Temperature response of parameters of a biochemically based model of photosynthesis. I. Seasonal changes in mature maritime pine (Pinus pinaster Ait.)

Responses of plant processes to temperature may vary according to the time scale on which they are measured. In this study, both short‐term and seasonal responses of photosynthesis to temperature were examined. A field study of seasonal changes in the temperature response of photosynthesis was conducted on two provenances, French and Moroccan, of mature maritime pine (Pinus pinaster Ait.). Measurements were made every 2 months over a 1‐year period and used to parameterize a mechanistic model of photosynthesis. Temperature responses of maximum Rubisco activity, V_cmax, and potential electron transport rate, J_max, were obtained for each measurement period, as was the response of stomatal conductance, g_s, to water vapour pressure deficit (VPD). Absolute values of V_cmax and J_max at 25 °C were related to needle nitrogen content, N_area.N_area, and thus V_cmax and J_max, were negatively correlated with the mean minimum temperature in the month preceding measurements. The ratio of J_max : V_cmax at 25 °C varied between 1 and 1·7 but did not show any seasonal trend. Nor was there any seasonal trend in the relative temperature response of V_cmax, which had an activation energy H_a of approximately 57 kJ mol^?1 throughout the experiment. The activation energy of J_max was also close to constant throughout the experiment, averaging 39 kJ mol^?1. For the French provenance, the optimal temperature of J_max was positively correlated with the maximum temperature of the previous day, but no such correlation was found for the Moroccan provenance. The response of g_s to VPD also varied seasonally, with much stronger stomatal closure in winter months. Taken together, these results implied a translational shift downwards of the photosynthetic temperature response curve with increasing T_prev, and a shift in the temperature optimum of photosynthesis of 5–10 °C between summer and winter. These results illustrate that the short‐term temperature response of photosynthesis varies significantly on a seasonal basis.     

Temperature response of parameters of a biochemically based model of photosynthesis. II. A review of experimental data

The temperature dependence of C₃ photosynthesis is known to vary with growth environment and with species. In an attempt to quantify this variability, a commonly used biochemically based photosynthesis model was parameterized from 19 gas exchange studies on tree and crop species. The parameter values obtained described the shape and amplitude of the temperature responses of the maximum rate of Rubisco activity (V_cmax) and the potential rate of electron transport (J_max). Original data sets were used for this review, as it is shown that derived values of V_cmax and its temperature response depend strongly on assumptions made in derivation. Values of J_max and V_cmax at 25 °C varied considerably among species but were strongly correlated, with an average J_max : V_cmax ratio of 1·67. Two species grown in cold climates, however, had lower ratios. In all studies, the J_max : V_cmax ratio declined strongly with measurement temperature. The relative temperature responses of J_max and V_cmax were relatively constant among tree species. Activation energies averaged 50 kJ mol^?1 for J_max and 65 kJ mol^?1 for V_cmax, and for most species temperature optima averaged 33 °C for J_max and 40 °C for V_cmax. However, the cold climate tree species had low temperature optima for both J_max(19 °C) and V_cmax (29 °C), suggesting acclimation of both processes to growth temperature. Crop species had somewhat different temperature responses, with higher activation energies for both J_max and V_cmax, implying narrower peaks in the temperature response for these species. The results thus suggest that both growth environment and plant type can influence the photosynthetic response to temperature. Based on these results, several suggestions are made to improve modelling of temperature responses.     

The relationship of leaf photosynthetic traits – Vcmax and Jmax – to leaf nitrogen,leaf phosphorus,and specific leaf area: a meta‐analysis and modeling study

Great uncertainty exists in the global exchange of carbon between the atmosphere and the terrestrial biosphere. An important source of this uncertainty lies in the dependency of photosynthesis on the maximum rate of carboxylation (V_cmax) and the maximum rate of electron transport (J_max). Understanding and making accurate prediction of C fluxes thus requires accurate characterization of these rates and their relationship with plant nutrient status over large geographic scales. Plant nutrient status is indicated by the traits: leaf nitrogen (N), leaf phosphorus (P), and specific leaf area (SLA). Correlations between V_cmax and J_max and leaf nitrogen (N) are typically derived from local to global scales, while correlations with leaf phosphorus (P) and specific leaf area (SLA) have typically been derived at a local scale. Thus, there is no global-scale relationship between V_cmax and J_max and P or SLA limiting the ability of global-scale carbon flux models do not account for P or SLA. We gathered published data from 24 studies to reveal global relationships of V_cmax and J_max with leaf N, P, and SLA. V_cmax was strongly related to leaf N, and increasing leaf P substantially increased the sensitivity of V_cmax to leaf N. J_max was strongly related to V_cmax, and neither leaf N, P, or SLA had a substantial impact on the relationship. Although more data are needed to expand the applicability of the relationship, we show leaf P is a globally important determinant of photosynthetic rates. In a model of photosynthesis, we showed that at high leaf N (3 gm⁻²), increasing leaf P from 0.05 to 0.22 gm⁻² nearly doubled assimilation rates. Finally, we show that plants may employ a conservative strategy of J_max to V_cmax coordination that restricts photoinhibition when carboxylation is limiting at the expense of maximizing photosynthetic rates when light is limiting.     

Interactive effects of nitrogen and phosphorus on the acclimation potential of foliage photosynthetic properties of cork oak,Quercus suber,to elevated atmospheric CO2 concentrations

Leaf gas-exchange and chemical composition were investigated in seedlings of Quercus suber L. grown for 21 months either at elevated (700 μmol mol^–1) or normal (350 μmol mol^–1) ambient atmospheric CO₂ concentrations, [CO₂], in a sandy nutrient-poor soil with either ‘high’ N (0.3 mol N m^–3 in the irrigation solution) or with ‘low’ N (0.05 mol N m^–3) and with a constant suboptimal concentration of the other macro- and micronutrients. Although elevated [CO₂] yielded the greatest total plant biomass in ‘high’ nitrogen treatment, it resulted in lower leaf nutrient concentrations in all cases, independent of the nutrient addition regime, and in greater nonstructural carbohydrate concentrations. By contrast, nitrogen treatment did not affect foliar N concentrations, but resulted in lower phosphorus concentrations, suggesting that under lower N, P use-efficiency in foliar biomass production was lower. Phosphorus deficiency was evident in all treatments, as photosynthesis became CO₂ insensitive at intercellular CO₂ concentrations larger than ≈ 300 μmol mol^–1, and net assimilation rates measured at an ambient [CO₂] of 350 μmol mol^–1 or at 700 μmol mol^–1 were not significantly different. Moreover, there was a positive correlation of foliar P with maximum Rubisco (Ribulose-1,5-bisphosphate carboxylase/oxygenase) carboxylase activity (V_cmax), which potentially limits photosynthesis at low [CO₂], and the capacities of photosynthetic electron transport (J_max) and phosphate utilization (P_max), which are potentially limiting at high [CO₂]. None of these potential limits was correlated with foliar nitrogen concentration, indicating that photosynthetic N use-efficiency was directly dependent on foliar P availability. Though the tendencies were towards lower capacities of potential limitations of photosynthesis in high [CO₂] grown specimens, the effects were statistically insignificant, because of (i) large within-treatment variability related to foliar P, and (ii) small decreases in P/N ratio with increasing [CO₂], resulting in balanced changes in other foliar compounds potentially limiting carbon acquisition. The results of the current study indicate that under P-deficiency, the down-regulation of excess biochemical capacities proceeds in a similar manner in leaves grown under normal and elevated [CO₂], and also that foliar P/N ratios for optimum photosynthesis are likely to increase with increasing growth CO₂ concentrations. Symbols: A, net assimilation rate (μmol m^–2 s^–1); A_max, light-saturated A (μmol m^–2 s^–1); α, initial quantum yield at saturating [CO₂] and for an incident Q (mol mol^–1); [CO₂], atmospheric CO₂ concentration (μmol mol^–1); C_i, intercellular CO₂ concentration (μmol mol^–1); C_a, CO₂ concentration in the gas-exchange cuvette (μmol mol^–1); F_B, fraction of leaf N in ‘photoenergetics’; F_L, fraction of leaf N in light harvesting; F_R, fraction of leaf N in Rubisco; Γ*, CO₂ compensation concentration in the absence of R_d (μmol mol^–1); J_max*, capacity for photosynthetic electron transport; J_mc, capacity for photosynthetic electron transport per unit cytochrome f (mol e^–[mol cyt f]^–1 s^–1); K_c, Michaelis-Menten constant for carboxylation (μmol mol^–1); K_o, Michaelis-Menten constant for oxygenation (mmol mol^–1); M_A, leaf dry mass per area (g m^–2); O, intercellular oxygen concentration (mmol mol^–1); [P_i], concentration of inorganic phosphate (mM); P_max*, capacity for phosphate utilization; Q, photosynthetically active quantum flux density (μmol m^–2 s^–1); R_d*, day respiration (CO₂ evolution from nonphotorespiratory processes continuing in the light); Rubisco, ribulose-1,5-bisphosphate carboxylase/oxygenase; RUBP, ribulose-1,5-bisphosphate; T_l, leaf temperature (°C); U_TPU*, rate of triose phosphate utilization; V_cmax*, maximum Rubisco carboxylase activity; V_cr, specific activity of Rubisco (μmol CO₂[g Rubisco]^–1 s^–1] *given in either μmol m^–2 s^–1 or in μmol g^–1 s^–1 as described in the text.     

The effects of elevated [CO2] and water availability on growth and physiology of peach (Prunus persica) plants

ABSTRACT

Peach (Prunus persica L.) seedlings were germinated and grown for two growing seasons either in open top chambers (OTC) with ambient (350 μmol mol^-1) or elevated (700 μmol mol^-1) [CO₂], or in an outside control plot, all located inside a glasshouse. The seedlings were grown in 10 dm³ pots and were fertilised once a week following Ingestad principles in order to supply mineral nutrients at free access rates. In the second growing season, rapid onset of water stress was imposed on rapidly growing peach seedlings by withholding water for a four-week drying cycle. In elevated [CO₂], seedlings had a total dry mass which was 33% higher than that in ambient [CO₂]. This increase was largely a consequence of increased height growth. [CO₂] and irrigation treatments had only small effects on allocation, and there was no increase in root allocation with low water availability possibly as consequence of the high-nutrient regime. Specific leaf area was significantly reduced in elevated [CO₂], and probably resulted from increases in starch concentrations. Stomatal conductance (g _s) was not affected by elevated [CO₂] both in well-watered and water-stressed seedlings. The combination of increased assimilation rate (A) and unchanged g _s led to large increases in intrinsic water use efficiency in response to elevated [CO₂]. The A/C _i curves were used to derive the parameters describing photosynthetic capacity, A_max, J_max and V_cmax . These parameters were similar among [CO₂] treatments; thus, there was no downward acclimation of photosynthesis in elevated [CO₂]. Moreover, A_max, J_max and V_cmax scaled linearly with leaf N content per unit leaf area. This indicates that the whole-plant source-sink balance of peach seedlings was not disrupted by growth in elevated [CO₂], because root volume and nutrient supply were non-restricting. These values may be used in scaling up models to improve their ability to predict the magnitude of tree responses to climate change in the Mediterranean area.     

Effect of leaf age and position on light-saturated CO<Subscript>2</Subscript> assimilation rate,photosynthetic capacity,and stomatal conductance in rubber trees

Shoots of the tropical latex-producing tree Hevea brasiliensis (rubber tree) grow according to a periodic pattern, producing four to five whorls of leaves per year. All leaves in the same whorl were considered to be in the same leaf-age class, in order to assess the evolution of photosynthesis with leaf age in three clones of rubber trees, in a plantation in eastern Thailand. Light-saturated CO₂ assimilation rate (A _max) decreased more with leaf age than did photosynthetic capacity (maximal rate of carboxylation, V _cmax , and maximum rate of electron transport, J _max), which was estimated by fitting a biochemical photosynthesis model to the CO₂-response curves. Nitrogen-use efficiency (A _max/N_a, N_a is nitrogen content per leaf area) decreased also with leaf age, whereas J _max and V _cmax did not correlate with N _a. Although measurements were performed during the rainy season, the leaf gas exchange parameter that showed the best correlation with A _max was stomatal conductance (g _s). An asymptotic function was fitted to the A _max-g _s relationship, with R ² = 0.85. A _max, V _cmax, J _max and g _s varied more among different whorls in the same clone than among different clones in the same whorl. We concluded that leaf whorl was an appropriate parameter to characterize leaves for the purpose of modelling canopy photosynthesis in field-grown rubber trees, and that stomatal conductance was the most important variable explaining changes in A _max with leaf age in rubber trees.     

Temperature responses of photosynthetic capacity parameters were not affected by foliar nitrogen content in mature Pinus sylvestris

A key weakness in current Earth System Models is the representation of thermal acclimation of photosynthesis in response to changes in growth temperatures. Previous studies in boreal and temperate ecosystems have shown leaf‐scale photosynthetic capacity parameters, the maximum rates of carboxylation (V_cmax) and electron transport (J_max), to be positively correlated with foliar nitrogen (N) content at a given reference temperature. It is also known that V_cmax and J_max exhibit temperature optima that are affected by various environmental factors and, further, that N partitioning among the foliar photosynthetic pools is affected by N availability. However, despite the strong recent anthropogenic influence on atmospheric temperatures and N deposition to forests, little is known about the role of foliar N contents in controlling the photosynthetic temperature responses. In this study, we investigated the temperature dependencies of V_cmax and J_max in 1‐year‐old needles of mature boreal Pinus sylvestris (Scots pine) trees growing under low and high N availabilities in northern Sweden. We found that needle N status did not significantly affect the temperature responses of V_cmax or J_max when the responses were fitted to a peaked function. If such N insensitivity is a common tree trait it will simplify the interpretation of the results from gradient and multi‐species studies, which commonly use sites with differing N availabilities, on temperature acclimation of photosynthetic capacity. Moreover, it will simplify modeling efforts aimed at understanding future carbon uptake by precluding the need to adjust the shape of the temperature response curves to variation in N availability.     

Co‐limitation of photosynthetic capacity by nitrogen and phosphorus in West Africa woodlands

Photosynthetic leaf traits were determined for savanna and forest ecosystems in West Africa, spanning a large range in precipitation. Standardized major axis fits revealed important differences between our data and reported global relationships. Especially for sites in the drier areas, plants showed higher photosynthetic rates for a given N or P when compared with relationships from the global data set. The best multiple regression for the pooled data set estimated V_cmax and J_max from N_DW and S. However, the best regression for different vegetation types varied, suggesting that the scaling of photosynthesis with leaf traits changed with vegetation types. A new model is presented representing independent constraints by N and P on photosynthesis, which can be evaluated with or without interactions with S. It assumes that limitation of photosynthesis will result from the least abundant nutrient, thereby being less sensitive to the allocation of the non‐limiting nutrient to non‐photosynthetic pools. The model predicts an optimum proportionality for N and P, which is distinct for V_cmax and J_max and inversely proportional to S. Initial tests showed the model to predict V_cmax and J_max successfully for other tropical forests characterized by a range of different foliar N and P concentrations.     

The relative impacts of daytime and night-time warming on photosynthetic capacity in Populus deltoides

In order to investigate the relative impacts of increases in day and night temperature on tree carbon relations, we measured night‐time respiration and daytime photosynthesis of leaves in canopies of 4‐m‐tall cottonwood (Populus deltoides Bartr. ex Marsh) trees experiencing three daytime temperatures (25, 28 or 31 °C) and either (i) a constant nocturnal temperature of 20 °C or (ii) increasing nocturnal temperatures (15, 20 or 25 °C). In the first (day warming only) experiment, rates of night‐time leaf dark respiration (R_dark) remained constant and leaves displayed a modest increase (11%) in light‐saturated photosynthetic capacity (A_max) during the day (1000–1300 h) over the 6 °C range. In the second (dual night and day warming) experiment, R_dark increased by 77% when nocturnal temperatures were increased from 15 °C (0·36 µmol m^?2 s^?1) to 25 °C (0·64 µmol m^?2 s^?1). A_max responded positively to the additional nocturnal warming, and increased by 38 and 64% in the 20/28 and 25/31 °C treatments, respectively, compared with the 15/25 °C treatment. These increases in photosynthetic capacity were associated with strong increases in the maximum carboxylation rate of rubisco (V_cmax) and ribulose‐1,5‐bisphosphate (RuBP) regeneration capacity mediated by maximum electron transport rate (J_max). Leaf soluble sugar and starch concentration, measured at sunrise, declined significantly as nocturnal temperature increased. The nocturnal temperature manipulation resulted in a significant inverse relationship between A_max and pre‐dawn leaf carbohydrate status. Independent measurements of the temperature response of photosynthesis indicated that the optimum temperature (T_opt) acclimated fully to the 6 °C range of temperature imposed in the daytime warming. Our findings are consistent with the hypothesis that elevated night‐time temperature increases photosynthetic capacity during the following light period through a respiratory‐driven reduction in leaf carbohydrate concentration. These responses indicate that predicted increases in night‐time minimum temperatures may have a significant influence on net plant carbon uptake.     

Hemp (Cannabis sativa L.) leaf photosynthesis in relation to nitrogen content and temperature: implications for hemp as a bio‐economically sustainable crop

Hemp (Cannabis sativa L.) may be a suitable crop for the bio‐economy as it requires low inputs while producing a high and valuable biomass yield. With the aim of understanding the physiological basis of hemp's high resource‐use efficiency and yield potential, photosynthesis was analysed on leaves exposed to a range of nitrogen and temperature levels. Light‐saturated net photosynthesis rate (A_max) increased with an increase in leaf nitrogen up to 31.2 ± 1.9 μmol m^?2 s^?1 at 25 °C. The A_max initially increased with an increase in leaf temperature (T_L), levelled off at 25–35 °C and decreased when T_L became higher than 35 °C. Based on a C₃ leaf photosynthesis model, we estimated mesophyll conductance (g_m), efficiency of converting incident irradiance into linear electron transport under limiting light (κ_2LL), linear electron transport capacity (J_max), Rubisco carboxylation capacity (V_cmax), triose phosphate utilization capacity (T_p) and day respiration (R_d), using data obtained from gas exchange and chlorophyll fluorescence measurements at different leaf positions and various levels of incident irradiance, CO₂ and O₂. The effects of leaf nitrogen and temperature on photosynthesis parameters were consistent at different leaf positions and among different growth environments except for κ_2LL, which was higher for plants grown in the glasshouse than for those grown outdoors. Model analysis showed that compared with cotton and kenaf, hemp has higher photosynthetic capacity when leaf nitrogen is <2.0 g N m^?2. The high photosynthetic capacity measured in this study, especially at low nitrogen level, provides additional evidence that hemp can be grown as a sustainable bioenergy crop over a wide range of climatic and agronomic conditions.     

Response of total night-time respiration to differences in total daily photosynthesis for leaves in a Quercus rubra L. canopy: implications for modelling canopy CO2 exchange

Measurements of photosynthesis and respiration were made on leaves in summer in a Quercus rubra L. canopy at approximately hourly intervals throughout 5 days and nights. Leaves were selected in the upper canopy in fully sunlit conditions (upper) and in the lower canopy (lower). In addition, leaves in the upper canopy were shaded (upper shaded) to decrease photosynthesis rates. The data were used to test the hypothesis that total night‐time respiration is dependent on total photosynthesis during the previous day and that the response is mediated through changes in storage in carbohydrate pools. Measurements were made on clear sunny days with similar solar irradiance and air temperature, except for the last day when temperature, especially at night, was lower than that for the previous days. Maximum rates of photosynthesis in the upper leaves (18.7 μmol m^?2 s^?1) were approximately four times higher than those in the lower leaves (4.3 μmol m^?2 s^?1) and maximum photosynthesis rates in the upper shaded leaves (8.0 μmol m^?2 s^?1) were about half those in the upper leaves. There was a strong linear relationship between total night‐time respiration and total photosynthesis during the previous day when rates of respiration were normalized to a fixed temperature of 20°C, removing the effects of temperature from this relationship. Measurements of specific leaf area, nitrogen and chlorophyll concentration and calculations of the maximum rate of carboxylation activity, V_cmax, were not significantly different between upper and upper shaded leaves 5 days after the shading treatment was started. There were small, but significant decreases in the rate of apparent maximum electron transport at saturating irradiance, J_max (P>0.05), and light use efficiency, ? (P<0.05), for upper shaded leaves compared with those for upper leaves. This suggests that the duration of shading in the experiment was sufficient to initiate changes in the electron transport, but not the carboxylation processes of photosynthesis. Support for the hypothesis was provided from analysis of soluble sugar and starch concentrations in leaves. Respiration rates in the upper shaded leaves were lower than those expected from a relationship between respiration and soluble sugar concentration for fully exposed upper and lower leaves. However, there was no similar difference in starch concentrations. This suggests that shading for the duration of several days did not affect sugar concentrations but reduced starch concentrations in leaves, leading to lower rates of respiration at night. A model was used to quantify the significance of the findings on estimated canopy CO₂ exchange for the full growing season. Introducing respiration as a function of total photosynthesis on the previous day resulted in a decrease in growing season night‐time respiration by 23% compared with the value when respiration was held constant. This highlights the need for a process‐based approach linking respiration to photosynthesis when modelling long‐term carbon exchange in forest ecosystems.     

Estimating photosynthetic traits from reflectance spectra: A synthesis of spectral indices,numerical inversion,and partial least square regression

The lack of efficient means to accurately infer photosynthetic traits constrains understanding global land carbon fluxes and improving photosynthetic pathways to increase crop yield. Here, we investigated whether a hyperspectral imaging camera mounted on a mobile platform could provide the capability to help resolve these challenges, focusing on three main approaches, that is, reflectance spectra-, spectral indices-, and numerical model inversions-based partial least square regression (PLSR) to estimate photosynthetic traits from canopy hyperspectral reflectance for 11 tobacco cultivars. Results showed that PLSR with inputs of reflectance spectra or spectral indices yielded an R² of ~0.8 for predicting V _cmax and J _max, higher than an R² of ~0.6 provided by PLSR of numerical inversions. Compared with PLSR of reflectance spectra, PLSR with spectral indices exhibited a better performance for predicting V _cmax (R² = 0.84 ± 0.02, RMSE = 33.8 ± 2.2 μmol m⁻² s⁻¹) while a similar performance for J _max (R² = 0.80 ± 0.03, RMSE = 22.6 ± 1.6 μmol m⁻² s⁻¹). Further analysis on spectral resampling revealed that V _cmax and J _max could be predicted with ~10 spectral bands at a spectral resolution of less than 14.7 nm. These results have important implications for improving photosynthetic pathways and mapping of photosynthesis across scales.     

Acclimation of photosynthesis in a boreal grass (Phalaris arundinacea L.) under different temperature, CO2, and soil water regimes

The aim of this work was to study the acclimation of photosynthesis in a boreal grass (Phalaris arundinacea L.) grown in controlled environment chambers under elevated temperature (ambient + 3.5°C) and CO₂ (700 μmol mol⁻¹) with varying soil water regimes. More specifically, we studied, during two development stages (early: heading; late: florescence completed), how the temperature response of light-saturated net photosynthetic rate (P _sat), maximum rate of ribulose-1,5-bisphosphate carboxylase/oxygenase activity (V _cmax) and potential rate of electron transport (J _max) acclimatized to the changed environment. During the early growing period, we found a greater temperature-induced enhancement of P _sat at higher measurement temperatures, which disappeared during the late stage. Under elevated growth temperature, V _cmax and J _max at lower measurement temperatures (5–15°C) were lower than those under ambient growth temperature during the early period. When the measurements were done at 20–30°C, the situation was the opposite. During the late growing period, V _cmax and J _max under elevated growth temperature were consistently lower across measurement temperatures. CO₂ enrichment significantly increased P _sat with higher intercellular CO₂ compared to ambient CO₂ treatment, however, elevated CO₂ slightly decreased V _cmax and J _max across measurement temperatures, probably due to down-regulation acclimation. For two growing periods, soil water availability affected the variation in photosynthesis and biochemical parameters much more than climatic treatment did. Over two growing periods, V _cmax and J _max were on average 36.4 and 30.6%, respectively, lower with low water availability compared to high water availability across measurement temperatures. During the late growing period, elevated growth temperature further reduced the photosynthesis under low water availability. V _cmax and J _max declined along with the decrease in nitrogen content of leaves as growing period progressed, regardless of climatic treatment and water regime. We suggest that, for grass species, seasonal acclimation of the photosynthetic parameters under varying environmental conditions needed to be identified to fairly estimate the whole-life photosynthesis.     

Contrasting acclimation responses to elevated CO2 and warming between an evergreen and a deciduous boreal conifer

Rising atmospheric carbon dioxide (CO₂) concentrations may warm northern latitudes up to 8°C by the end of the century. Boreal forests play a large role in the global carbon cycle, and the responses of northern trees to climate change will thus impact the trajectory of future CO₂ increases. We grew two North American boreal tree species at a range of future climate conditions to assess how growth and carbon fluxes were altered by high CO₂ and warming. Black spruce (Picea mariana, an evergreen conifer) and tamarack (Larix laricina, a deciduous conifer) were grown under ambient (407 ppm) or elevated CO₂ (750 ppm) and either ambient temperatures, a 4°C warming, or an 8°C warming. In both species, the thermal optimum of net photosynthesis (T_optA) increased and maximum photosynthetic rates declined in warm‐grown seedlings, but the strength of these changes varied between species. Photosynthetic capacity (maximum rates of Rubisco carboxylation, V_cmax, and of electron transport, J_max) was reduced in warm‐grown seedlings, correlating with reductions in leaf N and chlorophyll concentrations. Warming increased the activation energy for V_cmax and J_max (E_aV and E_aJ, respectively) and the thermal optimum for J_max. In both species, the T_optA was positively correlated with both E_aV and E_aJ, but negatively correlated with the ratio of J_max/V_cmax. Respiration acclimated to elevated temperatures, but there were no treatment effects on the Q₁₀ of respiration (the increase in respiration for a 10°C increase in leaf temperature). A warming of 4°C increased biomass in tamarack, while warming reduced biomass in spruce. We show that climate change is likely to negatively affect photosynthesis and growth in black spruce more than in tamarack, and that parameters used to model photosynthesis in dynamic global vegetation models (E_aV and E_aJ) show no response to elevated CO₂.     

Effects of elevated [CO2] on photosynthesis in European forest species: a meta-analysis of model parameters

The effects of elevated atmospheric CO₂ concentration on growth of forest tree species are difficult to predict because practical limitations restrict experiments to much shorter than the average life-span of a tree. Long-term, process-based computer models must be used to extrapolate from shorter-term experiments. A key problem is to ensure a strong flow of information between experiments and models. In this study, meta-analysis techniques were used to summarize a suite of photosynthetic model parameters obtained from 15 field-based elevated [CO₂] experiments on European forest tree species. The parameters studied are commonly used in modelling photosynthesis, and include observed light-saturated photosynthetic rates (A_max), the potential electron transport rate (J_max), the maximum Rubisco activity (V_cmax) and leaf nitrogen concentration on mass (N_m) and area (N_a) bases. Across all experiments, light-saturated photosynthesis was strongly stimulated by growth in elevated [CO₂]. However, significant down-regulation of photosynthesis was also observed; when measured at the same CO₂ concentration, photosynthesis was reduced by 10–20%. The underlying biochemistry of photosynthesis was affected, as shown by a down-regulation of the parameters J_max and V_cmax of the order of 10%. This reduction in J_max and V_cmax was linked to the effects of elevated [CO₂] on leaf nitrogen concentration. It was concluded that the current model is adequate to model photosynthesis in elevated [CO₂]. Tables of model parameter values for different European forest species are given.     

Photosynthetic parameters,dark respiration and leaf traits in the canopy of a Peruvian tropical montane cloud forest

Few data are available describing the photosynthetic parameters of the leaves of tropical montane cloud forests (TMCF). Here, we present a study of photosynthetic leaf traits (V _cmax and J _max), foliar dark respiration (R _d), foliar nitrogen (N) and phosphorus (P), and leaf mass per area (LMA) throughout the canopy for five different TMCF species at 3025 m a.s.l. in Andean Peru. All leaf traits showed a significant relationship with canopy height when expressed on an area basis, and V _cmax-area and J _max-area almost halved when descending through the TMCF canopy. When corrected to a common temperature, average V _cmax and J _max on a leaf area basis were similar to lowland tropical values, but lower when expressed on a mass basis, because of the higher TMCF LMA values. By contrast, R _d on an area basis was higher than found in tropical lowland forests at a common temperature, and similar to lowland forests on a mass basis. The TMCF J _max–V _cmax relationship was steeper than in other tropical biomes, and we propose that this can be explained by either the light conditions or the relatively low VPD in the studied TMCF. Furthermore, V _cmax had a significant—though relatively weak and shallow—relationship with N on an area basis, but not with P, which is consistent with the general hypothesis that TMCFs are N rather than P limited. Finally, the observed V _cmax–N relationship (i.e., maximum photosynthetic nitrogen use efficiency) was distinctly different from those in tropical and temperate regions, probably because the TMCF leaves compensate for reduced Rubisco activity in cool environments.     

Comparison of the A–Cc curve fitting methods in determining maximum ribulose 1·5-bisphosphate carboxylase/oxygenase carboxylation rate, potential light saturated electron transport rate and leaf dark respiration

A review of the literature revealed that a variety of methods are currently used for fitting net assimilation of CO₂–chloroplastic CO₂ concentration (A–C_c) curves, resulting in considerable differences in estimating the A–C_c parameters [including maximum ribulose 1·5‐bisphosphate carboxylase/oxygenase (Rubisco) carboxylation rate (V_cmax), potential light saturated electron transport rate (J_max), leaf dark respiration in the light (R_d), mesophyll conductance (g_m) and triose‐phosphate utilization (TPU)]. In this paper, we examined the impacts of fitting methods on the estimations of V_cmax, J_max, TPU, R_d and g_m using grid search and non‐linear fitting techniques. Our results suggested that the fitting methods significantly affected the predictions of Rubisco‐limited (A_c), ribulose 1,5‐bisphosphate‐limited (A_j) and TPU‐limited (A_p) curves and leaf photosynthesis velocities because of the inconsistent estimate of V_cmax, J_max, TPU, R_d and g_m, but they barely influenced the J_max : V_cmax, V_cmax : R_d and J_max : TPU ratio. In terms of fitting accuracy, simplicity of fitting procedures and sample size requirement, we recommend to combine grid search and non‐linear techniques to directly and simultaneously fit V_cmax, J_max, TPU, R_d and g_m with the whole A–C_c curve in contrast to the conventional method, which fits V_cmax, R_d or g_m first and then solves for V_cmax, J_max and/or TPU with V_cmax, R_d and/or g_m held as constants.     

Estimation of photosynthesis parameters for a modified Farquhar–von Caemmerer–Berry model using simultaneous estimation method and nonlinear mixed effects model

The aims of this paper was to modify the photosynthesis model of Farquhar, von Caemmerer and Berry (FvCB) to be able to predict light dependency of the carboxylation capacity (V_c) and to improve the prediction of temperature dependency of the maximum carboxylation capacity (V_cmax) and the maximum electron transport rate (J_max). The FvCB model was modified by adding a sub-model for Ribulose-1,5-bisphosphate carboxylase (Rubisco) activation and validating the parameters for temperature dependency of V_cmax and J_max. Values of parameters for temperature dependency of V_cmax and J_max were validated and adjusted based on data of the photosynthesis response to temperature. Parameter estimation was based on measurements under a wide range of environmental conditions, providing parameters with broad validity. The simultaneous estimation method and the nonlinear mixed effects model were applied to ensure the accuracy of the parameter estimation. The FvCB parameters, V_cmax, J_max, α (the efficiency of light energy conversion), θ (the curvature of light response of electron transport), and R_d (the non-photorespiratory CO₂ release) were estimated and validated on a dataset from two other years. Observations and predictions matched well (R² = 0.94). We conclude that incorporating a sub-model of Rubisco activation improved the FvCB model through predicting light dependency of carboxylation rate; and that estimating V_cmax, J_max, α, θ, and R_d requires data sets of both CO₂ and light response curves.     

Altitudinal variation in photosynthetic capacity, diffusional conductance and δ13C of butterfly bush (Buddleja davidii) plants growing at high elevations

In this study, we have examined several physiological, biochemical and morphological features of Buddleja davidii plants growing at 1300 m above sea level (a.s.l.) and 3400 m a.s.l., respectively, to identify coordinated changes in leaf properties in response to reduced CO₂ partial pressure (P_a). Our results confirmed previous findings that foliar δ¹³C, photosynthetic capacity and foliar N concentration on a leaf area basis increased, whereas stomatal conductance (g_s) decreased with elevation. The net CO₂ assimilation rate (A_max), maximum rate of electron transport (J_max) and respiration increased significantly with elevation, although no differences were found in carboxylation efficiency of Rubisco (V_cmax). Consequently, also the J_max to V_cmax ratio was significantly increased by elevation, indicating that the functional balance between Ribulose‐1,5‐biphosphate (RuBP) consumption and RuBP regeneration changes as elevation increases. Our results also indicated a homeostatic response of CO₂ transfer conductance inside the leaf (mesophyll conductance, g_m) to increasing elevation. In fact, with elevation, g_m also increased compensating for the strong decrease in g_s and, thus, in the P_i (intercellular partial pressure of CO₂) to P_a ratio, leading to similar chloroplast partial pressure of CO₂ (P_c) to P_a ratio at different elevations. Because there were no differences in V_cmax, also A measured at similar PPFD and leaf temperature did not differ statistically with elevation. As a consequence, a clear relationship was found between A and g_m, and between A and the sum of g_s and g_m. These data suggest that the higher dry mass δ¹³C of leaves at the higher elevation, indicative of lower long‐term P_c/P_a ratio, cannot be attributed to changes either in diffusional resistances or in carboxylation efficiency. We speculate that because temperature significantly decreases as the elevation increases, it dramatically affects CO₂ diffusion and hence P_c/P_a and, consequently, is the primary factor influencing ¹³C discrimination at high elevation.     

Plasticity of morphological and physiological traits in response to different levels of irradiance in seedlings of silver fir (Abies alba Mill)

The ability of silver fir ( Abies alba Mill.) to acclimate to different levels of irradiance was tested with 3-year-old seedlings, grown for 2 years in a nursery close to Nancy (eastern France) under 100, 48, 18 and 8% of incident irradiance (neutral shade nets). Growth, total nutrients in needles, maximal carboxylation rate ( V _cmax), maximal light driven electron flow ( J _max) and the relative amount of nitrogen allocated to photosynthetic processes (carboxylation, bioenergetics, light harvesting) were investigated. The sensitivity to drought stress was assessed among the phenotypes resulting from light acclimation. Leader-shoot and branch elongation were greatest under 18% irradiance. Total seedling biomass, root-to-total biomass ratio, total leaf area, leaf mass-to-area ratio and needle-area based nitrogen content responded positively to increasing irradiance while leaf area ratio decreased. Both V _cmax and J _max increased by a factor of 1.6 and 1.8, respectively, from the lowest to the highest irradiance but the ratio J _max/ V _cmax remained stable. All these parameters, expressed on a projected needle area basis, remained within the lower range of values measured for broadleaved trees. Relative allocation of needle N to the different components of the photosynthetic apparatus was very low: 12, 3 and 7% of total nitrogen were invested in carboxylation, bioenergetics and light harvesting, respectively. The relative allocation of nitrogen to carboxylation and bioenergetics remained stable while that to light harvesting decreased with increasing irradiance. During drought, seedlings pre-acclimated to shade closed their stomata at higher predawn needle water potential than those which were grown under higher irradiance. Critical temperature for PSII photochemistry in needles was unaffected by irradiance and was close to 47°C. Drought significantly increased the critical temperature up to 51°C. In general, the amplitude of responses of silver fir to changing irradiance (phenotypic plasticity) was smaller than that recorded in broadleaved species.