Short versus prolonged freezing differentially impacts freeze - thaw injury in spinach leaves: mechanistic insights through metabolite profiling

Exploring freeze-injury mechanism through ion-specific analysis of leachate from reversibly versus irreversibly injured spinach (Spinacia oleracea L.) leaves

The present study analyzed four cations (K+, Ca2+, Mg2+, Fe2+) in leachate from freeze-injured spinach (Spinacia oleracea L. 'Reflect') leaves exposed for four freezing-durations (FDs) (0.5, 3.0, 5.5, 10.5 h) at -4.8 °C. Comparison of electrolyte leakage from right-after-thaw with that after 6-d recovery revealed that injury at 0.5 or 3 h FDs was recoverable but irreversible at 5.5 or 10.5 h FDs. Data suggests leakage of K+, the most abundant cation in leachate, can serve as a proxy for total electrolyte-leakage in determining plant freezing-tolerance and an ionic marker discerning moderate vs. severe injury. Quantitative correspondence between Ca2+- and K+-leakage supports earlier proposition that leaked K+ induces loss of membrane-Ca2+, which, in turn, promotes further K+-leakage due to weakened membrane. Reduced/undetectable Fe2+ in leachate at longer FDs suggests activation of Fenton reaction converting soluble Fe2+ into insoluble Fe3+. Enhanced Mg2+-leakage at greater freeze-injury suggests structural/functional impairment of chlorophyll/chloroplast complex.

Freezing tolerance and tolerance to de-acclimation of European accessions of winter and facultative barley

Due to global warming, winter hardiness may seem to become less important for plant survival and yield. However, this is a superficial assumption, as probably only the most important factors locally affecting plant overwintering will change. For example, the frequency, degree, and length of extreme winter warming events may increase, leading to de-acclimation of plants. This study aimed to investigate existing variability in de-acclimation tolerance in Polish winter barley breeding materials and European winter and facultative barley cultivars, and to identify accessions with the highest and the lowest tolerance to de-acclimation by means of visual estimation of regrowth after freezing, measurements of electrolyte leakage and chlorophyll fluorescence, and LT50 assessment. The results of this study showed that freezing tolerance and tolerance to de-acclimation are independent traits, and even highly freezing tolerant plants can be susceptible to de-acclimation. Our results highlight the role of photosynthetic apparatus in de-acclimation, proving that chlorophyll fluorescence parameters, especially ET0/CS, can be useful indicators of tolerance to de-acclimation. This study also confirmed that although the mechanisms of response to de-acclimation seem to be common for susceptible barley accessions, the mechanisms of tolerance are different, and may be related to the accession's origin.

Dendrobium Multi-Omics Reveal Lipid Remodeling in Response to Freezing

Freezing damage is a common phenomenon responsible for reduced yields of economic crops. Regulation of lipid metabolism plays an important role in plant growth and adaptation during freezing. We previously carried out transcriptome and untargeted metabolome analyses to determine the regulation of flavonol and anthocyanin biosynthesis during freezing treatment (FT) and post-freezing recovery (FR) in Dendrobium catenatum. However, changes in lipid levels are hard to confirm by untargeted metabolomics analysis alone. Regulation of lipid metabolism in response to freezing is largely unknown in Dendrobium. In this study, a multi-omics strategy was used to offer a better means of studying metabolic flow during FT and FR. To this end, 6976 proteins were identified by the 4D_label-free proteome, including 5343 quantified proteins. For each of the two conditions, we enriched differentially accumulated proteins (DAPs) into 15 gene ontology (GO) terms, including primary metabolism, lipid metabolism, and photosynthesis processes. We also identified 7 lipid categories and 3672 lipid species using lipidome assays. We found significant remodeling occurring in the phospholipid category during FT and FR. We also found that most sphingolipids were significantly upregulated. An integrated multi-omics analysis revealed significant changes in the expression levels of 141 mRNAs and encoding proteins under both FT and FR conditions. During FT, phospholipase A (PLA) and phospholipase D (PLD) were associated with phospholipid editing and galactolipid remodeling. These results provide valuable new insights into how the freezing tolerance of D. catenatum might be improved by genetic engineering.

Impact of Plasma-Activated Water Treatment on Quality and Shelf-Life of Fresh Spinach Leaves Evaluated by Comprehensive Metabolomic Analysis

Fresh baby spinach leaves are popular in salads and are sold as chilled and plastic-packed products. They are of high nutritional value but very perishable due to microbial contamination and enzymatic browning resulting from leaf senescence. Therefore, innovative food processing methods such as plasma-activated water (PAW) treatment are being explored regarding their applicability for ensuring food safety. PAW’s impact on food quality and shelf-life extension has, however, not been investigated extensively in vegetables so far. In the present study, a comprehensive metabolomic analysis was performed to determine possible changes in the metabolite contents of spinach leaves stored in a refrigerated state for eight days. Liquid chromatography high-resolution mass spectrometry, followed by stringent biostatistics, was used to compare the metabolomes in control, tap-water-rinsed or PAW-rinsed samples. No significant differences were discernible between the treatment groups at the beginning or end of the storage period. The observed loss of nutrients and activation of catabolic pathways were characteristic of a transition into the senescent state. Nonetheless, the presence of several polyphenolic antioxidants and γ-linolenic acid in the PAW-treated leaves indicated a significant increase in stress resistance and health-promoting antioxidant capacity in the sample. Furthermore, the enhancement of carbohydrate-related metabolisms indicated a delay in the senescence development. These findings demonstrated the potential of PAW to benefit food quality and the shelf-life of fresh spinach leaves.

Impact of successive spring frosts on leaf phenology and radial growth in three deciduous tree species with contrasting climate requirements in central Spain

Rear-edge tree populations forming the equatorward limit of distribution of temperate species are assumed to be more adapted to climate variability than central (core) populations. However, climate is expected to become more variable and the frequency of climate extremes is forecasted to increase. Climatic extreme events such as heat waves, dry spells and spring frosts could become more frequent, and negatively impact and jeopardize rear-edge stands. To evaluate these ideas, we analyzed the growth response of trees to successive spring frosts in a mixed forest, where two temperate deciduous species, Fagus sylvatica L. (European beech) and Quercus petraea (Matt.) Liebl. (sessile oak), both at their southernmost edge, coexist with the Mediterranean Quercus pyrenaica Willd. (Pyrenean oak). Growth reductions in spring-frost years ranked across species as F. sylvatica > Q. petraea > Q. pyrenaica. Leaf flushing occurred earlier in F. sylvatica and later in Q. pyrenaica, suggesting that leaf phenology was a strong determinant of spring frost damage and stem growth reduction. The frost impact depended on prior climate conditions, since warmer days prior to frost occurrence predisposed to frost damage. Autumn Normalized Difference Vegetation Index data showed delayed leaf senescence in spring-frost years and subsequent years as compared with pre-frost years. In the studied forest, the negative impact of spring frosts on Q. petraea and especially on F. sylvatica growth, was considerably higher than the impacts due to drought. The succession of four spring frosts in the last two decades determined a trend of decreasing resistance of radial growth to frosts in F. sylvatica. The increased frequency of spring frosts might prevent the expansion and persistence of F. sylvatica in this rear-edge Mediterranean population.

Investigation on the stability in plant metabolomics with a special focus on freeze-thaw cycles: LC-MS and NMR analysis to Cassiae Semen (Cassia obtusifolia L.) seeds as a case study

Metabolomics is a rapid and sensitive tool for the detection of dynamic metabolic compositions in the study of systemic metabolic consequences. However, it is also susceptible to a tiny variation of pre-analytical handling procedures. To provide reproducible results, specific knowledge on metabolites perturbance along with different freeze-thaw cycles (FTCs) is needed for further metabolomics studies. In this paper, five FTCs of germinated Cassiae Semen (CS) were chosen as a case study to investigate the influence of FTC effect based on UHPLC-Q-Orbitrap-MS and NMR technologies. A total of 108 metabolites were relatively quantified by LC-MS and NMR analyses. Principal component analysis (PCA) showed that the first and second FTC samples are welly separated from the other groups; however, the extent of FTC-induced effects are smaller after the third cycle. Upon five consecutive FTCs, alterations which consisted of decreased stachyose, sucrose, norrubrofusarin-6-O-β-d-glucopyranoside, and quercetin 3-(3″-acetylgalactoside), as well as increased phenylalanine, leucine, isoleucine, methionine, phenylalanine, mannose, gluconic acid, and valine, could be observed. FTC does not exert the same effect on all metabolites. Although a large number of secondary metabolites were stable when subjected to five FTCs, FTC effects may lead to false-positive in the discovery of biomarker. In the case of reusing plant seed samples, no more than three consecutive freeze-thaw cycles were found advisable. This work provides unique perspectives on the FTC effects, which may fill in some existing gaps in the knowledge of the stability of plant metabolites during sample pre-handling.

The Effects of Biostimulants, Biofertilizers and Water-Stress on Nutritional Value and Chemical Composition of Two Spinach Genotypes (Spinacia oleracea L.)

In the present study, the effect of biostimulants application on the nutritional quality and bioactive properties of spinach cultivated in protected environment under water stress conditions was evaluated. For this purpose, four commercially available biostimulant products (Megafol (MEG), Aminovert (AM), Veramin Ca (V), Twin Antistress (TA), and two spinach genotypes (Fuji F1 and Viroflay) were tested under two irrigation regimes (normal irrigation (W+), and water-holding (W–). Fat and carbohydrates content was favored by water stress when Megafol (MEGW+) and Veramin (VW+) were applied on Fuji plants, while calorific value was also increased by MEGW+ treatment. In contrast, protein and ash content increased when AMW– and TAW+ were applied on Viroflay plants. Raffinose and glucose were the most abundant sugars, followed by sucrose and fructose, with the highest contents recorded for Fuji plants when AMW+ (fructose, glucose and total carbohydrates), CW– (sucrose), and TAW– (raffinose) treatments were applied. Regarding organic acids, oxalic and malic acid which had the highest contents for the TAW– (Viroflay plants) and AMW– (Fuji plants) treatments, respectively. α- and γ-tocopherol were the only isoforms detected with MEGW– and VW– inducing the biosynthesis of α-tocopherol, while AMW+ increased γ-tocopherol content in Fuji plants. The main fatty acids were α-linolenic and linoleic acids which were detected in the highest amounts in AMW–, AMW+, and TAW+ the former and in AMW–, VW–, and CW+ the latter. Regarding phenolic compounds content, peak 12 (5,3′,4′-Trihydroxy-3-methoxy-6:7-methylenedioxyflavone-4′-glucuronide) was the most abundant compound, especially in Viroflay plants under normal irrigation and no biostimulants added (CW–). The antioxidant and cytotoxic activity of the tested samples did not show promising results when compared with the positive controls, while a variable antibacterial activity was recorded depending on the tested biostimulant, irrigation regime and genotype. In conclusion, a variable effect of the tested biostimulants and irrigation regimes was observed on bioactive properties and chemical composition of both spinach genotypes which highlights the need for further research in order to make profound conclusions regarding the positive effects of biostimulants under water stress conditions.