The general aim of this work was to compare the leaf-level responses of different protective components to water deficit and high temperatures in Quercus cerris L. and Quercus robur L. Several biochemical components of the osmotic adjustment and antioxidant system were investigated together with changes in hormones. Q. cerris and Q. robur seedlings responded to water deficit and high temperatures by: (1) activating a different pattern of osmoregulation and antioxidant mechanisms depending on the species and on the nature of the stress; (2) upregulating the synthesis of a newly-explored osmoprotectant, dimethylsulphoniopropionate (DMSP); (3) trading-off between metabolites; and (4) modulating hormone levels. Under water deficit, Q. cerris had a higher antioxidant capacity compared to Q. robur, which showed a lower investment in the antioxidant system. In both species, exposure to high temperatures induced a strong osmoregulation capacity that appeared largely conferred by DMSP in Q. cerris and by glycine betaine in Q. robur. Collectively, the more stress-responsive compounds in each species were those present at a significant basal level in non-stress conditions. Our results were discussed in terms of pre-adaptation and stress-induced metabolic patterns as related to species-specific stress tolerance features.

The physiological and biochemical responses of pedunculate oaks (Quercus robur L.) to heat stress (HS) and mycorrhization (individually as well in combination) were estimated. One-year-old Q. robur seedlings were grown under controlled conditions in a pot experiment, inoculated with a commercial inoculum of ectomycorrhizal (ECM) fungi, and subjected to 72 h of heat stress (40 °C/30 °C day/night temperature, relative humidity 80%, photoperiod 16/8 h) in a climate chamber, and they were compared with seedlings that were grown at room temperature (RT). An in-depth analysis of certain well-known stress-related metrics such as proline, total phenolics, FRAP, ABTS, non-protein thiols, and lipid peroxidation revealed that mycorrhized oak seedlings were more resistant to heat stress (HS) than non-mycorrhized oaks. Additionally, levels of specific polyamines, total phenolics, flavonoids, and condensed tannins as well as osmotica (proline and glycine betaine) content were measured and compared between four treatments: plants inoculated with ectomycorrhizal fungi exposed to heat stress (ECM-HS) and those grown only at RT (ECM-RT) versus non-mycorrhized controls exposed to heat stress (NM-HS) and those grown only at room temperature (NM-RT). In ectomycorrhiza inoculated oak seedlings, heat stress led to not only a rise in proline, total phenols, FRAP, ABTS, non-protein thiols, and lipid peroxidation but a notable decrease in glycine betaine and flavonoids. Amounts of three main polyamines (putrescine, spermine, and spermidine) were quantified by using high-performance liquid chromatography coupled with fluorescent detection (HPLC/FLD) after derivatization with dansyl-chloride. Heat stress significantly increased putrescine levels in non-mycorrhized oak seedlings but had no effect on spermidine or spermine levels, whereas heat stress significantly increased all inspected polyamine levels in oak seedlings inoculated with ectomycorrhizal inoculum. Spermidine (SPD) and spermine (SPM) contents were significantly higher in ECM-inoculated plants during heat stress (approximately 940 and 630 nmol g−1 DW, respectively), whereas these compounds were present in smaller amounts in non-mycorrhized oak seedlings (between 510 and 550 nmol g−1 DW for Spd and between 350 and 450 nmol g−1 DW for Spm). These findings supported the priming and biofertilizer roles of ectomycorrhizal fungi in the mitigation of heat stress in pedunculate oaks by modification of polyamines, phenolics, and osmotica content.

The effects of drought simulated via osmotic stress induced by polyethylene glycol (PEG) in the growing medium were examined on two Wild cherry (Prunus avium L.) clones (6A and 8A) based on thirteen morphometric, physiological, and biochemical traits. The shoot tips were exposed to two PEG concentrations (20 and 50 g L -1 ) in growing medium designed for micropropagation with axillary buds. The results showed that all morphological and physiological traits were significantly reduced, indicating a strong detrimental effect of increased PEG concentrations. The significant decline of radical scavenging activity against ABTS •+ and total content of flavonoids (TFC) and phenols (TPC) were recorded in both clones as a response to high PEG concentrations, whereas opposite trends were noticed for ferric reducing antioxidant power (FRAP). Clone 8A achieved better performance, having more intensive growth, higher multiplication percentage, higher values of carotenoids, more intense decrement of ABTS and increment of FRAP values compared to the clone 6A. The results of the principal component analysis indicate that parameter TFC/ TPC in both stem and leaves, as well as TFC in the stems, achieved the strongest relation with morphometric parameters. Our results confirm the feasibility of in vitro evaluation of drought tolerance of Wild cherry, supporting further research on the variability of examined traits in this noble broadleaved tree species.

Phytoextraction is an efficient and cheap way to extract copper from soils in riparian zones. In this work five genotypes of the endangered tree species Populus nigra L. were tested for their copper tolerance and accumulation in vitro when cultivated on media with three Cu concentrations: 10-3 , 10-4 and 10-7 M (buffered with citric acid/Na-citrate buffer, pH 3 before sterilization). After five-weeks cultivation of rooted shoots, the highest increases in morphological and biomass parameters were observed at 10-7 M Cu 2+. As the medium with 10-3 M Cu 2+ exhibited a toxic effect, the effect of 10-4 M Cu 2+ and pH 3 was used for further genotype evaluation. According to the measured morphological and parameters of photosynthetic pigment contents, the best performance was achieved by the genotype Populus nigra cl. DN3. The highest copper accumulation on the same medium was achieved by genotype Populus nigra cl. BN5. The obtained data point to the considerable potential of the applied method in the evaluation of Populus nigra genotypes for use in projects of copper phytoextraction.

Besides anthropogenic factors, climate change causes altered precipitation patterns that indirectly affect the increase of heavy metals in soils due to hydrological effects and enhanced leaching (i.e., Cd and Ni), especially in the vicinity of mines and smelters. Phytoextraction is a well-known, powerful “green” technique for environmental clean-up that uses plants to extract, sequester, and/or detoxify heavy metals, and it makes significant contributions to the removal of persistent inorganic pollutants from soils. Poplar species, due to their growth features, high transpiration rate, large biomass, and feasible reproduction represent great candidates for phytoextraction technology. However, the consequences of concomitant oxidative stress upon plant metabolism and the mechanism of the poplar’s tolerance to heavy metal-induced stress are still not completely understood. In this study, cuttings of poplar species (Populus deltoides W. Bartram ex Marshall) were separately exposed to two heavy metals (Cd2+ and Ni2+) that were triple the maximum allowed amount (MAA) (according to national legislation). The aim of the study was to estimate the effects of heavy metals on: (I) the accumulation of free and conjugated polyamines, (II) plant hormones (including abscisic acid-ABA and indole-3-acetic acid-IAA), and (III) the activities of different antioxidant enzymes at root and leaf levels. By using the selected ion monitoring (SIM) mode of gas chromatography with mass spectrometry (GC/MS) coupled with the isotopically labeled technique, amounts of ABA and IAA were quantified, while polyamine amounts were determined by using high-performance liquid chromatography (HPLC) with fluorometric detection after derivatization. The results showed that P. deltoides responded to elevated concentrations of heavy metals in soils by exhibiting metal- and organ-specific tolerance. Knowledge about tolerance mechanisms is of great importance for the development of phytoremediation technology and afforestation programs for polluted soils.