Gas exchange and dry matter allocation responses to elevation of atmospheric CO(2) concentration in seedlings of three tree species

Photosynthetic rates of 13-month-old Pinus radiata D. Don, Nothofagus fusca (Hook f.) ?rst. and Pseudotsuga menziesii (Mirb.) Franco seedlings grown and measured at elevated atmospheric concentrations of CO(2) (~620 microl l(-1)) were 32 to 55% greater than those of seedlings grown and measured at ambient (~310 microl l(-1)) concentrations of CO(2). Seedlings grown in ambient and elevated concentrations of CO(2) had similar rates of photosynthesis when measured at ~620 microl l(-1) CO(2), but when measured at ~310 microl l(-1) CO(2), the P. radiata and N. fusca seedlings which were grown at elevated CO(2) had lower rates of photosynthesis than the seedlings grown at an ambient concentration of CO(2). Stomatal conductances in general were lower when measured at ~620 microl l(-1) CO(2) than at ~310 microl l(-1) CO(2). Stomatal conductances declined in all species grown at both CO(2) concentrations when the leaf-air water vapor concentration gradient (DeltaW) was increased from 10 to 20 mmol H(2)O mol(-1) air. The percent enhancement in photosynthesis for P. radiata and P. menziesii at elevated CO(2) was greater at 20 mmol than at 10 mmol DeltaW, suggesting that elevated CO(2) may moderate the effects of atmospheric water stress. Dry matter allocation patterns were not significantly different for plants grown in ambient or high CO(2) air.     

Chlorophyll and carotenoid concentrations in two varieties of Pinus ponderosa seedlings subjected to long-term elevated carbon dioxide

Two varieties of ponderosa pine (Pinus ponderosa Dougl. var. scopulorum (Rocky Mountain variety) and P. ponderosa var. ponderosa (Sierran variety)) seedlings were subjected to elevated atmospheric CO(2) for two and a half years. The CO(2) concentrations were ambient, ambient + 75 microl l(-1), ambient + 150 microl l(-1) and ambient + 300 microl l(-1), or approximately 350, 425, 500 and 650 microl l(-1) CO(2). After one and a half years of exposure to elevated CO(2) and until the end of the study, seedlings of both varieties showed symptoms of stress including mottling, mid-needle abscission and early senescence. In both varieties, exposure to CO(2) concentrations greater than ambient + 75 microl l(-1) resulted in lower chlorophyll a, chlorophyll b and carotenoid concentrations. At elevated CO(2) concentrations, the concentrations of pigments in needles of the Sierran variety were lower than those in the Rocky Mountain variety. Also, at elevated CO(2) concentrations, the pigment concentrations in the 1-year-old needles of both P. ponderosa varieties were lower than those in current-season needles.     

Carbon dioxide enrichment improves growth, water relations and survival of droughted honey mesquite (Prosopis glandulosa) seedlings

Low water availability reduces the establishment of the invasive shrub Prosopis on some grasslands. Water deficit survival and traits that may contribute to the postponement or tolerance of plant dehydration were measured on seedlings of P. glandulosa Torr. var. glandulosa (honey mesquite) grown at CO(2) concentrations of 370 (ambient), 710, and 1050 micro mol mol(-1). Because elevated CO(2) decreases stomatal conductance, the number of seedlings per container in the elevated CO(2) treatments was increased to ensure that soil water content was depleted at similar rates in all treatments. Seedlings grown at elevated CO(2) had a greater root biomass and a higher ratio of lateral root to total root biomass than those grown at ambient CO(2) concentration; however, these seedlings also shed more leaves and retained smaller leaves. These changes, together with a reduced transpiration/leaf area ratio at elevated CO(2), may have contributed to a slight increase in xylem pressure potentials of seedlings in the 1050 micro mol mol(-1) CO(2) treatment during the first 37 days of growth (0.26 to 0.40 MPa). Osmotic potential was not affected by CO(2) treatment. Increasing the CO(2) concentration to 710 and 1050 micro mol mol(-1) more than doubled the percentage survival of seedlings from which water was withheld for 65 days. Carbon dioxide enrichment significantly increased survival from 0% to about 40% among seedlings that experienced the lowest soil water content. By increasing seedling survival of drought, rising atmospheric CO(2) concentration may increase abundance of P. glandulosa on grasslands where low water availability limits its establishment.     

Effect of elevated [CO(2)] 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)].     

Analysis of the relationships among O(3) uptake, conductance, and photosynthesis in needles of Pinus ponderosa

We studied the effects of O(3) uptake on conductance (g(wv)) and photosynthesis (A) in needles of ponderosa pine (Pinus ponderosa Laws.) seedlings exposed for 70 days to one of three O(3) regimes-Low-O(3) (0.1 micro mol mol(-1) daily peak), High-O(3) (0.2 micro mol mol(-1) daily peak), and Low/High-O(3) (alternating 2 days Low-O(3) and 2 days High-O(3)). Seedlings exposed to charcoal-filtered air served as controls. Total O(3) exposures, expressed as ppm-h (the sum of the average hourly concentration in ppm ( micro mol mol(-1)) over the exposure period), were 77, 135, 105 and 4 for the Low-O(3), High-O(3), Low/High-O(3) and control treatments, respectively. Conductance (g(wv)) declined to about 60% of the value in control seedlings by Day 6 in seedlings in the High-O(3) treatment and by Day 37 in seedlings in the Low/High-O(3) treatment, but g(wv) did not decline at all in seedlings in the Low-O(3) treatment. At the end of the 70-day experiment, cumulative O(3) uptake, calculated from measured g(wv) values and assuming an internal O(3) concentration of zero, was 12.2, 13.5, and 14.7 mmol m(-2) for seedlings in the Low-O(3), Low/High-O(3), and High-O(3) treatments, respectively; however, O(3) uptake was reduced by 0, 24, and 36%, respectively, from that expected if there had been no decline in g(wv). With increasing total O(3) exposure, A declined, but the reduction was not strictly cumulative, i.e., A measured on Days 49 and 70 was similar for a given treatment even though both total O(3) exposure and uptake had increased. At the end of the experiment, A at near saturating CO(2) (1000 micro mol mol(-1)) and saturating photosynthetic photon flux density was reduced by about 25, 40 and 50% in seedlings in the Low-O(3), Low/High-O(3) and High-O(3) treatments, respectively, compared to the control seedlings. The ratio of internal to external CO(2) concentrations, an indicator of relative change in stomatal limitation of A, did not change over time and did not differ among treatments, suggesting that A and g(wv) decreased in parallel. After 40-60 days without O(3), A of seedlings in all O(3) treatments was not significantly different. Our data indicate that O(3)-induced stomatal closure was a result of reduced A and that decreased g(wv) reduced O(3) uptake to a rate that needles of ponderosa pine could tolerate without exhibiting further reductions in gas exchange capacity.     

Effect of elevated carbon dioxide concentration and root restriction on net photosynthesis, water relations and foliar carbohydrate status of loblolly pine seedlings

To determine the effects of CO(2)-enriched air and root restriction on photosynthetic capacity, we measured net photosynthetic rates of 1-year-old loblolly pine seedlings grown in 0.6-, 3.8- or 18.9-liter pots in ambient (360 micro mol mol(-1)) or 2x ambient CO(2) (720 micro mol mol(-1)) concentration for 23 weeks. We also measured needle carbohydrate concentration and water relations to determine whether feedback inhibition or water stress was responsible for any decreases in net photosynthesis. Across all treatments, carbon dioxide enrichment increased net photosynthesis by approximately 60 to 70%. Net photosynthetic rates of seedlings in the smallest pots decreased over time with the reduction occurring first in the ambient CO(2) treatment and then in the 2x ambient CO(2) treatment. Needle starch concentrations of seedlings grown in the smallest pots were two to three times greater in the 2x ambient CO(2) treatment than in the ambient CO(2) treatment, but decreased net photosynthesis was not associated with increased starch or sugar concentrations. The reduction in net photosynthesis of seedlings in small pots was correlated with decreased needle water potentials, indicating that seedlings in the small pots had restricted root systems and were unable to supply sufficient water to the shoots. We conclude that the decrease in net photosynthesis of seedlings in small pots was not the result of CO(2) enrichment or an accumulation of carbohydrates causing feedback inhibition, but was caused by water stress.     

Effect of elevated CO2 on monoterpene emission of young Quercus ilex trees and its relation to structural and ecophysiological parameters

We investigated growth, leaf monoterpene emission, gas exchange, leaf structure and leaf chemical composition of 1-year-old Quercus ilex L. seedlings grown in ambient (350 microl l(-1)) and elevated (700 microl l(-1)) CO2 concentrations ([CO2]). Monoterpene emission and gas exchange were determined at constant temperature and irradiance (25 degrees C and 1000 micromol m(-2) s(-1) of photosynthetically active radiation) at an assay [CO2] of 350 or 700 microl l(-1). Measurements were made on intact shoots after the end of the growing season between mid-October and mid-February. On average, plants grown in elevated [CO2] had significantly increased foliage biomass (about 50%). Leaves in the elevated [CO2] treatment were significantly thicker and had significantly higher concentrations of cellulose and lignin and significantly lower concentrations of nitrogen and minerals than leaves in the ambient [CO2] treatment. Leaf dry matter density and leaf concentrations of starch, soluble sugars, lipids and hemi-cellulose were not significantly affected by growth in elevated [CO2]. Monoterpene emissions of seedlings were significantly increased by elevated [CO2] but were insensitive to short-term changes in assay [CO2]. On average, plants grown in elevated [CO2] had 1.8-fold higher monoterpene emissions irrespective of the assay [CO2]. Conversely, assay [CO2] rapidly affected photosynthetic rate, but there was no apparent long-term acclimation of photosynthesis to growth in elevated [CO2]. Regardless of growth [CO2], photosynthetic rates of all plants almost doubled when the assay [CO2] was switched from 350 to 700 microl l(-1). At the same assay [CO2], mean photosynthetic rates of seedlings in the two growth CO2 treatments were similar. The percentage of assimilated carbon lost as monoterpenes was not significantly altered by CO2 enrichment. Leaf emission rates were correlated with leaf thickness, leaf concentrations of cellulose, lignin and nitrogen, and total plant leaf area. In all plants, monoterpene emissions strongly declined during the winter independently of CO2 treatment. The results are discussed in the context of the acquisition and allocation of resources by Q. ilex seedlings and evaluated in terms of emission predictions.     

Carbon exchange rates, chlorophyll content, and carbohydrate status of two forest tree species exposed to carbon dioxide enrichment

Seedlings of yellow-poplar (Liriodendron tulipifera L.) and white oak (Quercus alba L.) were exposed continuously to one of three CO(2) concentrations in open-top chambers under field conditions and evaluated after 24 weeks with respect to carbon exchange rates (CER), chlorophyll (Chl) content, and diurnal carbohydrate status. Increasing the CO(2) concentration from ambient to +150 or +300 microl l(-1) stimulated CER of yellow-poplar and white oak seedlings by 60 and over 35%, respectively, compared to ambient-grown seedlings. The increases in CER were not associated with a significant change in stomatal conductance and occurred despite a reduction in the amounts of Chl and accessory pigments in the leaves of plants grown in CO(2)-enriched air. Total Chl contents of yellow-poplar and white oak seedlings grown at +300 microl l(-1) were reduced by 27 and over 55%, respectively, compared with ambient-grown seedlings. Yellow-poplar and white oak seedlings grown at +300 microl l(-1) contained 72 and 67% more morning starch, respectively, than did ambient-grown plants. In contrast, yellow-poplar and white oak seedlings grown at +300 microl l(-1) contained 17 and 27% less evening sucrose, respectively, than did plants grown at ambient CO(2) concentration. Diurnal starch accumulation and the subsequent depletion of sucrose contributed to a pronounced increase in the starch/sucrose ratio of plants grown in CO(2)-enriched air. All seedlings exhibited a substantial reduction in dark respiration as CO(2) concentration increased, but the significance of this increase to the carbohydrate status and carbon economy of plants grown in CO(2)-enriched air remains unclear.     

Responses of foliar gas exchange to long-term elevated CO(2) concentrations in mature loblolly pine trees

Branches of field-grown mature loblolly pine (Pinus taeda L.) trees were exposed for 2 years (1992 and 1993) to ambient or elevated CO(2) concentrations (ambient + 165 micro mol mol(-1) or ambient + 330 micro mol mol(-1) CO(2)). Exposure to elevated CO(2) concentrations enhanced rates of net photosynthesis (P(n)) by 53-111% compared to P(n) of foliage exposed to ambient CO(2). At the same CO(2) measurement concentration, the ratio of intercellular to atmospheric CO(2) concentration (C(i)/C(a)) and stomatal conductance to water vapor did not differ among foliage grown in an ambient or enriched CO(2) concentration. Analysis of the relationship between P(n) and C(i) indicated no significant change in carboxylation efficiency of ribulose-1,5-bisphosphate carboxylase/oxygenase during growth in elevated CO(2) concentrations. Based on estimates derived from P(n)/C(i) curves, there were no apparent treatment differences in dark respiration, CO(2) compensation point or P(n) at the mean C(i). In 1992, foliage in the three CO(2) treatments yielded similar estimates of CO(2)-saturated P(n) (P(max)), whereas in 1993, estimates of P(max) were higher for branches grown in elevated CO(2) than in ambient CO(2). We conclude that field-grown loblolly pine trees do not exhibit downward acclimation of leaf-level photosynthesis in their long-term response to elevated CO(2) concentrations.     

Effects of season,needle age and elevated atmospheric CO(2) on photosynthesis in Scots pine (Pinus sylvestris)

Five-year-old Scots pine (Pinus sylvestris L.) seedlings were grown in open-top chambers at ambient and elevated (ambient + 400 &mgr;mol mol(-1)) CO(2) concentrations. Net photosynthesis (A), specific leaf area (SLA) and concentrations of nitrogen (N), carbon (C), soluble sugars, starch and chlorophyll were measured in current-year and 1-year-old needles during the second year of CO(2) enrichment. The elevated CO(2) treatment stimulated photosynthetic rates when measured at the growth CO(2) concentration, but decreased photosynthetic capacity compared with the ambient CO(2) treatment. Acclimation to elevated CO(2) involved decreases in carboxylation efficiency and RuBP regeneration capacity. Compared with the ambient CO(2) treatment, elevated CO(2) reduced light-saturated photosynthesis (when measured at 350 &mgr;mol mol(-1) in both treatments) by 18 and 23% (averaged over the growing season) in current-year and 1-year-old needles, respectively. We observed significant interactive effects of CO(2) treatment, needle age and time during the growing season on photosynthesis. Large seasonal variations in photosynthetic parameters were attributed to changes in needle chemistry, needle structure and feedbacks governed by whole-plant growth dynamics. Down-regulation of photosynthesis was probably a result of reduced N concentration on an area basis, although a downward shift in the relationship between photosynthetic parameters and N was also observed.     

Long-term elevation of atmospheric CO(2) concentration and the carbon exchange rates of saplings of Pinus taeda L. and Liquidambar styraciflua L

The relationship between carbon exchange rate (CER) and internal CO(2) concentration was measured in leaves of saplings of Liquidambar styraciflua L. (sweetgum) and Pinus taeda L. (loblolly pine) grown from seed for more than 14 months at atmospheric CO(2) concentrations of either 350 or 500 microl l(-1). An elevated concentration of CO(2) during growth reduced CER at any given internal CO(2) concentration in sweetgum, but not in loblolly pine. Stomatal limitation of CER showed little response to concentration of CO(2) during measurement, but was higher in both species when grown at 500 than at 350 microl l(-1) CO(2). The net effect of a long-term increase in CO(2) concentration from 350 to 500 microl l(-1) was an increase in CER of loblolly pine, but a slight decrease in CER of sweetgum. It is suggested that the depression of CER in sweetgum resulted from a reduction in the activity of ribulose-1,5-bisphosphate carboxylase-oxygenase.     

Higher growth temperatures decreased net carbon assimilation and biomass accumulation of northern red oak seedlings near the southern limit of the species range

If an increase in temperature will limit the growth of a species, it will be in the warmest portion of the species distribution. Therefore, in this study we examined the effects of elevated temperature on net carbon assimilation and biomass production of northern red oak (Quercus rubra L.) seedlings grown near the southern limit of the species distribution. Seedlings were grown in chambers in elevated CO(2) (700 μmol mol(-1)) at three temperature conditions, ambient (tracking diurnal and seasonal variation in outdoor temperature), ambient +3 °C and ambient +6 °C, which produced mean growing season temperatures of 23, 26 and 29 °C, respectively. A group of seedlings was also grown in ambient [CO(2)] and ambient temperature as a check of the growth response to elevated [CO(2)]. Net photosynthesis and leaf respiration, photosynthetic capacity (V(cmax), J(max) and triose phosphate utilization (TPU)) and chlorophyll fluorescence, as well as seedling height, diameter and biomass, were measured during one growing season. Higher growth temperatures reduced net photosynthesis, increased respiration and reduced height, diameter and biomass production. Maximum net photosynthesis at saturating [CO(2)] and maximum rate of electron transport (J(max)) were lowest throughout the growing season in seedlings grown in the highest temperature regime. These parameters were also lower in June, but not in July or September, in seedlings grown at +3 °C above ambient, compared with those grown in ambient temperature, indicating no impairment of photosynthetic capacity with a moderate increase in air temperature. An unusual and potentially important observation was that foliar respiration did not acclimate to growth temperature, resulting in substantially higher leaf respiration at the higher growth temperatures. Lower net carbon assimilation was correlated with lower growth at higher temperatures. Total biomass at the end of the growing season decreased in direct proportion to the increase in growth temperature, declining by 6% per 1 °C increase in mean growing season temperature. Our observations suggest that increases in air temperature above current ambient conditions will be detrimental to Q. rubra seedlings growing near the southern limit of the species range.     

Leaf physiology, production, water use, and nitrogen dynamics of the grassland invader Acacia smallii at elevated CO(2) concentrations

Invasion by woody legumes can alter hydrology, nutrient accumulation and cycling, and carbon sequestration on grasslands. The rate and magnitude of these changes are likely to be sensitive to the effects of atmospheric CO(2) enrichment on growth and water and nitrogen dynamics of leguminous shrubs. To assess potential effects of increased atmospheric CO(2) concentrations on plant growth and acquisition and utilization of water and nitrogen, seedlings of Acacia smallii Isely (huisache) were grown for 13 months at CO(2) concentrations of 385 (ambient), 690, and 980 micro mol mol(-1). Seedlings grown at elevated CO(2) concentrations exhibited parallel declines in leaf N concentration and photosynthetic capacity; however, at the highest CO(2) concentration, biomass production increased more than 2.5-fold as a result of increased leaf photosynthetic rates, leaf area, and N(2) fixation. Measurements of leaf gas exchange and aboveground biomass production and soil water balance indicated that water use efficiency increased in proportion to the increase in atmospheric CO(2) concentration. The effects on transpiration of an accompanying decline in leaf conductance were offset by an increase in leaf area, and total water loss was similar across CO(2) treatments. Plants grown at elevated CO(2) fixed three to four times as much N as plants grown at ambient CO(2) concentration. The increase in N(2) fixation resulted from an increase in fixation per unit of nodule mass in the 690 micro mol mol(-1) CO(2) treatment and from a large increase in the number and mass of nodules in plants in the 980 micro mol mol(-1) CO(2) treatment. Increased symbiotic N(2) fixation by woody invaders in response to CO(2) enrichment may result in increased N deposition in litterfall, and thus increased productivity on many grasslands.     

Effects of light availability on leaf gas exchange and expansion in lychee (Litchi chinensis)

Effects of photosynthetic photon flux density (PPFD) on leaf gas exchange of lychee (Litchi chinensis Sonn.) were studied in field-grown "Kwai May Pink" and "Salathiel" orchard trees and young potted "Kwai May Pink" plants during summer in subtropical Queensland (27 degrees S). Variations in PPFD were achieved by shading the trees or plants 1 h before measurement at 0800 h. In a second experiment, potted seedlings of "Kwai May Pink" were grown in a heated greenhouse in 20% of full sun (equivalent to maximum noon PPFD of 200 micromol m(-2)xs(-1)) and their growth over three flush cycles was compared with seedlings grown in full sun (1080 micromol m(-2)xs(-1)). Young potted plants of "Kwai May Pink" were also grown outdoors in artificial shade that provided 20, 40, 70 or 100% of full sun (equivalent to maximum PPFDs of 500, 900, 1400 and 2000 micromol m(-2)xs(-1)) and measured for shoot extension and leaf area development over one flush cycle. Net CO2 assimilation increased asymptotically in response to increasing PPFD in both orchard trees and young potted plants. Maximum rates of CO2 assimilation (11.9 +/- 0.5 versus 6.3 +/- 0.2 micromol CO2 m(-2) s(-1)), dark respiration (1.7 +/- 0.3 versus 0.6 +/- 0.2 micromol CO2 m(-2) s(-1)), quantum yield (0.042 +/- 0.005 versus 0.027 +/- 0.003 mol CO2 mol(-1)) and light saturation point (1155 versus 959 micromol m(-2) s(-1)) were higher in orchard trees than in young potted plants. In potted seedlings grown in a heated greenhouse, shoots and leaves exposed to full sun expanded in a sigmoidal pattern to 69 +/- 12 mm and 497 +/- 105 cm(2) for each flush, compared with 27 +/- 7 mm and 189 +/- 88 cm(2) in shaded seedlings. Shaded seedlings were smaller and had higher shoot:root ratios (3.7 versus 3.1) than seedlings grown in full sun. In the potted plants grown outdoors in 20, 40, 70 or 100% of full sun, final leaf area per shoot was 44 +/- 1, 143 +/- 3, 251 +/- 7 and 362 +/- 8 cm(2), respectively. Shoots were also shorter in plants grown in shade than in plants grown in full sun (66 +/- 5 mm versus 101 +/- 2 mm). Photosynthesis in individual leaves of lychee appeared to be saturated at about half full sun, whereas maximum leaf expansion occurred at higher PPFDs. We conclude that lychee plants can persist as seedlings on the forest floor, but require high PPFDs for optimum growth.     

Relationship between photosynthesis and leaf nitrogen concentration in ambient and elevated [CO2] in white birch seedlings

To study the effects of elevated CO2 concentration ([CO2]) on relationships between nitrogen (N) nutrition and foliar gas exchange parameters, white birch (Betula papyrifera Marsh.) seedlings were exposed to one of five N-supply regimes (10, 80, 150, 220, 290 mg N l(-1)) in either ambient [CO2] (360 micromol mol(-1)) or elevated [CO2] (720 micromol mol(-1)) in environment-controlled greenhouses. Foliar gas exchange and chlorophyll fluorescence were measured after 60 and 80 days of treatment. Photosynthesis showed a substantial down-regulation (up to 57%) in response to elevated [CO2] and the magnitude of the down-regulation generally decreased exponentially with increasing leaf N concentration. When measured at the growth [CO2], elevated [CO2] increased the overall rate of photosynthesis (P(n)) and instantaneous water-use efficiency (IWUE) by up to 69 and 236%, respectively, but decreased transpiration (E) and stomatal conductance (g(s)) in all N treatments. However, the degree of stimulation of photosynthesis by elevated [CO2] decreased as photosynthetic down-regulation increased from 60 days to 80 days of treatment. Elevated [CO2] significantly increased total photosynthetic electron transport in all N treatments at 60 days of treatment, but the effect was insignificant after 80 days of treatment. Both P(n) and IWUE generally increased with increasing leaf N concentration except at very high leaf N concentrations, where both P(n) and IWUE declined. The relationships of P(n) and IWUE with leaf N concentration were modeled with both a linear regression and a second-order polynomial function. Elevated [CO2] significantly and substantially increased the slope of the linear regression for IWUE, but had no significant effect on the slope for P(n). The optimal leaf N concentration for P(n) and IWUE derived from the polynomial function did not differ between the CO2 treatments when leaf N was expressed on a leaf area basis. However, the mass-based optimal leaf N concentration for P(n) was much lower in seedlings in elevated [CO2] than in ambient [CO2] (31.88 versus 37.00 mg g(-1)). Elevated [CO2] generally decreased mass-based leaf N concentration but had no significant effect on area-based leaf N concentration; however, maximum N concentration per unit leaf area was greater in elevated [CO2] than in ambient [CO2] (1.913 versus 1.547 g N m(-2)).     

Photosynthetic traits around budbreak in pre-existing needles of Sakhalin spruce (Picea glehnii) seedlings grown under elevated CO2 concentration assessed by chlorophyll fluorescence measurements

To assess the effects of elevated CO(2) concentration ([CO(2)]) on the photosynthetic properties around spring budbreak, we monitored the total leaf sugar and starch content, and chlorophyll fluorescence in 1-year-old needles of Sakhalin spruce (Picea glehnii Masters) seedlings in relation to the timing of budbreak, grown in a phytotron under natural daylight at two [CO(2)] levels (ambient: 360?μmol mol(-1) and elevated: 720?μmol mol(-1)). Budbreak was accelerated by elevated [CO(2)] accompanied with earlier temporal declines in the quantum yield of PSII electron transport (Φ(PSII)) and photochemical quenching (q(L)). Plants grown under elevated [CO(2)] showed pre-budbreak leaf starch content twice as high with no significant difference in Φ(PSII) from ambient-CO(2)-grown plants when compared at the same measurement [CO(2)], i.e., 360 or 720?μmol mol(-1), suggesting that the enhanced pre-budbreak leaf starch accumulation might not cause down-regulation of photosynthesis in pre-existing needles under elevated [CO(2)]. Conversely, lower excitation pressure adjusted for the efficiency of PSII photochemistry ((1?-?q(P)) F(v)'/F(m)') was observed in plants grown under elevated [CO(2)] around budbreak when compared at their growth [CO(2)] (i.e., comparing (1?-?q(P)) F(v)'/F(m)' measured at 720?μmol mol(-1) in elevated-CO(2)-grown plants with that at 360?μmol mol(-1) in ambient-CO(2)-grown plants), which suggests lower rate of photoinactivation of PSII in the elevated-CO(2)-grown plants around spring budbreak. The degree of photoinhibition, as indicated by the overnight-dark-adapted F(v)/F(m), however, showed no difference between CO(2) treatments, thereby suggesting that photoprotection during the daytime or the repair of PSII at night was sufficient to alleviate differences in the rate of photoinactivation.     

Elevated atmospheric CO2 concentration alters the effect of phosphate supply on growth of Japanese red pine (Pinus densiflora) seedlings

We demonstrated that the inorganic phosphate (P(i)) requirement for growth of Japanese red pine (Pinus densiflora Sieb. & Zucc.) seedlings is increased by elevated CO(2) concentration ([CO(2)]) and that responses of the ectomycorrhizal fungus Pisolithus tinctorius (Pers.) Coker & Couch to P(i) supply are also altered. To investigate the growth response of non-mycorrhizal seedlings to P(i) supply in elevated [CO(2)], non-mycorrhizal seedlings were grown for 73 days in ambient or elevated [CO(2)] (350 or 700 micromol mol(-1)) with nutrient solutions containing one of seven phosphate concentrations (0, 0.02, 0.04, 0.06, 0.08, 0.10 and 0.20 mM). In ambient [CO(2)], the growth response to P(i) was saturated at about 0.1 mM P(i), whereas in elevated [CO(2)], the growth response to P(i) supply did not saturate, even at the highest P(i) supply (0.2 mM), indicating that the P(i) requirement is higher in elevated [CO(2)] than in ambient [CO(2)]. The increased requirement was due mainly to an altered shoot growth response to P(i) supply. The enhanced P(i) requirement in elevated [CO(2)] was not associated with a change in photosynthetic response to P(i) or a change in leaf phosphorus (P) status. We investigated the effect of P(i) supply (0.04, 0.08 and 0.20 mM) on the ectomycorrhizal fungus P. tinctorius in mycorrhizal seedlings grown in ambient or elevated [CO(2)]. Root ergosterol concentration (an indicator of fungal biomass) decreased with increasing P(i) supply in ambient [CO(2)], but the decrease was far less in elevated [CO(2)]. In ambient [CO(2)] the ratio of extramatrical mycelium to root biomass decreased with increasing P(i) supply but did not change in elevated [CO(2)]. We conclude that, because elevated [CO(2)] increased the P(i) requirement for shoot growth, the significance of the ectomycorrhizal association was also increased in elevated [CO(2)].     

Responses of loblolly pine seedlings to elevated CO(2) and fluctuating water supply

Osmotic adjustment of loblolly pine (Pinus taeda L.) seedlings to fluctuating water supply in elevated CO(2) was investigated. Seedlings were grown in controlled-environment chambers in either 350 or 700 micro l l(-1) CO(2) with weekly watering for four months, after which they were either watered weekly (well-watered treatment) or every two weeks (water-stress treatment) for 59 days. Osmotic adjustment was assessed by pressure-volume analysis of shoots and by analysis of soluble carbohydrates and free amino acids in roots during the last drying cycle. In well-watered seedlings, elevated CO(2) increased the concentration of soluble sugars in roots by 68%. Water stress reduced the soluble sugar concentration in roots of seedling growing in ambient CO(2) to 26% of that in roots of well-watered seedlings. Elevated CO(2) mitigated the water stress-induced decrease in the concentration of soluble sugars in roots. However, this was probably due, in part, to carbohydrate loading during the first four months when all seedlings were grown in the presence of a high water supply, rather than to osmotic adjustment to water stress. Water stress caused a doubling in the concentration of free primary amino acids in roots, whereas elevated CO(2) reduced primary amino acid and nitrogen concentrations to 32 and 74%, respectively, of those in roots of seedlings grown in ambient CO(2). There was no indication of large-scale osmotic adjustment to water stress or that elevated CO(2) enhanced osmotic adjustment in loblolly pine.     

Diurnal and seasonal changes in the impact of CO(2) enrichment on assimilation, stomatal conductance and growth in a long-term study of Mangifera indica in the wet-dry tropics of Australia

We studied assimilation, stomatal conductance and growth of Mangifera indica L. saplings during long-term exposure to a CO(2)-enriched atmosphere in the seasonally wet-dry tropics of northern Australia. Grafted saplings of M. indica were planted in the ground in four air-conditioned, sunlit, plastic-covered chambers and exposed to CO(2) at the ambient or an elevated (700 micro mol mol(-1)) concentration for 28 months. Light-saturating assimilation (A(max)), stomatal conductance (g(s)), apparent quantum yield (phi), biomass and leaf area were measured periodically. After 28 months, the CO(2) treatments were changed in all four chambers from ambient to the elevated concentration or vice versa, and A(max) and g(s) were remeasured during a two-week exposure to the new regime. Throughout the 28-month period of exposure, A(max) and apparent quantum yield of leaves in the elevated CO(2) treatment were enhanced, whereas stomatal conductance and stomatal density of leaves were reduced. The relative impacts of atmospheric CO(2) enrichment on assimilation and stomatal conductance were significantly larger in the dry season than in the wet season. Total tree biomass was substantially increased in response to atmospheric CO(2) enrichment throughout the experimental period, but total canopy area did not differ between CO(2) treatments at either the first or the last harvest. During the two-week period following the change in CO(2) concentration, A(max) of plants grown in ambient air but measured in CO(2)-enriched air was significantly larger than that of trees grown and measured in CO(2)-enriched air. There was no difference in A(max) between trees grown and measured in ambient air compared to trees grown in CO(2)-enriched air but measured in ambient air. No evidence of down-regulation of assimilation in response to atmospheric CO(2) enrichment was observed when rates of assimilation were compared at a common intercellular CO(2) concentration. Reduced stomatal conductance in response to atmospheric CO(2) enrichment was attributed to a decline in both stomatal aperture and stomatal density.     

Interactive effects of elevated CO2 concentration and nitrogen supply on partitioning of newly fixed 13C and 15N between shoot and roots of pedunculate oak seedlings (Quercus robur)

Pedunculate oak (Quercus robur L.) seedlings were grown for 3 or 4 months (second- and third-flush stages) in greenhouses at two atmospheric CO2 concentrations ([CO2]) (350 or 700 micromol mol(-1)) and two nitrogen fertilization regimes (6.1 or 0.61 mmol N l(-1) nutrient solution). Combined effects of [CO2] and nitrogen fertilization on partitioning of newly acquired carbon (C) and nitrogen (N) were assessed by dual 13C and 15N short-term labeling of seedlings at the second- or third-flush stage of development. In the low-N treatment, root growth, but not shoot growth, was stimulated by elevated [CO2], with the result that shoot/root biomass ratio declined. At the second-flush stage, overall seedling biomass growth was increased (13%) by elevated [CO2] regardless of N fertilization. At the third-flush stage, elevated [CO2] increased growth sharply (139%) in the high-N but not the low-N treatment. Root/shoot biomass ratios were threefold higher in the low-N treatment relative to the high-N treatment. At the second-flush stage, leaf area was 45-51% greater in the high-N treatment than in the low-N treatment. At the-third flush stage, there was a positive interaction between the effects of N fertilization and [CO2] on leaf area, which was 93% greater in the high-N/elevated [CO2] treatment than in the low-N/ambient [CO2] treatment. Specific leaf area was reduced (17-25%) by elevated [CO2], whereas C and N concentrations of seedlings increased significantly in response to either elevated [CO2] or high-N fertilization. At the third-flush stage, acquisition of C and N per unit dry mass of leaf and fine root was 51 and 77% greater, respectively, in the elevated [CO2]/high-N fertilization treatment than in the ambient [CO2]/low-N fertilization treatment. However, there was dilution of leaf N in response to elevated [CO2]. Partitioning of newly acquired C and N between shoot and roots was altered by N fertilization but not [CO2]. More newly acquired C and N were partitioned to roots in the low-N treatment than in the high-N treatment.