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We compared ecophysiological characteristics of plant species that dominate during different stages of succession in fallow fields of a traditional agroecosystem in the Venezuelan High Andes. For each species we determined during the dry and wet seasons the photosynthetic light response and photosynthesis rate at light saturation (A[submax]), specific leaf area (SLA), stomatal conductance at light saturation (g[submax]), midday water potential (ψ), and intrinsic water use efficiency (WUE[subi], photosynthesis rate per unit of stomatal conductance). The species studied were the forbs Rumex acetosella (early succession dominant) and Lupinus meridanus (intermediate stages), the shrubs Acaena elongata and Baccharis prunifolia (late succession), the giant rosette Espeletia schultzii, and the shrub Hypericum laricifolium (mature ecosystem dominant). Clear ecophysiological trends were identified: early and intermediate successional species had higher A[submax], g[submax], and SLA but lower WUEp[subi]. E. schultzii maintained a high water potential during the dry season and, together with B. prunifolia, was the only species with no significant differences in A[submax] between seasons. The results indicate that traits generally linked to fast growth (high A[submax] and SLA) are associated with dominance during early succession, while traits linked with drought resistance (e.g., high WUE[subi] and thick xeromorphic leaves) are associated with dominance during late succession in this tropical mountain environment. [ABSTRACT FROM AUTHOR]

The branch bag method was used to monitor photosynthesis and transpiration of trembling aspen (Populus tremuloides Michx.) and hazelnut (Corylus cornuta Marsh.) over a 42-day midsummer period in 1996, as part of the Boreal Ecosystem–Atmosphere Study (BOREAS). During the same period, daytime measurements of stomatal conductance (gs) and leaf water potential (Ψleaf) were made on these species, and sap flow was monitored in aspen stems by the heat pulse method. Weather conditions during the study period were similar to the long-term average. Despite moist soils, both species showed an inverse relationship between daytime gs and vapor pressure deficit (D) when D was > 0.5 kPa. Daytime Ψleaf was below –2 MPa in aspen and near –1.5 MPa in hazelnut, except on rainy days. These results are consistent with the hypothesis that stomatal responses are constrained by hydraulic resistance from root to leaf, and by the need to maintain Ψleaf above a minimum threshold value. Reductions in gs on sunny afternoons with elevated ambient D (maximum 2.3 kPa) were associated with a significant decrease in photosynthetic rates. However, day-to-day variation in mean carbon assimilation rate was small in both species, and appeared to be governed more by solar radiation than D. These results may be generally applicable to healthy aspen stands under normal midsummer conditions in the southern boreal forest. However, strong reductions in carbon uptake may be expected at the more extreme values of D (> 4 kPa) that occur during periods of regional drought, even if soil water is not locally limiting. [ABSTRACT FROM PUBLISHER]

1. We present measurements of CO[sub 2] fluxes over 2 years above and within a young Beech stand in the east of France. This site is part of the Euroflux network set up to monitor fluxes over representative European forests. 2. The net ecosystem carbon (C) exchange was derived from continuous eddy flux measurements. Major components of the total flux (i.e. soil and above-ground biomass respiration and assimilation of leafy branches) were measured independently using chambers. The main C stocks (i.e. root, stem and branch biomass) were also quantified. 3. Daily minima of CO[sub 2] flux were typically around -20 µmol CO[sub 2] m[sup -2]s[sup -1] during the period of full leaf expansion, while night-time ecosystem respiration varied between 5 and 15 µmol CO[sub 2] m[sup -2]s[sup -1]. The seasonal pattern of net ecosystem assimilation was very close to that of net assimilation at the single branch scale. The seasonal variation of net ecosystem exchange was closely related to leaf expansion and soil water content during the dry year of 1996. 4. Measurements of ecosystem respiration (eddy flux) were corrected for CO[sub 2] storage within the stand. This C flux showed a seasonal pattern, the maximum rates (4–7 g C m[sup -2] day[sup -1]) occurring in spring and summer, and appeared to be correlated with soil temperature. Temporal variation of soil respiration showed the same pattern, and effects of both temperature and soil drying were found. Annual soil respiration was ≈70% of ecosystem respiration. Root respiration was 60% of the total below-ground respiration. 5. Annual net C exchange was -218 and -257 g C m[sup -2] in 1996 and 1997, respectively, corresponding to net C uptake by the forest. These values are much lower than the annual biomass increment (stems and large roots) of the stand: 427 and 471 g C m[sup -2] year[sup -1], respectively. The difference may be explained by a release of CO[sub 2] from the decomposition of woody debris... [ABSTRACT FROM AUTHOR]

A cheap CO2 enrichment system was designed to perform continuous gas exchange measurements of branches of mature European beech trees (Fagus sylvatica L.). Branches were grown at ambient (350 cm3 m-3) and elevated CO2 (700cm3 m-3) during the whole 1992 leafy period. Leaks resulting from airtightness defaults in the system appeared to be low enough to measure accurately net CO2 assimilation and transpiration rates during the day. However, the CO2 exchange rates during the night (respiration) were too low to allow accurate measurements. Elevated CO2 had a great effect on the net assimilation rate of branches via its influence on both the C3 photosynthetic pathway and the shade-tolerance of beech trees (85% increase). The A/Ca curves showed no acclimation effect to high CO2, both control and enriched branches increasing their net assimilation in the same way. The decrease of net assimilation rates in mature leaves was similar for both control and enriched branches, The pattern of daily transpiration rates remained the same for both control and enriched branches, hence we can assume that there was no visible CO2 effect on stomata. [ABSTRACT FROM AUTHOR]

The present study investigates the relationships between nitrogen uptake, transpiration, and carbon assimilation. Plants growing on nutrient solution were enclosed for 10-16d in a growth chamber, where temperature, photon flux density, vapour saturation deficit and CO[SUB2] concentration were controlled. One of these factors was modified every 4 to 5 d. Shoot photosynthesis and root and shoot respiration were recorded every half-hour. Nitrogen uptake from the root medium and plant transpiration were measured daily. In most cases, an increase in photon flux density led to increases in transpiration, net daily carbon assimilation, and nitrogen uptake. By modifying transpiration rate without changing photosynthesis (varying vapour saturation deflcit), or by modifying transpiration and carbon assimilation in opposite ways (varying CO[SUB2] air concentration), it was shown that nitrogen uptake does not follow transpiration, but is linked to the carbon uptake of the plant. When light was increased from low to interrnediate levels, the N uptake/C assimilation fatio remained constant. At higher photon flux density, this ratio declined markedly. It is proposed that in the first case, growth is limited by carbohydrate availability, thus any increase in carbon assimilation leads to a proportional increase in nitrogen uptake, in contrast to the second situation where carbohydrates may accumulate in the plant without further nitrogen requirement. [ABSTRACT FROM AUTHOR]

The internal parenchyma of the leaf and rhizome in 36 species of Sansevieria is made of dead cells and living cells arranged in a regular pattern. Intercellular spaces are lacking. The walls of dead cells consist of an inner amorphous layer positive to the fluorescence test for callose, a middle suberin-like layer and an outer fibrillar layer. In about half of the species examined, the inner layer forms distinctive thickenings. Detached leaves of Sansevieria lose water very slowly, and are able to recover it quickly. The pattern of leaf dehydration appears to be related to leaf morphology, whereas no relation is evident between the pattern of leaf rehydration and leaf morphology. Neither leaf dehydration nor leaf rehydration pattern is affected by the presence of wall thickenings in the dead parenchyma cells. The fresh weight per unit volume of both turgid and droughted leaves is nearly I, denoting that the dead cells are filled with water and do not undergo substantial cavitation during drought. The data indicate that the dead parenchyma cells of Sansevieria are a specialized water-storing system. [ABSTRACT FROM AUTHOR]

A model for the distribution of radiation incident on leaves in an isolated apple tree is presented. The simulated area of shadow cast by a tree compared well with measured values. The radiation model is combined with leaf models of photosynthesis and stomatal behaviour to simulate diurnal variations in the exchanges of carbon dioxide and water by the tree. Satisfactory correspondence was obtained when observed rates of transpiration and photosynthesis were compared with simulations. Further simulations indicated the diurnal patterns of transpiration and photosynthesis to be expected for trees with various shapes and leaf areas. [ABSTRACT FROM AUTHOR]

Microclimatic measurements and flux determinations were made on natural Agropyron­Koeleria grassland during two growing seasons. The calculated evaporation fluxes were analyzed in terms of the potential evaporation, soil moisture content, and plant water potential. The analysis fitted Monteith‘s canopy-resistance concept. The fraction of net radiation involved in evaporation was closely correlated with green leaf area index during the first half of the growing season, and with soil water content (0–30 cm) during the latter half of the growing season. Seasonal water-use estimates, by micrometeorological and soil moisture balance approaches, agreed to within 10%. [ABSTRACT FROM AUTHOR]

Presents a device for recording atmospheric carbon dioxide profiles. Operation of the system; Accuracy of the measurements; Typical calibration of the system.

Rose (Rosa hybrida cv. Madame G. Delbard) was micropropagated in vitro on agar at varying concentrations: 0, 4, 5.5, 6, 7.5, 8, 9 and 15 g l-1. Water status of the culture medium, leaf water potential, plant growth and propagation rate were investigated. Water potential of the medium was a linear function of agar concentration, and gel strength increased linearly from zero as agar concentration increased from 5 to 15 g l-1. Except for liquid medium, where shoot proliferation (total shoot number) was imparied by vitrification, fresh and dry weights and number of total shoots decreased linearly with increasing agar concentration. At low agar concentration, where water potential of the gelled medium was high, only few among numerous shoots that had attained a length of > 0.3 cm were usable. Number of usable (elongated) shoots was significantly higher at a moderate level of agar, namely 7.5 g l-1. This can be explained in terms of the antagonistic actions of cytokinin and water on shoot elongation. At low agar concentration, leaf water potential and absolute water content were high. At high agar concentration, under gel water deficit, leaf water potential and absolute water content were low but an osmotic adjustment maintained turgor pressure. At moderate agar concentration, when the number of usable shoots was high, lower leaf turgor was found. [ABSTRACT FROM AUTHOR]

Three independent methods were used to evaluate transpiration of a boreal forest: the branch bag, sap flow and eddy covariance methods. The branch bag method encloses several thousand needles and gives a continuous record of branch transpiration. The sap flow method provides a continuous record of sap velocity and an estimate of tree transpiration. The eddy covariance method typically measures evaporation rates between a forest and the atmosphere. We deployed an extra eddy covariance system below the forest to estimate canopy transpiration by difference.The three systems detected small water vapor fluxes despite a plentiful supply of energy to drive evaporation. We also observed that transpiration rates were low even when the soil was well supplied with water. Low rates of transpiration were attributed to the canopy's low leaf area index and the marked reduction in stomatal conductance as vapor pressure deficits increased. Water vapor fluxes, derived from the sap flow method, lagged behind those derived by the branch bag method by 1 to 2 h. The sap flow method also suffered from sampling errors caused by the non-uniformity of flow across the sapwood and the spatial variability of sapwood cross section throughout the forest. Despite technical difficulties associated with hourly measurements, daily totals of transpiration agreed well with values derived from micrometeorological systems. [ABSTRACT FROM AUTHOR]