CO2 Assimilation Capacity Research Articles

The role of agroforestry areas of the province of Bari in the absortion of carbon dioxide

Agroforestry areas have a significant and recognized productive, socio – economic, environmental and landscape role. An important ecological function performed by these areas is the net absorption of considerable quantities of atmospheric carbon dioxide. The scientific knowledge of the CO2 assimilation capacity of agroforestry ecosystems in a territory, is a useful and innovative means to support territorial planning. In the interests of environmental sustainability, emissions from human activities carried out in a specific local context, must be adapted by the simultaneous capacity of CO2 sequestration. For the protection of environmental quality, the choice of land use should therefore optimize the circuit of interaction between emissions and absorption. This work takes into account the agroforestry areas of the Province of Bari to estimate the potential capacity to absorb CO2 and compare it, with the current emission levels.

ELEVATED CARBON DIOXIDE DIFFERENTIALLY ALTERS THE PHOTOPHYSIOLOGY OF THALASSIOSIRA PSEUDONANA (BACILLARIOPHYCEAE) AND EMILIANIA HUXLEYI (HAPTOPHYTA)(1).

Increasing anthropogenic carbon dioxide is causing changes to ocean chemistry, which will continue in a predictable manner. Dissolution of additional atmospheric carbon dioxide leads to increased concentrations of dissolved carbon dioxide and bicarbonate and decreased pH in ocean water. The concomitant effects on phytoplankton ecophysiology, leading potentially to changes in community structure, are now a focus of concern. Therefore, we grew the coccolithophore Emiliania huxleyi (Lohmann) W. W. Hay et H. Mohler and the diatom strains Thalassiosira pseudonana (Hust.) Hasle et Heimdal CCMP 1014 and T. pseudonana CCMP 1335 under low light in turbidostat photobioreactors bubbled with air containing 390 ppmv or 750 ppmv CO2 . Increased pCO2 led to increased growth rates in all three strains. In addition, protein levels of RUBISCO increased in the coastal strains of both species, showing a larger capacity for CO2 assimilation at 750 ppmv CO2 . With increased pCO2 , both T. pseudonana strains displayed an increased susceptibility to PSII photoinactivation and, to compensate, an augmented capacity for PSII repair. Consequently, the cost of maintaining PSII function for the diatoms increased at increased pCO2 . In E. huxleyi, PSII photoinactivation and the counter-acting repair, while both intrinsically larger than in T. pseudonana, did not change between the current and high-pCO2 treatments. The content of the photosynthetic electron transport intermediary cytochrome b6/f complex increased significantly in the diatoms under elevated pCO2 , suggesting changes in electron transport function.

Effects of Formaldehyde Stress on Physiological Characteristics and Gene Expression Associated with Photosynthesis in Arabidopsis thaliana

Leaf chlorosis and bleach in plants under formaldehyde (HCHO) stress has been observed in previous studies. To better understand the mechanism of HCHO inhibition on photosynthesis, effects of HCHO stress on physiological characteristics and gene expression associated with photosynthesis in Arabidopsis thaliana were investigated in this study. The results indicated that HCHO stress reduced total chlorophyll (Chl) and Chl b content but increased Chl a content and a/b ratio in Arabidopsis. Suppression subtractive hybridization analysis identified 22 potential HCHO-repressive genes associated with photosynthesis. Furthermore, RT-PCR data showed that transcriptional levels of Lhcb2.1, Lhcb3, Lhca4, RBCS1A, RCA, Psao, Psbo, and Psbp were reduced to their lowest levels at 4 h after Arabidopsis was treated with 2–4 mM HCHO. The restoration in expression levels of Lhcb3, Lhca4, RBCS1A, and Psbp as well as up-regulation in Lhcb2.1 and RCA transcription was observed from 12 to 24 h after HCHO treatment. 13C-NMR analysis revealed that change patterns of endogenous glucose content were similar to expression patterns of these genes under HCHO stress from 0.5 to 24 h. The glucose content decreased to the lowest level at 4 h, accompanied by a potent inhibition on the capacity of CO2 assimilation caused by exogenous HCHO. From 4 to 24 h of HCHO stress, exogenous HCHO was efficiently metabolized and converted to glucose, which allowed the glucose content to be restored to its original level; thus, repressive effects of HCHO stress on expressions of photosynthesis-related genes were eliminated. These results suggested a correlation between HCHO metabolic level and expression levels of photosynthesis-related genes in vivo during this period. However, a second reduction in expressions of these genes after 36 h might be attributed to other effects triggered by HCHO stress.

Difference in photoinhibition and photoprotection between seedings and saplings leaves of Taxus cuspidata under high irradiance

The differences in chloroplast pigments, gas exchange and photosystemII (PSII) photochemistry as well as xanthophyll in seeding and sapling leaves of Taxus cuspidatagrown in full sunlight were examined. Compared with the sapling leaves, the chlorophyll content, photosynthetic capacity and light intensity for saturation of photosynthesis were lower in seeding leaves. The response curves of PSII photochemistry demonstrated that both seeding and sapling leaves occurred a down-regulation of PSII photochemistry at high irradiance, more serious down-regulation being examined in seeding leaves. And the down-regulation of PSII photochemistry occurred significantly when measured at midday, indicating that photoinhibition occurred heavily in seeding leaves when exposed to high light. The actual PSII efficiency (ΦPSII) and the efficiency of excitation capture by open PSII centers drastically decreased with the increase of non-photochemical quenching (NPQ) at midday. The photorespiration rate in seeding leaves was lower than that in sapling leaves under high irradiance. The results indicated that the xanthophlly cycle was activated in both the seeding and sapling leaves at midday and an increase of de-epoxidation were observed, but a little higher level of de-epoxidation was measured in seeding leaves. The xanthophyll cycle may play an important role in the dissipation of excess light energy associated with NPQ to avoid photodamage. Our results suggested that photoinhibition occurred in seeding leaves significantly due to lower capacity of CO2assimilation, photorespiration and the light intensity for saturation of photosynthesis, as well as the lower PSII photochemistry at high irradiance; therefore the T. cuspidataseeding could not adapt to growing at high irradiance. Key words: Japanese yew (Taxus cuspidata Sieb, et Zucc.), photosynthesis, chlorophyll fluorescence.

JIP analysis on rice (Oryza sativa cv Nipponbare) grown under limited nitrogen conditions

Nitrogen deficiency significantly reduces the CO2 assimilation capacity of plants and the quantum yield of photosynthesis. Here, we employed the JIP test to determine the effects of nitrogendeficiency on the plant’s photosysnthetic ability on the basis of chlorophyll fluorescence. Nitrogendeficient and nitrogen-replete rice plants were analyzed for the fluorescence transients of the plant leaves in comparison with the nitrogen-sufficient controls. Results showed that 7 day-replete plants behaved normally while 5, 3, and 1 day-replete plants were significantly affected from nitrogen starvation. More specifically, nitrogen starvation of plants resulted in an inactivation of photosystem II (PS II) reaction centers and a decline in electron transport beyond the reduced plastoquinone (QAA −), and a decrease in both the pool size and the reduction of end electron acceptors at the PS I. The affected plants were fully recovered from the deficiency after 7 days of nitrogen repletion, as evidenced by the similar level of fluorescence transients to the positive controls. Thus, our results demonstrated that the movement of electron carriers leading to the reduction of end electron acceptors was affected by nitrogen limitation leading to a more pronounced decrease in the reduction of end electron acceptors. Together with the fact that nitrogen-deficiency limits the CO2 assimilation of plants, this study indicates that nitrogen metabolism is tightly coupled with photosynthetic ability.

Effect of fertilizer application on photosynthesis and oil yield of Jatropha curcas L.

The use of Jatropha curcas oil as a source of biofuel has been well-explored. However, the physiological and growth studies of J. curcas have received considerably lesser attention. In this study, leaf gas exchange measurements and leaf nitrogen content were determined for four varieties of J. curcas, grown in the field or in pots. Based on stable carbon isotope analysis (δ13C) and gas-exchange studies, J. curcas is a C3 sun plant and the range of leaf photosynthetic rates (or CO2 assimilation rates, PNmax) were typically between 7 and 25 μmol(CO2) m−2 s−1 and light saturation generally occurred beyond 800 μmol(quanta) m−2 s−1. Higher rates of leaf photosynthesis were generally obtained with the mature leaves. In addition, increased foliar PNmax were recorded in potted J. curcas variety Indiana with increasing nitrogen (N) nutrition levels. These plants also showed greater growth, increased leaf N content, higher maximum CO2 assimilation capacity (PNhighCO2) and chlorophyll (Chl) content, indicating the potential of optimizing the growth of Jatropha by varying fertilizer nutrient levels. A rapid assessment for leaf N using a nondestructive and portable Chl meter had been established for J. curcas. This approach will allow repeated sampling of the same plant over time and thus enable the monitoring of the appropriate levels of soil fertility to achieve good Jatropha plantation productivity. High N nutrition improved the overall plant oil yield by increasing the total number of fruits/seeds produced per plant, while not affecting the intrinsic seed oil content.

Small chloroplast-targeted DnaJ proteins are involved in optimization of photosynthetic reactions in Arabidopsis thaliana

BackgroundDnaJ proteins participate in many metabolic pathways through dynamic interactions with various components of these processes. The role of three small chloroplast-targeted DnaJ proteins, AtJ8 (At1 g80920), AtJ11 (At4 g36040) and AtJ20 (At4 g13830), was investigated here using knock-out mutants of Arabidopsis thaliana. Photochemical efficiency, capacity of CO2 assimilation, stabilization of Photosystem (PS) II dimers and supercomplexes under high light illumination, energy distribution between PSI and PSII and phosphorylation of PSII-LHCII proteins, global gene expression profiles and oxidative stress responses of these DnaJ mutants were analyzed.ResultsKnockout of one of these proteins caused a series of events including a decrease in photosynthetic efficiency, destabilization of PSII complexes and loss of control for balancing the redox reactions in chloroplasts. Data obtained with DNA microarray analysis demonstrated that the lack of one of these DnaJ proteins triggers a global stress response and therefore confers the plants greater tolerance to oxidative stress induced by high light or methyl viologen treatments. Expression of a set of genes encoding enzymes that detoxify reactive oxygen species (ROS) as well as a number of stress-related transcription factors behaved in the mutants at growth light similarly to that when wild-type (WT) plants were transferred to high light. Also a set of genes related to redox regulation were upregulated in the mutants. On the other hand, although the three DnaJ proteins reside in chloroplasts, the expression of most genes encoding thylakoid membrane proteins was not changed in the mutants.ConclusionIt is proposed that the tolerance of the DnaJ protein knockout plants to oxidative stress occurs at the expense of the flexibility of photosynthetic reactions. Despite the fact that the effects of the individual protein knockout on the response of plants to high light treatment are quite similar, it is conceivable that both specific- and cross-talk functions exist between the three small chloroplast-targeted DnaJ proteins, AtJ8, AtJ11 and AtJ20.

Leaf Wilting Movement Can Protect Water-Stressed Cotton (Gossypium hirsutum L.) Plants Against Photoinhibition of Photosynthesis and Maintain Carbon Assimilation in the Field

Under severe water stress, leaf wilting is quite general in higher plants. This passive movement can reduce the energy load on a leaf. This paper reports an experimental test of the hypothesis that leaf wilting movement has a protective function that mitigates against photoinhibition of photosynthesis in the field. The experiments exposed cotton (Gossypium hirsutum L.) to two water regimes: water-stressed and well-watered. Leaf wilting movement occurred in water-stressed plants as the water potential decreased to −4.1 MPa, reducing light interception but maintaining comparable quantum yields of photosystem II (PS II; Yield for short) and the proportion of total PS II centers that were open (qP). Predrawn F v/F m (potential quantum yield of PS II) as an indicator of overnight recovery of PS II from photoinhibition was higher than or similar to that in well-watered plants. Compared with water-stressed cotton leaves for which wilting movement was permitted, water-stressed cotton leaves restrained from such movement had significantly increased leaf temperature and instantaneous CO2 assimilation rates in the short term, but reduced Yield, qP, and F v/F m. In the long term, predrawn F v/F m and CO2 assimilation capacity were reduced in water-stressed leaves restrained from wilting movement. These results suggest that, under water stress, leaf wilting movement could reduce the incident light on leaves and their heat load, alleviate damage to the photosynthetic apparatus due to photoinhibition, and maintain considerable carbon assimilation capacity in the long term despite a partial loss of instantaneous carbon assimilation in the short term.

Photosynthesis characteristics of three species of forages in the arid-hot valleys

By using TPS-1 portable photosynthesis measuring system, the photosynthesis characteristics of three kinds of high quality forages used in the arid-hot valley eco-agricultural models in Yuanmou were measured. The paper shows that Pennisetum purpureum Schumach is the heliophilous shade tolerance C4 plant and Styiosanthes guianensis is the heliophilous shade tolerance plant, both of them could make full use of the light energy under the weak light environment and have relatively strong CO2 assimilation capacity. Cajanus cajan(L.) Millspaugh is the strong heliophilous plant with high requirement for light and has a relatively weak CO2 assimilation capacity. Generally, these three kinds of high quality forages are drought resistant and show a relatively good adaptability to the arid-hot valley environment. To improve cultivation administration level and to interplant timely in the forest can be helpful to the ecological niche and to improve the productivity force.

Effects of Blue Light Deficiency on Acclimation of Light Energy Partitioning in PSII and CO2 Assimilation Capacity to High Irradiance in Spinach Leaves

Blue light effects on the acclimation of energy partitioning characteristics in PSII and CO2 assimilation capacity in spinach to high growth irradiance were investigated. Plants were grown hydroponically in different light treatments that were a combination of two light qualities and two irradiances,i.e. white light and blue-deficient light at photosynthetic photon flux densities (PPFDs) of 100 and 500 micromol m(-2) s(-1). The CO2 assimilation rate, the quantum efficiency of PSII(PhiPSII) and thermal dissipation activity (F(v)/F(m)-F'(v)/F'(m)) in young, fully expanded leaves were measured under 1,600 micromol m(-2) s(-1) white light. The CO2 assimilation rate and (PhiPSII) were higher, while F(v)/F(m)-F'(v)/F'(m) was lower in plants grown under high irradiance than in plants grown under low irradiance. These responses were observed irrespective of the presence or absence of blue light during growth. The extent of the increase in the CO2 assimilation rate and PhiPSII and the decrease in F(v)/F(m)-F'(v)/F'(m) by high growth irradiance was smaller under blue light-deficient conditions. These results indicate that blue light helps to boost the acclimation responses of energy partitioning in PSII and CO2 assimilation to high irradiance. Similarly, leaf N, Cyt f and Chl contents per unit leaf area increased by high growth irradiance, and the extent of the increment in leaf N, Cyt f and Chl was smaller under blue light-deficient conditions. Regression analysis showed that the differences in energy partitioning in PSIIand CO2 assimilation between plants grown under high white light and high blue-deficient light were closely related to the difference in leaf N.

Expression of the Algal Cytochrome c6 Gene in Arabidopsis Enhances Photosynthesis and Growth

Photosynthetic plants convert light energy into ATP and NADPH in photosynthetic electron transfer and photophosphorylation, and synthesize mainly carbohydrates in the Calvin-Benson cycle. Here we report the enhancement of photosynthesis and growth of plants by introducing the gene of an algal cytochrome c6, which has been evolutionarily eliminated from higher plant chloroplasts, into the model plant Arabidopsis thaliana. At 60 d after planting, the plant height, leaf length and root length of the transformants were 1.3-, 1.1- and 1.3-fold those in the wild-type plants, respectively. At the same time, in the transgenic plants, the amounts of chlorophyll, protein, ATP, NADPH and starch were 1.2-, 1.1-, 1.9-, 1.4- and 1.2-fold those in the wild-type plants, respectively. The CO2 assimilation capacity of the transgenic plants was 1.3-fold that of the wild type. Moreover, in transgenic Arabidopsis expressing algal cytochrome c6, the 1-qP, which reflects the reduced state of the plastoquinone pool, is 30% decreased compared with the wild type. These results show that the electron transfer of photosynthesis of Arabidopsis would be accelerated by the expression of algal cytochrome c6. Our results demonstrate that the growth and photosynthesis of Arabidopsis plants could be enhanced by the expression of the algal cytochrome c6 gene.

The "Kluge-Lüttge Kammer": A Preliminary Evaluation of an Enclosed, Crassulacean Acid Metabolism (CAM) Mesocosm that Allows Separation of Synchronized and Desynchronized Contributions of Plants to Whole System Gas Exchange

Crassulacean acid metabolism (CAM) is recognized as a photosynthetic adaptation of plants to arid habitats. This paper presents a proof-of-concept evaluation of partitioning net CO2 exchanges for soil and plants in an arid, exclusively CAM mesocosm, with soil depth and succulent plant biomass approximating that of natural Sonoran Desert ecosystems. We present the first evidence that an enclosed CAM-dominated soil and plant community exposed to a substantial day/night temperature difference (30/20 degrees C), exhibits a diel gas exchange pattern consisting of four consecutive phases with a distinct nocturnal CO2 uptake. These phases were modulated by plant assimilation and soil respiration processes. Day-time stomatal closure of the CAM cycle during phase III was used to eliminate aboveground photosynthetic assimilation and respiration and thereby to estimate belowground plant plus soil respiration. Rapid changes in temperature appeared to synchronize single plant gas exchange but individual plant gas exchange patterns were desynchronized at constant day/night temperatures (25 degrees C), masking the distinct mesocosm pattern. Overall, the mean carbon budget of this CAM model Sonoran Desert system was negative, releasing an average of 22.5 mmol CO2 m-2 d-1. The capacity for nocturnal CO2 assimilation in this exclusively CAM mesocosm was inadequate to recycle CO2 released by plant and soil respiration.

Nutritional and chlorophyll fluorescence responses of lucerne (Medicago sativa) to waterlogging and subsequent recovery

Periodic flooding of perennial crops such as lucerne (Medicago sativa,L) is a major cause of lowered productivity and leads in extreme cases to plant death. In this study, effects of waterlogging and subsequent recovery on plant nutrient composition and PSII photochemistry were studied to gain a better understanding of the mechanisms of recovery as they relate to leaf photochemistry (chlorophyll fluorescence) and nutrient dynamics. Three lucerne cultivars and one breeding line were flooded for 20 d, drained and left to recover for another 16 d under glasshouse conditions. Leaf and root nutrient composition (P, K, Ca, Mg, B, Cu and Zn) of waterlogged lucerne was significantly lower than in freely drained controls, leaf N concentrations were also significantly lower in waterlogged lucerne. At the same time, there were significantly (5-fold) higher concentrations of Fe in waterlogged roots and Na in leaves (2-fold) of stressed plants. PS II photochemistry, which was impaired due to waterlogging, recovered almost fully after 16 d of free drainage in all genotypes. Alongside fluorescence recovery, concentrations of several nutrients also increased in recovered plants. Growth parameters, however, remained suppressed after draining. The latter was due to both the smaller capacity of CO2 assimilation in previously waterlogged plants (caused in part by nutrient deficiency and associated inhibition of PSII) and the plant’s need to re-direct available nutrient and assimilate pools to repair the damage to the photosynthetic apparatus and roots. It is concluded, that for any lucerne-breeding program it is important to determine not only the degree of tolerance to waterlogging but also the potential for recovery of different genotypes, as well as look for ‘outstanding individuals’ within each population.

The response of nodulated alfalfa to water supply, temperature and elevated CO2: photosynthetic downregulation

Plants grown in an environment of elevated CO2 and temperature often show reduced CO2 assimilation capacity, providing evidence of photosynthetic downregulation. The aim of this study was to analyse the downregulation of photosynthesis in elevated CO2 (700 µmol mol−1) in nodulated alfalfa plants grown at different temperatures (ambient and ambient + 4°C) and water availability regimes in temperature gradient tunnels. When the measurements were taken in growth conditions, a combination of elevated CO2 and temperature enhanced the photosynthetic rate; however, when they were carried out at the same CO2 concentration (350 and 700 µmol mol−1), elevated CO2 induced photosynthetic downregulation, regardless of temperature and drought. Intercellular CO2 concentration measurements revealed that photosynthetic acclimation could not be accounted for by stomatal limitations. Downregulation of plants grown in elevated CO2 was a consequence of decreased carboxylation efficiency as a result of reduced rubisco activity and protein content; in plants grown at ambient temperature, downregulation was also induced by decreased quantum efficiency. The decrease in rubisco activity was associated with carbohydrate accumulation and depleted nitrogen availability. The root nodules were not sufficiently effective to balance the source–sink relation in elevated CO2 treatments and to provide the required nitrogen to counteract photosynthetic acclimation.

Effects of Light Intensity on Flower Life of Potted Delphinium Plants

Potted Delphinium hybrid ‘Bellamosum’ plants were held under a photosynthetic photon flux density (PPFD) of 7, 70 and 300 μmol · m-2 · s-1, their flower life, which terminated when sepal abscission occurred, was 6.4, 9.4 and 9.4 days, respectively. The capacity for CO2 assimilation on day 0 was negative at 7 μmol · m-2 · s-1. Four days after the onset of the light treatment, the concentrations of sucrose, glucose, fructose, and mannitol in sepals and gynoecia were positively correlated with light intensity. Flowers held at 7 μmol · m-2 · s-1 had a higher ethylene production on day 6, compared with flowers held at higher light intensity. The low light intensity that resulted in increased ethylene production may be attributed to the preceding reduction of CO2 assimilation and sugar content, which, in turn led to the acceleration of sepal abscission.

CO2 Assimilation, Photosynthetic Enzymes, and Carbohydrates of `Concord' Grape Leaves in Response to Iron Supply

Own-rooted 1-year-old `Concord' grapevines (Vitis labruscana Bailey) were fertigated twice weekly for 11 weeks with 1, 10, 20, 50, or 100 μm iron (Fe) from ferric ethylenediamine di (o-hydroxyphenylacetic) acid (Fe-EDDHA) in a complete nutrient solution. As Fe supply increased, leaf total Fe content did not show a significant change, whereas active Fe (extracted by 2,2′-dipyridyl) content increased curvilinearly. Chlorophyll (Chl) content increased as Fe supply increased, with a greater response at the lower Fe rates. Chl a: b ratio remained relatively constant over the range of Fe supply, except for a slight increase at the lowest Fe treatment. Both CO2 assimilation and stomatal conductance increased curvilinearly with increasing leaf active Fe, whereas intercellular CO2 concentrations decreased linearly. Activities of key enzymes in the Calvin cycle, ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco), NADP-glyceraldehyde-3-phosphate dehydrogenase (GAPDH), phosphoribulokinase (PRK), stromal fructose-1,6-bisphosphatase (FBPase), and a key enzyme in sucrose synthesis, cytosolic FBPase, all increased linearly with increasing leaf active Fe. No significant difference was found in the activities of ADP-glucose pyrophosphorylase (AGPase) and sucrose phosphate synthase (SPS) of leaves between the lowest and the highest Fe treatments, whereas slightly lower activities of AGPase and SPS were observed in the other three Fe treatments. Content of 3-phosphoglycerate (PGA) increased curvilinearly with increasing leaf active Fe, whereas glucose-6-phosphate (G6P), fructose-6-phosphate (F6P), and the ratio of G6P: F6P remained unchanged over the range of Fe supply. Concentrations of glucose, fructose, sucrose, starch, and total nonstructural carbohydrates (TNC) at both dusk and predawn increased with increasing leaf active Fe. Concentrations of starch and TNC at any given leaf active Fe content were higher at dusk than at predawn, but both glucose and fructose showed the opposite trend. No difference in sucrose concentration was found at dusk or predawn. The export of carbon from starch breakdown during the night, calculated as the difference between dusk and predawn measurements, increased as leaf active Fe content increased. The ratio of starch to sucrose at both dusk and predawn also increased with increasing leaf active Fe. In conclusion, Fe limitation reduces the activities of Rubisco and other photosynthetic enzymes, and hence CO2 assimilation capacity. Fe-deficient grapevines have lower concentrations of nonstructural carbohydrates in source leaves and, therefore, are source limited.

Canopy position affects photosynthetic adjustments to long-term elevated CO2 concentration (FACE) in aging needles in a mature Pinus taeda forest.

Few studies have examined the effects of elevated CO2 concentration ([CO2]) on the physiology of intact forest canopies, despite the need to understand how leaf-level responses can be aggregated to assess effects on whole-canopy functioning. We examined the long-term effects of elevated [CO2] (ambient + 200 ppm CO2) on two age classes of needles in the upper and lower canopy of Pinus taeda L. during the second through sixth year of exposure to elevated [CO2] in free-air (free-air CO2 enrichment (FACE)) in North Carolina, USA. Strong photosynthetic enhancement in response to elevated [CO2] (e.g., +60% across age classes and canopy locations) was observed across the years. This stimulation was 33% greater for current-year needles than for 1-year-old needles in the fifth and sixth years of treatment. Although photosynthetic stimulation in response to elevated [CO2] was maintained through the sixth year of exposure, we found evidence of concurrent down-regulation of Rubisco and electron transport capacity in the upper-canopy sunlit leaves. The lower canopy showed no evidence of down-regulation. The upper canopy down-regulated carboxylation capacity (Vcmax) and electron transport capacity (Jmax) by about 17-20% in 1-year-old needles; however, this response was significant across sampling years only for Jmax in 1-year-old needles (P < 0.02). A reduction in leaf photosynthetic capacity in aging conifer needles at the canopy top could have important consequences for canopy carbon balance and global carbon sinks because 1-year-old sunlit needles contribute a major proportion of the annual carbon balance of these conifers. Our finding of a significant interaction between canopy position and CO2 treatment on the biochemical capacity for CO2 assimilation suggests that it is important to take canopy position and needle aging into account because morphologically and physiologically distinct leaves could respond differently to elevated [CO2].

SEASONALITY OF WATER AND HEAT FLUXES OVER A TROPICAL FOREST IN EASTERN AMAZONIA

We used the eddy covariance technique from July 2000 to July 2001 to measure the fluxes of sensible heat, water vapor, and CO2 between an old‐growth tropical forest in eastern Amazonia and the atmosphere. Precipitation varied seasonally, with a wet season from mid‐December 2000 to July 2001 characterized by successive rainy days, wet soil, and, relative to the dry season, cooler temperatures, greater cloudiness, and reduced incoming solar and net radiation. Average evapotranspiration decreased from 3.96 ± 0.65 mm/d during the dry season to 3.18 ± 0.76 mm/d during the wet season, in parallel with decreasing radiation and decreasing water vapor deficit. The average Bowen ratio was 0.17 ± 0.10, indicating that most of the incoming radiation was used for evaporation. The Bowen ratio was relatively low during the early wet season (December–March), as a result of increased evaporative fraction and reduced sensible heat flux. The seasonal decline in Bowen ratio and increase in evaporative fraction coincided with an increase in ecosystem CO2 assimilation capacity, which we attribute to the growth of new leaves. The evaporative fraction did not decline as the dry season progressed, implying that the forest did not become drought stressed. The roots extracted water throughout the top 250 cm of soil, and water redistribution, possibly by hydraulic lift, partially recharged the shallow soil during dry season nights. The lack of drought stress during the dry season was likely a consequence of deep rooting, and possibly vertical water movement, which allowed the trees to maintain access to soil water year round.

CO2 Assimilation, Photosynthetic Enzymes, and Carbohydrates of Grape Leaves (Vitis labrusca L. cv. Concord) in Response to Iron Supply

Own-rooted one-year-old `Concord' grapevines were fertigated twice weekly for 11 weeks with 1, 10, 20, 50, OR 100 μmol iron (Fe) from ferric ethylenediamine di (o-hydroxyphenylacetic) acid in a complete nutrient solution. As Fe supply increased, leaf total Fe content did not change, whereas active Fe (extracted by 2, 2'-dipyridyl) and total chlorophyll content increased curvilinearly. CO2 assimilation and stomatal conductance increased curvilinearly with increasing active Fe, whereas intercellular CO2 concentrations decreased linearly. Activities of key Calvin cycle enzymes, Rubisco, NADP-glyceraldehyde-3-phosphate dehydrogenase, phosphoribulokinase, stromal fructose-1,6-bisphosphatase (FBPase), and a key enzyme in sucrose synthesis, cytosolic FBPase, all increased linearly with increasing active Fe. No difference was found in the activities of ADP-glucose pyrophosphorylase and sucrose phosphate synthase of leaves between the lowest and the highest treatments, whereas slightly lower activities were observed in the middle Fe treatments. Content of 3-phosphoglycerate increased curvilinearly with increased active Fe, whereas glucose-6-phosphate and fructose-6-phosphate did not change. Glucose, fructose, sucrose, starch, and total non-structural carbohydrates at both dusk and pre-dawn increased with increasing active Fe. Carbon export from starch breakdown during the night, calculated as the difference between dusk and predawn levels, increased as active Fe increased. In conclusion, Fe limitation reduces the activities of Rubisco and other photosynthetic enzymes, and hence CO2 assimilation capacity. Fe-deficient grapevines have lower concentrations of non-structural carbohydrates in source leaves, and therefore, are source limited.

Carbon Assimilation and Carbohydrate Metabolism of `Concord' Grape (Vitis labrusca L.) Leaves in Response to Nitrogen Supply

One-year-old grapevines (Vitis labrusca L. `Concord') were supplied twice weekly for 5 weeks with 0, 5, 10, 15, or 20 mm nitrogen (N) in a modified Hoagland's solution to generate a wide range of leaf N status. Both light-saturated CO2 assimilation at ambient CO2 and at saturating CO2 increased curvilinearly as leaf N increased. Although stomatal conductance showed a similar response to leaf N as CO2 assimilation, calculated intercellular CO2 concentrations decreased. On a leaf area basis, activities of key enzymes in the Calvin cycle, ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco), NADP-glyceraldehyde-3-phosphate dehydrogenase (GAPDH), phosphoribulokinase (PRK), and key enzymes in sucrose and starch synthesis, fructose-1,6-bisphosphatase (FBPase), sucrose phosphate synthase (SPS), and ADP-glucose pyrophosphorylase (AGPase), increased linearly with increasing leaf N content. When expressed on a leaf N basis, activities of the Calvin cycle enzymes increased with increasing leaf N, whereas activities of FBPase, SPS, and AGPase did not show significant change. As leaf N increased, concentrations of glucose-6-phosphate (G6P), fructose-6-phosphate (F6P), and 3-phosphoglycerate (PGA) increased curvilinearly. The ratio of G6P/F6P remained unchanged over the leaf N range except for a significant drop at the lowest leaf N. Concentrations of glucose, fructose, and sucrose at dusk increased linearly with increasing leaf N, and there was no difference between predawn and dusk measurements. As leaf N increased, starch concentration increased linearly at dusk, but decreased linearly at predawn. The calculated carbon export from starch degradation during the night increased with increasing leaf N. These results showed that 1) grapes leaves accumulated less soluble carbohydrates under N-limitation; 2) the elevated starch level in low N leaves at predawn was the result of the reduced carbon export from starch degradation during the night; and 3) the reduced capacity of CO2 assimilation in low N leaves was caused by the coordinated decreases in the activities of key enzymes involved in CO2 assimilation as a result of direct N limitation, not by the indirect feedback repression of CO2 assimilation via sugar accumulation.