Brassinosteroids (BRs) are steroidal phytohormones with a wide scale of effects. BRs, which play an essential role in plant growth and development, have been implicated in many physiological response. By exposing plants to the drought stress as well as to the heavy metals their survival is improved and also resistance and yield are increased. The plant response is regulated directly (by the synthesis of metabolites) or indirectly (by induction of antioxidizing compounds and enzymes), often in the interaction with other phytohormones. Experiments studying an impact of brassinosteroids on the reaction of plants stressed e.g. by water deficit are different in various parameters. The influence of application of 28-homobrassinolide (HBR) was studied in young plants of sunflowers (Helianthus annuus L.) cv. BELINDA,cv. MAS 97 and cv. SPIROV grown in the greenhouse under optimum and water deficit conditions. The leaves of 32-d-old plants sunflowers were sprayed with (28-HBR) onto the leaves at 0.01 and 1 μM concentrations for 3 days with a 1-day interval. Three levels of drought stress (0, 3, and 5 days withholding water) were applied. Thereafter, the effects of brassinosteroids and water stress were investigated on some biochemical and antioxidative parameters of sunflower plants. Lipid peroxidation, and proline content increased in plants subjected to drought stress. Based on our results it seems that brassinosteroids considerably alleviated oxidative damage that occurred under drought stress. The differences between drought-stressed and well-watered plants, brassinosteroid-treated and -nontreated plants were analyzed. According to the obtained results, it seems that in young sunflowers plants treated with lower concentrations (0.01 and 1µM) of 28-homo, drought had less negative impact on the monitored parameters compared to plants normally watered - growth rate did not change, differences in photosynthetic parameters were smaller. However, the observed differences between HBR-treated and non-treated plants were usually not statistically significant. Proline is accumulated in many plant species in response to osmotic stress, which is stimulated by drought. Results have been shown in sunflowers plants under drought stress, in which a reduction of root weight was correlated to stress severity. Treatment with BR fully compensated for the reduction in biomass caused by mild drought stress. On the other hand, in creases in biomass was correlated with increases in acid inverters activity in young leaves, which likely provided more assimilates to the plant due to their larger sizes. Furthermore, osmotic stress resulted in considerable reduction in the protein contents in all the three varieties of sunflowers. However, BRs not only restored but also stimulated the level of protein and free proline. In leaves of experimental plants cultivars MAS 97 and SPIROV after water stress treatment the level of malondialdehyde (MDA) content has been increased on 11% and 30% respectively. The higher MDA content has been observed in leaves of cultivar BELINDA. The combination of drought and HBR has been shown MDA content in leaves of all experimental plants on control level which can evidence about protection effect of BRs under water deficit treatment on the leaves of experimental cultivars of sunflower plants. BR-regulated stress response as a result of a complex sequence of biochemical reactions such as activation or suppression of key enzymatic reactions, induction of protein synthesis, and the production of various chemical defence compounds. BRs open up new approaches for plant resistance against hazardous environmental conditions.

28-brassinolidesteroids, drought stress, chlorophylls, Helianthus annuus L, MDA, proline

Presentation: oral