Changes in hydraulic conductance of citrus trees following a reduction in wetted soil volume

Abstract The effcct of the transition from fully to partially wetted soil voluine on transpiration rate and hydraulic conductance of mature citrus trees was examined in a 23-year-old, coninicrcial, sprinklerirrigated, Shanio u t i orange orchard. I rriga t i on frequency was determined by the rate of water loss from the soil, a s measured by neutron probes. The hydraulic conductance of tlic tree was coniputed from the rclationship between sap flow i n the trunk and leaf water potential. The diurnal valucs of leaf water potential and sap flow shifted towards lower levels as tlie water stored in the root zone was depleted. In the fully wetted soil volume the tree hydraulic conductance remained constant throughout the irrigation period, from June to Novcniber. However, partial wetting of the soil volume (40%) caused a reduction in the hydraulic conductance of the tree. Tlie decreased hydraulic conductance is attributed to tlie permanent interruption of water transport in part of tlie root system. Tlie rcsults of tlie experiment suggest that despite tlie increase of irrigation frequency, partial wetting intensifies water stress in tlie trees.     

Cell Cycle Control in Arabidopsis

Although the basic mechanism of cell cycle control is conservedamong eukaryotes, its regulation differs in each type of organism.Plants have unique developmental features that distinguish themfrom other eukaryotes. These include the absence of cell migration,the formation of organs throughout the entire life-span fromspecialized regions called meristems, and the potency of non-dividingcells to re-enter the cell cycle. The study of plant cell cyclecontrol genes is expected to contribute to the understandingof these unique developmental phenomena. The principal regulatorsof the eukaryotic cell cycle, the cyclin-dependent kinases (CDKs)and cyclins, are conserved in plants. This review focuses oncell cycle regulation in the plant Arabidopsis thaliana . Whileexpression of one Arabidopsis CDK gene, Cdc2aAt, was positivelycorrelated with the competence of cells to divide, expressionof a mitotic-like cyclin, cyc1At, was almost exclusively confinedto dividing cells. The expression of the Arabidopsis -type cyclinsappears to be an early stage in the response of plant cellsto external and internal stimuli. Arabidopsis thaliana (L.) Heynh.; cell cycle; CDK; cyclin; plant development; plant hormone     

Hydraulic control of stomatal conductance in Douglas fir [Pseudotsuga menziesii (Mirb.) Franco] and alder [Alnus rubra (Bong)] seedlings

Experiments were conducted on 1-year-old Douglas fir [Pseudotsuga menziesii (Mirb.) Franco] and 2- to 3-month-old alder [Alnus rubra (Bong)] seedlings growing in drying soils to determine the relative influence of root and leaf water status on stomatal conductance (g_c). The water status of shoots was manipulated independently of that of the roots using a pressure chamber that enclosed the root system. Pressurizing the chamber increases the turgor of cells in the shoot but not in the roots. Seedling shoots were enclosed in a whole-plant cuvette and transpiration and net photosynthesis rates measured continuously. In both species, stomatal closure in response to soil drying was progressively reversed with increasing pressurization. Responses occurred within minutes of pressurization and measurements almost immediately returned to pre-pressurization levels when the pressure was released. Even in wet soils there was a significant increase in g_c with pressurization. In Douglas fir, the stomatal response to pressurization was the same for seedlings grown in dry soils for up to 120 d as for those subjected to drought stress over 40 to 60 d. The stomatal conductance of both Douglas fir and alder seedlings was less sensitive to root chamber pressure at higher vapour pressure deficits (D), and stomatal closure in response to increasing D from 1.04 to 2.06 kPa was only partially reversed by pressurization. Our results are in contrast to those of other studies on herbaceous species, even though we followed the same experimental approach. They suggest that it is not always appropriate to invoke a ‘feedforward’ model of short-term stomatal response to soil drying, whereby chemical messengers from the roots bring about stomatal closure.     

Some properties of a previously undescribed virus from cassava: Cassava American Latent Virus

A virus with spherical particles c. 28 nm in diameter was sap-transmitted from different cassava (Manihot esculenta) cultivars to a limited range of species in the families Chenopodiaceae and Solanaceae. Cassava seedlings infected by inoculation with sap or with purified virus preparations did not show any symptom, although the virus was readily detected by ELISA or by further inoculations. Leaf extracts from infected Nicotiana benthamiana were infective after dilution of 10^--³but not 10^--⁴, and after heating for 10 min at 70°C, but not at 72°C. The virus was purified from N. benthamiana, N. clevelandii or from cassava. On sucrose gradients, the virus particles sediment as three components all containing a protein of mol. wt c. 57000. The genome of the virus is composed of two RNAs of mol. wt c. 2.54 times 10⁶(RNA-1) and 1.44 times 10⁶(RNA-2). RNA-2 was detected in the middle and the bottom nucleoprotein components, and RNA-1 only in the bottom component. An antiserum prepared to purified virus particles was used to readily detect the virus in cassava and other host plants by ELISA and by ISEM. No serological relationship was shown between this virus and eight nepoviruses, including the recently described cassava green mottle nepovirus infecting cassava in the Solomon Islands (Lennon, Aiton & Harrison, 1987). The virus described here is the first nepovirus isolated from cassava in South America, and is named cassava American latent virus.     

Row structure and foliage geometry as determinants of the interception of light rays in a sorghum row canopy

Abstract. The interception of light rays by the canopy of a widely spaced sorghum row crop was assessed from a series of hemispherical photographs taken on five occasions during its vegetative growth. Analysis of the photographs shows that most of the temporal and spatial variation in the canopy's interception could be accounted for by treating the row as an array of solid parallel walls with rectangular cross sections. Within the space occupied by rows, the effect of the distribution of foliage elements on interception could be assessed by the porosity of a single row. The north-south asymmetry observed in the distribution of ray interception indicates that this sorghum canopy, grown under unirrigated, arid summer conditions, adopted a light-avoiding growth pattern.     

European checkerspots (Melitaeini: Lepidoptera,Nymphalidae) are not aposematic – the point of view of great tits (Parus major)

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INDUCTION OF OVARIAN DEVELOPMENT IN AUTOGENOUS AEDES ATROPALPUS BY JUVENILE HORMONE AND 20-HYDROXYECDYSONE

Aedes atropalpus is an autogenous mosquito characterized by a first gonadotropic cycle which results in approximately 200 mature oocytes without a bloodmeal. Ovarian development is completely inhibited if these animals are decapitated or ligated between the thorax and abdomen shortly after adult emergence. Injection of 4.8 ng of 20-hydro- xyecdysone into decapitated females 12 h after eclosion restores ovarian development in all females so treated. However, the same amount of 20-hydroxyecdysone injected into isolated abdomens obtained shortly after adult emergence had no discernible effect on vitellogenesis. In contrast, all abdomens which received 0.5 ng of topically applied JH I followed by the injection of 4.8 ng 20-hydroxyecdysone produced mature oocytes. Isolated abdomens were also capable of oocyte maturation when treated with excess amounts of JH alone; JH I was the most effective followed by JH II and then JH III.

Polyacrylamide gel electrophoretic analysis of vitellin extracted from the ovaries of hormonally-treated animals did not reveal any qualitative differences compared to intact normal controls. However, less yolk protein was present in the former. This was verified by counting the number and measuring the size of ovarian follicles in individual females.     

Palaeoctopus pelagicus from the Turonian of Mexico reinterpreted as a coelacanth (Sarcopterygian) gular plate

Abstract: The supposed vestige of a cephalopod gladius from Turonian platy limestones at Vallecillo, north‐east Mexico, named Palaeoctopus pelagicus by Fuchs et al. in 2008, is reinterpreted and shown to be a gular plate of a coelacanth fish, possibly of the genus Megacoelacanthus. In addition to the gular plate, two extrascapulars and fin rays of all fins are preserved on one slab and its counterpart. This is the first record of a coelacanth from these lower Turonian strata at Vallecillo, which are rich in fish.     

Symbiotic streptomycetes in antennal glands of the South American digger wasp genus Trachypus (Hymenoptera,Crabronidae)

Several insect taxa have evolved symbioses with actinobacteria that protect the host or its nutritional resources against pathogens. Digger wasps of the genus Philanthus (‘beewolves’; Hymenoptera, Crabronidae) cultivate ‘Candidatus Streptomyces philanthi’ (Ca. S. philanthi) in specialized antennal glands and transfer them to subterranean brood cells, where the symbionts provide protection for the beewolf larva against pathogens by producing a cocktail of antibiotic substances. The present study investigates the occurrence of antennal symbionts in two species of the genus Trachypus, which is the closest relative to Philanthus. Cross sections of the antennae from females of both Trachypus denticollis and Trachypus boharti reveal bacteria‐containing gland reservoirs that are morphologically very similar to those found in Philanthus spp. Polymerase chain reaction‐based screens with specific primers, sequencing of the partial 16S rRNA gene, and fluorescence in situ hybridization confirm the presence of close relatives of ‘Ca. S. philanthi’ in T. denticollis and T. boharti. However, 16S rRNA sequence divergence between Trachypus spp. and Philanthus spp. symbionts is much higher for T. boharti than for T. denticollis, suggesting that horizontal transmission and/or de novo uptake of symbionts from the environment occurs occasionally. The results obtained indicate that the protective symbiosis with antennal actinobacteria is older and more widespread than previously recognized and occurs in at least two genera of digger wasps that comprise approximately 170 species.     

New Streptocarpus species (Gesneriaceae) from South Africa

Two new rosulate species of Streptocarpus are described from the eastern seaboard of South Africa. The first is endemic to Mpumalanga Province. This species has almost actinomorphic corollas with small cylindrical tubes and was previously included within Streptocarpus parviflorus. However, molecular and morphological data and habitat preference do not support this classification. The second species is from the Msikaba River Gorge in the Eastern Cape Province. It adds to the already impressive list of endemic plant species from this region and is allied to other rosulate species of the Eastern Cape. It approaches Streptocarpus rexii in flower size but differs in its much shorter corolla tubes, which lack purple nectar guides. In addition, the corolla floors are marked with yellow bars reminiscent of Streptocarpus cyaneus and also seen in sympatric populations of the small‐flowered Streptocarpus modestus. © 2008 The Linnean Society of London, Botanical Journal of the Linnean Society, 2008, 158 , 743–748.