Net CO2 Exchange Rate Research Articles

Effects of partial defoliation on carbon and nitrogen partitioning and photosynthetic carbon uptake by two-year-old cork oak (Quercus suber) saplings.

At the end of the growing season in late July, 20-month-old cork oak (Quercus suber L.) saplings were partially defoliated (63% of leaf area) to evaluate their ability to recover leaf area after defoliation. At 18 and 127 days after defoliation, changes in starch and nitrogen pools were determined in leaves and perennial organs, and variations in photosynthetic carbon uptake were investigated. To determine the role of stored nitrogen in regrowth after defoliation, plant nitrogen was labeled in the previous winter by enriching the nutrient solution with 15N. Plants recovered the lost leaf area in 127 days. Although there was remobilization of starch and nitrogen from leaves and perennial organs, the availability of resources for growth in the following spring was not decreased by defoliation. On the contrary, starch concentration in coarse roots was higher in defoliated saplings than in control saplings, presumably as a result of the higher net CO2 exchange rate in newly developed leaves compared with pre-existing leaves.

The contribution of bryophytes to the carbon exchange for a temperate rainforest

AbstractBryophytes blanket the floor of temperate rainforests in New Zealand and may influence a number of important ecosystem processes, including carbon cycling. Their contribution to forest floor carbon exchange was determined in a mature, undisturbed podocarp‐broadleaved forest in New Zealand, dominated by 100–400‐year‐old rimu (Dacrydium cupressimum) trees. Eight species of mosses and 13 species of liverworts contributed to the 62% cover of the diverse forest floor community. The bryophyte community developed a relatively thin (depth <30 mm), but dense, canopy that experienced elevated CO2 partial pressures (median 46.6 Pa immediately below the bryophyte canopy) relative to the surrounding air (median 37.6 Pa at 100 mm above the canopy). Light‐saturated rates of net CO2 exchange from 14 microcosms collected from the forest floor were highly variable; the maximum rate of net uptake (bryophyte photosynthesis – whole‐plant respiration) per unit ground area at saturating irradiance was 1.9 μmol m−2 s−1 and in one microcosm, the net rate of CO2 exchange was negative (respiration). CO2 exchange for all microcosms was strongly dependent on water content. The average water content in the microcosms ranged from 1375% when fully saturated to 250% when air‐dried. Reduction in water content across this range resulted in an average decrease of 85% in net CO2 uptake per unit ground area.The results from the microcosms were used in a model to estimate annual carbon exchange for the forest floor. This model incorporated hourly variability in average irradiance reaching the forest floor, water content of the bryophyte layer, and air and soil temperature. The annual net carbon uptake by forest floor bryophytes was 103 g m−2, compared to annual carbon efflux from the forest floor (bryophyte and soil respiration) of −1010 g m−2. To put this in perspective of the magnitude of the components of CO2 exchange for the forest floor, the bryophyte layer reclaimed an amount of CO2 equivalent to only about 10% of forest floor respiration (bryophyte plus soil) or ∼11% of soil respiration. The contribution of forest floor bryophytes to productivity in this temperate rainforest was much smaller than in boreal forests, possibly because of differences in species composition and environmental limitations to photosynthesis. Because of their close dependence on water table depth, the contribution of the bryophyte community to ecosystem CO2 exchange may be highly responsive to rapid changes in climate.

Degree of correlation between total light interception and whole-canopy net CO2exchange rate in two grapevine growth systems

This paper shows the degree of precision by which measurement of total canopy light interception (TCLI) can estimate whole-canopy net CO2 exchange rate (NCER). The test vines comprised a single 35° NE-SW oriented hedgerow, with vertically shoot positioned grapevine canopies at either a low shoot density (10 shoots/metre of row) or a high shoot density (20 shoots/metre of row). TCLI was measured on different dates and at various times on each date (during the 2001 growing season) by a multiple line sensor equipped with 64 phototransistors (35 mm spacing). Each sensor had a spectral sensitivity in the 300-1100 nm waveband and the line of sensors was moved horizontally in steps of 10 cm. Whole-canopy NCER was recorded over successive clear days using an enclosure method. TCLI and canopy NCER were highly correlated on a seasonal basis for both the low shoot density treatment (R2 = 0.97) and the high shoot density treatment (R2 = 0.94). The correlation became less precise (R2 = 0.80 and 0.79 for low and high, respectively), when data taken at various times of day were treated separately. Canopy NCER derived from single TCLI readings taken at solar noon tended to be under-estimated, while NCER derived from single TCLI readings taken later in the afternoon tended to be over-estimated. Nevertheless, for a given training system, our method was sufficiently precise to predict seasonal increase of canopy NCER as well as the total leaf area at which NCER approached a maximum thus setting a value above which additional leaf area resulted in mutual shading without enhancing carbon assimilation.

Heterosis for Photosynthesis, Dry Matter Production and Grain Yield in F1 Hybrid Rice (Oryza sativa L.) from Thermo-Sensitive Genic Male Sterile Line Cultivated at Different Soil Nitrogen Levels

This study was conducted to determine the effects of soil nitrogen levels (0.22, 0.44 and 0.88gN pot-1) on the heterosis for photosynthetic characters, dry matter production and grain yield in F1 hybrid rice obtained from thermo-sensitive genic male sterile line. Net CO2 exchange rate (CER) and other related characters viz., SPAD reading, leaf N content, soluble protein content and ribulose-1, 5-bisphosphate carboxylase/oxygenase (Rubisco) activity increased significantly with increasing N level in all the F1 hybrid and parental cultivars. With increasing N level, the number of tillers, leaf area, root weight, wholeplant dry weight and grain yield also increased. Under the low N level, the F1 hybrid did not show heterosis for most photosynthetic characters. However, at high N level, heterosis for CER over both the male parent and the mean of parents was ascribed to high leaf N content and several characters related to CER. The F1 hybrid exhibited substantial heterosis over the male parent for whole-plant dry weight, number of panicles per plant, number of spikelets per panicle and grain yield in all N levels. Among the yield components, the strengthening of heterosis for number of filled grains per panicle was the major cause of greater heterosis for grain yield in the F1 hybrid at high N level.

Net CO2 exchange rates in three different successional stages of the "Dark Taiga" of central Siberia

The net ecosystem exchange (NEE) of successional stages of the Abies -dominated dark taiga was measured in central Siberia (61°N 90°E) during the growing season of the year 2000 using the eddy covariance technique. Measurements started before snow melt and canopy activity in spring on day of year (DOY) 99 and lasted until a permanent snow cover had developed and respiration had ceased in autumn DOY 299. Three stands growing in close vicinity were investigated: 50 yr-old Betula pubescens (“Betula stand”, an early successional stage after fire), 250 yr-old mixed boreal forest, representing the transition from Betula -dominated to Abies -dominated canopies, and 200-yr-old Abies sibirica (“Abies stand”, representing a late successional stage following the mixed boreal forest). The mixed boreal forest had a multi-layered canopy with dense understory and trees of variable height and age below the main canopy, which was dominated by Abies sibirica , Picea obovata and few old Betula pubescens and Populus tremula trees. The Abies stand had a uniform canopy dominated by Abies sibirica . This stand appears to have established not after fire but after wind break or insect damage in a later successional stage. The stands differed with respect to the number of days with net CO 2 uptake (Betula stand 89 days, mixed boreal forest 109 days, and Abies stand 135 days), maximum measured LAI (Betula 2.6 m 2 m −2 , mixed boreal forest 3.5 m 2 m −2 and Abies stand 4.1 m 2 m −2 ) and basal area (Betula stand 30.2 m 2 ha −1 , mixed boreal forest 35.7 m 2 ha −1 , and Abies stand 46.5 m 2 ha −1 ). In the mixed boreal forest, many days with net daytime CO 2 release were observed in summer. Both other sites were almost permanent sinks in summer. Mean daytime CO 2 exchange rates in July were −8.45 μmol m −2 s −1 in the Betula stand, −4.65 μmol m −2 s −1 in the mixed boreal forest and −6.31 μmol m −2 s −1 in the Abies stand. Measured uptake for the growing season was −247.2 g C m −2 in the Betula stand, −99.7 g C m −2 in the mixed boreal forest and −269.9 g C m −2 in the Abies stand. The total annual carbon uptake might be slightly lower (i.e. less negative) due to some soil respiration under snow in winter. The study for the first time demonstrates that old forests in the “Dark Taiga” are carbon sinks and that sink activity is very similar in late and early successional stages. Canopy and crown structure with associated self-shading and available radiation are suggested as possible causes for the observed differences. DOI: 10.1034/j.1600-0889.2002.01351.x

Carbon dioxide and methane dynamics in a sub-Arctic peatland in northern Finland

We studied carbon dynamics on various surface parts of a highly patterned fen, typical in northern Finland, to examine the importance of different microsites to the areal carbon fluxes. The studies were carried out in June-September 1995 on a mesotrophic flark fen (an aapa mire) in Kaamanen (69°08?N, 27° 17?E). Wet flarks, moist lawns and dry strings accounted for 60%, 10% and 30% of the surface area, respectively. A static chamber technique was applied to measure the CH4 exchange, the instantaneous net ecosystem exchange (NEE, transparent chamber) and the ecosystem respiration (Rtot? opaque chamber) in several microsites. The static chamber results were compared with those obtained by the eddy covariance technique. The mean daytime areal net ecosystem CO2 exchange rate measurement in conditions where photosynthesis was light saturated (PAR>400 ?mol m-2 s-1) varied during the measurement period from ?59 mg CO2-C m?2h?1 (release) to 250 (uptake). The mean CH4 emission during the measuring period was 78 mg CH4-Cm?2 d?1 on the flarks, 68 mg on the lawn and 6.0 mg on the strings. The strings without shrubs (mainly Betula nana) were in general net sources of CO2, even during the middle of the growing season, whereas the lawns, flarks and also strings growing B. nana showed a daytime net uptake of CO2. Areally integrated chamber results showed lower CO2 and higher CH4 fluxes than predicted from the eddy covariance measurements.

Carbon dioxide and methane dynamics in a sub-Arctic peatland in northern Finland

We studied carbon dynamics on various surface parts of a highly patterned fen, typical in northern Finland, to examine the importance of different microsites to the areal carbon fluxes. The studies were carried out in June-September 1995 on a mesotrophic flark fen (an aapa mire) in Kaamanen (69°08?N, 27° 17?E). Wet flarks, moist lawns and dry strings accounted for 60%, 10% and 30% of the surface area, respectively. A static chamber technique was applied to measure the CH4 exchange, the instantaneous net ecosystem exchange (NEE, transparent chamber) and the ecosystem respiration (Rtot? opaque chamber) in several microsites. The static chamber results were compared with those obtained by the eddy covariance technique. The mean daytime areal net ecosystem CO2 exchange rate measurement in conditions where photosynthesis was light saturated (PAR>400 ?mol m-2 s-1) varied during the measurement period from ?59 mg CO2-C m?2h?1 (release) to 250 (uptake). The mean CH4 emission during the measuring period was 78 mg CH4-Cm?2 d?1 on the flarks, 68 mg on the lawn and 6.0 mg on the strings. The strings without shrubs (mainly Betula nana) were in general net sources of CO2, even during the middle of the growing season, whereas the lawns, flarks and also strings growing B. nana showed a daytime net uptake of CO2. Areally integrated chamber results showed lower CO2 and higher CH4 fluxes than predicted from the eddy covariance measurements.

Net CO2 exchange rates in three different successional stages of the “Dark Taiga” of central Siberia

The net ecosystem exchange (NEE) of successional stages of the Abies-dominated dark taiga was measured in central Siberia (61°N 90°E) during the growing season of the year 2000 using the eddy covariance technique. Measurements started before snow melt and canopy activity in spring on day of year (DOY) 99 and lasted until a permanent snow cover had developed and respiration had ceased in autumn DOY 299. Three stands growing in close vicinity were investigated: 50 yr-old Betula pubescens (“Betula stand”, an early successional stage after fire), 250 yr-old mixed boreal forest, representing the transition from Betula-dominated to Abies-dominated canopies, and 200-yr-old Abies sibirica (“Abies stand”, representing a late successional stage following the mixed boreal forest). The mixed boreal forest had a multi-layered canopy with dense understory and trees of variable height and age below the main canopy, which was dominated by Abies sibirica, Picea obovata and few old Betula pubescens and Populus tremula trees. The Abies stand had a uniform canopy dominated by Abies sibirica. This stand appears to have established not after fire but after wind break or insect damage in a later successional stage. The stands differed with respect to the number of days with net CO2 uptake (Betula stand 89 days, mixed boreal forest 109 days, and Abies stand 135 days), maximum measured LAI (Betula 2.6 m2 m−2, mixed boreal forest 3.5 m2 m−2 and Abies stand 4.1 m2 m−2) and basal area (Betula stand 30.2 m2 ha−1, mixed boreal forest 35.7 m2 ha−1, and Abies stand 46.5 m2 ha−1). In the mixed boreal forest, many days with net daytime CO2 release were observed in summer. Both other sites were almost permanent sinks in summer. Mean daytime CO2 exchange rates in July were −8.45 μmol m−2 s−1 in the Betula stand, −4.65 μmol m−2 s−1 in the mixed boreal forest and −6.31 μmol m−2 s−1 in the Abies stand. Measured uptake for the growing season was −247.2 g C m−2 in the Betula stand, −99.7 g C m−2 in the mixed boreal forest and −269.9 g C m−2 in the Abies stand. The total annual carbon uptake might be slightly lower (i.e. less negative) due to some soil respiration under snow in winter. The study for the first time demonstrates that old forests in the “Dark Taiga” are carbon sinks and that sink activity is very similar in late and early successional stages. Canopy and crown structure with associated self-shading and available radiation are suggested as possible causes for the observed differences.

Canopy Architecture and Cuvette Flow Patterns Influence Whole-canopy Net CO2 Exchange and Temperature in Sweet Cherry

To initiate photosynthetic studies of sweet cherry (Prunus avium L.) canopy architectures and cropping management under high light and temperature conditions (Yakima Valley, Wash.), we developed a whole-canopy research cuvette system with a variable airflow plenum that allowed different patterns of air delivery (in concentric circles around the trunk) into the cuvette. Air and leaf temperatures (Tair and Tleaf, respectively) were determined at four horizontal planes and four directional quadrants inside cuvette-enclosed canopies trained to a multiple leader/open-bush or a multiple leader/trellised palmette architecture. Air flow rate, air delivery pattern, and canopy architecture each influenced the whole-canopy temperature profile and net CO2 exchange rate (NCER) estimates based on CO2 differentials (inlet-outlet). In general, Tair and Tleaf were warmer (≈0 to 4 °C) in the palmette canopy and were negatively correlated with flow rate. The response of Tair and Tleaf to flow rate varied with canopy position and air delivery pattern. At a flow of 40 kL·min-1 (≈2 cuvette volume exchanges/min), mean Tair and Tleaf values were 2 to 3 °C warmer than ambient air temperature, and CO2 differentials were 15-20 μL·L-1. Tair and Tleaf were warmer than those in unenclosed canopies and increased with height in the canopy. Carbon differentials declined with increasing flow rate, and were greater in the palmette canopy and with a less dispersed (centralized) delivery. Dispersing inlet air delivery produced more consistent values of Tair and Tleaf in different canopy architectures. Such systematic factors must be taken into account when designing studies to compare the effects of tree architecture on whole-canopy photosynthesis.

Controls on carbon and energy exchange by a black spruce–moss ecosystem: Testing the mathematical model Ecosys with data from the BOREAS Experiment

Stomatal limitations to mass and energy exchange over boreal black spruce forests may be caused by low needle N concentrations that limit CO2 fixation rates. These low concentrations may be caused by low N uptake rates from cold boreal soils with high soil C:N ratios and by low N deposition rates from boreal atmospheres. A mathematical model of terrestrial ecosystems ecosys was used to examine the likelihood that slow N cycling could account for the low rates of mass and energy exchange measured over a 115‐year old boreal spruce/moss forest as part of the Boreal Ecosystem‐Atmosphere Study (BOREAS). In the model, net N mineralization was slowed by the high C:N ratios measured in the forest floor and by high lignin contents in spruce litterfall. Slow mineralization caused low N uptake rates and hence high C:N ratios in spruce and moss leaves that reduced specific activities and areal densities of rubisco and chlorophyll. Consequent low CO2 fixation rates caused low stomatal conductances and transpiration rates which in turn caused high soil water contents. Wet soils, in conjunction with large accumulations of surface detritus generated by slow litter mineralization, caused low soil temperatures that further slowed mineralization rates. Model outputs for ecosystem N status were corroborated by low needle N concentrations (< 10 mg g−1), stomatal conductances (< 0.05 mol m−2 s−1) and CO2 fixation rates (< 6 μmol m−2 s−1), and by high canopy Bowen ratios (1.5–2.0) and low canopy net CO2 exchange rates (< 10 μmol m−2 s−1) measured over the black spruce/moss forest at the BOREAS site. Modeled C accumulation rates of 60 (wood) + 10 (soil) = 70 g C m−2 yr−1 were consistent with estimates from aggregated CO2 fluxes measured over the spruce canopy and from allometric equations developed for black spruce in Canadian boreal forests. Model projections under IS92a climate change indicate that rates of wood C accumulation would rise and those of soil C accumulation would decline from those under current climate. Because these rates are N‐limited, they would be raised by increases in atmospheric N deposition.

Very high productivity of the C4 aquatic grass Echinochloa polystachya in the Amazon floodplain confirmed by net ecosystem CO2 flux measurements.

Fluxes of CO2 and H2O vapour from dense stands of the C4 emergent macrophyte grass Echinochloa polystachya were measured by eddy covariance in both the low water (LW) and high water (HW, flooded) phases of the annual Amazon river cycle at Manaus, Brazil. Typical clear-sky midday CO2 uptake rates by the vegetation stand (including detritus, sediment or water surface) were 30 and 35µmolCO2(ground)m-2s-1 in the LW and HW periods, respectively. A rectangular hyperbola model fitted the responses of "instantaneous" (20- or 30-min average) net CO2 exchange rates to incident photosynthetic photon flux densities (PFD) well. Stand evaporation rates were linearly related to PFD. The major difference in CO2 uptake rates between the two periods was the larger respiration flux during LW due to the CO2 efflux from sediment, roots and litter. Integrated 20- or 30-min fluxes were used to derive relationships between daily CO2 and H2O vapour fluxes and incident radiation. The daily CO2 fluxes were almost linearly related to incident radiation, but there was evidence of saturation at the highest daily radiation totals. Annual productivity estimated from the daily model in 1996-1997 agreed closely with that previously estimated for 1985-1986 from a leaf-scale photosynthetic model, but were some 15% less than those derived at that time from biomass harvests. Both CO2 uptake and water use efficiency were comparable with those found in fertilised maize fields in warm temperate conditions.

Carbohydrate Reserve Concentrations and Flower Bud Density Effects on Vegetative and Reproductive Development in Southern Highbush Blueberry

Vegetative budbreak, leaf area development, and fruit size in southern highbush blueberry (Vaccinium corymbosum L. interspecific hybrids) decrease as flower bud density increases. The effect on fruit size has been attributed to both insufficient carbohydrate reserves and reductions in current photoassimilates caused by decreased vegetative growth. Experiments were conducted with two southern highbush blueberry cultivars, `Misty' and `Sharpblue', to test the hypothesis that increased carbohydrate reserve concentrations can overcome the detrimental effects of high flower bud density by increasing vegetative budbreak, shoot development, and whole-canopy net CO2 exchange rate (NCER), which in turn will increase fruit size. Fully foliated plants were placed in greenhouses with either ambient (AMB) CO2 levels (≈360 μmol·mol-1) or enriched (ENR) CO2 levels (≈700 μmol·mol-1) for 38 d during fall. Plants were then moved outdoors, hand defoliated, and flower bud density (flower buds/cm cane length) adjusted to range from 0.07 to 0.31. Root starch and whole plant carbohydrate concentrations increased in ENR compared with AMB plants of both cultivars. Vegetative budbreak (number per centimeter cane length), leaf area, and whole-canopy NCER decreased as flower bud density increased in AMB and ENR plants of both cultivars; however, ENR `Sharpblue' plants had significantly greater vegetative growth and wholecanopy NCER at a given flower bud density compared with AMB `Sharpblue'. Concomitant with this was an increase in fruit fresh weight in ENR compared to AMB `Sharpblue'. This was not the case with `Misty', where vegetative development and fruit size were similar in ENR and AMB plants. Thus, the hypothesis that increased carbohydrate reserves will increase vegetative development and subsequent fruit size may be true only in certain cultivars of southern highbush blueberry. Alternatively, the increased carbohydrate reserve concentrations in ENR compared with AMB `Misty' plants may have been insufficient to affect subsequent vegetative or reproductive development.

Effects of Flower Bud Density on Vegetative and Reproductive Development and Carbohydrate Relations in Southern Highbush Blueberry

Vegetative budbreak and subsequent canopy development in some southern highbush blueberry (Vaccinium corymbosum L. interspecific hybrid) cultivars are delayed and/or reduced as flower bud density increases. This delay/reduction in vegetative growth has been correlated with decreased weight and soluble solids of individual fruit. In the present study, the effects of flower bud density (FBD) on vegetative budbreak and canopy development, starch reserves, and whole-canopy net CO2 exchange rate (NCER) were assessed to determine how FBD affected the source supply for fruit development. A range of flower bud densities was established in two cultivars of containerized southern highbush blueberry during dormancy. Vegetative budbreak was delayed and vegetative budbreak, leaf area, and leaf area to fruit ratios decreased as FBD increased. In general, increasing FBD increased the rate of root and cane starch depletion during the first four weeks after bloom. Whole-canopy NCER was similar across the range of FBD during early fruit development, but between four weeks after bloom and fruit ripening, NCER decreased as FBD increased. Although FBD explained only a small proportion of the variability observed in carbohydrate concentration and NCER, the data suggest that both the rapid depletion of starch reserves early in fruit development and the decrease in whole canopy NCER later in fruit development contribute to the detrimental effects of increased FBD on fruit development.

The Influence of Leaf "Windows" on Crassulacean Acid Metabolism in the South African Succulent Senecio Rowleyanus (Asteraceae)

Net CO2 exchange rate (PN) of shoots and diel fluctuations in titratable acidity of leaves of Senecio rowleyanus were measured to determine whether penetration of radiant energy through leaf “windows” (narrow, translucent strips on the leaf epidermis) resulted in increased CAM. Nocturnal PN and nighttime increases in acidity were compared among plants with windows covered with reflective adhesive tape, transparent adhesive tape (to control for potential effects of the adhesive), and no tape. The windows did not significantly enhance the degree of CAM in S. rowleyanus.

One Morphotype, Three Physiotypes: Sympatric Species of Clusia with Obligate C3 Photosynthesis, Obligate CAM and C3‐CAM Intermediate Behaviour

Abstract: Among about 150 species of the genus of Clusia, morphologically very similar neotropical shrubs and trees, 20 have been studied ecophysiologically and 39 original publications were screened here for key data on the potential performance of the various species. I have collected data on carbon isotope discrimination (873C), day‐night oscilliations of malate (Amal), citrate (Acitr) and titratable protons (AH), maximum rates of net CO2 exchange in the dark (Dark Jco2) and in the light (Light Jco2) apparent rate of photosynthetic electron transport at PS II (ETR), effective quantum yield of non‐cyclic electron transport at PS II (AF/F'm) at 1000 umol m‐2 s‐1 PPFD, and potential quantum yield of PS II of leaves adapted to short periods of darkness at midday (Fv/Fm midday) and after relaxation overnight (Fv/Fm predawn). The 20 species can be arranged from predominant C3 photosynthesis to principally CAM. There may be only a few bona fide obligatory CAM species in the genus, but it is doubtful if there are obligatory C3 species. The data were then used to provide comparisons of intrinsic capabilities for Clusia species occurring sympatrically at two sites each, in Brazil and Venezuela. No clear advantage of CAM emerges from these comparisons under the stress of high insolation and low water availability at these sites. Different Clusia species are successful in different ways and with different intrinsic ecophysiological capacities. The conventional expectations in CAM as a drought and light stress adaptation are confounded in Clusia to the extent that on some occasions C3 photosynthesis seems to be the superior strategy. However, it appears, that C3/CAM plasticity which is so widespread in the genus, with many species and potentially rapid speciation, allows a particularly wide ecological amplitude.

The effect of water level management on the soils and vegetation of two coastal Louisiana marshes

Wetland degradation and loss is the result of a combination of natural causes and anthropogenic activities and is a serious problem in coastal Louisiana, where approximately 80% of the total US coastal wetland loss since the 1930's has occurred. One method currently used to address this wetland loss problem is structural marsh management, which is the use of levees and water control structures to control hydroperiod. The effects of structural marsh management on two managed marshes in Southern Louisiana (Unit 4 of the Rockefeller Wildlife Refuge and the Fina LaTerre Mitigation Bank) were evaluated by comparing the soils and the dominant emergent marsh vegetation (Spartina patens) of the two managed marshes with those of two nearby unmanaged marshes. Soil redox potential, water depth, interstitial water sulfide concentration, salinity, NH4-N and elemental concentrations of Na, K, Ca, Mg, P, Fe and Zn were measured four times during 1989 which was a drawdown year. Net and total CO2 exchange rate, primary productivity, leaf area, stem density, and live, dead and total aboveground biomass were also measured. The managed marsh at Rockefeller had lower water levels, significantly less reduced surface and 15 cm deep soils and significantly lower interstitial sulfide concentrations and salinity levels. Na, K, Mg and Ca concentrations reflected the same pattern as salinity. Live aboveground biomass, primary productivity and leaf area were 3–4 times greater in the managed marsh. This indicates that marsh management improved soil conditions and provided an environment favorable to more vigorous plant growth. The management scheme at Fina LaTerre was also successful at maintaining lower water levels than in the adjacent unmanaged area. However, surface soils were more reduced and interstitial salinity higher, on average, in the managed marsh indicating generally poorer water circulation. Primary productivity was 50% less and stem density, leaf area, net CO2 and total CO2 exchange rates were significantly lower in the managed marsh, compared to the nearby reference marsh. Conditions in the managed marsh indicate that the management scheme was not successful at improving soil conditions when compared to those in the adjacent unmanaged marsh. This study indicates that structural marsh management is not the universal answer to problems faced by Louisiana's coastal wetlands, but may be of value in specific situations.

Evidence that Premature Senescence Affects Photosynthetic Decline of Wheat Flag Leaves during Growth in Elevated Carbon Dioxide

Net CO2 exchange (NCE) rates and various flag leaf constituents were measured in wheat plants (Triticum aestivum L.) grown in field chambers at either 350 or 700 p.L L-' CO2. Rates of NCE decreased with leaf age in both CO2 treatments during 1997. A stimulation of NCE initially occurred in response to CO2 enrichment, but this was not observed on the final eight of 33 experimental days. Net photosynthetic rates in response to growth at elevated CO2 were decreased ca. 22% on average for all measurement dates in 1997 and when all NCE rates measured at elevated CO2 for 1995-1997 were averaged (P ' 0.0001). Soluble protein, ct-amino nitrogen, and Chl a + b concentrations were significantly lower (P - 0.0001) in elevated compared with ambient C02-grown wheat flag leaves in 1997. The treatment by date interactions for these flag leaf constituents were nonsignificant (P 2 0.05). Flag leaf storage carbohydrates were measured on 10 dates in 1997, but only starch and sucrose were affected by the elevated CO2 treatment. An increase of acid proteinase activity was observed on the last two measurement dates of this study. However, changes of acid proteinase activity were unaffected by CO2 enrichment (P ? 0.05) and only occurred during late stages of senescence. These findings supported the suggestion that premature senescence contributed to the photosynthetic decline observed in wheat flag leaves during growth at elevated CO2. Changes of ox-amino nitrogen were correlated with photosynthetic decline, but acid proteinase activity probably was under endogenous control.

Growth and physiology of Picea abies populations from elevational transects: common garden evidence for altitudinal ecotypes and cold adaptation

1. There are conflicting reports concerning the adaptive features of tree populations originating from cold, high‐altitude environments. We hypothesize that such trees will possess adaptive features that will be demonstrated in a common environment, such as elevated rates of net CO2 exchange, elevated needle nitrogen concentration and high proportional biomass allocation to roots. To test this hypothesis we measured tree and seed properties of 54 populations of Norway spruce [Picea abies (L.) Karst.] located along eight altitudinal transects (from c.600 to 1500m) in southern Poland. We also measured growth, biomass partitioning, net photosynthetic capacity (Amax), needle dark respiration (RS) and carbohydrate, nitrogen (N) and chlorophyll concentration of seedlings originating from these populations grown for 2 to 7 years in a common garden at 150m elevation. Measured in situ along the elevational transects, there were linear declines in seed mass, average d.b.h. and height growth increment of seed trees with increased altitude or lower mean annual temperature. 2. In the common garden, the Norway spruce populations from colder, high‐altitude habitats had higher N concentration in needles than those from low altitudes. Both Amax and needle RS increased with altitude of seed origin and were significantly related to needle N concentration. High‐altitude populations also had higher concentrations of chlorophyll and carotene than those from low elevations. Despite higher photosynthetic rates in high‐altitude populations, seedling height and dry mass in the common garden declined with altitude of seed origin. Proportional dry mass partitioning to roots nearly doubled with increasing altitude of origin, while the length of the shoot‐growth period was reduced. The high respiration rates, high allocation to roots and reduced shoot‐growth period are probably responsible for the low growth rate potential of high‐altitude populations, more than offsetting their higher photosynthetic rates. 3. The results of this study showed that Norway spruce populations from cold mountain environments are characterized by several potentially adaptive features. Because these were similar to conifer population responses along a latitudinal gradient of origin, they are probably driven by climate. These climate‐driven differences were common to all transects: for a given altitude or mean annual temperature, plant traits were independent of mountain range of origin. However, populations originating from cold high‐elevation sites often differed per unit change in altitude or mean annual temperature more than did low elevation populations. The scaling of nitrogen, CO2 exchange and biomass and allocation patterns may be useful in modelling Norway spruce response on montane forest ecosystems under changing environments.

Photosynthetic capacity and dry mass partitioning in dwarf and semi-dwarf wheat ( Triticum aestivum L.)

Efficient use of space and high yields are critical for long-term food production aboard the International Space Station. The selection of a full dwarf wheat (less than 30 cm tall) with high photosynthetic and yield potential is a necessary prerequisite for growing wheat in the controlled, volume-limited environments available aboard long-term spaceflight missions. This study evaluated the photosynthetic capacity and carbon partitioning of a full-dwarf wheat cultivar, Super Dwarf, which is routinely used in spaceflight studies aboard U.S. space shuttle and NASA/Mir missions and made comparisons with other dwarf and semi-dwarf wheat cultivars utilized in other ground-based studies in plant space biology. Photosynthetic capacity of the flag leaf in two dwarf (Super Dwarf, BB-19), and three semi-dwarf (Veery-10, Yecora Rojo, IBWSN 199) wheat cultivars (Triticum aestivum L.) was assessed by measuring: net maximum photosynthetic rate, RuBP carboxylation efficiency, chlorophyll concentration and flag leaf area. Dry mass partitioning of carbohydrates to the leaves, sheaths, stems and ear was also assessed. Plants were grown under controlled environmental conditions in three replicate studies: slightly enriched CO2 (370 micromoles mol-1), high photosynthetic photon flux (1000 micromoles m-2 s-1; 58 mol m-2 d-1) for a 16 h photoperiod, 22/15 degrees C day/night temperatures, ample nutrients and water provided by one-half strength Hoagland's nutrient solution (Hoagland and Arnon, 1950). Photosynthetic capacity of the flag leaf was determined at anthesis using net CO2 exchange rate versus internal CO2 concentration curves measured under saturating light (2000 micromoles m-2 s-1) and CO2 (1000 micromoles mol-1). Dwarf wheat cultivars had greater photosynthetic capacities than the taller semi-dwarfs, they averaged 20% higher maximum net photosynthetic rates compared to the taller semi-dwarfs, but these higher rates occurred only at anthesis, had slightly greater carboxylation efficiencies and significantly increased chlorophyll concentrations per unit leaf area. The reduced-height wheat had significantly less dry mass fraction in the stem but greater dry mass partitioned to the ear than the taller semi-dwarfs (Yecora rojo, IBWSN-199). Studies with detached heads confirm that the head is a significant sink in the shorter wheat cultivars.

A STUDY ON THE RELATIONSHIP BETWEEN LEAF CONDUCTANCE, CO2 CONCENTRATION AND CARBOXYLATION RATE IN VARIOUS SPECIES

Leaf conductance gL is strongly influenced by environmental factors like CO2, irradiance and air humidity. According to Ball et al. (1987), gL is correlated with an index calculated as the product of net CO2 exchange rate A and ambient water vapour concentration Wa, divided by ambient CO2 concentration ca. However, this empirical model does not apply to high values of gL observed at ca below CO2 compensation concentration Γ. Therefore, we applied modified indices in which A is replaced by estimates for the rate of carboxylation. Such estimates, P1 and P2, were determined by adding to A the quotient of Γ and the sum of gas phase resistance rg and intracellular resistance for CO2 exchange ri, P1 = A+Γ/(rg + ri), or the quotient of Γ and ri, P2 = A + Γ/ri. If P2 is chosen, ca in the Ball index has to be replaced by the intercellular CO2 concentration ci. By using the modified indices P1·Wa/ca and P2·Wa/ci, we analysed data from the C3 species Nicotiana tabacum and Nicotiana plumbaginifolia, the C3–C4 intermediate species Diplotaxis tenuifolia, and the C4 species Zea mays. The data were collected at widely varying levels of irradiance and CO2 concentration. For all species uniform relationships between gL and the new indices were found for the whole range of CO2 concentrations below and above Γ. Correlations between gL and P1·Wa/ca were closer than those between gL and P2·Wa/ci because P1/ca implicitly contains gL. Highly significant correlations were also obtained for the relationships between gL and the ratios P1/ca and P2/ci.