Seedlings of “Xuegan” (Citrus sinensis) and “Sour pummelo” (Citrus grandis) were irrigated daily with a nutrient solution at a pH of 2.5, 3, 4, 5, or 6 for 9 months. Thereafter, the following responses were investigated: seedling growth; root, stem, and leaf concentrations of nutrient elements; leaf gas exchange, pigment concentration, ribulose-1,5-bisphosphate carboxylase/oxygenase activity and chlorophyll a fluorescence; relative water content, total soluble protein level, H2O2 production and electrolyte leakage in roots and leaves. This was done (a) to determine how low pH affects photosynthesis, related physiological parameters, and mineral nutrient profiles; and (b) to understand the mechanisms by which low pH may cause a decrease in leaf CO2 assimilation. The pH 2.5 greatly inhibited seedling growth, and many physiological parameters were altered only at pH 2.5; pH 3 slightly inhibited seedling growth; pH 4 had almost no influence on seedling growth; and seedling growth and many physiological parameters reached their maximum at pH 5. No seedlings died at any given pH. These results demonstrate that citrus survival is insensitive to low pH. H+-toxicity may directly damage citrus roots, thus affecting the uptake of mineral nutrients and water. H+-toxicity and a decreased uptake of nutrients (i.e., nitrogen, phosphorus, potassium, calcium, and magnesium) and water were likely responsible for the low pH-induced inhibition of growth. Leaf CO2 assimilation was inhibited only at pH 2.5. The combinations of an impaired photosynthetic electron transport chain, increased production of reactive oxygen species, and decreased uptake of nutrients and water might account for the pH 2.5-induced decrease in CO2 assimilation. Mottled bleached leaves only occurred in the pH 2.5-treated C. grandis seedlings. Furthermore, the pH 2.5-induced alterations of leaf CO2 assimilation, water-use efficiency, chlorophylls, polyphasic chlorophyll a fluorescence (OJIP) transients and many fluorescence parameters, root and leaf total soluble proteins, H2O2 production, and electrolyte leakage were all slightly greater in C. grandis than in C. sinensis seedlings. Hence, C. sinensis was slightly more tolerant to low pH than C. grandis. In conclusion, our findings provide novel insight into the causes of low pH-induced inhibition of seedling growth and leaf CO2 assimilation.

Knowing how acid soils and aluminum in soils may limit the growth of Eucalyptus trees in plantations is important because these plantations grow in many tropical and subtropical regions. Seedlings of four vegetatively propagated Eucalyptus clones, E. grandis × E. urophylla ‘GLGU9’(G9), E. grandis × E. urophylla ‘GLGU12’ (G12), E. urophylla × E. camaldulensis ‘GLUC3’ (G3) and E. urophylla ‘GLU4’(G4), were subjected to liquid culture with Hoagland nutrient solution for 40 days, then treated with four different treatments of acid and aluminum for 1 day. The four treatments used either pH 3.0 or 4.0 with or without added aluminum (4.4 mM) in all possible combinations; a control used no added aluminum at pH 4.8. Subsequently, the photosynthetic parameters and morphology of leaves from eucalypt seedlings were determined and observed. The results showed that the tested chlorophyll content, net photosynthetic rate, transpiration rate and water use efficiency were apparently inhibited by aluminum. Under uniform Al concentration (4.4 mM), the Al-induced limitation to photosynthetic parameters increased with pH, indicating acid stimulation to Al toxicity. Among all treatments, the most significant reduction was found in the combination of pH 3.0 and 4.4 mM Al. The photosynthetic and transpiration rates showed similar trends with G9 > G12 > G3 > G4, suggesting that G9 and G12 had higher Al-tolerance than other two clones. Microscopic observation revealed changes in leaf morphology when exposed to Al stress; for example, a reduced thickness of leaf epidermis and palisade tissue, the descendant palisade tissue/spongy tissue ratio and leaf tissue looseness. Overall, the acid and aluminum stress exerted negative effects on the photosynthetic activity of eucalypt seedlings, but the differences in tolerance to Al toxicity between the clones were favorable, offering potential to improve Eucalyptus plantation productivity by selecting Al tolerant clones.

No abstract

Transcriptomic analysis of cultured fungi suggests that many genes for secondary metabolite synthesis are presumably silent under standard laboratory condition. In order to investigate the expression of silent genes in symbiotic systems, 136 fungi-fungi symbiotic systems were built up by co-culturing seventeen basidiomycetes, among which the co-culture of Trametes versicolor and Ganoderma applanatum demonstrated the strongest coloration of confrontation zones. Metabolomics study of this co-culture discovered that sixty-two features were either newly synthesized or highly produced in the co-culture compared with individual cultures. Molecular network analysis highlighted a subnetwork including two novel xylosides (compounds 2 and 3). Compound 2 was further identified as N-(4-methoxyphenyl)formamide 2-O-β-D-xyloside and was revealed to have the potential to enhance the cell viability of human immortalized bronchial epithelial cell line of Beas-2B. Moreover, bioinformatics and transcriptional analysis of T. versicolor revealed a potential candidate gene (GI: 636605689) encoding xylosyltransferases for xylosylation. Additionally, 3-phenyllactic acid and orsellinic acid were detected for the first time in G. applanatum, which may be ascribed to response against T.versicolor stress. In general, the described co-culture platform provides a powerful tool to discover novel metabolites and help gain insights into the mechanism of silent gene activation in fungal defense.

No abstract