Maximum Carbon Assimilation Rate Research Articles

Optimization of leaf morphology in relation to leaf water status: A theory.

The leaf economic traits such as leaf area, maximum carbon assimilation rate, and venation are all correlated and related to water availability. Furthermore, leaves are often broad and large in humid areas and narrower in arid/semiarid and hot and cold areas. We use optimization theory to explain these patterns. We have created a constrained optimization leaf model linking leaf shape to vein structure that is integrated into coupled transpiration and carbon assimilation processes. The model maximizes net leaf carbon gain (NPPleaf) over the loss of xylem water potential. Modeled relations between leaf traits are consistent with empirically observed patterns. As the results of the leaf shape–venation relation, our model further predicts that a broadleaf has overall higher NPPleaf compared to a narrowleaf. In addition, a broadleaf has a lower stomatal resistance compared to a narrowleaf under the same level of constraint. With the same leaf area, a broadleaf will have, on average, larger conduits and lower total leaf xylem resistance and thus be more efficient in water transportation but less resistant to cavitation. By linking venation structure to leaf shape and using water potential as the constraint, our model provides a physical explanation for the general pattern of the covariance of leaf traits through the safety–efficiency trade‐off of leaf hydraulic design.

Soil water content effects on net ecosystem CO2 exchange and actual evapotranspiration in a Mediterranean semiarid savanna of Central Chile

Biosphere-atmosphere water and carbon fluxes depend on ecosystem structure, and their magnitudes and seasonal behavior are driven by environmental and biological factors. We studied the seasonal behavior of net ecosystem CO2 exchange (NEE), Gross Primary Productivity (GPP), Ecosystem Respiration (RE), and actual evapotranspiration (ETa) obtained by eddy covariance measurements during two years in a Mediterranean Acacia savanna ecosystem (Acacia caven) in Central Chile. The annual carbon balance was −53 g C m−2 in 2011 and −111 g C m−2 in 2012, showing that the ecosystem acts as a net sink of CO2, notwithstanding water limitations on photosynthesis observed in this particularly dry period. Total annual ETa was of 128 mm in 2011 and 139 mm in 2012. Both NEE and ETa exhibited strong seasonality with peak values recorded in the winter season (July to September), as a result of ecosystem phenology, soil water content and rainfall occurrence. Consequently, the maximum carbon assimilation rate occurred in wintertime. Results show that soil water content is a major driver of GPP and RE, defining their seasonal patterns and the annual carbon assimilation capacity of the ecosystem, and also modulating the effect that solar radiation and air temperature have on NEE components at shorter time scales.

Evolutionary trade-offs between drought resistance mechanisms across a precipitation gradient in a seasonally dry tropical oak (Quercus oleoides)

In seasonally dry tropical forest regions, drought avoidance during the dry season coupled with high assimilation rates in the wet season is hypothesized to be an advantageous strategy for forest trees in regions with severe and long dry seasons. In contrast, where dry seasons are milder, drought tolerance coupled with a conservative resource-use strategy is expected to maximize carbon assimilation throughout the year. Tests of this hypothesis, particularly at the intraspecific level, have been seldom conducted. In this study, we tested the extent to which drought resistance mechanisms and rates of carbon assimilation have evolved under climates with varying dry season length and severity within Quercus oleoidesCham. and Schlect., a tropical dry forest species that is widely distributed in Central America. For this purpose, we conducted a greenhouse experiment where seedlings originating from five populations that vary in rainfall patterns were grown under different watering treatments. Our results revealed that populations from xeric climates with more severe dry seasons exhibited large mesophyllous leaves (with high specific leaf area, SLA), and leaf abscission in response to drought, consistent with a drought-avoidance strategy. In contrast, populations from more mesic climates with less severe dry seasons had small and thick sclerophyllous leaves with low SLA and reduced water potential at the turgor loss point (πtlp), consistent with a drought-tolerance strategy. Mesic populations also showed high plasticity in πtlp in response to water availability, indicating that osmotic adjustment to drought is an important component of this strategy. However, populations with mesophyllous leaves did not have higher maximum carbon assimilation rates under well-watered conditions. Furthermore, SLA was negatively associated with mass-based photosynthetic rates, contrary to expectations of the leaf economics spectrum, indicating that drought-resistance strategies are not necessarily tightly coupled with resource-use strategies. Overall, our study demonstrates the importance of considering intraspecific variation in analyses of the vulnerability of tropical trees to climate change.

Photoprotection of evergreen and drought-deciduous tree leaves to overcome the dry season in monsoonal tropical dry forests in Thailand

In tropical dry forests, uppermost-canopy leaves of evergreen trees possess the ability to use water more conservatively compared with drought-deciduous trees, which may result from significant differences in the photoprotective mechanisms between functional types. We examined the seasonal variations in leaf gas exchange, chlorophyll fluorescence and the amounts of photosynthetic pigments within lamina of the uppermost-canopy leaves of three drought-deciduous trees (Vitex peduncularis Wall., Xylia xylocarpa (Roxb.) W. Theob., Shorea siamensis Miq.), a semi-deciduous tree (Irvingia malayana Miq.) and two evergreen trees (Hopea ferrea Lanessan and Syzygium cumini (L.) Skeels) in Thailand. Area-based maximum carbon assimilation rates (Amax) decreased during the dry season, except in S. siamensis. The electron transport rate (ETR) remained unchanged in deciduous trees, but decreased during the dry season in evergreen and semi-deciduous trees. In the principal component analysis, the first axis (Axis 1) accounted for 44.3% of the total variation and distinguished deciduous from evergreen trees. Along Axis 1, evergreen trees were characterized by a high Stern-Volmer non-photochemical quenching coefficient (NPQ), high xanthophyll cycle pigments/chlorophyll and a high de-epoxidation state of the xanthophyll cycle, whereas the deciduous trees were characterized by a high ETR, a high quantum yield of PSII (ΦPSII = (Fm(') -F)/Fm(')) and a high mass-based Amax under high-light conditions. These findings indicate that drought-deciduous trees showing less conservative water use tend to dissipate a large proportion of electron flow through photosynthesis or alternative pathways. In contrast, the evergreens showed more conservative water use, reduced Amax and ETR and enhanced NPQ and xanthophyll cycle pigments/chlorophyll during the dry season, indicating that down-regulated photosynthesis with enhanced thermal dissipation of excess light energy played an important role in photoprotection. Trees with different water uses and leaf lifespans appear to employ different photoprotective mechanisms to overcome the unfavorable dry-season drought. Our data may suggest that future changes in precipitation will strongly impinge on forest structure and functions.

Physiological responses of Spartina alterniflora to varying environmental conditions in Virginia marshes

Physiological measurements were used to investigate the dependence of photosynthesis on light, temperature, and intercellular carbon dioxide (CO2) levels in the C4 marsh grass Spartina alterniflora. Functional relationships between these environmental variables and S. alterniflora physiological responses were then used to improve C4-leaf photosynthesis models. Field studies were conducted in monocultures of S. alterniflora in Virginia, USA. On average, S. alterniflora exhibited lower light saturation values (~1000 μmol m−2 s−1) than observed in other C4 plants. Maximum carbon assimilation rates and stomatal conductance to water vapor diffusion were 36 μmol (CO2) m−2 s−1 and 200 mmol (H2O) m−2 s−1, respectively. Analysis of assimilation-intercellular CO2 and light response relationships were used to determine Arrhenius-type temperature functions for maximum rate of carboxylation (Vcmax), phosphoenolpyruvate carboxylase activity (Vpmax), and maximum electron transport rate (Jmax). Maximum Vcmax values of 105 μmol m−2 s−1 were observed at the leaf temperature of 311 K. Optimum Vpmax values (80.6 μmol m−2 s−1) were observed at the foliage temperature of 308 K. The observed Vpmax values were lower than those in other C4 plants, whereas Vcmax values were higher, and more representative of C3 plants. Optimum Jmax values reached 138 μmol (electrons) m−2 s−1 at the foliage temperature of 305 K. In addition, the estimated CO2 compensation points were in the range of C3 or C3–C4 intermediate plants, not those typical of C4 plants. The present results indicate the possibility of a C3–C4 intermediate or C4-like photosynthetic mechanism rather than the expected C4-biochemical pathway in S. alterniflora under field conditions. In a scenario of atmospheric warming and increased atmospheric CO2 concentrations, S. alterniflora will likely respond positively to both changes. Such responses will result in increased S. alterniflora productivity, which is uncharacteristic of C4 plants.

Precipitation as driver of carbon fluxes in 11 African ecosystems

Abstract. This study reports carbon and water fluxes between the land surface and atmosphere in eleven different ecosystems types in Sub-Saharan Africa, as measured using eddy covariance (EC) technology in the first two years of the CarboAfrica network operation. The ecosystems for which data were available ranged in mean annual rainfall from 320 mm (Sudan) to 1150 mm (Republic of Congo) and include a spectrum of vegetation types (or land cover) (open savannas, woodlands, croplands and grasslands). Given the shortness of the record, the EC data were analysed across the network rather than longitudinally at sites, in order to understand the driving factors for ecosystem respiration and carbon assimilation, and to reveal the different water use strategies in these highly seasonal environments. Values for maximum net carbon assimilation rates (photosynthesis) ranged from −12.5 μmol CO2 m−2 s−1 in a dry, open Millet cropland (C4-plants) up to −48 μmol CO2 m−2 s−1 for a tropical moist grassland. Maximum carbon assimilation rates were highly correlated with mean annual rainfall (r2=0.74). Maximum photosynthetic uptake rates (Fpmax) were positively related to satellite-derived fAPAR. Ecosystem respiration was dependent on temperature at all sites, and was additionally dependent on soil water content at sites receiving less than 1000 mm of rain per year. All included ecosystems dominated by C3-plants, showed a strong decrease in 30-min assimilation rates with increasing water vapour pressure deficit above 2.0 kPa.

Interactive effects of shade and irrigation on the performance of seedlings of three Mediterranean Quercus species

Shade and irrigation are frequently used to increase the success of Mediterranean Quercus spp. plantations. However, there is controversy about the combined effects of these treatments on plant performance. We assessed the effects of two irradiances (full sunlight and moderate shade) and two summer watering regimes (high (daily) and low (alternate days)) on leaf and whole-plant traits of 1-year-old seedlings of Quercus coccifera, Q. ilex subsp. ballota and Q. faginea grown outdoors for 8.5 months. Leaf traits included measures of morphology, nitrogen concentration, gas exchange and photochemical efficiency, and measures of whole-plant traits included biomass allocation patterns, growth phenology, across-summer leaf area change and relative growth rate (RGR). Moderate shade reduced leaf mass per area, increased photochemical efficiency, maximum carbon assimilation rate (Amax) and allocation to leaves, and prolonged the growing period in one or more of the species. Daily watering in summer increased Amax of Q. ilex and prolonged the growing period of Q. ilex and Q. faginea. Both treatments tended to increase RGR. The effect of shade was greater in the low-watering regime than in the high-watering regime for two of the 15 studied traits, with treatment effects being independent for the remaining 13 traits. Leaf nitrogen and the ability to maintain leaf area after the arid period, rather than biomass allocation traits, explained the variation in seedling RGR. Trait responsiveness to the treatments was low and similar among species and between study scales, being unexpectedly low in Q. faginea leaves. This may be because selective pressures on leaf plasticity act differently in deciduous and evergreen species. We conclude that moderate shade and daily summer watering enhance the performance of Mediterranean Quercus seedlings through species-specific mechanisms.

Parameterization of Canopy Structure and Leaf-Level Gas Exchange for an Eastern Amazonian Tropical Rain Forest (Tapajós National Forest, Pará, Brazil)

Abstract Carbon flux of Amazonian primary forest vegetation has been shown to vary both spatially and temporally. Process-based models are adequate tools to understand the basis of such variation and can also provide projections to future scenarios. The parameterization of such process-based models requires information from the vegetation in question simply because ecosystem-level gas exchange is a direct result of the tightly coupled interaction between local vegetation and regional climate. In this study, data are presented concerning canopy structure [leaf area index (LAI), and the ratio of leaf dry mass to leaf area (LMA)], leaf chemistry [area-based foliar nitrogen content (Narea) and carbon isotope composition (δ13C)], and photosynthetic gas exchange [maximum carbon assimilation rates (Amax), stomatal conductance (gs@Amax), maximum carboxylation capacity (Vcmax), and respiration rates (Rd)] versus relative height from an extensive survey of primary forest vegetation of the Santarém region (eastern Amazon, Santarém, Federal State of Pará, Brazil). Ground-level LAI values ranged between 4.5 and 5.9. Both Amax and Vcmax showed large variations within the canopy profile with values ranging between 2.4 and 15.7 μmol m−2 s−1 and between 10.1 and 105.7 μmol m−2 s−1, respectively. Also, Narea varied between 0.75 to 4.19 gN m−2, and similar to Amax and Vcmax, showed higher values at the top of the canopy. Variations were detected among sites in patterns of vertical distribution of Narea and LAI, indicating spatial heterogeneity of the forest. Also, no statistically significant evidence of seasonal variations on parameters was observed, indicating that there is limited gas exchange acclimation by the vegetation to wet or dry seasons.

Photosynthesis of coppicing poplar clones in a free-air CO2 enrichment (FACE) experiment in a short-rotation forest

Photosynthetic capacity was assessed in coppices of three poplar clones (Populus alba L. genotype 2AS11, P. × euramericana (Dode) Guinier genotype I-214 and P. nigra L. genotype Jean Pourtet) growing in the POPFACE/EUROFACE free-air CO2 enrichment experiment in central Italy. Plants were grown either at an elevated CO2 concentration of 550 μmol mol-1 or in control conditions for 3 years and were then harvested and allowed to coppice. Plants were either fertilised with the addition of liquid fertiliser at a level of 212 kg N ha-1 year-1 or unfertilised after harvesting. No evidence was found of changes in the maximum Rubisco carboxylation rate (VCmax) and thus there was no photosynthetic downregulation caused by the FACE treatment in either P. × euramericana or P. nigra, but there was a marginally significant reduction in VCmax of fertilised P. alba (P<0.09). Carbon assimilation rates were significantly higher in FACE plants than control plants. Maximum carbon assimilation rate was stimulated by an average of 32.8% in these clones, with individual stimulation values of 27.6% for P. alba, 32.1% for P. × euramericana and 49.5% for P. nigra. No significant interactions between the FACE and fertilisation treatments were found for any of the photosynthetic variables measured. The day respiration rate in leaves of P.×euramericana was significantly increased by FACE treatment, but it was unaffected in the other clones. This work shows that photosynthesis remains stimulated at elevated CO2 concentration in these plants following harvesting, although to a lesser extent than seen normally, which may be related to a reduction in sink strength.

Leaf Gas Exchange Characteristics in Relation to Leaf Canopy Position of Myrica faya in its Native Environment (Tenerife, Canary Islands)

Abstract: Diurnal courses of gas exchange were measured throughout one year in fully expanded current‐year leaves in the uppermost canopy (sun leaves, 18 m above ground) and in the lower canopy (shade leaves, 12 m above ground) of Myrica faya Ait., a dominant component of the Canarian laurel forest in Tenerife, Canary Islands, Spain. M. faya showed large differences between sun and shade leaves in gas exchange characteristics (about 50 % of maximum carbon assimilation rate (Amax) reduction in shade leaves, but this reduction can be higher on specific days) that were modulated by strong light attenuation and high leaf area index (LAI) of the stand. This species presented low Amax, about 10 μmol m‐2 s‐1, high maximum transpiration (E, 8 mmol m‐2 s‐1) and stomatal conductance (gs, 750 mmol m‐2 s‐1) and very low instantaneous water use efficiency (WUE, mean maximum 1.1 mmol mol‐1) and A/gs (mean maximum 23.5 μmol mol‐1). M. faya responded to high air vapour pressure deficit (VPD), decreasing its gs but maintaining relatively high values of A and E during the studied period. Stomatal response to VPD showed a higher sensitivity than its congeners, M. cerifera, and Laurus azorica, tree species co‐occurring in the Canarian laurel forest. In general, all these gas exchange characteristics lead us to consider this species more similar to subtropical plants of humid regions than to species of the Mediterranean region.

Leaf gas exchange characteristics of a Canarian laurel forest tree species [ Persea indica (L.) K. Spreng.] under natural conditions

Persea indica (L.) K. Spreng. is a tree species of the Canarian laurel forest. Diurnal courses of gas exchange were measured during the period of one year in fully expanded current-year leaves in the uppermost canopy (sun leaves, 18 m above ground) and in the lower canopy (shade leaves, 12 m above ground) in order to understand the general behaviour of this tree species in relation with the environmental conditions of its habitat. Persea indica showed a great plasticity in gas exchange characteristics with large differences between sun and shade leaves (50 percnt; of maximum carbon assimilation rate (Amax) reduction in the shade) that were modulated by the strong light attenuation and high leaf area index (LAI) of the stand. This species presented low Amax, about 11 μmol m−2 s−1, high maximum transpiration (E, above 8 mmol m−2 s−1) and high stomatal conductance (gs, 800 mmol m−2 s−1) and therefore very low instantaneous water use efficiency (WUE, mean maximum 1.5 mmol mol−1) and A/gs (mean maximum between 20-35 μmol mol−1) and high mean minimum Ci value (around 237 μmol mol−1), measured in sun leaves. All these gas exchange characteristics fit well with a non-conservative strategy of water use efficiency, which is appropriate for species that are subject to mild or short duration droughts. Overall, this species is more similar to subtropical plants of humid regions than to sclerophyllous plants of Mediterranean-type climate. However, under strong sunlight and dry conditions, sun leaves of P. indica show midday stomatal closure and downregulation of photochemical efficiency, effective ways to avoid excess water loss and photodamage of photosynthetic apparatus.

Gas exchange characteristics of a Canarian laurel forest tree species (Laurus azorica) in relation to environmental conditions and leaf canopy position.

Diurnal courses of gas exchange were measured over a 1-year period in fully expanded current-year leaves in the upper (sun-exposed, 18 m above ground) and the lower (shaded, 12 m above ground) canopy of Laurus azorica (Seub.) Franco, a major canopy species of the Canarian laurel forest in Tenerife, Canary Islands, Spain. Laurus azorica exhibited high leaf plasticity in gas exchange characteristics, with a maximum carbon assimilation rate (Amax) of shade leaves about 50% that of sun leaves. This difference reflects the high leaf area index (LAI) of the stand and the correspondingly sharp light attenuation with increasing canopy depth. In sun leaves, Amax peaked at about 11 micromol m-2 s-1 and maximum transpiration (E) was about 8 mmol m-2 s-1, which corresponded with a maximum stomatal conductance (gs) of about 650 mmol m-2 s-1. Mean maximum instantaneous water-use efficiency (WUE) was 1.5 mmol mol-1 and the mean maximum A/gs was 20-35 micromol mol-1. Mean minimum internal CO2 concentration (Ci) was 225 micromol mol-1. Although high air vapor pressure deficit (VPD) caused a small decrease in gs, it remained high enough to maintain relatively high A and E. These gas exchange characteristics indicate a non-conservative use of water, which is appropriate for a species subject to droughts that are mild or of short duration. In this respect, Laurus azorica differs from its congener, L. nobilis L., of the Mediterranean region and other shrubs growing in Mediterranean-type climates in California and Chile that have to withstand more severe or more prolonged droughts.

THE UNUSUAL LIFE HISTORY OFALSEIS BLACKIANA: A SHADE-PERSISTENT PIONEER TREE?

Classifications of tree species into regeneration guilds or functional groups are often based on a limited number of life history characteristics indicative of requirements for early establishment, with the assumption that these traits are correlated with growth requirements of larger individuals. We tested the validity of this assumption for Alseis blackiana, a common canopy tree in central Panama, by examining its seed germination, seedling growth, and sapling distribution, growth, and mortality. We found that the early life history of Alseis is characteristic of pioneer species: the seed germination rate was much higher in forest gaps than in the forest understory, small seedlings were absent from the understory, and seedlings showed strong plastic responses to variation in light. Seedling relative growth rate, leaf area production rate, and maximum carbon assimilation rates of Alseis are much more similar to those of the pioneer tree Miconia argentea than to those of the non-pioneer tree Tetragastris panamensis. However, in contrast to Miconia, estab- lished seedlings of Alseis are capable of remarkable persistence in the understory; in a 1- ha plot on Barro Colorado Island, the mortality rate for 722 saplings ,1 cm dbh was ,4% per year between 1986 and 1998. We suspect that shade tolerance in Alseis is achieved by both physiological and morphological adaptations, e.g., efficient light interception by thin, nonoverlapping leaves, tolerance to litterfall damage, and a high resprouting capacity. These results show that existing functional group classifications do not account for the full range of seedling trait combinations found in tropical trees and indicate that a gap requirement for seedling establishment need not limit the abundance of adult individuals in a stand.

EFFECT OF VITAMIN CONCENTRATIONS ON GROWTH AND DEVELOPMENT OF VITAMIN-REQUIRING ALGAE(1).

Vitamin-requiring marine algae, Cyclotella nana, Monochrysis lutheri, and Amphidinium carterae, were grown in batch culture with limiting concentrations of vitamin B12 , thiamine, and biotin, respectively. Cell numbers, average cell volumes, biomasses, (11) CO2 uptake rates, and chlorophyll a contents were determined daily. Maximum (14) CO2 uptake rates in most vitamin concentrations were obtained at 2 days with C. nana and M. lutheri and at 4 days with A. carterae after starved cultures were exposed to the vitamin. Radiocarbon uptake rates approximately reflect biomass increases. Cell numbers were proportional to vitamin concentrations when cells were incubated for 2 to 3 more days. Cell sizes varied depending on time of incubation. Chlorophyll a content did not always reflect vitamin concentrations. Maximum carbon assimilation rates (Km ) and saturation constants (Ks ) determined from (14) CO2 , uptake rates in different vitamin concentrations during early incubation were higher than when determined from cell number in log phase growth. Dissolved vitamin B12 , thiamine, and biotin in many samples of seawaters were in the ranges which influence the growth rate, cell size, and chlorophyll a content of C. nana, M. lutheri, and A. carterae, respectively, in laboratory studies. The effects of vitamins on these algae in situ may be similar.