Proteolysis of ribulose-1,5-bisphosphate carboxylase/oxygenase in leaves of Citrus explants throughout the annual cycle and its regulation by ethylene

Leaf senescence and ribulose-1,5-bisphosphate carboxylase/oxygenase (RuBP carboxylase, EC 4.1.1.39) degradation in orange [ Citrus sinensis (L.) Osbeck cv. Washington Navel] explants have been investigated. Explants consisted of a segment of stem (ca 15 cm) and 5 mature leaves. In vitro RuBP carboxylase degradation was determined by culturing the explants in water for different periods of time (3 days usually) and quantifying the two RuBP carboxylase subunits in the extracts following sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE). In vitro RuBP carboxylase degradation was estimated by autodigestion of leaf extracts and SDS-PAGE. The extent of in vivo RuBP carboxylase degradation in explants cultured under 16 h light/8 h dark photoperiod varied throughout the year and showed a cyclic behaviour correlated with the growth cycle of Citrus. The highest proteolytic activity both in vivo and in vitro was found in explants made from April to August coinciding with the maximum vegetative growth period of the tree.
Leaf senescence and abscission could be retarded significantly at any time of the year by maintaining the explants continuously in the dark. Treatment of the explants in the dark with a continuous flow of ethylene enhanced both leaf abscission and rate of RuBP carboxylase degradation, proportionally to ethylene concentration (0.1-0.6 ppm). Ethylene-induced senescence of Citrus leaf explants in the dark appears to be a convenient model system to study the regulation of the proteolytic degradation of RuBP carboxylase.     

Total root length as estimated from small sub-samples

A method is proposed for estimating the total length of a root system from sub-samples. This method is based on the measurement of the length and diameter of small pieces of roots, and on the measurements of the bulk density of root sub-samples. It is assumed that roots are cylinders with a given bulk density. The length and diameter of small root pieces are measured by image analysis. A weighted quadratic mean (W.Q.M.) root diameter is then calculated and used in estimating the root length. This W.Q.M. diameter is defined as the real mean diameter of an equivalent single root with the same length and volume as the tested root system. The accuracy of prediction is demonstrated for one theoretical root system. The standard deviation of estimation can be calculated using sampling simulations.     

Primary structure of chain I of the heterodimeric hemoglobin from the blood clamBarbatia virescens

The blood clamBarbatia virescens has a heterodimeric hemoglobin in erythrocytes. Interestingly, the congeneric clamsB. reeveana andB. lima contain quite different hemoglobins: tetramer and polymeric hemoglobin consisting of unusual didomain chain. The complete amino acid sequence of chain I ofB. virescens has been determined. The sequence was mainly determined from CNBr peptides and their subpeptides, and the alignment of the peptides was confirmed by sequencing of PCR-amplified cDNA forB. virescens chain I. The cDNA-derived amino acid sequence matched completely with the sequence proposed from protein sequencing.B. virescens chain I is composed of 156 amino acid residues, and the molecular mass was calculated to be 18,387 D, including a heme group. The sequence ofB. virescens chain I showed 35–42% sequence identity with those of the related clamAnadara trapezia and the congeneric clamB. reeveana. An evolutionary tree forAnadara andBarbatia chains clearly indicates that all of the chains are evolved from one ancestral globin gene, and that the divergence of chains has occurred in each clam after the speciation. The evolutionary rate for clam hemoglobins was estimated to be about four times faster than that of vertebrate hemoglobin. We suggest that blood clam hemoglobin is a physiologically less important molecule when compared with vertebrate hemoglobins, and so it evolved rapidly and resulted in a remarkable diversity in quaternary and subunit structure within a relatively short period.     

Nutrient requirements of exotic tree species in Zimbabwe

Pot and field experiments were conducted in the greenhouse and at three field sites (Marondera, Domboshawa and Makoholi) in Zimbabwe to examine the effects of soils and fertilizers on nutrient uptake and growth of 6 exotic tree species (Eucalyptus camaldulensis, E. grandis, E. tereticornis, Leucaena leucocephala, Casuarina cunninghamiana, and Acacia holosericea). Plant growth, N and P contents of all species were increased by the application of N, P, K and micronutrient fertilizers. The effect of individual nutrients (N, P, K and micronutrients) and their combination on Eucalyptus species was further investigated in a pot experiment using soil from Domboshawa. Eucalyptus species responded only to N application and no significant interactions were found between N and the other elements. Nutrient uptake results showed that E. camaldulensis and E. tereticornis removed more cations than the N-fixing trees but only in the fertilized treatments. L. leucocephala and C. cunninghamiana were higher in P, but no clear trends were observed for N. Plant growth and nutrient uptake by E. camaldulensis. C. cunninghamiana and A. holosericea were assessed in the field at the three sites. Plant species grown in the Marondera site had greater height and diameter at breast height (DBH) than those in the two other sites. These results followed trends in soil nutrient contents. The analysis of foliage revealed differences in the nutrient concentration of leaves from different trees, but no effect of site was found. The interrelationships between plant characteristics, soil and foliage nutrients were examined. In a pot experiment, mineral N was the only variable correlated with growth response and nutrient uptake, while in the field the only significant correlation was obtained with soil pH.     

Responses of stomata of senescing and nonsenescing leaves of Nicotians glauca to changes in intercellular concentrations of leaf carbon dioxide

Abstract. Gas exchange measurements were performed to test the hypothesis that failure of stomata to open in senescing leaves of Nicotiana glauca is caused by elevated concentrations of carbon dioxide in the intercellular spaces of leaf mesophyll tissue (c_i). Senescing leaves selected for experiments were completely chlorotic and lacked positive rates of photosynthesis. When stomata in detached epidermis from senescing leaves were illuminated in CO2-free air, they opened to similar apertures as those in detached epidermis from nonsenescing leaves. To compare the effects of changes in c_i on stomatal responses of the two leaf types, leaf 'flags' of either nonsenescing or senescing leaves were illuminated at a photosynthetic photon flux density of 500 μmol m⁻² s⁻¹ in a gas exchange cuvette. Leaf temperatures were maintained at 23.5 ± 0.5°C, and vapour pressure differences between leaves and the air were maintained between 0.70 and 0.75kPa. C_i was adjusted by changing external concentrations of carbon dioxide in air circulating through the cuvette. Conductances and photosynthetic rates of nonsenescing leaves changed in response to changes in c_i, but neither the conductances nor the photosynthetic rates of senescing leaves were affected significantly by changes in q. We conclude that guard cells of senescing leaves of Nicotiana glauca do not lose the capacity to respond to changes in carbon dioxide concentration and that increases in c_i resulting from declining rates of mesophyll photosynthesis are not the sole cause of maintenance of stomatal closure during leaf senescence. The data suggest that factors external to guard cells may prevent them from responding to changes in carbon dioxide concentrations in intact senescing leaves.     

Conservative and variable regions of homologous snake phospholipases A2 sequences: Prediction of the taxonspecific peptides structure

Homologous amino acid sequences of phospholipases A₂ (PLA₂) of snakes belonging to the families Elapidae, Viperidae, and Colubridae were considered in order to study the conservative and variable regions location. The PLA₂ sequences were divided into two groups (taxons) according to the phylogenetic tree reconstructed from the pair similarity matrix. Results of the intergroup comparison were plotted to facilitate the identification of significant conservative and variable regions. It was shown that the results of the comparison between two phylogenetic groups of snake PLA₂ did not much depend on the number of each group representatives and did not markedly change if one of the groups was represented by the single sequence. The knowledge of the number and location of conservative and variable regions and their dependence on the phylogenetic relations between compared taxa may be used to predict a synthetic peptide structure to obtain specific antibodies against PLA₂ of one of these taxons. Such prediction is possible if there is a specific region conservative for one taxon but variable for two of them.     

Determinants in the perceptions of visual blight

Research on visual blight has been predominantly qualitative and has pointed out salient attributes of blight: that it is durable, visually demeaning, and somehow aesthetically depressing. Data from a national sample (n=3005) are used in this study to measure the effect of demographic variation on the perception of visual blight. The perception of blight shows marked and regular variation with socioeconomic status, marital status, and religiosity.Harris data for this study was purchased with funds from the Shell Foundation. The author wishes to acknowledge Robert Q. Hanham, whose assistance proved invaluable.     

Effects of stormwater sewer discharges on the aquatic community in urban canals in Lelystad

In urban areas with a separate sewerage system, the stormwater runoff is discharged into surface waters. A study on the effects of stormwater sewer discharges on the composition of the aquatic community in urban waters was carried out in two areas with a primarily residential use in the new town of Lelystad. The aquatic organisms considered included hydrophytes, epiphytic diatoms, filamentous algae and macroinvertebrates.The results indicate that the stormwater sewer discharges cause a slight change in the composition of the aquatic community. The water in the urban surface waters can be characterized as eutrophic and - toa-mesosaprobic. Near stormwater sewer outfalls the water tends to the more polluteda-mesosaprobic state. The shifts in the composition of the aquatic community could be traced primarily on the basis of a number of epiphytic diatoms and macroinvertebrates (in particular some Diptera and Trichoptera). Most hydrophytes, filamentous algae and the remaining epiphytic diatoms and macroinvertebrates (in particular water beetles and water mites) turned out to be of little or no use in this respect.Because the investigation was carried out in an unusually dry summer, the results probably underestimate the effects of stormwater discharges on the aquatic community.     

Population structure and dynamics of Pinus massoniana Lamb. on mount Jinyun,Sichuan, China

Masson Pine (Pinus massoniana Lamb.) is a pioneer in forest succession in the subtropics of East Asia. However, the species persits, though with decreasing abundance, throughout the various successional phases. Agestructure, spatial pattern, density, population biomass, and their dynamics are described for a population in Sichuan, China, on the basis of a census of all individuals in the population while substituting space with time. In the course of succession, the population density increases and its rate of growth decreases until self-thinning starts; during the phase of self-thinning density decrease and continues to decrease even afterwards, but the rate of growth increase markedly after self-thinning has stopped. The development of population biomass (Bp) during the early succession from shrub-grassland to the early stages of mixed pine and broad-leaved forest can be described by a logistic equation. Later, Bp decrease rapidly. These changes are governed partly by inherent biological features of P. massoniana and partly by the invasion, establishment and development of shade-tolerant evergreen broad-leaved trees. Both self- and alien-thinning occur. Soil conditions affect the rates of these processes.     

Toxic tetranortriterpenes of the fruit of Melia azedarach

Four new tetranortriterpenes, meliatoxins A₁, A₂, B₁ and B₂ have been isolated and identified as toxic constituents of the fruit of Melia azedarach L. var. australasica. Toxicity and pathological results confirm that the meliatoxins are responsible for most but not all of the symptoms resulting from the ingestion of whole fruit.