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Mmol M (mmol + m)
Selected AbstractsHeight-related trends in leaf xylem anatomy and shoot hydraulic characteristics in a tall conifer: safety versus efficiency in water transportNEW PHYTOLOGIST, Issue 1 2008D. R. Woodruff Summary ,,Hydraulic vulnerability of Douglas-fir (Pseudotsuga menziesii) branchlets decreases with height, allowing shoots at greater height to maintain hydraulic conductance (Kshoot) at more negative leaf water potentials (,l). ,,To determine the basis for this trend shoot hydraulic and tracheid anatomical properties of foliage from the tops of Douglas-fir trees were analysed along a height gradient from 5 to 55 m. ,,Values of ,l at which Kshoot was substantially reduced, declined with height by 0.012 Mpa m,1. Maximum Kshoot was reduced by 0.082 mmol m,2 MPa,1 s,1 for every 1 m increase in height. Total tracheid lumen area per needle cross-section, hydraulic mean diameter of leaf tracheid lumens, total number of tracheids per needle cross-section and leaf tracheid length decreased with height by 18.4 µm2 m,1, 0.029 µm m,1, 0.42 m,1 and 5.3 µm m,1, respectively. Tracheid thickness-to-span ratio (tw/b)2 increased with height by 1.04 × 10,3 m,1 and pit number per tracheid decreased with height by 0.07 m,1. ,,Leaf anatomical adjustments that enhanced the ability to cope with vertical gradients of increasing xylem tension were attained at the expense of reduced water transport capacity and efficiency, possibly contributing to height-related decline in growth of Douglas fir. [source] Photosynthetic limitations in olive cultivars with different sensitivity to salt stressPLANT CELL & ENVIRONMENT, Issue 4 2003F. LORETO ABSTRACT Olive (Olea europea L) is one of the most valuable and widespread fruit trees in the Mediterranean area. To breed olive for resistance to salinity, an environmental constraint typical of the Mediterranean, is an important goal. The photosynthetic limitations associated with salt stress caused by irrigation with saline (200 mm) water were assessed with simultaneous gas-exchange and fluorescence field measurements in six olive cultivars. Cultivars were found to possess inherently different photosynthesis when non-stressed. When exposed to salt stress, cultivars with inherently high photosynthesis showed the highest photosynthetic reductions. There was no relationship between salt accumulation and photosynthesis reduction in either young or old leaves. Thus photosynthetic sensitivity to salt did not depend on salt exclusion or compartmentalization in the old leaves of the olive cultivars investigated. Salt reduced the photochemical efficiency, but this reduction was also not associated with photosynthesis reduction. Salt caused a reduction of stomatal and mesophyll conductance, especially in cultivars with inherently high photosynthesis. Mesophyll conductance was generally strongly associated with photosynthesis, but not in salt-stressed leaves with a mesophyll conductance higher than 50 mmol m,2 s,1. The combined reduction of stomatal and mesophyll conductances in salt-stressed leaves increased the CO2 draw-down between ambient air and the chloroplasts. The CO2 draw-down was strongly associated with photosynthesis reduction of salt-stressed leaves but also with the variable photosynthesis of controls. The relationship between photosynthesis and CO2 draw-down remained unchanged in most of the cultivars, suggesting no or small changes in Rubisco activity of salt-stressed leaves. The present results indicate that the low chloroplast CO2 concentration set by both low stomatal and mesophyll conductances were the main limitations of photosynthesis in salt-stressed olive as well as in cultivars with inherently low photosynthesis. It is consequently suggested that, independently of the apparent sensitivity of photosynthesis to salt, this effect may be relieved if conductances to CO2 diffusion are restored. [source] Control of transpiration in an irrigated Eucalyptus globulus Labill. plantationPLANT CELL & ENVIRONMENT, Issue 2 2000D. A. White ABSTRACT Stomatal conductance and transpiration were measured concurrently in an irrigated Eucalyptus globulus Labill. plantation. Canopy stomatal conductance, canopy boundary layer conductance and the dimensionless decoupling coefficient (,) were calculated (a) summing the conductance of three canopy layers (gc) and (b) weighting the contribution of foliage according to the amount of radiation received (gc,). Canopy transpiration was then calculated from gc and gc, for , = 1 (Eeq), , = 0 (Eimp) and by weighting Eeq and Eimp using , (E,). Eeq, Eimp and E, were compared to transpiration estimated from measurements of heat pulse velocity. The mean value of , was 0·63. Transpiration calculated using gc and assuming perfect coupling (12·5 ± 0·9 mmol m,2 s,1) significantly overestimated measured values (8·7 ± 0·8 mmol m,2 s,1). Good estimates of canopy transpiration were obtained either (a) calculating E, separately for the individual canopy layers or (b) treating the canopy as a single layer and using gc, in a calculation of Eimp (, = 0). The latter approach only required measurement of stomatal conductance at a single canopy position but would be unsuitable for use in combined models of canopy transpiration and assimilation. It should however, be suitable for estimating transpiration in forests regardless of the degree of coupling. [source] Nutritional niche separation in coexisting bog species demonstrated by 15N-enriched simulated rainfallAUSTRAL ECOLOGY, Issue 4 2009BEVERLEY R. CLARKSON Abstract Empodisma minus and Sporadanthus ferrugineus (both Restionaceae) coexist in New Zealand raised bogs, yet Sporadanthus have significantly more depleted 15N natural abundance signatures than coexisting Empodisma. Their root systems are spatially separated with Empodisma having a thick surface layer of about 50 mm of cluster roots overlying the deeper Sporadanthus roots. We hypothesized this root displacement allows Empodisma to preferentially access the primary N input from rainfall, thus establishing niche separation, and tested this using tracer stable isotopes. We aerially applied 1.6 mmol m,2 of 15N as (NH4)2SO4 chased by deionized water to simulate a rainfall event of 34 L m,2. Root/peat matrix cores were harvested after 5 h and analysed for 15N uptake. Approximately 80% of the tracer applied was recovered in the cores, with 90% of this recovered in the upper Empodisma cluster root layer. Seven weeks after application, young shoots of Empodisma were significantly enriched (mean ,15N = +7.21,; reference = ,0.42,), whereas those of coexisting Sporadanthus were not (mean ,15N = ,2.76,; reference = ,4.24,). However, we were unable to quantify the 15N uptake because of the dilution effect of the large biomass. We calculated the contribution of biological nitrogen fixation as a possible alternative source of N in achieving niche separation. The acetylene reduction assay showed minor amounts of nitrogenase activity are associated with Empodisma and Sporadanthus roots (equivalent to 0.045 ± 0.019 and 0.104 ± 0.017 kg N ha,1 year,1 respectively). Our results suggest that the species acquire nutrients from different rooting zones, with Empodisma accessing nutrients at the surface from rainfall and Sporadanthus accessing nutrients from mineralization in deeper peat layers. Such niche differentiation probably facilitates species coexistence and may provide a mechanism for slowing the rate of competitive displacement during long-term succession. [source] Exploration of the hydrogen producing potential of Rhodobacter capsulatus chemostat cultures: The application of deceleration-stat and gradient-stat methodologyBIOTECHNOLOGY PROGRESS, Issue 5 2009Sebastiaan Hoekema Abstract In this work, the dependency of the volumetric hydrogen production rate of ammonium-limited Rhodobacter capsulatus chemostat cultures on their imposed biomass concentration and dilution rate was investigated. A deceleration-stat experiment was performed by lowering the dilution rate from 1.0 d,1 to zero aimed at a constant biomass concentration of 4.0 g L,1 at constant incident light intensity. The results displayed a maximal volumetric hydrogen production rate of 0.6 mmol m,3 s,1, well below model predictions. Possibly the high cell density limited the average light availability, resulting in a sub-optimal specific hydrogen production rate. To investigate this hypothesis, a gradient-stat experiment was conducted at constant dilution rate of 0.4 d,1 at constant incident light intensity. The biomass concentration was increased from 0.7 to 4.0 g L,1 by increasing the influent ammonium concentration. Up to a biomass concentration of 1.5 g L,1, the volumetric hydrogen production rate of the system increased according to model predictions, after which it started to decline. The results obtained provide strong evidence that the observed decline in volumetric hydrogen production rate at higher biomass concentrations was at least partly caused by a decrease in light availability. © 2009 American Institute of Chemical Engineers Biotechnol. Prog., 2009 [source] | ||||||||||