Primary production, light and vertical mixing in Potter Cove, a shallow bay in the maritime Antarctic

Phytoplankton photosynthesis was measured during spring-summer 1991-1992 in the inner and outer part of the shallow Potter Cove, King George Island. Strong winds characterise this area. Wind-induced turbulent mixing was quantified by means of the root-mean square expected vertical displacement depth of cells in the water column, Z_t. The light attenuation coefficient was used as a measure of the influence of the large amount of terrigenous particles usually present in the water column; 1% light penetration ranged between 30 and 9 m, and between 30 and 15 m for the inner and outer cove, respectively. Obvious differences between photosynthetic capacity [P*_max; averages 2.6 and 0.6 µg C (µg chlorophyll-a)^-1 h^-1] and photosynthetic efficiency {!*; 0.073 and 0.0018 µg C (µg chlorophyll-a)^-1 h^-1 [(µmol m^-2 s^-1)^-1]} values were obtained for both sites during low mixing conditions (Z_t from 10 to 20 m), while no differences were found for high mixing situations (Z_t>20 m). This suggests different photoacclimation of phytoplankton responses, induced by modifications of the light field, which in turn are controlled by physical forcing. Our results suggest that although in experimental work P*_max can be high, wind-induced mixing and low irradiance will prevent profuse phytoplankton development in the area.     

Modelling mixing parameters in concentric-tube airlift bioreactors

Axial dispersion of the liquid phase was investigated in a concentric-tube airlift bioreactor (RIMP: V _L=0.70?m³) as a whole and in the separate zones (riser, downcomer, gas-separator) using the axial dispersion model. The axial dispersion number Bo and the axial dispersion coefficient, D _ax were determined from the output curves to an initial Dirac pulse, using the tracer response technique. They were analyzed in relation to process and geometrical parameters, such as: gas superficial velocity, ν_SGR; top clearance, h _S; bottom clearance, h _B, and resistances at downcomer entrance expressed as A _d/A _R ratio. Correlations between Bodenstein numbers in the overall bioreactor and riser and downcomer sections (Bo_T,Bo_R,Bo_D) and the geometrical and process parameters were developed, which can allow to assess the complex influence of these parameters on liquid axial dispersion.     

Concentric-tube airlift bioreactors

Liquid circulation velocity was investigated in three concentric-tube airlift reactors of different scales (RIMP, V _L =0.07 m³; RIS-1, V _L =2.5 m³; RIS-2, V _L =5.20 m³). The effects of top and bottom clearance and resistance in flow pathway at downcomer entrance on the riser liquid superficial velocity, the circulation time, the friction coefficient and flow radial profiles of the gas holdup and the liquid superficial velocity in riser, using water-air as a biphasic system, were studied. It was found that the riser liquid superficial velocity is affected by the analyzed geometrical parameters in different ways, depending on their effects on the pressure loss. The riser liquid superficial velocity, the friction coefficient and the parameters of the drift-flux model were satisfactorily correlated with the bottom spatial ratio (B), gas separation ratio (Y) and downcomer flow resistance ratio (A _d /A _D ), resulting empirical models, with correlation coefficients greater than 0.85.     

Influence of climate-driven shifts in biomass allocation on water transport and storage in ponderosa pine

Conifers decrease the amount of biomass apportioned to leaves relative to sapwood in response to increasing atmospheric evaporative demand. We determined how these climate-driven shifts in allocation affect the aboveground water relations of ponderosa pine growing in contrasting arid (desert) and humid (montane) climates. To support higher transpiration rates, a low leaf:sapwood area ratio (A_L/A_S) in desert versus montane trees could increase leaf-specific hydraulic conductance (K_L). Alternatively, a high sapwood volume:leaf area ratio in the desert environment may increase the contribution of stored water to transpiration. Transpiration and hydraulic conductance were determined by measuring sap flow (J_S) and shoot water potential during the summer (June-July) and fall (August-September). The daily contribution of stored water to transpiration was determined using the lag between the beginning of transpiration from the crown at sunrise and J_S. In the summer, mean maximum J_S was 31.80LJ.74 and 24.34Dž.05 g m^-2 s^-1 for desert and montane trees (a 30.6% difference), respectively. In the fall, J_S was 25.33NJ.52 and 16.36dž.64 g m^-2 s^-1 in desert and montane trees (a 54.8% difference), respectively. J_S was significantly higher in desert relative to montane trees during summer and fall (P<0.05). Predawn and midday shoot water potential and sapwood relative water content did not differ between environments. Desert trees had a 129% higher K_L than montane trees in the summer (2.41᎒^-5 versus 1.05᎒^-5 kg m^-2 s^-1 MPa^-1, P<0.001) and a 162% higher K_L in the fall (1.97᎒^-5 versus 0.75᎒^-5 kg m^-2 s^-1 MPa^-1, P<0.001). Canopy conductance decreased with D in all trees at all measurement periods (P<0.05). Maximum g_C was 3.91 times higher in desert relative to montane trees averaged over the summer and fall. Water storage capacity accounted for 11 kg (11%) and 10.6 kg (17%) of daily transpiration in the summer and fall, respectively, and did not differ between desert and montane trees. By preventing xylem tensions from reaching levels that cause xylem cavitation, high K_L in desert ponderosa pine may facilitate its avoidance. Thus, the primary benefit of low leaf:sapwood allocation in progressively arid environments is to increase K_L and not to increase the contribution of stored water to transpiration.     

In situ oxygen microelectrode measurements of bottom-ice algal production in McMurdo Sound, Antarctica

In-situ estimates of fast-ice algal productivity at Cape Evans, McMurdo Sound, in 1999 were lower than at the same site in previous years. Under-ice irradiance was between 0 and 8 µmol photons m^-2 s^-1; the ice was between 1.9 and 2.0 m thick and the algal biomass averaged 150 mg chl a m^-2, although values as high as 378 mg chl a m^-2 were recorded. Production on 11 and 12 November was between 0.053 and 1.474 mg C m^-2 h^-1. When the data from 11 November were fitted to a hyperbolic tangent function, a multilinear regression gave estimates for P_max of 0.571 nmol O₂ cm^-2 s^-1, an ! of 0.167 nmol O₂ cm^-2 s^-1 µmol^-1 photons m^-2 s^-1 and an E_k of 3.419 µmol photons m^-2 s^-1. A P_max of 2.674 nmol O₂ cm^-2 s^-1, an ! of 0.275 nmol O₂ cm^-2 s^-1 µmol^-1 photons m^-2 s^-1, r of 0.305 nmol O₂ cm^-2 s^-1 and an E_k of 9.724 µmol^-1 photons m^-2 s^-1 were estimated from the 12 November data. The sea-ice algal community was principally comprised of Nitzschia stellata, Entomoneis kjellmanii and Berkeleya adeliensis. Other taxa present included N. lecointei, Fragilariopsis spp., Navicula glaciei, Pleurosigma spp. and Amphora spp. Variations in the method for estimating the thickness of the diffusive boundary layer were not found to significantly affect the measurements of oxygen flux. However, the inability to accurately measure fine-scale variations in biomass is thought to contribute to the scatter of the P versus E data.     

Lactate dehydrogenase from the Antarctic eelpout, Lycodichthys dearborni

As part of our studies to examine the molecular basis of cold-adaptation, we have determined the kinetic properties, thermal stability and deduced amino acid sequence of the enzyme lactate dehydrogenase (LDH) from an Antarctic zoarcid fish, Lycodichthys dearborni. Unlike Antarctic notothenioid fish which are endemic to the Southern Ocean, zoarcid fish are cosmopolitan and have a substantially longer evolutionary history as a sub-order. The A₄-LDH isoform was isolated and purified from the white muscle of L. dearborni. The kinetic parameters K_m^PYR and k_cat were determined at temperatures from 0 to 25°C. K_m^PYR was substantially higher at low temperatures than those from Antarctic and temperate notothenioid fish, whereas k_cat at these temperatures was essentially the same as those of the other fish LDH in this study. The sequence of L. dearborni A₄-LDH was determined from cDNA derived from white muscle RNA and found to be similar to, but distinct from, the A₄-LDH sequences of Antarctic notothenioid fish. Molecular modelling based on the structure of the A₄-LDH from Pagothenia borchgrevinki suggested that three conservative amino acid changes within the core of the protein that are not directly part of the active site but which might nonetheless influence the active site, may be important in cold-adaptation in L. dearborni A₄-LDH, and that several other changes on the surface of the protein might also play a role in cold-adaptation.     

Physiological energetics of the protobranch bivalve <Emphasis Type="Italic">Yoldia hyperborea</Emphasis> in a cold ocean environment

Yoldia hyperborea is a deposit-feeding circumpolar protobranch that also inhabits muddy sediments of the cold water boreal system of Conception Bay, Newfoundland, Canada. Little is known about this species, despite its wide distribution and frequent high density in the benthos. The present work deals with oxygen consumption and ammonia excretion under cold ambient conditions. Y. hyperborea showed low basal metabolism [0.051 ml O₂ h^у·(g dry weight)^у, T=у°C] and low ammonia excretion rates [4.212 µg·NH₄-N·h^у·(g dry weight)^у, T=у°C]. Low metabolic activity could prove a useful strategy during periods of low food availability. In addition, Y. hyperborea was able to regulate its O₂ consumption rate at very low pO₂ levels, which may be advantageous for a species that may experience periods of hypoxia.     

Soil microbial community and bacterial functional diversity at Machu Picchu, King George Island, Antarctica

The objective of this study was to evaluate the potential contribution of the soil microbial community in the vicinity of two plant covers, Sanionia uncinata and Deschampsia antarctica, at Machu Picchu Station, King George Island, Antarctica. Soil samples were collected at the study site during the southern (pole) summer period from 0-5 cm and 5-10 cm depths, for chemical and biological analyses. Soil microbial biomass reached a maximal value of 144 µg g^-1 in soil samples taken from under the S. uncinata upper layer plant. qCO₂ ranged from 167 to 239 µg CO₂^.mgC_mic^.h^-1 at the 0-5 and 5-10 cm depths, respectively. CO₂ evolution showed values of 54.3 mg^.m^-2 h^-1 beneath plant cover and 55.9 mg^.m^-2 h^-1 in the open space. CO₂ evolved by substrate induced respiration in the soil samples taken under the plant cover in the summer period, oscillated between 0.25 and 4.78 µg CO₂ g^-1 h^-1. The data obtained from this short study may provide evidence that both activity and the composition and substrate utilization of the microbial community appear to change substantially across the moisture level and sample location.     

Influence of k L a on bioconversion of rice straw hemicellulose hydrolysate to xylitol

The effect of overall oxygen mass transfer coefficient (k_La) on the conversion of xylose to xylitol by Candida guilliermondii FTI 20037 was investigated in batch experiments. Rice straw hemicellulose hydrolysate obtained by acid hydrolysis was employed as a xylose-rich medium. The results showed that this bioconversion strongly depended on the aeration rate. The maximum volumetric productivity (0.52 g/l h^у) and the highest xylitol yield (0.73 g/g) were achieved at an overall oxygen mass transfer coefficient of 15 h^у. Under these conditions 80% efficiency in relation to theoretical yield was attained.     

Photosynthetic induction in saplings of three shade-tolerant tree species: comparing understorey and gap habitats in a French Guiana rain forest

The photosynthetic induction response under constant and fluctuating light was examined in naturally occurring saplings (about 0.5-2 m in height) of three shade-tolerant tree species, Pourouma bicolor spp digitata, Dicorynia guianensis, and Vouacapoua americana, growing in bright gaps and in the shaded understorey in a Neotropical rain forest. Light availability to saplings was estimated by hemispherical photography. Photosynthetic induction was measured in the morning on leaves that had not yet experienced direct sunlight. In Dicorynia, the maximum net photosynthesis rate (A_max) was similar between forest environments (ca 4 µmol m^-2 s^-1), whereas for the two other species, it was twice as high in gaps (ca 7.5) as in the understorey (ca 4.5). However, the time required to reach 90% of A_max did not differ among species, and was short, 7-11 min. Biochemical induction was fast in leaves of Pourouma, as about 3 min were needed to reach 75% of maximum carboxylation capacity (V_cmax); the two other species needed 4-5 min. When induction continued after reaching 75% of V_cmax, stomatal conductance increased in Pourouma only (ca 80%), causing a further increase in its net photosynthesis rate. When fully induced leaves were shaded for 20 min, loss of induction was moderate in all species. However, gap saplings of Dicorynia had a rapid induction loss (ca 80%), which was mainly due to biochemical limitation as stomatal conductance decreased only slowly. When leaves were exposed to a series of lightflecks separated by short periods of low light, photosynthetic induction increased substantially and to a similar extent in all species. Although A_max was much lower in old than in young leaves as measured in Dicorynia and Vouacapoua, variables of the dynamic response of photosynthesis to a change in light tended to be similar between young and old leaves. Old leaves, therefore, might remain important for whole-plant carbon gain, especially in understorey environments. The three shade-tolerant species show that, particularly in low light, they are capable of efficient sunfleck utilization.     

Mapping of a thermo-sensitive earliness <Emphasis Type="Italic">per se</Emphasis> gene on <Emphasis Type="Italic">Triticum monococcum</Emphasis> chromosome 1A<Superscript>m</Superscript>

An earliness per se gene, designated Eps-A^m1, was mapped in diploid wheat in F₂ and single-seed descent mapping populations from the cross between cultivated (DV92) and wild (G3116) Triticum monococcum accessions. A QTL with a peak on RFLP loci Xcdo393 and Xwg241, the most distal markers on the long arm of chromosome 1A^m, explained 47% of the variation in heading date (LOD score 8.3). Progeny tests for the two F_2:3 families with critical recombination events between Xcdo393 and Xwg241 showed that the gene was distal to Xcdo393 and linked to Xwg241. Progeny tests and replicated experiments with line #3 suggested that Eps-A^m1 was distal to Xwg241. This gene showed a large effect on heading date in the controlled environment experiments, and a smaller, but significant, effect under natural conditions. Eps-A^m1 showed significant epistatic interactions with photoperiod and vernalization treatments, suggesting that the different classes of genes affecting heading date interact as part of a complex network that controls the timing of flowering induction. Besides its interactions with other genes affecting heading date, Eps-A^m1 showed a significant interaction with temperature. The effect of temperature was larger in plants carrying the DV92 allele for late flowering than in those carrying the G3116 allele for early flowering. Average differences in heading date between the experiments performed at 16 °C and 23 °C were approximately 11 days (P < 0.001) for the lines carrying the Eps-A^m1 allele for early flowering but approximately 50 days (P < 0.0001) for the lines carrying the allele for late flowering. The large differences in heading time (average 80 days) observed between plants carrying the G3116 and DV92 alleles when grown at 16 °C, suggest that it would be possible to produce very detailed maps for this gene to facilitate its future positional cloning.     

Effect of drought on leaf gland secretion of the mangrove Avicennia germinans L.

This study assessed the effects of salinity changes over space and time upon leaf gland secretion in Avicennia germinans trees growing naturally in an area featuring markedly seasonal rainfall. Soil ? (, soil N MPa) during the wet season was -0.95ǂ.05 and -2.12ǂ.08 at low and high salinity sites, respectively. During the dry season, these values decreased to -3.24ǂ.09 at low salinity and to -5.75ǂ.06 at high salinity. Consequently, predawn and midday plant water potential were lowered during drought at both sites. The rates of secretion (mmol m^-2 h^-1 ) increased during drought from 0.91ǂ.12 during the wet season to 1.93ǂ.12 at low salinity, and from 1.69ǂ.12 during the wet season to 2.81ǂ.15 at high salinity. Conversely, stomatal conductance (g_s) was lowered by both salinity and drought. As xylem osmolality increased during drought, secretion tended to rise exponentially, and g_s decayed hyperbolically. Thus, a trade-off is obtained between enhancement in salt secretion and control of water loss suggested by g_s.     

Considerations of correlated fertility between genders on genetic diversity: the Pinus densiflora seed orchard as a model

The correlation between 99 clone female and male fertilities in a first generation seed orchard of Pinus densiflora was studied over 6 years. The effective number of the parent (N_p) and the variance effective population number [N_e^(v)] were used to assess the impact of total (O_T), female (N_f) and male (N_m) fertility variation. A theoretical framework was developed to account for female and male fertility correlations as well as the impact of possible pollen contamination. Total fertility variation was described by the sibling coefficient (O_T: the probability that two genes randomly chosen from the gamete gene pool originate from the same parent), which was further subdivided into N_f and N_m. These parameters were compared under various conditions including the total seed harvest, imposing on equal seed harvest among the orchard's clones and two contamination scenarios (M = 0 and 20%). Fertility variations among females, males and clones were observed within and among years. Sibling coefficients (O_T) were lower, but the effective number of parent (N_p) and variance effective population number (N_e^(v)) were higher in years with moderate female and good male strobilus production. N_p for female and male reproductive outputs varied from 49 to 82 and from 57 to 93, respectively. N_p was higher for males than females. When the crop of the 6 years was pooled, N_p for female, male and the clone were 73, 87 and 85, respectively. The impact of female-male fertility correlation for conditions with no-, positive- and negative-correlations were assessed and their impact on O_T, N_p and N_e^(v) was also evaluated. It was demonstrated that the practice of equal seed harvesting from every clone, or the mixing of seeds from several years, would substantially improve the genetic diversity and the genetic representation of the seed orchard population when a positive correlation between gender fertilities was observed. The relevance of these results to supplemental-mass-pollination was discussed under two cases where equal- and un-equal amounts of pollen from clones were included in the pollen mixes.     

An evaluation of the uniform stress hypothesis based on stem geometry in selected North American conifers

The uniform stress hypothesis of stem formation was evaluated by comparing stem taper of Abies balsamea, Abies lasiocarpa, Picea rubens, Pinus contorta, Pinus elliottii, Pinus palustris, Pinus ponderosa, Pinus taeda, and Pseudotsuga menziesii to the taper expected if stems develop to uniformly distribute bending stress. The comparison was conducted by regressing stem diameter at height h (D_h) against bending moment at h (M_h) using the model D_h=J (M_h)^' where J and ' are fitted coefficients, and testing for '=0.333, the hypothesized value. Twelve curves were fitted with the model. Seven of the fitted values of ' were significantly different from 0.333, but eight of the values were within ᆞ% of 0.333 and eleven values were within ᆣ% of 0.333. Where the fitted value of ' was >15% of 0.333, residuals were biased with height. Fit by relative height, values of ' were within ᆞ% of 0.333 for large portions of these stems. While most of the fitted values of ' support the uniform-stress hypothesis, the values of ' for Pseudotsuga menziesii trees clearly did not. Many of the fitted values of J were inversely related to the modulus of elasticity (E) of green wood reported for these species. With the exception of Pseudotsuga menziesii, growing conditions appeared to account for extraordinary values of J. Increases in J with stem height corresponded with reported decreases in E with height. The covariance between J and E suggests some regulation of bending curvature by adjustments in cross-sectional area. These results suggest that stems taper to maintain a uniform bending curvature and that when E is relatively constant within and among stems, diameter along the stem or across stems can be predicted from bending moment using a simple power function.     

K L a Evaluation during the course of fermentation using data reconciliation techniques

The oxygen mass transfer coefficient often serves to compare the efficiency of bioreactors and their mixing devices as well as being an important scale-up factor. In submerged fermentation, four methods are available to estimate the overall oxygen mass transfer coefficient (K_La): the dynamic method, the stationary method based on a previous determination of the oxygen uptake rate (Q_O2X), the gaseous oxygen balance and the carbon dioxide balance. Each method provides a distinct estimation of the value of K_La. Data reconciliation was used to obtain a more probable value of K_La during the production of Saccharomyces cerevisiae, performed in 22.5-l fed-batch bioreactor. The estimate of K_La is obtained by minimising an objective function that includes measurement terms and oxygen conservation models, each being weighted according to their level of confidence. Weighting factors of measurement terms were taken as their respective inverse variance whereas weighting factors of oxygen conservation models were obtained using Monte Carlo simulations. Results show that more coherent and precise estimations of K_La are obtained.     

A mesocosm study of physical-biological interactions in artificial sea ice: effects of brine channel surface evolution and brine movement on algal biomass

The impact of changing physico-chemical boundary conditions in sea ice on biological processes was investigated during a 20-day-long simulated freeze-melt cycle in an 180-m³ mesocosm filled with artificial seawater and addition of a mixed Arctic sea-ice community. Ice formation started at T_air of -15°C with a growth rate of 0.7-1.2 mm h^-1 for 10 days. The last 10 days (T_air of=-5°C), ice thickness remained around 20 cm. Ice temperature gradients inside the ice were linear and determined brine salinities. Brine was collected by means of centrifugation and its volume ranged from 5 to 30% of total ice volume. Surface areas of interconnected brine channels were determined with two similar techniques and maximum values ranged between 1.5 and 4.8 m² kg^-1ice. Measurements determined with a modified method varied considerably and differed by a maximal factor of 2.0-6.5. Brine channel surfaces increased during the experiment as a result of the warming of the ice. The inoculated algal community was dominated by flagellates <10 µm. The low diatom biomass increased in the ice after the air temperature rise with rates comparable to field data (µ=0.2-0.3 day^-1). Comparison with brine salinities points towards the hypothesis of vertical brine stability being a controlling factor for ice algal growth. We infer from brine channel surface measurements that persistence of brine channel surfaces during spring might be an important prerequisite for the commencement of net diatom biomass accumulation. Advantages and limitations of mesoscale mesocosms as alternatives in ice biological work are discussed.     

Water availability and carbon isotope discrimination in conifers

The stable C isotope composition ('¹³C) of leaf and wood tissue has been used as an index of water availability at both the species and landscape level. However, the generality of this relationship across species has received little attention. We compiled literature data for a range of conifers and examined relationships among landscape and environmental variables (altitude, precipitation, evaporation) and '¹³C. A significant component of the variation in '¹³C was related to altitude (discrimination decreased with altitude in stemwood, 2.53‰ km^-1 altitude, r²=0.49, and in foliage, 1.91‰ km^-1, r²=0.42), as has been noted previously. The decrease in discrimination with altitude was such that the gradient in CO₂ partial pressure into the leaf (P_a-P_i) and altitude were generally unrelated. The ratio of precipitation to evaporation (P/E) explained significant variation in P_a-P_i of stemwood (r²=0.45) and foliage (r²=0.27), but only at low (<0.8) P/E. At greater P/E there was little or no relationship, and other influences on '¹³C probably dominated the effect of water availability. We also examined the relationship between plant drought stress (O) and '¹³C within annual rings of stemwood from Pinus radiata and Pinus pinaster in south-western Australia. Differential thinning and fertiliser application produced large differences in the availability of water, nutrients and light to individual trees. At a density of 750 stems ha^-1, O and '¹³C were less (more negative) than at 250 stems ha^-1 indicating greater drought stress and less efficient water use, contrary to what was expected in light of the general relationship between discrimination and P/E. The greater '¹³C of trees from heavily thinned plots may well be related to an increased interception of radiation by individual trees and greater concentrations of nutrients in foliage - attributes that increase rates of photosynthesis, reduce P_i and increase '¹³C. '¹³C was thus modified to a greater extent by interception of radiation and by nutrient concentrations than by water availability and the '¹³C-O relationship varied between thinning treatments. Within treatments, the relationship between '¹³C and O was strong (0.38<r²<0.58). We conclude that '¹³C may well be a useful indicator of water availability or drought stress, but only in seasonally dry climates (P/E<1) and where variation in other environmental factors can be accounted for.     

Dwarf mistletoe affects whole-tree water relations of Douglas fir and western larch primarily through changes in leaf to sapwood ratios

Dwarf mistletoes induce abnormal growth patterns and extreme changes in the biomass allocation of their hosts as well as directly parasitizing them for resources. Because biomass allocation can affect the resource use and efficiency of conifers, we studied the influences of dwarf mistletoe infection on above-ground biomass allocation of Douglas fir and western larch, and the consequences of such changes on whole-tree water use and water relations. Sap flow, tree water potentials, leaf:sapwood area ratios (A_L:A_S), leaf carbon isotope ratios, and nitrogen content were measured on Douglas fir and western larch trees with various degrees of mistletoe infection during the summer of 1996 in western Montana. Heavy dwarf mistletoe infection on Douglas fir and western larch was related to significant increases in A_L:A_S. Correspondingly, water transport dynamics were altered in infected trees, but responses were different for the two species. Higher A_L:A_S ratios in heavily infected Douglas firs were offset by increases in sapwood area-based sap flux densities (Q_SW) such that leaf area-based sap flux densities (Q_L) and predawn leaf water potentials at the end of the summer did not change significantly with mistletoe infection. Small (but statistically insignificant) decreases of Q_L for heavily infected Douglas firs were enough to offset increases in leaf area such that whole-tree water use was similar for uninfected and heavily infected trees. Increased A_L:A_S ratios of heavily infected western larch were not offset by increases of Q_SW. Consequently, Q_L was reduced, which corresponded with significant decreases of water potential at the end of the summer. Furthermore, mistletoe-infection-related changes in A_L:A_S as a function of tree size resulted in greater whole-tree water use for large infected larches than for large uninfected trees. Such changes may result in further depletion of limited soil water resources in mature infected stands late in the growing season. Foliage from infected trees of both species had lower water use efficiencies than non-infected trees. Our results demonstrate substantial changes of whole-tree processes related to mistletoe infection, and stress the importance of integrating whole-tree physiological and structural processes to fully understand the mechanisms by which pathogens suppress forest productivity.     

Water relations of coastal and estuarine Rhizophora mangle: xylem pressure potential and dynamics of embolism formation and repair

Physiological traits related to water transport were studied in Rhizophora mangle (red mangrove) growing in coastal and estuarine sites in Hawaii. The magnitude of xylem pressure potential (P_x), the vulnerability of xylem to cavitation, the frequency of embolized vessels in situ, and the capacity of R. mangle to repair embolized vessels were evaluated with conventional and recently developed techniques. The osmotic potential of the interstitial soil water (?_sw) surrounding the roots of R. mangle was c. -2.6LJ.52᎒^-3 and -0.4Lj.13᎒^-3 MPa in the coastal and estuarine sites, respectively. Midday covered (non-transpiring) leaf water potentials (O_L) determined with a pressure chamber were 0.6-0.8 MPa more positive than those of exposed, freely-transpiring leaves, and osmotic potential of the xylem sap (?_x) ranged from -0.1 to -0.3 MPa. Consequently, estimated midday values of P_x (calculated by subtracting ?_x from covered O_L) were about 1 MPa more positive than O_L determined on freely transpiring leaves. The differences in O_L between covered and transpiring leaves were linearly related to the transpiration rates. The slope of this relationship was steeper for the coastal site, suggesting that the hydraulic resistance was larger in leaves of coastal R. mangle plants. This was confirmed by both hydraulic conductivity measurements on stem segments and high-pressure flowmeter studies made on excised leafy twigs. Based on two independent criteria, loss of hydraulic conductivity and proportions of gas- and liquid-filled vessels in cryo-scanning electron microscope (cryo-SEM) images, the xylem of R. mangle plants growing at the estuarine site was found to be more vulnerable to cavitation than that of plants growing at the coastal site. However, the cryo-SEM analyses suggested that cavitation occurred more readily in intact plants than in excised branches that were air-dried in the laboratory. Cryo-SEM analyses also revealed that, in both sites, the proportion of gas-filled vessels was 20-30% greater at midday than at dawn or during the late afternoon. Refilling of cavitated vessels thus occurred during the late afternoon when considerable tension was present in neighboring vessels. These results and results from pressure-volume relationships suggest that R. mangle adjusts hydraulic properties of the water-transport system, as well as the leaf osmotic potential, in concert with the environmental growing conditions.     

Viable bacterial biomass and functional diversity in fresh and marine waters in the Canadian Arctic

Bacterial biomass and functional diversity in four marine and four freshwater samples, collected from Resolute Bay, Nunavut, Canada, were studied using fluorescent nucleic-acid staining and sole-carbon-source utilization. Viable microbial counts using the LIVE/DEAD BacLight Viability Kit estimated viable marine bacterial numbers from 0.7 to 1.8᎒⁶ cells/l, which were lower than viable bacterial numbers in freshwater samples (2.1-9.9᎒⁶ cells/l) (RCBD-ANOVA). Calculations of the Shannon-Wiener diversity index and average well colour development were based on substrate utilization in ECO-Biolog plates incubated at 4°C and 20°C for 38 and 24 days, respectively. The Shannon-Wiener diversity of the marine water samples was significantly greater ( x _H'=2.40ǂ.08, P <0.005; RCBD-ANOVA) than that of freshwater samples ( x _H'=1.20ǂ.00, P <0.005; RCBD-ANOVA). Differences in microbial diversity between fresh and marine water samples at 4°C ( x _4°C =2.01) and 20°C (x_20°C =2.31) were also detected by RCBD-ANOVA analysis. Interactions between water type and incubation temperature were not significant ( F =1.926, F _c=5.12). Principal component analysis revealed differences in metabolic substrate utilization patterns and, consequently, the microbial diversity between water types and samples.