Better salinity tolerance in tetraploid vs diploid volkamer lemon seedlings is associated with robust antioxidant and osmotic adjustment mechanisms

Effects of biochar types on seed germination, growth, chlorophyll contents, grain yield, sodium, and potassium uptake by wheat (Triticum aestivum L.) under salt stress

Soil salinity is a significant challenge in agriculture, particularly in arid and semi-arid regions such as Pakistan, leading to soil degradation and reduced crop yields. The present study assessed the impact of different salinity levels (0, 25, and 50 mmol NaCl) and biochar treatments (control, wheat-straw biochar, rice-husk biochar, and sawdust biochar applied @ 1% w/w) on the germination and growth performance of wheat. Two experiments: a germination study and a pot experiment (grown up to maturity), were performed. The results showed that NaCl-stress negatively impacted the germination parameters, grain, and straw yield, and agronomic and soil parameters. Biochar treatments restored these parameters compared to control (no biochar), but the effects were inconsistent across NaCl levels. Among the different biochars, wheat-straw biochar performed better than rice-husk and sawdust-derived biochar regarding germination and agronomic parameters. Biochar application notably increased soil pHs and electrical conductivity (ECe). Imposing NaCl stress reduced K concentrations in the wheat shoot and grains with concomitant higher Na concentrations in both parts. Parameters like foliar chlorophyll content (a, b, and total), stomatal and sub-stomatal conductance, and transpiration rate were also positively influenced by biochar addition. The study confirmed that biochar, particularly wheat-straw biochar, effectively mitigated the adverse effects of soil salinity, enhancing both soil quality and wheat growth. The study highlighted that biochar application can minimize the negative effects of salinity stress on wheat. Specifically, the types and dosages of biochar have to be optimized for different salinity levels under field conditions.

Genome-wide identification and stress response analysis of BcaCPK gene family in amphidiploid Brassica carinata

BACKGROUND

Calcium-dependent protein kinases (CPKs) are crucial for recognizing and transmitting Ca2+ signals in plant cells, playing a vital role in growth, development, and stress response. This study aimed to identify and detect the potential roles of the CPK gene family in the amphidiploid Brassica carinata (BBCC, 2n = 34) using bioinformatics methods.

RESULTS

Based on the published genomic information of B. carinata, a total of 123 CPK genes were identified, comprising 70 CPK genes on the B subgenome and 53 on the C subgenome. To further investigate the homologous evolutionary relationship between B. carinata and other plants, the phylogenetic tree was constructed using CPKs in B. carinata and Arabidopsis thaliana. The phylogenetic analysis classified 123 family members into four subfamilies, where gene members within the same subfamily exhibited similar conserved motifs. Each BcaCPK member possesses a core protein kinase domain and four EF-hand domains. Most of the BcaCPK genes contain 5 to 8 introns, and these 123 BcaCPK genes are unevenly distributed across 17 chromosomes. Among these BcaCPK genes, 120 replicated gene pairs were found, whereas only 8 genes were tandem duplication, suggesting that dispersed duplication mainly drove the family amplification. The results of the Ka/Ks analysis indicated that the CPK gene family of B. carinata was primarily underwent purification selection in evolutionary selection. The promoter region of most BcaCPK genes contained various stress-related cis-acting elements. qRT-PCR analysis of 12 selected CPK genes conducted under cadmium and salt stress at various points revealed distinct expression patterns among different family members in response to different stresses. Specifically, the expression levels of BcaCPK2.B01a, BcaCPK16.B02b, and BcaCPK26.B02 were down-regulated under both stresses, whereas the expression levels of other members were significantly up-regulated under at least one stress.

CONCLUSION

This study systematically identified the BcaCPK gene family in B. carinata, which contributes to a better understanding the CPK genes in this species. The findings also serve as a reference for analyzing stress responses, particularly in relation to cadmium and salt stress in B. carinata.

Water stress modulates terpene biosynthesis and morphophysiology at different ploidal levels in Lippia alba (Mill.) N. E. Brown (Verbenaceae)

Monoterpenes are the main component in essential oils of Lippia alba. In this species, the chemical composition of essential oils varies with genome size: citral (geraniol and neral) is dominant in diploids and tetraploids, and linalool in triploids. Because environmental stress impacts various metabolic pathways, we hypothesized that stress responses in L. alba could alter the relationship between genome size and essential oil composition. Water stress affects the flowering, production, and reproduction of plants. Here, we evaluated the effect of water stress on morphophysiology, essential oil production, and the expression of genes related to monoterpene synthesis in diploid, triploid, and tetraploid accessions of L. alba cultivated in vitro for 40 days. First, using transcriptome data, we performed de novo gene assembly and identified orthologous genes using phylogenetic and clustering-based approaches. The expression of candidate genes related to terpene biosynthesis was estimated by real-time quantitative PCR. Next, we assessed the expression of these genes under water stress conditions, whereby 1% PEG-4000 was added to MS medium. Water stress modulated L. alba morphophysiology at all ploidal levels. Gene expression and essential oil production were affected in triploid accessions. Polyploid accessions showed greater growth and metabolic tolerance under stress compared to diploids. These results confirm the complex regulation of metabolic pathways such as the production of essential oils in polyploid genomes. In addition, they highlight aspects of genotype and environment interactions, which may be important for the conservation of tropical biodiversity.

Effects of combined extreme cold and drought stress on growth, photosynthesis, and physiological characteristics of cool-season grasses

Abiotic stress is an important factor affecting turf establishment and limiting the sustainability of the turf industry. To alleviate the effects of combined cold and drought stress in cold- and drought-prone regions, it is essential to select and introduce turfgrass germplasms that are suitable for these conditions for successful turf establishment. Thus, we evaluated the effects of combined extreme cold and drought stress on the morphological, plant leaf functional, photosynthetic, and physiological and biochemical traits of 16 wild annual bluegrass (Poa annua) germplasms. We found that there were significant differences (P < 0.05) among different provenances, combined cold and drought stress, and the main interaction factors. Combined cold and drought stress altered the morphological characteristics of the 16 germplasms to varying degrees. Furthermore, combined cold and drought stress significantly reduced the net photosynthetic rate (Pn), stomatal conductance (gs), transpiration rate (Tr), instantaneous water use efficiency (WUE), chlorophyll content, chlorophyll fluorescence parameters, accumulated intercellular CO2 concentration (Ci), and relative electrical conductivity (REC) and malondialdehyde (MDA), proline (Pro), soluble protein (SP), soluble sugar (SS), superoxide anion (O2.-), hydrogen peroxide (H2O2), and hydroxyl radical (·OH) and other active oxygen, and increased the superoxide dismutase activity (SOD), peroxidase activity (POD), catalase activity (CAT), ascorbate peroxidase activity (APX) and glutathione reductase (GR) activities. Comprehensive evaluation using principal component analysis (PCA), membership function analysis, and clustered heatmaps indicated that the 'HZ' germplasm had stronger combined cold and drought tolerance, whereas the 'ZQ' germplasm was more sensitive to combined cold and drought, which was roughly consistent with the order of morphological damage symptoms. Therefore, it is recommended to use the 'HZ' germplasm for planting projects in cold- and drought-prone areas, while the 'ZQ' germplasm is more suitable for use under warmer and non-water-deficient conditions.

Foliar application of ascorbic acid enhances growth and yield of lettuce (Lactuca sativa) under saline conditions by improving antioxidant defence mechanism

Lettuce (Lactuca sativa L.) production is low because of different environmental stresses. Salt stress significantly reduces lettuce growth and yield. Foliar application of ascorbic acid is considered as a possible way to mitigate the adverse salinity effects on plants. This current study investigated the effect of foliar spray of ascorbic acid (control, 100, 200, 300 and 400mg/L) to mitigate negative effects of salinity (0, 50, 100 and 150mMNaCl) in lettuce plants in 2019 and 2020. Salinity level of 200mMNaCl significantly reduced growth and yield traits; i.e. leaf length and diameter, number of leaves, fresh plant weight, number of roots, root length and root dry weight and these traits increased under foliar application of ascorbic acid concentration of 400mg/L. Two salinity levels (150 and 200mMNaCl)×400mg/L ascorbic acid enhanced superoxide dismutase (SOD) content in lettuce plants. Peroxidase (POD) content increased in 200mMNaCl and 400mg/L ascorbic acid. Catalase (CAT) content increased in 100, 150 and 200mMNaCl and 400mg/L ascorbic acid. Ascorbic acid was significantly greater in 200mMNaCl and 400mg/L ascorbic acid. Phenolic content was the maximum in 200mMNaCl and 300mg/L and 400mg/L ascorbic acid. Titratable acidity was higher in 0, 50, 100, 150 and 200mMNaCl and control of ascorbic acid. We conclude that ascorbic acid had potential to mitigate the adverse effects of salinity by reducing oxidative injury in agricultural crops especially lettuce.

Polyploidy and zinc oxide nanoparticles alleviated Cd toxicity in rice by modulating oxidative stress and expression levels of sucrose and metal-transporter genes

The Cd toxicity causes severe perturbations to the plant's growth and development. Here, polyploid and diploid rice lines were treated with zinc-oxide nanoparticles (ZnO-NPs) and Cd, and physiological, cytological and molecular changes were observed. The Cd toxicity significantly reduced plant's growth attributes (such as shoot length, biological yield, dry matter, and chlorophyll contents, which decreased by 19%, 18%, 16%, 19% in polyploid and 35%, 43%, 45% and 43% in diploid rice, respectively), and disturbed the sugar level through the production of electrolytes, hydrogen peroxide, and malondialdehyde. The application of ZnO-NPs significantly alleviated the Cd toxicity in both lines by improving the antioxidant enzymes activities and physiochemical attributes. Semi-thin sections and transmission electron microscope revealed more and different types of abnormalities in diploid rice compared to polyploid rice under Cd stress. Moreover, RNA-seq analysis identified several differentially expressed genes between polyploid and diploid rice, especially metal and sucrose transporter genes. The GO, COG, and KEGG analyses revealed ploidy-specific pathways associated with plant growth and development. In conclusion, ZnO-NPs application to both rice lines significantly improved plant growth and decreased Cd accumulation in plants. We inferred that polyploid rice is more resistant to Cd stress than diploid rice.

Meiotic Behaviors of Allotetraploid Citrus Drive the Interspecific Recombination Landscape, the Genetic Structures, and Traits Inheritance in Tetrazyg Progenies Aiming to Select New Rootstocks

Sexual breeding at the tetraploid level is a promising strategy for rootstock breeding in citrus. Due to the interspecific origin of most of the conventional diploid citrus rootstocks that produced the tetraploid germplasm, the optimization of this strategy requires better knowledge of the meiotic behavior of the tetraploid parents. This work used Genotyping By Sequencing (GBS) data from 103 tetraploid hybrids to study the meiotic behavior and generate a high-density recombination landscape for their tetraploid intergenic Swingle citrumelo and interspecific Volkamer lemon progenitors. A genetic association study was performed with root architecture traits. For citrumelo, high preferential chromosome pairing was revealed and led to an intermediate inheritance with a disomic tendency. Meiosis in Volkamer lemon was more complex than that of citrumelo, with mixed segregation patterns from disomy to tetrasomy. The preferential pairing resulted in low interspecific recombination levels and high interspecific heterozygosity transmission by the diploid gametes. This meiotic behavior affected the efficiency of Quantitative Trait Loci (QTL) detection. Nevertheless, it enabled a high transmission of disease and pest resistance candidate genes from P. trifoliata that are heterozygous in the citrumelo progenitor. The tetrazyg strategy, using doubled diploids of interspecific origin as parents, appears to be efficient in transferring the dominant traits selected at the parental level to the tetraploid progenies.

Comparative Analysis of Antioxidant System and Salt-Stress Tolerance in Two Hibiscus Cultivars Exposed to NaCl Toxicity

Hibiscus (Hibiscus syriacus L.) is known as a horticultural plant of great ornamental and medicinal value. However, the effect of NaCl stress on hibiscus seedlings is unclear. Little is known about H. syriacus 'Duede Brabaul' (DB) and H. syriacus 'Blueberry Smoothie' (BS). Here, the effects of solutions with different concentrations of NaCl on the organic osmolytes, ion accumulation, and antioxidant enzyme activity of hibiscus seedling leaves were determined. The results showed that the Na⁺/K⁺ ratio was imbalanced with increasing NaCl concentration, especially in BS (range 34% to 121%), which was more sensitive than DB (range 32% to 187%) under NaCl concentrations of 50 to 200 mM. To cope with the osmotic stress, the content of organic osmolytes increased significantly. Additionally, NaCl stress caused a large increase in O₂·^- and H₂O₂, and other reactive oxygen species (ROS), and antioxidant enzyme activity was significantly increased to remove excess ROS. The expression level of genes related to salt tolerance was significantly higher in DB than that in BS under different NaCl concentrations. Taken together, DB possessed a stronger tolerance to salt stress and the results suggest membrane stability, Na⁺/K⁺, H₂O₂, catalase and ascorbate peroxidase as salt tolerance biomarkers that can be used for gene transformation and breeding in future hibiscus research.

A Comparative Study of Morphology, Photosynthetic Physiology, and Proteome between Diploid and Tetraploid Watermelon (Citrullus lanatus L.)

Watermelon is an important fruit that is widely distributed around the world. In particular, the production and consumption of watermelon in China ranks first in the world. Watermelon production is severely affected by a variety of biotic and abiotic stresses during cultivation, and polyploidization can promote stress resistance and yield. However, the morphological and physiological characteristics of tetraploid watermelon and the underlying molecular mechanisms are still poorly understood. In this study, we revealed that the leaves, fruits, and seeds of tetraploid watermelon were significantly larger than those of the diploid genotype. Some physiological characteristics, including photosynthetic rate (Pn) and stomatal conductance (Gs), were greater, whereas the intercellular CO₂ concentration (Ci) and transpiration rate (Tr) were lower in tetraploid than in diploid watermelon. Two-dimensional gel electrophoresis combined with tandem mass spectrometry (MALDI-TOF/TOF) was performed to compare proteomic changes between tetraploid and diploid watermelon. A total of 21 differentially expressed proteins were identified; excluding the identical proteins, 8 proteins remained. Among them, four proteins were upregulated and four were downregulated in tetraploid versus diploid genotypes. qRT-PCR results showed inconsistencies in gene expression and protein accumulation, indicating a low correlation between gene expression and protein abundance. Generally, this study extends our understanding of the traits and molecular mechanisms of tetraploid watermelon and provides a theoretical basis for watermelon polyploid breeding.

Nanoparticles: The Plant Saviour under Abiotic Stresses

Climate change significantly affects plant growth and productivity by causing different biotic and abiotic stresses to plants. Among the different abiotic stresses, at the top of the list are salinity, drought, temperature extremes, heavy metals and nutrient imbalances, which contribute to large yield losses of crops in various parts of the world, thereby leading to food insecurity issues. In the quest to improve plants' abiotic stress tolerance, many promising techniques are being investigated. These include the use of nanoparticles, which have been shown to have a positive effect on plant performance under stress conditions. Nanoparticles can be used to deliver nutrients to plants, overcome plant diseases and pathogens, and sense and monitor trace elements that are present in soil by absorbing their signals. A better understanding of the mechanisms of nanoparticles that assist plants to cope with abiotic stresses will help towards the development of more long-term strategies against these stresses. However, the intensity of the challenge also warrants more immediate approaches to mitigate these stresses and enhance crop production in the short term. Therefore, this review provides an update of the responses (physiological, biochemical and molecular) of plants affected by nanoparticles under abiotic stress, and potentially effective strategies to enhance production. Taking into consideration all aspects, this review is intended to help researchers from different fields, such as plant science and nanoscience, to better understand possible innovative approaches to deal with abiotic stresses in agriculture.

Cytogenetics and Consequences of Polyploidization on Different Biotic-Abiotic Stress Tolerance and the Potential Mechanisms Involved

The application of polyploidy in sustainable agriculture has already brought much appreciation among researchers. Polyploidy may occur naturally or can be induced in the laboratory using chemical or gaseous agents and results in complete chromosome nondisjunction. This comprehensive review described the potential of polyploidization on plants, especially its role in crop improvement for enhanced production and host-plant resistance development against pests and diseases. An in-depth investigation on techniques used in the induction of polyploidy, cytogenetic evaluation methods of different ploidy levels, application, and current research trends is also presented. Ongoing research has mainly aimed to bring the recurrence in polyploidy, which is usually detected by flow cytometry, chromosome counting, and cytogenetic techniques such as fluorescent in situ hybridization (FISH) and genomic in situ hybridization (GISH). Polyploidy can bring about positive consequences in the growth and yield attributes of crops, making them more tolerant to abiotic and biotic stresses. However, the unexpected change in chromosome set and lack of knowledge on the mechanism of stress alleviation is hindering the application of polyploidy on a large scale. Moreover, a lack of cost-benefit analysis and knowledge gaps on the socio-economic implication are predominant. Further research on polyploidy coupling with modern genomic technologies will help to bring real-world market prospects in the era of changing climate. This review on polyploidy provides a solid foundation to do next-generation research on crop improvement.

Effects of Salt Stress on the Morphology, Growth and Physiological Parameters of Juglansmicrocarpa L. Seedlings

In this study, to screen for walnut salt-tolerant rootstocks, Juglans microcarpa L. seedlings were treated in different NaCl concentrations (0, 50, 100, 200, and 300 mmol/L), and the growth situation of seedlings was observed. Moreover, we determined the physiological indexes of seedlings on different days (6, 12, 18, and 24 d) after treatment. The results showed that after salt stress, the external morphology of seedlings displayed salt injury, which manifested as yellowing, withering, curling, and falling off of leaves. High concentrations and long-term stress led to more serious damage, with numerous leaves undergoing withering and shedding. Salt stress significantly inhibited the growth of seedlings. With the increase in salt concentration and stress time, the chlorophyll content and photosynthetic parameters of seedlings reduced to varying degrees; the relative electrical conductivity (REC) and malondialdehyde (MDA) increased. Superoxide dismutase (SOD), peroxidase (POD), and catalase (CAT) activities generally increased, followed by a decrease; proline (Pro) accumulated; and soluble sugar (SS) content first increased and then decreased. In addition, it promoted the production of abscisic acid (ABA) and inhibited the synthesis of indole-3-acetic acid (IAA), gibberellic acid 3 (GA3), and zeatin riboside (ZR). It was found that J.microcarpa L. seedlings were more tolerant under 100 mmol/L salt stress, whereas the damage to growth was more severe at 200 mmol/L to 300 mmol/L salt stress.

Phenotypic Differences and Physiological Responses of Salt Resistance of Walnut with Four Rootstock Types

Walnut is one of the world’s four largest nuts. Currently, the bottleneck in walnut breeding is the production of resistant variants. Soil salinization is a global problem, and the use of salt-tolerant rootstocks is a basic strategy to overcome the challenge of sustained walnut production. Providing a scientific basis for the selection of walnut salt-tolerant rootstocks is possible by studying the physiological and biochemical response characteristics and salt tolerance variations of different walnut genotypes under salt stress. In the present study, seedlings of four genotypes of walnut rootstocks, including J1 (Juglans hindsii), J2 (J. mandshurica), J3 (J. regia × J. mandshurica), and J4 (J. regia × J. hindsii), were employed as test materials to conduct a 28-day pot experiment under NaCl stress with five NaCl concentrations (0, 50, 100, 200, and 300 mmol/L). Under different NaCl treatment concentrations, seedling morphology, growth indices, chlorophyll content, photosynthetic parameters, relative electrical conductivity (REC), malondialdehyde (MDA), proline (Pro), soluble sugar (SS), and the activity of superoxide dismutase (SOD) and peroxidase (POD) in the leaves were examined. Salt stress altered the morphological characteristics and growth indices of seedlings from four genotypes to varying degrees. In addition, according to the analysis of physiological and biochemical data, salt stress had a considerable impact on both the physiological and biochemical processes of seedlings. Salt stress decreased the chlorophyll content and photosynthetic parameters of four genotypes, the REC, MDA content, Pro content, and SS content of each genotype increased by different degrees, and the enzymatic activities showed different trends. The salt tolerance of rootstocks was evaluated thoroughly using principal component analysis and membership function analysis based on the 16 parameters. The results of a comprehensive evaluation of salt tolerance showed that the order of salt tolerance of the four genotypes was J4 > J1 > J3 > J2, which corresponded to the order of the morphological symptoms of salt injury. In conclusion, J4 has strong salt tolerance and is an important germplasm resource for walnut salt-tolerant rootstock breeding.

Hydrogen Peroxide Variation Patterns as Abiotic Stress Responses of Egeria densa

In vegetation management, understanding the condition of submerged plants is usually based on long-term growth monitoring. Reactive oxygen species (ROS) accumulate in organelles under environmental stress and are highly likely to be indicators of a plant's condition. However, this depends on the period of exposure to environmental stress, as environmental conditions are always changing in nature. Hydrogen peroxide (H2O2) is the most common ROS in organelles. The responses of submerged macrophytes, Egeria densa, to high light and iron (Fe) stressors were investigated by both laboratory experiments and natural river observation. Plants were incubated with combinations of 30-200 μmol m-2 s-1 of photosynthetically active radiation (PAR) intensity and 0-10 mg L-1 Fe concentration in the media. We have measured H2O2, photosynthetic pigment concentrations, chlorophyll a (Chl-a), chlorophyll b (Chl-b), carotenoid (CAR), Indole-3-acetic acid (IAA) concentrations of leaf tissues, the antioxidant activity of catalase (CAT), ascorbic peroxidase (APX), peroxidase (POD), the maximal quantum yield of PSII (Fv Fm -1), and the shoot growth rate (SGR). The H2O2 concentration gradually increased with Fe concentration in the media, except at very low concentrations and at an increased PAR intensity. However, with extremely high PAR or Fe concentrations, first the chlorophyll contents and then the H2O2 concentration prominently declined, followed by SGR, the maximal quantum yield of PSII (Fv Fm -1), and antioxidant activities. With an increasing Fe concentration in the substrate, the CAT and APX antioxidant levels decreased, which led to an increase in H2O2 accumulation in the plant tissues. Moreover, increased POD activity was proportionate to H2O2 accumulation, suggesting the low-Fe independent nature of POD. Diurnally, H2O2 concentration varies following the PAR variation. However, the CAT and APX antioxidant activities were delayed, which increased the H2O2 concentration level in the afternoon compared with the level in morning for the same PAR intensities. Similar trends were also obtained for the natural river samples where relatively low light intensity was preferable for growth. Together with our previous findings on macrophyte stress responses, these results indicate that H2O2 concentration is a good indicator of environmental stressors and could be used instead of long-term growth monitoring in macrophyte management.

Antioxidative and osmoprotecting mechanisms in carrot plants tolerant to soil salinity

Soil salinization is a growing problem for agriculture worldwide and carrot is one the most salt-sensitive vegetable species. However, some varieties are capable of withstanding high salt concentrations due to unknown genetic and physiological mechanisms. The aim of this work was to reveal protecting mechanisms against osmotic and ionic stresses that contribute to salt tolerance in carrot. For this purpose, changes in biochemical traits due to soil salinity occurring in the salt-tolerant and salt-sensitive plants were determined. The obtained results showed that the tolerance of the salt-tolerant variety was partially determined constitutively, however, the exposition to saline soil triggered a physiological response that was more evident in the root than in the leaves. The most noticeable changes were the high increase in the content of osmoprotective proline and other low molecular antioxidants such as glutathione and ascorbic acid, and the decrease in the ratio of reduced to oxidized glutathione forms. These changes imply an efficient operation of the ascorbate–glutathione cycle that together with a high activity of antioxidative enzymes such as peroxidases, indicate on the induction of mechanisms associated mainly with protection against excessive reactive oxygen species.

Enhanced Photosynthetic Capacity, Osmotic Adjustment and Antioxidant Defenses Contribute to Improve Tolerance to Moderate Water Deficit and Recovery of Triploid Citrus Genotypes

Currently, drought stress is a major issue for crop productivity, and future climate models predict a rise in frequency and severity of drought episodes. Polyploidy has been related to improved tolerance of plants to environmental stresses. In Citrus breeding programs, the use of triploidy is an effective way to produce seedless fruits, one of the greatest consumer expectations. The current study used physiological and biochemical parameters to assess the differential responses to moderate water deficit of 3x genotypes compared to 2x genotypes belonging to the same hybridization. Both parents, the mandarin Fortune and Ellendale tangor, were also included in the experimental design, while the 2x common clementine tree was used as reference. Water deficit affects leaf water status, as well as physiological and detoxification processes. Triploid genotypes showed a better ability to maintain water status through increased proline content and photosynthetic capacity. Moreover, less oxidative damage was associated with stronger antioxidant defenses in triploid genotypes. We also found that triploidy improved the recovery capacity after a water deficit episode.

Tetraploidy Confers Superior in vitro Water-Stress Tolerance to the Fig Tree (Ficus carica) by Reinforcing Hormonal, Physiological, and Biochemical Defensive Systems

The fig tree is a well-adapted and promising fruit tree for sustainable production in arid and semi-arid areas worldwide. Recently, Iran's dryland fig orchards have been severely damaged due to prolonged severe and consecutive drought periods. As emphasized in many studies, ploidy manipulated plants have a significantly enhanced drought tolerance. In the current study, we compared the induced autotetraploid explants of two fig cultivars ('Sabz' and 'Torsh') with their diploid control plants for their water stress tolerance under in vitro conditions using different polyethylene glycol (PEG) concentrations (0, 5, 10, 15, 20, and 25%). After 14 days of implementing water stress treatments, the results revealed that both tetraploid genotypes survived at 20% PEG treatments. Only 'Sabz' tetraploid explants survived at 25% PEG treatment, while both diploid control genotypes could tolerate water stress intensity only until 15% PEG treatment. The results also demonstrated that the tetraploid explants significantly had a higher growth rate, more leaf numbers, and greater fresh and dry weights than their diploid control plants. Under 15% PEG treatment, both tetraploid genotypes could maintain their relative water content (RWC) at a low-risk level (80-85%), while the RWC of both diploid genotypes drastically declined to 55-62%. The ion leakage percentage also was significantly lower in tetraploid explants at 15% PEG treatment. According to the results, these superiorities could be attributed to higher levels of stress response hormones including abscisic acid, salicylic acid, and jasmonic acid at different PEG treatments, the robust osmotic adjustment by significantly increased total soluble sugar (TSS), proline, and glycine betaine contents, and augmented enzymatic defense system including significantly increased superoxide dismutase (SOD), catalase (CAT), ascorbate peroxidase (APX), and glutathione peroxidase (GPX) activities in tetraploid genotypes, compared to their diploid control genotypes. Consequently, the current study results demonstrated that the 'Sabz' tetraploid genotype had a significantly higher water stress tolerance than other tested genotypes.

Different strategies lead to a common outcome: different water-deficit scenarios highlight physiological and biochemical strategies of water-deficit tolerance in diploid versus tetraploid Volkamer lemon

Water scarcity restricts citrus growth and productivity worldwide. In pot conditions, tetraploid plants tolerate water deficit more than their corresponding diploids. However, their tolerance mechanisms remain elusive. In this study, we focused on which mechanisms (i.e., hydraulic, osmotic or antioxidative) confer water-deficit tolerance to tetraploids. We exposed diploid and tetraploid Volkamer lemon rootstock (Citrus volkameriana Tan. and Pasq.) to quickly (fast) and slowly (slow) developing water-deficit conditions. We evaluated their physiological, antioxidative defense and osmotic adjustment responses, and mineral distribution to leaves and roots. Water-deficit conditions decreased the photosynthetic variables of both diploid and tetraploid plants. Moreover, the corresponding decrease was greater in diploids than tetraploids. Higher concentrations of antioxidant enzymes, osmoprotectants and antioxidant capacity were found in the leaves and roots of tetraploids than diploids under water deficit. Diploid plants showed fast response in slow water-deficit condition, but that response did not persist as the deficit intensified. Meanwhile, tetraploids had lower water loss, which slowed the onset of slow water deficit relative to diploids. This response allowed stronger photosynthesis, while antioxidant and osmoprotectant production allowed for further tolerance once desiccation began. Overall, our results concluded that Volkamer lemon tetraploid plants tolerate rapid and slow water deficit by maintaining their photosynthesis due to low conductance (stem or roots), which helps to avoid desiccation, and stronger biochemical defense machinery than their corresponding diploids.

Transcriptome and proteome analysis suggest enhanced photosynthesis in tetraploid Liriodendron sino-americanum

Polyploidy generally provides an advantage in phenotypic variation and growth vigor. However, the underlying mechanisms remain poorly understood. The tetraploid Liriodendron sino-americanum (Liriodendron × sinoamericanum P.C Yieh ex C.B. Shang & Zhang R.Wang) exhibits altered morphology compared with its diploid counterpart, including larger, thicker and deeper green leaves, bigger stomata, thicker stems and increased tree height. Such characteristics can be useful in ornamental and industrial applications. To elucidate the molecular mechanisms behind this variation, we performed a comparative transcriptome and proteome analysis. Our transcriptome data indicated that some photosynthesis genes and pathways were differentially altered and enriched in tetraploid L. sino-americanum, mainly related to F-type ATPase, the cytochrome b6/f complex, photosynthetic electron transport, the light harvesting chlorophyll protein complexes, and photosystem I and II. Most of the differentially expressed proteins we could identify are also involved in photosynthesis. Our physiological results showed that tetraploids have an enhanced photosynthetic capacity, concomitant with great levels of sugar and starch in leaves. This suggests that tetraploid L. sino-americanum might experience comprehensive transcriptome reprogramming of genes related to photosynthesis. This study has especially emphasized molecular changes involved in photosynthesis that accompany polyploidy, and provides a possible explanation for the altered phenotype of polyploidy plants in comparison with their diploid form.

Improved response of triploid citrus varieties to water deficit is related to anatomical and cytological properties

Polyploidy plays a major role in citrus plant breeding to improve the adaptation of polyploid rootstocks as well as scions to adverse conditions and to enhance agronomic characteristics. In Citrus breeding programs, triploidy could be a useful tool to react to environmental issues and consumer demands because the produced fruits are seedless. In this study, we compared the physiological, biochemical, morphological, and ultrastructural responses to water deficit of triploid and diploid citrus varieties obtained from 'Fortune' mandarin and 'Ellendale' tangor hybridization. One diploid clementine tree was included and used as a reference. All studied scions were grafted on C-35 citrange rootstock. Triploidy decreased stomatal density and increased stomata size. The number of chloroplasts increased in 3x varieties. These cytological properties may explain the greater photosynthetic capacity (P_net, g_s, F_v/F_m) and enhanced water-holding capacity (RWC, proline). In addition, reduced degradation of ultrastructural organelles (chloroplasts and mitochondria) and thylakoids accompanied by less photosynthetic activity and low oxidative damages were found in 3x varieties. Triploid varieties, especially T40-3x, had a better ability to limit water loss and dissipate excess energy (NPQ) to protect photosystems. Higher starch reserves in 3x varieties suggest a better carbon and energy supply and increases in plastoglobuli size suggest less oxidative damage (H₂O₂, MDA), especially in T40-3x, and preservation of photosynthetic apparatus. Taken together, our results suggest that desirable cytological and ultrastructural traits induced by triploidy improve water stress response and could be a useful stress marker during environmental constraints.

Physiological and biochemical responses of Kinnow mandarin grafted on diploid and tetraploid Volkamer lemon rootstocks under different water-deficit regimes

Water shortage is among the major abiotic stresses that restrict growth and productivity of citrus. The existing literature indicates that tetraploid rootstocks had better water-deficit tolerance than corresponding diploids. However, the associated tolerance mechanisms such as antioxidant defence and nutrient uptake are less explored. Therefore, we evaluated physiological and biochemical responses (antioxidant defence, osmotic adjustments and nutrient uptake) of diploid (2x) and tetraploid (4x) volkamer lemon (VM) rootstocks grafted with kinnow mandarin (KM) under two water-deficit regimes. The KM/4xVM (VM4) and KM/2xVM (VM2) observed decrease in photosynthetic variables, i.e., photosynthetic rate (Pn), stomatal conductance (gs), transpiration rate (E), leaf greenness (SPAD), dark adopted chlorophyll fluorescence (Fv/Fm), dark adopted chlorophyll fluorescence (Fv´/Fm´), relative water contents (RWC) and leaf surface area (LSA), and increase in non-photochemical quenching (NPQ) under both water-deficit regimes. Moreover, oxidative stress indicators, i.e., malondialdehyde (MDA) and hydrogen peroxide, and activities of antioxidant enzymes, i.e., superoxide dismutase (SOD), peroxidase (POD), catalase (CAT), ascorbate peroxidase (APx), glutathione reductase (GR) were increased under both water-deficit regimes. Nonetheless, increase was noted in osmoprotectants such as proline (PRO) and glycine betaine (GB) and other biochemical compounds, including antioxidant capacity (AC), total phenolic content (TPC) and total soluble protein (TSP) in VM2 and VM4 under both water-deficit regimes. Dry biomass (DB) of both rootstocks was decreased under each water-deficit condition. Interestingly, VM4 showed higher and significant increase in antioxidant enzymes, osmoprotectants and other biochemical compounds, while VM2 exhibited higher values for oxidative stress indicators. Overall, results indicated that VM4 better tolerated water-deficit stress by maintaining photosynthetic variables associated with strong antioxidant defence machinery as compared to VM2. However, nutrient uptake was not differed among tested water-deficit conditions and rootstocks. The results conclude that VM4 can better tolerate water-deficit than VM2. Therefore, VM4 can be used as rootstock in areas of high-water deficiency for better citrus productivity.

Triploidy in Citrus Genotypes Improves Leaf Gas Exchange and Antioxidant Recovery From Water Deficit

The triploidy has proved to be a powerful approach breeding programs, especially in Citrus since seedlessness is one of the main consumer expectations. Citrus plants face numerous abiotic stresses including water deficit, which negatively impact growth and crop yield. In this study, we evaluated the physiological and biochemical responses to water deficit and recovery capacity of new triploid hybrids, in comparison with diploid hybrids, their parents ("Fortune" mandarin and "Ellendale" tangor) and one clementine tree used as reference. The water deficit significantly decreased the relative water content (RWC) and leaf gas exchange (P _net and g _s ) and it increased the levels of oxidative markers (H₂O₂ and MDA) and antioxidants. Compared to diploid varieties, triploid hybrids limited water loss by osmotic adjustment as reflected by higher RWC, intrinsic water use efficiency (iWUE P_net/g_s ) iWUE and leaf proline levels. These had been associated with an effective thermal dissipation of excess energy (NPQ) and lower oxidative damage. Our results showed that triploidy in citrus enhances the recovery capacity after a water deficit in comparison with diploids due to better carboxylation efficiency, restored water-related parameters and efficient antioxidant system.

Specific Physiological and Anatomical Traits Associated With Polyploidy and Better Detoxification Processes Contribute to Improved Huanglongbing Tolerance of the Persian Lime Compared With the Mexican Lime

Huanglongbing (HLB) is presently a major threat to the citrus industry. Because of this disease, millions of trees are currently dying worldwide. The putative causal agent is a motile bacteria belonging to Candidatus Liberibacter spp., which is transmitted by psyllids. The bacteria is responsible for the synthesis of callose at the phloem sieve plate, leading to the obstruction of the pores that provide connections between adjacent sieve elements, thus limiting the symplastic transport of the sugars and starches synthesized in leaves to the other plant organs. The Persian triploid lime (Citrus latifolia) is one of the most HLB-tolerant citrus varieties, but the determinants associated with the tolerance are still unknown. HLB-infected diploid Mexican lime (Citrus aurantiifolia) and Persian lime were investigated. The leaf petiole was analyzed using scanning electron microscopy (SEM) to observe callose deposition at the phloem sieve plate. Leaf starch contents and detoxification enzyme activities were investigated. In the field, Persian lime leaves present more limited symptoms due to HLB than the Mexican lime leaves do. Photosynthesis, stomatal conductance, and transpiration decreased compared with control plants, but values remained greater in the Persian than in the Mexican lime. Analysis of the petiole sieve plate in control petiole samples showed that pores were approximately 1.8-fold larger in the Persian than in the Mexican lime. SEM analyses of petiole samples of symptomatic leaves showed the important deposition of callose into pores of Mexican and Persian limes, whereas biochemical analyses revealed better detoxification in Persian limes than in Mexican limes. Moreover, SEM analyses of infected petiole samples of asymptomatic leaves showed much larger callose depositions into the Mexican lime pores than in the Persian lime pores, whereas biochemical traits revealed much better behavior in Persian limes than in Mexican limes. Our results reveal that polyploids present specific behaviors associated with important physiological and biochemical determinants that may explain the better tolerance of the Persian lime against HLB compared with the Mexican lime.

Salinity-Induced Physiological Responses of Three Putative Salt Tolerant Citrus Rootstocks

Our study aimed to evaluate the physiological responses following salinity treatment of three putatively salt-tolerant Citrus rootstocks recently developed by the University of Florida’s Citrus breeding program. Four-month-old seedlings from each of the three rootstocks (HS1, HS17, and HC15) were irrigated with 0, 60, 80, and 100 mm NaCl solution. The seedlings were evaluated together with the salt-tolerant Cleopatra mandarin as a positive control, Volkamer lemon as a moderately salt-tolerant rootstock, and the salt-sensitive Carrizo rootstock as a negative control. Our results demonstrated that chlorophyll content, net CO2 assimilation rate (A), transpiration rate (E), and stomatal conductance (gsw) significantly decreased in response to salinity. Na+ and Cl− levels were higher in leaf tissues than in the roots. Relatively little damage to the cellular membrane was recorded in HC15 and Cleopatra rootstocks under the 100 mm NaCl treatment, along with high accumulation of total phenolic content (TPC), while HS17 had the highest proline levels. Our results indicate that HC15 and HS17 rootstocks exhibited salt tolerance capacity via different strategies under salt stress and could be suitable replacements to the commercially available, salt-tolerant Cleopatra rootstock.

Identification of the NaCl-responsive metabolites in Citrus roots: A lipidomic and volatomic signature

It is known that the first osmotic phase affects the growth rates of roots immediately upon addition of salt; thus, dissecting metabolites profiling provides an opportunity to throw light into the basis of plant tolerance by searching for altered signatures that may be associated with tolerance at this organ. This study examined the influence of salt treatment on fatty acid composition and chemical composition of the essential oil of C. aurantium roots. Results proved that, under salt treatment, an increase of double bond index and linoleic desaturation ratio was pointed out. On the other hand, the reduction of saturated fatty acids was spotted. Such treatment also induced quantitative changes in the chemical composition of the essential oils from C. aurantium roots and increased markedly the rates of monoterpenes, while the sesquiterpenes decreased significantly. Both primary and secondary metabolites were found to be significantly salt responsive, including one fatty acid (palmitoleic acid) and six volatiles (E-2-dodecenal, tetradecanal, γ-Elemene, trans-caryophyllene, α-Terpinene and germacrene D). Plasticity at the metabolic level may allow Citrus plants to acclimatize their metabolic ranges in response to changing environmental conditions.

From physiology to salt marsh management challenges with sea level rise: the case of native Spartina foliosa, invasive S. densiflora and their hybrid

Sea level rise (SLR) imposes increasing salinity and inundation stresses in salt marshes which simultaneously face invasions by exotic plant species. We aimed to improve and apply knowledge on the ecophysiological responses of halophytes to SLR to conservation management of salt marshes. In a mesocosm experiment, we measured and compared phosphoenolpyruvate carboxylase (PEPC) activity and related functional traits of the California-native Spartina foliosa, invasive S. densiflora and their hybrid S. densiflora × foliosa in response to increasing levels of salinity and inundation. S. foliosa was moderately sensitive to salinity, showing a 57% reduction in PEPC specific activity from freshwater to hypersalinity. This native species compensated for the reduction of PEPC activity with increased salinity through 80% higher enzyme activation by phosphorylation. PEPC functional trait responses of S. foliosa were mostly independent of inundation depth. In view of these results, managers should conserve undeveloped lands for accommodation space above current high tide lines to facilitate colonization of stress-tolerant S. foliosa. Our results on functional responses of PEPC traits recorded high sensitivity to salinity for S. densiflora. This was reflected by 65% lower PEPC specific activity together with increasing accumulation of free proline (+96%) and total proteins (+23%) with elevated salinity. These results suggest prioritized eradication of S. densiflora populations in brackish habitats. Measured PEPC responses support the high stress tolerance of the S. densiflora × foliosa hybrid. PEPC traits for the hybrid were mostly independent of salinity and inundation. The hybrid showed higher PEPC-specific activity than S. foliosa (+70%) and S. densiflora (+15%) in freshwater under intermediate inundation. Results suggest that eradication of the hybrid should be the highest management priority. Our study shows that the responses of key functional physiological traits to environmental stresses serve as biological indicators that can guide ecosystem management practices in a scenario of climate change.

Synthetic Polyploidy in Grafted Crops

Synthetic polyploids have been extensively studied for breeding in the last decade. However, the use of such genotypes at the agronomical level is still limited. Polyploidization is known to modify certain plant phenotypes, while leaving most of the fundamental characteristics apparently untouched. For this reason, polyploid breeding can be very useful for improving specific traits of crop varieties, such as quality, yield, or environmental adaptation. Nevertheless, the mechanisms that underlie polyploidy-induced novelty remain poorly understood. Ploidy-induced phenotypes might also include some undesired effects that need to be considered. In the case of grafted or composite crops, benefits can be provided both by the rootstock's adaptation to the soil conditions and by the scion's excellent yield and quality. Thus, grafted crops provide an extraordinary opportunity to exploit artificial polyploidy, as the effects can be independently applied and explored at the root and/or scion level, increasing the chances of finding successful combinations. The use of synthetic tetraploid (4x) rootstocks may enhance adaptation to biotic and abiotic stresses in perennial crops such as apple or citrus. However, their use in commercial production is still very limited. Here, we will review the current and prospective use of artificial polyploidy for rootstock and scion improvement and the implications of their combination. The aim is to provide insight into the methods used to generate and select artificial polyploids and their limitations, the effects of polyploidy on crop phenotype (anatomy, function, quality, yield, and adaptation to stresses) and their potential agronomic relevance as scions or rootstocks in the context of climate change.