Magnesium deficiency treatment causes reductions in photosynthesis of well-nourished Norway spruce

In order to investigate effects of magnesium deficiency on Norway spruce [Picea abies (L.) Karst.] photosynthesis, 100 well-nourished 5-year-old spruce trees were grown in sand culture, individually supplied with circulating nutrient solutions. Mineral nutrients were added to the nutrient solutions in optimal quantities and optimal relations to nitrogen. Magnesium was supplied at 0.203, 0.041 and 0.005 mM in order to simulate optimal nutrition, moderate deficiency and severe deficiency. Parameters of photosynthetic gas exchange, chlorophyll, magnesium and starch concentrations were determined in current-year and 1-year-old needles during one growing season. By mid May — 6 months after onset of the Mg deficiency treatments in late autumn — CO₂-assimilation rates of 1-year-old needles were significantly decreased independent of the severity of the deficiency treatment, whereas the chlorophyll concentrations did not differ from the controls. The occurrence of yellowing symptoms during July did not further influence the Mg deficiency effect on photosynthesis. In contrast to 1-year-old needles, significant reductions of photosynthesis and chlorophyll in current-year needles were only caused by severely deficient Mg supply. Mg deficiency affected carboxylation efficiency but not light use efficiency. From the accumulation of starch in the needles, up to 30-fold of the controls, the conclusion has been drawn that reactions of CO₂-fixation were affected by reduced carbohydrate export. The light-dependent pigment reduction, leading to the typical tipyellowing of needles, clearly reflects a secondary effect of Mg deficiency.     

Down co-regulation of light absorption, photochemistry, and carboxylation in Fe-deficient plants growing in different environments

The regulation of photosynthesis through changes in light absorption, photochemistry, and carboxylation efficiency has been studied in plants grown in different environments. Iron deficiency was induced in sugar beet (Beta vulgaris L.) by growing plants hydroponically in controlled growth chambers in the absence of Fe in the nutrient solution. Pear (Pyrus communis L.) and peach (Prunus persica L. Batsch) trees were grown in field conditions on calcareous soils, in orchards with Fe deficiency-chlorosis. Gas exchange parameters were measured in situ with actual ambient conditions. Iron deficiency decreased photosynthetic and transpiration rates, instantaneous transpiration efficiencies and stomatal conductances, and increased sub-stomatal CO₂ concentrations in the three species investigated. Photosynthesis versus CO₂ sub-stomatal concentration response curves and chlorophyll fluorescence quenching analysis revealed a non-stomatal limitation of photosynthetic rates under Fe deficiency in the three species investigated. Light absorption, photosystem II, and Rubisco carboxylation efficiencies were down-regulated in response to Fe deficiency in a coordinated manner, optimizing the use of the remaining photosynthetic pigments, electron transport carriers, and Rubisco.     

Time course of antioxidant responses of Capsicum annuum subjected to a progressive magnesium deficiency

The effect of magnesium (Mg²⁺)‐deficiency on the antioxidant responses of Capsicum annuum was investigated over a 60‐day period under controlled conditions. This Mg²⁺‐deficiency aimed to mimic the physiological conditions that plants may experience in the field. At each harvest time, five different leaf‐levels (L2 to L6) were distinguished. L2 and L6 correspond to the second and sixth youngest leaves, respectively. The following parameters were determined: Mg²⁺, chlorophyll and protein contents, total and redox pools of ascorbate and glutathione, and the activities of superoxide dismutase, ascorbate peroxidase, dehydroascorbate reductase, and glutathione reductase. Under Mg²⁺‐deficiency, leaf Mg²⁺ contents decreased over time in all leaf‐levels except in the second youngest leaves (L2), where they remained constant at about 0.25% (dry weight basis). Mg²⁺‐deficiency led to an increase in the antioxidant enzyme activities concomitant with an increase in the ascorbate and glutathione pools, whereas total chlorophyll and soluble protein contents decreased. The L2 leaves showed an increase in glutathione reductase activity and in the ascorbate redox state whereas no difference was observed for the other parameters. Superoxide dismutase activities increased in L5 leaves from day 15 and, afterwards, in L3 to L5 leaves, irrespective of Mg²⁺ content. At day 30, glutathione reductase activities increased in L2 to L4 leaves and dehydroascorbate reductase activities in L4 leaves. At day 45, we observed an increase in the ascorbate peroxidase activities in L3 to L5 leaves. At the same time, ascorbate and glutathione pools increased in intermediate leaves, whereas chlorophyll content decreased in L3 and L4 leaves, and protein content decreased in L4 leaves. Results suggest that pepper leaves enhance their defence capacities against oxidative stress by increasing ascorbate more than glutathione synthesis. However, cells showed higher regeneration rates for the glutathione redox state than for the ascorbate redox state.     

Magnesium deficiency in expanding leaves of Phaseolus vulgaris-gas exchange and nutrient concentrations

Phaseolus vulgaris was cultured either with or without magnesium in an aerated nutrient solution in growth chambers from 21 days after germination. Five days after transfer to Mg-deficient nutrient solution, terminal leaflets of first trifoliate leaves stopped expansion. From the fifth day after transfer, the net assimilation rate, the transpiration rate and the leaf water vapour conductance of first trifoliate leaves of the deficient plants declined. Following resupply of Mg on the seventh day after transfer to the Mg-deficient solution, the assimilation rate increased to 93% by the 12th day, the transpiration rate to 76% and the leaf water vapour conductance to 50% of the control plants.     

Characterisation of magnesium nutrition and interaction of magnesium and potassium in rice

Magnesium (Mg) is known as one of the essential nutrients for higher plants; yet, the preliminary physiological responses of field crops to its deficiency or excess, particularly to its interaction with potassium (K), remain largely unknown. In this study, we observed that Mg deficiency in rice (Oryza sativa) [less than 1.1 mg g^?1 dry weight (DW) in the shoot] resulted in significant reduction in shoot biomass, decrease in total chlorophyll concentration and net photosynthetic rate and reduction in activities of both nitrate reductase [NR; enzyme classification (EC) 1.6.6.1] and glutamine synthetase (EC 6.3.1.2) in the leaves. However, the Mg‐deficient plant contained higher starch in the leaves, and partitioned larger biomass into roots. Excess of Mg (more than 3.0 mg g^?1 DW in the shoot), together with low K supply, suppressed NR activity and decreased concentration of soluble sugar in the leaves. There were great antagonistic and moderately synergistic effects between K and Mg, but the effects of K were much more significant than those of Mg on their uptake and translocation, NR activity and net photosynthetic rate in the leaves. The optimum weight ratio of K to Mg ranged between 22 and 25 in the leaves at tillering stage. Mg deficiency was not compensated for by moderate supply of K but was aggravated by excess supply of K, suggesting specific roles of Mg in both dry matter production and partition of carbon assimilates in rice.     

Responses to flooding and drought stress by two citrus rootstock seedlings with different water-use efficiency

Leaf water relations, net gas exchange and leaf and root constituent responses to 9 days of drought stress (DS) or soil flooding were studied in 6‐month‐old seedlings of Carrizo citrange [Citrus sinensis (L.) Osb. ×Poncirus trifoliata L.; Carr] and Cleopatra mandarin (Citrus resnhi Hort. ex Tanaka; Cleo) growing in containers of native sand in the greenhouse. At the end of the drought period, both species had similar minimum stem water potentials but Cleo had higher leaf relative water content (RWC) and higher leaf osmotic potential at full turgor () than Carr. Flooding had no effect on RWC but osmotic adjustment (OA) and were higher in Cleo than in Carr. Net CO₂ assimilation rate (A_CO2) in leaves was decreased more by drought than by flooding in both species but especially in Carr. Leaf water‐use efficiency (A_CO2/transpiration) was lower in Carr and was decreased more by DS and flooding stress than in Cleo. Higher values of intercellular CO₂ concentration (C_i) in stressed plants than in control plants indicated that non‐stomatal factors including chlorophyll degradation and chlorophyll fluorescence [maximum quantum efficiency of PSII (Fv/Fm, where Fm is the maximum fluorescence and F₀, minimum fluorescence in dark‐adapted leaves)] were more important limitations on A_CO2 than stomatal conductance. In both genotypes, leaf proline was increased by drought but not by flooding, whereas both stresses increased proline in roots. Soluble sugars in leaves were increased by DS, and flooding decreased leaf sugars in Cleo. In general, DS tended to increase the concentrations of Ca, K, Mg, Na and Cl in both leaves and roots, whereas flooding tended to decrease these ions with the exception of leaf Ca in Cleo. Based on water relations and net gas exchange, Cleo was more tolerant to short‐term DS and flooding stress than Carr.     

Light induced transpiration in a chlorophyll deficient mutant of Hordeum vulgare

A chlorophyll deficient mutant of Hordeum vulgare L. was investigated with respect to its transpiration response to light pulses. Broad band blue light. 380–500 nm, caused a significant transpiration response, while broad band red light did not. The transpiration response to changes in the ambient CO₂-concentration was the same in the chlorophyll deficient mutant as in green plants. The absence of a transpiration response to red light in the mutant was therefore not the result of a defective CO₂-response. It is concluded that the specific blue light response is not mediated via photosynthetic CO₂-fixation. The nature of the blue light response is discussed.     

Influences of different nitrate to ammonium ratios on chlorosis,cation concentrations and the binding forms of Mg and Ca in needles of Mg-deficient Norway spruce

Effects of Mg deficiency and variations of the NO₃ ^-/NH₄ ⁺-ratio on chlorophyll, mineral nutrient concentrations and the binding forms of Mg and Ca were investigated in current-year, one- and two-year-old needles of clonal Norway spruce (Picea abies [L.] Karst.). Six-year-old spruce plants were grown for one year in sand culture with circulating nutrient solutions containing sufficient (0.2 mt M) or limiting (0.04 mt M) concentrations of Mg. The NO₃ ^-/NH₄ ⁺-ratio in the nutrient solutions administered to the experimental trees was adjusted to 0.76 in the Mg-sufficient treatment and to 1.86, 0.76 and 0.035 in Mg-limited treatments. Mg and chlorophyll concentrations, were strongly influenced by the applied nitrogen source in current-year needles and - to a less extent - also in one-year-old needles. NH₄ ⁺-dominated nutrition resulted in decreased height growth and significantly lower Mg and chloropyhll concentrations in current-year and one-year-old needles compared to NO₃ ^--dominated nutrition. Decreases in total Mg were linearly correlated to reductions of water-soluble Mg and water-unsoluble Mg not bound to chlorophyll. Mg bound to chlorophyll, however, was only reduced, when total Mg decreased below a physiological threshold value of 2% of the total nitrogen concentration in the respective needles. Total Ca concentrations in the needles, which were reduced by Mg deficiency especially when nutrition was NH₄ ⁺-dominated, were strongly correlated to the portion of Ca bound to oxalate. The amount of water-soluble Ca and pectate-bound Ca remained nearly constant, independent from changes of total Ca concentrations. Negative effects of increasing NH₄ ⁺ supply on concentrations of Mg and other cations in the needles can be attributed to an inhibition of cation uptake induced by ion antagonism and/or reduced root growth.     

Analysis of acid-soil stress in sorghum genotypes with emphasis on aluminium and magnesium interactions

Acid-soil stress in 12 sorghum (Sorghum bicolor (L.) Moench) genotypes was attributed mainly to aluminium (Al) toxicity. Root damage and magnesium (Mg) deficiency are two independent aspects of plant sensitivity to Al, either in acid soil or in nutrient solution. At moderate soil acidity, Mg deficiency dominantly limited growth whilst at high acidity root damage overruled the effect of Mg deficiency on the growth response. In nutrient solutions containing Al, increased Mg supply improved both root development and Mg nutrition of plants, whereas increased calcium (Ca) supply, or nutrition with ammonium (NH₄) instead of nitrate (NO₃), alleviated root damage but amplified Mg deficiency. At lowered pH the syndrome of Al toxicity was more profound. The implications of Mg-Al interactions, root damage, Mg supply and genotype selection are elucidated.     

Responses of benthic algae to nutrient enrichment in a shallow lake: Linking community production,biomass, and composition

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Alteration of nutrient allocation and transporter genes expression in rice under N,P, K,and Mg deficiencies

Nitrogen (N), phosphorus (P), potassium (K), and magnesium (Mg) have essential physiological functions in plants. Their interactions in plants are not fully understood especially at the molecular level. In this study, we detected the physiological and molecular responses of rice plants at the vegetative growth phase to N, P, K, and Mg starvations. Deficiencies of N and P resulted in accumulation of soluble sugar and starch in the leaves. The root to shoot ratio increased under N and P deficiencies, but decreased under K and Mg deficiencies. In addition, deficiency of either K or Mg resulted in accumulation of the other cation in shoots. Moreover, K starvation decreased both K and soluble sugar contents in the roots pronouncedly. RT-PCR analysis showed that several sugar transporter genes in the leaves orchestrated with sugar accumulation induced by the nutrient shortages. Expression of a high affinity K transporter gene (OsHAK1) and a putative Mg transporter gene (OsMGT) showed opposite down- and up-regulation in the roots by K starvation. These findings suggest that deficiencies of the major nutrients suppressed the export of carbohydrates from source leaves. The regulated sugar and nutrient transporter genes investigated in this study could be used for elucidating the molecular mechanism of plants in their adaptation to varied nutrient supply.     

Magnesium deficiency in sugar beets alters sugar partitioning and phloem loading in young mature leaves

Magnesium deficiency has been reported to affect plant growth and biomass partitioning between root and shoot. The present work aims to identify how Mg deficiency alters carbon partitioning in sugar beet (Beta vulgaris L.) plants. Fresh biomass, Mg and sugar contents were followed in diverse organs over 20 days under Mg-sufficient and Mg-deficient conditions. At the end of the treatment, the aerial biomass, but not the root biomass, of Mg-deficient plants was lower compared to control plants. A clear inverse relationship between Mg and sugar contents in leaves was found. Mg deficiency promoted a marked increase in sucrose and starch accumulation in the uppermost expanded leaves, which also had the lowest content of Mg among all the leaves of the rosette. The oldest leaves maintained a higher Mg content. [¹⁴C]Sucrose labelling showed that sucrose export from the uppermost expanded leaves was inhibited. In contrast, sucrose export from the oldest leaves, which are close to, and export mainly to, the roots, was not restricted. In response to Mg deficiency, the BvSUT1 gene encoding a companion cell sucrose/H⁺ symporter was induced in the uppermost expanded leaves, but without further enhancement of sucrose loading into the phloem. The observed increase in BvSUT1 gene expression supports the idea that sucrose loading into the phloem is defective, resulting in its accumulation in the leaf.     

Nutrient enrichment experiments in three central Florida lakes of different trophic states

Seasonal nutrient enrichment experiments (short-term bioassays) were conducted in three Florida lakes of different trophic states to determine the effects of addition of various nutrient combinations upon chlorophyll a and phytoplankton standing crops. Nutrient enriched surface water samples with crustacean zooplankton removed were incubated in situ in clear polyethylene bags for 3 to 6 days. The 2⁵ factorial design employed two levels (ambient and enriched) of each of five nutrients [NH₄ ⁺, PO _inf4 ^sup3− , Fe⁻ -EDTA, SiO _inf3 ^sup2− and a cation (Ca²⁺ or K⁺) or trace elements]. Ammonium produced significant increases in chlorophyll a and phytoplankton standing crops in all experiments. Phosphate produced similar results in the mesotrophic lake, but the eutrophic lakes had both positive and nonsignificant responses which varied seasonally between lakes. Iron increased chlorophyll a in most experiments but affected total phytoplankton standing crop only during the summer and fall. Silicon had negative effects in some experiments. Cations and trace elements produced marked differences between lakes for chlorophyll a, but total phytoplankton standing crop showed few significant responses. Synergistic responses to two- and three-factor interactions were observed in all lakes. Differences in the responses of phytoplankton taxonomic divisions to enrichment may be responsible for much of the between lake variation in chlorophyll a and total phytoplankton volume responses. Nutrient limitations in these lakes are discussed and related to limnological factors and predictive models.     

Effects of upstream lakes and nutrient limitation on periphytic biomass and nitrogen fixation in oligotrophic, subalpine streams

1. We conducted bioassays of nutrient limitation to understand how macronutrients and the position of streams relative to lakes control nitrogen (N₂) fixation and periphytic biomass in three oligotrophic Rocky Mountain catchments. We measured periphytic chlorophyll‐a (chl‐a) and nitrogen‐fixation responses to nitrogen (N) and phosphorus (P) additions using nutrient‐diffusing substrata at 19 stream study sites, located above and below lakes within the study catchments. 2. We found that periphytic chl‐a was significantly co‐limited by N and P at 13 of the 19 sites, with sole limitation by P observed at another four sites, and no nutrient response at the final two sites. On average, the addition of N, P and N + P stimulated chl‐a 35%, 114% and 700% above control values respectively. The addition of P alone stimulated nitrogen fixation by 2500% at five of the 19 sites. The addition of N, either with or without simultaneous P addition, suppressed nitrogen fixation by 73% at nine of the 19 sites. 3. Lake outlet streams were warmer and had higher dissolved organic carbon concentrations than inlet streams and those further upstream, but position relative to lakes did not affect chl‐a and nitrogen fixation in the absence of nutrient additions. Chl‐a response to nutrient additions did not change along the length of the study streams, but nitrogen fixation was suppressed more strongly by N, and stimulated more strongly by P, at lower altitude sites. The responses of chl‐a and nitrogen fixation to nutrients were not affected by location relative to lakes. Some variation in responses to nutrients could be explained by nitrate and/or total N concentration. 4. Periphytic chl‐a and nitrogen fixation were affected by nutrient supply, but responses to nutrients were independent of stream position in the landscape relative to lakes. Understanding interactions between nutrient supply, nitrogen fixation and chl‐a may help predict periphytic responses to future perturbations of oligotrophic streams, such as the deposition of atmospheric N.     

Physiological characterisation of magnesium deficiency in sugar beet: acclimation to low magnesium differentially affects photosystems I and II

Magnesium deficiency in plants is a widespread problem, affecting productivity and quality in agriculture, yet at a physiological level it has been poorly studied in crop plants. Here, a physiological characterization of Mg deficiency in Beta vulgaris L., an important crop model, is presented. The impact of Mg deficiency on plant growth, mineral profile and photosynthetic activity was studied. The aerial biomass of plants decreased after 24 days of hydroponic culture in Mg-free nutrient solution, whereas the root biomass was unaffected. Analysis of mineral profiles revealed that Mg decreased more rapidly in roots than in shoots and that shoot Mg content could fall to 3 mg g^–1 DW without chlorosis development and with no effect on photosynthetic parameters. Sucrose accumulated in most recently expanded leaves before any loss in photosynthetic activity. During the development of Mg deficiency, the two photosystems showed sharply contrasting responses. Data were consistent with a down-regulation of PSII through a loss of antenna, and of PSI primarily through a loss of reaction centres. In each case, the net result was a decrease in the overall rate of linear electron transport, preventing an excess of reductant being produced during conditions under which sucrose export away from mature leaf was restricted.     

Magnesium deficiency enhances resistance to paraquat toxicity in bean leaves

Effects of deficient (20mmol m^?3) and sufficient (1000 mmol m^?3) magnesium (Mg) supply and of varied light intensity (100 μmol m^?2 s^?1 to 580 μmol m^?2 s^?1) on paraquat-dependent chlorophyll destruction in bean (Phaseolus vulgaris) plants grown in nutrient solution were studied over a 12-d period using leaf discs or intact primary leaves. Treatment of leaf discs with 10mmol m ³ paraquat for 15h caused severe chlorophyll loss, especially with increasing light intensity. This chlorophyll destruction by paraquat was very much higher in Mg-sufficient than Mg-deficient leaves. The occurrence of paraquat resistance in Mg deficient leaves was already apparent after 6d growth in nutrient solution, i.e. before any decrease in chlorophyll or growth by Mg deficiency was evident. Also, following foliar application of paraquat (10–140 mmol m^?3) to intact plants, Mg-deficient plants were much more resistant to paraquat, even following longer exposure duration (72 h) and four to 14 times higher paraquat concentrations than those received by Mg sufficient plants. From experiments where exogenous scavengers of superoxide radical (O₂^.-), hydroxyl radical (OH^·) and singlet oxygen (¹O₂) were applied to leaf discs, it appears that O₂^.-, and partly, OH^· are the main O₂ species which contribute to chlorophyll destruction by paraquat. The results demonstrate that Mg-deficient bean plants become highly resistant to O₂^.--mediated and light-induced paraquat injury. The mode of this paraquat resistance is attributed to well-known stimulative effects of Mg deficiency on O₂^.- and H₂O₂ scavenging enzymes and antioxidants.     

不同非生物胁迫对麻疯树幼苗光合速率等生理指标的影响

对麻疯树(Jatropha curcas L.)幼苗在不同非生物胁迫下的净光合速率(Pn)和蒸腾速率(Tr)等生理指标的变化进行了研究。结果表明,在缺磷处理中,麻疯树叶片Pn保持在对照的90%左右,气孔导度(Gs)和胞间CO2浓度(Ci)在处理2 d后显著增加,Gs上升20%～40%,Ci升高4%～16%,Tr变化不大;处理17 d时,麻疯树的P含量下降55%～85%,而干重只下降3%。缺氮处理9 d时麻疯树叶片的Pn下降到最低,之后维持在对照的64%左右,Gs在处理2～7 d显著高出对照15%～57%,处理9～16 d恢复到对照水平,Ci从第2天开始上升,高出对照4%～24%,Tr变化不大;处理17 d时组织中N含量显著下降47%～78%,植株干重下降23%。盐胁迫处理5 d后,麻疯树叶片Pn降低到对照的54%,之后维持在对照的48%左右,Gs、Ci和Tr与Pn的变化一致,均呈下降趋势;处理17 d,叶柄和茎中P含量增加37%～54%,组织中K+/Na+下降87%～96%,植株干重下降18%。干旱胁迫处理6 d,叶片Pn快速下降至29%,Gs、Ci和Tr与Pn整体变化趋势一致;处理第7天,叶片细胞膜透性增加67%,停止浇水17 d后植株干重下降55%,同时叶片卷曲下垂,老叶脱落。麻疯树植株Pn在缺磷胁迫过程中最早达到相对稳定状态,其次为盐胁迫和缺氮胁迫。这说明麻疯树植株对缺磷环境具有良好的适应性,而对缺氮环境适应性相对较差;耐盐类型可能属于逃避盐害中的聚盐植物,适应干旱环境的机制属于御旱性类型。     

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.