The effect of combined stresses, photoinhibition, and nutrient depletion on the oxidative stress of cyanobacteria was measured in laboratory experiments to develop the biomass prediction model. Phormidium ambiguum was exposed to various photosynthetically active radiation (PAR) intensities and phosphorous (P) concentrations with fixed nitrogen concentrations. The samples were subjected to stress assays by detecting the hydrogen peroxide (H₂O₂) concentration and antioxidant activities of catalase (CAT) and superoxide dismutase (SOD). H₂O₂ concentrations decreased to 30 µmol m⁻² s⁻¹ of PAR, then increased with higher PAR intensities. Regarding P concentrations, H₂O₂ concentrations (nmol L⁻¹) generally decreased with increasing P concentrations. SOD and CAT activities were proportionate to the H₂O₂ protein⁻¹. No H₂O₂ concentrations detected outside cells indicated the biological production of H₂O₂, and the accumulated H₂O₂ concentration inside cells was parameterized with H₂O₂ concentration protein⁻¹. With over 30 µmol m⁻² s⁻¹ of PAR, H₂O₂ concentration protein⁻¹ had a similar increasing trend with PAR intensity, independently of P concentration. Meanwhile, with increasing P concentration, H₂O₂ protein⁻¹ decreased in a similar pattern regardless of PAR intensity. Protein content decreased with gradually increasing H₂O₂ up to 4 nmol H₂O₂ mg⁻¹ protein, which provides a threshold to restrict the growth of cyanobacteria. With these results, an empirical formula—protein (mg L⁻¹) = − 192*Log((H₂O₂/protein)/4.1), where H₂O₂/protein (nmol mg⁻¹) = − 0.312*PAR²/(50² + PAR²)*((25/PAR)⁴ + 1)*Log(P/133,100), as a function of total phosphorus concentration, P (µg L⁻¹)—was developed to obtain the cyanobacteria biomass.

There is an information gap regarding heat stress-induced oxidative damage and the species-specific behavior of plants under stress conditions. The present study was designed with the hypothesis that heat stress may induce species-specific oxidative damage that determines the competitive capacity of common submerged macrophytes. We conducted two laboratory experiments to simulate mono- and mixed cultures of three submerged macrophytes with the application of two heat shock treatments. The results showed that both heat shocks had significant effects on growth, photosynthetic pigments and the ability to induce strong oxidative damage for all three species. The comparative results of mono- and mixed cultures showed that P. crispus had an advantage in both the control and high-temperature treatments over the other two species as a strong competitor in the mixed culture. Further, the competitive capacity of P. crispus increased in the moderately high-temperature condition compared to the control.

Light is an important factor that affects cyanobacterial growth and changes in light can influence their growth and physiology. However, an information gap exists regarding light-induced oxidative stress and the species-specific behavior of cyanobacteria under various light levels. This study was conducted to evaluate the comparative effects of different light intensities on the growth and stress responses of two cyanobacteria species, Pseudanabaena galeata (strain NIES 512) and Microcystis aeruginosa (strain NIES 111), after periods of two and eight days. The cyanobacterial cultures were grown under the following different light intensities: 0, 10, 30, 50, 100, 300, and 600 μmol m⁻² s⁻¹. The optical density (OD₇₃₀), chlorophyll a (Chl-a) content, protein content, H₂O₂ content, and the antioxidative enzyme activities of catalase (CAT) and peroxidase (POD) were measured separately in each cyanobacteria species. P. galeata was negatively affected by light intensities lower than 30 μmol m⁻² s⁻¹ and higher than 50 μmol m⁻² s⁻¹. A range of 30 to 50 μmol m⁻² s⁻¹ light was favorable for the growth of P. galeata, whereas M. aeruginosa had a higher tolerance for extreme light conditions. The favorable range for M. aeruginosa was 10 to 100 μmol m⁻² s⁻¹.

We explored the allelopathic potential of kudzu as a function of its phenolics. Aqueous and methanol extracts of different kudzu organs (leaf, stem, root, and seed) were assayed for allelopathy with the use of lettuce and radish seeds. Both leaf and root extracts significantly inhibited all of the measured germination indices (total germination, speed of germination, and coefficient of the rate of germination) (all P < 0.01). When treated with leaf extract, the total germination of both species was ∼ 20% less than the control. Furthermore, the leaf extract significantly reduced the speed of germination to 38 and 53% that of the lettuce and radish controls, respectively. Lettuce and radish seeds soaked in leaf and root extracts for 24 h imbibed less water (∼ 30% for both species) than those soaked in distilled water (control), suggesting that a reduction of water imbibition might be one of the mechanisms of germination retardation. Stem and seed extracts affected neither the water uptake nor the germination indices of radish and lettuce seeds. Kudzu leaves and roots contain higher amounts of total phenolics (P = 0.001) and soluble phenolics (P = 0.005) than stems and seeds, consistent with the results of the germination bioassays. In agar plate bioassays, both litter and rhizosphere soil had phytotoxic effects on the radicle growth of radish (P = 0.003) and perennial ryegrass (P = 0.001) seedlings. Perennial ryegrass and cobbler's pegs seedlings grown on leaf and root leachate-amended soil gave ∼ 40% shorter roots and shoots and ∼ 50% less dry weight than those grown in leachate-free soil. Kudzu litter was incorporated and incubated in soil for 6 wk, at which point the soluble phenolics in the soil solution were at a level (> 150 ppm) considered to be allelopathic.