Photosynthetic response of seedlings of the sub-tropical tree Schima superba with exposure to elevated carbon dioxide and temperature

Seedlings of Schima superba were exposed to both ambient (375 ppm) and 720 ppm levels of CO₂ in combination with two incubation temperatures (25/20, 30/25°C, day/night) for a six-month period. Net height growth of seedlings was enhanced in the early period of exposure to high levels of CO₂. However, when seedlings were exposed for a longer period of time to this high concentration, net height growth was inhibited. Decreased photosynthetic rate with elevated CO₂ was observed when measured in the ambient CO₂ over a long-term exposure of 6 months. In contrast, a significant increase in photosynthesis was noted for seedlings exposed to higher incubation temperature in either ambient or 720 ppm CO₂ concentrations. The response of CO₂ assimilation to internal Ci was indicated by the lower sensitivity in seedlings grown in elevated CO₂ concentration. Though this response could also be found in a higher sensitivity in seedlings grown at higher temperature, the seedlings grown in normal conditions (ambient CO₂ and temperature) were still more sensitive to CO₂ assimilation response to internal Ci. This experiment suggests that: (1) exposure of seedlings to higher CO₂ levels for longer periods may lead to a decrease in seedling height growth and photosynthetic rate, as well as decreasing sensitivity to changing internal CO₂ concentrations; (2) the optimum temperature for photosynthesis of seedlings grown in an elevated CO₂ concentration was higher than that for seedlings grown in ambient concentration.     

Monoterpene emission from coniferous trees in response to elevated CO2 concentration and climate warming

It was hypothesized that high CO₂ availability would increase monoterpene emission to the atmosphere. This hypothesis was based on resource allocation theory which predicts increased production of plant secondary compounds when carbon is in excess of that required for growth. Monoterpene emission rates were measured from needles of (a) Ponderosa pine grown at different CO₂ concentrations and soil nitrogen levels, and (b) Douglas fir grown at different CO₂ concentrations. Ponderosa pine grown at 700 μmol mol^–1 CO₂ exhibited increased photosynthetic rates and needle starch to nitrogen (N) ratios when compared to trees grown at 350 μmol mol^–1 CO₂. Nitrogen availability had no consistent effect on photosynthesis. Douglas fir grown at 550 μmol mol^–1 CO₂ exhibited increased photosynthetic rates as compared to growth at 350 μmol mol^–1 CO₂ in old, but not young needles, and there was no influence on the starch/N ratio. In neither species was there a significant effect of elevated growth CO₂ on needle monoterpene concentration or emission rate. The influence of climate warming and leaf area index (LAI) on monoterpene emission were also investigated. Douglas fir grown at elevated CO₂ plus a 4 °C increase in growth temperature exhibited no change in needle monoterpene concentration, despite a predicted 50% increase in emission rate. At elevated CO₂ concentration the LAI increased in Ponderosa pine, but not Douglas fir. The combination of increased LAI and climate warming are predicted to cause an 80% increase in monoterpene emissions from Ponderosa pine forests and a 50% increase in emissions from Douglas fir forests. This study demonstrates that although growth at elevated CO₂ may not affect the rate of monoterpene emission per unit biomass, the effect of elevated CO₂ on LAI, and the effect of climate warming on monoterpene biosynthesis and volatilization, could increase canopy monoterpene emission rate.     

Adaptations of the Photosynthetic Mechanism of Sugar Maple (Acer saccharum) Seedlings Grown in Various Light Intensities

Sugar maple (Acer saccharum Marsh.) seedlings were grown in a nursery for three years in 13, 25, 45 and 100 per cent of full daylight. During the third year of growth, the rates of their apparent photosynthesis and respiration were measured periodically with an infra-red gas analyzer at various light intensities and normal CO₂ concentration. In addition, the rates of apparent photosynthesis of a single attached leaf of the same seedlings were measured at saturating light intensity, hut varying CO₂ concentrations. An increase in the light intensity in which seedlings were grown had no effect on their height or mean leaf area, hut resulted in thicker leaves, an increase in the total leaf area per seedling due to an increase in the number of leaves, an increase in the dry weight especially of roots and a decrease in the chlorophyll content of leaves. Throughout the growing season seedlings grown in full daylight, as compared with those grown in lower light intensities, had the lowest rates of apparent photosynthesis measured at standard conditions (21,600 lux light intensity and 300 ul/l of CO₂), when this was expressed per unit leaf area, hut the highest rates on a per seedling basis. Thus dry matter production attained at the end of the growing season correlated positively with the photosynthetic rate per seedling, but not per unit leaf area. The rates of apparent photosynthesis of seedlings grown at lower light intensities were more responsive to changes in light intensity or CO₂ concentration than those of seedlings grown in full daylight intensity.     

Photosynthesis in willows (Salix × dasyclados) grown at different CO2 concentrations and fertilization levels

Summary Willows (Salix x dasyclados) were grown for 4 months in growth chambers at four nutrient and CO₂ levels, and photosynthesis measurements were made during the latter half of this period. Photosynthesis became saturated at lower light intensities at low CO₂ concentrations than at higher ones. The effect of CO₂ concentration on photosynthesis was greater at higher temperatures. The willows grown at the highest CO₂ concentration (1000 ppm) had a lower photosynthetic capacity than the others when measured at various concentrations. The effect of nutrient status on photosynthesis clearly increased with rising CO₂ concentrations. Although photosynthetic acclimation took place to a certain extent at higher CO₂ concentrations, photosynthesis still remained higher the higher the growth concentration was. At each CO₂ level photosynthesis increased contemporaneously with leaf nitrogen content, but at each fertilization level a rise in CO₂ concentration slightly increased photosynthesis and reduced the nitrogen content. The relative increase in photosynthesis achieved by a rise in CO₂ was greater than the corresponding increase in biomass growth, whereas the effect of fertilization was greater on biomass growth than on the rate of photosynthesis in the same willows.     

Atmospheric CO2 enrichment increases growth and photosynthesis of Pinus taeda: a 4 year experiment in the field

Forest trees are major components of the terrestrial biome and their response to rising atmospheric CO₂ plays a prominent role in the global carbon cycle. In this study, loblolly pine seedlings were planted in the field in recently disturbed soil of high fertility, and CO₂ partial pressures were maintained at ambient CO₂ (Amb) and elevated CO₂ (Amb + 30 Pa) for 4 years. The objective of the study was to measure seasonal and long-term responses in growth and photosynthesis of loblolly pine exposed to elevated CO₂ under ambient field conditions of precipitation, light, temperature and nutrient availability. Loblolly pine trees grown in elevated CO₂ produced 90% more biomass after four growing seasons than did trees grown in ambient CO₂. This large increase in final biomass was primarily due to a 217% increase in leaf area in the first growing season which resulted in much higher relative growth rates for trees grown in elevated CO₂. Although there was not a sustained effect of elevated CO₂ on relative growth rate after the first growing season, absolute production of biomass continued to increase each year in trees grown in elevated CO₂ as a consequence of the compound interest effect of increased leaf area on the production of more new leaf area and more biomass. Allometric analyses of biomass allocation patterns demonstrated size-dependent shifts in allocation, but no direct effects of elevated CO₂ on partitioning of biomass. Leaf photosynthetic rates were always higher in trees grown in elevated CO₂, but these differences were greater in the summer (60–130% increase) than in the winter (14–44% increase), reflecting strong seasonal effects of temperature on photosynthesis. Our results suggest that seasonal variation in the relative photosynthetic response to elevated CO₂ will occur in natural ecosystems, but total non-structural carbohydrate (TNC) levels in leaves indicate that this variation may not always be related to sink activity. Despite indications of canopy-level adjustments in carbon assimilation, enhanced levels of leaf photosynthesis coupled with increased total leaf area indicate that net carbon assimilation for the whole tree was greater for trees grown under elevated CO₂ compared with ambient CO₂. If the large growth enhancement observed in loblolly pine were maintained after canopy closure, then these trees could be a large sink for fossil carbon emitted to the atmosphere and produce a negative feedback on atmospheric CO₂.     

Root restriction as a factor in photosynthetic acclimation of cotton seedlings grown in elevated carbon dioxide

Interactive effects of root restriction and atmospheric CO₂ enrichment on plant growth, photosynthetic capacity, and carbohydrate partitioning were studied in cotton seedlings (Gossypium hirsutum L.) grown for 28 days in three atmospheric CO₂ partial pressures (270, 350, and 650 microbars) and two pot sizes (0.38 and 1.75 liters). Some plants were transplanted from small pots into large pots after 20 days. Reduction of root biomass resulting from growth in small pots was accompanied by decreased shoot biomass and leaf area. When root growth was less restricted, plants exposed to higher CO₂ partial pressures produced more shoot and root biomass than plants exposed to lower levels of CO₂. In small pots, whole plant biomass and leaf area of plants grown in 270 and 350 microbars of CO₂ were not significantly different. Plants grown in small pots in 650 microbars of CO₂ produced greater total biomass than plants grown in 350 microbars, but the dry weight gain was found to be primarily an accumulation of leaf starch. Reduced photosynthetic capacity of plants grown at elevated levels of CO₂ was clearly associated with inadequate rooting volume. Reductions in net photosynthesis were not associated with decreased stomatal conductance. Reduced carboxylation efficiency in response to CO₂ enrichment occurred only when root growth was restricted suggesting that ribulose-1,5-bisphosphate carboxylase/oxygenase activity may be responsive to plant source-sink balance rather than to CO₂ concentration as a single factor. When root-restricted plants were transplanted into large pots, carboxylation efficiency and ribulose-1,5-bisphosphate regeneration capacity increased indicating that acclimation of photosynthesis was reversible. Reductions in photosynthetic capacity as root growth was progressively restricted suggest sink-limited feedback inhibition as a possible mechanism for regulating net photosynthesis of plants grown in elevated CO₂.     

Effects of Atmospheric CO(2) Enrichment on the Growth and Mineral Nutrition of Quercus alba Seedlings in Nutrient-Poor Soil

One-year-old dormant white oak (Quercus alba L.) seedlings were planted in a nutrient-deficient forest soil and grown for 40 weeks in growth chambers at ambient (362 microliters per liter) or elevated (690 microliters per liter) levels of CO₂. Although all of the seedlings became severely N deficient, CO₂ enrichment enhanced growth by 85%, with the greatest enhancement in root systems. The growth enhancement did not increase the total water use per plant, so water-use efficiency was significantly greater in elevated CO₂. Total uptake of N, S, and B was not affected by CO₂, therefore, tissue concentrations of these nutrients were significantly lower in elevated CO₂. An increase in nutrient-use efficiency with respect to N was apparent in that a greater proportion of the limited N pool in the CO₂-enriched plants was in fine roots and leaves. The uptake of other nutrients increased with CO₂ concentration, and P and K uptake increased in proportion to growth. Increased uptake of P by plants in elevated CO₂ may have been a result of greater proliferation of fine roots and associated mycorrhizae and rhizosphere bacteria stimulating P mineralization. The results demonstrate that a growth response to CO₂ enrichment is possible in nutrient-limited systems, and that the mechanisms of response may include either increased nutrient supply or decreased physiological demand.     

System-level adjustments to elevated CO2 in model spruce ecosystems

Atmospheric carbon dioxide enrichment and increasing nitrogen deposition are often predicted to increase forest productivity based on currently available data for isolated forest tree seedlings or their leaves. However, it is highly uncertain whether such seedling responses will scale to the stand level. Therefore, we studied the effects of increasing CO₂ (280, 420 and 560 μL L^-1) and increasing rates of wet N deposition (0, 30 and 90 kg ha^-1 y^-1) on whole stands of 4-year-old spruce trees (Picea abies). One tree from each of six clones, together with two herbaceous understory species, were established in each of nine 0.7 m² model ecosystems in nutrient poor forest soil and grown in a simulated montane climate for two years. Shoot level light-saturated net photosynthesis measured at growth CO₂ concentrations increased with increasing CO₂, as well as with increasing N deposition. However, predawn shoot respiration was unaffected by treatments. When measured at a common CO₂ concentration of 420 μL L^-1 37% down-regulation of photosynthesis was observed in plants grown at 560 μL CO₂ L^-1. Length growth of shoots and stem diameter were not affected by CO₂ or N deposition. Bud burst was delayed, leaf area index (LAI) was lower, needle litter fall increased and soil CO₂ efflux increased with increasing CO₂. N deposition had no effect on these traits. At the ecosystem level the rate of net CO₂ exchange was not significantly different between CO₂ and N treatments. Most of the responses to CO₂ studied here were nonlinear with the most significant differences between 280 and 420 μL CO₂ L^-1 and relatively small changes between 420 and 560 μL CO₂ L^-1. Our results suggest that the lack of above-ground growth responses to elevated CO₂ is due to the combined effects of physiological down-regulation of photosynthesis at the leaf level, allometric adjustment at the canopy level (reduced LAI), and increasing strength of below-ground carbon sinks. The non-linearity of treatment effects further suggests that major responses of coniferous forests to atmospheric CO₂ enrichment might already be under way and that future responses may be comparatively smaller.     

The influence of increasing growth temperature and CO2 concentration on the ratio of respiration to photosynthesis in soybean seedlings

Using controlled environmental growth chambers, whole plants of soybean, cv. ‘Clark’, were examined during early development (7–20 days after sowing) at both ambient (≈ 350 μL L^–1) and elevated (≈ 700 μL L^–1) carbon dioxide and a range of air temperatures (20, 25, 30, and 35 °C) to determine if future climatic change (temperature or CO₂ concentration) could alter the ratio of carbon lost by dark respiration to that gained via photosynthesis. Although whole-plant respiration increased with short-term increases in the measurement temperature, respiration acclimated to increasing growth temperature. Respiration, on a dry weight basis, was either unchanged or lower for the elevated CO₂ grown plants, relative to ambient CO₂ concentration, over the range of growth temperatures. Levels of both starch and sucrose increased with elevated CO₂ concentration, but no interaction between CO₂ and growth temperature was observed. Relative growth rate increased with elevated CO₂ concentration up to a growth temperature of 35 °C. The ratio of respiration to photosynthesis rate over a 24-h period during early development was not altered over the growth temperatures (20–35 °C) and was consistently less at the elevated relative to the ambient CO₂ concentration. The current experiment does not support the proposition that global increases in carbon dioxide and temperature will increase the ratio of respiration to photosynthesis; rather, the data suggest that some plant species may continue to act as a sink for carbon even if carbon dioxide and temperature increase simultaneously.     

Carbon partitioning and rhizosphere C-flow in Lolium perenne as affected by CO2 concentration, irradiance and below-ground conditions

Plant responses to increasing atmospheric CO₂ concentrations have received considerable interest. However, major uncertainties in relation to interactive effects of CO₂ with above- and below-ground conditions remain. This microcosm study investigated the impacts of CO₂ concentration on plant growth, dry matter partitioning and rhizodeposition as affected by: (i) photon flux density (PFD), and (ii) growth matrix. Plants were grown in a sandy loam soil for 28 d under two photon flux densities: 350 (low PFD) and 1000 μmol m^–2 s^–1 (high PFD) and two CO₂ concentrations: 450 (low CO₂) and 720 μmol mol^–1 (high CO₂). Partitioning of recent assimilate amongst plant and rhizosphere C-pools was determined by use of ¹⁴CO₂ pulse-labelling. In treatments with high PFD and/or high CO₂, significant (P < 0.05) increases in dry matter production were found in comparison with the low PFD/low CO₂ treatment. In addition, significant (P < 0.05) reductions in shoot %N and SLA were found in treatments imposing high PFD and/or high CO₂. Root weight ratio (RWR) was unaffected by CO₂ concentration, however, partitioning of ¹⁴C to below ground pools was significantly (P < 0.05) increased. In a separate study, L. perenne was grown for 28 d in microcosms percolated with nutrient solution, in either a sterile sand matrix or nonsterile soil, under high or low CO₂. Dry matter production was significantly (P < 0.01) increased for both sand and soil grown seedlings. Dry matter partitioning was affected by matrix type. ¹⁴C-allocation below ground was increased for sand grown plants. Rhizodeposition was affected by CO₂ concentration for growth in each matrix, but was increased for plants grown in the soil matrix, and decreased for those in sand. The results illustrate that plant responses to CO₂ are potentially affected by (i) PFD, and (ii) by feedbacks from the growth matrix. Such feedbacks are discussed in relation to soil nutrient status and interactions with the rhizosphere microbial biomass.     

Elevated CO2 influences the responses of two birch species to soil moisture: implications for forest community structure

Increased levels of atmospheric CO₂ may alter the structure and composition of plant communities by affecting how species respond to their physical and biological environment. We investigated how elevated CO₂ influenced the response of paper birch ( Betula papyrifera Marsh.) and yellow birch (Betula alleghaniensis Britt.) seedlings to variation in soil moisture. Seedlings were grown for four months on a soil moisture gradient, individually and in mixed species stands, in controlled environment facilities at ambient (375 μL L^–1) and elevated (700 μL L^–1) atmospheric CO₂. For both individually and competitively grown paper birch seedlings, there was a greater CO₂ growth enhancement for seedlings watered less frequently than for well-watered seedlings. This differential change in CO₂ responsiveness across the moisture gradient reduced the difference in seedling growth between high and low water levels and effectively broadened the regeneration niche of paper birch. In contrast, for yellow birch seedlings, elevated CO₂ only produced a significant growth enhancement at the wet end of the soil moisture gradient, and increased the size difference between seedlings at the two ends of the gradient. Gas exchange measurements showed that paper birch seedlings were more sensitive than yellow birch seedlings to declines in soil moisture, and that elevated CO₂ reduced this sensitivity. Additionally, elevated CO₂ improved survival of yellow birch seedlings growing in competition with paper birch in dry stands. Thus, elevated CO₂ may influence regeneration patterns of paper birch and yellow birch on sites of differing soil moisture. In the future, as atmospheric CO₂ levels rise, growth of paper birch seedlings and survival of yellow birch seedlings may be enhanced on xeric sites, while yellow birch may show improved growth on mesic sites.     

Increased CO2 and nutrient status changes affect phytomass and the production of plant defensive secondary chemicals in Salix myrsinifolia (Salisb.)

The effect of CO₂ enrichment (700 and 1050 ppm) on phytomass, soluble sugars, leaf nitrogen and secondary chemicals of three Salix myrsinifolia clones was studied in plants cultivated at very poor (sand seedlings) and moderate (peat seedlings) nutrient availability and under low illumination. The total shoot phytomass production of sand scedlings was less than 10% of that of the peat seedlings. Carbon dioxide increased the total shoot phytomass of peat seedlings. When the ambient carbon supply was doubled (to 700 ppm) the growth of sand seedlings was slightly enhanced but 1050 ppm CO₂ gave growth figures similar to those at the control CO₂ level. Leaf nitrogen content and total soluble sugar contents were significantly higher in peat seedlings than in sand seedlings. Leaf nitrogen showed a decreasing trend in relation to CO₂ increase. On the other hand, CO₂ did not have any clear-cut effect on total sugars. At the control CO₂ level the content of salicortin, which is a dynamic phenolic, was higher in the peat seedlings than in the sand seedlings, but salicin showed the opposite trend. CO₂ enrichment considerably decreased these phenolics in the peat seedlings. At the control CO₂ level, the content of more static phenolics, such as proanthocyanidins, was higher in sand seedlings. An increased carbon supply considerably increased static phenolics in the peat seedlings. Willow defence against generalist herbivores is moderately decreased by enhancement of atmospheric carbon dioxide.     

Effects of atmospheric CO2 enrichment and root restriction on leaf gas exchange and growth of banana (Musa)

The effects of atmospheric CO₂ enrichment and root restriction on photosynthetic characteristics and growth of banana (Musa sp. AAA cv. Gros Michel) plants were investigated. Plants were grown aeroponically in root chambers in controlled environment glasshouse rooms at CO₂ concentrations of 350 or 1 000 μmol CO₂ mol^-1. At each CO₂ concentration, plants were grown in large (2001) root chambers that did not restrict root growth or in small (20 1) root chambers that restricted root growth. Plants grown at 350 μmol CO₂ mol^-1 generally had a higher carboxylation efficiency than plants grown at 1 000 μmol CO₂ mol^-1 although actual net CO₂ assimilation (A) was higher at the higher ambient CO₂ concentration due to increased intercellular CO₂ concentrations (C_i resulting from CO₂ enrichment. Thus, plants grown at 1 000 μmol CO₂ mol^-1 accumulated more leaf area and dry weight than plants grown at 350 μmol CO₂ mol^-1. Plants grown in the large root chambers were more photosynthetically efficient than plants grown in the small root chambers. At 350 μmol CO₂ mol^-1, leaf area and dry weights of plant organs were generally greater for plants in the large root chambers compared to those in the small root chambers. Atmospheric CO₂ enrichment may have compensated for the effects of root restriction on plant growth since at 1 000 μmol CO₂ mol^-1 there was generally no effect of root chamber size on plant dry weight.     

The effect of high levels of carbon dioxide on dark respiration and growth of plants

Abstract Raising ambient levels of CO₂ during the night, between 350 and 950cm³m^?3, reduced the dark respiration rate of Medicago sativum seedlings. The percentage effect was greater for maintenance respiration than for dark respiration as a whole, and when the plants were in a low photosynthate status. Twenty-four h carbon balance studies confirmed a reduction in night time respiration and an increase of net carbon gain when night time [CO₂] was high. Growth experiments showed a small but significant increase of dry weight in Medicago sativum seedlings exposed to high [CO₂] (～ 1200 cm³m^?3) at night. This effect was greater for plants grown with Rhizobium nodules than for plants grown with nitrate in the absence of Rhizobium. A similar, but smaller and statistically non-significant effect of high night time [CO₂] on growth was found for Xanthium strumarium seedlings. The significance of these findings is discussed in relation to the rising CO₂ content of the atmosphere.     

Estimation of the whole-plant CO2 compensation point of tobacco (Nicotiana tabacum L.)

The whole-plant CO₂ compensation point (Γ_plant) is the minimum atmospheric CO₂ level required for sustained growth. The minimum CO₂ requirement for growth is critical to understanding biosphere feedbacks on the carbon cycle during low CO₂ episodes; however, actual values of Γ_plant remain difficult to calculate. Here, we have estimated Γ_plant in tobacco by measuring the relative leaf expansion rate at several low levels of atmospheric CO₂, and then extrapolating the leaf growth vs. CO₂ response to estimate CO₂ levels where no growth occurs. Plants were grown under three temperature treatments, 19/15, 25/20 and 30/25°C day/night, and at CO₂ levels of 100, 150, 190 and 270 μmol CO₂ mol⁻¹ air. Biomass declined with growth CO₂ such that Γ_plant was estimated to be approximately 65 μmol mol⁻¹ for plants grown at 19/15 and 30/25°C. In the first 19 days after germination, plants grown at 100 μmol mol⁻¹ had low growth rates, such that most remained as tiny seedlings (canopy size <1 cm²). Most seedlings grown at 150 μmol mol⁻¹ and 30/25°C also failed to grow beyond the small seedling size by day 19. Plants in all other treatments grew beyond the small seedling size within 3 weeks of planting. Given sufficient time (16 weeks after planting) plants at 100 μmol mol⁻¹ eventually reached a robust size and produced an abundance of viable seed. Photosynthetic acclimation did not increase Rubisco content at low CO₂. Instead, Rubisco levels were unchanged except at the 100 and 150 μmol mol⁻¹ where they declined. Chlorophyll content and leaf weight per area declined in the same proportion as Rubisco, indicating that leaves became less expensive to produce. From these results, we conclude that the effects of very low CO₂ are most severe during seedling establishment, in large part because CO₂ deficiency slows the emergence and expansion of new leaves. Once sufficient leaf area is produced, plants enter the exponential growth phase and acquire sufficient carbon to complete their life cycle, even under warm conditions (30/25°C) and CO₂ levels as low as 100 μmol mol⁻¹.     

The effects of elevated CO2 and temperature on the resistance of winter-dormant birch seedlings (Betula pendula) to hares and voles

The effects of elevated CO₂ and temperature on the resource allocation pattern and resistance against mammalian herbivores of silver birch (Betula pendula Roth) were studied. Birch seedlings were grown through two growing seasons in closed‐top chambers exposed to four different treatments: ambient CO₂ and temperature, elevated atmospheric CO₂ (700 ppm) and ambient temperature, elevated temperature (+3°C above ambient) and ambient CO₂, and a combination of elevated CO₂ and temperature. After winter hardening of the seedlings, the growth of the seedlings was measured and the concentration of secondary compounds such as phenolics and papyriferic acid determined. The top parts of the stem were fed to hares, and the basal parts of the same stems were offered to voles. Elevated CO₂ increased the height and basal diameter of the shoots, shoot biomass and total biomass of the seedlings but did not have any effect on secondary chemistry. Elevated temperature increased the height and shoot biomass, but did not have a significant effect on the total biomass of the seedlings. Elevated temperature decreased the concentration of condensed tannins and their precursor, (+)‐catechin, in the top part of the stems, but only the concentration of (+)‐catechin in the basal part of the stems. There were no significant interactive effects between CO₂ and temperature on phenolics in the stems, while the concentration of papyriferic acid showed significant interaction in the top part of the stems. This indicates high accumulation of papyriferic acid in ambient CO₂ under increased temperature. Consequently, elevated temperature increased the resistance of birch against hares, but did not affect the resistance of the basal parts of the same birches to voles. Our results indicate that the predicted climatic change will not necessarily lead to increased browsing damage by the mountain hare and the field vole to silver birch.     

Effects of elevated CO2 and temperature on plant growth and herbivore defensive chemistry

Concentration of atmospheric CO₂ and temperature have both been rising for the last three decades. In this century, the temperature has been predicted to rise by 2–5 °C and the CO₂ concentration to double. These changes may affect the primary and secondary metabolism of plants and thus have implications for other trophic levels. However, the biotic interactions in changing climate conditions are poorly known. In this study, two questions were addressed: (i) How will climate change affect growth and the amounts of secondary compounds in flexible plant species? and (ii) How will this affect herbivores living on this species. Four clones of the dark‐leaved willow (Salix myrsinifolia (Salisb.)) seedlings were grown in closed‐top chambers with two controlled factors: concentration of atmospheric CO₂ and temperature (T). There were four combinations of these factors, each combination replicated four times (total of 16 chambers): (i) Control CO₂ (350 ppm) and control T, (ii) Elevated CO₂ (700 ppm) and control T, (iii) Control CO₂ and elevated T (2 °C), and (iv) Elevated CO₂ and elevated T. Stem growth and aerial biomass of the plants were determined; and the leaf phenolics, nitrogen and water concentrations were analysed. In addition the growth rate of larvae and feeding preference of adults of a specialist herbivore, the chrysomelid beetle Phratora vitellinae (L.), on the treated willow leaves were measured. Elevated temperature and CO₂ concentration increased the stem biomass and elevated CO₂ increased leaf biomass and total aerial biomass of the willows. Patterns of biomass allocation were different in different temperature treatments. At elevated temperature there was less branch and leaf material in relation to stems than at the control temperature. Moreover, patterns of biomass allocation differed among clones. CO₂ enhancement increased the specific leaf weight (SLW) and reduced both water and nitrogen content of the leaves, however, leaf area was unaffected by the treatments. Carbon dioxide (CO₂) and T enhancement reduced the concentrations of several phenolic compounds in the leaves. Phenolic compounds, nutrients, and water in the leaves might be diluted partly due to increased carbon allocation to different structures (e.g. thickening of cell wall and increase of trichomes, etc.). In some cases plant clones showed specific responses to treatments. The CO₂ enhancement reduced the relative growth rate (RGR) of the beetle larvae, and in contrast, temperature elevation increased it. Adult beetles did not clearly discriminate between willow leaves grown in different T and CO₂ environments, but tended to eat more leaf material from chambers with doubled CO₂ concentration. At elevated CO₂ adult beetles may need to eat more leaf material in order to reproduce, which may in turn prolong the life cycles, increasing the risk of being eaten and possibly affecting ability to overwinter successfully. Overall, climate change may significantly modify the dynamic interaction between willow and beetle populations.     

Effects of solar ultraviolet-B radiation,temperature and CO2 on growth and physiology of sunflower and maize seedlings

The effects of solar UV-B radiation, in combination with elevated temperature (4 °C ) and CO₂ (680 L L^-1 concentration, on sunflower and maize seedlings were studied from May to August in 1991 at the research station Quinta de São Pedro in Portugal (38.7°N). The ambient solar radiation of Portugal was reduced to levels of Central European latitudes by using the ozone filter technique. This radiation served as control, while the ambient solar radiation of Portugal was to simulate intense UV-B treatment (+30%). All plants were grown up to 18 days in 4 climate controlled growth chambers simulating a daily course of temperature with T_max=28 °C or 32 °C , resp., and ambient CO₂ concentrations (340 L L^-1); in one chamber the CO₂ concentration was twice as high (680 L L^-1). Under intense UV-B and at 28 °C (T_max) all growth parameters (height, leaf area, fresh and dry weight, stem elongation rate, relative growth rate) of sunflower and maize seedlings were reduced down to 35% as compared to controls. An increase in growing temperature by 4 °C , alone or in combination with doubled CO₂, compensated or even overcompensated the UV-B effect so that the treated plants were comparable to controls. Chlorophyll content, on a leaf area basis, increased under intense UV-B radiation. This increase was compensated by lower leaf areas, resulting in comparable chlorophyll contents. Similar to growth, also the net photosynthetic rates of sunflower and maize seedlings were reduced down to 29% by intense UV-B calculated on a chlorophyll basis. This reduction was compensated by an increased temperature. Doubling of CO₂ concentration had effects only on sunflower seedlings in which the photosynthetic rates were higher than in the controls. Dark respiration rates of the seedlings were not influenced by any experimental condition. Transpiration and water use efficiency (wue) were not influenced by intense UV-B. Higher temperatures led to higher transpiration rates and lower water use efficiencies, resp.. Doubling of CO₂ reduced the transpiration rate drastically while for wue maximum values were recorded.     

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

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

Effect of nitrogen and phosphorus availability on the growth response of Eucalyptus grandis to high CO2

The response of Eucalyptus grandis seedlings to elevated atmospheric CO₂ concentrations was examined by growing seedlings at either 340 or 660 n mol CO₂ mol^-1 for 6 weeks. Graded increments of phosphorus and nitrogen fertilizers were added to a soil deficient in these nutrients to establish if the growth response to increasing nutrient availability was affected by CO₂ concentration. At 660 μmol CO₂ mol^-1, seedling dry weight was up to five times greater than at 340 μmol CO₂ mol^-1. The absolute response was largest when both nitrogen and phosphorus availability was high but the relative increase in dry weight was greatest at low phosphorus availability. At 340 μmol CO₂ mol^-1 and high nitrogen availability, growth was stimulated by addition of phosphorus up to 76 mg kg 1 soil. Further additions of phosphorus had little effect. However, at 660 μmol CO₂ mol^-1, growth only began to plateau at a phosphorus addition rate of 920mg kg^-1 soil. At 340 μmol CO₂ mol^-1 and high phosphorus availability, increasing nitrogen from 40 to 160mg kg^-1 soil had little effect on plant growth. At high CO₂, growth reached a maximum at between 80 and 160mg nitrogen kg^-1 soil. Total uptake of phosphorus was greater at high CO₂ concentration at all fertilizer addition rates, but nitrogen uptake was either lower or unchanged at high CO₂ concentration except at the highest nitrogen fertilizer rate. The shoot to root ratio was increased by CO₂ enrichment, primarily because the specific leaf weight was greater. The nitrogen and phosphorus concentration in the foliage was lower at elevated CO₂ concentration partly because of the higher specific leaf weight. These results indicate that critical foliar concentrations currently used to define nutritional status and fertilizer management may need to be reassessed as the atmospheric CO₂ concentration rises.