Distribution,properties, and functions of midgut carboxypeptidases and dipeptidases from Musca domestica larvae

Dipeptidase and carboxypeptidase A activities were determined in cells and luminal contents of the fore-, mid-, and hind-midgut of Musca domestica larvae. Dipeptidase activity was found mainly in hind-midgut cells, whereas carboxy-peptidase activity was recovered in major amounts in both cells and in luminal contents of hind-midguts. The subcellular distribution of dipeptidase and part of the carboxypeptidase A activities is similar to that of a plasma membrane enzyme marker (aminopeptidase), suggesting that these activities are bound to the microvillar membranes. Soluble carboxypeptidase A seems to occur both bound to secretory vesicles and trapped in the cell glycocalyx. Based on density-gradient ultracentrifugation and thermal inactivation, there seems to be only one molecular species of each of the following enzymes (soluble in water or solubilized in Triton X-100): membrane-bound dipeptidase (pH optimum 8.0; Km 3.7 mM GlyLeu, M_r 111,000), soluble carboxypeptidase (pH optimum 8.0; Km 1.22 mM N-carbobenzoxy-glycyl-L-phenylalanine (ZGlyPhe), M_r45,000) and membrane-bound carboxypeptidase (pH optimum 7.5, Km 2.3 mM ZGlyPhe, M_r58,000). The results suggest that protein digestion is accomplished sequentially by luminal trypsin and luminal carboxypeptidase, by membrane-bound carboxypeptidase and aminopeptidase, and finally by membrane-bound dipeptidase.     

Water-storage capacity ofThuja,Tsuga andAcer stems measured by dehydration isotherms

Water-storage capacity was measured inThuja occidentalis L.,Tsuga canadensis (L.) Carr., andAcer saccharum Marsh. during the dehydration of stem segments 1.5–2.5 cm in diameter. Stem water potential was measured with a temperature-corrected stem hygrometer and cavitations were detected acoustically. Water loss was measured by weight change. Dehydration isotherms consistently displayed three phases. The first phase, from water potential (Ψ) 0 to about −0.2 MPa, had a high capacitance (C>0.4kg water lost· (1 of tissue)⁻¹· MPa⁻¹) and we have attributed this high C to capillary water as defined by Zimmermann (1983, Xylem structure and the ascent of sap, Springer-Verlag). The second phase from Ψ=−0.5 to about −2.0 had the lowest C values (<0.02 kg·l⁻¹·MPa⁻¹) and was accompanied by a few cavitation events. This phase may have been a transition zone between capillary storage and water released by cavitation events as well as water drawn from living cells of the bark. The third phase also had a high C (about 0.07–0.22kg·l⁻¹·MPa⁻¹) and was associated with many cavitation events while Ψ declined below about −2.5 MPa; we presume the high capacitance was the consequence of water released by cavitation events. We discuss the ecological adaptive advantage of these three phases of water-storage in trees. In moist environments, water withdrawn from capillary storage may be an important fraction of transpiration, but may be of little adaptive advantage. For most of the growth season trees draw mainly on elastic storage, but stem elastic storage is less than leaf elastic storage and therefore unlikely to be important. In very dry environments, water relased by cavitation events might be important to the short-term survival of trees.     

Validation of age readings from otoliths of juvenile roundnose grenadier, Coryphaenoides rupestris, a deep-water macrourid fish

Validation of the ageing of deep-water fish is difficult and there are only a few instances where the rings on the otoliths have been shown to be laid down annually. Roundnose grenadier have been fished commercially in the North Atlantic since the 1960s and the adult fish have frequently been aged by counting the rings in otoliths or scales. All the ageing was done on the assumption that the rings in the otoliths or scales were annual. Between 1975 and 1992, the Scottish Association for Marine Science carried out seasonal trawling surveys in the Rockall Trough using a fine-meshed trawl, and collected otoliths from a wide size range of roundnose grenadier. An examination of the growing edge of otoliths from juvenile fish from these collections suggests that the rings in the otoliths are laid down annually. The broader, opaque zones which represent the growth phase were dominant between September and March. The thinner, hyaline zones were dominant between April and July. The apparent delay in the growth phase compared with most shallow-water species is discussed in relation to the availability of mesopelagic prey.     

Identification of ATP-sensitive Potassium Channel in Frog Ventricular Myocytes

Nuclear magnetic resonance (NMR) microimaging and proton relaxation times were used to monitor differences between the hydration state of the nucleus and cytoplasm in the Rana pipiens oocyte. Individual isolated ovarian oocytes were imaged in a drop of Ringer's solution with an in-plane resolution of 80 μm. Proton spin echo images of oocytes arrested in prophase I indicated a marked difference in contrast between nucleoplasm and cytoplasm with additional intensity gradations between the yolk platelet-rich region of the cytoplasm and regions with little yolk. Neither shortening τ_e (spin echo time) to 9 msec (from 18 msec) nor lengthening τ_r (spin recovery time) to 2 sec (from 0.5 sec) reduced the observed contrast between nucleus and cytoplasm. Water proton T₁ (spin-lattice) relaxation times of oocyte suspensions indicated three water compartments that corresponded to extracellular medium (T₁= 3.0 sec), cytoplasm (T₁= 0.8 sec) and nucleoplasm (T₁= 1.6 sec). The 1.6 sec compartment disappeared at the time of nuclear breakdown. Measurements of plasma and nuclear membrane potentials with KCl-filled glass microelectrodes demonstrated that the prophase I oocyte nucleus was about 25 mV inside positive relative to the extracellular medium. A model for the prophase-arrested oocyte is proposed in which a high concentration of large impermeant ions together with small counter ions set up a Donnan-type equilibrium that results in an increased distribution of water within the nucleus in comparison with the cytosol. This study indicates: (i) a slow exchange between two or more intracellular water compartments on the NMR time-scale, (ii) an increased rotational correlation time for water molecules in both the cytoplasmic and nuclear compartments compared to bulk water, and (iii) a higher water content (per unit dry mass) of the nucleus compared to the cytoplasm, and (iv) the existence of a large (about 75 mV positive) electropotential difference between the nuclear and cytoplasmic compartments. Received: 18 January 1996/Revised: 29 April 1996     

Noxious floating weeds of Malaysia

After ten years of field surveys on various water bodies ranging from stagnant water ponds, pools and man-made lakes to flowing waters such as rivers, streams and canals, there is a clear evidence of four problematic weeds in Malaysia. These species are Eichhornia crassipes, Salvinia molesta, Lemna perpusilla, and Pistia stratiotes. Among these weeds, E. crassipes and S. molesta are widely distributed througout Malaysia. E. crassipes generally dominates canals and rivers although, recently, this species has spread to man-made lakes. The favourable tropical climate and conducive environmental factors help to trigger the massive growth of these weeds. The high nutrient concentrations, notably phosphate which has a soluble reactive concentration greater than 0.1 mg l^–1, initiate a high productivity. Manual control methods are generally used and several herbicides including 2,4-D and glyphosate are frequently employed to eradicate these weeds.     

Use of an alternative method for monitoring total nitrogen concentrations in Florida lakes

We studied a recently described method for the determination of total nitrogen in natural waters involving sample oxidation with persulfate and subsequent determination of nitrate-nitrogen with second derivative spectroscopy and compared it to the USEPA approved method involving the sum of nitrate-nitrogen and Kjeldahl-nitrogen as measured with an automated analyzer. The overall objective was to determine if the two methods gave the same estimates of total nitrogen and if the detection limits, precision and accuracy of the new method were as good as those of the USEPA method. The new method was used to make measurements on replicated blanks, standards and lake water samples covering a range of concentrations. We also collaborated with certified laboratories to make comparative measurements on 5 standards and 21 lake water samples that were run by us with the new method and by them with the USEPA method. The new method had an instrument detection limit of 0.07 mg 1^–1, and the standard deviation of 20 sets of lake water samples averaged 0.03 mg 1^–1. The new method gave concentrations equivalent to those found with the USEPA method, was more sensitive, and had a higher degree of precision. We concluded that the new method is suitable as a substitute for the USEPA method. We also found that the addition of acid to lake water samples stored under refrigeration was not necessary to preserve them for later determinations of nitrate-nitrogen and total nitrogen and that freezing was an effective means of sample preservation for 90 days.     

Molecular dynamics study of interaction of dimyristoyl phosphotidyl choline with water

We present here results of molecular dynamics (MD) simulations on hydrated bilayers of 40 molecules of 1-2-dimyristoyl-sn-glycero-3-phosphatidyl choline (DMPC) in liquid crystalline (Lα) phase using two different models (i) with same (A) conformation for all DMPC molecules, (ii) with alternate rows having different (A and B reported in crystallographic studies on DMPC) conformations. The bilayers were hydrated using 776 and 1064 water molecules. Simulations have been carried out at 310K with AMBER 4.0 program, using united atom force field for 200 pico seconds (ps) after equilibration. During heating and equilibration constant pressure temperature (PT) conditions were maintained while in simulation of equillibrated bilayers constant volume temperature (VT) conditions were used. Subaveraged atomic coordinates were used to calculate geometric parameters of lipid molecules and lipid water interaction. Our results show larger flexibility of polar head group and glycerol region in Lα phase compared to gel or non-hydrated bilayers. Chain disorder was more towards end. Sn-2 chains were more disordered. Use of two types of starting conformations increased disorder. Trans fraction of chain torsional angle was higher in non-hydrated bilayer. However it was more disordered due to ‘swing’ movement of chains because of distortion in torsional angles α2 and 03 due to absence of water molecules. Trans fraction of the chains, order parameter and water penetration showed general agreement with the available experimental results. On the whole MD technique was found to be quite useful for depicting microscopic behaviour of liquid crystalline system and correlating the same with macroscopic changes observed experimentally.     

Sap flow in wheat under free-air CO2 enrichment

The effects of elevated carbon dioxide (CO₂) concentration on plant water use are best evaluated on plants grown under field conditions and with measurement techniques that do not disturb the natural function of the plant. Heat balance sap flow gauges were used on individual main stems of wheat (Triticum aestivum L. cv Yecora rojo) grown under normal ambient conditions (control) and in a free-air CO₂ enrichment (FACE) system in Arizona with either high (control + high H₂O = CW; FACE + high H₂O = FW) or low (control + low H₂O = CD; FACE + low H₂O = FD) irrigation regimens. Over a 30d period (stem elongation to anthesis), combinations of treatments were monitored with,10–40 gauges per treatment. The effects of increased CO₂ on tiller water use were inconsistent in both the diurnal patterns of sap flow and the statistical analyses of daily sap flow (F_tot). Initial results suggested that the reductions in F_tot, from CO₂ enrichment were small (,0–10%) in relation to the H₂O treatment effect (,20–30%). For a 3d period, F_tot of FW was,19–26% less than that of CW (P = 0.10). Examination of the different sources of variation in the study revealed that the location of gauges within the experimental plots influenced the variance of the sap flow measurements. This variation was probably related to positional variation in subsurface drip lines used to irrigate plots. A sampling design was proposed for use of sap flow gauges in FACE systems with subsurface irrigation that takes into account the main treatment effects of CO₂ enrichment and the other sources of variation identified in this study. Despite the small and often statistically non-significant differences in F_tot between the CW and FW treatments, cumulative water use of the FW treatment at the end of the first three test periods ranged from 7 to 23% lower than that of the CW treatment. Differences in sap flow between FW and CW compared well with treatment differences in evapotranspiration. The results of the study, based on the first reported sap flow measurements of wheat, suggest that irrigation requirements for wheat production, in the present climatic regimen of the south-western US, may be predicted to decrease slightly because of increasing atmospheric CO₂.     

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.