Allelopathy of red pine: isolation and identification of an allelopathic substance in red pine needles

Since red pine (Pinus densiflora Sieb. et Zucc.) often forms sparse forest floors where herbaceous plants do not grow well, allelopathy of red pine was investigated. A growth inhibitory substance was isolated from an aqueous methanol extract of red pine needles and determined by spectral data as abscisic acid-β-d-glucopyranosyl ester (ABA-GE). This substance inhibited root and shoot growth of cress and E. crus-galli seedlings at concentrations greater than 0.1 μM. The concentrations required for 50% growth inhibition on roots and shoots of cress were 0.23 and 0.61 μM, respectively, and those of E. crus-galli were 1.1 and 2.8 μM, respectively. The activity of ABA-β-d-glucosidase, which liberates free ABA from ABA-GE, in cress and E. crus-galli seedlings was 13–29 nmol mg⁻¹ protein min⁻¹. Endogenous concentration of ABA-GE in the pine needles was 4.1–21.5 μmol kg⁻¹ and the concentration in soil water of the pine forest was 2.5 μM. The effectiveness of ABA-GE on growth inhibition and the occurrence of ABA-GE in pine needles and soil water suggest ABA-GE may play an important role in the allelopathy of red pine resulting in the formation of sparse forest floors.     

Characterization of the photosynthetic apparatus in cortical bark chlorenchyma of Scots pine

Winter-induced inhibition of photosynthesis in Scots pine (Pinus sylvestris L.) needles is accompanied by a 65% reduction of the maximum photochemical efficiency of photosystem II (PSII), measured as F _v/F _m, but relatively stable photosystem I (PSI) activity. In contrast, the photochemical efficiency of PSII in bark chlorenchyma of Scots pine twigs was shown to be well preserved, while PSI capacity was severely decreased. Low-temperature (77 K) chlorophyll fluorescence measurements also revealed lower relative fluorescence intensity emitted from PSI in bark chlorenchyma compared to needles regardless of the growing season. Nondenaturating SDS-PAGE analysis of the chlorophyll–protein complexes also revealed much lower abundance of LHCI and the CPI band related to light harvesting and the core complex of PSI, respectively, in bark chlorenchyma. These changes were associated with a 38% reduction in the total amount of chlorophyll in the bark chlorenchyma relative to winter needles, but the Chl a/b ratio and carotenoid composition were similar in the two tissues. As distinct from winter pine needles exhibiting ATP/ADP ratio of 11.3, the total adenylate content in winter bark chlorenchyma was 2.5-fold higher and the estimated ATP/ADP ratio was 20.7. The photochemical efficiency of PSII in needles attached to the twig recovered significantly faster (28–30 h) then in detached needles. Fluorescence quenching analysis revealed a high reduction state of Q _A and the PQ-pool in the green bark tissue. The role of bark chlorenchyma and its photochemical performance during the recovery of photosynthesis from winter stress in Scots pine is discussed.     

Ectomycorrhizal root growth in scots pine stands in response to manipulation of litter and humus layers

 The effect on ectomycorrhizal root growth in a nitrogen-enriched planted stand of Scots pine (Pinus sylvestris L.) on podzolic sandy soil to manipulation of litter and humus layers (removal, doubling and control treatments) was examined, and compared to ectomycorrhizal root growth in an untreated naturally established Scots pine stand on nutrient-poor non-podzolic sandy soil. Half a year after manipulation of litter and humus layers in the planted stand, ingrowth-cores to a depth of 60 cm were installed in both stands. Scots pine roots were sampled four times during two growing seasons. Ectomycorrhizal roots were found at all sampled soil depths to 60 cm in all plots. Root growth and ectomycorrhizal development were greater in the naturally established stand than in all plots in the planted stand. Numbers of ectomycorrhizal root tips in the litter and humus removal plots were generally higher than in the control plots in the planted stand until May 1992. Doubling litter and humus did not significantly affect root length or the numbers of ectomycorrhizal root tips. The N _dissolved , NH₄ ⁺ and NO₃ ^– concentrations and the organic matter content in the upper 5 cm of the mineral soil in the planted stand on podzolic sandy soil were generally higher and the pH significantly lower than in the naturally established stand on non-podzolic sandy soil. Root growth and ectomycorrhizal development in the secondary stand may have been negatively affected by the chemical composition of the podzolic sandy soil. Accepted: 19 March 1997     

Seasonal changes in photosynthetic activity and chlorophylls in the Scots pine and Siberian spruce with optimal or insufficient moistening

The paper studies the seasonal changes in photosynthetic activity and concentration of chlorophylls in the needles of the Scots pine Pinus sylvestris L. and Siberian spruce Picea obovata Ledeb. depending on the environmental factors. In drought, the daily peaks of photosynthesis are regulated by the level of moisture in the needles and concentration of chlorophylls. The work of chlorophyll is 2–6 times more effective with optimal moistening than in drought.     

Early growth invigoration of jack pine seedlings by natural plant growth regulators

 Survival of conifer transplants is often poor on exposed planting sites in the boreal forest. More than one-third of all conifers do not become established. To enhance the competitive ability of jack pine seedlings, seeds were treated with natural plant growth regulators (PGRs; viz., homobrassinolide, salicylic acid, and two polyamines, spermine and spermidine) and growth promotion was studied for 16 days. Homobrassinolide (5 ng l^–1), salicylic acid (100 μg l^–1) and spermine (10 μg l^–1) enhanced elongation growth and elongation rate of whole plant. Homobrassinolide (5 ng l^–1) and salicylic acid (100 μg l^–1) stimulated root elongation by 38% and 10% respectively while spermine (1000 μg l^–1) increased needle growth by 14%. Homobrassinolide stimulated dry weight growth and growth rate. Homobrassinolide recorded over 20% increase in dry matter production, apportioned equally to root and needles, whereas spermine enhanced total dry matter production by almost 10%, mostly by increasing needle weight. Homobrassinolide facilitated nearly 19% increase in growth rate while spermine recorded only a 7% growth promotion. Spermidine inhibited both elongation and dry weight growth at all concentrations. Growth promotion by homobrassinolide, salicylic acid and spermine may be through an acceleration of processes connected to cell elongation, cell division and C allocation and these PGRs showed most promise for the early invigoration and improvement of jack pine seedling growth. Received: 27 August 1997 / Accepted: 8 January 1998     

Ecogeographical characteristics of the seed-bearing capacity and natural regeneration of pine on the fire-sites in pine forests of Transbaikalia

The paper presents the results of the studying of the parameters of post-fire structure and seed-bearing capacity of tree stands, factors of surface medium (thickness of burnt duff, projective cover of herb and moss vegetation) as well as number, vitality, and age structure of self-seeded Scots pine (Pinus sylvestris L.) and accompanying small-leaved species in the Lower Selenga pine forest massif of the forest-steppe in Southwestern Transbaikalia. The seed harvests were 1.5–2 times higher than in the geographically replacing forest types in the forest-steppe of West Siberia, and vitality of the undergrowth of the pine under the canopy of fire-affected stands was extremely low. It is shown that the pine reforests successfully on the fire-sites in the zone of insemination from the forest walls and on the thin fire-sites in the cowberry-rhododendron pine forests, where it is 2–3 times more abundant than in forest-steppe of the West Siberia. The reforestation is insufficient on the fire-sites in the bearberry-lichen pine forests.     

UV-B induction of flavonoid biosynthesis in Scots pine (Pinus sylvestris L . ) seedlings

 Cultivation of Scots pine (Pinus sylvestris L.) seedlings under simulated global radiation including the UV-B band (280 – 320 nm; 220 mW m^–2 UV-B_BE) led to increased formation of the diacylated flavonol glucosides 3″,6″-di-p-coumaroyl-astragalin and 3″,6″-di-p-coumaroyl-isoquercitrin in primary and cotyledonary needles, respectively. 3″,6″-Di-p-coumaroyl-astragalin was also the main constitutive diacylated flavonol glucoside in both needle types. This compound predominantly accumulated in primary needles upon UV-B irradiation, and reached concentrations of 2.4 μmol g^–1 fresh weight (fw). Its concentration was only weakly affected in cotyledonary needles. 3″,6″-Di-p-coumaroyl-isoquercitrin was mainly induced in cotyledonary needles with maximum concentrations of 0.8 to 0.9 μmol g^–1 fw, but was virtually unaffected in primary needles under the same irradiation conditions. Pulse labelling with L-(U-¹⁴C)phenylalanine revealed that these metabolites were formed de novo. Phenylalanine ammonia-lyase (EC 4.3.1.5) and chalcone synthase (EC 2.3.1.74) were only slightly induced by the UV-B treatment. The results described here represent the first report on UV-B-induced flavonoid biosynthesis in a conifer species. Received: 5 December 1995 / Accepted: 20 March 1996     

The uptake of and structural changes induced by trichloroacetic acid in the needles of Scots pine seedlings

The uptake of trichloroacetic acid (TCA) and structural changesinduced in the needles of Scots pine (Pinus sylvestris L.) seedlingswere studied. Two exposure set-ups, a root route and an atmosphericroute through the surfaces simulating the wet deposition offog, were used. Both set-ups included two dose levels and correspondingcontrol treatments. The temperature and the relative humidityin the climate chambers were adjusted to represent the conditionsof June–July in a subarctic area in central Finland. Theseedlings were exposed three times a week for two months. Theresults showed that the uptake of TCA in needles occurred bothvia roots and via needle surface. However, most of the TCA viathe atmospheric route was absorbed on the surface of the needles.The structural responses in pine needles depended partly onthe treatment method: TCA applied via the atmospheric routedisintegrated the structures of the epicuticular waxes and thatof the stomatal cells, which was not seen in the exposures viaroots. A common feature was the decrease in size of the chloroplastsin concert with the increasing TCA concentrations inside theneedles. Key words: Pinus sylvestris L., climate chamber, effects, microscopic structure, secondary pollutant, TCA, trichloroacetic acid     

Okadaic acid and trifluoperazine enhance <Emphasis Type="Italic">Agrobacterium</Emphasis>-mediated transformation in eastern white pine

Mature zygotic embryos of recalcitrant Christmas tree species eastern white pine (Pinus strobus L.) were used as explants for Agrobacterium tumefaciens strain GV3101-mediated transformation using the uidA (β-Glucuronidase) gene as a reporter. Influence of the time of sonication and the concentrations of protein phosphatase inhibitor (okadaic acid) and kinase inhibitor (trifluoperazine) on Agrobacterium-mediated transformation have been evaluated. A high transformation frequency was obtained after embryos were sonicated for 45–50 s, or treated with 1.5–2.0 μM okadaic acid or treated with 100–200 μM trifluoperazine, respectively. Protein phosphatase and kinase inhibitors enhance Agrobacterium-mediated transformation in eastern white pine. A 2–3.5-fold higher rate of hygromycin-resistant callus was obtained with an addition of 2 μM okadaic acid or 150 μM trifluoperazine or sonicated embryos for 45 s. Stable integration of the uidA gene in the plant genome of eastern white pine was confirmed by polymerase chain reaction (PCR), Southern and northern blot analyses. These results demonstrated that a stable and enhanced transformation system has been established in eastern white pine and this system would provide an opportunity to transfer economically important genes into this Christmas tree species. Communicated by W. H. Wu     

Effects of increasing saturation vapour pressure deficit on growth and ABA levels in black spruce and jack pine

 Plant responses to saturation vapour pressure deficit (SVPD) were studied by subjecting black spruce [Picea mariana (Mill) B.S.P.] and jack pine seedlings (Pinus banksiana Lamb.) to humid (0.3 – 0.8 kPa) or dry (2.0 – 2.5 kPa SVPD) regimes for 4 weeks using a computer-controlled environmental system to control diurnal variation in SVPD. Dry matter accumulation in needles was not altered by increasing SVPD. However, root growth declined by 60% which increased shoot to root ratio and reduced total seedling dry weight in both black spruce and jack pine. Relative growth rate of jack pine also declined to about half the rate of plants grown under humid conditions. In situ root marking studies showed that the decline in root growth of jack pine under the high SVPD was the result of reduced lateral root initiation, whereas root elongation was unaffected by humidity. A 4-week exposure to dry air increased abscisic acid (ABA) levels in needles, but not roots, of jack pine whereas ABA levels in black spruce were not altered. A short (3-day) exposure failed to increase needle ABA levels in either species. These results suggest that the responses of conifers to dry air were not the result of ABA accumulation. Received: 24 March 1996 / Accepted: 30 May 1996     

Effects of long-term open-air exposure to fluoride,nitrogen compounds and SO2 on visible symptoms,pollutant accumulation and ultrastructure of Scots pine and Norway spruce seedlings

 Effects of SO₂, aqueous fluoride (NaF) and a solution of nitrogen compounds (NH₄NO₃) on the visible symptoms, pollutant accumulation and ultrastructure of Scots pine (Pinus sylvestris L.) and Norway spruce [Picea abies (L.) Karst.] seedlings were studied in an open-air experiment lasting for 3 consecutive years. Visible injury symptoms were most pronounced in combination exposures and whenever F was applied. Visible symptoms correlated well with needle pollutant concentrations. Exposure to NaF increased needle F contents particularly when F was applied with SO₂ or NH₄NO₃. This suggests that a reduction in N or SO₂ emissions, in F polluted areas, could improve the condition of conifers via decreased accumulation of phytotoxic F in the needles. Norway spruce needles accumulated 2 – 10 times as much S and F as those of Scots pine. Microscopic observations showed various changes in the needle mesophyll cell ultrastructure. In both species, exposure to SO₂ increased significantly the amount of cytoplasmic vacuoles, suggesting detoxification of excess sulphate or low pH. F treatments resulted in a significant enlargement of plastoglobuli in Scots pine and a darkening of plastoglobuli in Norway spruce. All exposures enhanced the accumulation of lipid bodies. An increased portion of translucent plastoglobuli was most pronounced in N treatments. Many of the ultrastructural changes and visible symptoms appeared only as number of years exposed increased, indicating that long-term experiments are needed. Both visible symptoms and ultrastructural changes pointed to the more pronounced sensitivity of Norway spruce compared to Scots pine. Ultrastructural results mostly supported earlier qualitative observations of F, N and SO₂ effects on needle mesophyll cell ultrastructure. However, no reduction of thylakoids in SO₂ containing exposure or curling of thylakoids in F exposure could be detected in the present study. Received: 5 December 1994 / Accepted: 28 April 1995     

Ectomycorrhizal community structure of different genotypes of Scots pine under forest nursery conditions

In this paper, we report the effect of Scots pine genotypes on ectomycorrhizal (ECM) community and growth, survival, and foliar nutrient composition of 2-year-old seedlings grown in forest bare-root nursery conditions in Lithuania. The Scots pine seeds originated from five stands from Latvia (P1), Lithuania (P2 and P3), Belarus (P4), and Poland (P5). Based on molecular identification, seven ECM fungal taxa were identified: Suillus luteus and Suillus variegatus (within the Suilloid type), Wilcoxina mikolae, Tuber sp., Thelephora terrestris, Cenococcum geophilum, and Russuloid type. The fungal species richness varied between five and seven morphotypes, depending on seed origin. The average species richness and relative abundance of most ECM morphotypes differed significantly depending on pine origin. The most essential finding of our study is the shift in dominance from an ascomycetous fungus like W. mikolae in P2 and P4 seedlings to basidiomycetous Suilloid species like S. luteus and S. variegatus in P1 and P5 seedlings. Significant differences between Scots pine origin were also found in seedling height, root dry weight, survival, and concentration of C, K, Ca, and Mg in the needles. The Spearman rank correlation coefficient revealed that survival and nutritional status of pine seedlings were positively correlated with abundance of Suilloid mycorrhizas and negatively linked with W. mikolae abundance. However, stepwise multiple regression analysis showed that only survival and magnesium content in pine needles were significantly correlated with abundance of ECM fungi, and Suilloid mycorrhizas were a main significant predictor. Our results may have implications for understanding the physiological and genetic relationship between the host tree and fungi and should be considered in management decisions in forestry and ECM fungus inoculation programs.     

Altered root growth and plant chemistry ofPinus sylvestris seedlings subjected to aluminum in nutrient solution

One-year-old Scots pine (Pinus sylvestris L.) seedlings were grown for 9 weeks in nutrient solutions containing 0, 0.5, 1, 2 and 4 mM aluminum nitrate (Al(NO3)3) at pH 4.2. Nine weeks exposure to Al significantly reduced total plant, shoot and root mass and caused a linear decline in proportional allocation of biomass to roots. Relative growth rate of roots declined to as low as zero. Aluminum treatment decreased calcium and magnesium uptake and increased Al content in roots and needles. After 3 weeks of exposure a 10–60% increase in total phenols in roots and a 20–40% increase in o-diphenols in roots and needles were noted. Roots affected by Al showed degeneration of meristematic cells, fewer cell divisions, deformation in cell walls and higher lignification and suberization. The majority of root apices were structurally similar to dormant roots, and a premature senescence of the entire root system was observed. Net photosynthetic rate after 6 weeks of treatment was negatively correlated with needle Al content and Al/Ca ratio (r < -0.9, P < 0.1). The results suggest that Scots pine may be more susceptible to Al than was expected based on previous experiments.     

Frost hardening and photosynthetic performance of Scots pine (Pinus sylvestris L.). II. Seasonal changes in the fluidity of thylakoid membranes

The fluidity of chloroplast thylakoid membranes of frost-tolerant and frost-sensitive needles of␣three- to four-year-old Scots pine (Pinus sylvestris L.) trees, of liposomes produced from the lipids of the thylakoids of these needles, and of liposomes containing varying amounts of light-harvesting complex (LHC) II protein was investigated by means of electron paramagnetic resonance (EPR) measurements using spin-labelled fatty acids as probes. Broadening of the EPR-resonance signals of 16-doxyl stearic acid in chloroplast membranes of frost-sensitive needles and changes in the amplitudes of the peaks were observed upon a decrease in temperature from +30 °C to −10 °C, indicating a drastic loss in rotational mobility. The lipid molecules of the thylakoid membranes of frost-tolerant needles exhibited greater mobility. Moderate frost resistance could be induced in Scots pine needles by short-day treatment (Vogg et al., 1997, Planta, this issue), and growth of the trees under short-day illumination (9 h) resulted in a higher mobility of the chloroplast membrane lipids than did growth under long-day conditions (16 h). The EPR spectrum of thylakoids from frost-tolerant needles at −10 °C was typical of a spin label in highly fluid surroundings. However, an additional peak in the low-field range appeared in the subzero temperature range for the chloroplast membranes of frost-sensitive needles, which represents spin-label molecules in a motionally restricted surrounding. The EPR spectra of thylakoids and of liposomes of thylakoid lipids from frost-hardy needles were identical at +30 °C and −10 °C. The corresponding spectra from frost-sensitive plants revealed an additional peak for the thylakoids, but not for the pure liposomes. Hence, the domains with restricted mobility could be attributed to protein-lipid interactions in the membranes. Broadening of the spectrum and the appearance of an additional peak was observed with liposomes of pure distearoyl phosphatidyl glycerol modified to contain increasing amounts of LHC II. These results are discussed with respect to a loss of chlorophyll and chlorophyll-binding proteins in thylakoids of Scots pine needles under winter conditions. Received: 3 March 1997 / Accepted: 16 July 1997     

Development of scots pine seedlings and functioning of antioxidant systems under the chronic action of lead ions

This study has shown that the effect of Pb²⁺ ions (10–150 μM) on Scots pine seedlings is manifested by a biomass decrease and a delay in development of the root system, including shortening of the main root, reduction of the lateral root formation zone, and reduction of the number of lateral roots. The ability of the root system to deposit Pb²⁺ ions and to perform a barrier function, preventing Pb²⁺ uptake into assimilating organs, has been revealed. This ability is blocked if the Pb²⁺ concentration in the nutrient medium exceeds 80 μM. In the case of elevated Pb²⁺ concentrations, the content of photosynthetic pigments in pine needles decreases, whereas that in cotyledons increases. Analysis of the proline content and the functioning of the antioxidant enzyme system (superoxide dismutase, catalase, ascorbate peroxidase, and peroxidase) shows that the presence of Pb²⁺ ions in a wide concentration range does not induce intensive oxidative stress in pine seedlings.     

The effects of UV-B radiation on epidermal anatomy in loblolly pine (Pinus taeda L.) and Scots pine (Pinus sylvestris L.)

The effects of enhanced UV‐B radiation on the needle anatomy of loblolly pine (Pinus taeda L.) and Scots pine (Pinus sylvestris L.) were studied in the field under supplemental UV‐B radiation supplied by a modulated irradiation system. The supplemental UV‐B levels were designed to simulate either a 16 or 25% loss of stratospheric ozone over College Park, Maryland. Enhanced UV‐B radiation caused different responses in these two species. The needles of loblolly pine had larger amounts of tannin in the lumen of epidermal cells and more wall‐bound phenolics in the outer epidermal walls of UV‐B‐treated needles, whereas the most pronounced effect on Scots pine needles was increased cutinization. In both species, the outer epidermal cell walls thickened and the needle cross‐sectional and mesophyll areas decreased (statistically significantly only in Scots pine). This suggests that more carbon may have been allocated to the protection mechanisms at the expense of photosynthetic area. The difference in response between these species suggests that the response to UV‐B radiation is not mediated by a single mechanism and that no generalization with regard to the effects of UV‐B on conifers can be made.     

Carbon balance and allocation of assimilated CO2 in Scots pine, Norway spruce, and Silver birch seedlings determined with gas exchange measurements and 14C pulse labelling

Carbon dioxide is released from the soil to the atmosphere in heterotrophic respiration when the dead organic matter is used for substrates for soil micro-organisms and soil animals. Respiration of roots and mycorrhiza is another major source of carbon dioxide in soil CO₂ efflux. The partitioning of these two fluxes is essential for understanding the carbon balance of forest ecosystems and for modelling the carbon cycle within these ecosystems. In this study, we determined the carbon balance of three common tree species in boreal forest zone, Scots pine, Norway spruce, and Silver birch with gas exchange measurements conducted in laboratory in controlled temperature and light conditions. We also studied the allocation pattern of assimilated carbon with ¹⁴C pulse labelling experiment. The photosynthetic light responses of the tree species were substantially different. The maximum photosynthetic capacity (P _max) was 2.21 μg CO₂ s⁻¹ g⁻¹ in Scots pine, 1.22 μg CO₂ s⁻¹ g⁻¹ in Norway spruce and 3.01 μg CO₂ s⁻¹ g⁻¹ in Silver birch seedlings. According to the pulse labelling experiments, 43–75% of the assimilated carbon remained in the aboveground parts of the seedlings. The amount of carbon allocated to root and rhizosphere respiration was about 9–26%, and the amount of carbon allocated to root and ectomycorrhizal biomass about 13–21% of the total assimilated CO₂. The ¹⁴CO₂ pulse reached the root system within few hours after the labelling and most of the pulse had passed the root system after 48 h. The transport rate of carbon from shoot to roots was fastest in Silver birch seedlings.     

Aluminium causes nutrient imbalance and structural changes in the needles of Scots pine without inducing clear root injuries

The effects of aluminium chloride (AICI3) treatments (50 and 150 mg/l) on 3-year-old Scots pine (Pinus sylvestris L.) seedlings were studied in a sand culture during 2 growing periods in an open field experiment. Even by the end of the first growing period, a decline was observed in the concentrations of Ca, Mg and P within the needles, and of Ca and Mg in the roots. After the second growing period, increased N and K concentrations were observed in the needles of Al-treated seedlings. Both the needles and roots of Al-treated seedlings showed, after the second growing period, a decline in growth and increased concentrations of AI as the amount of AICI3 in the nutrient solution increased. Al-induced changes in needle structure were found to be symptomatic of a nutrient imbalance, particularly of Mg and P. Al-stress did not result in any observable changes in root anatomy or in the number of mycorrhizas. Scots pine proved to be rather resistant to Al-stress, indicating that direct Al-injuries are not likely in the field, though Al-stress may be a contributing factor in the formation of nutrient imbalances.     

Scots pine as a model plant for studying the mechanisms of conifers adaptation to heavy metal action: 2. Functioning of antioxidant enzymes in pine seedlings under chronic zinc action

Functioning of antioxidant enzymes (superoxide dismutase (SOD), catalase (CAT), ascorbate peroxidase (APO), and guaiacol peroxidases (GPO)) and low-molecular organic ROS scavengers (proline and phenolic compounds) in various organs (roots, cotyledons, stem, and needle) of 6-week-old seedlings of pine (Pinus sylvestris L.) developing in the chronic presence of ZnSO₄ (50, 100, and 150 μM). Pine seedlings were grown in water culture in the climate-controlled chamber at an irradiance of 37.6 W/m² with a 16-h photoperiod, an air temperature of 23 ± 1/15 ± 1°C (day/night), and a relative humidity of 55/70% (day/night). Endogenous Zn content was a key factor determining SOD activity decomposing superoxide into H₂O₂ and O₂. Hydrogen peroxide produced is efficiently destroyed by CAT and also by APO and GPO. At the same time, the content of proline increased (especially at 150 μM ZnSO₄), but the content of phenolic compounds remained unchanged. All these processes help to maintain stable intracellular levels of O₂^⊙− and H₂O₂ at elevated zinc concentrations and to prevent generation of hydroxyl radical and development of oxidative stress.