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It is a common opinion that main challenge for plant production is to cope with environmental stresses, both abiotic and biotic. Although biostimulator Asahi SL has been used for years to improve plant status and obtain possibly the highest and best quality yield, especially under conditions unfavourable for plant cultivation, its mode of action is still not understood. In this work the effect of Asahi SL on Arabidopsis thaliana plants grown under drought stress was studied. Plants grown under drought stress conditions and treated with Asahi SL were, as compared to untreated, higher and more advanced in development, particularly generative. Biomass accumulation was stimulated by biostimulator mainly due to better photosynthetic apparatus efficiency manifested by a higher (i) assimilation area, (ii) chlorophyll content, (iii) intensity of photosynthesis and (iv) improvement of chlorophyll a fluorescence parameters. Asahi SL treated plants were characterized by a lower concentration of biological active ABA. Despite the higher transpiration and lowered stomatal resistance RWC and water content were unchanged in Asahi SL treated plants which can be explained by increased water uptake in biostimulator sprayed plants. The obtained results clearly showed that the application of biostimulator played a protective role against drought stress.
Festuca arundinacea (Fa) is one of the most drought-tolerant species within the Lolium-Festuca complex. In the current work the protein level of chloroplastic Cu-Zn SOD (superoxide dismutase) in two Fa plants with extreme values of drought tolerance during exposition to water deficit was investigated. The obtain results revealed higher level of enzyme accumulation in more drought-tolerant Fa genotype. In less-drought tolerant plant the increase of dismutase level during stress treatment was only slight.
Photoinhibitory processes in the photosynthetic apparatus of the seedlings of Abies alba (Mill.), Picea abies (Karst.), and Pinus mugo (Turra) growing under strong shade (5 % of full solar irradiance) or full irradiance conditions were investigated in winter and spring using chlorophyll a fluorescence techniques. The extent of photoinhibition in needles as indicated by a decrease in maximum quantum yield of PS II photochemistry (Fv/Fm) depended on species, air temperature and acclimation to the light environment. Unexpectedly, shade-tolerant Abies alba was less affected by low-temperature photoinhibition compared to the other species. Fv/Fm recovered with increasing air temperature. During winter, the seedlings of Picea abies growing in shade showed higher Fv/Fm than those from full light. Non-photochemical quenching of fluorescence (NPQ) measured at the same levels of actinic light was higher in needles acclimated to full light except for Abies alba in February. Photosynthetic performance in term of ETR (apparent electron transferrate) was also higher in full light-acclimated needles. In April, at ambient temperature, recovery of PS II efficiency from the stress induced by illumination with saturating light was faster in the needles of Picea abies than in those of Abies alba. The shade-acclimated needles of Abies alba and Picea abies showed greater down-regulation of PS II induced by high light stress.
Detached leaves of 14 day-old dark-grown pea seedlings were immersed with their cut ends either in water (control) or in 20 mM Pb(NO₃)₂ solution. They were exposed to continuous illumination during 24 and 48 h. The formation of PSII primary photochemistry in thylakoids was determined in vivo by measuring changes in values of parameters of chlorophyll a fast fluorescence kinetics: Fo, Fm, Fv, Fv/Fm and t 1/2. The amount of lead accumulation in leaves, content of chlorophylls and carotenoids and rates of CO₂ uptake in light and evolution in darkness (Pn-net photosynthesis and DR - dark respiration respectively) were determined. It has been found that with the exception of Fo, values of Fv, Fm and Fv/Fm were reduced by Pb²⁺. The values of t 1/2 were significantly larger in Pb²⁺ treated leaves. Decrease in the chlorophyll a fluorescence parameters was paralleled with the strong inhibition by this metal the biosynthesis of chlorophyll a and b but less of the carotenoids. Pb²⁺ drastically reduced Pn but had a stimulatory action on DR after 24 h and small inhibition of DR after 48 h exposure of leaves to this metal. As a consequence, after 48 h of greening the ratio of DR/Pn of control leaves was 0.45 whereas in Pb²⁺ treated leaves 2.7. It is proposed that DR in leaves plays a protective role against damage of Pn by Pb²⁺. Protection can be due to the supply the respiratory derived reductant and ATP to carry out cell metabolism upon reduced photosynthesis.
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