Snejana Doncheva

The effect of zinc supply and succinate treatment on plant growth and mineral uptake in pea plant

The influence of succinate treatment on Zn toxicity was investigated using plant growth and mineral uptake as stress indicators. Pea plants (Pisum sativum L., cv. Citrine) were treated with various Zn concentrations (0.67 to 700 mM Zn) in the presence and absence of 0.2 mM Na-succinate. Plants pre-treated with succinate and then exposed to Zn exhibited higher dry root, stem and leaf weight than the plants treated with Zn alone. An increase in Zn supply resulted in a decrease in the concentrations of Ca, Mg, P in the roots and an increase of Ca and N levels in the stems and leaves. The amount of Zn in the roots, stems and leaves increased with greater Zn rates. The succinate treatment increased P in the roots but did not affect the Ca, N and Mg contents in Zn-treated plants. Most of the Zn taken up was retained in the roots after succinate treatment. The ameliorative effect of succinate on plant growth could be due to a lower Zn translocation in the leaves and stems and increased Zn accumulation in the roots. Lower Zn translocation in aboveground parts seemed to result from Zn complexing by organic anion in the roots. This probably caused less Zn transport to the stems and leaves and suggested that succinate has potential for complexing with Zn and may play a role in tolerance to high Zn levels.

INFLUENCE OF SUCCINATE ON ZINC TOXICITY OF PEA PLANTS

The response of pea plants (Pisum sativum, cv. Citrine) to various zinc (Zn) concentrations (0.67 to 1000 μM Zn) in the presence and absence of succinate (200 μM Na-succinate) were investigated. Treatment of pea plants alone with excess of Zn reduced plant growth, chlorophyll content and induced alterations in the structure of the chloroplast, resulting mainly in decreased granal thylakoids. The photochemical activity of photosystem II estimated by the ratios Fv/Fm and Fv/Fo was less affected by Zn treatment. The presence of succinate lead to an increase in plant growth and chlorophyll content, improved chloroplast structure of and recovered photosystem II activity in Zn-treated plants. This stimulation was accompanied by an increased zinc root concentration and a decreased zinc shoot concentration. The higher root zinc concentration and decreased zinc translocation from root to shoot by succinate treatment suggest that succinate facilitates the formation of metal-succinate complexes in the roots and may play a role in zinc accumulation.These results provide indirect evidence for a possible role of succinate in Zn-resistance of plants.

Copper-induced alterations in structure and proliferation of maize root meristem cells

Summary Maize plants ( Zea mays L., cv. Kneja 2L 611) were treated with various copper concentrations to investigate the effect of high copper concentrations on root growth, cell proliferation and morphology of meristem root cells. Electron microscopy and light microscopic autoradiography were used to study the influence of the copper excess on these parameters. The higher concentrations of copper inhibited root growth, shortening root length and decreasing fresh and dry root weight. Treatments with the copper excess led to a decrease in the number of labelled meristem cells after 3 H-thymidine administration. The strongest inhibition was achieved by 78 μmol/L copper when only 6% of the nuclei were in the S-phase of the cell cycle compared with 47% in. the control cells. These data show that copper interrupts the progression of nuclei at the crucial G 1 /S transition point of the cell cycle, when it prevents their entry into mitosis. The decreased root growth could be due to the effect of copper on root meristem cell proliferation. Ultrastructural investigation of root meristem cells in the case of copper excess (31–78 μmol/L) showed disturbance of plasmalemma, endoplasmic reticulum and mitochondrial membranes.

Effects of Succinate on Manganese Toxicity in Pea Plants

ABSTRACT Pea (Pisum sativum cv. Citrine) plants were grown in nutrient solution containing various manganese (Mn) concentrations in the presence or absence of succinate to evaluate the potential role of succinate in the plant tolerance to Mn excess. Supplying pea plants with excess Mn led to a reduction in the relative growth rate (RGR), chlorophyll a and b content, photosynthetic O2 evolution activity, and photosystem II (PSII) activity, as measured in the light-adapted state (φPSII) in comparison to the control. The primary photochemical efficiency of PSII, estimated by the Fv/Fm ratio, was less affected by increasing Mn concentration. Chloroplasts from Mn-treated leaves exhibited significant changes in their ultrastructure, depending on the strength of Mn toxicity. The concentration of Mn in roots, stem, and leaves increased with the increase of Mn in the nutrient solution. Addition of succinate before and after Mn treatment did not reduce the inhibitory effect of Mn on the plant growth, chlorophyll fluorescence parameters, photosynthetic O2 evolution activity, and chloroplast structure of the pea plants. It was found that supply of exogenous succinate at a high Mn concentration (over 1500 μM) in the nutrient solution led to an increase of Mn uptake in the roots accompanied by a decrease in a Mn translocation to the leaves and stems compared to Mn-treated pea-plants. However, differences in the toxicity effect of Mn in both Mn and Mn/Succinate-treated pea plants were not detected. Thus, such changes in Mn distribution within the Mn/succinate-treated plant did not confer tolerance of Mn excess to pea plants. These results suggest that succinate probably has an affinity for Mn and may function as a “terminal acceptor” of large amounts of Mn, decreasing Mn transport to the stem and leaves, but does not contribute to Mn tolerance.

Response of sun- and shade-adapted plants of Haberlea rhodopensis to desiccation

The differences in some morphological and physiological characteristics of sun- and shade-adapted Haberlea rhodopensis plants were compared. Changes in the photosynthetic activity, electrolyte leakage from leaf tissues, malondialdehyde content (MDA) and leaf anatomy were studied at different degrees of desiccation as well as after rehydration of plants. The MDA content in well-watered sun Haberlea plants was higher compared to shade plants suggesting higher lipid peroxidation, which is commonly regarded as an indicator of oxidative stress, but desiccation of plants at high light did not cause additional oxidative damage as judged by the unaffected MDA content. The electrolyte leakage from dried leaves (8% RWC) from both shade and sun plants increased fourfold indicating similar membrane damage. However, the recovery after rehydration showed that this damage was reversible. Well-watered sun plants had higher photosynthetic activity probably due to the larger thickness of the mesophyll layer in such plants. On the other hand, desiccation at high light reduced CO2 assimilation which was in accordance with the stronger reduction of stomatal conductance. Stomata were visible only on the abaxial side of sun leaves having also higher abundance of non-glandular trichomes. Increased trichomes density and epicuticular waxes and filaments upon desiccation could help plants to increase reflection, reduce net radiation income, slow down the rate of water loss and survive adverse conditions.

Senescence progression in a single darkened cotyledon depends on the light status of the other cotyledon in Cucurbita pepo (zucchini) seedlings: potential involvement of cytokinins and cytokinin oxidase/dehydrogenase activity.

Darkness mediates different senescence-related responses depending on the targeting of dark treatment (whole plants or individual leaves) and on the organs that perceive the signal (leaves or cotyledons). As no data are available on the potential role of darkness to promote senescence when applied to individual cotyledons, we have investigated how darkness affects the progression of senescence in either a single or both individually darkened cotyledons of young 10-day-old Cucurbita pepo (zucchini) seedlings. Strong acceleration of senescence was observed when both cotyledons were darkened as judged by the damage in their anatomical structure, deterioration of chloroplast ultrastructure in parallel with decreased photosynthetic rate and photochemical quantum efficiency of PSII. In addition, the endogenous levels of cytokinins (CKs) and IAA were strongly reduced. In a single individually darkened cotyledon, the structure and function of the photosynthetic apparatus as well as the contents of endogenous CKs and IAA were much less affected by darkness, thus suggesting inhibitory effect of the illuminated cotyledon on the senescence of the darkened one. Apparently, the effect of darkness to accelerate/delay senescence in a single darkened cotyledon depends on the light status of the other cotyledon from the pair. The close positive correlation between CK content and the activity of CK oxidase/dehydrogenase (CKX; EC 1.4.3.18/1.5.99.12) suggested that CKX was essentially involved in the mechanisms of downregulation of endogenous CK levels. Our results indicated that CKX-regulated CK signaling could be a possible regulatory mechanism controlling senescence in individually darkened cotyledons.

Ultrastructural localization of Ag-NOR proteins in root meristem cells after copper treatment

Summary The effect of a 2-week exposure to various copper concentrations on the structure of nucleoli and ultra-structural localization of Ag-NOR proteins was investigated in root meristem cells of Zea mays. Transmission electron microscopy and silver staining were applied on interphasic nucleoli to measure the influence of copper on these parameters. Morphological results indicate that copper treatment induces alterations in the structure of the nucleoli, which was manifested by segregation of its main nucleolar components. After nucleolar segregation the nucleoli underwent a gradual degranulation, which was completed at the highest Cu concentration (78μmol/L). Morphometric analyses showed that with the increase of the Cu concentrations the nucleolar area decreases, whereas the Ag-stained are a increases, resulting in an increase in the Ag-stained/nucleolar area from 30 to nearly 100%. The results suggest that nucleolar morphological alterations in copper-treated root meristem cells are at least partly due to the copper effect on Ag-NOR proteins.

Characterization of the tolerance to excess manganese in four maize varieties

Abstract Manganese (Mn) is an essential micronutrient in all organisms, but may become toxic when present in excess. Four maize (Zea mays L.) varieties, Kneja 605, Kneja 434, Kneja 509 and Kneja 537, were studied with respect to their responses to excess Mn in hydroponic solution. In the varieties Kneja 605, Kneja 509 and Kneja 537, increasing Mn concentrations in the nutrient solution negatively affected biomass accumulation, photosynthetic rate, transpiration, stomatal conductance and chlorophyll content. In addition, these varieties showed increased electrolyte leakage and lipid peroxidation (malondialdehyde [MDA] content). Increased Mn leaf concentrations, higher contents of chlorophyll a and chlorophyll b, higher photosynthetic rate and transpiration, lower concentrations of MDA and insignificant changes in the electrolyte leakage in the leaves were found in var. Kneja 434 compared with the other maize varieties studied. This variety appeared to possess a stronger ability to cope with Mn phytotoxicity, suggesting high potential for Mn detoxification and var. Kneja 434 could be a good candidate for improving maize productivity on acid soils under non-tropical conditions.

The effect of silicon on the symptoms of manganese toxicity in maize plants

The effect of exogenously applied silicon (Si) on plant growth, lipid peroxidation, total phenolic compounds and non-protein thiols was studied in two maize varieties (Zea mays L. vars. Kneja 605, 434) differing in sensitivity to excess manganese (Mn). Based on the density of brown spots per leaf area and relative shoot weight (RSW) used to define Mn tolerance var. Kneja 434 was found to be more Mn-tolerant than Kneja 605. The lipid peroxidation level and total phenolic compounds were enhanced with increasing Mn concentration in the nutrient solution. In addition, the Mn-sensitive var. Kneja 605 with markedly expressed first visible Mn toxicity symptoms had higher levels of total phenolic acids than var. Kneja 434 thus supporting the hypothesis that a stimulating effect of Mn on phenol content reflected rather a stress response to Mn excess than a tolerance mechanism. In contrast, non-protein SH content increased to a higher extent in the Mn-tolerant var. Kneja 434. The increased amount of non-protein SH compounds was accompanied by a much stronger oxidative stress in the Mn-sensitive plants when compared with the Mn-tolerant variety, thus suggesting that non-protein SH compounds may play a role in Mn tolerance in maize. The addition of silicon (Si) reduced the density of brown spots per leaf area as well as lipid peroxidation level and improved plant growth in Mn-treated plants.

Impact of Different Substrate Moisture Levels on Lettuce Plants during Ground Based Experiment in SVET-2 Space Greenhouse

Plant experiments carried out in space has proved that microgravity alters conditions in the plant growth facilities especially in the root modules and thus affects plant growth and development. Microgravity changes behavior of fluids and gases in the porous media used as plant growth substrates which causes problems with the control of water supply systems and this often leads to excess water input (overmoistening) and oxygen deficiency. The pattern of fluid and gas distribution in substrate medium in microgravity could not be repeated on Earth but some processes could be imitated. Overmoistening of the substrate medium and the subsequent oxygen deficiency could be replaced with the waterlogging on Earth. Ground experiment was carried out in the laboratory prototype of SVET-2 Space Greenhouse (SVET-2 SG) to study the effect of different root-zone moisture conditions and waterlogging on growth, photosynthesis and chlorophyll content of lettuce plants (Lactuca sativa L. cv. Lolo Rossa). The increase in height and biomass was suppressed while leaf dry matter increased during the waterlogging treatment suggesting assimilates accumulation in the leaves and slow translocation to the roots. Waterlogging caused a rapid decline in net photosynthetic rate (Pn). The reduction of Pn could not be attributed only to diffusion limitations resulting from stomatal closure but also to metabolic inhibition due to accumulation of assimilates in the leaves. The chlorophyll content decreased during the waterlogging and slowly recovered after termination of the waterlogging treatment. Lettuce plants showed decline in Pn and overall growth during waterlogging and demonstrated fast recovery after waterlogging removal. The results suggested that lettuce plants were waterlogging-resistant to a certain degree but yield is greatly affected.