Lethal heat stress-dependent volatile emissions from tobacco leaves: what happens beyond the thermal edge?

Natural vegetation is predicted to suffer from extreme heat events as a result of global warming. In this study, we focused on the immediate response to heat stress. Photosynthesis and volatile emissions were measured in the leaves of tobacco (Nicotiana tabacum cv. Wisconsin 38) after exposure to heat shock treatments between 46 °C and 55 °C. Exposure to 46 °C decreased photosynthetic carbon assimilation rates (A) by >3-fold. Complete inhibition of A was observed at 49 °C, together with a simultaneous decrease in the maximum quantum efficiency of PSII, measured as the Fv/Fm ratio. A large increase in volatile emissions was observed at 52 °C. Heat stress resulted in only minor effects on the emission of monoterpenes, but volatiles associated with membrane damage such as propanal and (E)-2-hexenal+(Z)-3-hexenol were greatly increased. Heat induced changes in the levels of methanol and 2-ethylfuran that are indicative of modification of cell walls. In addition, the oxidation of metabolites in the volatile profiles was strongly enhanced, suggesting the acceleration of oxidative processes at high temperatures that are beyond the thermal tolerance limit.

Salt-stress induced modulation of chlorophyll biosynthesis during de-etiolation of rice seedlings.

Chlorophyll biosynthesis in plants is subjected to modulation by various environmental factors. To understand the modulation of the chlorophyll (Chl) biosynthesis during greening process by salt, 100-200 mM NaCl was applied to the roots of etiolated rice seedlings 12 h prior to the transfer to light. Application of 200 mM NaCl to rice seedlings that were grown in light for further 72 h resulted in reduced dry matter production (-58%) and Chl accumulation (-66%). Ionic imbalance due to salinity stress resulted in additional downregulation (41-45%) of seedling dry weight, Chl and carotenoid contents over and above that of similar osmotic stress induced by polyethylene glycol. Downregulation of Chl biosynthesis may be attributed to decreased activities of Chl biosynthetic pathway enzymes, i.e. 5-aminolevulinic acid (ALA) dehydratase (EC-2.4.1.24), porphobilinogen deaminase (EC-4.3.1.8), coproporphyrinogen III oxidase (EC-1.3.3.3), protoporphyrinogen IX oxidase (EC-1.3.3.4), Mg-protoporphyrin IX chelatase (EC-6.6.1.1) and protochlorophyllide oxidoreductase (EC-1.3.33.1). Reduced enzymatic activities were due to downregulation of their protein abundance and/or gene expression in salt-stressed seedlings. The extent of downregulation of ALA biosynthesis nearly matched with that of protochlorophyllide and Chl to prevent the accumulation of highly photosensitive photodynamic tetrapyrroles that generates singlet oxygen under stress conditions. Although, ALA synthesis decreased, the gene/protein expression of glutamyl-tRNA reductase (EC-1.2.1.70) increased suggesting it may play a role in acclimation to salt stress. The similar downregulation of both early and late Chl biosynthesis intermediates in salt-stressed seedlings suggests a regulatory network of genes involved in tetrapyrrole biosynthesis.

Salt and genotype impact on antioxidative enzymes and lipid peroxidation in two rice cultivars during de-etiolation.

Crop yield is severely affected by soil salinity, as salt levels that are harmful to plant growth occur in large terrestrial areas of the world. The present investigation describes the studies of enzymatic activities, in-gel assays, gene expression of some of the major antioxidative enzymes, tocopherol accumulation, lipid peroxidation, ascorbate and dehydroascorbate contents in a salt-sensitive rice genotype PB1, and a relatively salt-tolerant cultivar CSR10 in response to 200 mM NaCl. Salt solution was added to the roots of hydroponically grown 5-day-old etiolated rice seedlings, 12 h prior to transfer to cool white fluorescent + incandescent light (100 µmol photons m(-2) s(-1)). Total tocopherol and ascorbate contents declined in salt-stressed rice seedlings. Among antioxidative enzymes, an increase in the activities of superoxide dismutase (EC 1.15.1.1), catalase (EC 1.11.1.6), ascorbate peroxidase (EC 1.11.1.11), glutathione reductase (EC 1.6.4.2), and their gene expression was observed in both cultivars in response to salt stress. The salt-tolerant cultivar CSR10 resisted stress due to its early preparedness to combat oxidative stress via upregulation of gene expression and enzymatic activities of antioxidative enzymes and a higher redox status of the antioxidant ascorbate even in a non-stressed environment.