Mesembryanthemum crystallinum L. (common ice plant) is an edible halophyte. However, if ice plants are used to phytoremediate salinity soil, there are problems of slow initial growth, and a long period before active NaCl uptake occurs under higher salinity conditions. Application of endophytic bacteria may improve the problem, but there remain gaps in our understanding of how endophytic bacteria affect the growth and the biochemical and physiological characteristics of ice plants. The aims of this study were to identify growth-promoting endophytic bacteria from the roots of ice plants and to document the metabolomic response of ice plants after application of selected endophytic bacteria. Two plant growth-promoting endophytic bacteria were selected on the basis of their ability to promote ice plant growth. The two strains putatively identified as Microbacterium spp. and Streptomyces spp. significantly promoted ice plant growth, at 2-times and 2.5-times, respectively, compared with the control and also affected the metabolome of ice plants. The strain of Microbacterium spp. resulted in increased contents of metabolites related to the tricarboxylic acid cycle and photosynthesis. The effects of salt stress were alleviated in ice plants inoculated with the endobacterial strains, compared with uninoculated plants. A deeper understanding of the complex interplay among plant metabolites will be useful for developing microbe-assisted soil phytoremediation strategies, using Mesembryanthemum species.

The current level of tropospheric ozone (O₃) is expected to reduce the net primary production of forest trees. Here, we evaluated the negative effects of O₃ on the photosynthetic CO₂ uptake of Japanese forest trees species based on their cumulative stomatal O₃ uptake, defined as the phytotoxic O₃ dose (POD). Seedlings of four representative Japanese deciduous broad-leaved forest tree species (Fagus crenata, Quercus serrata, Quercus mongolica var. crispula and Betula platyphylla var. japonica) were exposed to different O₃ concentrations in open-top chambers for two growing seasons. The photosynthesis–light response curves (A-light curves) and stomatal conductance were measured to estimate the leaf-level cumulative photosynthetic CO₂ uptake (ΣP_{n_est}) and POD, respectively. The whole-plant-level ΣP_{n_est} were highly correlated with the whole-plant dry mass increments over the two growing seasons. Because whole-plant growth is largely determined by the amount of leaf area per plant and net photosynthetic rate per leaf area, this result suggests that leaf-level ΣP_{n_est}, which was estimated from the monthly A-light curves and hourly PPFD, could reflect the cumulative photosynthetic CO₂ uptake of the seedlings per unit leaf area. Although the O₃-induced reductions in the leaf-level ΣP_{n_est} were well explained by POD in all four tree species, species-specific responses of leaf-level ΣP_{n_est} to POD were observed. In addition, the flux threshold appropriate for the linear regression of the responses of relative leaf-level ΣP_{n_est} to POD was also species-specific. Therefore, species-specific responses of cumulative photosynthetic CO₂ uptake to POD could be used to accurately evaluate O₃ impact on the net primary production of deciduous broad-leaved trees.

Photosynthesis by trees is expected to contribute to preventing climate change including global warming. However, the current levels of tropospheric ozone (O₃) reduce the uptake of photosynthetic carbon dioxide (CO₂) by forest trees in Japan, which is a concern. Furthermore, it is unknown how much O₃ should be reduced to prevent plants’ O₃-induced damage. The objective of the present study was to assess the negative effects of the current levels of O₃ absorbed via stomata and the impact of its mitigation on the CO₂ uptake by trees in Japanese forests. Impact assessment, targeted in 2011–2015, was performed for four deciduous broad-leaved trees: Fagus crenata, Quercus serrata, Q. mongolica var. crispula, and Betula platyphylla var. japonica. The assessment was based on species-specific cumulative stomatal O₃ uptake (COU) and species-specific responses of annual photosynthetic CO₂ uptake to COU. Annual COU differed between the four trees; the average COU of F. crenata, Q. serrata, Q. mongolica var. crispula, and B. platyphylla var. japonica across Japan was 41.7, 26.5, 33.0, and 29.1 mmol m⁻², respectively, and the reductions in CO₂ uptake by the four trees were 14.0%, 10.6%, 8.6%, and 15.4%, respectively. Further analysis revealed that reducing the atmospheric O₃ concentration by approximately 28%, 20%, 17%, and 49% decreased the O₃-induced reductions in photosynthetic CO₂ uptake to 5% in F. crenata, Q. serrata, Q. mongolica var. crispula, and B. platyphylla var. japonica, respectively. In the near future, implementing mitigation measures for the O₃ damage in plants is expected to enhance the photosynthetic capacity of Japanese forest tree species.