The observation of acclimation in leaf photosynthetic capacity to differences in growth irradiance has been widely used as support for a hypothesis that enables a simplification of some soil-vegetation-atmosphere transfer (SVAT) photosynthesis models. The acclimation hypothesis requires that relative leaf nitrogen concentration declines with relative irradiance from the top of a canopy to the bottom, in 1 : 1 proportion. In combination with a light transmission model it enables a simple estimate of the vertical profile in leaf nitrogen concentration (which is assumed to determine maximum carboxylation capacity), and in combination with estimates of the fraction of absorbed radiation it also leads to simple 'big-leaf' analytical solutions for canopy photosynthesis. We tested how forests deviate from this condition in five tree canopies, including four broadleaf stands, and one needle-leaf stand: a mixed-species tropical rain forest, oak ( Quercus petraea (Matt.) Liebl), birch ( Betula pendula Roth), beech ( Fagus sylvatica L.) and Sitka spruce ( Picea sitchensis (Bong.) Carr). Each canopy was studied when fully developed (mid-to-late summer for temperate stands). Irradiance ( Q , µ µ µ µ mol m Relative V a also declined linearly with relative Q , but with a significant intercept at zero irradiance ( P < 0·01). This intercept was strongly related to L a of the lowest leaves in each canopy ( P < 0·01, r 2 = 0·98, n = 5). For each canopy, daily ln Q was also linearly related with ln V a (P < 0·05), and the intercept was correlated with the value for photosynthetic capacity per unit nitrogen (PUN: Key-words :Acclimation; beech; birch; canopy photosynthesis model; leaf mass per unit area; leaf nitrogen; oak; photosynthetic capacity; Sitka spruce; tropical rain forest.Abbreviations : A max , photon saturated leaf photosynthetic rate at ambient CO 2 concentration ( µ mol m − 2 s − 1 ); A can , canopy photosynthetic rate ( µ mol m − 2 s − 1 ); H , mean canopy height (m); H r , height in canopy relative to H ; J a , maximum electron transfer rate (conventionally J max ) at 25 ° C, on an area basis ( µ mol m ; Q dr , the diffuse radiation component of Q r , obtained from hemispherical photographs; R da , daytime leaf respiration rate at 25 °C (µmol m −2 s −1 ); V a , V m , V ar , V ml , maximum carboxylation rate (conventionally V cmax ) at 25 °C on an area basis (µmol m −2 s −1 ), and a mass basis (nmol g −1 s −1 ), V a relative to V a of the highest measured leaves, V m of the lowest leaves in the canopy; α, is the apparent quantum efficiency, or initial slope of the J/ 344 P. Meir et al.
Maximum Rubisco activities (V(cmax)), rates of photosynthetic electron transport (J(max)), and leaf nitrogen and chlorophyll concentrations were studied along a light gradient in the canopies of four temperate deciduous species differing in shade tolerance according to the ranking: Populus tremula L. < Fraxinus excelsior L. < Tilia cordata Mill. = Corylus avellana L. Long-term light environment at the canopy sampling locations was characterized by the fractional penetration of irradiance in the photosynthetically active spectral region (I(sum)). We used a process-based model to distinguish among photosynthesis limitations resulting from variability in fractional nitrogen investments in Rubisco (P(R)), bioenergetics (P(B), N in rate-limiting proteins of photosynthetic electron transport) and light harvesting machinery (P(L), N in chlorophyll and thylakoid chlorophyll-protein complexes). On an area basis, V(cmax) and J(max) (V(a) (cmax) and J(a) (max)) increased with increasing growth irradiance in all species, and the span of variation within species ranged from two (T. cordata) to ten times (C. avellana). Examination of mass-based V(cmax) and J(max) (V(m) (cmax) and J(m) (max)) demonstrated that the positive relationships between area-based quantities and relative irradiance mostly resulted from the scaling of leaf dry mass per area (M(A)) with irradiance. Although V(m) (cmax) and J(m) (max) were positively related to growth irradiance in C. avellana, and J(m) (max) was positively related to irradiance in P. tremula, the variation range was only a factor of two. Moreover, V(m) (cmax) and J(m) (max) were negatively correlated with relative irradiance in T. cordata. Rubisco activity in crude leaf extracts generally paralleled the gas-exchange data, but it was independent of light in T. cordata, suggesting that declining V(m) (cmax) with increasing relative irradiance was related to increasing diffusive resistances from the intercellular air spaces to the sites of carboxylation in this species. Because irradiance had little effect on foliar nitrogen concentration, the relationships of P(B) and P(R) with irradiance were similar to those of V(m) (cmax) and J(m) (max). Shade-intolerant species tended to have greater P(B) and P(R) and also larger V(a) (cmax) and J(a) (max) than more shade-tolerant species. However, for the whole material, P(B) and P(R) varied only about 50%, whereas V(a) (cmax) and J(a) (max) varied more than 15-fold, further emphasizing the importance of leaf anatomical plasticity in determining photosynthetic acclimation to high irradiance. Leaf chlorophyll concentrations and fractional nitrogen investments in light harvesting increased hyperbolically with decreasing irradiance to improve quantum use efficiency for incident irradiance. The effect of irradiance on P(L) was of the same order as its effect in the opposite direction on M(A), leading to either a constant model estimate of leaf absorptance with I(sum) or a slightly positive correlation. We conclude that leaf morphological plasticity is a mo...
Transpiration and ozone uptake rates were measured simultaneously in sunflower leaves at different stomatal openings and various ozone concentrations. Ozone uptake rates were proportional to the ozone concentration up to 1500 nanoliters per liter. The leaf gas phase diffusion resistance (stomatal plus boundary layer) to water vapor was calculated and converted to the resistance to ozone multiplying it by the theoretical ratio of diffusion coefficients for water vapor and ozone in air (1.67). The ozone concentration in intercellular air spaces calculated from the ozone uptake rate and diffusion resistance to ozone scattered around zero. The ozone concentration in intercellular air spaces was measured directly by supplying ozone to the leaf from one side and measuring the equilibrium concentration above the other side, and it was found to be zero. The total leaf resistance to ozone was proportional to the gas phase resistance to water vapor with a coefficient of 1.68. It is concluded that ozone enters the leaf by diffusion through the stomata, and is rapidly decomposed in cell walls and plasmalemma. diffusion resistance of the whole gaseous pathway from cell surfaces to ambient air:where E2 is the transpiration rate (minus cuticular transpiration); rg, the diffusion resistance in the leaf gaseous phase to water vapor; wi, the water vapor concentration at evaporating cell surfaces; and wa, that in the ambient air. CO2 is a heavier gas (M = 44) than water vapor (M = 18); therefore, CO2 moves more slowly than water vapor through the same diffusion pathway and at the same concentration difference. The ratio of the diffusion coefficients of H20 and CO2 in the leaf gaseous pathway was measured to be 1.62 (9).We could not find a value of the diffusion constant for 03 in air, DZ, in the literature. However, diffusion constants for various gas mixtures may be calculated using the molecular parameters of component gases (1) 0.43 X (T)00) X Abbreviations: E, transpiration rate; wa, wi, water vapor concentration in ambient air (a) and on evaporating cell surfaces (i); r8, rg, leaf gas phase diffusion resistance to water vapor (w) and to ozone (z); M, molecular weight; DW, Dz, diffusion constant for water vapor (w) and for ozone (z); T, temperature, Tk, critical temperature; Vk, critical volume; P, atmospheric pressure; Za, zi, ozone concentration in ambient air (a) and in the leaf intercellular air space (i); Q, ozone uptake rate; v, gas flow rate; S, leaf area; gz, total leaf conductance for ozone; rz, total leaf resistance to ozone; gge ggz, leaf gas phase diffusion conductance for water vapor (w) and for ozone (z).
The present study was undertaken to test for the hypothesis that the rate of development in the capacity for photosynthetic electron transport per unit area ( J max;A ), and maximum carboxylase activity of Rubisco ( V cmax;A ) is proportional to average integrated daily quantum flux density ( Q int ) in a mixed deciduous forest dominated by the shade-intolerant species Populus tremula L., and the shade-tolerant species Tilia cordata Mill. We distinguished between the age-dependent changes in net assimilation rates due to modifications in leaf dry mass per unit area ( M A ), foliar nitrogen content per unit dry mass ( N M ), and fractional partitioning of foliar nitrogen in the proteins of photosynthetic electron transport ( F B ), Rubisco ( F R ) and in light-harvesting chlorophyll-protein complexes. In both species, increases in J max;A and V cmax;A during leaf development were primarily determined by nitrogen allocation to growing leaves, increases in leaf nitrogen partitioning in photosynthetic machinery, and increases in M A . Canopy differences in the rate of development of leaf photosynthetic capacity were mainly controlled by the rate of change in M A . There was only small within-canopy variation in the initial rate of biomass accumulation per unit Q int (slope of M A versus leaf age relationship per unit Q int ), suggesting that canopy differences in the rate of development of J max;A and V cmax;A are directly proportional to Q int . Nevertheless, M A , nitrogen, J max;A and V cmax;A of mature leaves were not proportional to Q int because of a finite M A in leaves immediately after budburst (light-independent component of M A ). M A , leaf chlorophyll contents and chlorophyll : N ratio of mature leaves were best correlated with the integrated average quantum flux density during leaf development, suggesting that foliar photosynthetic apparatus, once developed, is not affected by day-to-day fluctuations in Q int . However, for the upper canopy leaves of P. tremula and for the entire canopy of T. cordata , there was a continuous decline in N contents per unit dry mass in mature non-senescent leaves on the order of 15-20% for a change of leaf age from 40 to 120 d, possibly manifesting nitrogen reallocation to bud formation. The decline in N contents led to similar decreases in leaf photosynthetic capacity and foliar chlorophyll contents. These data demonstrate that light-dependent variation in the rate of developmental changes in M A determines canopy differences in photosynthetic capacity, whereas foliar photosynthetic apparatus is essentially constant in fully developed leaves.
Within a time-scale of several days photosynthesis can acclimate to light by variation in the capacity for photosynthesis with depth in a canopy or by variation in the stoichiometry of photosynthetic components at each position within the canopy. The changes in leaf photosynthetic capacity are usually related to and expressed as changes in leaf nitrogen content. However, photosynthetic capacity and leaf nitrogen never match exactly the photon flux density (PFD) gradient within a canopy. As a result, photosynthetic light use efficiency, i.e. photosynthetic performance per incident PFD, increases considerably from the top of the canopy to the lower shaded part. Many of existing optimisation models fail to express the actual pattern of nitrogen or photosynthetic capacity distribution within a canopy. This failure occurs because these optimisation models do not consider that the quantitative aspect of photosynthesis acclimation is a whole plant phenomenon. Although turnover models, which describe the distribution of the photosynthetic apparatus within a canopy as a dynamic equilibrium between breakdown and regeneration of apparatus with respect to nitrogen availability, photosynthetic rate and export of carbohydrates, produce realistic results, these models require confirmation. The mechanism responsible for changes in the relative share of light-harvesting apparatus as acclimation to irradiance remains unknown. Ability of the photosynthetic apparatus to balance properly the light harvesting capacity with electron transport and biochemical capacities is limited. As a result of this fundamental limitation, photosynthetic light use efficiency always increases with increasing thickness of the photosynthetic apparatus.
Needle dimensions, needle surface area, needle dry weight per area (LWA) and needle density (ND, needle weight per volume) were measured in terminal current-year shoots in a natural canopy of variably sized Picea abies (L.) Karst. trees growing along a light gradient. Needle shape was described as a rhomboid. Needle width (D(2)) increased with increasing diffuse site factor, a(d) (relative amount of penetrating diffuse solar radiation), whereas needle thickness (D(1)) remained nearly constant, resulting in an inverse relationship between D(1)/D(2) and a(d) and an increase in the ratio of total (TLA) to projected needle surface area (PLA) with increasing a(d). Because of the variations in needle morphology with respect to light availability, the shoot parameters used in present canopy models are also expected to be light-sensitive, and studies involving shoot morphology should also consider the variability in needle geometry. Needle dimensions and total tree height were not correlated. However, LWA increase with both increasing a(d) and total tree height. When LWA was expressed as the product of ND and needle height (NH, height of the rhomboidal transverse section of a needle), LWA appeared to increase with irradiance, because of changing NH, and with total tree height, because of changing needle density.
energy for mitochondrial respiration rate were also observed, indicating that acclimation to temperature of mitochondrial and chloroplastic electron transport proceeds in a co-ordinated manner, and possibly involves longterm changes in membrane fluidity properties. We conclude that, because of correlations between temperature and light, the shapes of J max versus T, and R d versus T response curves vary within tree canopies, and this needs to be taken account in modelling whole canopy photosynthesis.Key-words: Populus tremula; Tilia cordata; chlorophyll fluorescence; feedback limitations; light gradients; photosynthetic electron transport; temperature acclimation. INTRODUCTIONIn temperate trees, foliar maximum photosynthesis rates (A max ) increase with long-term integrated quantum flux density (e.g. Walters & Field 1987;Ellsworth & Reich 1993;Pearcy & Sims 1994;Niinemets & Tenhunen 1997;, and the light acclimation of leaf photosynthetic characters is the major factor optimizing whole canopy carbon gain (Gutschick 1988;Gutschick & Wiegel 1988;. Although the positive scaling of A max with irradiance improves canopy carbon gain under moderate environmental stresses that always occur in natural ecosystems, in long-term, canopy carbon accumulation is also dependent on the ability of leaves to maintain this increased capacity over periods of more severe and long-lasting stresses that may frequently accompany light gradients in tree canopies. For example, daily average integrated quantum flux density incident on the leaves (Q int ) correlates positively with daily average air temperature and vapour pressure deficit (Chiariello 1984;Shuttleworth et al. 1985;Margolis & Ryan 1997) within the canopy. Because of these correlations, the leaves exposed to higher irradiance frequently also suffer from greater water ( ABSTRACTResponses of foliar light-saturated net assimilation rate (A max ), capacity for photosynthetic electron transport (J max ) and mitochondrial respiration rate (R d ) to long-term canopy light and temperature environment were investigated in a temperate deciduous canopy composed of Populus tremula L. in the upper (17-28 m) and of Tilia cordata Mill. in the lower canopy layer (4-17 m). Climatic measurements indicated that seasonal average daily maximum air temperature (T max ) was 5·5°C (range 0·7-10·5°C) higher in the top than in the bottom of the canopy, and strong positive correlations were observed between T max and seasonal average integrated quantum flux density (Q int ), as well as between seasonal average daily mean temperature and Q int . Because of changes in leaf dry mass and nitrogen per unit area, A max , J max , and R d scaled positively with Q int in both species at a common leaf temperature (T). According to J max versus T response curves and dark chlorophyll fluorescence transients, photosynthetic electron transport was less heat resistant in P. tremula with optimum temperature of J max , T opt , of 33·5 ± 0·6°C than in T. cordata with T opt of 40·7 ± 0·6°C. This difference was suggested t...
The quickly rising atmospheric carbon dioxide (CO 2 )-levels, justify the need to explore all carbon (C) sequestration possibilities that might mitigate the current CO 2 increase. Here, we report the likely impact of future increases in atmospheric CO 2 on woody biomass production of three poplar species (Populus alba L. clone 2AS-11, Populus nigra L. clone Jean Pourtet and Populus  euramericana clone I-214). Trees were growing in a high-density coppice plantation during the second rotation (i.e., regrowth after coppice; 2002-2004; POPFACE/EUROFACE). Six plots were studied, half of which were continuously fumigated with CO 2 (FACE; free air carbon dioxide enrichment of 550 ppm). Half of each plot was fertilized to study the interaction between CO 2 and nutrient fertilization. At the end of the second rotation, selective above-and belowground harvests were performed to estimate the productivity of this bio-energy plantation. Fertilization did not affect growth of the poplar trees, which was likely because of the high rates of fertilization during the previous agricultural land use. In contrast, elevated CO 2 enhanced biomass production by up to 29%, and this stimulation did not differ between above-and belowground parts. The increased initial stump size resulting from elevated CO 2 during the first rotation (1999)(2000)(2001) could not solely explain the observed final biomass increase. The larger leaf area index after canopy closure and the absence of any major photosynthetic acclimation after 6 years of fumigation caused the sustained CO 2 -induced biomass increase after coppice. These results suggest that, under future CO 2 concentrations, managed poplar coppice systems may exhibit higher potential for C sequestration and, thus, help mitigate climate change when used as a source of C-neutral energy.
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