Heat-induced oxidative activity protects suspension-cultured plant cells from low temperature damage.

A short heat pre-treatment (1 h at 38°C) was found to protect both suspension-cultured apple fruit cells and tobacco cells from cold-induced cell death. Tobacco cells were more sensitive to low temperatures than apple cells, with significant cell death after 48 h at 0 or -2°C. Real-time measurements of H2O2 levels during the heat pre-treatment revealed a substantial burst of this reactive oxygen species in both cell types. Real-time and longer-term measurements also showed a large burst of H2O2 production from tobacco cells, but not apple cells, when exposed to low temperatures. Lower temperatures reduced levels of peroxidase activity (both total and intracellular), with the heat pre-treatment preventing some of the cold-induced reduction of this activity in both apple and tobacco cells. The greater sensitivity to low temperature of the tobacco cells may be related to higher H2O2 production, with the heat treatment maintaining higher peroxidase activity. The lesser sensitivity of the apple cells may be due to the lack of a H2O2 burst and maintenance of peroxidase activity by the heat treatment. These results support a role for oxidative metabolism in the beneficial effects of heat in inducing low temperature tolerance.

Anthocyanins in leaves: light attenuators or antioxidants?

Anthocyanins have the potential to mitigate photooxidative injury in leaves, both by shielding chloroplasts from excess high-energy quanta, and by scavenging reactive oxygen species. To distinguish between the impacts of these two putative mechanisms, superoxide (O2•-) concentration and chlorophyll oxidation were measured for Lactuca sativa L. chloroplast suspensions under various light and antioxidant-supplemented environments. A red cellulose filter, the optical properties of which approximated that of anthocyanin, effected a 33% decline in rate of O2•- generation and 37% reduction in chlorophyll bleaching, when used to shield irradiated chloroplasts. Colourless and blue tautomers of cyanidin 3-(6-malonyl)glucoside at pH 7 removed up to 17% of O2•- generated by chloroplasts, indicating that cytosolic anthocyanins can serve as effective antioxidants. Red tautomers, typical of vacuolar anthocyanins, also showed strong reducing potentials as indicated by cyclic voltammetry. These potentials declined by 40% after 15 min exposure to O2•-. Maximum quantum efficiencies of photosynthesis were similar for red and green portions of intact L. sativa leaves, but the red regions were less photoinhibited, and recovered more extensively after exposures to strong light. Anthocyanins evidently offer effective and versatile protection to leaves without significantly compromising photosynthesis.

Antioxidant capacities of green and cyanic leaves in the sun species, Quintinia serrata.

Anthocyanins scavenge harmful reactive oxygen species (ROS), and may provide antioxidative protection within plant cells. For the shade species Elatostema rugosum A. Cunn., it was recently established that red-leaved morphs held a significant antioxidant advantage over green morphs. It is not known whether anthocyanins confer the same protection in sun species that are exposed to elevated levels of irradiation and thus greater photooxidative stress. Quintinia serrata A. Cunn., a canopy tree species, is polymorphic for anthocyanin expression. Levels of antioxidants in leaves of red and green morphs of Q. serrata were identified, and activities quantified using enzymatic and α,α-diphenyl-ß-picrylhydrazyl (DPPH) assays and cyclic voltammetry. Both morphs contained similar levels of superoxide dismutase, catalase, ascorbic peroxidase and hydroxycinnamic acids, and showed similar ranges in DPPH scavenging activity and voltammetric profiles. Anthocyanin concentrations increased with leaf age in red morphs and were positively correlated with the extent of DPPH scavenging. However, hydroxycinnamic acids were the most active methanol-extractable fraction in both morphs, contributing 27% more to the overall low-molecular-weight antioxidant pool in the green morph than in the red. We conclude that anthocyanin biosynthesis can enhance, but is not a prerequisite for, oxidative protection in Q. serrata leaves. The possibility that anthocyanins provide a versatile mechanism for photoprotection is discussed.