Irina R. Fomina

Effects of silicon on growth processes and adaptive potential of barley plants under optimal soil watering and flooding

Barley (Hordeum vulgare L.) was grown in pots with brown loess soil and highly soluble amorphous silicon dioxide as the source of monosilicic acid to examine its influence on plant growth and adaptive potential under optimal soil watering and flooding. The adaptive potential of plants was estimated by the concentration of the thiobarbituric acid reactive substances (TBARs) as well as superoxide dismutase (SOD), guaiacol peroxidase (GPX) and ascorbate peroxidase (AsP) activities. Application of amorphous silica to the soil increased the Si content in barley shoots and roots and stimulated their growth and biomass production under optimal soil watering. Soil flooding suppressed the growth both of the (−Si)- and (+Si)-plants. The intensity of oxidative destruction estimated by the concentration of TBARs was lower in the roots and leaves of the (+Si)-plants. Soil flooding induced SOD activity in the roots and in the leaves of the (−Si;+flooding) and (+Si;+flooding)-plants, but no significant differences were observed due to the Si treatment. GPX activity in the roots of (+Si)-plants was higher than in the (−Si)-ones under optimal soil watering, but under soil flooding no differences between (+Si)- and (−Si)-treatments were observed. AsP activity was not influenced by Si treatment neither under optimal soil watering nor under flooding. Thus, application of Si stimulates growth processes of barley shoots and roots under optimal soil watering and decreases intensity of oxidative destruction under soil flooding without significant changes in the activities of antioxidant enzymes.

Structural features of the salt glands of the leaf of Distichlis spicata 'Yensen 4a' (Poaceae)

The epidermal salt glands of the leaf of Distichlis spicata ‘Yensen 4a’ (Poaceae) have a direct contact with one or two water-storing parenchyma cells, which act as collecting cells. A vacuole occupying almost the whole volume of the collecting cell has a direct exit into the extracellular space (apoplast) through the invaginations of the parietal layer of the cytoplasm, which is interrupted in some areas so that the vacuolar-apoplastic continuum is separated only by a single thin membrane, which looks as a valve. On the basis of ultrastructural morphological data (two shapes of the extracellular channels, narrow and extended, are found in basal cells), the hypothesis on the mechanical nature of the salt pump in the basal cell of Distichlis leaf salt gland is proposed. According to the hypothesis, a driving force giving ordered motion to salt solution from the vacuole of the collecting cell through the basal cell of the salt gland to cap cell arises from the impulses of a mechanical compression–expansion of plasma membrane, which penetrates the basal cell in the form of extracellular channels. The acts of compression–expansion of these extracellular channels can be realized by numerous microtubules present in the basal cell cytoplasm.

Molecular Mechanisms of Stress Resistance of Photosynthetic Machinery 2

The mechanisms of action of stressors, such as high light intensity and heat stress, on the photosynthetic machinery, primarily on the photosystem II, are reviewed. First of all, stressors alter the chemical composition of thylakoid membranes and decrease the activity of photosynthesis. Photodamage is caused by the direct effect of light on oxygen-evolving complex, whereas accumulation of reactive oxygen species due to high light or high temperatures causes suppression of the de novo synthesis of the reaction center proteins and, ultimately, leads to the inhibition of the recovery of photosystem II. In addition to their destructive and inhibitory action, the reactive oxygen species and products of lipid peroxidation trigger protective processes that lead to acclimatization. Particular attention is paid to the mechanisms that protect photosynthetic machinery from injury and to the inhibitory effect of stressors in the light of varying intensity. The known stress sensory systems of cyanobacteria are also reviewed.

Untangling metabolic and spatial interactions of stress tolerance in plants. 1. Patterns of carbon metabolism within leaves

The localization of the key photoreductive and oxidative processes and some stress-protective reactions within leaves of mesophytic C3 plants were investigated. The role of light in determining the profile of Rubisco, glutamate oxaloacetate transaminase, catalase, fumarase, and cytochrome-c-oxidase across spinach leaves was examined by exposing leaves to illumination on either the adaxial or abaxial leaf surfaces. Oxygen evolution in fresh paradermal leaf sections and CO2 gas exchange in whole leaves under adaxial or abaxial illumination was also examined. The results showed that the palisade mesophyll is responsible for the midday depression of photosynthesis in spinach leaves. The photosynthetic apparatus was more sensitive to the light environment than the respiratory apparatus. Additionally, examination of the paradermal leaf sections by optical microscopy allowed us to describe two new types of parenchyma in spinach—pirum mesophyll and pillow spongy mesophyll. A hypothesis that oxaloacetate may protect the upper leaf tissue from the destructive influence of active oxygen is presented. The application of mathematical modeling shows that the pattern of enzymatic distribution across leaves abides by the principle of maximal ecological utility. Light regulation of carbon metabolism across leaves is discussed.

Stress responses of spring rape plants to soil flooding

Abstract Stress responses of spring rape to soil hypoxia were investigated during 8-days flooding. Soil air-filled porosity decreased from 25-30% to 0%, oxygen diffusion rate - from 2.6-3.5 to 0.34 μmol O2 m-2 s-1, and redox potential - from 460 to 150mVwithin few hours. Alcohol dehydrogenase activity in roots increased up to 7-fold after one day of flooding and then decreased to 170% of control. Superoxide dismutase activity in roots increased by 27% during first 3 days and then dropped to 60% of control; in the leaves superoxide dismutase activity increased in average by 44%. Ascorbate peroxidase activity in leaves increased by 37% during first 3 days and then decreased to control value. Glutathione reductase activity increased by 45% in roots of flooded plants but did not change in leaves. Proline concentration in leaves increased up to 4-fold on the 3d day of flooding and then decreased to control value. Thus soil flooding induces increase of alcohol dehydrogenase activity and subsequent increase of superoxide dismutase and glutathione reductase activities in roots while the leaves display a few days increase of free proline concentration and ascorbate peroxidase activity, and a long-term increase of superoxide dismutase activity.

An Optimization Model of the Photosynthetic Leaf: The Model of Optimal Photosynthetic CO2 Fixation within Leaves of Mesophytic C3 Plants

The main function of the photosynthetic leaf is to convert light energy into chemical energy and to assimilate CO 2 . Coordination of these functions as related to the heterogeneous leaf structure and the distribution of metabolism within the leaf has been investigated in many studies [1‐5, etc.]. Presumably, after hundreds of millions of years of selection, the typical mesophytic leaf structure and the distribution of metabolic activity within it are relatively optimal. The stimuli that result in the formation of specific structural or functional features of the leaf may emerge through generalization of the experimental data using the unified theory of ecological utility. To describe the functioning of biological systems on the basis of the rule of maximal ecological utility (MEU) [6], the function of partial ecological utility is entered for each defined parameter of the system. It is possible to use any limited monomial function [7]. At optimal functioning of a system, the general utility as a product of partial utilities (in the case of their independence from each other) should be maximized. Some authors suppose that photosynthesis follows the light absorption gradient [8]. However, light absorption decreases from the top, illuminated surface to the bottom of the leaf [5], while the maximal CO 2 fixation occurs in the middle of palisade mesophyll [9]. The results imply that a significant amount of absorbed light energy may not be utilized directly for CO 2 fixation at the top of the leaf. It is possible that low photosynthesis in the upper layer of the palisade is due to excess illumination, which induces the formation of oxygen radicals (O 2 uptake under saturated light might reach 50% of its maximal evolution [10]). If the presumption is correct, the optimization of CO 2 fixation within the leaf should take place when the total utility of the two functions, (1) light absorption for photosynthesis and (2) protection from light intensities that can cause photodestruction, is maximized. The goal of the present work was to use the MEU rule to describe an optimal model of photosynthesis in a bifacial, mesophytic leaf of a C 3 plant. Let us consider this on a simplified model, where the leaf is a flat plate, and its surface is illuminated by light ( I 0 ; ν in the PAR range). The light is absorbed by chlorophyll ( a / b = 3/1 [9]) in a layer of phototrophic cells in the depth of the plate; the plate itself has the same absorption spectrum. In such a leaf, the function r ( z ) (partial utility of protection from photodestruction) and the function s ( z ) (partial utility of light absorption for photosynthesis) depend on the depth of the photosynthetic layer within the relative thickness ( h ) of the leaf (the variable h is incorporated by variable z , where

Effect of temperature on oxidative stress induced by lead in the leaves of Plantago major L.

Abstract Fluctuation of the summer day-time temperatures in the mid-latitudes in a range from 16 to 30°C should not have irreversible negative effects on plants, but may influence metabolic processes including the oxidative stress. To test the effect of moderately high temperature on oxidative stress induced by lead in the leaves of Plantago major L.; the plants were incubated in a water solution of 0, 150, 450, and 900 μM Pb (NO3)2 at 20 and 28°C. Plant reactions were evaluated by the content of thiobarbituric acid reactive substances and ascorbate peroxidase and glutathione reductase activities in leaves after 2, 24, 48, and 72 h. The Pb concentration in the leaves rose with the increase in the Pb content and was higher at 20°C. The increase in stomatal resistance caused by Pb was higher at 28°C. The contents of TBARS increased after 2 h of plant exposure to Pb and the increase was the highest at 900 μM Pb, 28°C. The AsP activity increased up to 50% after 24 h of Pb-treatment at 28°C; the highest increase in glutathione reductase activity was observed after 72 h at 20°C. Thus, the moderately high temperature 28°C compared with optimal 20°C caused a decrease in Pb accumulation in Plantago leaves but amplified the negative effects of lead, especially in the beginning of stress development.

Influence of oxidative stressors on the photosynthetic apparatus of the methyl viologen-resistant mutant Prq20 of cyanobacterium Synechocystis sp. PCC 6803

Delayed Chl a fluorescence and the CO2-dependent O2 exchange were measured to assess the effect of oxidative stress inducers methyl viologen and benzyl viologen, cumene hydroperoxide, menadione, and H2O2 as well as high irradiance on the photosynthetic apparatus of Synechocystis sp. PCC 6803 wild type and its methyl viologen-resistant mutant Prq20 with impaired regulatory gene prqR. The extent of damage upon exposure to viologens proved much smaller in the mutant; the causes of this are analyzed.

NaCl-induced photoinhibition and recovery of the photosynthetic activity of a katG− mutant of cyanobacterium Synechocystis sp. PCC 6803

The joint effects of 0.5 M NaCl and light of different intensities on the activity of the photosynthetic apparatus and ATP content in cells of the katG− mutant of cyanobacterium Synechocystis sp. PCC 6803 have been studied. The mutant demonstrated a higher photoinhibition rate and a slower rate of recovery compared with the wild type, as shown by measurements of the CO2-dependent O2 production and delayed fluorescence of Chl a. The presence of 0.5 M NaCl in the incubation medium caused equal photoinhibition of the photosynthetic apparatus at I = 1200 μE m−2 s−1 in the mutant and wild-type cells. At I = 2400 μE m−2 s−1, we observed stronger inhibition and slower recovery of the photosynthetic apparatus in the katG− mutant than in wild-type cells. The data obtained evidence an important role of catalase-peroxidase in the system of reparation of the photosynthetic apparatus damaged by high-intensity light, especially at the background of NaCl stress.