Growth rate—reserve content relationship as influenced by irradiance,CO2 concentration,and temperature

The patterns of the CO₂ exchange of single vegetative bean plants were monitored during steady state exchange and after lowering the irradiance, the CO₂ concentration, or the temperature. The measured patterns were used to calculate the dynamics of the rate of synthesis of structural dry matter and of the amount of the reserve materials during the experiments. The rate of synthesis of structural dry matter was assumed to be proportional to growth respiration (total minus maintenance). The growth conversion efficiency was assumed to be independent of the treatments. The maintenance respiration coefficient was taken to be dependent only on the temperature. Change in the amounts of reserve materials was calculated as a difference between the net CO₂ input and the amount converted into new structural dry matter.During the first day of a low CO₂ uptake a substantial depletion of reserve materials took place also during light hours, since the rate of synthesis of structural dry matter lagged behind the decrease of photosynthesis. On the second day the rate of synthesis was adapted to the low CO₂ input and there occurred little change in the amount of reserve materials. There was a rapid increase in the amount of reserve materials after the irradiance was increased again or after temperature was lowered.A saturating dependence of the specific growth rate on the content of reserve materials was found to exist irrespective of the mode of changing the content of reserve materials. A hysteresis-like retardation of the specific growth rate took place after the reserve had already been exhausted for some time. During retardation a replenishment of reserve materials took place.It is suggested that adaptation processes tend to keep the content of reserve materials within a certain (probably optimal) range.     

Short-term CO2 exchange response to temperature,irradiance, and CO2 concentration in strawberry

Relative importance of short-term environmental interaction and preconditioning to CO₂ exchange response was examined in Fragaria ananasa (strawberry, cv. Quinault). Tests included an orthogonal comparison of 15 to 60-min and 6 to 7-h exposures to different levels of temperature (16 to 32°C), photosynthetically active radiation (PAR, 200 to 800 E m² s^-1), and CO₂ (300 to 600 l/l) on successive days of study. Plants were otherwise maintained at 21°C, 300 E m² s^-1 PAR and 300–360 l/l CO₂ as standard conditions. Treatment was restricted to the mean interval of 14 h daily illumination and the first 3–4 days of each test week over a 12-week cultivation period. CO₂ exchange rates were followed with each step-change in environmental level including ascending/descending temperature/PAR within a test period, initial response at standard conditions on successive days of testing, and measurement at reduced O₂. Response generally supported prior concepts of leaf biochemical modeling in identifying CO₂ fixation as the major site of environmental influence, while overall patterns of whole plant CO₂ exchange suggested additional effects for combined environmental factors and preconditioning. These included a positive interaction between temperature and CO₂ concentration on photosynthesis at high irradiance and a greater contribution by dark respiration at lower PAR than previously indicated. The further importance of estimating whole plant CO₂ exchange from repetitive tests and measurements was evidenced by a high correlation of response to prior treatment both during the daily test period and on consecutive days of testing.Abbreviations C₃ plant a plant in which the product of CO₂ fixation is a 3-carbon acid (3-phosphoglyceric acid) - IRGA intra-red gas analyzer - PAR photosynthetically active radiation - RH relative humidity - RuBisCO ribulose-1,5-bisphosphate carboxylase/oxygenase Reference to a company and/or product named by the Department is only for purposes of information and does not imply approval or recommendation of the product to the exclusion of others which may also be suitable.     

Effects of atmospheric CO2 concentration, irradiance, and soil nitrogen availability on leaf photosynthetic traits of Polygonum sachalinense around natural CO2 springs in northern Japan

Long-term exposure to elevated CO₂ concentration will affect the traits of wild plants in association with other environmental factors. We investigated multiple effects of atmospheric CO₂ concentration, irradiance, and soil N availability on the leaf photosynthetic traits of a herbaceous species, Polygonum sachalinense, growing around natural CO₂ springs in northern Japan. Atmospheric CO₂ concentration and its interaction with irradiance and soil N availability affected several leaf traits. Leaf mass per unit area increased and N per mass decreased with increasing CO₂ and irradiance. Leaf N per area increased with increasing soil N availability at higher CO₂ concentrations. The photosynthetic rate under growth CO₂ conditions increased with increasing irradiance and CO₂, and with increasing soil N at higher CO₂ concentrations. The maximal velocity of ribulose 1,5-bisphosphate carboxylation (V _cmax) was affected by the interaction of CO₂ and soil N, suggesting that down-regulation of photosynthesis at elevated CO₂ was more evident at lower soil N availability. The ratio of the maximum rate of electron transport to V _cmax (J _max/V _cmax) increased with increasing CO₂, suggesting that the plants used N efficiently for photosynthesis at high CO₂ concentrations by changes in N partitioning. To what extent elevated CO₂ influenced plant traits depended on other environmental factors. As wild plants are subject to a wide range of light and nutrient availability, our results highlight the importance of these environmental factors when the effects of elevated CO₂ on plants are evaluated.     

Changes in photosynthetic capacity,carboxylation efficiency,and CO2 compensation point associated with midday stomatal closure and midday depression of net CO2 exchange of leaves of Quercus suber

The carbon-dioxide response of photosynthesis of leaves of Quercus suber, a sclerophyllous species of the European Mediterranean region, was studied as a function of time of day at the end of the summer dry season in the natural habitat. To examine the response experimentally, a standard time course for temperature and humidity, which resembled natural conditions, was imposed on the leaves, and the CO₂ pressure external to the leaves on subsequent days was varied. The particular temperature and humidity conditions chosen were those which elicited a strong stomatal closure at midday and the simultaneous depression of net CO₂ uptake. Midday depression of CO₂ uptake is the result of i) a decrease in CO₂-saturated photosynthetic capacity after light saturation is reached in the early morning, ii) a decrease in the initial slope of the CO₂ response curve (carboxylation efficiency), and iii) a substantial increase in the CO₂ compensation point caused by an increase in leaf temperature and a decrease in humidity. As a consequence of the changes in photosynthesis, the internal leaf CO₂ pressure remained essentially constant despite stomatal closure. The effects on capacity, slope, and compensation point were reversed by lowering the temperature and increasing the humidity in the afternoon. Constant internal CO₂ may aid in minimizing photoinhibition during stomatal closure at midday. The results are discussed in terms of possible temperature, humidity, and hormonal effects on photosynthesis.Abbreviations and symbols CE carboxylation efficiency - NP net photosynthesis rate - PAR photosynthetically active radiation - P_i leaf internal CO₂ partial pressure - W water vapor mole fraction difference between leaf and air - T CO₂ compensation pressure Dedicated to Professor Dr. Hubert Ziegler on the occasion of his 60th birthday     

Effects of temperature and CO2 concentration on photosynthetic CO2 fixation by Chlorella

The rates of photosynthetic ¹⁴CO₂ fixation by Chlorella vulgarisllh, grown under high CO₂, were determined between 4 to 37°Cwith air containing from 300 to 13,000 ppm ¹⁴CO₂. When the CO₂level was increased, both the rate of photosynthesis and theoptimum temperature for maximum photosynthesis increased. Themaximum photosynthetic rate was reached at 12°C with 300ppm ^l4CO₂. Among the photosynthetic products fromed at 300 ppm ¹⁴CO₂, glycolatedecreased greatly when the temperature was raised from 20 to30°C. At 3,000 ppm ¹⁴CO₂ an insignificant amount of glycolatewas formed at all temperatures, whereas ¹⁴C-incorporation intothe insoluble fraction, sucrose, and the lipid fraction wassignificantly higher than at 300 ppm ¹⁴CO₂. The ¹⁴C in sucrosewas greatly increased and the radioactivity in the insolublefraction decreased when the temperature was raised from 28 to36°C. (Received April 8, 1980; )     

CO2 efflux, CO2 concentration and photosynthetic refixation in stems of Eucalyptus globulus (Labill.)

In spite of the importance of respiration in forest carbon budgets,the mechanisms by which physiological factors control stem respirationare unclear. An experiment was set up in a Eucalyptus globulusplantation in central Portugal with monoculture stands of 5-year-oldand 10-year-old trees. CO₂ efflux from stems under shaded andunshaded conditions, as well as the concentration of CO₂ dissolvedin sap [CO₂^*], stem temperature, and sap flow were measuredwith the objective of improving our understanding of the factorscontrolling CO₂ release from stems of E. globulus. CO₂ effluxwas consistently higher in 5-year-old, compared with 10-year-old,stems, averaging 3.4 versus 1.3 µmol m^–2 s^–1,respectively. Temperature and [CO₂^*] both had important, andsimilar, influences on the rate of CO₂ efflux from the stems,but neither explained the difference in the magnitude of CO₂efflux between trees of different age and size. No relationshipwas found between efflux and sap flow, and efflux was independentof tree volume, suggesting the presence of substantial barriersto the diffusion of CO₂ from the xylem to the atmosphere inthis species. The rate of corticular photosynthesis was thesame in trees of both ages and only reduced CO₂ efflux by 7%,probably due to the low irradiance at the stem surface belowthe canopy. The younger trees were growing at a much fasterrate than the older trees. The difference between CO₂ effluxfrom the younger and older stems appears to have resulted froma difference in growth respiration rather than a differencein the rate of diffusion of xylem-transported CO₂. Key words: Eucalyptus globulus, refixation, stem respiration Received 19 May 2008; Revised 14 September 2008 Accepted 8 October 2008     

Interaction between CO2 concentration and flow rate on peripheral airway resistance

In the present study, we investigated the interaction between CO2 concentration and rate of delivered flow on peripheral airway resistance (Rp) in the intact canine lung. Dogs were anesthetized, intubated, paralyzed, and mechanically ventilated with room air to maintain end-tidal CO2 between 4.8 and 5.2%. Using a wedged bronchoscope technique, we measured Rp at functional residual capacity. The relationship between CO2 concentration and Rp was measured at flow rates of 100 and 400 ml/min with 5, 3, 2, 1, and 0% CO2 in air. Measurements were made at the end of a 3-min exposure to each gas. At low flow rates (100 ml/min) responses to hypocapnia were small, whereas at high flow rates (400 ml/min) responses were large. The PC50 (defined as the CO2 concentration required to produce a 50% increase in Rp above baseline Rp established on 5% CO2) at 400 ml/min (1.73%) was significantly larger than that at 100 ml/min (0.38%). We also directly measured the relationship between Rp and flow rate with 5% CO2 (normocapnia) or 1% CO2 (hypocapnia) delivered into the wedged segment. Increases in normocapnic flow caused small but significant decreases in Rp. In contrast, increases in hypocapnic flow from 100 to 400 ml/min caused a 108% increase in Rp. Thus the response to hypocapnia is augmented by increasing flow rate. This interaction can be explained by a simple model that considers the effect of local ventilation-perfusion ratio and gas mixing on the local CO2 concentration at the site of peripheral airway contraction.     

Analysing the responses of photosynthetic CO2 assimilation to long-term elevation of atmospheric CO2 concentration

Understanding how photosynthetic capacity acclimatises when plants are grown in an atmosphere of rising CO₂ concentrations will be vital to the development of mechanistic models of the response of plant productivity to global environmental change. A limitation to the study of acclimatisation is the small amount of material that may be destructively harvested from long-term studies of the effects of elevation of CO₂ concentration. Technological developments in the measurement of gas exchange, fluorescence and absorption spectroscopy, coupled with theoretical developments in the interpretation of measured values now allow detailed analyses of limitations to photosynthesisin vivo. The use of leaf chambers with Ulbricht integrating spheres allows separation of change in the maximum efficiency of energy transduction in the assimilation of CO₂ from changes in tissue absorptance. Analysis of the response of CO₂ assimilation to intercellular CO₂ concentration allows quantitative determination of the limitation imposed by stomata, carboxylation efficiency, and the rate of regeneration of ribulose 1:5 bisphosphate. Chlorophyll fluorescence provides a rapid method for detecting photoinhibition in heterogeneously illuminated leaves within canopies in the field. Modulated fluorescence and absorption spectroscopy allow parallel measurements of the efficiency of light utilisation in electron transport through photosystems I and IIin situ.Abbreviations A net rate of CO₂ uptke per unit leaf area (µmol m^–2 s^–1) - A_sat light-saturated A - A₈₂₀ change in absorptance of PSI on removal of illumination (OD) - c CO₂ concentration in air (µmol mol^–1) - c_a c in the bulk air; c_i, c in the intercellular spaces - ce carboxylation efficiency (mol m^–2 s^–1) - E transpiration per unit leaf area (mol m^–2 s^–1) - F fluorescence emission of PSII (relative units) - F_m maximal level of F - F_o minimal level of F upon illumination when PSII is maximally oxidised - F_s the steady-state F following the m peak - F_v the difference between F_m and F_o - F'_m maximal F' generated after the m peak by addition of a saturating light pulse - F'_o the minimal level of F' after the m peak determined by re-oxidising PSII by far-red light - g₁ leaf conductance to CO₂ diffusion in the gas phase (mol m^–2 s^–1) - g'₁ leaf conductance to water vapour diffusion in the gas phase (mol m^–2 s^–1) - k_c and k_o the Michaelis constants for CO₂ and O₂, respectively, (µmol mol^–1); - J_max the maximum rate of regeneration of rubP (µmol m^–2 s^–1) - l stomatal limitation to CO₂ uptake (dimensionless, 0–1) - LCP light compensation point of photosynthesis (µmol m^–2 s^–1) - o_i the intercellular O₂ concentration (mmol mol^–1) - Pi cytosol inorganic phosphate concentration - PSI photosystem I - PSII photosystem II - Q photon flux (µmol m^–2 s^–1) - Q_abs Q absorbed by the leaf - rubisCO ribulose 1:5 bisphosphate carboxylase/oxygenase; rubP, ribulose 1:5 bisphosphate; s, projected surface area of a leaf (m²) - V_c,max is the maximum rate of carboxylation (µmol m^–2 s^–1) - W_c the rubisCO limited rate of carboxylation (µmol m^–2 s¹) - W_j the electron transport limited rate of regeneration of rubP (µmol m^–2 s^–1) - W_p the inorganic phosphate limited rate of regeneration of rubP (µmol m^–2 s^–1) - absorptance of light (dimensionless, 0–1) - _a of standard black absorber ₁, of leaf - _s of integrating sphere walls - , CO₂ compensation point of photosynthesis (µmol mol^–1) - the specificity factor for rubisCO carboxylation (dimensionless) - , convexity of the response of A to Q (dimensionless 0–1) - the quantum yield of photosynthesis on an absorbed light basis (A/Q_abs; dimensionless) - the quantum yield of photosynthesis on an incident light basis (A/Q; dimensionless) - _app the maximum - _m the maximum - _m,app the photochemical efficiency of PSII (dimensionless, 0–1) - _PSII,m the maximum      

Suitable indicators and an altered empirical equation for calculating the CO2 concentration in colorimetric determinations of photosynthetic rate

Cresol red was found to be the most suitable of the six colour pH indicators tested for the colorimetry of CO₂ concentration within the range of 100 to 2000 p.p.m. In the region near pH 8-2 (corresponding approximately to 300 p.p.m. CO₂)this indicator is most suitablein view of its low unit concentration, although somewhat greater differences in absorbancy can be found on changing pH by one unit if an approximately six-fold higher concentration of thymol blue is used. For higher concentrations of CO₂ (pH below 7-5) phenol red is most appropriate. Tropaeolin 000 and aurin (rosolic acid) are not suitable. It is more convenient in spectrophotometry and colorimetry using filters with narrow spectral ranges to apply any of the above indicators in the green-yellow-orange spectral region rather than in blue radiation. When the air to be measured is bubbled through a bicarbonate solution, an equilibrium between the CO₂ concentration in the air and pH value is established in less than 10 min for differences in CO₂ concentration from 100 to 400 p.p.m. Partial pressure of CO₂ and the experimentally established pH value are related by the altered equation: pH =α- 0-934 log P. The values of α at different temperatures were determined.     

Biochemical components of the photosynthetic CO2 compensation point of higher plants.

Barley, Panicum milioides and Panicum maximum were exposed to ¹⁴CO₂ near their photosynthetic CO₂ compensation points and their respective ¹⁴C-products were determined. In short exposure times Panicum maximum had 100% of its ¹⁴C in malate and aspartate whereas Panicum milioides and barley had 16 and 3% of their respective ¹⁴C in C₄ organic acids. Near the respective CO₂ compensation points a linear relationship occurs in plotting the ratio of glycine, serine, and glycerate to C₄ organic acids. The ratio of ribulose 1,5-bisphosphate oxygenase to phosphoenolpyruvate carboxylase is linear with their CO₂ compensation points. The photosynthetic CO₂ compensation point apparently is controlled by the activity of enzymes producing photorespiration metabolites and the activity of phospheonolpyruvate carboxylase.     

A study on the relationship between leaf conductance,CO2 concentration and carboxylation rate in various species

Leaf conductance g_L is strongly influenced by environmental factors like CO₂, irradiance and air humidity. According to Ball et al. (1987), g_L is correlated with an index calculated as the product of net CO₂ exchange rate A and ambient water vapour concentration W_a, divided by ambient CO₂ concentration c_a. However, this empirical model does not apply to high values of g_L observed at c_a below CO₂ compensation concentration . Therefore, we applied modified indices in which A is replaced by estimates for the rate of carboxylation. Such estimates, P₁ and P₂, were determined by adding to A the quotient of and the sum of gas phase resistance r_g and intracellular resistance for CO₂ exchange r_i, P₁ = A+/(r_g + r_i), or the quotient of and r_i, P₂ = A + /r_i. If P₂ is chosen, c_a in the Ball index has to be replaced by the intercellular CO₂ concentration c_i. By using the modified indices P₁·W_a/c_a and P₂·W_a/c_i, we analysed data from the C₃ species Nicotiana tabacum and Nicotiana plumbaginifolia, the C₃–C₄ intermediate species Diplotaxis tenuifolia, and the C₄ species Zea mays. The data were collected at widely varying levels of irradiance and CO₂ concentration. For all species uniform relationships between g_L and the new indices were found for the whole range of CO₂ concentrations below and above . Correlations between g_L and P₁·W_a/c_a were closer than those between g_L and P₂·W_a/c_i because P₁/c_a implicitly contains g_L. Highly significant correlations were also obtained for the relationships between g_L and the ratios P₁/c_a and P₂/c_i.     

Effects of CO2 concentration during growth on subsequent photosynthetic CO2 fixation in Chlorella1

The maximum rate of photosynthetic ¹⁴CO₂ fixation (V_max) aswell as the concentration of CO₂ at which the rate of photosynthetic¹⁴CO₂ fixation attains one-half its maximum velocity (K_m) inChlorella vulgaris 11h cells was strongly dependent on the concentrationof CO₂ continuously provided during the algal growth. The V_max (µmoles ¹⁴CO₂ fixed/ml pcv?min) and K_m (% CO₂)of the algal cells which had been grown in air containing 4%CO₂ (by volume) were ca. 10 and 0.15–0.17, while thosein the cells which had been grown in ordinary air (containing0.04% CO₂) were 7 and 0.05–0.06, respectively. When the concentration of CO₂ in the bubbling gas was loweredfrom 4 to 0.04% during the algal growth, their photosynthetickinetics attained the respective lower steady levels after 5–10hr. On the other hand, when the photosynthetic kinetics weredetermined 24 hr after raising the concentration of CO₂ from0.04 to 4%, the V_max and K_m-values were found to have alreadyattained the respective higher levels. (Received October 15, 1976; )     

The responses of photosynthetic rate and stomatal conductance of Fraxinus rhynchophylla to differences in CO2 concentration and soil moisture

The photosynthetic parameters in leaves of three-year-old seedlings of Fraxinus rhynchophylla L. were studied under different soil water conditions and CO₂ concentrations ([CO₂]) with a LI-COR 6400 portable photosynthesis system. The objective was to investigate the response of photosynthesis and stomatal conductance (g _s) to various [CO₂] and soil water conditions, and to understand the adaptability of F. rhynchophylla to such conditions. The results showed that the soil water content (RWC) required to maintain high photosynthetic productivity in F. rhynchophylla was 49.5–84.3%; in this range, net photosynthetic rate (P _N) rose with [CO₂] increasing from 500 to 1,400 μmol mol^?1. Outside this RWC range, P _N decreased significantly. The apparent maximum photosynthetic rate (P _max,c) and carboxylation velocity (V _c) increased with increasing RWC and remained relatively high, when RWC was between 49.5 and 96.2%. CO₂ compensation points and photorespiration rate exhibited a trend opposite to that of P _max,c and V _c, indicating that moderate water stress was beneficial for increasing plant assimilation, decreasing photorespiration, and increasing production of photosynthates. g _s declined significantly with increasing [CO₂] under different water supplies, but the RWC range maintaining high g _s increased. g _s reached its maximum, when RWC was approximately 73% and then decreased with declining RWC. The maximal g _s was found with increasing RWC. Thus, based on photosynthetic characteristics in artificial, vegetation construction in semiarid loess hill and gully area, F. rhynchophylla could be planted in habitats of low soil water content.     

高大气CO2浓度下氮素对小麦叶片光能利用的影响

关于氮素对高大气CO₂浓度下C₃植物光合作用适应现象的调节机理已有较为深入的研究, 但对其光合作用适应现象的光合能量转化和分配机制缺乏系统分析。该文以大气CO₂浓度和施氮量为处理手段, 通过测定小麦(Triticum aestivum)抽穗期叶片的光合作用-胞间CO₂浓度响应曲线以及荧光动力学参数来测算光合电子传递速率和分配去向, 研究了长期高大气CO₂浓度下小麦叶片光合电子传递和分配对施氮量的响应。结果表明, 与正常大气CO₂浓度处理相比, 高大气CO₂浓度下小麦叶片较多的激发能以热量的形式耗散, 增施氮素可使更多的激发能向光化学反应方向的分配, 降低光合能量的热耗散速率; 大气CO₂浓度升高后小麦叶片光化学淬灭系数无明显变化, 高氮叶片的非光化学猝灭降低而低氮叶片明显升高, 施氮促进PSII反应中心的开放比例, 降低光能的热耗散; 高大气CO₂浓度下高氮叶片通过PSII反应中心的光合电子传递速率(J_F)较高, 而且参与光呼吸的非环式电子流速率(J₀)显著降低, 较正常大气CO₂浓度处理的高氮叶片下降了88.40%, 光合速率增加46.47%; 高大气CO₂浓度下小麦叶片J_F-J₀升高而J₀/J_F显著下降, 光呼吸耗能被抑制, 更多的光合电子分配至光合还原过程。因此, 大气CO₂浓度增高条件下, 小麦叶片激发能的热耗散速率增加, 但增施氮素后小麦叶片PSII反应中心开放比例提高, 光化学速率增加, 进入PSII反应中心的电子流速率明显升高, 光呼吸作用被抑制, 光合电子较多地进入光化学过程, 这可能是高氮条件下光合作用适应性下调被缓解的一个原因。     

Spring photosynthetic onset and net CO2 uptake in Alaska triggered by landscape thawing

The springtime transition to regional‐scale onset of photosynthesis and net ecosystem carbon uptake in boreal and tundra ecosystems are linked to the soil freeze–thaw state. We present evidence from diagnostic and inversion models constrained by satellite fluorescence and airborne CO₂ from 2012 to 2014 indicating the timing and magnitude of spring carbon uptake in Alaska correlates with landscape thaw and ecoregion. Landscape thaw in boreal forests typically occurs in late April (DOY 111 ± 7) with a 29 ± 6 day lag until photosynthetic onset. North Slope tundra thaws 3 weeks later (DOY 133 ± 5) but experiences only a 20 ± 5 day lag until photosynthetic onset. These time lag differences reflect efficient cold season adaptation in tundra shrub and the longer dehardening period for boreal evergreens. Despite the short transition from thaw to photosynthetic onset in tundra, synchrony of tundra respiration with snow melt and landscape thaw delays the transition from net carbon loss (at photosynthetic onset) to net uptake by 13 ± 7 days, thus reducing the tundra net carbon uptake period. Two global CO₂ inversions using a CASA‐GFED model prior estimate earlier northern high latitude net carbon uptake compared to our regional inversion, which we attribute to (i) early photosynthetic‐onset model prior bias, (ii) inverse method (scaling factor + optimization window), and (iii) sparsity of available Alaskan CO₂ observations. Another global inversion with zero prior estimates the same timing for net carbon uptake as the regional model but smaller seasonal amplitude. The analysis of Alaskan eddy covariance observations confirms regional scale findings for tundra, but indicates that photosynthesis and net carbon uptake occur up to 1 month earlier in evergreens than captured by models or CO₂ inversions, with better correlation to above‐freezing air temperature than date of primary thaw. Further collection and analysis of boreal evergreen species over multiple years and at additional subarctic flux towers are critically needed.     

The photosynthetic induction response in wheat leaves: net CO2 uptake,enzyme activation,and leaf metabolites

The photosynthetic induction response was studied in whole leaves of wheat (Triticum aestivum L.) following 5-min, 30-min and 10-h dark periods. After the 5-min dark treatment there was a rapid burst in the rate of photosynthesis upon illumination (half of maximum after 30s), followed by a slight decrease after 1.5 more min and then a gradual rise to the maximum rate. During this initial burst in photosynthesis, there was a rapid rise in the level of 3-phosphoglycerate (PGA) and a high PGA/triose-phosphate (triose-P) ratio was obtained. In addition, after the 5-min dark treatment, ribulose-1,5-bisphosphate carboxylase (Rubisco, EC 4.1.1.39), ribulose-5-phosphate kinase (EC 2.7.1.19) and chloroplastic fructose-1,6-bisphosphatase (EC 3.1.3.11) maintained a relatively high state of activation, and maximum activation occurred within 1 min of illumination. The results indicate there is a high capacity for CO₂ fixation in the cycle upon illumination but attaining maximum rates requires an increase in the ribulose-1,5-bisphosphate (RuBP) pool (adjustment in triose-P utilization for carbohydrate synthesis versus RuBP synthesis). With both the 30-min and 10-h dark pretreatments there was only a slight rise in photosynthesis upon illumination, followed by a lag, then a gradual increase to steady-state (half-maximum rate after 6 min). In contrast to the 5-min dark treatment, the level of PGA was low and actually decreased initially, whereas the level of RuBP increased and was high during induction, indicating that Rubisco is limiting. This regulation via the carboxylase was not reflected in the initial extractable activity, which reached a maximum by 1 min after illumination. The light activation of chloroplastic fructose-1,6-bisphosphatase in leaves darkened for 30 min and 10 h prior to illumination was relatively slow (reaching a maximum after 8 min). However, this was not considered to limit carbon flux through the carbon-fixation cycle during induction since RuBP was not limiting. When photosynthesis approached the maximum steady-state rate, a high PGA/triose-P ratio and a high PGA/RuBP ratio were obtained. This may allow a high rate of photosynthesis by producing a favorable mass-action ratio for the reductive phase (the conversion of PGA to triose phosphate) while stimulating starch and sucrose synthesis.Abbreviations Chl chlorophyll - FBP fructose-1,6-bisphosphate - FBPase fructose-1,6-bisphosphatase - Fru6P fructose-6-phosphate - Glc6P glucose-6-phosphate - PGA 3-phosphoglycerate - Pi inoganic phosphate - Rubisco RuBP carboxylase/oxygenase - RuBP ribulose-1,5-bisphosphate - Ru5P ribulose-5-phosphate - triose-P triose phosphates (dihydroxyacetone phosphate+glyceraldehyde-3-phosphate)     

The mechanisms of plant photosynthetic acclimation to elevated CO2 concentration]

When measured at a same CO(2) concentration, net photosynthetic rate is often significantly lower in long-term high CO(2)-grown plants than the ambient CO(2)-grown ones. This phenomenon is termed photosynthetic acclimation or down-regulation. Although there have been many reports and reviews, the mechanism(s) of the photosynthetic acclimation is not very clear. Combining the work of the authors' group, this paper briefly reviews the progress in studies on the mechanism(s) of the photosynthetic acclimation to elevated CO(2). It is suggested that besides the possible effects of respiration enhancement and excessive photosynthate accumulation, RuBP carboxylation limitation and RuBP regeneration limitation are probably the main factors leading to the photosynthetic acclimation.