Limited findings have been reported on using nanomaterials to improve tree fruit growth, development, and productivity under various stress conditions. To assess the effect of nanoparticles (NPs) like nano-zinc oxide (nZnO) and nano-silicon (nSi) on mango tree growth, yield, and fruit quality under salinity conditions, a factorial experiment was conducted using twelve treatments; three replicates each. Foliar spray of nZnO (50, 100, and 150 mg/L), nSi (150 and 300 mg/L), their combinations, and distilled water as a control was applied at full bloom and one month after of salt-stressed “Ewais” mango trees. Trees positively responded to different levels of nZnO and nSi. Plant growth, nutrients uptake, and carbon assimilation have improved with all treatments, except the higher concentration of nSi. Plant response to stress conditions was represented by a high level of proline content with all treatments, but changes in the activity of the antioxidant enzymes were positively related to the lower and medium concentrations of NPs. Flower malformation has significantly decreased, and the annual fruit yield and physiochemical characteristics have improved with all treatments. It could be recommended that a combination of 100 mg/L nZnO and 150 mg/L nSi improves mango tree resistance, annual crop load, and fruit quality under salinity conditions.

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Mango production faces several challenges, such as nutrient deficiency, physiological stress, and alternate bearing, which eventually affect tree productivity. This study was carried out during the 2017 and 2018 seasons to evaluate the effect of single and combined applications of humic acid (as potassium humate; 0.15%, 0.30%, 0.45%) and boron (as boric acid; 300, 600 mg·L−1) on ‘Zebda’ mango trees grown at Dir AlMalak region, Sharkeya Governorate, Egypt. Foliar spray was applied twice before flowering (first week of January and first week of February), and a third spray was applied by the beginning of flowering (first week of March) in both seasons. Humic acid and boron effectively enhanced tree growth, flowering, yield, and fruit quality. Humic acid was more effective than boron in this respect. Combined application of both materials surpassed the single application of each material on overall tree physiology and annual productivity. The observed results may be a consequence of the increase in tree photosynthetic pigments, nutrients, organic solutes, and phytohormones such as auxins, gibberellins, and cytokinins. The reduction in abscisic acid content may be related to the role of humic acid and boron protecting the plant against destructive oxidative reactions; improving the ability of the trees to withstand environmental stresses; thereby reduce floral malformation percentage, minimize the incidence of alternate bearing, and improve annual tree productivity and fruit quality. The most pronounced effect in this regard was noted with the application of 0.30% humic acid + 600 mg·L−1 boric acid.

Polyethylene glycol (PEG) can be used in somatic embryogenesis to enhance embryogenic development and improve the success of in vitro culture but PEG also causes osmotic stress in developing embryos. The effects of PEG on embryo growth and development in date palm cell suspension culture and associated antioxidant enzyme activities were evaluated. Callus maintained on MS basal media was transferred to regeneration liquid media supplemented with increasing levels (0-20%) of polyethylene glycol 6000 (PEG) to induce osmotic stress. The degree of embryogenic callus formation, its fresh weight, and the percentage of normal embryo callus shapes were increased with an increase in the level of PEG up to 10%. Total soluble protein (TSP), proline, glycine betaine (GB), total soluble phenol (TSPh), total sugars (TS), and total soluble organic acids (TOA) also increased whereas superoxide dismutase (SOD) activity decreased in response to PEG supplementation. Raising the PEG level increased malondialdehyde (MDA) concentration up to 10% PEG and thereafter decreased. Glutathione reductase (GR) and catalase (CAT) activities decreased at the highest levels of PEG. The proportion of normal embryo developmental shapes were about 50% compared with 20% abnormal shapes at optimum levels of PEG. Proliferation of somatic embryos was influenced by their developmental shapes. Cv. Samani accumulated more organic solutes compared with cv. Sewi in both control and stress inducing media. In contrast, lipid peroxidation, GR, SOD, and CAT activities were significantly higher in cv. Sewi than in cv. Samani indicating that the cv. Samani had the ability to tolerate a higher level of osmotic stress compared to cv. Sewi due to the enhanced osmotic rebalancing within its tissues.

Plant growth, development, and productivity are adversely affected under drought conditions. Previous findings indicated that 5-aminolevulinic acid (ALA) and 24-epibrassinolide (EBL) play an important role in the plant response to adverse environmental conditions. This study demonstrated the role of ALA and EBL on oxidative stress and photosynthetic capacity of drought-stressed ‘Williams’ banana grown under the Egyptian semi-arid conditions. Exogenous application of either ALA or EBL at concentrations of 15, 30, and 45 mg·L−1 significantly restored plant photosynthetic activity and increased productivity under reduced irrigation; this was equivalent to 75% of the plant’s total water requirements. Both compounds significantly reduced drought-induced oxidative damages by increasing antioxidant enzyme activities (superoxide dismutase ‘SOD’, catalase ‘CAT’, and peroxidase ‘POD’) and preserving chloroplast structure. Lipid peroxidation, electrolyte loss and free non-radical H2O2 formation in the chloroplast were noticeably reduced compared to the control, but chlorophyll content and photosynthetic oxygen evolution were increased. Nutrient uptake, auxin and cytokinin levels were also improved with the reduced abscisic acid levels. The results indicated that ALA and EBL could reduce the accumulation of reactive oxygen species and maintain the stability of the chloroplast membrane structure under drought stress. This study suggests that the use of ALA or EBL at 30 mg·L−1 can promote the growth, productivity and fruit quality of drought-stressed banana plants.

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