Tsonko Tsonev

Plant responses to drought, acclimation, and stress tolerance

At the whole plant level, the effect of stress is usually perceived as a decrease in photosynthesis and growth. That is why this review is focused mainly on the effect of drought on photosynthesis, its injury, and mechanisms of adaptation. The analysed literature shows that plants have evolved a number of adaptive mechanisms that allow the photochemical and biochemical systems to cope with negative changes in environment, including increased water deficit. In addition, the acquisition of tolerance to drought includes both phenotypic and genotypic changes. The approaches were made to identify those metabolic steps that are most sensitive to drought. Some studies also examined the mechanisms controlling gene expression and putative regulatory pathways.

A Possible Role for Jasmonic Acid in Adaptation of Barley Seedlings to Salinity Stress

Abstract. The changes caused by NaCl salinity and jasmonic acid (JA) treatment (8 days) on growth and photosynthesis of barley plants (Hordeum vulgare L., var. Alfa) have been studied. Gas exchange measurements and analysis of enzyme activities were used to study the reactions of photosynthesis to salinity and JA. Both 100 mm NaCl and 25 μm JA treatment led to a noticeable decrease in both the initial slope of the curves representing net photosynthetic rate vs intercellular CO2 concentration and the maximal rate of photosynthesis. The calculated values of the intercellular CO2 concentration, CO2 compensation point, and maximal carboxylating efficiency of ribulose-1,5-bisphosphate carboxylase support the suggestion that biochemical factors are involved in the response of photosynthesis to JA and salinity stress. The activities of phosphoenolpyruvate carboxylase and carbonic anhydrase increased more than twofold. Pretreatment with JA for 4 days before salinization diminished the inhibitory effect of high salt concentration on the growth and photosynthesis. The results are discussed in terms of a possible role of JA in increasing salinity tolerance of the barley plants.

Phenylamides in plants

Phenylamides, secondary metabolites, conjugates of aliphatic polyamines or arylmonoamines and hydroxycinnamic acids, combining the properties of both parent compounds, are a subject of increasing interest, as follows from their ubiquitous distribution and multiplicity of functions. In this paper the occurrence, distribution, localization, metabolism, as well as chemical structure and properties of phenylamides underlying their physiological functions are briefly reviewed, with an emphasis on their involvement in the developmental and stress-defense phenomena of plants. Basic knowledge on phenylamides is supplemented with recent data pointing to the structural similarity of plant phenylamides with those found in insect toxins. This finding challenges the view of phenylamides as plant-specific molecules and suggests their role as a common defense tool in various organisms.

Effects of Water Stress and High-Temperature Stress on the Structure and Activity of Photosynthetic Apparatus of Zea Mays and Helianthus Annuus

Effects of high-temperature stress (HTS) and PEG-induced water stress (WS), applied separately or in combination, on the functional activity and ultrastructure of the photosynthetic apparatus (PSA) of maize (Zea mays L.) and sunflower (Helianthus annuus L.) plants were investigated. In maize plant tissues WS provoked the decrease in RWC by 10.9 %, HTS by 7.0 %, and after simultaneous application of the both treatments the decrease was 32.7 % in comparison with control plants. Similar but more expressed changes were observed in sunflower plants. Sunflower was more sensitive to these stresses. Net photosynthetic rate decreased significantly after all treatments, more in sunflower. In mesophyll chloroplasts after separately applied WS and HTS the number of grana and thylakoids was reduced and electron-transparent spaces appeared. At combined stress (WS+HTS) granal and stromal thylakoids were considerably affected and chloroplast envelope in many of them was partially disrupted.

The impact of blue light on leaf mesophyll conductance

Blue light has many direct and indirect effects on photosynthesis. The impact of blue light on mesophyll conductance (g(m)), one of the main diffusive limitation to photosynthesis, was investigated in leaves of Nicotiana tabacum and Platanus orientalis, characterized by high and low g(m), respectively. Leaves were exposed to blue light fractions between 0% and 80% of incident light intensity (300 micromol photons m(-2) s(-1)), the other fraction being supplied as red light. Leaves exposed to blue light showed reduced photosynthesis and unaltered stomatal conductance. The g(m), measured using the chlorophyll fluorescence-based method, was strongly reduced in both plant species. Such a reduction of g(m) may not be real, as several assumptions used for the calculation of g(m) by fluorescence may not hold under blue light. To assess possible artefacts, the electron transport rate measured by fluorescence (J(f)) and by gas-exchange (J(c)) were compared in leaves exposed to different fractions of blue light under non-photorespiratory conditions. The two values were only equal, a prerequisite for correct g(m) measurements, when the illumination was totally provided as red light. Under increasing blue light levels an increasing discrepancy was observed, which suggests that J(f) was not correctly calculated, and that such an error could also upset g(m) measurements. Blue light was not found to change the absorbance of light by leaves, whereas it slightly decreased the distribution of light to PSII. To equate J(f) and J(c) under blue light, a further factor must be added to the J(f) equation, which possibly accounted for the reduced efficiency of energy transfer between the pigments predominantly absorbing blue light (the carotenoids) and the chlorophylls. This correction reduced by about 50% the effect of blue light on g(m). However, the residual reduction of g(m) under blue light was real and significant, although it did not appear to limit the chloroplast CO(2) concentration and, consequently, photosynthesis. Reduction of g(m) might be caused by chloroplast movement to avoid photodamage, in turn affecting the chloroplast surface exposed to intercellular spaces. However, g(m) reduction occurred immediately after exposure to blue light and was complete after less than 3 min, whereas chloroplast relocation was expected to occur more slowly. In addition, fast g(m) reduction was also observed after inhibiting chloroplast movement by cytochalasin. It is therefore concluded that g(m) reduction under blue light is unlikely to be caused by chloroplast movement only, and must be elicited by other, as yet unknown, factors.

Changes in photosynthesis, mesophyll conductance to CO2, and isoprenoid emissions in Populus nigra plants exposed to excess nickel

Poplar (Populus nigra) plants were grown hydroponically with 30 and 200 μM Ni (Ni30 and Ni200). Photosynthesis limitations and isoprenoid emissions were investigated in two leaf types (mature and developing). Ni stress significantly decreased photosynthesis, and this effect depended on the leaf Ni content, which was lower in mature than in developing leaves. The main limitations to photosynthesis were attributed to mesophyll conductance and metabolism impairment. In Ni-stressed developing leaves, isoprene emission was significantly stimulated. We attribute such stimulation to the lower chloroplastic [CO2] than in control leaves. However chloroplastic [CO2] did not control isoprene emission in mature leaves. Ni stress induced the emission of cis-β-ocimene in mature leaves, and of linalool in both leaf types. Induced biosynthesis and emission of isoprenoids reveal the onset of antioxidant processes that may also contribute to reduce Ni stress, especially in mature poplar leaves.

ABA as a Modulator of the Response of Pisum sativum to Salt Stress

Summary Ten-day-old pea plants ( Pisum sativum L., cultivar Ran 1) were treated for 24h with 10 -5 M or 10 -6 M ABA before a salinity treatment with 50 mM NaCl lasting 48 h. The sodium chloride treatment alone resulted in an increase of endogenous proline content, CO 2 compensation point, photorespiration and glycolate oxidase activity. Photosynthesis was significantly inhibited. ABA eliminated the inhibitory effect of NaCl and the values of the observed parameters were similar to those of the control. 14 CO 2 fixation of protoplasts isolated from salt stressed plants was inhibited about 60 %; 14 CO 2 fixation by protoplasts isolated from plants treated with ABA before salinization was almost equal to that of the control. 10 -5 M or 10 -6 M ABA added exogenously to the protoplast reaction medium did not affect 14 CO 2 fixation. The reduced CO 2 fixation of protoplasts isolated from NaCl stressed plants indicated a direct effect of NaCl on the photosynthetic process, which was not dependent upon stomatal closure.

Isoprene prevents the negative consequences of high temperature stress in Platanus orientalis leaves

The phenomenon of enhanced plant thermotolerance by isoprene was studied in leaves of the same age of 1- or 2-year-old Platanus orientalis plants. Our goals were to determine whether the isoprene emission depends on the age of the plant, and whether different emission rates can influence heat resistance in plants of different age. Two-year-old plants emit greater amounts of isoprene and possess better capacity to cope with heat stress than 1-year-old plants. After a high temperature treatment (38°C for 4 h), photosynthetic activity, hydrogen peroxide content, lipid peroxidation and antiradical activity were preserved in isoprene emitting leaves of 1- and 2-year-old plants. However, heat inhibited photosynthesis and PSII efficiency, caused accumulation of H2O2, and increased all indices of membrane damage and antioxidant capacity in leaves of plants of both ages in which isoprene was inhibited by fosmidomycin. In isoprene-inhibited leaves fumigated with exogenous isoprene during the heat treatment, the negative effects on photosynthetic capacity were reduced. These results further support the notion that isoprene plays an important role in protecting photosynthesis against damage at high temperature. It is suggested that isoprene is an important compound of the non-enzymatic defence of plants against thermal stress, possibly contributing to scavenging of reactive oxygen species (ROS) and membrane stabilising capacity, especially in developed plants.

Ultradian variation of isoprene emission, photosynthesis, mesophyll conductance, and optimum temperature sensitivity for isoprene emission in water-stressed Eucalyptus citriodora saplings

Water availability is a major limiting factor on plant growth and productivity. Considering that Eucalyptus spp. are among the few plant species able to produce both isoprene and monoterpenes, experiments were designed to investigate the response of isoprene emission and isoprenoid concentrations in Eucalyptus citriodora saplings exposed to decreasing fraction of transpirable soil water (FTSW). In particular, this study aimed to assess: (a) the kinetic of water stress-induced variations in photosynthesis, isoprene emission, and leaf isoprenoid concentrations during progressive soil water shortage as a function of FTSW; (b) the ultradian control of isoprene emission and photosynthesis under limited soil water availability; and (c) the optimum temperature sensitivity of isoprene emission and photosynthesis under severe water stress. The optimum temperature for isoprene emission did not change under progressive soil water deficit. However, water stress induced a reallocation of carbon through the MEP/DOXP pathway resulting in a qualitative change of the stored isoprenoids. The ultradian trend of isoprene emission was also unaffected under water stress, and a similar ultradian trend of stomatal and mesophyll conductances was also observed, highlighting a tight coordination between diffusion limitations to photosynthesis during water stress. The kinetics of photosynthetic parameters and isoprene emission in response to decreasing FTSW in E. citriodora are strikingly similar to those measured in other plant functional types. These findings may be useful to refine the algorithms employed in process-based models aiming to precisely up-scale carbon assimilation and isoprenoid emissions at regional and global scales.

BVOC emissions, photosynthetic characteristics and changes in chloroplast ultrastructure of Platanus orientalis L. exposed to elevated CO2 and high temperature

To investigate the interactive effects of increasing [CO(2)] and heat wave occurrence on isoprene (IE) and methanol (ME) emissions, Platanus orientalis was grown for one month in ambient (380 micromol mol(-1)) or elevated (800 micromol mol(-1)) [CO(2)] and exposed to high temperature (HT) (38 degrees C/4 h). In pre-existing leaves, IE emissions were always higher but ME emissions lower as compared to newly-emerged leaves. They were both stimulated by HT. Elevated [CO(2)] significantly reduced IE in both leaf types, whereas it increased ME in newly-emerged leaves only. In newly-emerged leaves, elevated [CO(2)] decreased photosynthesis and altered the chloroplast ultrastructure and membrane integrity. These harmful effects were amplified by HT. HT did not cause any unfavorable effects in pre-existing leaves, which were characterized by inherently higher IE rates. We conclude that: (1) these results further prove the isoprenes putative thermo-protective role of membranes; (2) HT may likely outweigh the inhibitory effects of elevated [CO(2)] on IE in the future.