precipitation differed significantly. Precipitation was the largest in 2005 (147 mm above normal), intermediate in 2006 (48 mm below) and lowest in 2007 (486 mm below normal). The forest was a strong C sink during all years, sequestering 361 ± 67 (2005), 835 ± 55 (2006) and 724 ± 55 (2007) g C m^?2 yr^?1 according to eddy covariance measurements of net ecosystem CO₂ exchange (NEE). The interannual differences in NEE were traced to drought‐induced declines in canopy and whole tree hydraulic conductances, which declined with growing precipitation deficit and decreasing soil volumetric water content (VWC). In contrast, the interannual differences were small in gross ecosystem productivity (GEP) and ecosystem respiration (ER), both seemingly insensitive to drought. However, the drought sensitivity of GEP was masked by higher leaf area index and higher photosynthetically active radiation during the dry year. Normalizing GEP by these factors enhanced interannual differences, but there were no signs of suppressed GEP at low VWC during any given year. Although ER was very consistent across the 3 years, and not suppressed by low VWC, the total respiratory cost as a fraction of net primary production increased with annual precipitation and the contribution of heterotrophic respiration (R_h) was significantly higher during the wettest year, exceeding new litter inputs by 58%. Although the difference was smaller during the other 2 years (R_h : litterfall ratio was 1.05 in 2006 and 1.10 in 2007), the soils lost about 109 g C m^?2 yr^?1, outlining their potential vulnerability to decomposition, and pointing to potential management considerations to protect existing soil C stocks.     

Reconciling the optimal and empirical approaches to modelling stomatal conductance

Models of vegetation function are widely used to predict the effects of climate change on carbon, water and nutrient cycles of terrestrial ecosystems, and their feedbacks to climate. Stomatal conductance, the process that governs plant water use and carbon uptake, is fundamental to such models. In this paper, we reconcile two long‐standing theories of stomatal conductance. The empirical approach, which is most commonly used in vegetation models, is phenomenological, based on experimental observations of stomatal behaviour in response to environmental conditions. The optimal approach is based on the theoretical argument that stomata should act to minimize the amount of water used per unit carbon gained. We reconcile these two approaches by showing that the theory of optimal stomatal conductance can be used to derive a model of stomatal conductance that is closely analogous to the empirical models. Consequently, we obtain a unified stomatal model which has a similar form to existing empirical models, but which now provides a theoretical interpretation for model parameter values. The key model parameter, g₁, is predicted to increase with growth temperature and with the marginal water cost of carbon gain. The new model is fitted to a range of datasets ranging from tropical to boreal trees. The parameter g₁ is shown to vary with growth temperature, as predicted, and also with plant functional type. The model is shown to correctly capture responses of stomatal conductance to changing atmospheric CO₂, and thus can be used to test for stomatal acclimation to elevated CO₂. The reconciliation of the optimal and empirical approaches to modelling stomatal conductance is important for global change biology because it provides a simple theoretical framework for analyzing, and simulating, the coupling between carbon and water cycles under environmental change.     

Genetic differentiation amongst fragrant orchids (Gymnadenia conopsea s.l.) in the British Isles

Genetic variation was examined in five microsatellite loci to seek evidence of genetic differentiation and restricted gene flow that would support the taxonomic division of Gymnadenia into three species ( G. borealis , G. conopsea , and G. densiflora ). A total of 107 alleles was detected in 17 populations from England, Scotland, and Ireland. The mean expected heterozygosities ranged from 0.48 to 0.81. The differentiation in allele frequencies amongst populations that had been assigned to each taxon on the basis of morphology was sufficiently large to support the taxa as distinct species. Phylogenetic trees based on microsatellite allele frequencies, as well as assignment tests, supported the existence of three distinct groups with at least partial restriction of gene flow between them. There was substantial homozygote excess, leading to high F _IS estimates, for most loci in most populations. This is unlikely to have been a result of widespread null alleles, and more probably reflects a high level of inbreeding in G. conopsea . This inference requires further investigation. The implications of the results of this and other taxonomic studies for the conservation of Gymnadenia in Britain are discussed.  © 2007 The Linnean Society of London, Botanical Journal of the Linnean Society , 2007, 155 , 349–360.     

The Onset of Germination Ability in Developing Wheat

The ability of primary wheat grains (cv. Sappo) to germinatewas determined at different times from anthesis. Fresh, prematurelyharvested grains germinated best at low temperatures (<10°C). The temperature range over which appreciable germinationtook place broadened and the maximum percentage germinationachieved increased with lengthening time between anthesis andharvest. Chilling and exogenous applications of GA₃ each affectedthe rate of germination and the maximum percentage germinationof grains in their own ways. The results are discussed in relationto the identification of ‘dormancy’ in wheat grains.     

Visual and Acoustical Signalling during Courtship by Fiddler Crabs (Genus Uca)

Male fiddler crabs (Genus Uca) employ both visual and acousticalsignals to attract females for mating. In U. pugilator and severalother American species, the males attract females during theday first by waving, then by producing sounds just within theirburrows. At night, the males produce sounds at low rates, butwhen touched by a female, they increase their rate of soundproduction. In the European species, U. tangeri, many elements of courtshipare similar to those in U. pugilator, but two types of soundsare produced. One of these, the short drumwhirl, appears tosubstitute for waving when the male is temporarily obscuredfrom the female during his diurnal courtship activities. Thelong drumwhirl is used under different circumstances. The acoustical responses of a male to a female influence thecourtship behavior of other males in the area. When sounds fromstimulated males are played back to test males during the day,their lates of waving increase. At night, the playbacks elicitincreases in rates of sound production. The influence of tidal oscillations, temperature, and lightcycles on the behavior of males is discussed. Courtship activities of aquatic crabs are compared to thoseof terrestrial Brachyura. In aquatic forms, courtship may beabsent or, if present, does not involve elaborate signallingby the male. Chemical or visual cues at close range are themost important stimuli. In several genera of terrestrial crabs,visual signalling for prolonged periods is common, and soundsare often emitted by males to "call" females from their burrowsto the surface for mating. Some of the factors that may accountfor differences in courtship activities in aquatic and terrestrialspecies are discussed.