The response of photosynthetic model parameters to temperature and nitrogen concentration in Pinus radiata D. Don

Responses of photosynthesis (A) to intercellular CO₂ concentration (c_i) in 2-year-old Pinus radiata D. Don seedlings were measured at a range of temperatures in order to parametrize a biophysical model of leaf photosynthesis. Increasing leaf temperature from 8 to 30°C caused a 4-fold increase in V_cmax, the maximum rate of carboxylation (10.7–43.3 μol m^?2 s^?1 and a 3-fold increase in J_max, the maximum electron transport rate (20.5–60.2 μmol m ^?2 s^?1). The temperature optimum for J_max was lower than that for V_cmax, causing a decline in the ratio J_max:V_cmax from 2.0 to 1.4 as leaf temperature increased from 8 to 30°C. To determine the response of photosynthesis to leaf nitrogen concentration, additional measurements were made on seedlings grown under four nitrogen treatments. Foliar N concentrations varied between 0.36 and 1.27 mol kg^?1, and there were linear relationships between N concentration and both V_cmax and J_max. Measurements made throughout the crown of a plantation forest tree, where foliar N concentrations varied from 0.83 mol kg^?1 near the base to 1.54 mol kg^?1 near the leader, yielded similar relationships. These results will be useful in scaling carbon assimilation models from leaves to canopies.     

Strong thermal acclimation of photosynthesis in tropical and temperate wet‐forest tree species: the importance of altered Rubisco content

Understanding of the extent of acclimation of light‐saturated net photosynthesis (A_n) to temperature (T), and associated underlying mechanisms, remains limited. This is a key knowledge gap given the importance of thermal acclimation for plant functioning, both under current and future higher temperatures, limiting the accuracy and realism of Earth system model (ESM) predictions. Given this, we analysed and modelled T‐dependent changes in photosynthetic capacity in 10 wet‐forest tree species: six from temperate forests and four from tropical forests. Temperate and tropical species were each acclimated to three daytime growth temperatures (T_growth): temperate – 15, 20 and 25 °C; tropical – 25, 30 and 35 °C. CO₂ response curves of A_n were used to model maximal rates of RuBP (ribulose‐1,5‐bisphosphate) carboxylation (V_cmax) and electron transport (J_max) at each treatment's respective T_growth and at a common measurement T (25 °C). SDS‐PAGE gels were used to determine abundance of the CO₂‐fixing enzyme, Rubisco. Leaf chlorophyll, nitrogen (N) and mass per unit leaf area (LMA) were also determined. For all species and T_growth, A_n at current atmospheric CO₂ partial pressure was Rubisco‐limited. Across all species, LMA decreased with increasing T_growth. Similarly, area‐based rates of V_cmax at a measurement T of 25 °C (V_cmax²⁵) linearly declined with increasing T_growth, linked to a concomitant decline in total leaf protein per unit leaf area and Rubisco as a percentage of leaf N. The decline in Rubisco constrained V_cmax and A_n for leaves developed at higher T_growth and resulted in poor predictions of photosynthesis by currently widely used models that do not account for T_growth‐mediated changes in Rubisco abundance that underpin the thermal acclimation response of photosynthesis in wet‐forest tree species. A new model is proposed that accounts for the effect of T_growth‐mediated declines in V_cmax²⁵ on A_n, complementing current photosynthetic thermal acclimation models that do not account for T sensitivity of V_cmax²⁵.     

Estimation of vegetation photosynthetic capacity from space‐based measurements of chlorophyll fluorescence for terrestrial biosphere models

Photosynthesis simulations by terrestrial biosphere models are usually based on the Farquhar's model, in which the maximum rate of carboxylation (V_cmax) is a key control parameter of photosynthetic capacity. Even though V_cmax is known to vary substantially in space and time in response to environmental controls, it is typically parameterized in models with tabulated values associated to plant functional types. Remote sensing can be used to produce a spatially continuous and temporally resolved view on photosynthetic efficiency, but traditional vegetation observations based on spectral reflectance lack a direct link to plant photochemical processes. Alternatively, recent space‐borne measurements of sun‐induced chlorophyll fluorescence (SIF) can offer an observational constraint on photosynthesis simulations. Here, we show that top‐of‐canopy SIF measurements from space are sensitive to V_cmax at the ecosystem level, and present an approach to invert V_cmax from SIF data. We use the Soil‐Canopy Observation of Photosynthesis and Energy (SCOPE) balance model to derive empirical relationships between seasonal V_cmax and SIF which are used to solve the inverse problem. We evaluate our V_cmax estimation method at six agricultural flux tower sites in the midwestern US using spaced‐based SIF retrievals. Our V_cmax estimates agree well with literature values for corn and soybean plants (average values of 37 and 101 μmol m^?2 s^?1, respectively) and show plausible seasonal patterns. The effect of the updated seasonally varying V_cmax parameterization on simulated gross primary productivity (GPP) is tested by comparing to simulations with fixed V_cmax values. Validation against flux tower observations demonstrate that simulations of GPP and light use efficiency improve significantly when our time‐resolved V_cmax estimates from SIF are used, with R² for GPP comparisons increasing from 0.85 to 0.93, and for light use efficiency from 0.44 to 0.83. Our results support the use of space‐based SIF data as a proxy for photosynthetic capacity and suggest the potential for global, time‐resolved estimates of V_cmax.     

Seasonal variation in leaf properties and ecosystem carbon budget in a cool-temperate deciduous broad-leaved forest: simulation analysis at Takayama site, Japan

Seasonal changes in gross primary production (GPP) and net ecosystem production (NEP) in temperate deciduous forests are mostly driven by environmental conditions and the phenology of leaf demography. This study addresses another factor, temporal changes in leaf properties, i.e., leaf aging from emergence to senescence. A process-based model was used to link the ecosystem-scale carbon budget with leaf-level properties on the basis of field observation and scaling procedures; temporal variations in leaf thickness (leaf mass per area, LMA), photosynthetic rubisco (V_cmax) and electron-transport (J_max) capacity, and dark respiration (R_d) were empirically parameterized. The model was applied to a cool-temperate deciduous broad-leaved forest at Takayama, in central Japan, and validated with data of net ecosystem CO₂ exchange (NEE=–NEP) measured using the eddy-covariance method. NEP of the Takayama site varied seasonally from 3 g C m^–2 day^–1 net source in late winter to 5 g C m^–2 day^–1 net sink in early to mid-summer. A sensitivity experiment showed that removing the leaf-aging effect changed the seasonal CO₂ exchange pattern, and led to overestimation of annual GPP by 6% and annual NEP by 38%. We found that seasonal variation in V_cmax affected the seasonal pattern and annual budget of CO₂ exchange most strongly; LMA and R_d had moderate influences. The rapid change in V_cmax and R_d during leaf emergence and senescence was important in evaluating GPP and NEP of the temperate deciduous forest.     

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.     

Changes in needle nitrogen partitioning and photosynthesis during 80 years of tree ontogeny in Picea abies

Needle nitrogen partitioning and photosynthesis of Norway spruce were studied in a forest chronosequence in Järvselja Experimental Forest, Estonia. Current- and previous-year shoots were sampled from upper and lower canopy positions in four stands, ranging in age from 13 to 82 years. A/c _i curves were determined to obtain maximum carboxylation rate (V _cmax) and maximum rate of electron transport (J _max), whereas needle nitrogen partitioning into carboxylation (P _R), bioenergetics associated with electron transport (P _B) and thylakoid light harvesting components (P _L) was calculated from the values of V _cmax, J _max and leaf chlorophyll concentration. The greatest changes in studied needle characteristics took place between tree ages of 13 and 26 years, and this pattern was independent of needle age and canopy position. Needle mass per projected area (LMA) was lowest in the 13-year-old stand and mass-based nitrogen concentration (N^M) was generally highest in that stand. The values of LMA were significantly higher and those of N^M lower in the 26-year-old stand. Mass-based V _cmax and J _max were highest in the 13-year-old stand. Area-based photosynthetic capacity was independent of tree age. The proportion of photosynthetic nitrogen (P _R, P _B and P _L) was highest and that of non-photosynthetic nitrogen lowest in the 13-year-old stand. Current-year needles had lower LMA and P _L, but higher photosynthetic capacity compared to 1-year-old foliage. Needles from lower canopy positions exhibited lower LMA, area-based nitrogen concentration and photosynthetic capacity than needles from upper canopy. The period of substantial reductions in needle photosynthetic capacity and changes in nitrogen partitioning coincides with the onset of reproductive phase during tree ontogeny.     

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.     

Chlorophyll fluorescence and gas exchange measurements in field research: an ecological case study

We tested whether cheap and quick chlorophyll (Chl) fluorescence can be used in ecophysiological field studies as proxies for gas-exchange measurements. We measured net photosynthetic rate at saturating irradiance and ambient atmospheric CO₂ concentrations (P_Nsat), maximum carboxylation rate (V_cmax), maximum quantum yield of PSII (F_v/F_m), the performance index (PIabs), leaf nitrogen (Narea), and carbon isotope discrimination (Δ¹³C) within four herbaceous species along two elevational gradients. We analysed the relationship between Chl fluorescence and gas-exchange parameters and their link to indirect assessment of plant performance via ecophysiological traits. F_v/F_m showed no relationship to P_Nsat and only weak relationships to V_cmax. PI_abs was positively related to P_Nsat and V_cmax. PI_abs, P_Nsat, and V_cmax were positively associated with N_area and negatively to Δ¹³C, whereas F_v/F_m showed no relationship to Narea and a positive to Δ¹³C. Thus, PIabs might be suitable to characterize the photosynthetic activity when aiming on large numbers of samples.     

Inter-specific variation of the biochemical limitation to photosynthesis and related leaf traits of 30 species from mountain grassland ecosystems under different land use

The maximum rate of carboxylation (V_cmax) and the potential rate of RuBP regeneration (P_ml, which equals J_max/4), as well as leaf nitrogen content (N_L) and specific leaf area (SLA), were studied in sun leaves of 30 species from differently managed mountain grassland ecosystems (abandoned areas, pastures and meadows) at three study areas in the Eastern Alps. A significant correlation between V_cmax and P_ml across the investigated species was observed. In comparison to a previous survey on the relationship between P_ml and V_cmax, the investigated species were found to invest a proportionally smaller amount of available resources into P_ml. Despite this close correlation between V_cmax and P_ml, variation in the V_cmax–N_L and P_ml–N_L relationships was large, indicating that the investigated species differed widely in their respective nitrogen use efficiencies. Among the investigated species, dwarf shrubs were characterized by significantly lower values of V_cmax, P_ml and N_L compared to graminoids and forbs, which did not differ significantly from each other. Species from abandoned areas were found to be lower with respect to V_cmax, P_ml, N_L and SLA than species from pastures and meadows, which were not significantly different from each other.     

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.     

Photosynthetic parameters in seedlings of Eucalyptus grandis as affected by rate of nitrogen supply

Seedlings of Eucalyptus grandis were grown at five different rates of nitrogen supply. Once steady‐state growth rates were established, a detailed set of CO₂ and water vapour exchange measurements were made to investigate the effects of leaf nitrogen content (N), as determined by nitrogen supply rate, on leaf structural, photosynthetic, respiratory and stomatal properties. Gas exchange data were used to parametrize the Farquhar–von Caemmerer photosynthesis model. Leaf mass per area (LMA) was negatively correlated to N. A positive correlation was observed between both day (R_d) and night respiration (R_n) and N when they were expressed on a leaf mass basis, but no correlation was found on a leaf area basis. An R_d/R_n ratio of 0·59 indicated a significant inhibition of dark respiration by light. The maximum net CO₂ assimilation rate at ambient CO₂ concentration (A_max), the maximum rate of potential electron transport (J_max) and the maximum rate of carboxylation (V_cmax) significantly increased with N, particularly when expressed on a mass basis. Although the maximum stomatal conductance to CO₂ (g_scmax) was positively correlated with A_max, there was no relationship between g_scmax and N. Leaf N content influenced the allocation of nitrogen to photosynthetic processes, resulting in a decrease of the J_max/V_cmax ratio with increasing N. It was concluded that leaf nitrogen concentration is a major determinant of photosynthetic capacity in Eucalyptus grandis seedlings and, to a lesser extent, of leaf respiration and nitrogen partitioning among photosynthetic processes, but not of stomatal conductance.     

Leaf photosynthetic capacity is regulated by the interaction of nitrogen and potassium through coordination of CO2 diffusion and carboxylation

Combined application of nitrogen (N) and potassium (K) fertilizer could significantly enhance crop yield. Crop yield and photosynthesis are inseparable. However, the influence of N and K interaction on photosynthesis is still not fully understood. Field and hydroponic experiments were conducted to examine the effects of N and K interaction on leaf photosynthesis characteristics and to explore the mechanisms in the hydroponic experiment. CO₂ conductance and carboxylation characteristic parameters of oilseed leaves were measured under different N and K supplies. Results indicated that detectable increases in leaf area, biomass and net photosynthetic rate (A_n) were observed under optimal N and K supply in field and hydroponic experiments. The ratio of total CO₂ diffusion conductance to the maximum carboxylation rate (g_tot/V_cmax) and A_n presented a linear‐plateau relationship. Under insufficient N, increased K contributed to the CO₂ transmission capacity and improved the proportion of N used for carboxylation, promoting g_tot/V_cmax. However, the low V_cmax associated with N insufficiency limited the A_n. High N supply obviously accelerated V_cmax, yet K deficiency led to a reduction of g_tot, which restricted V_cmax. Synchronous increases in N and K supplementation ensured the appropriate ratio of N to K content in leaves, which simultaneously facilitated g_tot and V_cmax and preserved a g_tot/V_cmax suitable for guaranteeing CO₂ transmission and carboxylation coordination; the overall effect was increased A_n and leaf area. These results highlight the suitable N and K nutrients to coordinate CO₂ diffusion and carboxylation, thereby enhancing photosynthetic capacity and area to obtain high crop yield.     

Photosynthesis, carboxylation and leaf nitrogen responses of 16 species to elevated pCO2 across four free-air CO2 enrichment experiments in forest, grassland and desert

The magnitude of changes in carboxylation capacity in dominant plant species under long‐term elevated CO₂ exposure (elevated pC_a) directly impacts ecosystem CO₂ assimilation from the atmosphere. We analyzed field CO₂ response curves of 16 C₃ species of different plant growth forms in favorable growth conditions in four free‐air CO₂ enrichment (FACE) experiments in a pine and deciduous forest, a grassland and a desert. Among species and across herb, tree and shrub growth forms there were significant enhancements in CO₂ assimilation (A) by +40±5% in elevated pC_a (49.5–57.1 Pa), although there were also significant reductions in photosynthetic capacity in elevated pC_a in some species. Photosynthesis at a common pC_a (A_a) was significantly reduced in five species growing under elevated pC_a, while leaf carboxylation capacity (V_cmax) was significantly reduced by elevated pC_a in seven species (change of ?19±3% among these species) across different growth forms and FACE sites. Adjustments in V_cmax with elevated pC_a were associated with changes in leaf N among species, and occurred in species with the highest leaf N. Elevated pC_a treatment did not affect the mass‐based relationships between A or V_cmax and N, which differed among herbs, trees and shrubs. Thus, effects of elevated pC_a on leaf C assimilation and carboxylation capacity occurred largely through changes in leaf N, rather than through elevated pC_a effects on the relationships themselves. Maintenance of leaf carboxylation capacity among species in elevated pC_a at these sites depends on maintenance of canopy N stocks, with leaf N depletion associated with photosynthetic capacity adjustments. Since CO₂ responses can only be measured experimentally on a small number of species, understanding elevated CO₂ effects on canopy N_m and N_a will greatly contribute to an ability to model responses of leaf photosynthesis to atmospheric CO₂ in different species and plant growth forms.     

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.     

Predicting dark respiration rates of wheat leaves from hyperspectral reflectance

Greater availability of leaf dark respiration (R_dark) data could facilitate breeding efforts to raise crop yield and improve global carbon cycle modelling. However, the availability of R_dark data is limited because it is cumbersome, time consuming, or destructive to measure. We report a non‐destructive and high‐throughput method of estimating R_dark from leaf hyperspectral reflectance data that was derived from leaf R_dark measured by a destructive high‐throughput oxygen consumption technique. We generated a large dataset of leaf R_dark for wheat (1380 samples) from 90 genotypes, multiple growth stages, and growth conditions to generate models for R_dark. Leaf R_dark (per unit leaf area, fresh mass, dry mass or nitrogen, N) varied 7‐ to 15‐fold among individual plants, whereas traits known to scale with R_dark, leaf N, and leaf mass per area (LMA) only varied twofold to fivefold. Our models predicted leaf R_dark, N, and LMA with r² values of 0.50–0.63, 0.91, and 0.75, respectively, and relative bias of 17–18% for R_dark and 7–12% for N and LMA. Our results suggest that hyperspectral model prediction of wheat leaf R_dark is largely independent of leaf N and LMA. Potential drivers of hyperspectral signatures of R_dark are discussed.     

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.     

Season effects on leaf nitrogen partitioning and photosynthetic water use efficiency in mango

The key parameters of photosynthetic capacity (maximum carboxylation rate (V_cmax), electron transport capacity (J_max) and dark respiration rate (R_d)) and the slope (m) of the stomatal conductance model of Ball et al. [Progress in photosynthetic research, Martinus Nijhoff, Dordrecht, 1987] were measured for a whole growing season in fully expanded leaves of 12-year-old mango trees cv. Cogshall in La Réunion island. Leaf nitrogen partitioning into carboxylation (P_c) and bioenergetic (P_b) pools were computed according to the model of Niinemets and Tenhunen [Plant Cell Environ 1997;20: 845–66]. V_cmax, J_max, R_d, P_c and P_b remained relatively stable over the whole study period, with the exception of the period of linear fruit growth when J_max, R_d and P_b were slightly lower, and leaf non-structural carbohydrate content higher. During the pre-floral and floral periods, m decreased by more than 50%, indicating an increase in photosynthetic water use efficiency and m increased again during the period of linear fruit growth. Our results show that, in tropical orchard conditions characterized by mild seasonal climatic changes and non-limiting water supply, leaf nitrogen partitioning is rather stable. Our results also advocate for more studies on the effect of phenology on m and photosynthetic water use efficiency, which is of paramount importance for building coupled biochemical models of photosynthetic carbon assimilation.     

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.     

玉米、花生及其间作茬口与施磷对冬小麦光合特性及产量的影响机制

该试验在玉米单作茬口、玉米-花生间作茬口(间作茬口)、花生单作茬口共3种茬口,以及0 kg P_2O_5·hm~(-2)(P_0)和180 kg P_2O_5·hm~(-2)(P_1) 2个磷水平下,研究了间作茬口与施磷对冬小麦分蘖、叶面积指数(LAI)、干物质积累、光合特性及产量的影响机制,为玉米花生间作与小麦-玉米复种轮作提供理论依据。结果表明:(1)间作茬口较玉米茬口显著提高了冬小麦有效分蘖数、LAI、净光合速率和干物质积累量,并提高了冬小麦旗叶的SPAD值、CO_2饱和点、光饱和点及最大净光合速率(P_(nmax))、表观量子效率(AQY)、羧化效率(CE)、最大羧化速率(V_(cmax))、最大RUBP再生的电子传递速率(J_(max))和最大磷酸丙糖利用速率(V_(TPU)),且CE、V_(cmax)、V_(TPU)的增幅均达到显著水平(P0.05),有效改善了冬小麦产量构成,显著提高籽粒产量(P0.05)。(2)间作茬口较花生茬口提高了冬小麦乳熟期的P_(nmax)、AQY、CE,增加了穗粒数和粒重,提高了产量。(3)与不施磷相比,施磷180 kg P_2O_5·hm~(-2)显著促进间作茬口冬小麦生长,显著提高冬小麦旗叶的SPAD值、P_(nmax)、AQY、CE、V_(cmax)、J_(max)、V_(TPU)和籽粒产量(P0.05)。研究发现,间作茬口较玉米茬口能有效增强冬小麦旗叶表观量子效率和CO_2羧化能力,显著提高小麦花后光合能力,促进冬小麦生长,从而增加穗粒数、粒重和籽粒产量,且间作茬口结合施磷180 kg P_2O_5·hm~(-2)效果更好。     

Comparison of parameters estimated from A/Ci and A/Cc curve analysis

The parameters estimated from traditional A/C _i curve analysis are dependent upon some underlying assumptions that substomatal CO₂ concentration (C _i) equals the chloroplast CO₂ concentration (C _c) and the C _i value at which the A/C _i curve switches between Rubisco- and electron transport-limited portions of the curve (C _i-t) is set to a constant. However, the assumptions reduced the accuracy of parameter estimation significantly without taking the influence of C _i-t value and mesophyll conductance (g _m) on parameters into account. Based on the analysis of Larix gmelinii’s A/C _i curves, it showed the C _i-t value varied significantly, ranging from 24 Pa to 72 Pa and averaging 38 Pa. t-test demonstrated there were significant differences in parameters respectively estimated from A/C _i and A/C _c curve analysis (p<0.01). Compared with the maximum ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) carboxylation rate (V_cmax), the maximum electron transport rate (J_max) and J_max/V_cmax estimated from A/C _c curve analysis which considers the effects of g _m limit and simultaneously fits parameters with the whole A/C _c curve, mean V_cmax estimated from A/C _i curve analysis (V_cmax-C _i) was underestimated by 37.49%; mean Jmax estimated from A/C _i curve analysis (J_max-C _i) was overestimated by 17.8% and (J_max-C _i)/(V_cmax-C _i) was overestimated by 24.2%. However, there was a significant linear relationship between V_cmax estimated from A/C _i curve analysis and V_cmax estimated from A/C _c curve analysis, so was it J_max (p<0.05).