The effects of SO2 and O3 on the foliar nutrition of Scots pine, Norway spruce and Sitka spruce in the Liphook open-air fumigation experiment

Foliar elements were analysed in Scots pine, Sitka spruce and Norway spruce over a 6 year period before and during continuous exposure to SO₂ and O₃ in an open-air fumigation experiment. Sulphur dioxide treatment elevated foliar sulphur concentration in all species, and there were increases in foliar nitrogen in the two spruce species but not in pine. The concentrations of cations were frequently increased by SO₂ treatment, but there was no correlation between the sulphur concentration of needles and their total cation charge. SO₂-related elevations of foliar magnesium were correlated with the concentration of this element in soil solution, but the mechanism by which other cations were enhanced remains unclear. The only consistent effects on nutrient ratios were for SO₂ treatments to increase sulphur/cation ratios.     

Growth of coniferous trees exposed to SO2 and O3 using an open-air fumigation system

Scots pine (Pinus sylvestris L.), Norway spruce (Picea abies (L.) Karst.) and Sitka spruce (Picea sitchensis Bong. Carr.) were planted as 2-year-old seedlings in an open-air fumigation facility at Liphook in southern England in March 1985. The soil was a humoferric podzol of pH 4. SO₂ fumigation began in May 1987 and continued until December 1990. Long-term mean SO₂ concentrations were 4,13 and 22 nmol mo^?1. Three plots, one at each SO₂ level, were also exposed to O₃ at an average of 1–3.times the ambient level. O₃ fumigation ran from March to December 1988, May to December 1989 and February to December 1990. Each species reacted differently to treatment. Scots pine showed no growth response to either pollutant, although other work on the site demonstrated a number of deleterious effects of SO₂ on this species, including increased leaf loss and foliar injury. Stem basal diameter growth of Norway spruce was depressed in SO₂-treated plots. In contrast, extension growth of shoots of Sitka spruce increased in SO₂-treated plots, in apparent response to codeposition of NH₃-N. However, diameter growth of Sitka spruce main stems did not increase. No effects of O₃ on growth were recorded for any species.     

The effect of open-air fumigation with SO2 and O3 on carbohydrate metabolism in Scots pine (Pinus sylvestris) and Norway spruce (Picea abies)

The carbohydrate metabolism of the needles of Scots pine (Pinus sylvestris) and Norway spruce (Picea abies) has been examined in trees that were exposed to SO₂, and O₃, in an open-air fumigation experiment located in the Liphook forest in southern England. Two-year-old seedlings were planted in 1985 in seven experimental plots. Five plots received fumigation treatments of SO₂, O₃ or a combination of these gases to give a 2 × 3 factorial design with one additional ambient plot Fumigation with SO₂, occurred from May 1987 to December 1990 and O₃, fumigation occurred from March to December 1988, May to December 1989 and February to December 1990. Five samples of needles for investigation of carbohydrate metabolism were taken between February and July 1989. The concentrations of soluble carbohydrates (including sucrose and hexoses) were greatly reduced in the needles taken from Scots pine growing in the treated plots, and were also reduced, but to a lesser extent, in the needles taken from Norway spruce. Little variation in the concentration of starch in the needles of either species was detected. The activities of the two final enzymes of sucrose synthesis, sucrose phosphate synthase and sucrose 6-phos-phate phosphatase, were greatly reduced in the needles of Scots pine and were also reduced, but to a lesser extent, in the needles of Norway spruce in the fumigated plots. These reductions could be correlated with decreases in rates of photosynthetic CO₂ assimilation determined by independent groups of researchers working on the Liphook site.     

The effects of SO2 and O3 fumigation on acid deposition and foliar leaching in the Liphook forest fumigation experiment

The ambient pollution climate at the Liphook forest fumigation site, where coniferous trees were fumigated with SO₂ and O₃, for 4 years under field conditions, was characteristic of the fringes of the areas where pollutant effects are a problem. Experimental treatments increased SO₂ concentrations to levels more characteristic of Eastern Europe, and summer O₃ concentrations by 30%. Deposition of SO₂ to the soil between the trees (inferred from shallow lysimeters) was significant, the deposition velocity being 2–1 mms^?1. Deposition to Scots pine and Sitka spruce canopies was greater, deposition velocities being 8.5 and 9.4 mm s^?1, respectively. These high values may perhaps be explained by co-deposition with NH₃. Calculations assume that dry deposition was the sole source of SO₄^2? gain in throughfall, and that there was no significant retention by the trees. There was a trend for O₃ to enhance SO₂ deposition to both soil and trees. Fumigation with SO₂ led to a significant increase in leaching of cations from foliage. Each species neutralized about 63% of the dry-deposited SO₂, predominantly by ion exchange for Ca and K. Equations are provided which allow calculation of foliar leaching given SO₂ concentrations or SO₄^2? deposition. Fumigation increased the rate of nutrient cycling considerably, without affecting foliar concentrations or damaging the trees. Ozone treatments did not enhance foliar leaching, calling into question some suggested mechanisms for the causes of forest decline.     

The influence of elevated CO2 and O3 on fine roots and mycorrhizas of naturally growing young Scots pine trees during three exposure years

Young Scots pine trees naturally established at a pine heath were exposed to two concentrations of CO₂ (ambient and doubled ambient) and two O₃ regimes (ambient and doubled ambient) and their combination in open-top field chambers during growing seasons 1994, 1995 and 1996 (late May to 15 September). Filtered ozone treatment and chamberless control trees were also included in the treatment comparisons. Root ingrowth cores were inserted to the undisturbed soil below the branch projection of each tree at the beginning of the fumigation period in 1994 and were harvested at the end of the fumigation periods in 1995 and 1996. Root biomasses were determined from different soil layers in the ingrowth cores, and the infection levels of different mycorrhizal types were calculated. Elevated O₃ and CO₂ did not have significant effects on the biomass production of Scots pine coarse (Ø > 2 mm) or fine roots (Ø < 2 mm) and roots of grasses and dwarf shrubs. Elevated O₃ caused a transient stimulation, observable in 1995, in the proportion of tuber-like mycorrhizas, total mycorrhizas and total short roots but this stimulation disappeared during the last study year. Elevated CO₂ did not enhance carbon allocation to root growth or mycorrhiza formation, although a diminishing trend in the mycorrhiza formation was observed. In the combination treatment increased CO₂ inhibited the transient stimulating effect of ozone, and a significant increase of old mycorrhizas was observed. Our conclusion is that doubled CO₂ is not able to increase carbon allocation to growth of fine roots or mycorrhizas in nutrient poor forest sites and realistically elevated ozone does not cause a measurable limitation to roots within a period of three exposure years.     

Effects of elevated O3 and CO2 concentrations on photosynthesis and stomatal conductance in Scots pine

Naturally regenerated Scots pines (Pinus sylvestris L.), aged 28–30 years old, were grown in open-top chambers and subjected in situ to three ozone (O₃) regimes, two concentrations of CO₂, and a combination of O₃ and CO₂ treatments From 15 April to 15 September for two growing seasons (1994 and 1995). The gas exchanges of current-year and 1-year-old shoots were measured, along with the nitrogen content of needles. In order to investigate the factors underlying modifications in photosynthesis, five parameters linked to photosynthetic performance and three to stomatal conductance were determined. Elevated O₃ concentrations led to a significant decline in the CO₂ compensation point (Г^*), maximum RuP₂-saturated rate of carboxylation (V_cmax), maximum rate of electron transport (J_max), maximum stomatal conductance (g_smax), and sensitivity of stomatal conductance to changes in leaf-to-air vapour pressure difference (?g_s/?D_v) in both shoot-age classes. However, the effect of elevated O₃ concentrations on the respiration rate in light (R_d) was dependent on shoot age. Elevated CO₂(700 μmol mol^?1) significantly decreased J_max and g_smax but increased R_d in 1-year-old shoots and the ?g_s/?D_v in both shoot-age classes. The interactive effects of O₃ and CO₂ on some key parameters (e.g. V_cmax and J_max) were significant. This may be closely related to regulation of the maximum stomatal conductance and stomatal sensitivity induced by elevated CO₂. As a consequence, the injury induced by O₃ was reduced through decreased ozone uptake in 1-year-old shoots, but not in the current-year shoots. Compared to ambient O₃ concentration, reduced O₃ concentrations (charcoal-filtered air) did not lead to significant changes in any of the measured parameters. Compared to the control treatment, calculations showed that elevated O₃ concentrations decreased the apparent quantum yield by 15% and by 18%, and the maximum rate of photosynthesis by 21% and by 29% in the current-year and 1-year-old shoots, respectively. Changes in the nitrogen content of needles resulting from the various treatments were associated with modifications in photosynthetic components.     

Competition increasingly dominates the responsiveness of juvenile beech and spruce to elevated CO2 and/or O3 concentrations throughout two subsequent growing seasons

Saplings of Fagus sylvatica and Picea abies were grown in mono‐ and mixed cultures in a 2‐year phytotron study under all four combinations of ambient and elevated ozone (O₃) and carbon dioxide (CO₂) concentrations. The hypotheses tested were (1) that the competitiveness of beech rather than spruce is negatively affected by the exposure to enhanced O₃ concentrations, (2) spruce benefits from the increase of resource availability (elevated CO₂) in the mixed culture and (3) that the responsiveness of plants to CO₂ and O₃ depends on the type of competition (i.e. intra vs. interspecific). Beech displayed a competitive disadvantage when growing in mixture with spruce: after two growing seasons under interspecific competition, beech showed significant reductions in leaf gas exchange, biomass development and crown volume as compared with beech plants growing in monoculture. In competition with spruce, beech appeared to be nitrogen (N)‐limited, whereas spruce tended to benefit in terms of its plant N status. The responsiveness of the juvenile trees to the atmospheric treatments differed between species and was dominated by the type of competition: spruce growth benefited from elevated CO₂ concentrations, while beech growth suffered from the enhanced O₃ regime. In general, interspecific competition enhanced these atmospheric treatment effects, supporting our hypotheses. Significant differences in root : shoot biomass ratio between the type of competition under both elevated O₃ and CO₂ were not caused by readjustments of biomass partitioning, but were dependent on tree size. Our study stresses that competition is an important factor driving plant development, and suggests that the knowledge about responses of plants to elevated CO₂ and/or O₃, acquired from plants growing in monoculture, may not be transferred to plants grown under interspecific competition as typically found in the field.     

NMR imaging of roots: effects after root freezing of containerised conifer seedlings

Seedlings of Scots pine ( Pinus sylvestris L.) and Norway spruce [ Picea abies (L) Karst.] were subjected to low root temperatures, and 10 days later the roots were examined by NMR imaging. The amount of NMR detectable roots decreased with decreasing temperature, with the signal from the younger roots at the bottom of the container being the first to disappear. The origin of the loss of NMR signal is unclear but may be due to changes in the NMR properties of root water after cold damage. A recent method is discussed for obtaining unbiased estimates of root lengths from a series of total vertical projections; the method is particularly suited to evaluating NMR projection images. Since NMR imaging methods can apparently distinguish between control and cold damaged roots, it may be possible to design more routine applications using low resolution NMR methods.     

Photosynthetic performance, chloroplast pigments and mineral content of Norway spruce (Picea abies (L.) Karst.) exposed to SO2 and O3 in an open-air fumigation experiment

Photosynthetic performance, mineral content and chloroplast pigments were investigated in August-September 1988 and 1989 in Norway spruce trees (Picea abies (L.) Karst.) exposed to SO₂, and O₃ in an open-air fumigation facility at Liphook, England. The data do not suggest a treatment effect on the mineral content of the needles in terms of nutrient leaching from the foliage. In addition, there were no direct SO₂ and/or O₃ effects on the content and/or composition of the chloroplast pigments. However, the long-term application of SO₂ resulted in a depression of net photosynthesis under light saturation and ambient CO₂ (A ₃₄₀) which was probably caused by a treatment-related depression of the carboxylation efficiency (CE). In 1989, the supposed treatment effects were apparently masked by an insufficient N-supply and probably also by low water availability during summer. However, fumigation appeared to accelerate an N-deficiency-related decrease of CE, stomatal closure and the age-dependent development of the chlorophyll content of the needles. In 1989, an observed depression of the photosynthetic capacity (A₂₅₀₀) was in part accompanied by a decrease in light use efficiency (α), suggesting an enhanced photosensitivity resulting from the impact of several possible interacting stresses (drought, N deficiency and fumigation). The results support the general conclusion that long-term low-level SO₂ dosage adversely affects the photosynthetic performance of the needle, whether directly or indirectly, and may also interact with other environmental stresses. The findings of our investigations are discussed with regard to the hypothesis of forest decline in the mountain regions of the Fichtelgebirge (north-eastern Bavaria, Germany).     

Diagnostic parameters for selecting against novel spruce (Picea abies) decline: III. Response of photosynthesis and transpiration to O3 exposures

The dose- and time-response effects of single 4 h day-time exposures of 0.064, 0.166, 0.336, 0.452 or 0.693 μl l^?1 (ppm) O₃ followed by single 4 h night-time exposures of 0.078, 0.198, 0.378, 0.502 or 0.747 μl l^?1 O₃ on photosynthesis, transpiration and dark respiration were examined for nine Carpatho-Ukrainian (‘Rachovo’) half-sib families and for two populations. ‘Westerhof’ from the FRG and ‘Schmiedefeld’ from the GDR, of Norway spruce [Picea abies (L.) Karst.], all in their 4th growing season. Needles were scorched by 4 h exposures to 0.336 μl l^?1 O₃ and higher. The lag before photosynthesis and transpiration responded significantly to O₃ decline took from a few minutes at the highest concentration to several hours at the lower concentrations. Recovery of photosynthesis and transpiration was absent or extremely slow. Photosynthesis of the different spruce types was affected significantly differently, the most sensitive spruce having its photosynthesis suppressed 1.9 times and its transpiration 1.6 times more than the most tolerant spruce. The physiological responses of ‘Westerhof’ were less sensitive than the average ‘Rachovo’ half-sibs. Neither night transpiration nor dark respiration were affected by high doses of night O₃, preceded by day O₃ exposures. The gradients of different photosynthesis and transpiration sensitivities of the young half-sibs (and ‘Westerhof’) demonstrated a significant, positive, mutual correlation, and significant positive correlations with the gradient of novel decline symptoms of their parents growing in Danish forests. The relative photosynthesis and transpiration sensitivities may thus serve as diagnostic parameters in laboratory tests for selection against novel spruce decline.     

Identification of endogenous gibberellins, and metabolism of tritiated and deuterated GA4, GA9 and GA20, in Scots pine (Pinus sylvestris) shoots

The application of gibberellin A_4/7 (GA_4/7) to the stem of previous-year (1-year-old) terminal shoots of Scots pine (Pinus sylvestris) seedlings has been observed to stimulate cambial growth locally, as well as at a distance in the distal current-year terminal shoot, but the distribution and metabolic fate of the applied GA_4/7, as well as the pathway of endogenous GA biosynthesis in this species, has not been investigated. As a first step, we analysed for endogenous GAs and monitored the transport and metabolism of labelled GAs 4, 9 and 20. Endogenous GAs from the elongating current-year terminal shoot of 2-year-old seedlings were purified by column chromatography and high-performance liquid chromatography and analysed by combined gas chromatography-mass spectrometry (GC-MS). GAs 1, 3, 4, 9, 12 and 20 were identified in the stem, and GAs 1, 3 and 4 in the needles, by full-scan mass spectrometry (GAs 1, 3, 4, 9 and 12) or selected-ion monitoring (GA₂₀) and Kovats retention index. Tritiated and deuterated GA₄, GA₉ or GA₂₀ were applied around the circumference at the midpoint of the previous-year terminal shoot, and metabolites were extracted from the elongating current-year terminal shoot, the application point, and the 1-year-old needles and the cambial region above and below the application point. After purification, detection by liquid scintillation spectrometry and analysis by GC-MS, it was evident that, for each applied GA, unmetabolised [²H₂]GA and [³H]radioactivity were present in every seedling part analysed. Most of the radioactivity was retained at the application point when [³H]GA₉ and [³H]GA₂₀ were applied, whereas the largest percentage of radioactivity derived from [³H]GA₄ was recovered in the current-year terminal shoot. It was also found that [²H₂]GA₉ was converted to [²H₂]GA₂₀ and to both [²H₂]GA₄ and [²H₂]GA₁, [²H₂]GA₄ was metabolised to [²H₂]GA₁, and [²H₂]GA₂₀ was converted to [²H₂]GA₂₉. The data indicate that for Pinus sylvestris shoots (1) GAs applied laterally to the outside of the vascular system of previous-year shoots not only are absorbed and translocated extensively throughout the previous-year and current-year shoots, but also are readily metabolised, (2) the GA metabolic pathways found are closely related to the endogenous GAs identified, and (3) GA₉ metabolism follows two distinctly different routes: in one, GA₉ is converted to GA₁ through GA₄, and in the other it is converted to GA₂₀, which is then metabolised to GA₂₉. The results suggest that the late 13-hydroxylation pathway is an important route for GA biosynthesis in shoots of Pinus sylvestris, and that the stimulation of cambial growth in Scots pine by exogenous GA_4/7 may be due to its conversion to GA₁, rather than to it being active per se.     

Relationship between respiratory depletion of sugars and loss of cold hardiness in coniferous seedlings over-wintering at raised temperatures: indications of different sensitivities of spruce and pine

Long-term effects of elevated winter temperatures on cold hardiness were investigated for Norway spruce (Picea abies L. Karst.), lodgepole pine (Pinus contorta Dougl.) and Scots pine (Pinus sylvestris L.). Two-year-old seedlings with the same pre-history of growth and cold hardening in the field were maintained from early December to late March at two field sites in northern Sweden and in a cold room. The temperatures at these locations averaged –13·5, –8·9 and 5·5°C, respectively. Following treatments, carbohydrate contents and cold tolerances were assessed. Needle respiration was also analysed during the 5·5°C treatment. Cold tolerance of lodgepole pine and Scots pine was much reduced following the 5·5°C treatment. Cold tolerance was somewhat reduced in lodgepole pine following the –8·9 °C treatment, but was essentially maintained in spruce throughout all treatments. The cold tolerance of needles was strongly correlated with their soluble sugar contents. Spruce maintained cold hardiness by having larger reserves of sugars and lower rates of respiration which decreased more rapidly as sugars were depleted. Tolerance of lodgepole pine to frost desiccation was also much reduced following the 5·5°C treatment.