Limitation of plant water use by rhizosphere and xylem conductance: results from a model

Hydraulic conductivity ( K ) in the soil and xylem declines as water potential ( Ψ ) declines. This results in a maximum rate of steady-state transpiration ( E _crit) and corresponding minimum leaf Ψ ( Ψ _crit) at which K has approached zero somewhere in the soil–leaf continuum. Exceeding these limits causes water transport to cease. A model determined whether the point of hydraulic failure (where K = 0) occurred in the rhizosphere or xylem components of the continuum. Below a threshold of root:leaf area ( A _R: A _L), the loss of rhizosphere K limited E _crit and Ψ _crit. Above the threshold, loss of xylem K from cavitation was limiting. The A _R: A _L threshold ranged from > 40 for coarse soils and/or cavitation-resistant xylem to < 0·20 in fine soils and/or cavitation-susceptible xylem. Comparison of model results with drought experiments in sunflower and water birch indicated that stomatal regulation of E reflected the species' hydraulic potential for extracting soil water, and that the more sensitive stomatal response of water birch to drought was necessary to avoid hydraulic failure. The results suggest that plants should be xylem-limited and near their A _R: A _L threshold. Corollary predictions are (1) within a soil type the A _R: A _L should increase with increasing cavitation resistance and drought tolerance, and (2) across soil types from fine to coarse the A _R: A _L should increase and maximum cavitation resistance should decrease.     

Phosphorylation and Palmitoylation of the Human D2L Dopamine Receptor in Sf9 Cells

Abstract: We have expressed and biochemically characterized the human D2_long (D2_L) dopamine receptor isoform using the baculovirus/Sf9 cell system. The expressed receptor bound ligands with a pharmacological profile similar to that reported for neuronal and cloned D2_L receptors expressed in mammalian cell lines. Dopamine binding to D2_L receptor was sensitive to guanine nucleotides, indicating receptor coupling to endogenous G proteins. A D2_L receptor-specific antibody identified two major protein species at ∼44 kDa and at ∼93 kDa in immunoblots, suggesting the presence of D2_L receptor monomers and dimers. Both species were purified by immunoprecipitation from digitonin-solubilized preparation of cells expressing D2_L receptor prelabeled with ³²P_i or [³H]-palmitate. These results constitute the first direct evidence for D2_L receptor phosphorylation and palmitoylation.     

Reversal of Mu gem2ts-induced mutations

Abstract: Mutations induced by the integration of a Mu gem 2ts mutant prophage can revert at frequencies around 1 × 10⁻⁶, more than 10⁴-fold higher than that obtained with Mu wild-type. Several aspects characterize Mu gem 2ts precise excision: (i) the phage transposase is not involved; (ii) the RecA protein is not necessary; and (iii) revertants remain lysogenic with the prophage inserted elsewhere in the host genome. In addition, prophage re-integration seems to be non-randomly distributed, whereas Mu insertion into the host genome is a transposition event without any sequence specificity. In this paper, we describe that the site of re-integration somehow depends on the original site of insertion. Two alternative models are proposed to explain the strong correlation between donor and receptor sites.     

Nitrogen/phosphorus leaf stoichiometry and the scaling of plant growth

We adopted previous N : P stoichiometric models for zooplankton relative growth to predict the relative growth rates of the leaves μ _L of vascular plants assuming that annual leaf growth in dry mass is dictated by how leaf nitrogen N _L is allocated to leaf proteins and how leaf phosphorus P _L is allocated to rRNA. This model is simplified provided that N _L scales as some power function of P _L across the leaves of different species. This approach successfully predicted the μ _L of 131 species of vascular plants based on the observation that, across these species, N _L scaled, on average, as the 3/4 power of P _L, i.e. N _L ∝  P     . When juxtaposed with prior allometric theory and observations, our findings suggest that a transformation in N : P stoichiometry occurs when the plant body undergoes a transition from primary to secondary growth.     

Effects of temperature, body size and feeding on rates of metabolism in young‐of‐the‐year haddock

The mean rate of oxygen consumption (routine respiration rate, R _R, mg O₂ fish⁻¹ h⁻¹), measured for individual or small groups of haddock Melanogrammus aeglefinus (3–12 cm standard length, L _S) maintained for 5 days within flow‐through respiratory chambers at four different temperatures, increased with increasing dry mass ( M _D). The relationship between R _R and M _D was allometric ( R _R = α  M ^b ) with b values of 0·631, 0·606, 0·655 and 0·650 at 5·0, 8·0, 12·0 and 15·0° C, respectively. The effect of temperature ( T ) and M _D on mean R _R was described by     indicating a Q ₁₀ of 2·27 between 5 and 15° C. Juvenile haddock routine metabolic scope, calculated as the ratio of the mean of highest and lowest deciles of R _R measured in each chamber, significantly decreased with temperature such that the routine scope at 15° C was half that at 5° C. The cost of feeding ( R _SDA) was c . 3% of consumed food energy, a value half that found for larger gadoid juveniles and adults.     

Polypeptide Elongation Factors of the Developing Chick Brain

Abstract: The polypeptide elongation factors (EF-1_L, EF-1_H, and EF-2) of the developing chick brain were separated and purified by means of a combination of gel chromatographic methods. The molecular weight of EF-1_H of the chick brain ranged from 5 to 10 × 10⁵, and was different from that of the chick liver (about 7 × 10⁵). The molecular weight of other purified factors was about 5 × 10⁴ for EF-1_L. and 9.4 × 10⁴ for EF-2. High activities of polyphenylalanine (poly-Phe) synthesis per mg protein in the developing chick brain were observed between the 3rd embryonic week and the 1st post-hatch week and declined afterwards. On the other hand, the levels of both EF-1 and EF-2 per mg protein in the brain were observed to be high in an early embryonic stage, gradually declining afterwards to the adult level. The brain EF-1_L was a major component of EF-1 in an early embryonic stage, while EF-1_H became recognizable in the 3rd embryonic week. Moreover, the EF-1_H activities were found to be more than double with regard to the binding reaction and to be more than 10-fold as active in respect to poly-Phe synthesis in comparison with the activities of EF-1_L. It is proposed that the brain EF-1_H could be due to aggregates consisting of EF-1_L, a stimulatory factor, and other components.     

Correlation among dominance status, metabolic rate and otolith size in masu salmon

In spatial competition between individuals, neither fish sex nor body mass affected dominance status in masu salmon Oncorhynchus masou . In contrast, resting metabolic rate ( M _R) was significantly correlated with dominance status, indicating that a high metabolic rate can increase the dominance rank of juvenile salmon. Whole animal growth rate was significantly correlated with M _R, but not with initial body weight. This suggests that the body size of masu salmon is not a cause, but rather a consequence, of dominance status which is closely related to M _R. The increment width between otolith daily rings was also significantly correlated with M _R. Thus, the size of the Otolith may indicate the degree of M _R.     

Effects of Amphetamine on Protein Synthesis and Energy Metabolism in Mouse Brain: Role of Drug-Induced Hyperthermia

Abstract: Changes in brain protein synthesis activity, and in brain levels of glucose, glycogen, and several high-energy phosphate metabolites, were evaluated under conditions of amphetamine-induced hyperthermia in mice. Protein synthesis showed a striking dependence on rectal temperature ( T _R), falling abruptly at T _R above 40°C. A similar result was obtained following direct heating of the animals. Protein synthesis activity in liver showed the same temperature dependence observed for brain. Increased synthesis of a protein with characteristics of the major mammalian stress protein, hsp 70, was demonstrated in both brain and liver following amphetamine administration. Brain protein synthesis showed significant recovery within 2 h after amphetamine administration whereas that of liver remained below 30% of control activity, suggesting significant temporal and quantitative differences in the response of individual tissues to elevated temperatures. Brain glycogen levels after amphetamine administration were significantly lower under conditions of ambient temperature which resulted in more severe drug-induced hyperthermia but did not correlate as strikingly as protein synthesis with the temperatures of individual animals. Brain glycogen also fell in animals whose temperatures were increased by brief exposure at high ambient temperature. Brain glucose levels did not consistently change with hyperthermia. Slight decreases in high-energy phosphates with increasing T _R were likely the result of fixation artifact. These results demonstrate the fundamental role of hyperthermia in the reduction of protein synthesis in brain and other tissues by amphetamine, and suggest that temperature also constitutes a significant source of variability in the effects of this drug on brain energy metabolism, in particular glycogenolysis.     

Competitive strategies for water availability in two Mediterranean Quercus species

Competition for water availability was studied in a mixed natural stand of Quercus suber L. and Quercus cerris L. growing in Sicily by measuring diurnal changes of leaf conductance to water vapour ( g _L), water potential ( Ψ _L) and relative water content ( RWC ) in April, July and October 1997 as well as the seasonal changes in root hydraulic conductance per unit leaf surface area ( K _RL). Quercus cerris behaved as a drought-tolerant species, with strong reductions of K _RL, Ψ _L, and RWC in the summer. By contrast, Q. suber appeared to withstand summer drought by an avoidance strategy based on reducing g _L, maintaining Ψ _L and RWC high and K _RL at the same level as that measured in the spring. A 'conductance ratio' ( CR ) was calculated in terms of the ratio of g _L to K _RL. Seasonal changes of this ratio contrasted in the two species, thus suggesting that Q. suber and Q. cerris did not really compete for available water. In the summer, when Q. suber was extracting water from the soil to maintain high leaf hydration, Q. cerris had restricted water absorption, thus suffering drought but tolerating its effects. The possibility that cohabitation of drought-tolerant with drought-avoiding species can be generalized is also discussed.     

Changes in leaf hydraulics and stomatal conductance following drought stress and irrigation in Ceratonia siliqua (Carob tree)

Changes in leaf hydraulic conductance (K) were measured using the vacuum chamber technique during dehydration and rehydration of potted plants of Ceratonia siliqua . K of whole, compound leaves as well as that of rachides and leaflets decreased by 20–30% at leaf water potentials (Ψ_L) of −1.5 and −2.0 MPa, i.e. at Ψ_L values commonly recorded in field-growing plants of the species. Higher K losses (up to 50%) were measured for leaves at Ψ_L of −2.5 and −3.0 MPa, i.e. near or beyond the leaf turgor loss point. Leaves of plants rehydrated while in the dark for 30 min, 90 min and 12 h recovered from K loss with characteristic times and to extents inversely proportional to the initial water stress applied. Leaf conductance to water vapour of plants dehydrated to decreasing Ψ_L and rehydrated at low transpiration was inversely related to loss of K, thus suggesting that leaf vein embolism and refilling (and related changes in leaf hydraulics) may play a significant role in the stomatal response.     

Effect of water availability on leaf water isotopic enrichment in beech seedlings shows limitations of current fractionation models

Current models of leaf water enrichment predict that the differences between isotopic enrichment of water at the site of evaporation (Δ_e) and mean lamina leaf water enrichment (Δ_L) depend on transpiration rates ( E ), modulated by the scaled effective length ( L ) of water isotope movement in the leaf. However, variations in leaf parameters in response to changing environmental conditions might cause changes in the water path and thus L . We measured the diel course of Δ_L for ¹⁸O and ²H in beech seedlings under well-watered and water-limited conditions. We applied evaporative enrichment models of increasing complexity to predict Δ_e and Δ_L, and estimated L from model fits. Water-limited plants showed moderate drought stress, with lower stomatal conductance, E and stem water potential than the control. Despite having double E , the divergence between Δ_e and Δ_L was lower in well-watered than in water-limited plants, and thus, L should have changed to counteract differences in E . Indeed, L was about threefold higher in water-limited plants, regardless of the models used. We conclude that L changes with plant water status far beyond the variations explained by water content and other measured variables, thus limiting the use of current evaporative models under changing environmental conditions.     

Effect of body size on the standard metabolism of horse mackerel

The routine metabolic rate R _R and standard metabolic rate R _S were measured in horse mackerel Trachurus trachurus at 13°C over weight range of 1·4–390 g. A data extraction method rather than the more commonly used method of extrapolating the swimming speed-metabolic rate curves back to zero swimming speed was developed to measure the R _S. The relation between R _R and R _S and weight was expressed as a linear regression with the log transformed data. The mean slope of the regression was 0·752 for R _S and 0·725 for R _S.     

Buoyancy effect on forced convection in the leaf boundary layer

Abstract. Mixed convection (forced convection plus free convection) in the leaf boundary layer was examined by air flow visualization and by evaluation of the boundary layer conductance at different leaf-air temperature differences ( T _L- T _A) under low wind velocities. The visualized air flow was found to become more unstable and buoyant at higher T _L- T _A. An ascending longitudinal plume was induced along the upper surface, and the air flow along the lower surface ascended after passing the trailing leaf edge. The air flow modified by buoyancy was considered to result in an increase in boundary layer conductance ( G _A) for mixed convection, which became higher with higher T _L- T _A as compared with the conductance for pure forced convection without buoyancy. This increase in G _A appeared larger at larger Grashof number (Gr) and at smaller Reynolds number (Re). The dependences of buoyancy effect on Gr and Re were related to 'edge-effects'.     

Modelling environmental controls on ecosystem photosynthesis and the carbon isotope composition of ecosystem-respired CO2 in a coastal Douglas-fir forest

We developed and applied an ecosystem-scale model that calculated leaf CO₂ assimilation, stomatal conductance, chloroplast CO₂ concentration and the carbon isotope composition of carbohydrate formed during photosynthesis separately for sunlit and shaded leaves within multiple canopy layers. The ecosystem photosynthesis model was validated by comparison to leaf-level gas exchange measurements and estimates of ecosystem-scale photosynthesis from eddy covariance measurements made in a coastal Douglas-fir forest on Vancouver Island. A good agreement was also observed between modelled and measured δ ¹³C values of ecosystem-respired CO₂ ( δ _R). The modelled δ _R values showed strong responses to variation in photosynthetic photon flux density (PPFD), air temperature, vapour pressure deficit (VPD) and available soil moisture in a manner consistent with leaf-level studies of photosynthetic ¹³C discrimination. Sensitivity tests were conducted to evaluate the effect of (1) changes in the lag between the time of CO₂ fixation and the conversion of organic matter back to CO₂; (2) shifts in the proportion of autotrophic and heterotrophic respiration; (3) isotope fractionation during respiration; and (4) environmentally induced changes in mesophyll conductance, on modelled δ _R values. Our results indicated that δ _R is a good proxy for canopy-level C _c/ C _a and ¹³C discrimination during photosynthetic gas exchange, and therefore has several applications in ecosystem physiology.     

Respiratory responses of Oreochromis niloticus (Pisces, Cichlidae) to environmental hypoxia under different thermal conditions

The oxygen uptake ( V O₂), breathing frequency ( f _R), breath volume ( V _S.R), gill ventilation ( V G) and oxygen extraction (%) from the ventilatory current of four groups of Oreochromis niloticus during graded hypoxia were measured under the following acclimation temperatures: 20. 25. 30 and 35°C. The critical oxygen tensions ( P O₂), determined from V O₂ v. P O₂ of inspired water at each experimental temperature were, respectively. 19±1±3±1. 18±0±4±9, 29±7± 4±1 and 30±2± 0.6 mmHg. The f _R remained nearly constant during the reductions of O₂ at all the temperatures studied. V G increased discretely from normoxic levels until the P O₂ was reached, below which it assumed extremely high values (17-fold higher or more). The increases observed in V G resulted, at all the acclimation temperatures, in an elevation in V _S.R rather than in f _R. The extraction of O₂ decreased gradually from normoxia until the P O₂ was reached, below which an abrupt reduction of extraction was recorded, except at 35°C when fish showed a gradual reduction in extraction just below the tension of 80 mmHg.