Roots of clonal birches (Betula pendula) were inoculated with the ectomycorrhizal fungi Paxillus involutus (isolates P0 and Mi) and Hebeloma cylindrosporum (strains D1 and D105). These fungi showed different rates of mycorrhiza formation in vitro. Mature mycorrhizas were obtained after only 2–4 d with H. cylindrosporum, whereas 6–8 d were necessary with P. involutus isolate P0, and P. involutus isolate Mi was not able to form mature mycorrhiza during the 10 d of the experiment. Temporal changes in PAL activity and the expression of genes encoding intracellular pathogenesis-related proteins were followed after inoculating birch roots with these fungi. Transient increase of PAL activity, and transient induction of expression of the wound-inducible Bet v1-SC1 gene, were observed in roots challenged with both H. cylindrosporum strains and the P. involutus isolate P0. These changes were found to coincide with hyphal penetration between root cells during Hartig net formation, and were never observed in roots inoculated with the poorly aggressive P. involutus isolate Mi. Examination of mycorrhizal root sections under u.v. light indicated the presence of phenolic compounds in the host cell walls at the vicinity of the Hartig net. These results strongly suggest that hyphal penetration between the root cells triggers a transient defence response which, in turn, could limit Hartig net formation to the outer layer of the root cortex.

Birch (Betula pendula Roth) seedlings were grown under enhanced u.v.-B radiation and simulated forest-soil conditions, after which individual secondary metabolites were determined in the leaves. It was found that not all of the u.v.-absorbing secondary metabolites of the seedlings responded to supplementary u.v.-B radiation. Under increased u.v.-B radiation, significant increases in concentration were observed only for the major flavonoid, quercitrin, the minor flavonoid, myricetin-3-galactoside, and for chlorogenic acid. On the other hand, 3,4′dihydroxypropiophenone-3-β-D-glucopyranoside decreased under u.v.-B irradiation. The concentration of phenolic compounds in the leaves changed during the growing season (between two harvests) but this change was not related to u.v.-B enhancement. A low availability of mineral nutrients did not impair the capacity of the seedlings to accumulate u.v.-protecting phenolic compounds under increased u.v.-B radiation. The growth conditions used might have affected the intracellular concentrations of secondary metabolites, and thus furnished the birch seedlings with an increased tolerance of u.v.-B radiation. These findings point to the significance of certain phenolic components in the protection of deciduous trees against u.v.-B radiation.

Cloned cuttings of Betula pendula Roth were grown in field fumigation chambers at Birmensdorf throughout one growing season in filtered air with either <3 (control) or 90/40 nl l−1 O3 (day/night; ozone generated from pure oxygen). Each ozone regime was split into high and low soil nutrient regimes by watering plants with either a 0·05% or a 0·005% solution of a fertilizer which contained macronutrients and micronutrients.

Fertilization had a strong effect on plant growth, enzyme activities and the expression of ozone-induced effects at the biochemical level. The activities of PEPC and Rubisco were enhanced about threefold in the plants with high fertilization (HF). Significant effects of ozone were in most cases found only in the older leaves of the plants with low fertilization (LF). There, sucrose, glucose and fructose levels were enhanced. In both fertilization treatments, the number of starch granules along the minor veins was increased. These ozone effects point to a decreased or inhibited phloem loading. The increased PEPC activity and the enhanced malate levels in the ozone-exposed plants might be the result of a redirection of carbon flow from sucrose synthesis and translocation towards anapleurotic processes, which can feed detoxification and repair of ozone injury as indicated by enhanced respiration. These findings agree well with the observed effects of ozone in lowering the root[ratio ]shoot biomass ratio. Although there was a marked reduction in the O3/LF plants, O3/HF plants showed no significant response. Inositol was decreased under ozone exposure in both fertilizer treatments, contrasting with the pattern for carbohydrates.

These results demonstrate the role of fertilization as an important modifier of ozone-induced effects at the plant biochemical level. Well fertilized plants appear to cope better with the impact of ozone on metabolism.

Copper and zinc tolerances of 10 micropropagated birch (Betula pendula and B. pubescens) clones were studied in hydroponic culture. Tolerance indices were determined, based on the mean growth rate of the longest root in 1 wk. A seed-derived clone (142A), from a lead/Zn-contaminated site showed more tolerance to Cu and Zn than bud-derived clones (HA02 and HA18) from a Cu/nickel-contaminated site or an ozone-tolerant clone (KL-2-M) from an uncontaminated area. For Cu, the EC50 values were 30, 14, 8 and 11 μm in clones 142A, HA02, HA18 and KL-2-M, respectively. For Zn, the EC50s were 4000 and 350 μm in clones 142A and KL-2-M, respectively. The relative Cu and Zn tolerances of the other clones were estimated by growing the plants in 30 μm CuSO4, and in 2000 or 350 μm ZnSO4, respectively. It is of interest that the Zn-tolerant clone 142A was tolerant to Cu, although this metal was present at a very low concentration in the soil where the parent tree grows. Another clone (142B), from another seed of the same parent tree, was tolerant neither to Zn nor Cu. Compared with their own EC50s for root growth for Cu, 142A took up more Cu than KL-2-M, suggesting that the higher tolerance of the former clone is not explained by reduced uptake of Cu. The Zn uptake in clones 142A and KL-2-M was studied at 4000 μm and 800 μm Zn, respectively. Interestingly, the roots of both clones contained the same amount of Zn, even though clone 142A was exposed to a fivefold concentration of Zn.