Seasonal changes in canopy structure in two mediterranean dune shrubs

Abstract. The annual cycle of canopy structure in two mediterranean shrubs in a pioneer zone of the mobile dune system in the Donana National Park, Scrophularia frutescens and Halimium halimifolium, has been analyzed. Destructive methods were used as well as a new non-destructive method, based on frequency analysis of organ distribution within the plant canopy. S. frutescens shows strong seasonal changes of photo-synthetic biomass, but little annual increment in dry weight. In H. halimifolium, seasonal changes are not as strongly differentiated as in S. frutescens, but a higher annual increment is shown. The canopy structure of both species and its temporal changes are compared with existingplant strategy models.     

Phytoecological importance,mutual redundancy and phytological threshold values of certain climatic factors

A combination of methods (intensity of indication, floristic and mesological redundancy analysis, beta-diversity analysis, principal components analysis and Wildi's interactive ranking procedure) were used to evaluate redundancy and relative phytoecological importance among 80 climatic variables in Galicia (N.W. Spain). The information they contained was found to be adequately summarized by just 3 factors thought to play a major role in regulating the distribution of the species considered in the study area and similar areas: Baudiere's QE index, mean minimum temperature in the coldest month and mean temperature range in the coldest month. For these three factors, phytoclimatic thresholds were determined by examining beta-diversity and were used to define phytoclimatic zone types.

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Resumen Se valora la redundancia e importancia fitoecológica relative de 80 variables climáticas en Galicia (N.O. de España) empleando una combinacion de diferentes metodos (intensidad indicadora, análisis de la redundancia florística y mesológica, análisis de beta-diversidad, análisis de componentes principales y el método de ordenación interactiva de Wildi). La información contenida en esta variables es adecuadamente resumida por 3 factores que juegan un papel predominante en regular la distribución de las especies consideradas en el area de estudio y areas similares: el índice QE de Baudiere, la temperatura media de las mínimas del mes más frío y la oscilación térmica del mes más frío. Para estos factores el análisis de la beta-diversidad permitió determinar los umbrales fitoclimáticos utilizados en la definición y cartografía de las zonas fitoclimáticas.

     

External Protons Enhance the Activity of the Hyperpolarization-activated K Channels in Guard Cell Protoplasts of Vicia faba

Hyperpolarization-activated K channels (K _H channels) in the plasmalemma of guard cells operate at apoplastic pH range of 5 to over 7. Using patch clamp in a whole-cell mode, we characterized the effect of varying the external pH between 4.4–8.1 on the activity of the K _H channels in isolated guard cell protoplasts from Vicia faba leaves. Acidification from pH 5.5 to 4.4 increased the macroscopic conductance of the K _H channels by 30–150% while alkalinization from pH 5.5 to 8.1 decreased it only by roughly 15%. The voltage-independent maximum cell conductance, increased by ∼60% between pH 8.1 and 4.4 with an apparent pK _a of 5.3, most likely owing to the increased availability of channels. Voltage-dependent gating was affected only between pH 5.5 and 4.4. Acidification in this range shifted the voltage-dependent open probability by over 10 mV. We interpret this shift as an increase of the electrical field sensed by the gating subunits caused by the protonation of external negative surface charges. Within the framework of a surface charge model the mean spacing of these charges was ∼30 ? and their apparent dissociation constant was 10^−4.6. The overall voltage sensitivity of gating was not altered by pH changes. In a subgroup of protoplasts analyzed within the framework of a Closed-Closed-Open model, the effect of protons on gating was limited to shifting of the voltage-dependence of all four transition rate constants. Received: 26 April 1996/Revised: 29 June 1996     

Genetic analysis of two tomato mutants affected in the regulation of iron metabolism

Iron is one of the most important micronutrients for plants. Like other organisms, plants have developed active mechanisms for the acquisition of sufficient iron from the soil. Nevertheless, very little is known about the genetic mechanisms that control the active uptake. In tomato, two spontaneously derived mutants are available, which are defective in key steps that control this process. The recessive mutationchloronerva (chln) affects a gene which controls the synthesis of the non-protein amino acid nicotianamine (NA), a key component in the iron physiology of plants. The root system of the recessive mutantfer is unable to induce any of the characteristic responses to iron deficiency and iron uptake is thus completely blocked. We present a characterization of the double mutant, showing that thefer gene is epistatic over thechln gene and thus very likely to be one of the major genetic elements controlling iron physiology in tomato. In order to gain access to these two genes at the molecular level, both mutants were precisely mapped onto the high density RFLP map of tomato. Thechln gene is located on chromosome 1 and thefer gene is on chromosome 6 of tomato. Using this high-resolution map, a chromosome walk has been started to isolate thefer gene by map-based cloning. The isolation of thefer gene will provide new insights into the molecular mechanisms of iron uptake control in plants.     

Genetically-based plant resistance traits affect arthropods,fungi, and birds

We examine how the distribution of a leafgalling aphid (Pemphigus betae) affects other species associated with natural stands of hybrid cottonwoods (Populus angustifolia x P. fremontii). Aphid transfers on common-garden clones and RFLP analysis show that resistance to aphids in cottonwoods is affected by plant genotype. Because susceptible trees typically support thousands of galls, while adjacent resistant trees have few or none, plant resistance traits that affect the distribution of this abundant herbivore may directly and/or indirectly affect other species. We found that the arthropod community of aphid-susceptible trees had 31% greater species richness and 26% greater relative abundance than aphid-resistant trees. To examine direct and indirect effects of plant resistance traits on other organisms, we experimentally excluded aphids and found that abundances and/or foraging behavior of arthropods, fungi, and birds were altered. First, exclusion of gall aphids on susceptible trees resulted in a 24% decrease in species richness and a 28% decrease in relative abundance of the arthropod community. Second, exclusion of aphids also caused a 2- to 3-fold decrease in foraging and/or presence of three taxa of aphid enemies: birds, fungi, and insects. Lastly, aphidexclussion resulted in a 2-fold increase in inquilines (animals who live in abodes properly belonging to another). We also found that fungi and birds responded to variation in gall density at the branch level. We conclude plant resistance traits affect diverse species from three trophic levels supporting a bottom-up influence of plants on community structure.     

A simple, accurate method for determining wet and dry weight concentrations of plant cell suspension cultures using microcentrifuge tubes

Summary A simple method using microcentrifuge tubes for determining fresh and dry weights, and collecting cell-free supernatant from plant suspension cultures is described. This method offers improvements in accuracy, precision, and time efficiency over traditional filtration methods. Using 4-day-old Nicotinia tabacum cultures, the centrifuge method was shown to remove 25% more of the interstitial water from cell aggregates compared to a suction filtration method, with significantly less variation in fresh weight data.     

Influence of BA and sucrose on the competence and determination of pepper (Capsicum annuum L. var. Sweet Banana) hypocotyl cultures during shoot formation

Summary The simultaneous presence of 6-benzyladenine (BA) and sucrose in a Murashige and Skoog medium (SIM) during the initial stages of shoot initiation have been found to be obligatory for high-frequency shoot formation in the Capsicum annuum L. var. Sweet Banana upper hypocotyl explants. The explants are determined for shoot formation following a minimum of 8 days of culture on SIM. Deprivation of exogenous sucrose from day 6 to day 20 of culture had no effect on the shoot forming response of the explants. BA and sucrose appear to act independently on different aspects of the competence of explants to respond to SIM during shoot initiation.Abbreviations BA N⁶-benzyladenine - MS Murashige and Skoog medium - SIM shoot induction medium - HFM hormone free medium - SUC sucrose minus medium     

Plant regeneration from somatic embryos ofTaxus brevifolia

Summary Taxusbrevifolia is the source of paclitaxel (Taxol®), an anticancer drug. A method for regeneration ofTaxus brevifolia from immature zygotic embryos via somatic embryogenesis is described. Embryogenic callus tissues were obtained by culturing immature zygotic embryos on Lloyd and McCown medium (MCM) supplemented with 160 M 2,4-dichlorophenoxyacetic acid (2,4-D) + 5 M benzylaminopurine (BA) + 5 M naphthaleneacetic acid (NAA) for 4 weeks. Putative embryoids were obtained following transfer of cultures to MCM medium supplemented with 4 M BA + 5 M kinetin + 1 M NAA for 6 to 8 weeks. Conversion of embryos was obtained on MCM medium supplemented with 40 M abscisic acid (ABA) + 1% activated charcoal. Development of bipolar structures with recognizable shoot and root apices was observed in somatic embryos. Five percent of somatic embryos were regenerated into plantlets on half-strength growth regulator-free MCM medium.