Search

The effects of mycotoxin zearalenone (ZEN) on the photochemical activity of photosystem II (PSII) in wheat and soybean leaf discs incubated in ZEN solutions as well as the after-effects of pre-sowing soaking of seeds in solutions containing ZEN on the photochemical activity of PSII and on the seedlings growth under salt stress (NaCl solutions were investigated). The incubation of wheat leaf discs in ZEN solutions strongly inhibited the energy flux per cross section (CS) for absorption (ABS/CS), trapping (TRo/CS) and electron transport (ETo/CS), while the effects of ZEN action on soybean discs were opposite and the values of those parameters significantly increased with the increase in ZEN concentration. Incubation of seeds in a ZEN solution resulted in an increase in photochemical efficiency of PSII in soybean seedlings, but did not induce any response of PSII in those of wheat at medium illuminations. Only at the stronger illumination for both species did ZEN induce an increase in efficiency of excitation energy capture by open PSII reaction centers, photochemical quenching of chlorophyll a fluorescence and quantum yield of PSII electron transport. Pre-sowing soaking of seeds in a ZEN solution decreased the photoinhibitory injuries of PSII in wheat and soybean due to safe scattering of the excess excitation energy through an increase in energy-dependent quenching (qE) and state transition quenching (qT). ZEN when added to NaCl solutions during the period of germination contributed to reduction in the growth inhibition of wheat seedlings. The incubation of wheat leaf discs in ZEN solutions strongly inhibited CS, ABS/CS, TRo/CS and ETo/CS. Possible effects of ZEN on some physiological processes in plants have been discussed especially in the context with photochemical activity of PSII and a salt stress.

The aim of the study was to verify the hypothesis that brassinosteroids would modify the cold acclimation process through the stimulation of photosynthesis and carbohydrate metabolism, and would consequently cause an increase in the plant’s capacity for winter survival. Seedlings of two winter rye cultivars (winter-resistant and moderately winter-resistant types) were sprayed with 24-epibrassinolide (EBR) and then subjected to cold acclimation at 4 °C. The foliar hormone application was studied for its effects on two components of winter resistance: tolerance to frost; and resistance to snow mould. These were examined after medium (3 weeks) and long (6 weeks) terms of cold acclimation. Additionally, as a control group, the level of endogenous brassinosteroids was measured in non-treated plants. The maximum acquired snow mould resistance was observed in the EBR-treated plants after 3 weeks of cold acclimation, while the maximum frost tolerance was recorded after 6 weeks of cold acclimation, as compared with control. In the winter-resistant cultivars, the EBR-induced snow mould resistance coincided with a lower photosynthetic efficiency and a lack of change in their Rubisco activity and level of total soluble carbohydrates. The elevated EBR-induced frost tolerance in the winter-resistant cultivars was associated with a carbohydrate metabolism that promoted a decrease in the monosaccharides of the leaves that stimulate photosynthetic efficiency and Rubisco activity. For both cultivars, the control plants developed frost tolerance between the third and sixth week of cold acclimation. However, the winter-resistant cultivar was characterised by a constant castasterone (CS) level, while the moderately winter-resistant one experienced a sharp increase in its endogenous CS after 3 weeks, and then after 6 weeks of cold acclimation.

We investigated the modification of cold-induced mechanisms of photosynthetic apparatus adjustment and phytohormone response by brassinosteroid 24-epibrassinolide (EBR) and its consequences for frost tolerance of perennial ryegrass (Lolium perenne L.). We recorded the responses of two cultivars with contrasting frost tolerances to foliar hormone application, both in non-acclimated plants and plants cold acclimated for 3 and 6 weeks at 4 °C. In non-cold-acclimated plants of both cultivars, EBR induced increases in carbon fixation and lowered sucrose levels. Temporary suppression in quantum efficiency of PSII of photosystem II and activities of ribulose-1,5-bisphosphate carboxylase/oxygenase and sucrose phosphate synthase, a consequence of energy dissipation in non-photochemical quenching, was observed in the leaves of the highly frost-tolerant cultivar after 3 weeks of cold acclimation. After 6 weeks of cold acclimation, EBR accelerated recovery of photosynthesis, reflecting adjustment to cold conditions, and increased frost tolerance. As carbohydrate export from leaves is favored during cold acclimation, EBR application did not increase frost tolerance of the moderately tolerant cultivar, reflecting the downregulation of photosynthesis due to high leaf sucrose concentrations. It is also likely that EBR participated in the enhancement of frost tolerance by regulation of stress-related signaling compounds such as JA and ethylene but not SA, in winter ryegrass undergoing cold acclimation. Taken together, our results demonstrate that EBR-induced changes are temperature dependent. The beneficial effect of EBR is not universal under cold conditions, as genetically determined mechanisms are apparently dominant relative to EBR action.