Nitrogen nutrition of C3 plants at elevated atmospheric CO2 concentrations

The atmospheric CO₂ concentration has risen from the preindustrial level of approximately 290 μl l⁻¹ to more than 350 μl l⁻¹ in 1993. The current rate of rise is such that concentrations of 420 μl l⁻¹ are expected in the next 20 years. For C₃ plants, higher CO₂ levels favour the photosynthetic carbon reduction cycle over the photorespiratory cycle, resulting in higher rates of carbohydrate production and plant productivity. The change in balance between the two photosynthetic cycles appears to alter nitrogen and carbon metabolism in the leaf, possibly causing decreases in nitrogen concentrations in the leaf. This may result from increases in the concentration of storage carbohydrates of high molecular weight (soluble or insoluble) and/or changes in distribution of protein or other nitrogen containing compounds. Uptake of nitrogen may also be reduced at high CO₂ due to lower transpiration rates. Decreases in foliar nitrogen levels have important implications for production of crops such as wheat, because fertilizer management is often based on leaf chemical analysis, using standards estimated when the CO₂ levels were considerably lower. These standards will need to be re-evaluated as the CO₂ concentration continues to rise. Lower levels of leaf nitrogen will also have implications for the quality of wheat grain produced, because it is likely that less nitrogen would be retranslocated during grain filling.     

Does down‐regulation of photosynthetic capacity by elevated CO2 depend on N supply in Dactylis glomerata?

Dactylis glomerata was grown hydroponically in a controlled environment at ambient (360 μl l⁻¹) or elevated (680 μl l⁻¹) CO₂ and four concentrations of nitrogen (0.15, 0.6, 1.5 and 6.0 m M NO₃⁻), to test the hypothesis that reduction of photosynthetic capacity at elevated [CO₂] is dependent on N availability and mediated by a build-up of non-structural carbohydrates. Photosynthetic capacity of the youngest fully expanded leaf (leaf 5, 2 days after full expansion) was reduced in CO₂-enriched plants at low, but not high N supply and so the stimulation of net photosynthesis by CO₂ enhancement was less at low than at high N supply. CO₂ enrichment resulted in a decrease in ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) content on a leaf area basis at 0.6 and 1.5 m M NO₃⁻, but not at 0.15 and 6.0 m M NO₃⁻, and had no effect on the total N content of the leaf on an area basis. However, decreases in Rubisco content could be primarily accounted for by a decrease in total N content of leaves, independent of [CO₂]. A doubling of the Rubisco content by increasing the N supply beyond 0.6 m M had only a marginal effect on the maximum carboxylation velocity in vivo, suggesting that the fraction of inactive Rubisco increased with increasing N supply. Although CO₂-enriched plants accumulated more non-structural carbohydrates in the leaf, the reduction of photosynthetic capacity at low N supply was not mediated simply by a build-up of carbohydrates. In D . glomerata , the photosynthetic capacity was mainly determined by the total N content of the leaf.     

The role of temperature in determining the stimulation of CO2 assimilation at elevated carbon dioxide concentration in soybean seedlings

Soybean ( Glycine max cv. Clark) was grown at both ambient (ca 350 μmol mol⁻¹) and elevated (ca 700 μmol mol⁻¹) CO₂ concentration at 5 growth temperatures (constant day/night temperatures of 20, 25, 30, 35 and 40°C) for 17–22 days after sowing to determine the interaction between temperature and CO₂ concentration on photosynthesis (measured as A, the rate of CO₂ assimilation per unit leaf area) at both the single leaf and whole plant level. Single leaves of soybean demonstrated increasingly greater stimulation of A at elevated CO₂ as temperature increased from 25 to 35°C (i.e. optimal growth rates). At 40°C, primary leaves failed to develop and plants eventually died. In contrast, for both whole plant A and total biomass production, increasing temperature resulted in less stimulation by elevated CO₂ concentration. For whole plants, increased CO₂ stimulated leaf area more as growth temperature increased. Differences between the response of A to elevated CO₂ for single leaves and whole plants may be related to increased self-shading experienced by whole plants at elevated CO₂ as temperature increased. Results from the present study suggest that self-shading could limit the response of CO₂ assimilation rate and the growth response of soybean plants if temperature and CO₂ increase concurrently, and illustrate that light may be an important consideration in predicting the relative stimulation of photosynthesis by elevated CO₂ at the whole plant level.     

Interaction of enriched CO2 and water stress on the physiology of and biomass production in sweet potato grown in open-top chambers

Abstract. The objective of this study was to investigate the effects of water stress in sweet potato ( Ipomoea batatas L. [Lam] 'Georgia Jet') on biomass production and plant-water relationships in an enriched CO₂ atmosphere. Plants were grown in pots containing sandy loam soil (Typic Paleudult) at two concentrations of elevated CO₂ and two water regimes in open-top field chambers. During the first 12 d of water stress, leaf xylem potentials were higher in plants grown in a CO₂ concentration of 438 and 666 μmol mol⁻¹ than in plants grown at 364 μmol mol⁻¹. The 364 μmol mol⁻¹ CO₂ grown plants had to be rewatered 2 d earlier than the high CO₂-grown plants in response to water stress. For plants grown under water stress, the yield of storage roots and root: shoot ratio were greater at high CO₂ than at 364 μmol mol⁻¹; the increase, however, was not linear with increasing CO₂ concentrations. In well-watered plants, biomass production and storage root yield increased at elevated CO₂, and these were greater as compared to water-stressed plants grown at the same CO₂ concentration.     

CO2 enrichment of soybeans. Effects of leaf/pod ratio

The effect of varying leaf number on response of soybean ( Glycine max (L.) Merr. cv. Fiskeby V) to CO₂ enrichment was studied. Plants were trimmed at pod set to 15 pods and 1 or 3 leaves (15:1 and 5:1 pod/leaf ratio) and placed in 350 or 1000 μl/l CO₂ growth chambers. Photosynthetic rates and dry weights were measured 6 times in all plants at each CO₂ concentration over a period of 39 days. Measured at treatment CO₂ concentration, photosynthetic rates deelined rapidly in enriched plants, but remained higher than those of non-enriched plants. When all plants were measured at the same CO₂ concentration, for most sampling dates, neither growth, CO₂ concentration or pod/leaf ratio significantly affected rates of photosynthesis per unit area of comparable leaves. CO₂ enrichment significantly increased total weights and pod weights in 15:1 but not 5:1 pod/leaf ratio plants. Plants with a 5:1 pod/leaf ratio had significantly higher total and pod weights than 15:1 ratio plants. Both the photosynthesis and dry weight data suggest that plants in the 5:1 ratio enriched treatment were sink-limited, but plants in all other treatments were source limited.     

Growth, dry matter partitioning, and diurnal activities of RuBP carboxylase in citrus seedlings maintained at two levels of CO2

The long-term response of citrus rootstock seedlings to CO₂ enrichment was examined in Carrizo estrange ( Poncirua trifoliata (L.) Raf. x Citrus sinensis (L.) Osbeck] and Swingle citrumelo ( P. trifoliate x C. parodist Macf.]. Plaotlets 14 weeks old were transferred to outdoor controlled-environment chambers and maintained for 5 months from Feb. 14 to July 21. During this period, new growth (cm) of citrange and citrumelo shoots at 660 μl1⁻¹ was 94 and 69% greater, respectively, than at 330 μ1 1⁻¹. Total dry weight of both rootstock shoots had increased by over 100%. Growth of few species is affected this markedly by elevated CO₂ levels.
More carbon was partitioned to above-ground organs in CO₂-enriched citrus seedlings. Stem dry matter per unit length was also 32 and 44% greater in citrange and citrumelo, respectively. Total leaf area was increased by 124% in citrange and 85% in citrumelo due to greater leaf number and size. Variations in overall relative growth rate appeared to be related to the rapid, sequential, flush-type growth in citrus, in which an entire shoot segment with its associated leaves remains an active sink until fully expanded. RuBP carboxylase (EC 4.1.1.39) activity in leaves of recently-expanded flushes was higher in citrumelo plants grown at 660 vs 330 μ1 1⁻¹ CO₂ and changed diurnally for citrange (but not citrumelo) leaves at both CO₂ levels. The results are consistent with the hypothesis that positive long-term effects of CO² enrichment may be greater in species or during growth periods where sink capacity for carbon utilization is high.     

The role of light and CO2 in optimising the conditions for shoot proliferation of Actinidia deliciosa in vitro

Proliferating cultures of Actinidia deliciosa A. Chev., C. F. Liang and A. R. Ferguson cv. Tomuri (♂) were grown under photosynthetic photon flux density (PPFD) rates ranging from 30 to 250 μmol m⁻² s⁻¹ in order to determine certain physiological parameters in vitro: CO₂ evolution, photosynthesis at three CO₂ atmospheric concentrations (330, 1450 and 4500 μl l⁻¹), fresh and dry matter accumulation and proliferation rate.
A proportional response in dry weight, dry/fresh weight ratios and PPFD was found. The proliferation rate increased up to 120 μmol m⁻² s⁻¹ but decreased at higher rates. At the highest PPFD, the CO² released from cultures and accumulated in the vessels reached 200 μl l⁻¹ of; at the lowest rate the CO₂ concentration reached 10500 μl l⁻¹ after 28 days of culture. The photosynthetic rate at 1450 and 4500 μl l⁻¹ of CO₂ was nearly 4 times higher than at the lowest concentration tested.     

Inhibition of photosynthesis and export in geranium grown at two CO2 levels and infected with Xanthomonas campestris pv. Pelargonii

The effects of CO₂ enrichment on growth of Xanthomonas campestris pv. pelargonii and the impact of infection on the photosynthesis and export of attached, intact, 'source' leaves of geranium ( Pelargonium x domesticum, 'Scarlet Orbit Improved' ) are reported. Two experiments were performed, one with plants without flower buds, and another with plants which were flowering. Measurements were made on healthy and diseased leaves at the CO₂ levels (35 Pa or 90 Pa) at which the plants were grown. There were no losses of chlorophyll, or any signs of visible chlorosis or necrosis due to infection. Lower numbers of bacteria were found in leaves at high CO₂, suggesting growth at elevated CO₂ created a less favourable condition in the leaf for bacterial growth. Although high CO₂ lowered the bacterial number in infected leaves, reductions in photosynthesis and export were greater than at ambient CO₂. The capacity of infected source leaves to export photoassimilates at rates observed in the controls was reduced in both light and darkness. In summary, the severity of infection on source leaf function by the bacteria was increased, rather than reduced by CO₂ enrichment, underscoring the need for further assessment of plant diseases and bacterial virulence in plants growing under varying CO₂ levels.     

Photosynthesis, growth and structural characteristics of holm oak resprouts originated from plants grown under elevated CO2

The physiological characteristics of holm oak ( Quercus ilex L.) resprouts originated from plants grown under current CO₂ concentration (350 μl l⁻¹) (A-resprouts) were compared with those of resprouts originated from plants grown under elevated CO₂ (750 μl l⁻¹) (E-resprouts). At their respective CO₂ growth concentration, no differences were observed in photosynthesis and chlorophyll fluorescence parameters between the two kinds of resprout. E-resprouts appeared earlier and showed lower stomatal conductance, higher water-use efficiency and increased growth (higher leaf, stem and root biomass and increased height). Analyses of leaf chemical composition showed the effect of elevated [CO₂] on structural polysaccharide (higher cellulose content), but no accumulation of total non-structural carbohydrate on area or dry weight basis was seen. Four months after appearance, downregulation of photosynthesis and electron transport components was observed in E-resprouts: lower photosynthetic capacity, photosystem II quantum efficiency, photochemical quenching of fluorescence and relative electron transport rate. Reduction in ribulose-1,5-bisphosphate carboxylase/oxygenase (RuBisCo) activity, deduced from the maximum carboxylation velocity of RuBisCo, accounts for the observed acclimation. Increased susceptibility of photosynthetic apparatus to increasing irradiance was detected in E-resprouts.     

The impact of elevated ozone and carbon dioxide on young Acer saccharum seedlings

The effects of high O₃ (200 nl l⁻¹ during the light period) and high CO₂ (650 μl l⁻¹ CO₂, 24 h a day) alone and in combination were studied on 45-day-old sugar maple ( Acer saccharum Marsh.) seedlings for 61 days in growth chambers. After 2 months of treatment under the environmental conditions of the experiment, sugar maple seedlings did not show a marked response to the elevated CO₂ treatment: the effect of high CO₂ on biomass was only detected in the leaves which developed during the treatment, and assimilation rate was not increased. Under high O₃ at ambient CO₂, assimilation rate at days 41 and 55 and Rubisco content at day 61 decreased in the first pair of leaves; total biomass was reduced by 43%. In these seedlings large increases (more than 2-fold) in glucose 6-phosphate dehydrogenase (G6PDH, EC 1.1.1.49) activity and in anaplerotic CO₂ fixation by phosphoenolpyruvate carboxylase (PEPC, EC 4.1.1.31) were observed, suggesting that an enhanced reducing power and carbon skeleton production was needed for detoxification and repair of oxidative damage. Under high O₃ at elevated CO₂, a stimulation of net CO₂ assimilation was observed after 41 days but was no longer observed at day 55. However, at day 61, the total biomass was only reduced by 21% and stimulation of G6PDH and PEPC was less pronounced than under high O₃ at ambient CO₂. This suggests that high CO₂ concentration protects, to some extent, against O₃ by providing additional carbon and energy through increased net assimilation.     

Effect of elevated CO2 on the stomatal distribution and leaf physiology of Alnus glutinosa

Variation in stomatal development and physiology of mature leaves from Alnus glutinosa plants grown under reference (current ambient, 360 μmol mol⁻¹ CO₂) and double ambient (720 μmol mol⁻¹ CO₂) carbon dioxide (CO₂) mole fractions is assessed in terms of relative plant growth, stomatal characters (i.e. stomatal index and density) and leaf photosynthetic characters. This is the first study to consider the effects of elevated CO₂ concentration on the distribution of stomata and epidermal cells across the whole leaf and to try to ascertain the cause of intraleaf variation. In general, a doubling of the atmospheric CO₂ concentration enhanced plant growth and significantly increased stomatal index. However, there was no significant change in relative stomatal density. Under elevated CO₂ concentration there was a significant decrease in stomatal conductance and an increase in assimilation rate. However, no significant differences were found for the maximum rate of carboxylation ( V _cmax) and the light saturated rate of electron transport ( J _max) between the control and elevated CO₂ treatment.     

Acclimation of Arabidopsis thaliana to long-term CO2 enrichment and nitrogen supply is basically a matter of growth rate adjustment

The long-term response of Arabidopsis thaliana to increasing CO₂ was evaluated in plants grown in 800 μl l⁻¹ CO₂ from sowing and maintained, in hydroponics, on three nitrogen supplies: "low,""medium" and "high." The global response to high CO₂ and N-supply was evaluated by measuring growth parameters in parallel with photosynthetic activity, leaf carbohydrates, ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) messenger RNA and protein, stomatal conductance (g_s) and density. CO₂ enrichment was found to stimulate biomass production, whatever the N-supply. This stimulation was transient on low N-supply and persisted throughout the whole vegetative growth only in high N-supply. Acclimation on low N–high CO₂ was not associated with carbohydrate accumulation or with a strong reduction in Rubisco amount or activity. At high N-supply, growth stimulation by high CO₂ was mainly because of the acceleration of leaf production and expansion while other parameters such as specific leaf area, root/shoot ratio and g_s appeared to be correlated with total leaf area. Our results thus suggest that, in strictly controlled and stable growing conditions, acclimation of A. thaliana to long-term CO₂ enrichment is mostly controlled by growth rate adjustment.     

Light dependent activation of CO2 assimilation and the ratio of Rubisco activase to Rubisco during leaf aging of rice (Oryza sativa)

The effects of the ratio of Rubisco activase to Rubisco (activase/Rubisco ratio) on light dependent activation of CO₂ assimilation were investigated during leaf aging of rice. Changes of photosynthetic CO₂ gas exchange rates in relation to step increases of light intensity from two photon flux densities of 60 µmol m⁻² s⁻¹ (low initial PFD) and 500 µmol m⁻² s⁻¹ (high initial PFD) to saturated PFD of 1 800 µmol m⁻² s⁻¹ were measured. These photosynthetic activation processes were considered to be limited by the Rubisco activation rate when analyzed by the relaxation method. The relaxation time of low initial PFD gradually declined from 3 to 33 days after leaf emergence and showed high and negative correlation to the activase/Rubisco ratio. The initial rate of Rubisco activation under low initial PFD linearly correlated to the amounts of Rubisco activase, whereas these were almost constant from 3 to 23 days after leaf emergence. But these correlations could not be recognized in the case of high initial PFD. Moreover, the relaxation times were more sensitive to intercellular CO₂ concentration (C_i) under high initial PFD than under low initial PFD, especially, at C_i below 300 µl l⁻¹. These results suggest the involvement of the activase/Rubisco ratio in the photosynthetic activation under relatively low initial PFD, and the limitation of photosynthetic activation under relatively high initial PFD by Rubisco carbamylation during leaf aging of rice.     

Physiological effects of long-term exposure to low and moderate concentrations of atmospheric NH3 on poplar leaves

Abstract. Poplar shoots ( Populus euramericana L.) obtained from cuttings were exposed for 6 or 8 weeks to NH₃ concentrations of 50 and 100 μgm⁻³ or filtered air in fumigation chambers. After this exposure the rates of NH₃ uptake, transpiration, CO₂ assimilation and respiration of leaves were measured using a leaf chamber. During the long-term exposure also modulated chlorophyll fluorescence measurements were carried out to obtain information about the photosynthetic performance of individual leaves. Both fluorescence and leaf chamber measurements showed a higher photosynthetic activity of leaves exposed to 100 μg NH₃ m⁻³. These leaves showed also a larger leaf conductance and a larger uptake rate of NH₃ than leaves exposed to 50 μg m⁻³ NH₃ or filtered air. The long-term NH₃ exposure did not induce an internal resistance against NH₃ transport in the leaf, nor did it affect the leaf cuticle. So, not only at a short time exposure, but also at a long-term exposure NH₃ uptake into leaves can be calculated from data on the boundary layer and stomatal resistance for H₂O and ambient NH₃-concentration. Furthermore, the NH₃ exposure had no effect on the relation between CO₂-assimilation and stomatal conductance, indicating that NH₃ in concentrations up to 100 μg m⁻³ has no direct effect on stomatal behaviour; for example, by affecting the guard or contiguous cells of the stomata.     

Mycorrhiza formation and elevated CO2 both increase the capacity for sucrose synthesis in source leaves of spruce and aspen

The effects of mycorrhiza formation in combination with elevated CO₂ concentrations on carbon metabolism of Norway spruce ( Picea abies ) seedlings and aspen ( Populus tremula × Populus tremuloides ) plantlets were analysed. Plants were inoculated for 6 wk with the ectomycorrhizal fungi Amanita muscaria and Paxillus involutus (aspen only) in an axenic Petri-dish culture at 350 and 700 μl l⁻¹ CO₂ partial pressure. After mycorrhiza formation, a stimulation of net assimilation rate was accompanied by decreased activities of sucrose synthase, an increased activation state of sucrose-phosphate synthase, decreased fructose-2,6-bisphosphate and starch, and slightly elevated glucose-6-phosphate contents in source leaves of both host species, independent of CO₂ concentration. Exposure to elevated CO₂ generally resulted in higher net assimilation rates, increased starch as well as decreased fructose-2,6-bisphosphate (aspen only) content in source leaves of both mycorrhizal and nonmycorrhizal plants. Our data indicate only slightly improved carbon utilization by mycorrhizal plants at elevated CO₂. They demonstrate however, that both factors which modulate the sink-source properties of plants increase the capacity for sucrose synthesis in source leaves mainly by allosteric enzyme regulation.     

Intraspecific variation in the response of rice (Oryza sativa) to increased CO2– photosynthetic, biomass and reproductive characteristics

Two rice ( Oryza sativa L.) cultivars of contrasting morphologies, IR-36 and Fujiyama-5, were exposed to ambient (360 μl l⁻¹) and ambient plus 300 μl l⁻¹ CO₂ from time of emergence until ca 50% grain fill at the Duke University Phytotron, Durham, North Carolina. Exposure to increased CO₂ resulted in about a 50% increase in the photosynthetic rate for both cultivars and photosynthetic enhancement was still evident after 3 months of exposure to a high CO₂ environment. The photosynthetic response at 5% CO₂ and the response of CO₂ assimilation (A) to internal CO₂ (C_i) suggest a reallocation of biochemical resources from RuBP carboxylation to RuBP regeneration. Increases in total plant biomass at elevated CO₂ were approximately the same in both cultivars, although differences in allocation patterns were noted in root/shoot ratio. Differences in reproductive characteristics were also observed between cultivars at an elevated CO₂ environment with a significant increase in harvest index for IR-36 but not for Fujiyama-5. Changes in carbon allocation in reproduction between these two cultivars suggest that lines of rice could be identified that would maximize reproductive output in a future high CO₂ environment.     

Interaction of elevated CO2 and O3 on growth, photosynthesis and respiration of three perennial species grown in low and high nitrogen

Seedlings of three species native to central North America, a C₃ tree, Populus tremuloides Michx., a C₃ grass, Agropyron smithii Rybd., and a C₄ grass, Bouteloua curtipendula Michx., were grown in all eight combinations of two levels each of CO₂, O₃ and nitrogen (N) for 58 days in a controlled environment. Treatment levels consisted of 360 or 674 μmol mol^-1 CO₂, 3 or 92 nmol mol^-1 O₃, and 0.5 or 6.0 m M N. In situ photosynthesis and relative growth rate (RGR) and its determinants were obtained at each of three sequential harvests, and leaf dark respiration was measured at the second and third harvests. In all three species, plants grown in high N had significantly greater whole-plant mass, RGR and photosynthesis than plants grown in low N. Within a N treatment, elevated CO₂ did not significantly enhance any of these parameters nor did it affect leaf respiration. However, plants of all three species grown in elevated CO₂ had lower stomatal conductance compared to ambient CO₂-exposed plants. Seedlings of P. tremuloides (in both N treatments) and B. curtipendula (in high N) had significant ozone-induced reductions in whole-plant mass and RGR in ambient but not under elevated CO_2. This negative O₃ impact on RGR in ambient CO₂ was related to increased leaf dark respiration, decreased photosynthesis and/or decreased leaf area ratio, none of which were noted in high O₃ treatments in the elevated CO₂ environment. In contrast, A. smithii was marginally negatively affected by high O_3.     

Effects of manganese toxicity on photosynthesis of white birch (Betula platyphylla var. japonica) seedlings

The effects of manganese (Mn) toxicity on photosynthesis in white birch ( Betula platyphylla var. japonica ) leaves were examined by the measurement of gas exchange and chlorophyll fluorescence in hydroponically cultured plants. The net photosynthetic rate at saturating light and ambient CO₂ (C_a) of 35 Pa decreased with increasing leaf Mn concentrations. The carboxylation efficiency, derived from the difference in CO₂ assimilation rate at intercellular CO₂ pressures attained at C_a of 13 Pa and O Pa, decreased with greater leaf Mn accumulation. Net photosynthetic rate at saturating light and saturating CO₂ (5%) also declined with leaf Mn accumulation while the maximum quantum yield of O₂ evolution at saturating CO₂ was not affected. The maximum efficiency of PSII photochemistry (F_v/F_m) was little affected by Mn accumulation in white birch leaves over a wide range of leaf Mn concentrations (2–17 mg g₋₁ dry weight). When measured in the steady state of photosynthesis under ambient air at 430 μmol quanta m₋₂ s₋₁, the levels of photochemical quenching (qP) and the excitation capture efficiency of open PSII (F'^v/F'^m) declined with Mn accumulation in leaves. The present results suggest that excess Mn in leaves affects the activities of the CO² reduction cycle rather than the potential efficiency of photochemistry, leading to increases in Q_A reduction state and thermal energy dissipation, and a decrease in quantum yield of PSII in the steady state.     

In situ responses to elevated CO2 in tropical forest understorey plants

1. Plants growing in deep shade and high temperature, such as in the understorey of humid tropical forests, have been predicted to be particularly sensitive to rising atmospheric CO₂. We tested this hypothesis in five species whose microhabitat quantum flux density (QFD) was documented as a covariable. After 7 (tree seedlings of Tachigalia versicolor and Beilschmiedia pendula ) and 18 months (shrubs Piper cordulatum and Psychotria limonensis, and grass Pharus latifolius ) of elevated CO₂ treatment ( c. 700 μl litre^–1) under mean QFD of less than 11 μmol m^–2 s^–1, all species produced more biomass (25–76%) under elevated CO₂.
2. Total plant biomass tended to increase with microhabitat QFD (daytime means varying from 5 to 11μmol m^–2 s^–1) but the relative stimulation by elevated CO₂ was higher at low QFD except in Pharus .
3. Non-structural carbohydrate concentrations in leaves increased significantly in Pharus (+ 27%) and Tachigalia (+ 40%).
4. The data support the hypothesis that tropical plants growing near the photosynthetic light compensation point are responsive to elevated CO₂. An improved plant carbon balance in deep shade is likely to influence understorey plant recruitment and competition as atmospheric CO₂ continues to rise.     

Do tomatoes on the plant behave as climacteric fruits?

I considered the possibility that changes in fruit photosynthesis obscure the occurrence of the climacteric rise in respiration in tomato fruits attached to the plant. Internal CO₂ and ethylene concentrations in tomatoes ( Lycopersicon esculentum Mill. cv. OH 7814) were analyzed after direct sampling through polyethylene tubes implanted in the external pericarp. Fruits which were shaded with aluminium foil contained up to 60 ml 1⁻¹ CO₂, until the internal ethylene concentration exceeded 1 μl l⁻¹, when CO₂ concentration declined to below 40 ml l⁻¹; the CO₂ concentration in fruits exposed to light only occasionally exceeded 40 ml 1⁻¹. The internal CO₂ concentration of detached fruits first declined and then increased along with ethylene concentration, as expected for the climacteric. Detached green fruits under continuous low photosynthetic photon flux density (100 μmol m⁻² s⁻¹) contained almost no internal CO₂ and produced no CO₂. Changes in photosynthesis and an associated CO₂-generating system in green fruits are thought to obscure the climacteric rise in tomato fruits developing on the plant.