The need for exposure to low, but non-freezing temperatures is a common aspect of both cold acclimation and vernalization, suggesting a possible link between these two processes. Cold hardiness levels are regulated by the C-repeat binding factor (CBF) regulon, whereas vernalization requirements are influenced by vernalization genes (VRNs). The VRN1 gene has an epistatic effect on the CBF regulon, reducing frost tolerance during postvernalization. It is widely acknowledged that, apart from low temperature, light also serves as an external signal influencing the expression of CBF genes indicating that photoperiod and light quality play important roles in regulating cold acclimation processes. For instance, frost-tolerant winter wheat illuminated by white light with additional far-red light increases frost resistance due to low red:far-red (R:FR) ratio. However, information regarding the regulation or influence of VRN1 gene on the light quality induced frost tolerance is currently lacking. In the present study, reciprocal near-isogenic lines (NILs) produced from crossing the non-hardy springhabit (Vrn-A1) cultivar 'Manitou' with the very cold-hardy winter-habit (vrn-A1) cultivar 'Norstar' were used. Our objective was to investigate how winter/spring VRN1 alleles (vrn-A1/Vrn-A1), inserted in the same genetic background, affect wheat frost tolerance under different spectral illuminations at temperatures of 15 degrees C and 5 degrees C. Based on freezing tests and the cold-related gene expressions patterns, it appears that the light-induced frost tolerance does not completely depend on VRN1 gene expression but is strongly dependent on the background. Additionally, the presence of the spring allele is capable of sensitizing an otherwise frost-tolerant genotype to frost.

Reduced height (Rht) genes have revolutionised wheat cultivation, but they can compromise freezing tolerance, and only a few alleles are in use. Thus, evaluating the role of other Rht alleles in stress responses is crucial. Far-red supplementation of white light (W+FR) can induce pre-hardening in cereals at 15°C. However, the relevant effect of blue light enrichment (W+B) is poorly described. This study investigates the influence of W+FR or W+B exposure in young winter wheat leaves of a tall (wild-type, rht12) and a dwarf, gibberellin-deficient (near-isogenic line, Rht12) genotype in cv. Maris Huntsman background over 10 days at 15°C. The main objectives were to investigate the relationship between light quality, gibberellin homeostasis, and freezing tolerance. Key parameters such as frost injury, hormonal pools and the expression of relevant genes were examined. Results provided evidence about the involvement of Rht alleles in the basal freezing tolerance of wheat leaves from the side of gibberellin availability. It was revealed that W+FR and W+B treatments partially rescued the freezing-sensitive phenotype of Rht12 leaves, suggesting a potential compensatory mechanism. Analysis of gibberellic acid (GA) metabolism indicated differential responses to light treatments between the Rht12 and wild-type leaves, with implications for freezing tolerance. Moreover, alterations in hormone levels, including jasmonic acid (JA) and salicylic acid (SA), were observed, highlighting the complex interplay between light signalling and hormonal regulation in wheat. Overall, these findings suggest that manipulating light responses may offer a strategy to enhance freezing tolerance in gibberellin-deficient dwarf wheat genotypes.

Hybrid vigor and polyploidy are genetic events widely utilized to increase the productivity of crops. Given that bioenergy usage needs to be expanded, we investigated triploid hybrid vigor in terms of the biology of biomass-related willow traits and their relevance to the control of biomethane production. To produce triploid hybrid genotypes, we crossed two female diploid Swedish cultivars (Inger, Tordis) with two male autotetraploid willow (Salix viminalis) variants (PP-E7, PP-E15). Field studies at two locations and in two successive years recorded considerable midparent heterosis (MPH%) in early shoot length that ranged between 11.14 and 68.85% and in the growth rate between 34.12 and 97.18%. The three triploid hybrids (THs) developed larger leaves than their parental cultivars, and the MPH% for their CO2 assimilation rate varied between 0.84 and 25.30%. The impact of hybrid vigor on the concentrations of plant hormones in these TH genotypes reflected essentially different hormonal statuses that depended preferentially on maternal parents. Hybrid vigor was evinced by an elevated concentration of jasmonic acid in shoot meristems of all the three THs (MPH:29.73; 67.08; 91.91%). Heterosis in auxin-type hormones, such as indole-3-acetic acid (MPH:207.49%), phenylacetic acid (MPH:223.51%), and salicylic acid (MPH:27.72%) and benzoic acid (MPH:85.75%), was detectable in the shoots of TH21/2 plants. These hormones also accumulated in their maternal Inger plants. Heterosis in cytokinin-type hormones characterized the shoots of TH3/12 and TH17/17 genotypes having Tordis as their maternal parent. Unexpectedly, we detected abscisic acid as a positive factor in the growth of TH17/17 plants with negative MPH percentages in stomatal conductance and a lower CO2 assimilation rate. During anaerobic digestion, wood raw materials from the triploid willow hybrids that provided positive MPH% in biomethane yield (6.38 and 27.87%) showed negative MPH in their acid detergent lignin contents (from -8.01 to -14.36%). Altogether, these insights into controlling factors of above-ground growth parameters of willow genotypes support the utilization of triploid hybrid vigor in willow breeding to expand the cultivation of short rotation energy trees for renewable energy production.

Effects of ascorbate (ASA) and hydrogen peroxide (H 2 O 2 ) on metabolite profile was compared in wheat. Interestingly, the redox environment became more oxidized after ASA treatment and more reduced after H 2 O 2 addition based on the ratios of oxidised and reduced ascorbate and glutathione. The excess of ASA could inhibit, while H 2 O 2 could induce the oxidative pentose phosphate pathway producing reducing power as shown by the unchanged and decreased glucose-6-phosphate content, respectively. This different effect on glucose-6-phosphate content can also explain the reduced formation of several amino acids from the intermediate products of glycolysis after ASA treatment and their constant or greater levels after H 2 O 2 addition. In contrast to most amino acids, the accumulation of Pro was greatly induced by ASA, and this change was fivefold greater than after H 2 O 2 addition. This difference could also contribute to the distinct redox shifts after the two treatments, since NADPH is oxidised during Pro synthesis. The more oxidising environment after ASA treatment activated several transcripts related to the ascorbate–glutathione cycle and the pentose phosphate pathway. Our results indicate the overcompensating effect of ASA and H 2 O 2 on the redox environment in leaf tissues and the subsequent different adjustment of metabolite profile and the related transcript levels.

Both light intensity and spectrum (280–800 nm) affect photosynthesis and, consequently, the formation of reactive oxygen species (ROS) during photosynthetic electron transport. ROS, together with antioxidants, determine the redox environment in tissues and cells, which in turn has a major role in the adjustment of metabolism to changes in environmental conditions. This process is very important since there are great spatial (latitude, altitude) and temporal (daily, seasonal) changes in light conditions which are accompanied by fluctuations in temperature, water supply, and biotic stresses. The blue and red spectral regimens are decisive in the regulation of metabolism because of the absorption maximums of chlorophylls and the sensitivity of photoreceptors. Based on recent publications, photoreceptor-controlled transcription factors such as ELONGATED HYPOCOTYL5 (HY5) and changes in the cellular redox environment may have a major role in the coordinated fine-tuning of metabolic processes during changes in light conditions. This review gives an overview of the current knowledge of the light-associated redox control of basic metabolic pathways (carbon, nitrogen, amino acid, sulphur, lipid, and nucleic acid metabolism), secondary metabolism (terpenoids, flavonoids, and alkaloids), and related molecular mechanisms. Light condition-related reprogramming of metabolism is the basis for proper growth and development of plants; therefore, its better understanding can contribute to more efficient crop production in the future.

It is well established that cold acclimation processes are highly influenced, apart from cold ambient temperatures, by light-dependent environmental factors. In this study we investigated whether an extra blue (B) light supplementation would be able to further improve the well-documented freezing tolerance enhancing effect of far-red (FR) enriched white (W) light. The impact of B and FR light supplementation to white light (WFRB) on hormone levels and lipid contents were determined in winter barley at moderate (15 °C) and low (5 °C) temperatures. Low R:FR ratio effectively induced frost tolerance in barley plantlets, but additional B light further enhanced frost hardiness at both temperatures. Supplementation of WFR (white light enriched with FR light) with B had a strong positive effect on abscisic acid accumulation while the suppression of salicylic acid and jasmonic acid levels were observed at low temperature which resembles the shade avoidance syndrome. We also observed clear lipidomic differences between the individual light and temperature treatments. WFRB light changed the total lipid content negatively, but monogalactosyldiacylglycerol (MGDG) content was increased, nonetheless. Our results prove that WFRB light can greatly influence phytohormone dynamics and lipid contents, which eventually leads to more efficient pre-hardening to avoid frost damage.

Selenium (Se) enrichment of Stevia rebaudiana Bertoni can serve a dual purpose, on the one hand to increase plant biomass and stress tolerance and on the other hand to produce Se fortified plant-based food. Foliar Se spraying (0, 6, 8, 10 mg/L selenate, 14 days) of Stevia plantlets resulted in slightly decreased stevioside and rebaudioside A concentrations, and it also caused significant increment in stem elongation, leaf number, and Se content, suggesting that foliar Se supplementation can be used as a biofortifying approach. Furthermore, Se slightly limited photosynthetic CO2 assimilation (AN, gsw, Ci/Ca), but exerted no significant effect on chlorophyll, carotenoid contents and on parameters associated with photosystem II (PSII) activity (FV/FM, F0, Y(NO)), indicating that Se causes no photodamage in PSII. Further results indicate that Se is able to activate PSI-cyclic electron flow independent protection mechanisms of the photosynthetic apparatus of Stevia plants. The applied Se activated superoxide dismutase (SOD) isoenzymes (MnSOD1, FeSOD1, FeSOD2, Cu/ZnSOD1, Cu/ZnSOD2) and down-regulated NADPH oxidase suggesting the Se-induced limitation of superoxide anion levels and consequent oxidative signalling in Stevia leaves. Additionally, the decrease in S-nitrosoglutathione reductase protein abundance and the intensification of protein tyrosine nitration indicate Se-triggered nitrosative signalling. Collectively, these results suggest that Se supplementation alters Stevia shoot morphology without significantly affecting biomass yield and photosynthesis, but increasing Se content and performing antioxidant effects, which indicates that foliar application of Se may be a promising method in Stevia cultivation.

Soil salinity can limit the use of marginal lands for biomass production based on cultivation of short-rotation woody crops as energy willow. Here, we compare salt stress responses of the diploid, productive cultivar (Energo), and its artificially produced autotetraploid (PPE-2; PPE-7; PPE-13) variants. After pre-testing the effects of various salt concentrations, willow plants with different genome sizes were exposed to 1.5 g NaCl kg(-1) soil (electrical conductivity (EC) value: 7.04 mS/cm). Digital imaging of shoot surface area (green pixel) and root surface (white pixel) indicated variable improvements in growth responses of tetraploids relative to diploid ones in saline soils. After nine weeks of salt stress, increase in salt adaptation capability of tetraploid plants was indicated by larger biomass, leaf and root weights under salinity (1.5 g NaCl kg(-1) soil) relative to diploids. Biomass weights were significantly higher in the case of tetraploid PPE-2 plants with increased water consumption and leaf water content than of diploid plants. The inhibitory effect of salt stress on photosynthetic assimilation rates was less significant in plants with doubled genome. The Na+ accumulation was reduced in leaves of tetraploids and increased in their roots, while the K+ ion content was higher in their leaves than in diploids. Tetraploidy improved K+/Na+ ratio in leaves and roots of willow plants under normal soil condition. This parameter was less reduced in tetraploid leaves exposed to salt stress. The described tetraploid energy willow genotypes with salt tolerance can play a role in the extended use of green energy.

Although ethylene (ET) is an important participant in plant responses to salt stress, its role in the early period of acclimation, especially in the case of photosynthesis has not been revealed in detail. In this study, the effects of tolerable (100 mM) or lethal (250 mM) NaCl concentrations were investigated in hydroponically grown tomato (Solanum lycopersicum L. cv. Ailsa Craig) plants of different ET status, in wild type (WT) plants, in WT plants pretreated with the ET generator 1-aminocyclopropane-1-carboxylic acid (ACC) and in ET insensitive, Never ripe (Nr/Nr) mutants for 1-, 6- and 24 h. In the leaves ACC treatment reduced the osmotic effect of salt stress, while Nr mutation enhanced not only osmotic but ionic component of salt stress at 100 mM NaCl. ET insensitivity caused greater decline in stomatal conductance and photosynthetic CO2 assimilation rate than in the controls under tolerable salt stress, but both ACC treatment and Nr mutation helped to maintain positive carbon assimilation under lethal salt stress after 24 h. Nr mutant leaves showed highly enhanced regulated non-photochemical quenching (NPQ) and therefore lower quantum yield of photosystem II (PSII), due to more intensive cyclic electron flow around photosystem I (CEF-PSI), which was further increased under high salinity. Exogenous ACC treatment lowered CEF-PSI and enhanced PSII photochemistry after 6 h of lethal salt stress. Controlling PSI photoinhibition, ET is suggested to be an important regulator of CEF-PSI and photoprotection under salt stress. Furthermore, the altered ET status could cause contrasting effects under different stress severity.

Exogenous ethylene (ET) or its precursor, 1-aminocyclopropane-1-carboxylic acid (ACC) may increase stress tolerance of plants in long-term experiments. ACC applied in hydroponic culture of tomato ( Solanum lycopersicum L. cv. Rio Fuego) brought about concentration-dependent differences in plant responses. ACC at 0.01 µM changed K + distribution between shoots and roots but at higher concentrations K + / 86 Rb + uptake was inhibited. Surprisingly, excess Na + and reduced K + accumulation could be measured in root and leaf tissues, respectively, which at 100 µM ACC led to a decreased K + /Na + ratio and to a weak ionic stress in these plant organs. ACC at low concentrations increased the net CO 2 fixation rate (A N ), however, at 100 µM, it triggered a parallel decline in stomatal conductance, A N and in the actual quantum yield of photosystem II. Photosystem I (PSI) was less sensitive to ACC, lower concentrations caused an increasing tendency in the effective quantum yield and changed the non-photochemical quenching profile of PSI from donor-side dominant to acceptor-side-dominant limitation. Photoinhibition was avoided via the dissipation of light energy by regulated non-photochemical quenching and by transiently enhanced cyclic electron flow (CEF) around PSI. The decrease in CEF at 100 µM ACC correlated well with the accumulation of O 2 •− and H 2 O 2 in the leaf tissues suggesting that molecular oxygen may function as an electron acceptor when natural acceptors of the linear electron transport were over-reduced. Concentration-dependent changes in soluble sugars and sorbitol as osmoprotectants may also contribute to the stress resistance of plants elicited by ET/ACC.

Salicylic acid (SA) is a key component of plant defence, which exerts a concentration-dependent effect on photosynthesis under multi-faceted influence of light.Photosynthetic activities and chloroplast morphology were studied in tomato plants after treatment with a sublethal, 0.1 mM, and a cell death-inducing, 1 mM concentrations of SA under normal photoperiod during light phase and after a prolonged dark phase.SA (1 mM) decreased the maximal (Fv/Fm) and effective quantum yields of PSII [Y(II)] and PSI [Y(I)] under both environmental conditions, however, the photoprotective processes were not significantly different between light and dark samples.Decrease in grana height, thylakoid dilation and deformation of lumen were also observed in the light.In contrast to illuminated samples, 0.1 mM SA decreased Y(II) and Y(I) after dark incubation, but nonphotochemical energy dissipation and cyclic electron flow increased, suggesting that the photoprotective mechanisms could be activated in plants exposed to prolonged darkness.

The presence of solar radiation is one of the most important environmental factors, which is required for the optimal growth and development of plants. The absence of it (e.g. in the night period or artificially prolonged darkness) can alter the light-dependent signalling and regulation pathways and may induce new defence responses. Antioxidant enzymes as components of the plant defence system play a crucial role in the detoxification of reactive oxygen species (ROS) induced by several stressors; however, their regulation can be different in the light or in the dark. In this review we summarize the current knowledge about the physiological and molecular aspects of dark-modulated key antioxidant enzymes (superoxide dismutase, catalase, and ascorbate peroxidase) in different plant species and discuss their roles in different developmental processes (seedling growth and development or senescence) and in responses to environmental stresses (cold, chilling, heat, and biotic stress). Moreover, the hormonal regulation of respective gene transcription and the changes in activity of various isoenzymes at subcellular level are also summarized. Based on this knowledge, modification of these antioxidant enzymes may be used to increase the yield and stress tolerance of cultivated plants in the changing environment.

Exposure of tomato plants to dark period promotes leaf senescence, which takes place at different speed in young, mature and old leaves of intact plants. Dark-induced senescence is accompanied with decreased glucose levels, chlorophyll content and photosynthetic activity and it induces changes in sugar metabolism, in which hexokinases (HXKs) play a prominent role. The aim of this work was to reveal changes in the expression of various HXK genes and in HXK activity in different leaf positions of tomato kept in darkness. A 24-h-long dark period reduced the expression of the mitochondrial SlHXK1 and chloroplastic SlHXK4 in the young and mature leaves, but induced the expression of all SlHXKs especially that of SlHXK3 in the old leaves. In contrast to HXK expression, HXK activity decreased in all leaf positions, however the smallest changes were observed in young, sink leaves. In addition, cessation of CO2 assimilation in the dark led to low glucose levels, which can also participate in the early induction of dark-induced leaf senescence. With the exception of the mitochondrial SlHXK3 expression, this decline in the activity and relative transcript abundance of HXKs, as well as in the photosynthetic parameters was more pronounced after a 7-day-long dark treatment. It can be concluded that slower rate of dark-induced chlorophyll loss and senescence was accompanied with higher HXK activities. A single HXK gene, SlHXK3 was up-regulated during dark starvation suggesting that it can play a role in the maintenance of HXK activity and integrity of mitochondrial functions in young and mature leaves.

Ethylene proved to be an important modulator of salicylic acid (SA) signalling pathway. Since SA may regulate both the production and scavenging of hydrogen peroxide (H2O2), which show light-dependency, the aim of this study was to compare H2O2 metabolism in the leaves of SA-treated wild-type (WT) tomato (Solanum lycopersicum L. cv. Ailsa Craig) and in ethylene receptor Never-ripe (Nr) mutants grown in normal photoperiod or in prolonged darkness. H2O2 accumulation was higher in the WT than in the mutants in normal photoperiod after 1 mM SA treatment, while Nr leaves contained more H2O2 after light deprivation. The expression of certain superoxide dismutase (SOD) genes and activity of the enzyme followed the same tendency as H2O2, which was scavenged by different enzymes in the two genotypes. Catalase (CAT, EC 1.11.1.6) activity was inhibited by SA in WT, while the mutants maintained enhanced enzyme activity in the dark. Thus, in WT, CAT inhibition was the major component of the H2O2 accumulation elicited by 1 mM SA in a normal photoperiod, since the expression and/or activity of ascorbate (APX, EC 1.11.1.11) and guaiacol peroxidases (POD, EC 1.11.1.7) were induced in the leaves. The absence of APX and POD activation in mutant plants suggests that the regulation of these enzymes by SA needs functional ethylene signalling. While the block of ethylene perception in Nr mutants was overwritten in the transcription and activity of certain SOD and CAT isoenzymes during prolonged darkness, the low APX and POD activities led to H2O2 accumulation in these tissues.

Abscisic acid (ABA) regulates many salt stress-related processes of plants such as water balance, osmotic stress tolerance and photosynthesis. In this study we investigated the responses of wild type (WT) and the ABA-deficient sitiens mutant of tomato (Solanum lycopersicum cv. Rheinlands Ruhm) to sublethal and lethal salt stress elicited by 100 mM and 250 mM NaCl, respectively. Sitiens mutants displayed much higher decrease in water potential, stomatal conductance and net CO2 assimilation rate under high salinity, especially at lethal salt stress, than the WT. However, ABA deficiency in sitiens caused more severe osmotic stress and more moderate ionic stress, higher K+/Na+ ratio, in leaf tissues of plants exposed to salt stress. The higher salt concentration caused irreversible damage to Photosystem II (PSII) reaction centres, severe reduction in the linear photosynthetic electron transport rate and in the effective quantum yields of PSII and PSI in sitiens plants. The cyclic electron transport (CET) around PSI, which is an effective defence mechanism against the damage caused by photoinhibition in PSI, decreased in sitiens mutants, while WT plants were able to increase CET under salt stress. This suggests that the activation of CET needs active ABA synthesis and/or signalling. In spite of ABA deficiency, proline accumulation could alleviate the stress injury at sublethal salt stress in the mutants but its accumulation was not sufficient at lethal salt stress.

The salt stress triggered by sublethal, 100 mM and lethal, 250 mM NaCl induced ethylene production as well as rapid accumulation of superoxide radical and H2O2 in the root tips of tomato (Solanum lycopersicum cv. Ailsa Craig) wild type and ethylene receptor mutant, Never ripe (Nr/Nr) plants. In the wild type plants superoxide accumulation confined to lethal salt concentration while H2O2 accumulated more efficiently under sublethal salt stress. However, in Nr roots the superoxide production was higher and unexpectedly, H2O2 level was lower than in the wild type under sublethal salt stress. Nitric oxide production increased significantly under sublethal and lethal salt stress in both genotypes especially in mutant plants, while peroxynitrite accumulated significantly under lethal salt stress. Thus, the nitro-oxidative stress may be stronger in Nr roots, which leads to the programmed death of tissues, characterized by the DNA and protein degradation and loss of cell viability under moderate salt stress. In Nr mutants the cell death was induced in the absence of ethylene perception. Although wild type roots could maintain their potassium content under moderate salt stress, K(+) level significantly declined leading to small K(+)/Na(+) ratio in Nr roots. Thus Nr mutants were more sensitive to salt stress than the wild type and the viability of root cells decreased significantly under moderate salt stress. These changes can be attributed to a stronger ionic stress due to the K(+) loss from the root tissues.

Reed canarygrass (Phalaris arundinacea) is a good candidate for bioenergy production in Northern and Middle Europe. The crop is well-adapted to cold and drought stress but its resistance to high salinity has not been revealed in details. In this study the effects of 75 and 150 mM NaCl treatments were investigated on the ion accumulation, water potential changes and photosynthetic activity of three Romanian reed canarygrass genotypes, Tardin, Romanesti diverse and Timpuriu. Since cv. Tardin was able to maintain high K+ level and relatively low Na+ concentration in leaf tissues, high stomatal conductance and net CO2 fixation rate under salt stress and as it could maintain the water potential of tissues at control level, this genotype can be defined as salt tolerant. Salt stress induced significant Na+ accumulation, very low K /Na+ ratio, and severe reduction in stomatal conductance and photosynthetic activity in the leaf tissues of Timpuriu cultivar, which proved to be sensitive to high salinity.

The plant hormone ethylene or the gaseous signalling molecule nitric oxide (NO) may enhance salt stress tolerance by maintaining ion homeostasis, first of all K+/Na+ ratio of tissues. Ethylene and NO accumulation increased in the root apices and suspension culture cells of tomato at sublethal salt stress caused by 100 mM NaCl, however, the induction phase of programmed cell death (PCD) was different at lethal salt concentration. The production of ethylene by root apices and the accumulation of NO in the cells of suspension culture did not increase during the initiation of PCD after 250 mM NaCl treatment. Moreover, cells in suspension culture accumulated higher amount of reactive oxygen species which, along with NO deficiency contributed to cell death induction. The absence of ethylene in the apical root segments and the absence of NO accumulation in the cell suspension resulted in similar ion disequilibrium, namely K+/Na+ ratio of 1.41 ± 0.1 and 1.68 ± 0.3 in intact plant tissues and suspension culture cells, respectively that was not tolerated by tomato.