Leaf defense capacity of Japanese elm (Ulmus davidiana var. japonica) seedlings subjected to a nitrogen loading and insect herbivore dynamics in a free air ozone-enriched environment.

Japanese elm (Ulmus davidiana var. japonica) is a native species in cool-temperate forests in Japan. We investigated growth, physiological reactions, and leaf defense capacity of Japanese elm seedlings under nitrogen (N) loading (45.3 kg N ha-1 year-1) and seasonal insect dynamics in a free-air ozone (O3)-enriched environment (about 54.5 nmol O3 mol-1) over a growing season. Higher leaf N content and lower condensed tannin content in the presence of N loading and lower condensed tannin content in elevated O3 were observed, suggesting that both N loading and elevated O3 decreased the leaf defense capacity and that N loading further enhanced the leaf quality as food resource of insect herbivores. Two major herbivores were observed on the plants, elm leaf beetle (Pyrrhalta maculicollis) and elm sawfly (Arge captiva). The peak number of observed insects was decreased by N loading. Visible foliar injury caused by N loading might directly induce the reduction of number of the observed elm sawfly individuals. While elevated O3 slightly suppressed the chemical defense capacity, significantly lower number of elm leaf beetle was observed in elevated O3. We conclude that N loading and elevated O3 can alter not only the leaf defense capacity of Japanese elm seedlings but also the dynamics of elm leaf beetle and sawfly herbivores.

Nitrogen loading increases the ozone sensitivity of larch seedlings with higher sensitivity to nitrogen loading.

Larch (Larix sp.) tree is a critical species for the future afforestation in Northeast Asia. The impacts of elevated concentrations of ground-level ozone (O3) and nitrogen (N) deposition are raising concerns. However, knowledge of the combined effects of elevated O3 and N loading are still limited. We investigated whether nitrogen loading mitigates the negative impacts of ozone on two larch species: the Japanese larch (L. kaempferi) and its hybrid larch F1 (L. gmelinii var. japonica × L. kaempferi) or not. We used open-top cambers and compared responses of the larch seedlings. Results showed the N loading mitigated the negative effects of O3 on Japanese larch. However, in hybrid larch F1, N loading did not mitigate O3-induced inhibition of growth and photosynthetic capacity. Mitigation effect of N loading on negative O3 impacts may vary between the two Larix spp., Hybrid larch F1 could be more affected by the combined effects of O3 and N loading due to its higher growth response to N loading. Elevated O3 also reduced leaf nitrogen/phosphorus (N/P) ratio by elevated O3, with significant effects in hybrid larch F1, particularly under N loading. In the present study, leaf N/P ratio was utilized to validate the hypothesis that a positive effect of N loading may be observed if O3 does not induce P limitation in Larix spp. We demonstrated a potential leaf N/P ratio function, which could reflect responses to O3 and N loading in hybrid larch F1.

Impacts of ethylenediurea (EDU) soil drench and foliar spray in Salix sachalinensis protection against O3-induced injury.

It is widely accepted that elevated levels of surface ozone (O3) negatively affect plants. Ethylenediurea (EDU) is a synthetic substance which effectively protects plants against O3-caused phytotoxicity. Among other questions, the one still open is: which EDU application method is more appropriate for treating fast-growing tree species. The main aims of this study were: (i) to test if chronic exposure of Salix sachalinensis plants to 200-400mgEDUL-1, the usually applied range for protection against O3 phytotoxicity, is beneficial to plants; (ii) to evaluate the effects of chronic exposure to elevated O3 on S. sachalinensis; (iii) to assess the efficacy of two methods (i.e. soil drench and foliar spray) of EDU application to plants; (iv) to investigate the appropriate concentration of EDU to protect against elevated O3-induced damage in S. sachalinensis; and (v) to compare the two methods of EDU application in terms of effectiveness and EDU consumption. Current-year cuttings grown in infertile soil free from organic matter were exposed either to low ambient O3 (AOZ, 10-h≈28.3nmolmol-1) or to elevated O3 (EOZ, 10-h≈65.8nmolmol-1) levels during daylight hours. Over the growing season, plants were treated every nine days with 200mL soil drench of 0, 200 or 400mgEDUL-1 or with foliar spray of 0, 200 or 400mgEDUL-1 (in two separate experiments). We found that EDU per se had no effects on plants exposed to AOZ. EOZ practically significantly injured S. sachalinensis plants, and the impact was indifferent between the experiments. EDU did not protect plants against EOZ impact when applied as soil drench but it did protect them when applied as 200-400mgL-1 foliar spray. We conclude that EDU may be more effective against O3 phytotoxicity to fast-growing species when applied as a spray than when applied as a drench. Keymessage: Soil-drenched EDU was ineffective in protecting willow plants against O3-induced injury, whereas foliar-sprayed EDU was effective even at the concentration of 200mgL-1.