Water Relations, Gas Exchange, Chlorophyll Fluorescence and Electrolyte Leakage of Ectomycorrhizal Pinus halepensis Seedlings in Response to Multi-Heavy Metal Stresses (Pb, Zn, Cd)

Juvenile Plant-Microbe Interactions Modulate the Adaptation and Response of Forest Seedlings to Rapid Climate Change

The negative impacts of climate change on native forest ecosystems have created challenging conditions for the sustainability of natural forest regeneration. These challenges arise primarily from abiotic stresses that affect the early stages of forest tree development. While there is extensive evidence on the diversity of juvenile microbial symbioses in agricultural and fruit crops, there is a notable lack of reports on native forest plants. This review aims to summarize the critical studies conducted on the diversity of juvenile plant-microbe interactions in forest plants and to highlight the main benefits of beneficial microorganisms in overcoming environmental stresses such as drought, high and low temperatures, metal(loid) toxicity, nutrient deficiency, and salinity. The reviewed studies have consistently demonstrated the positive effects of juvenile plant-microbiota interactions and have highlighted the potential beneficial attributes to improve plantlet development. In addition, this review discusses the beneficial attributes of managing juvenile plant-microbiota symbiosis in the context of native forest restoration, including its impact on plant responses to phytopathogens, promotion of nutrient uptake, facilitation of seedling adaptation, resource exchange through shared hyphal networks, stimulation of native soil microbial communities, and modulation of gene and protein expression to enhance adaptation to adverse environmental conditions.

Drought responsiveness in six wheat genotypes: identification of stress resistance indicators

Introduction

Wheat (Triticum aestivum L.) is among the world's most important staple food crops. In the current climate change scenario, a better understanding of wheat response mechanisms to water stress could help to enhance its productivity in arid ecosystems.

Methods

In this study, water relations, gas exchange, membrane integrity, agronomic traits and molecular analysis were evaluated in six wheat genotypes (D117, Syndiouk, Tunisian durum7 (Td7), Utique, Mahmoudi AG3 and BT) subjected to drought-stress.

Results and discussion

For all the studied genotypes, drought stress altered leaf area, chlorophyll content, stomatal density, photosynthetic rate and water-use efficiency, while the relative water content at turgor loss point (RWC0) remained stable. Changes in osmotic potential at turgor loss point (Ψπ0), bulk modulus of elasticity (Ɛmax) and stomatal regulation, differed greatly among the studied genotypes. For the drought-sensitive genotypes AG3 and BT, no significant changes were observed in Ψπ0, whereas the stomatal conductance (gs) and transpiration rate (E) decreased under stress conditions. These two varieties avoided turgor loss during drought treatment through an accurate stomatal control, resulting in a significant reduction in yield components. On the contrary, for Syndiouk, D117, Td7 and Utique genotypes, a solute accumulation and an increase in cell wall rigidity were the main mechanisms developed during drought stress. These mechanisms were efficient in enhancing soil water uptake, limiting leaf water loss and protecting cells membranes against leakage induced by oxidative damages. Furthermore, leaf soluble sugars accumulation was the major component of osmotic adjustment in drought-stressed wheat plants. The transcriptional analysis of genes involved in the final step of the ABA biosynthesis (AAO) and in the synthesis of an aquaporin (PIP2:1) revealed distinct responses to drought stress among the selected genotypes. In the resistant genotypes, PIP2:1 was significantly upregulated whereas in the sensitive ones, its expression showed only a slight induction. Conversely, the sensitive genotypes exhibited higher levels of AAO gene expression compared to the resistant genotypes. Our results suggest that drought tolerance in wheat is regulated by the interaction between the dynamics of leaf water status and stomatal behavior. Based on our findings, Syndiouk, D117, Utique and Td7, could be used in breeding programs for developing high-yielding and drought-tolerant wheat varieties.

Environmental stress - what can we learn from chlorophyll a fluorescence analysis in woody plants? A review

Chlorophyll a fluorescence (ChF) signal analysis has become a widely used and rapid, non-invasive technique to study the photosynthetic process under stress conditions. It monitors plant responses to various environmental factors affecting plants under experimental and field conditions. Thus, it enables extensive research in ecology and benefits forestry, agriculture, horticulture, and arboriculture. Woody plants, especially trees, as organisms with a considerable life span, have a different life strategy than herbaceous plants and show more complex responses to stress. The range of changes in photosynthetic efficiency of trees depends on their age, ontogeny, species-specific characteristics, and acclimation ability. This review compiles the results of the most commonly used ChF techniques at the foliar scale. We describe the results of experimental studies to identify stress factors that affect photosynthetic efficiency and analyse the experience of assessing tree vigour in natural and human-modified environments. We discuss both the circumstances under which ChF can be successfully used to assess woody plant health and the ChF parameters that can be useful in field research. Finally, we summarise the advantages and limitations of the ChF method in research on trees, shrubs, and woody vines.

Ectomycorrhizal Fungi Modulate Pedunculate Oak's Heat Stress Responses through the Alternation of Polyamines, Phenolics, and Osmotica Content

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.

Effects of High Temperature-Triggered Transcriptomics on the Physiological Adaptability of Cenococcum geophilum, an Ectomycorrhizal Fungus

High temperature stress caused by global warming presents a challenge to the healthy development of forestry. Cenococcum geophilum is a common ectomycorrhizal fungus (ECMF) in the forest system and has become an important fungus resource with application potential in forest vegetation restoration. In this study, three sensitive isolates of C. geophilum (ChCg01, JaCg144 and JaCg202) and three tolerant isolates of C. geophilum (ACg07, ChCg28 and ChCg100) were used to analyze the physiological and molecular responses to high temperature. The results showed that high temperature had a significant negative effect on the growth of sensitive isolates while promoting the growth of tolerant isolates. The antioxidative enzymes activity of C. geophilum isolates increased under high temperature stress, and the SOD activity of tolerant isolates (A07Cg and ChCg100) was higher than that of sensitive isolates (ChCg01 and JaCg202) significantly. The tolerant isolates secreted more succinate, while the sensitive isolates secreted more oxalic acid under high temperature stress. Comparative transcriptomic analysis showed that differentially expressed genes (DEGs) of six C. geophilum isolates were significantly enriched in "antioxidant" GO entry in the molecular. In addition, the "ABC transporters" pathway and the "glyoxylate and dicarboxylic acid metabolic" were shared in the three tolerant isolates and the three sensitive isolates, respectively. These results were further verified by RT-qPCR analysis. In conclusion, our findings suggest that C. geophilum can affect the organic acid secretion and increase antioxidant enzyme activity in response to high temperature by upregulating related genes.

Photosynthetic Characteristics and Chloroplast Ultrastructure Responses of Citrus Leaves to Copper Toxicity Induced by Bordeaux Mixture in Greenhouse

Bordeaux mixture (Bm) is a copper (Cu)-based pesticide that has been widely used for controlling citrus scab and citrus canker. However, frequent spraying of Bm is toxic to citrus. To our knowledge, few studies are available that discuss how the photosynthetic characteristics and chloroplast ultrastructure of citrus leaves are affected by Cu toxicity induced by excessive Bm. In the study, two-year-old seedlings of Citrus grandis (C. grandis) and Citrus sinensis (C. sinensis), which were precultured in pots, were foliar-sprayed with deionized water (as control) or Bm diluted 500-fold at intervals of 7 days for 6 times (4 times as recommended by the manufacturer) to investigate the leaf Cu absorption, photosynthesis, chloroplast ultrastructure and antioxidant enzymatic activities. Bm foliar-sprayed 6 times on citrus seedlings increased the leaf Cu content, decreased the photosynthetic pigments content and destroyed the chloroplast ultrastructure, which induced leaf chlorosis and photosynthetic inhibition. A lower Cu absorption, a higher light photon-electron transfer efficiency, a relative integrity of chloroplast ultrastructure and a promoted antioxidant protection contributed to a higher photosynthetic activity of C. grandis than C. sinensis under excessive spraying of Bm. The present study provides crucial references for screening and selecting citrus species with a higher tolerance to Cu toxicity induced by excessive Bm.