LC-MS/MS-based metabolomics approach identified novel antioxidant flavonoids associated with drought tolerance in citrus species.

Citrus fruits are cultivated around the world, and they face drought stress frequently during their growth and development. Previous studies showed that citrus plants biosynthesized flavonoid compounds in response to abiotic stress. In this study, we have quantified 37 flavonoid compounds from the leaves of three distinct citrus species including sour orange (drought-tolerant), pummelo 'Majia you pummelo' (drought-sensitive), and lemon (drought-sensitive). The 37 flavonoids consisted of 12 flavones, 10 flavonols, 6 flavanones, 5 isoflavanones, and 1 each for chalcone, flavanol, flavanonol, and flavone glycoside. Drought stress differentially altered the flavonoid metabolism in drought-tolerant and drought-sensitive citrus species. The kaempferol 3-neohesperidoside was 17-fold higher in sour orange (124.41 nmol/L) after 18 days of drought stress than lemon (7.33 nmol/L). In sour orange, neohesperidin (69.49 nmol/L) was 1,407- and 37-fold higher than pummelo and lemon, respectively. In sour orange, some flavonoids were significantly increased, such as vitexin, neohesperidin, cynaroside, hyperoside, genistin, kaempferol 3-neohesperidoside, eriocitrin, and luteolin, in response to drought stress, whereas in lemon, these flavonoids were significantly decreased or not altered significantly in response to drought stress. Moreover, the total contents of flavonoids and antioxidant activity were increased in sour orange as compared with pummelo and lemon. The genes associated with flavonoid biosynthesis (PAL, CHI, FLS, GT1, F3H, F3'M, C4H, 4CL, FLS, FG2, FG3, and CYP81E1) were more highly expressed in sour orange leaves than in pummelo and lemon after drought stress. These outcomes showed that pummelo and lemon failed to biosynthesize antioxidant flavonoids to cope with the prolonged drought stress, whereas the sour orange biosynthesized fortified flavonoid compounds with increased antioxidant activity to detoxify the harmful effects of reactive oxygen species produced during drought stress.

Influence of bio fabricated manganese oxide nanoparticles for effective callogenesis of Moringa oleifera Lam.

The use of nanoscale fertilizers to boost crop output has increased in recent years. Nanoparticles (NPs) can stimulate the biosynthesis of bioactive compounds in plants. It is the first report on biosynthesized manganese oxide nanoparticles (MnO-NPs) that mediate in-vitro callus induction of Moringa oleifera. To achieve better biocompatibility the leaf extract of Syzygium cumini was used to synthesize MnO-NPs. Scanning electron microscope SEM revealed spherical shaped morphology of MnO-NPs with an average diameter of 36 ± 0.3 nm. Energy-dispersive X-ray spectroscopy (EDX) depicted the formation of pure MnO-NPs. X-ray diffraction (XRD) and Fourier Transform Infrared (FTIR) authenticate the crystalline structure. UV-visible absorption spectroscopy depicted the activity of MnO-NPs under visible light. The biosynthesized MnO-NPs were concentration-dependent and revealed promising results in callus induction of Moringa oleifera. It was found that MnO-NPs enhance callus production of Moringa oleifera and keep the callus infection free by providing an optimum environment for rapid growth and development. Therefore MnO-NPs synthesized through the green process can be utilized in tissue culture studies. This study concludes that MnO is one of the essential plant nutrients that have tailored nutritive properties at a nano scale.

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.

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.

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.

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

Tetraploids are usually more tolerant to environmental stresses than diploids. Citrus plants face numerous abiotic stresses, including salinity, which negatively affect growth and yield. Double diploid citrus rootstocks have been shown to be more tolerant to abiotic stresses than their diploid relatives. In this study, we evaluated the antioxidative and osmotic adjustment mechanisms of diploid (2x) and double diploid (4x) volkamer lemon (Citrus volkameriana Tan. and Pasq.) rootstocks, which act against salt stress (75 and 150 mM). Results indicated that, under salt stress, all physiological variables (photosynthesis, stomatal conductance, transpiration rate, and leaf greenness) decreased, and these decreases were more noticeable in 2x plants than in 4x plants. On the other hand, accumulation of oxidative markers (malondialdehyde and hydrogen peroxide) was greater in the leaves and roots of 2x seedlings than in 4x seedlings. Similarly, the activities of antioxidative enzymes (peroxidase, ascorbate peroxidase, glutathione reductase, and catalase) were higher in the leaves and roots of 4x plants than in 2x plants. However, superoxide dismutase activity was higher in the roots of 2x seedlings than 4x seedlings. Double diploid plants affected by salt stress accumulated more osmolytes (i.e. proline and glycine betaine) in their leaves and roots than that by 2x plants. Total protein content, antioxidant capacity, and total phenolic content were also higher in 4x plants than 2x plants under salinity. At 150 mM, both 2x and 4x plants showed more symptoms of stress than those at 75 mM. Sodium content was the highest in the roots of 2x plants and in the leaves of 4x plants, while chloride content peaked in the leaves of 2x plants and in the roots of 4x plants. Overall, our results demonstrate that the active antioxidative defence mechanisms of 4x plants increase their tolerance to salinity compared to their corresponding 2x relatives. Thus, the use of newly developed tetraploid rootstocks may be a strategy for enhancing crop production in saline conditions.

Effect of different combinations of antibiotics on fruit quality and antioxidant defense system in Huanglongbing infected Kinnow orchards.

Huanglongbing (HLB), also known as citrus greening disease, is the most devastating disease of citrus across the world, caused by the phloem limited fastidious bacterium 'Candidatus Liberibacter spp.'. This research was conducted on HLB infected 10-year-old Kinnow orchard located at Multan, Pakistan. Different classes of antibiotics in various combinations were applied on HLB-infected trees. The antibiotic treatments were applied before flowering in February, during fruit setting in April and at fruit growth stage in June. The different antibiotics combinations used were Ampicillin sodium + Rifampicin, Cefalexin + Rifampicin, Ampicillin sodium + Cefalexin, Ampicillin sodium + Cefalexin + Rifampicin and Control (distilled water). Different fruit qualitative and quantitative attributes were examined. The application of antibiotics significantly decreased 2-11% in flower, June and pre-harvest drops as compared to control. Further, antibiotics increased fruit weight and yield by five times while the juice content, total soluble solids, ripening index, total sugars, phenolic and vitamin C content were also increased in fruits. In addition, total soluble proteins, peroxidase and catalase activities were increased in fruits harvested from antibiotic treated plants compared to control, however the superoxidase dismutase activity was decreased in fruits of antibiotic treated plants. Finally, it is concluded that application of different antibiotics combinations helps in improving the fruit yield and different quality attributes of HLB infected Kinnow trees.