Georgi I. Georgiev

Physiological Response of Wheat Seedlings to Mild and Severe Osmotic Stress

In the present study the physiological status of two wheat (Triticum aestivum L.) cultivars subjected to polyethylene glycol-induced dehydration is evaluated. Wheat seedlings were exposed to either 8-d-long mild (15% PEG) or 24-h-long severe (30% PEG) osmotic stress by immersing their roots in PEG-supplemented Knop nutrient solution. Relative water content in the leaves and the levels of free proline, malondialdehyde, and hydrogen peroxide were chosen as indicative parameters corresponding to the degree of stress of the treated plants. Electrolyte leakage from leaf tissues of control and stressed plants was compared in terms of the common parameter Injury index used for characterizing cell membrane stability. In addition, a model test system was established for preliminary stress evaluation based on the kinetics of ion leakage. Short-term exposure to higher concentration of PEG was considered to be more harmful than prolonged mild stress as judged by RWC, proline and hydrogen peroxide accumulation, and injury index. The two cultivars demonstrated more obvious dissimilarities under conditions of prolonged mild stress than under severe stress.

An improvement of the diffusion model for assessment of drought stress in plant tissues

The article discusses an improvement of a previously developed method for assessment of ion leakage from plant tissues as a gauge of membrane and cell wall performance under stressful environment. It employs conductometric measurements of the ion efflux from leaves and their quantitative interpretation by a theoretical model based on the laws of diffusion. Experimental data are readily fit with the model and results are in accordance with relative water content of dehydrated barley (Hordeum vulgare) seedlings of two distinct cultivars. Some new parameters obtained from fitting are proposed as reliable indicators of the leaf status. They appear to be helpful in further distinguishing the behavior of two separate cellular structures with respect to their electrolyte permeability. It is concluded that the established method based on the kinetics of ion leakage is adequate for evaluation of contrasting genotypes under normal and stress conditions. Furthermore, it could be used as a simple and powerful tool for routine analysis and screening for drought tolerance in crops.

Changes in foliar proline concentration of osmotically stressed barley.

The amino acid proline is accumulated in plant tissues in response to a variety of stresses. The existence of two routes for its biosynthesis is well documented. However, little is known about the contribution of each pathway to the accumulation of free proline under stress conditions. In the present study young barley plants were subjected to osmotic stress by treating their roots with 25% polyethylene glycol. Prior to stress imposition roots were incubated for 24 h in nutrient solution containing proline or one of its metabolic precursors: glutamate and ornithine. Free proline quantity in the leaves was measured before and after stress. Relative water content (RWC) was used as a measure of the plant water status. Foliar proline levels showed a significant increase in ornithine- and proline-pretreated plants compared to the control. Nevertheless, no considerable changes in leaf RWC were observed. It was shown that before stress application only ornithine but not glutamate was immediately metabolized to proline. Under stress conditions, however, both precursors were converted into proline. The possible role of this amino acid in the processes of post stress recovery is discussed.

Variation in ion leakage parameters of two wheat genotypes with different Rht-B1 alleles in response to drought

The reaction to soil drying was evaluated in two Triticum aestivum near-isogenic lines carrying different alleles of the height-reducing gene Rht-B1 based on an improved method for assessment of electrolyte leakage. The two lines were previously shown to differ in their physiological responses to induced water deficit stress. Drought was imposed for 6 days on 10-day-old seedlings. Ion efflux from leaves was measured conductometrically in multiple time points during the 24 h incubation period, and the obtained biphasic kinetics was interpreted according to a previously developed theoretical model proposing different leakage rates through the apoplast and the symplast. Most of the model parameters were able to properly differentiate the two closely related genotypes. The mutant Rht-B1c displayed lower and slower electrolyte leakage in comparison with the wild-type Rht-B1a. It was speculated that the Rht genes expressing defective DELLA proteins might be involved in water stress response through modulation of cell wall stiffness, which influences its capacity for ions retention, and also by their contribution to ROS detoxification, thus indirectly stabilizing cellular membranes. The presented analytical approach relating processes of ion and water flow in and out of the cell could be used for characterization of membrane and cell wall properties of different genotypes under normal and stress conditions.

Physiological and anatomical responses of wheat to induced dehydration and rehydration

Hydroponically grown wheat seedlings of two prominent Bulgarian cultivars (Katya and Prelom) were subjected to 48 h osmotic stress with PEG 8000 and were then rehydrated. The degree of stress was evaluated by monitoring relative water content, lipid peroxidation level, and accumulation of free proline and hydrogen peroxide in the leaves. Anatomy and ultrastructure of leaf tissue were observed under light microscopy. After imposition of stress, drought tolerant cultivar Katya displayed higher free proline content and significantly lower malondialdehyde and peroxide concentration in leaves than in the leaves of susceptible cultivar Prelom. After 24 h of rehydration Katya showed better ability to restore leaf water status and an apparent tendency towards recovery, whereas Prelom sustained higher levels of hydrogen peroxide, lipid peroxidation products and free proline and markedly low relative water content. Here, we have uncovered some of the characteristics displayed by cultivar Katya that enable it to survive and recover from severe osmotic stress. Interestingly, there was congruence between our results and the high level of cultivar Katya drought tolerance observed in the field.

CHANGES IN PHOSPHATE FRACTIONS, GROWTH RATE, NODULATION AND NITROGEN2 FIXATION OF PHOSPHORUS-STARVED SOYBEAN PLANTS

The objective of this study was to elucidate the effects of phosphorus (P) starvation on the internal P status expressed as P fractions (acid soluble P, sugarP, inorganic P, nucleotide P, and insolubleP) in different plant organs (leaves, roots and nodules), at different plant growth rates and dinitrogen(N2) fixation rate of soybean plants. The symptoms of P starvation differed during early 10 days and late 28 days of starvation. There were close relationships between nodulation and N2 fixation rate, and parameters of growth rate and internal P status. Although growth rate and N2 fixation of P starved plants were reduced their nodules showed ability to accumulate more soluble P in comparison with the leaves. The decreased sugar-P and inorganic P accounted for accumulation of more soluble sugars and starch in the stressed leaves. In contrast, stressed nodules accumulated higher quantities of sugar P, inorganic P, and nucleotide P and had reduced quantities of starch and sucrose. The increased accumulation of phosphorylated sugars in the nodules was regarded as a manifestation of the mechanism of stress tolerance of soybean nodules to P limited nutrition.

A SIMPLE IMPEDIMETRIC DEVICE FOR IN SITU ANALYSIS OF PLANT TISSUES

Construction and major features of a set-up suitable for impedance analysis of plant tissues are disclosed in the paper. The measuring device is based on a simple scheme operating in conjunction with the PC sound card. While many programmes running directly on sound card are capable of simulating impedance analyzer, their performance is not consistent when chemical or biological systems are explored. The main reason is a relatively low input impedance of the card itself. An addition of coordinating preamplifier between the sample and the audio input of the PC drastically improves the implementation of the set-up. On the other hand, a specially designed measuring head gives possibilities for straightforward assessment of plant tissue’s electrical parameters in situ at various ambient conditions. Some examples of impedance analysis of intact leaves are given and their behaviour in regard to a preliminary imposed desiccation is briefly discussed.

Application of impedance spectroscopy and conductometry for assessment of varietal differences in wheat

The potentials of an electrochemical and a physical technique for detection of physiological differences in three wheat cultivars under optimal growth conditions were outlined in the study. Electrolyte leakage kinetics was established by continuous measurements of conductivity of solutions in which leaf pieces were incubated for 24 hours. Impedance spectra were obtained from intact leaves at frequency range from 7 to 2010 Hz and 250 mV measuring voltage applied between two gold plated silicon substrates serving as electrodes. The obtained spectra were approximated by a model employing two ARC elements connected in series. Parameters of the previously described diffusion model based on time course conductivity measurements were inversely correlated with electrical impedance spectroscopy data, thus the genotype with highest ion leakage (cultivar Prelom) exhibited lowest impedance magnitude. It was concluded that the two methods were able not merely to distinguish the three studied cultivars but also to rank them in the same order based on their electrical properties.