Photosynthetic Stimulation Research Articles

Canopy position affects photosynthetic adjustments to long-term elevated CO2 concentration (FACE) in aging needles in a mature Pinus taeda forest.

Few studies have examined the effects of elevated CO2 concentration ([CO2]) on the physiology of intact forest canopies, despite the need to understand how leaf-level responses can be aggregated to assess effects on whole-canopy functioning. We examined the long-term effects of elevated [CO2] (ambient + 200 ppm CO2) on two age classes of needles in the upper and lower canopy of Pinus taeda L. during the second through sixth year of exposure to elevated [CO2] in free-air (free-air CO2 enrichment (FACE)) in North Carolina, USA. Strong photosynthetic enhancement in response to elevated [CO2] (e.g., +60% across age classes and canopy locations) was observed across the years. This stimulation was 33% greater for current-year needles than for 1-year-old needles in the fifth and sixth years of treatment. Although photosynthetic stimulation in response to elevated [CO2] was maintained through the sixth year of exposure, we found evidence of concurrent down-regulation of Rubisco and electron transport capacity in the upper-canopy sunlit leaves. The lower canopy showed no evidence of down-regulation. The upper canopy down-regulated carboxylation capacity (Vcmax) and electron transport capacity (Jmax) by about 17-20% in 1-year-old needles; however, this response was significant across sampling years only for Jmax in 1-year-old needles (P < 0.02). A reduction in leaf photosynthetic capacity in aging conifer needles at the canopy top could have important consequences for canopy carbon balance and global carbon sinks because 1-year-old sunlit needles contribute a major proportion of the annual carbon balance of these conifers. Our finding of a significant interaction between canopy position and CO2 treatment on the biochemical capacity for CO2 assimilation suggests that it is important to take canopy position and needle aging into account because morphologically and physiologically distinct leaves could respond differently to elevated [CO2].

Is stimulation of leaf photosynthesis by elevated carbon dioxide concentration maintained in the long term? A test with Lolium perenne grown for 10 years at two nitrogen fertilization levels under Free Air CO2Enrichment (FACE)

ABSTRACTPhotosynthesis is commonly stimulated in grasslands with experimental increases in atmospheric CO2 concentration ([CO2]), a physiological response that could significantly alter the future carbon cycle if it persists in the long term. Yet an acclimation of photosynthetic capacity suggested by theoretical models and short‐term experiments could completely remove this effect of CO2. Perennial ryegrass (Lolium perenne L. cv. Bastion) was grown under an elevated [CO2] of 600 µmol mol−1 for 10 years using Free Air CO2Enrichment (FACE), with two contrasting nitrogen levels and abrupt changes in the source : sink ratio following periodic harvests. More than 3000 measurements characterized the response of leaf photosynthesis and stomatal conductance to elevated [CO2] across each growing season for the duration of the experiment. Over the 10 years as a whole, growth at elevated [CO2] resulted in a 43% higher rate of light‐saturated leaf photosynthesis and a 36% increase in daily integral of leaf CO2 uptake. Photosynthetic stimulation was maintained despite a 30% decrease in stomatal conductance and significant decreases in both the apparent, maximum carboxylation velocity (Vc,max) and the maximum rate of electron transport (Jmax). Immediately prior to the periodic (every 4–8 weeks) cuts of the L. perenne stands, Vc,max and Jmax, were significantly lower in elevated than in ambient [CO2] in the low‐nitrogen treatment. This difference was smaller after the cut, suggesting a dependence upon the balance between the sources and sinks for carbon. In contrast with theoretical expectations and the results of shorter duration experiments, the present results provide no significant change in photosynthetic stimulation across a 10‐year period, nor greater acclimation in Vc,max and Jmax in the later years in either nitrogen treatment.

Soil moisture effects determine CO2 responses of grassland species.

It has been suggested that positive biomass responses of grassland to elevated CO2 result from moisture savings in the soil as opposed to direct photosynthetic stimulation. In order to test this hypothesis for calcareous grassland we subjected experimental communities consisting of two important graminoid components of such grasslands (Carex flacca and Bromus erectus) on natural substrate to a fully factorial treatment of ambient (360ppm) and elevated (600ppm) CO2 concentration and four irrigation regimes (9mm, 18mm, 27mm and 36mmweek-1). Biomass stimulation under elevated CO2 was higher the lower the irrigation rate was. Superimposed on the effects of irrigation on soil moisture, elevated CO2-induced higher soil water contents in all irrigation treatments via reduced plant water consumption (on average one-third lower stomatal conductance). This led to eight different soil moisture regimes instead of the intended four. When growth parameters were plotted against the effective soil water content rather than irrigation treatment, the "pure" CO2 effect on total biomass and other traits became much smaller and completely disappeared for biomass per tiller, leaf area per ground area, leaf mass fraction (LMF) and root mass fraction (RMF). We conclude that the CO2 response observed in this graminoid system consisted of a small primary CO2 effect and a large secondary, CO2-induced, soil moisture effect. Thus, it is difficult to use responses to CO2 from experiments in which CO2-induced soil moisture savings occur to predict CO2 effects as long as future soil moisture regimes are not defined. We suggest that direct and indirect (moisture driven) CO2 effects should be strictly separated, which requires data to be tested against soil moisture.

Effect of elevated [CO2] and varying nutrient application rates on physiology and biomass accumulation of Sitka spruce (Picea sitchensis)

Sitka spruce (Picea sitchensis (Bong.) Carr.) seedlings were supplied with solutions containing nitrogen (N) at 0.1 x or 2 x the optimum rate (low-N and high-N supply, respectively) and grown either outside in a control plot or inside open-top chambers and exposed to ambient (355 &mgr;mol mol(-1)) or elevated (700 &mgr;mol mol(-1)) CO(2) concentration ([CO(2)]). Gas exchange measurements, chlorophyll determinations and nutrient analysis were made on current-year (< 1-year-old) shoots of the upper whorl after the seedlings had been growing in the [CO(2)] treatments for 17 months and the nutrient treatments for 6 months. Total seedling biomass and biomass allocation were assessed at the end of the experiment. Nutrient treatment had a significant effect on the light response curves, irrespective of [CO(2)] or chamber treatment; seedlings supplied with high-N rates had higher net photosynthetic rates than seedlings supplied with low-N rates. The degree of photosynthetic stimulation in response to elevated [CO(2)] was larger in seedlings receiving high-N rates than in seedlings receiving low-N rates. Light-saturated net photosynthesis of seedlings grown and measured in elevated [CO(2)] was 26% higher than that of seedlings grown and measured in ambient [CO(2)]. There was no significant effect of [CO(2)] or chamber treatment on the CO(2) response curves of seedlings receiving High-N supply rates. In contrast, analysis of the CO(2) response curves of seedlings receiving Low-N supply rates showed acclimation to elevated [CO(2)]. Both maximum rate of carboxylation (V(cmax)) and maximum electron transport capacity (J(max)) were lower and J(max)/V(cmax) higher in seedlings in the elevated [CO(2)] treatment. There was no effect of elevated [CO(2)] on stomatal conductance, although it was highly dependent on foliar [N], ranging from ~60 mmol m(-2) s(-1) at ~1.5 g N m(-2) to 200 mmol m(-2) s(-1) at ~5 g N m(-2). In the high-N and low-N treatments, foliar N concentration was 10 and 28% lower in seedlings grown in elevated [CO(2)] than in seedlings grown in ambient [CO(2)], respectively. There was no [CO(2)] effect on foliar phosphorus concentration ([P]). Chlorophyll concentration increased with increasing N supply in all treatments. There was no significant effect of elevated [CO(2)] on specific leaf area. Chlorophyll concentration expressed either on an area or dry mass basis for a given foliar [N] was higher in seedlings grown in elevated [CO(2)] than in seedings grown in ambient [CO(2)]. Elevated [CO(2)] increased total biomass accumulation by 37% in seedlings in the high-N treatment but had no effect in seedlings in the low-N treatment. There was a proportionally bigger allocation of biomass to roots of seedlings in the elevated [CO(2)] + low-N supply rate treatment compared with seedlings in other treatments. This resulted in a reduction in aboveground biomass compared with corresponding seedlings grown in ambient [CO(2)].

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.

The Influence of Root Zone Temperature on Photosynthetic Acclimation to Elevated Carbon Dioxide Concentrations

Soybean (Glycine max‘Clark’) was grown from germination to 21 d after sowing (DAS) at ambient (∼360 μmol mol−1) or elevated (∼720 μmol mol−1) carbon dioxide (CO2) at either one of two soil temperatures, 25 or 30 °C to determine the influence of root zone temperature on root growth and photosynthetic stimulation at ambient and elevated concentrations of carbon dioxide. Although the photosynthetic rate became less stimulate over time, a significant stimulation of whole plant photosynthesis and plant dry weight was observed at the elevated CO2concentration during the experimental period irrespective of soil temperature. At neither carbon dioxide concentration did the warmer soil temperature (30 °C) stimulate whole plant growth compared to a soil temperature of 25 °C, but it did increase root growth relative to shoot (top) growth with a subsequent increase in root/shoot ratio. Increasing soil temperature at either carbon dioxide concentration also significantly stimulated whole plant photosynthetic rate. However, the degree of stimulation was reduced with time irrespective of carbon dioxide concentration so that at 21 DAS no difference in photosynthesis between ambient and elevated soil temperatures was observed. Data from this experiment indicate that for soybean, a higher soil temperature stimulates root/shoot ratio and enhances photosynthetic response to elevated carbon dioxide in the short-term (i.e. days), but increasing root/shoot ratios does not provide a satisfactory explanation of long-term stimulation of photosynthesis at elevated levels of carbon dioxide.

Inhibition of pteridine biosynthesis eliminates blue-light dependent stimulation of red-light saturated photosynthesis in Laminaria saccharina (L.) Lamouroux

Blue-light stimulation of red light-saturated photosynthetic oxygen evolution in Laminaria saccharina (L.) Lamouroux could be abolished within 5 days by incubation in a solution of 2,4-diamino-6-hydroxypyrimidine (DAHP), an inhibitor of GTP-cyclohydrolase I (E.C.3.5.4.16) activity. Photosynthesis in red light was not detectably affected. GTP-cyclohydrolase I, which catalyses the first step in the biosynthetic pathway of pteridines, was shown to be active in Laminaria. Under conditions that lead to complete inhibition of the photosynthetic stimulation, DAHP reduced the content of the pteridines in the tissue considerably. The amount of pterin was about 14%, that of biopterin was about 45% and that of an unidentified pteridine was about 27% of those of the controls. By contrast, the total concentration of flavins (FAD + FMN − riboflavin) was not significantly affected. The results suggest that pterins may be involved in the response of photosynthesis to blue light, possibly participating in photoreception.

Influence of increasing carbon dioxide concentration on the photosynthetic and growth stimulation of selected C4 crops and weeds

Plants of six weedy species (Amaranthus retroflexus, Echinochloa crus-galli, Panicum dichotomiflorum, Setaria faberi, Setaria viridis, Sorghum halapense) and 4 crop species (Amaranthus hypochondriacus, Saccharum officinarum, Sorghum bicolor and Zea mays) possessing the C4type of photosynthesis were grown at ambient (38 Pa) and elevated (69 Pa) carbon dioxide during early development (i.e. up to 60 days after sowing) to determine: (a) if plants possessing the C4photosynthetic pathway could respond photosynthetically or in biomass production to future increases in global carbon dioxide and (b) whether differences exist between weeds and crops in the degree of response. Based on observations in the response of photosynthesis (measured as A, CO2assimilation rate) to the growth CO2condition as well as to a range of internal CO2(Ci) concentrations, eight of ten C4species showed a significant increase in photosynthesis. The largest and smallest increases observed were for A. retroflexus (+30%) and Z. mays (+5%), respectively. Weed species (+19%) showed approximately twice the degree of photosynthetic stimulation as that of crop species (+10%) at the higher CO2concentration. Elevated carbon dioxide also resulted in significant increases in whole plant biomass for four C4weeds (A. retroflexus, E. crus-galli, P. dichotomiflorum, S. viridis) relative to the ambient CO2condition. Leaf water potentials for three selected species (A. retroflexus, A. hypochondriacus, Z. mays) indicated that differences in photosynthetic stimulation were not due solely to improved leaf water status. Data from this study indicate that C4plants may respond directly to increasing CO2concentration, and in the case of some C4weeds (e.g. A. retroflexus) may show photosynthetic increases similar to those published for C3species.

The interaction of high temperature and elevated CO2 on photosynthetic acclimation of single leaves of rice in situ

Rice (Oryza sativa L. cv. IR72) was grown at three different CO2 concentrations (ambient, ambient + 200 μmol mol−1, ambient + 300 μmol mol−1) at two different growth temperatures (ambient, ambient + 4°C) from sowing to maturity to determine longterm photosynthetic acclimation to elevated CO2 with and without increasing temperature. Single leaves of rice showed a cooperative enhancement of photosynthetic rate with elevated CO2 and temperature during tillering, relative to the elevated CO2 condition alone. However, after flowering, the degree of photosynthetic stimulation by elevated CO2 was reduced for the ambient + 4°C treatment. This increasing insensitivity to CO2 appeared to be accompanied by a reduction in ribulose‐1.5‐bisphosphate carboxylase/oxygenase (Rubisco) activity and/or concentration as evidenced by the reduction in the assimilation (A) to internal CO2 (C1) response curve. The reproductive response (e.g. percent filled grains, panicle weight) was reduced at the higher growth temperature and presumably reflects a greater increase in floral sterility. Results indicate that while CO2 and temperature could act synergistically at the biochemical level, the direct effect of temperature on floral development with a subsequent reduction in carbon utilization may change sink strength so as to limit photosynthetic stimulation by elevated CO2 concentration.

Reversibility of photosynthetic acclimation of swiss chard and sugarbeet grown at elevated concentrations of CO2

Although leaf photosynthesis and plant growth are initially stimulated by elevated CO2 concentrations, increasing insensitivity to CO2 (acclimation) is a frequent occurrence. In order to examine the acclimation process, we studied photosynthesis and whole plant development in swiss chard (Beta vulgaris L. Koch ssp. ciela) and sugarbeet (Beta vulgaris L. ssp. vulgaris) grown at either ambient or twice ambient concentrations of CO2. In an initial controlled environment study, photosynthetic acclimation to elevated CO2 levels was observed in both subspecies 24 days after sowing (DAS) but was not observed at 42 and 49 DAS for sugarbeet or at 49 DAS for swiss chard. Although sugarbeet and swiss chard differed in root size and morphology, this was not a factor in the onset of photosynthetic acclimation. The reversal of photosynthetic acclimation that was observed in older plants grown at elevated CO2, concentrations was associated with a rapid increase in root development (i.e. increased root: shoot [R/S] ratio), increased sucrose levels in sinks (roots) and no differences in total soluble leaf protein of either subspecies relative to the ambient CO2 condition. In a second set of experiments, swiss chard and sugarbeet were grown in outdoor Plexiglass chambers at different times of the year (i.e. summer and early fall). Average 24‐h temperature was 30.7 and 19.4°C for the summer and fall plantings, respectively. In agreement with the controlled environment study, lack of photosynthetic acclimation, determined from the response of photosynthesic rate to internal CO2 concentration, was correlated with increased root biomass and sucrose concentration relative to the ambient condition. However, photo‐synthetic acclimation was observed depending on the season, i.e. summer (swiss chard) or fall (sugarbeet), suggesting that acclimation was affected by environmental factors, such as temperature. Data from both experiments suggest that continued long‐term photosynthetic stimulation may be dependent upon the ability of increased CO2 to stimulate new sink development which would allow full utilization of the additional carbon made available in a high CO2 environment.

Atmospheric CO 2 enrichment: Kinetics of chlorophyll a fluorescence and photosynthetic CO 2 uptake in individual, attached cotton leaves

Chl fluorescence and gas exchange of attached cotton leaves ( Gossypium hirsutum L.) were measured in ambient air and in a highly CO 2-enriched atmosphere (4000 μl 1 −1 CO 2; photosynthetic saturation). In the short term (hours to one day), net CO 2 uptake approximately doubled in all leaves examined. Photochemical (q P) and nonphotochemical (q NP) quenching of chlorophyll fluorescence, and calculated linear photosynthetic electron flow, did not change significantly when CO 2 rose from 250 to 4000 μl 1 −1 CO 2. These results show that high CO 2 concentration did not inhibit photosynthesis in any leaf. In contrast, the long-term response of leaves to atmospheric CO 2-enrichment was variable. Some leaves sustained the initial high level of photosynthetic stimulation for more than a week while in others photosynthetic CO 2-uptake declined more or less. These leaves turned yellowish-green although chlorophyll content declined little. Variance in the degree of leaf yellowing was also encountered in experiments with clover when sets of plants were CO 2-enriched. Gas exchange and chl fluorescence results suggest that yellowing of cotton leaves in high CO 2 was not equivalent to ‘natural’ senescence although some chlorophyll fluorescence parameters changed similarly. During extended high CO 2 treatment the level of q NP increased notably in the yellowing leaves. The high levels of q NP and relaxation kinetics of chl fluorescence quenching recorded upon darkening demonstrate that thylakoid energization increased during the decline of photosynthetic CO 2 uptake in high CO 2. This shows that the photosynthetic decline was not caused by decreasing thylakoid energization because of physical damage by oversized starch grains. Calculated photosynthetic electron flow declined little suggesting that CO 2 at ribulosebisphosphate carboxylase-oxygenase fell and thus photorespiration rose. With regard to growth limitation in high CO 2 concentration, these results support the concept that high CO 2 concentration tends to induce low inorganic phosphate concentrations (Morin et al. Plant Physiol. 99, 89–95, 1992; Duchein et al. J. Exp. Bot. 44, 17–22, 1993) which can limit chloroplast ATP synthase and thus increase thylakoid energization. It is proposed that the different responses of individual leaves to atmospheric CO 2 enrichment reflects variety among leaves in the phosphate status or in the capacity for Pi-recycling (assimilate utilization).

Nitrogen nutrition and wheat growth in relation to absorbed solar radiation

During two growing seasons (1980–1982), the influence of nitrogen fertilizer on the growth of winter and spring wheat was assessed. An analysis was devised which related grain yield to harvest index, mean daily incident irradiance, the duration of the growing season, fraction of radiation absorbed by the foliage ( f), the efficiency of conversion of absorbed radiation into biomass (ε) during vegetative growth ( ε m ), and the influence of the reproductive/senescent phase on ε. Levels of nitrogen fertilizer application did not influence the proportion of photons to total solar radiation transmitted ( τ q , τ t ) and a general relation τ q = τ t 1.35 closely described all observations. Nitrogen also failed to influence the light extinction factor and differences in f, which increased with increasing nitrogen availability, were caused by variations in canopy size. Fertilizer nitrogen accelerated the rate of canopy expansion, giving a greater maximum canopy size and hence improved f. Nitrogen also slightly increased the growth period. Rates of canopy senescence were related to the maximum area of canopy achieved, thus the rate and duration of canopy expansion had a dominant influence on senescence. Conversion efficiencies for moles of quanta into moles of anhydrous hexose monomer were calculated and their values closely followed trends in ε for absorbed energy flux into biomass. For both winter and spring wheat, successive increments of applied nitrogen consistently increased ε m to a maximum level. During reproductive growth, a complex interaction between photosynthetic stimulation in response to actively developing grains and intracellular disassembly via canopy senescence, will dictate the influence of the reproductive phase on ε. Consequently, post-anthesis trends in ε are difficult to interpret. Rates of crop growth from stem elongation to near maturity were constant, while the quantity of irradiance absorbed increased. An ontogenetic decline in ε was inferred during this phase and thus the response of biomass accumulation to absorbed radiation may be curvilinear. However, a linear approximation proved useful in producing a seasonal constant.

The development of whole cell biosensors for on‐line screening of herbicide pollution of surface waters

AbstractThe use of mediator assisted amperometric whole‐cell biosensors, monitoring microbial photosynthetic electron transfer, to screen for herbicides in intake protection by the water industry is described. Cells harvested from axenic cultures are loaded onto bacteriological filters and held against a working carbon electrode polarised against the silver chloride reference. Redox mediators are reduced by the microorganisms and then reoxidised at the working electrode resulting in a flow of current. For on‐line applications the sensors are controlled by a programmable electrochemical analyser (Artek) and housed in specially designed perspex flow cells which incorporate light emitting diodes (LEDs) for photosynthetic stimulation. A range of cyanobacteria and algae have been tested, but biosensors incorporating unicellular cyanobacteria have proven the most successful. Results obtained using the cyanobacterium Synechococcus show detection levels of less than 200 ppb with response times of less than ten minutes for selected herbicides. Sensor stability in the absence of pollutant poisoning has been found to be greatly influenced by the choice of mediator. Membrane penetrating mediators such as ‐benzoquinone reduce sensor life to about 24 hours, while a sensor life of up to six days has been achieved with ferricyanide.

Role of O2 and Mitochondrial Respiration in a Photosynthetic Stimulation of Oat Protoplast Acidification of a Surrounding Medium

Some photosynthetically stimulated acidification of the medium by oat (Avena sativa L. cv Garry) leaf protoplasts required respiration. The requisite respiration (a) had a low apparent affinity for O(2), (b) was blocked by cyanide plus salicylhydroxamic acid, (c) characterized protoplasts and mitochondria isolated from protoplasts, (d) could be induced in leaf segments, and (e) appeared to result from an inhibition of mitochondrial respiration that included the cytochrome pathway.Carbon monoxide and cyanide prevented acidification of weakly photosynthesizing suspensions. Salicylhydroxamic acid had no effect on acidification, indicating a specific dependence upon cyanide-sensitive respiration. Photosynthesis stimulated acidification through stable products, and exogenously supplied O(2) stimulated acidification. The acidification response to O(2) was additive to the response to photosynthesis at subsaturating levels of light, indicating a common mode of action. Oligomycin prevented stimulation of acidification by low levels of photosynthetic activity; this stimulation appeared to be due to O(2)-induced increases in mitochondrial energy production. Oligomycin only partially inhibited stimulation of acidification by higher levels of light; this stimulation appeared to be partially dependent upon photophosphorylation. Therefore, oligomycin-sensitive acidification of the medium appeared to reflect changes in mitochondrial energy production in photosynthesizing protoplasts.