Exposure of red spruce seedlings to acid mist: importance of droplet composition just prior to drying periods

Two-year-old red spruce (Picea rubens Sarg.), raised from seed collected at five locations from Maine to North Carolina, were exposed in a greenhouse to either sulphuric or nitric acid mist (median pH 2-85). Exposures were performed three times a day, four days a week over an eleven-week period during the growing season. Each exposure consisted of 4 h of misting followed by a 3.75 h drying period. Immediately before each drying period, acidic droplets were either left to dry directly on the foliage or were removed by a 15 min deionized water (diw) rinse. When sulphuric acid droplets dried directly on the foliage, visible injury developed and to a lesser degree the same was true for nitric acid. When acidic droplets were replaced with dim before each drying period, foliar injury did not develop despite misting for hundreds of hours at pH values well within the range that has caused visible injury in previous experiments. When acidic droplets dried directly on foliage, needle dry weight was reduced, but differences between sulphuric and nitric acid exposure were not found. No treatment effects were found on terminal shoot length. Treatment responses among seedlings from different seed resources were similar, with greater variation found within provenances than between provenances. We conclude that the composition of liquid on foliage at the end of wet deposition events and the processes occurring during drying periods are important factors in determining the response of red spruce to acidic cloudwater.

Causal mechanisms by which sulphate, nitrate and acidity influence frost hardiness in red spruce: review and hypothesis

This paper summarizes results from four experiments in which red spruce seedlings (Picea rubens Sarg.) were exposed to simulated acid mist containing SO₄ ^2- , NH_4- , NO₃ ^- and H⁺ ions. Seedlings were grown in compost, with or without fertilizer, in charcoal filtered air in open-top chambers near Edinburgh, Scotland. Plants were sprayed from bud burst between May and November with mist containing different concentrations and combinations of the four major ions to provide a range of doses, which were applied at different frequencies. Reductions in frost hardiness expressed in terms of the temperature which killed 50% of shoots (LT₅₀ ) were significantly correlated with the dose of S received by the seedlings. Differences in foliar S concentrations between the controls and treated plants were correlated with S dose. Absolute S concentrations were, however, of limited use for predictive purposes. Seedlings appear to be more sensitive than older trees to S toxicity because the former have the greatest proportion of newly expanding needles which optimize conditions for foliar uptake. Seedlings are also least well equipped to export SO₄ ^2- ions since they have a smaller resource of older foliage to supplement their assimilate pool. In conditions which promote uncontrolled SO₄ ^2- ion uptake by foliage, i.e. high external SO₄ ^2- concentrations and incompletely formed cuticles, the potential exists for the internal build up of SO₄ ^2- ions. It is proposed that in the absence of sufficient assimilate and K the presence of these high concentrations of SO₄ ^2- ions in the apoplast or cytosol can lead to protein denaturation and loss of membrane integrity. Reductions in frost hardiness appear to result through direct attack by SO₄ ² ions on membrane proteins which impairs their function. Indirect effects on hardiness occur through both an increased consumption of sugars reducing the'pool'available for cryoprotection and a reduction in photosynthetic function, the ability to produce sugars. The presence of NO_3- N mitigates the toxic effects of SO₄ ² because SO₄ ² ions are consumed in assimilation processes which both utilize and are facilitated by the presence of large amounts of fertilizer N. High concentrations of SO₄ ² and H+ are found to be particularly toxic because of the synergistic effects these ions exert on their mutual uptake with devastating consequences for the control of cellular pH. Trees growing at high altitude sites are likely to be particularly sensitive to SO₄ ^2- toxicity because (1) their carbon balance is low, (2) cuticle development is poor and (3) levels of soil available Ca² tend to be low relative to Al³⁺ so that membranes may already be weakened as a result of insufficient Ca²⁺ ions for protein bridging.

Exposure of the foliage and the rooting environment

Two-year-old red spruce (Picea rubens Sarg.) was grown in replicated open-top chambers supplied with charcoal-filtered air near Edinburgh, Scotland. Between May and November 1989, plants were exposed to four mist treatments, three containing sulphuric acid and ammonium nitrate in equimolar concentrations at 0.005 mol m^-3 (pH 5) or 1.0 mol m^-3 (pH 27), and a fourth treatment with sulphuric acid alone at 1.0 mol m³ (equivalent to 2 mm precipitation). Two dose rates were used for the pH 2.7 treatment equivalent to 2 and 8 mm of rain per week. Three subtreatments (soil surface exposed to mist, addition of extra sulphuric acid to the soil surface, exclusion of mist from the soil) were included in each chamber. Frost hardiness was assessed by measuring rates of electrolyte leakage after controlled freezing of detached shoots. At the end of October, frost hardiness, expressed as the lethal temperature for 50% of shoots (LT₅₀ ), was decreased by 8 °C in the 8 mm wk^-1 treatment at pH 27, compared to pH 5. The 2 mm wk^-1 treatment at pH 2.7 had no effect on frost hardiness either when ammonium nitrate was present or absent (i.e. sulphuric acid only). Excluding mist from the soil, and adding extra sulphuric acid, both increased frost hardiness by about 3 °C when compared with uncovered soil. Excluding mist from the soil increased the amount of foliage initiated and produced inside the chambers but neither subtreatment, excluding the mist nor providing additional sulphuric acid to the soil affected foliar nutrient concentrations. Mist of pH 27 as sulphuric acid alone and in combination with ammonium nitrate both enhanced N uptake. Several observations concerning the effect of acidic mist on frost hardiness were confirmed by this study: (i) preventing mist from reaching the soil/roots, improving conditions for root growth can ameliorate the effects of acidic mist on shoot growth and frost hardiness; (ii) the effect was determined by the ion dose but not by the ion concentration in the mist; (iii) the effect was primarily mediated through foliar absorption; (iv) the presence of high foliar nitrogen concentrations did not increase frost hardiness when foliar sulphur concentrations were also high; (v) low N concentrations were more important for frost hardiness than high foliar N concentrations.

Effects of misting frequency and rainfall exclusion

Two-year-old red spruce (Picea rubens Sarg.) of Pittston provenance and 3-yr-old plants of Chatham provenance were exposed to acid mist in replicated open-top chambers supplied with charcoal-filtered air near Edinburgh, Scotland. Plants of Chatham provenance had already been exposed to acid mist throughout the previous growing season. The plants were exposed to mist, equivalent to 4 mm rainfall per week, containing an equimolar mixture of sulphuric acid and ammonium nitrate at pH 2.5 or pH 5.0 (1.6 or 0.01 mol m³ ) from May to November. This weekly dose was delivered at a low frequency (2 mm twice a week), or high frequency (1 mm on 4 consecutive days each week) to chambers fitted with ceilings to exclude rain. The low frequency dose was also applied to chambers without ceilings, to examine the effect of natural washing by rain. Frost hardiness, estimated by exposing detached shoots to controlled freezing and then measuring rates of electrolyte leakage, was determined during the misting period at the end of October and in December. Foliar nutrient concentrations were measured during the dormant period after treatment had ceased. At the end of October, plants which had received acid mist were less frost hardy than plants receiving mist at pH 5. The temperature causing 50% shoot death (LT₅₀ ) increased by 6 °C for low frequency application, and by 10 °C at high frequency, relative to the plants receiving mist at pH 5. Exclusion of ambient rainfall had no detectable effect on the frost hardiness response to acid mist. In December, 3 wk after the cessation of misting, all plants were more frost hardy than in October. Significant effects of the acid mist treatment could no longer be detected. Differences in nutrient concentrations were small among treatments, although K⁺ concentrations in the low frequency treatment with acid mist with rain exclusion were 50 % below those in other treatments. Ca concentrations were 50% larger in the acid mist treatment with rain exclusion than without. The data suggested enhanced sulphate uptake resulting from increasing the frequency of exposure, but the increase was not significant. There was no clear relationship between the pattern of frost hardiness and nutrient concentrations except for S, which was 30% smaller in the control plants (pH 5) than in the high frequency pH 2.5 treatments. It is concluded that excluding rainfall, an experimental artifact introduced in evaluating effects of acid mist, does not influence the frost hardiness response of red spruce seedlings. The much greater effect of exposure to the same dose at double the frequency suggests that such experiments may underestimate effects in the field, if those trees are exposed to more frequent episodes of polluted cloud water than experimental plants.

Amended Description of Pratylenchus macrostylus Wu, 1971 with SEM Observations

The description of Pratylenchus macrostylus Wu is amended using specimens collected from Fraser fir and red spruce in the Black Mountains of North Carolina. Measurements of females in North Carolina overlap those of the type series. However, stylet length (21.8-27.8 mum, 24.7 ñ 1.1) is greater in North Carolina specimens, which also have a longer body length and greater C ratio. Heads of the North Carolina specimens are divided into lateral and submedian segments which taper and fuse with oral discs. Males are rare and not important in species diagnosis. Previously described specimens in Japan differed from those in North America in key diagnostic characters of stylet and body length. This discrepancy suggests that the Japanese species may be distinct from the North American.

Quantification of frost damage in plant tissues by rates of electrolyte leakage

A method for assessing frost hardiness of plant tissues [using shoots of Picea rubens Sarg. syn P. rubra (Du Roi) Link] has been developed based upon the rate of electrolyte leakage from shoots immersed in distilled water after exposure to a range of freezing treatments. The relationship between conductivity (the electrolyte concentration in solution) and time has been shown to follow an asymptotic curve, which may be represented by a first-order equation: C_t -C_o = C_auto -C_o - C_-kt ) where C₁ is the conductivity at time t, C_o is the initial conductivity, C_auto is the conductivity after autoclaving and k is the first-order rate constant (units time^-1 ). The rate of electrolyte leakage (k) varies directly with the extent of tissue damage. In P. rebens a rate of 0-4%, h^-1 distinguished between shoots which eventually died, and shoots which remained alive. A minimum of 3 conductivity measurements (after 1 day, 5 days and after autoclaving) is required for a reliable estimate of k. This objective, quantitative method of assessing frost hardiness may therefore be used directly to estimate LT₅₀ values within a population.

Effects of acid mist on the frost hardiness of red spruce seedlings

Seedlings of red spruce [Picea rubens Sarg. syn. P. rubra (Du Roi) Link] were exposed to mists containing equimolar (NH₄ , SO₄ and HNO₃ at pHs of 2.5, 2.7, 3.0, 35, 4.0 or 5.0. The mists were applied twice each week, amounting to 2 mm precipitation equivalent on each occasion, between July and December, to open-top chambers supplied with charcoal-filtered air. Frost hardiness of shoots excised from seedlings was determined on 6 occasions starting on 21 September, and was found to be strongly influenced by acid mist treatments, seedlings subject to the most acidic mists being the least frost hardy. On 21 September when the first sample was taken the lethal temperature for killing 50% of shoots (LT₅₀ ) was - 11 °C for the least acidic (pH 5.0) mist and - 7 °C for the most acidic (pH 2.5). By 19 October, the LT₅₀ s of pH 5.0 and pH 2.5 mists were -27 and -15 °C respectively. All intermediate treatments ranked according to treatment concentration, with the smaller concentrations causing lower LT.₅₀ values. The treatment at pH 30 provided S and N inputs to the seedlings similar to those experienced by red spruce at elevations of about 1000 m in the southern Appalachians. At pH 3.0, the frost hardiness LT₁₀ during October was typically 8 °C higher than the pH 5.0 treatment, leading to a significant increase in the probability of frost damage at the LT₁₀ level in an average October. The proximity of minimum night temperatures during September to December to the LT₁₀ temperatures of red spruce shoots receiving large inputs of SO₄ ^2- , NO₃ ^- , NH₄ ⁺ and H⁺ suggests that decreases in frost hardiness caused by intercepted cloud water containing large concentrations of these ions may play a significant part in the observed decline at mountain-top locations.

Visible foliar injury of red spruce seedlings subjected to simulated acid mist

Two-year-old red spruce seedlings [Picea rubens Sarg. syn. P. rubra (Du Roi) Link] were subjected to 6 simulated acid mist treatments (pH 2.5, 2.7, 3.0, 3.5, 4.0, 5.0) in a replicated experiment using open-top chambers. Acid mist solutions containing equimolar (NH₄ )₂ SO₄ , and HNO₃ were applied twice weekly for 22 weeks, each application being equivalent to 2 mm of precipitation. Visible symptoms of foliar damage were observed on the 3 most acidic mist treatments (pH 2.5, 2.7, 3.0). The inputs of nitrogen, sulphur and acidity in the most acidic treatment were 55, 42, 1,3 kg ha^-1 , respectively, over a 10 week period. The plants subjected to the pH 2.5 treatment were found to be most severely damaged with approximately 40% foliar necrosis after 10 weeks of treatment. On approximately 80% of seedlings, necrosis was confined to current year needles only. These damaged needles were initially light brown or light orange in colour turning a deeper red 3 to 5 weeks after initial necrosis. Percentage foliar damage was linearly related to concentration (of NH₄ ⁺ , NO₃ ^- , SO₄ ^2- and H⁺ ) with 62% foliar damage in the pH 2.5 treatment after a 22-week period. Spray application stopped in December 1987. Observations during the following spring showed that the pH 2.5 and pH 3 treatments induced earlier Hushing, requiring 60 day °C less thermal time than the pH 5-0 treatment. In 1988, this decrease in thermal requirement was equivalent to flushing 11 days earlier. There was no evidence of acid mist treatments inducing bud mortality.