EFFECTS OF ELEVATED CARBON DIOXIDE ON ESTIMATION OF LEAF AREA AND LEAF DRY WEIGHT OF SOYBEAN

The objectives of this study were to determine the effects of elevated CO₂ on relationships between leaf area (A) and linear leaf dimensions (length [L] and width [W]) and leaf dry weight (M) in soybeans (Glycine max (L.) Merr. cv. Bragg). Based on dimensional measurements made on trifoliolates 1–6 for plants grown under three CO₂ levels (348, 502 and 645 μl l^−-1), the best predictor for both trifoliolate leaf area and for fully expanded central leaflets of the trifoliolates was an equation of the form A = b_o + b₁L·W; these relationships were unaffected by CO₂, although there was a small effect of leaf position. For expanding central leaflets of the fifth trifoliolate, no CO₂, leaf size (age) or CO₂ × leaf size effect was found. Specific leaf weight (i.e., M/A) was significantly affected by CO₂, increasing with increasing CO₂. Hence, trifoliolate dry weight can be nondestructively estimated from trifoliolate area using the equation M = 0.097 + (6.71 × 10^−-3 + 1.04 × 10^−-6[CO₂])A, where [CO₂] is mean daytime CO₂ concentration of the growth environment.     

RESPONSE OF TWO OLD FIELD PERENNIALS TO INTERACTIONS OF CO2 ENRICHMENT AND DROUGHT STRESS

Aster pilosus Willd. (aster, C₃) and Andropogon virginicus L. (broomsedge, C₄) were grown in growth chambers at 26/20 C day/night temperatures with a PPFD of 1,000 μmol s^–1 m^–2. Water was withheld for a 2-wk drought period under three CO₂ concentrations (approximately 380, 500, and 650 μl 1^–1). There were significant effects of CO₂ enrichment on aster so that drought stress did not occur in plants grown with CO₂ enrichment. Non-watered plants with CO₂ enrichment had greater leaf water potentials, greater photosynthetic rates, and greater total dry wt than non-watered plants grown at 380 μl 1^–1 CO₂. The response of broomsedge to drought was the same in all CO₂ treatments and there was no significant interaction of CO₂ enrichment and drought. The decreased severity of drought stress and the increased growth of aster with CO₂ enrichment may increase its competitive ability during droughts, allowing it to persist for longer periods during succession in abandoned fields.     

Growth and physiological responses of cotton (Gossypium hirsutum L.) to elevated carbon dioxide and ultraviolet-B radiation under controlled environmental conditions

Better understanding of crop responses to projected changes in climate is an important requirement. An experiment was conducted in sunlit, controlled environment chambers known as soil–plant–atmosphere–research units to determine the interactive effects of atmospheric carbon dioxide concentration [CO₂] and ultraviolet‐B (UV‐B) radiation on cotton (Gossypium hirsutum L.) growth, development and leaf photosynthetic characteristics. Six treatments were used, comprising two levels of [CO₂] (360 and 720 µmol mol^?1) and three levels of 0 (control), 7.7 and 15.1 kJ m^?2 d^?1 biologically effective UV‐B radiations within each CO₂ level. Treatments were imposed for 66 d from emergence until 3 weeks after the first flower stage. Plants grown in elevated [CO₂] had greater leaf area and higher leaf photosynthesis, non‐structural carbohydrates, and total biomass than plants in ambient [CO₂]. Neither dry matter partitioning among plant organs nor pigment concentrations was affected by elevated [CO₂]. On the other hand, high UV‐B (15.1 kJ m^?2 d^?1) radiation treatment altered growth resulting in shorter stem and branch lengths and smaller leaf area. Shorter plants at high UV‐B radiation were related to internode lengths rather than the number of mainstem nodes. Fruit dry matter accumulation was most sensitive to UV‐B radiation due to fruit abscission. Even under 7.7 kJ m^?2 d^?1 of UV‐B radiation, fruit dry weight was significantly lower than the control although total biomass and leaf photosynthesis did not differ from the control. The UV‐B radiation of 15.1 kJ m^?2 d^?1 reduced both total (43%) and fruit (88%) dry weights due to smaller leaf area and lower leaf net photosynthesis. Elevated [CO₂] did not ameliorate the adverse effects of UV‐B radiation on cotton growth and physiology, particularly the boll retention under UV‐B stress.     

Altered night-time CO2 concentration affects the growth, physiology and biochemistry of soybean

Soybean plants (Glycine max (L.) Merr. c.v. Williams) were grown in CO₂ controlled, natural-light growth chambers under one of four atmospheric CO₂ concentrations ([CO₂]): (1) 250 μmol mol^–1 24 h d^–1[250/250]; (2) 1000 μmol mol^–1 24 h d^–1[1000/1000]; (3) 250 μmol mol^–1 during daylight hours and 1000 μmol mol^–1 during night-time hours [250/1000] or (4) 1000 μmol mol^–1 during daylight hours and 250 μmol mol^–1 during night-time hours [1000/250]. During the vegetative growth phase few physiological differences were observed between plants exposed to a constant 24 h [CO₂] (250/250 and 1000/1000) and those that were switched to a higher or lower [CO₂] at night (250/1000 and 1000/250), suggesting that the primary physiological responses of plants to growth in elevated [CO₂] is apparently a response to daytime [CO₂] only. However, by the end of the reproductive growth phase, major differences were observed. Plants grown in the 1000/250 regime, when compared with those in the 1000/1000 regime, had significantly more leaf area and leaf mass, 27% more total plant dry mass, but only 18% of the fruit mass. After 12 weeks of growth these plants also had 19% higher respiration rates and 32% lower photosynthetic rates than the 1000/1000 plants. As a result the ratio of carbon gain to carbon loss was reduced significantly in the plants exposed to the reduced night-time [CO₂]. Plants grown in the opposite switching environment, 250/1000 versus 250/250, showed no major differences in biomass accumulation or allocation with the exception of a significant increase in the amount of leaf mass per unit area. Physiologically, those plants exposed to elevated night-time [CO₂] had 21% lower respiration rates, 14% lower photosynthetic rates and a significant increase in the ratio of carbon gain to carbon loss, again when compared with the 250/250 plants. Biochemical differences also were found. Ribulose-1,5-bisphosphate carboxylase/ oxygenase concentrations decreased in the 250/ 1000 treatment compared with the 250/250 plants, and phosphoenolpyruvate carboxylase activity decreased in the 1000/250 compared with the 1000/1000 plants. Glucose, fructose and to a lesser extent sucrose concentrations also were reduced in the 1000/250 treatment compared with the 1000/1000 plants. These results indicate that experimental protocols that do not maintain elevated CO₂ levels 24 h d^–1 can have significant effects on plant biomass, carbon allocation and physiology, at least for fast-growing annual crop plants. Furthermore, the results suggest some plant processes other than photosynthesis are sensitive to [CO₂] and under ecologically relevant conditions, such as high night-time [CO₂], whole plant carbon balance can be affected.     

The effect of elevated CO2 on diel leaf growth cycle, leaf carbohydrate content and canopy growth performance of Populus deltoides

Image sequence processing methods were applied to study the effect of elevated CO₂ on the diel leaf growth cycle for the first time in a dicot plant. Growing leaves of Populus deltoides, in stands maintained under ambient and elevated CO₂ for up to 4 years, showed a high degree of heterogeneity and pronounced diel variations of their relative growth rate (RGR) with maxima at dusk. At the beginning of the season, leaf growth did not differ between treatments. At the end of the season, final individual leaf area and total leaf biomass of the canopy was increased in elevated CO₂. Increased final leaf area at elevated CO₂ was achieved via a prolonged phase of leaf expansion activity and not via larger leaf size upon emergence. The fraction of leaves growing at 30–40% day^?1 was increased by a factor of two in the elevated CO₂ treatment. A transient minimum of leaf expansion developed during the late afternoon in leaves grown under elevated CO₂ as the growing season progressed. During this minimum, leaves grown under elevated CO₂ decreased their RGR to 50% of the ambient value. The transient growth minimum in the afternoon was correlated with a transient depletion of glucose (less than 50%) in the growing leaf in elevated CO₂, suggesting diversion of glucose to starch or other carbohydrates, making this substrate temporarily unavailable for growth. Increased leaf growth was observed at the end of the night in elevated CO₂. Net CO₂ exchange and starch concentration of growing leaves was higher in elevated CO₂. The extent to which the transient reduction in diel leaf growth might dampen the overall growth response of these trees to elevated CO₂ is discussed.     

Nitrogen balance for wheat canopies (Triticum aestivum cv. Veery 10) grown under elevated and ambient CO2 concentrations

We examined the hypothesis that elevated CO₂ concentration would increase NO₃^– absorption and assimilation using intact wheat canopies (Triticum aestivum cv. Veery 10). Nitrate consumption, the sum of plant absorption and nitrogen loss, was continuously monitored for 23 d following germination under two CO₂ concentrations (360 and 1000 μmol mol^–1 CO₂) and two root zone NO₃^– concentrations (100 and 1000 mmol m³ NO₃^–). The plants were grown at high density (1780 m^–2) in a 28 m³ controlled environment chamber using solution culture techniques. Wheat responded to 1000 μmol mol^–1 CO₂ by increasing carbon allocation to root biomass production. Elevated CO₂ also increased root zone NO₃^– consumption, but most of this increase did not result in higher biomass nitrogen. Rather, nitrogen loss accounted for the greatest part of the difference in NO₃^– consumption between the elevated and ambient [CO₂] treatments. The total amount of NO₃^–-N absorbed by roots or the amount of NO₃^–-N assimilated per unit area did not significantly differ between elevated and ambient [CO₂] treatments. Instead, specific leaf organic nitrogen content declined, and NO₃^– accumulated in canopies growing under 1000 μmol mol^–1 CO₂. Our results indicated that 1000 μmol mol^–1 CO₂ diminished NO₃^– assimilation. If NO₃^– assimilation were impaired by high [CO₂], then this offers an explanation for why organic nitrogen contents are often observed to decline in elevated [CO₂] environments.     

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.     

Profiles of isoprene emission and photosynthetic parameters in hybrid poplars exposed to free‐air CO2 enrichment†

Poplar (Populus × euroamericana) saplings were grown in the field to study the changes of photosynthesis and isoprene emission with leaf ontogeny in response to free air carbon dioxide enrichment (FACE) and soil nutrient availability. Plants growing in elevated [CO₂] produced more leaves than those in ambient [CO₂]. The rate of leaf expansion was measured by comparing leaves along the plant profile. Leaf expansion and nitrogen concentration per unit of leaf area was similar between nutrient treatment, and this led to similar source–sink functional balance. Consequently, soil nutrient availability did not cause downward acclimation of photosynthetic capacity in elevated [CO₂] and did not affect isoprene synthesis. Photosynthesis assessed in growth [CO₂] was higher in plants growing in elevated than in ambient [CO₂]. After normalizing for the different number of leaves over the profile, maximal photosynthesis was reached and started to decline earlier in elevated than in ambient [CO₂]. This may indicate a [CO₂]‐driven acceleration of leaf maturity and senescence. Isoprene emission was adversely affected by elevated [CO₂]. When measured on the different leaves of the profile, isoprene peak emission was higher and was reached earlier in ambient than in elevated [CO₂]. However, a larger number of leaves was emitting isoprene in plant growing in elevated [CO₂]. When integrating over the plant profile, emissions in the two [CO₂] levels were not different. Normalization as for photosynthesis showed that profiles of isoprene emission were remarkably similar in the two [CO₂] levels, with peak emissions at the centre of the profile. Only the rate of increase of the emission of young leaves may have been faster in elevated than in ambient [CO₂]. Our results indicate that elevated [CO₂] may overall have a limited effect on isoprene emission from young seedlings and that plants generally regulate the emission to reach the maximum at the centre of the leaf profile, irrespective of the total leaf number. In comparison with leaf expansion and photosynthesis, isoprene showed marked and repeatable differences among leaves of the profile and may therefore be a useful trait to accurately monitor changes of leaf ontogeny as a consequence of elevated [CO₂].     

The influence of elevated CO2 on non-structural carbohydrate distribution and fructan accumulation in wheat canopies

We grew 2.4 m² wheat canopies in a large growth chamber under high photosynthetic photon flux (1000 μmol m⁻² s⁻¹) and using two CO₂ concentrations, 360 and 1200 μmol mol⁻¹. Photosynthetically active radiation (400–700 nm) was attenuated slightly faster through canopies grown in 360μmol mol⁻¹ than through canopies grown in 1200μmol mol⁻¹, even though high-CO₂ canopies attained larger leaf area indices. Tissue fractions were sampled from each 5-cm layer of the canopies. Leaf tissue sampled from the tops of canopies grown in 1200μmol mol⁻¹ accumulated significantly more total non-structural carbohydrate, starch, fructan, sucrose, and glucose (p≤ 0.05) than for canopies grown in 360μmol mol⁻¹. Non-structural carbohydrate did not significantly increase in the lower canopy layers of the elevated CO₂ treatment. Elevated CO₂ induced fructan synthesis in all leaf tissue fractions, but fructan formation was greatest in the uppermost leaf area. A moderate temperature reduction of 10 °C over 5d increased starch, fructan and glucose levels in canopies grown in 1200μmol mol⁻¹, but concentrations of sucrose and fructose decreased slightly or remained unchanged. Those results may correspond with the use of fructosyl-residues and release of glucose when sucrose is consumed in fructan synthesis.     

CO2 enrichment in a maturing pine forest: are CO2 exchange and water status in the canopy affected?

A _net, leaf net CO₂ assimilation
c_a, CO₂ concentration of air surrounding a leaf
c_i, leaf intercellular CO₂ concentration
Δ, ¹³C isotope discrimination
δ¹³C, relative stable carbon isotope content
?, ratio of A_net at c_a = 560μmol mol^–1 to A_net at c_a = 360 μmol mol^–1
FACE, free-air CO₂ enrichment
g_w, stomatal conductance to water vapour
Π_i, initial leaf osmotic potential
R_t, relative water content at incipient turgor loss
Ψ_l, xylem water potential of leaves
Ψ_m, soil matric potential

Elevated CO₂ is expected to reduce forest water use as a result of CO₂-induced stomatal closure, which has implications for ecosystem-scale phenomena controlled by water availability. Leaf-level CO₂ and H₂O exchange responses and plant and soil water relations were examined in a maturing loblolly pine (Pinus taeda L.) stand in a free-air CO₂ enrichment (FACE) experiment in North Carolina, USA to test if these parameters were affected by elevated CO₂. Current-year foliage in the canopy was continuously exposed to elevated CO₂ (ambient CO₂+200μmol mol^–1) in free-air during needle growth and development for up to 400 d. Photosynthesis in upper canopy foliage was stimulated by 50–60% by elevated CO₂ compared with ambient controls. This enhancement was similar in current-year, ambient-grown foliage temporarily measured at elevated CO₂ compared with long-term elevated CO₂ grown foliage. Significant photosynthetic enhancement by CO₂ was maintained over a range of conditions except during peak drought. There was no evidence of water savings in elevated CO₂ plots in FACE compared to ambient plots under drought and non-drought conditions. This was supported by evidence from three independent measures. First, stomatal conductance was not significantly different in elevated CO₂ versus ambient trees of P. taeda. Calculations of time-integrated c_i/c_a ratios from analysis of foliar δ¹³C showed that these ratios were maintained in foliage under elevated CO₂. Second, soil moisture was not significantly different between ambient and elevated CO₂ plots during drought. Third, pre-dawn and mid-day leaf water potentials were also unaffected by the seasonal CO₂ exposure, as were tissue osmotic potentials and turgor loss points. Together the results strongly support the hypothesis that maturing P. taeda trees have low stomatal responsiveness to elevated CO₂. Elevated CO₂ effects on water relations in loblolly pine-dominated forest ecosystems may be absent or small apart from those mediated by leaf area. Large photosynthetic enhancements in the upper canopy of P. taeda by elevated CO₂ indicate that this maturing forest may have a large carbon sequestration capacity with limiting water supply.     

Effects of Elevated CO2 and Defoliation on Compensatory Growth and Photosynthesis of Seedlings in a Tropical Tree,Copaifera aromatica1

After defoliation by herbivores, some plants exhibit enhanced rates of photosynthesis and growth that enable them to compensate for lost tissue, thus maintaining their fitness relative to competing, undefoliated plants. Our aim was to determine whether compensatory photosynthesis and growth would be altered by increasing concentrations of atmospheric CO₂. Defoliation of developing leaflets on seedlings of a tropical tree, Copaifera aromatica, caused increases in photosynthesis under ambient CO₂, but not under elevated CO₂. An enhancement in the development of buds in the leaf axils followed defoliation at ambient levels of CO₂. In contrast, under elevated CO₂, enhanced development of buds occurred in undefoliated plants with no further enhancement in bud development due to exposure to elevated CO₂. Growth of leaf area after defoliation was increased, particularly under elevated CO₂. Despite this increase, defoliated plants grown under elevated CO₂ were further from compensating for tissue lost during defoliation after 5¹/₂ weeks than those grown under ambient CO₂ concentrations.     

Growth dynamics and population development in an alpine grassland under elevated CO2

Leaf expansion, population dynamics and reproduction under elevated CO₂ were studied for two dominant and four subdominant species in a high alpine grassland (2500 above sea level, Swiss Central Alps). Plots of alpine heath were exposed to 335 l l^-1 and 680 l l^-1 CO₂ in open-top chambers over three growing seasons. Treatments also included natural and moderately improved mineral nutrient supply (40 kg N ha^-1 year^-1 in an NPK fertilizer mix). Seasonal dynamics of leaf expansion, which was studied for the dominant graminoid Carex curvula only, were not affected by elevated CO₂ during two warm seasons or during a cool season. Improved nutrient supply increased both the expansion rate and the duration of leaf growth but elevated CO₂ did not cause any further stimulation. Plant and tiller density (studied in all species) increased under elevated CO₂ in the codominant Leontodon helveticus and the subdominant Trifolium alpinum, remained unchanged in two other minor species Poa alpina and Phyteuma globulariifolium, and decreased in Carex curvula. In Potentilla aurea elevated CO₂ compensated for a natural decline in shoot number. By year 3 the number of fertile shoots in Leontodon and individual seed weight in Carex were slightly increased under elevated CO₂, indicating CO₂ effects on sexual reproduction in these two dominant species. The results suggest that the effects of elevated CO₂ on the population dynamics of the species studied were not general, but species-specific and rather moderate effects. However, the reduction of tiller density in Carex curvula, in contrast to the increases observed in Leontodon helveticus and Trifolium alpinum, indicates that elevated CO₂ may negatively affect the abundance of the species most characteristic of this alpine plant community.     

Interactive effects of low atmospheric CO2 and elevated temperature on growth, photosynthesis and respiration in Phaseolus vulgaris

For most of the past 250 000 years, atmospheric CO₂ has been 30–50% lower than the current level of 360 μmol CO₂ mol^–1 air. Although the effects of CO₂ on plant performance are well recognized, the effects of low CO₂ in combination with abiotic stress remain poorly understood. In this study, a growth chamber experiment using a two-by-two factorial design of CO₂ (380 μmol mol^–1, 200 μmol mol^–1) and temperature (25/20 °C day/night, 36/29 °C) was conducted to evaluate the interactive effects of CO₂ and temperature variation on growth, tissue chemistry and leaf gas exchange of Phaseolus vulgaris. Relative to plants grown at 380 μmol mol^–1 and 25/20 °C, whole plant biomass was 36% less at 380 μmol mol^–1× 36/29 °C, and 37% less at 200 μmol mol^–1× 25/20 °C. Most significantly, growth at 200 μmol mol^–1× 36/29 °C resulted in 77% less biomass relative to plants grown at 380 μmol mol^–1× 25/20 °C. The net CO₂ assimilation rate of leaves grown in 200 μmol mol^–1× 25/20 °C was 40% lower than in leaves from 380 μmol mol^–1× 25/20 °C, but similar to leaves in 200 μmol mol^–1× 36/29 °C. The leaves produced in low CO₂ and high temperature respired at a rate that was double that of leaves from the 380μmol mol^–1× 25/20 °C treatment. Despite this, there was little evidence that leaves at low CO₂ and high temperature were carbohydrate deficient, because soluble sugars, starch and total non-structural carbohydrates of leaves from the 200μmol mol^–1× 36/29 °C treatment were not significantly different in leaves from the 380μmol mol^–1× 25/20 °C treatment. Similarly, there was no significant difference in percentage root carbon, leaf chlorophyll and leaf/root nitrogen between the low CO₂× high temperature treatment and ambient CO₂ controls. Decreased plant growth was correlated with neither leaf gas exchange nor tissue chemistry. Rather, leaf and root growth were the most affected responses, declining in equivalent proportions as total biomass production. Because of this close association, the mechanisms controlling leaf and root growth appear to have the greatest control over the response to heat stress and CO₂ reduction in P. vulgaris.     

Does growth under elevated CO2 moderate photoacclimation in rice?

Acclimation of plant photosynthesis to light irradiance (photoacclimation) involves adjustments in levels of pigments and proteins and larger scale changes in leaf morphology. To investigate the impact of rising atmospheric CO₂ on crop physiology, we hypothesize that elevated CO₂ interacts with photoacclimation in rice (Oryza sativa). Rice was grown under high light (HL: 700 µmol m^?2 s^?1), low light (LL: 200 µmol m^?2 s^?1), ambient CO₂ (400 µl l^?1) and elevated CO₂ (1000 µl l^?1). Leaf six was measured throughout. Obscuring meristem tissue during development did not alter leaf thickness indicating that mature leaves are responsible for sensing light during photoacclimation. Elevated CO₂ raised growth chamber photosynthesis and increased tiller formation at both light levels, while it increased leaf length under LL but not under HL. Elevated CO₂ always resulted in increased leaf growth rate and tiller production. Changes in leaf thickness, leaf area, Rubisco content, stem and leaf starch, sucrose and fructose content were all dominated by irradiance and unaffected by CO₂. However, stomata responded differently; they were significantly smaller in LL grown plants compared to HL but this effect was significantly suppressed under elevated CO₂. Stomatal density was lower under LL, but this required elevated CO₂ and the magnitude was adaxial or abaxial surface‐dependent. We conclude that photoacclimation in rice involves a systemic signal. Furthermore, extra carbohydrate produced under elevated CO₂ is utilized in enhancing leaf and tiller growth and does not enhance or inhibit any feature of photoacclimation with the exception of stomatal morphology.     

Effects of CO(2) Concentration on Rubisco Activity, Amount, and Photosynthesis in Soybean Leaves

Growth at an elevated CO₂ concentration resulted in an enhanced capacity for soybean (Glycine max L. Merr. cv Bragg) leaflet photosynthesis. Plants were grown from seed in outdoor controlled-environment chambers under natural solar irradiance. Photosynthetic rates, measured during the seed filling stage, were up to 150% greater with leaflets grown at 660 compared to 330 microliters of CO₂ per liter when measured across a range of intercellular CO₂ concentrations and irradiance. Soybean plants grown at elevated CO₂ concentrations had heavier pod weights per plant, 44% heavier with 660 compared to 330 microliters of CO₂ per liter grown plants, and also greater specific leaf weights. Ribulose 1,5-bisphosphate carboxylase/oxygenase (rubisco) activity showed no response (mean activity of 96 micromoles of CO₂ per square meter per second expressed on a leaflet area basis) to short-term (~1 hour) exposures to a range of CO₂ concentrations (110-880 microliters per liter), nor was a response of activity (mean activity of 1.01 micromoles of CO₂ per minute per milligram of protein) to growth CO₂ concentration (160-990 microliters per liter) observed. The amount of rubisco protein was constant, as growth CO₂ concentration was varied, and averaged 55% of the total leaflet soluble protein. Although CO₂ is required for activation of rubisco, results indicated that within the range of CO₂ concentrations used (110-990 microliters per liter), rubisco activity in soybean leaflets, in the light, was not regulated by CO₂.     

Selection of soybean plant leaves which yield mesophyll cell isolates with maximal rates of CO2 and NO inf2 sup− photoassimilation

A problem often encountered when assaying mesophyll cell isolates prepared from mature soybean leaves, was that of poor reproducibility in rates of net ¹⁴CO₂ photoassimilation and NO₂ ^– photoreduction. It was known that soybean source leaves repeatedly displayed their most active net CO₂ photoassimilation in the period from attainment of maximal leaf area to approximately two to five days subsequent to that point. Advantage was taken of the fact that when soybean leaflets of each leaf reach their maximal area they also have reached their maximal leaf length from base to tip. This facilitates a more rapid determination of the point in time in which leaflet areas had reached A_max. Soybean plants (Glycine max cv. Williams) were propagated in the growth chamber with a 12 h light-12 h dark cycle, 25C, 65% RH, and 700 microeinsteins per meter squared per second. At 24 d post-emergence, the third leaf (numbered acropetally from the unifoliates) of each plant had just attained maximum leaflet areas (110 cm²) and lengths (13 cm). For this study, leaf mesophyll cells were enzymatically isolated, using commercially prepared pectinase, from leaflet sets of leaves selected from each of the second, third, and fourth leaf positions. Maximal rates of net ¹⁴CO₂ photoassimilation (with 5 mM HCO₃ ^–) for the second, third and fourth leaf (leaflet) isolates were, respectively, 27.0, 57.0, and 41.7 mol ¹⁴CO₂ assimilated per milligram chlorophyll per hour; simultaneously maximal rates of NO _inf2 ^sup– photoreduction (1 mM NO _inf2 ^sup– ) were, respectively, 4.4, 8.1, and 0.0 mol NO _inf2 ^sup– reduced per milligram chlorophyll per hour. These studies made it clear that in order repeatedly to attain reproducible maximal rates of leaf cell isolate net ¹⁴CO₂ photoassimilation and NO _inf2 ^sup– photoreduction, it always was necessary to select the newest, fully expanded leaves (e.g. leaf number 3) for cell isolation. Leaves from several plants only were pooled if they were excised from identically the same node on each of the plants.Abbreviations A_max - maximum leaflet (trifoliolate) area attained during ontogeny - CO₂ - CO₂ gas dissolved in solution - HCO _inf3 ^sup– - bicarbonate - L_max - maximum leaf blade length (midvein) attained during ontogeny - NiRase - chloroplast nitrite reductase (reduced ferredoxin) - NiPR - nitrite photoreduction - PE - post-emergence - Pn - net CO₂ photoassimilation (for leaflets and mesophyll cell isolates) - PPRC - pentose phosphate reductive cycle     

The impact of elevated carbon dioxide on the growth and gas exchange of three C4 species differing in CO2 leak rates

Recent work has suggested that the photosynthetic rate of certain C₄ species can be stimulated by increasing CO₂ concentration, [CO₂], even under optimal water and nutrients. To determine the basis for the observed photosynthetic stimulation, we tested the hypothesis that the CO₂ leak rate from the bundle sheath would be directly related to any observed stimulation in single leaf photosynthesis at double the current [CO₂]. Three C₄ species that differed in the reported degree of bundle sheath leakiness to CO₂, Flaveria trinervia, Panicum miliaceum, and Panicum maximum, were grown for 31–48 days after sowing at a [CO₂] of 350 μl l^?1 (ambient) or 700 μl l^?1 (elevated). Assimilation as a function of increasing [CO₂] at high photosynthetic photon flux density (PPFD, 1 600 μmol m^?2 s^?1) indicated that leaf photosynthesis was not saturated under current ambient [CO₂] for any of the three C₄ species. Assimilation as a function of increasing PPFD also indicated that the response of leaf photosynthesis to elevated [CO₂] was light dependent for all three C₄ species. The stimulation of leaf photosynthesis at elevated [CO₂] was not associated with previously published values of CO₂ leak rates from the bundle sheath, changes in the ratio of activities of PEP-carboxylase to RuBP carboxylase/oxgenase, or any improvement in daytime leaf water potential for the species tested in this experiment. In spite of the simulation of leaf photosynthesis, a significant increase in growth at elevated [CO₂] was only observed for one species, F. trinervia. Results from this study indicate that leaf photosynthetic rates of certain C₄ species can respond directly to increased [CO₂] under optimal growth conditions, but that the stimulation of whole plant growth at elevated carbon dioxide cannot be predicted solely on the response of individual leaves.     

Acclimation of photosynthesis to elevated CO2 in onion (Allium cepa) grown at a range of temperatures

Onion (Allium cepa) was grown in the field within temperature gradient tunnels (providing about ‐2.5°C to +2.5°C from outside temperatures) maintained at either 374 or 532 μmol mol^?1 CO₂. Plant leaf area was determined non‐destructively at 7 day intervals until the time of bulbing in 12 combinations of temperature and CO₂ concentration. Gas exchange was measured in each plot at the time of bulbing, and the carbohydrate content of the leaf (source) and bulb (sink) was determined. Maximum rate of leaf area expansion increased with mean temperature. Leaf area duration and maximum rate of leaf area expansion were not significantly affected by CO₂. The light‐saturated rates of leaf photosynthesis (A_sat) were greater in plants grown at normal than at elevated CO₂ concentrations at the same measurement CO₂ concentration. Acclimation of photosynthesis decreased with an increase in growth temperature, and with an increase in leaf nitrogen content at elevated CO₂. The ratio of intercellular to atmospheric CO₂ (C₁/C₃ ratio) was 7.4% less for plants grown at elevated compared with normal CO₂. A_sat in plants grown at elevated CO₂ was less than in plants grown at normal CO₂ when compared at the same C₁. Hence, acclimation of photosynthesis was due both to stomatal acclimation and to limitations to biochemical CO₂ fixation. Carbohydrate content of the onion bulbs was greater at elevated than at normal CO₂. In contrast, carbohydrate content was less at elevated compared with normal CO₂ in the leaf sections in which CO₂ exchange was measured at the same developmental stage. Therefore, acclimation of photosynthesis in fully expanded onion leaves was detected despite the absence of localised carbohydrate accumulation in these field‐grown crops.     

THE INFLUENCE OF WATER STRESS ON STEM AND LEAF PHOTOSYNTHESIS IN GLYCINE MAX AND SPARTEUM JUNCEUM (LEGUMINOSAE)

Stem and leaf photosynthesis were measured in Glycine max var. essex (soybean) and Sparteum junceum (Spanish broom). The significance of stem photosynthesis to whole plant growth was evaluated by blocking stem photosynthesis with black straw sections. The growth of S. junceum was reduced by 18% when black straws were used in comparison to clear straws. The whole plant growth of G. max was not influenced by blocking the stem carbon contribution. Mean midday leaf photosynthesis was 12 μmol CO₂ m^–2 s^–1 and 17 μmol CO₂ m^–2 s^–1 for G. max and 5. junceum, respectively. Mean midday stem photosynthesis of S. junceum was 6.5 μmol CO₂ m^–2 s^–1; however, positive net photosynthesis did not occur in G. max stems. Water stress caused a proportionally greater decrease in leaf photosynthesis compared to that of stems during diurnal cycles of photosynthesis in S. junceum. As a result the contribution to canopy carbon gain by stem photosynthesis increased from 38% to 48% of the total plant carbon gain under reduced water availability.