Knowledge about the effect of chemical conditioners on the reduction of the deleterious effects caused by salinity on crops is of great importance for the expansion of the cultivation of vegetable crops such as zucchini in the semi-arid region of the Northeast. In this context, the objective of the present study was to evaluate the effect of the foliar application of hydrogen peroxide as a mitigator of salt stress on the gas exchange, production, and postharvest fruit quality of zucchini cultivated in a hydroponic system. The study was conducted in NFT-type (Nutrient Film Technique) hydroponic system in a greenhouse, in Pombal — PB, Brazil. The experimental design was completely randomized, in a 4 × 4 factorial scheme, corresponding to four levels of electrical conductivity of the nutrient solution - ECns (2.1 (control); 3.6; 5.1 and 6.6 dS m−1), and four concentrations of hydrogen peroxide - H2O2 (0; 20; 40 and 60 μM), with three replicates. Nutrient solution with electrical conductivity above 2.1 dS m−1 caused a reduction in gas exchange and the total number of fruits of zucchini. An increase in nutrient solution salinity levels increased the total soluble solids content of the fruits and the initial fluorescence of zucchini. Under conditions of nutrient solution salinity above 2.1 dS m−1, hydrogen peroxide could not mitigate the effects of salt stress. Application of 20 μM of H2O2 when the plants were grown in a nutrient solution of 2.1 dS m−1 promoted higher total fruit weight and basal diameter of the fruits.

Salt-induced redox and ion imbalance may lead to increased oxidative stress and impair the plant growth. However, among the many reactive oxygen species (ROS) produced under stress, hydrogen peroxide (H₂O₂) plays a central role in several signal transduction pathways triggering a series of physiological mechanisms and increase plant tolerance to salt stress. Thus, this study aimed to evaluate the changes in the balance of inorganic and organic solutes induced by seed priming with H₂O₂ in leaves and roots of sunflower plants grown under salt stress. In this study, six treatments were tested: T1 - control (absence of NaCl and absence of H₂O₂); T2 - salt control (presence of 100 mM NaCl and absence of H₂O₂); T3 − 100 mM H₂O₂ (12 h) + 100 mM NaCl; T4 − 10 mM H₂O₂ (12 h) + 100 mM NaCl; T5 − 1 mM H₂O₂ (24 h) + 100 mM NaCl; T6 − 0.1 mM H₂O₂ (36 h) + 100 mM NaCl. Our results show that, the role of H₂O₂ in increase of the salt tolerance is directly associated with reduction on Na⁺ (on average 46%) and Cl^- (on average 24%) contents in the leaves, besides a positive regulation of K⁺ and NO₃ ^- absorption (for three harvests). In addition, H₂O₂ priming also decreased the oxidative damage, mainly by increasing the activity of catalase enzyme (CAT) in leaves (on average 49% at 2^nd and 3^rd harvests) and roots (on average 39% at 3^rd harvest).

Salinity is still one of the main factors that limit the growth and production of crops. However, currently, hydrogen peroxide (H2O2) priming has become a promising technique to alleviate the deleterious effects caused by salt. Therefore, this study aimed to test different leaf spraying strategies with H2O2 for acclimation of sunflower plants to salt stress, identifying the main physiological and biochemical changes involved in this process. The experiment was conducted in a completely randomized design, with four replications. Initially, four concentrations of H2O2 were tested (0.1; 1; 10 and 100 mM) associated with different applications: 1AP - one application (48 h before exposure to NaCl); 2AP - two applications (1AP + one application 7 days after exposure to NaCl) and 3AP - three applications (2AP + one application 14 days after exposure to NaCl), besides this two reference treatments were also added: control (absence of NaCl and absence of H2O2) and salt control (presence of 100 mM of NaCl and absence of H2O2). The experiment was conducted in hydroponic system containing Furlani's nutrient solution. Salt stress reduced the growth of sunflower plants, however, the H2O2 priming through leaf spraying was able to reduce the deleterious effects caused by salt, especially in the 1 mM H2O2 treatment with one application. H2O2 acts as a metabolic signal assisting in the maintenance of ionic and redox homeostasis, and consequently increasing the tolerance of plants to salt stress.

Salinity commonly affects photosynthesis and crop production worldwide. However, some studies have shown that hydrogen peroxide seed priming can increase the tolerance of plants to salt stress. In this context, this study aimed to test the hypothesis that the H₂O₂ seed priming can improve photosynthetic efficiency in sunflower plants under salt stress. The experiment was conducted in a completely randomized design, with four replicates. Six treatments were tested: control (deionized water); salt control (100 mM NaCl); 0.1 mM H₂O₂ (36 h) + 100 mM NaCl; 1 mM H₂O₂ (24 h) + 100 mM NaCl; 10 mM H₂O₂ (12 h) + 100 mM NaCl; and 100 mM H₂O₂ (12 h) + 100 mM NaCl. Compared to the control treatment, leaf, stem and root dry mass of stressed plants showed 73, 76, and 71% reduction, respectively. However, this effect was less pronounced in plants whose seeds were primed with H₂O₂. After five weeks, H₂O₂ seed priming increased the CO₂ assimilation rate by 26%, instantaneous carboxylation efficiency by 38%, potential quantum yield and quantum yield of photochemical quenching of photosystems II by 22%, electron rate transport by 43%, and the chlorophyll b content by 71%. In addition, it reduced the non-photochemical quenching parameters by 22% compared to plants under salt control treatment.

Salinity is one of the main limiting factors for crop growth. The metabolic responses to salt stress are variable and depend on species characteristics. This study aimed to select sunflower genotypes tolerant to salt stress and evaluate some mechanisms of salt tolerance in two contrasting (salt-tolerant and salt-sensitive) genotypes. In the first assay, the biomass production and the accumulation of Na+ and K+ in 26 sunflower genotypes were evaluated. Genotypes AG963, AG967, AG972, BRS321, BRS324, H251, H360 and H863 showed lower biomass production and were characterized as salt-sensitive and the genotypes BRS323, Catisol, EXP11-26, EXP44-49, EXP60050, EXP887, HLA860HO and Olisun 5 showed higher biomass production and were considered salt-tolerant. The high K+ content and the low Na+ content in the leaves were the ion traits related to salt tolerance and can be used in sunflower breeding programs for this purpose. In the second assay, the plants of salt-tolerant BRS323 had lower Na+ and Cl- contents and higher levels of K+ than plants of salt-sensitive AG967. A better homeostasis in the mechanisms of transport, distribution and accumulation of inorganic solutes in conjunction with a more efficient osmoregulation mechanism through the synthesis of organic solutes may, at least in part, explain the greater salt-tolerance of BRS323 genotype in comparison to AG967.

Salt stress is abiotic stress that directly affects the gas exchange and redox metabolism of the plants. However, some studies claim that the combination of potassium-phosphorus fertilization may increase salt tolerance in plants. In this context, the objective of this study was to evaluate gas exchange and the antioxidant enzyme activity responses to combined potassium-phosphorus fertilization in the tissues of West Indian cherry during the fructification phase, 240 days after application of treatments (DAT) in the second year of brackish water irrigation production. The treatments were distributed in randomized blocks, in a 5 × 4 factorial scheme, with five levels of irrigation water electrical conductivity – (ECw equal to 0.6 – control, 1.4; 2.2; 3.0 or 3.8 dS m⁻¹) and four potassium-phosphorus fertilization combinations, 200/120; 170/102; 120/72 or 90/54 g K₂O/P₂O₅ plant⁻¹ per year, with three replicates. Irrigation water salinity impaired the gas exchange, showing a reduction of 47.94, 47.21, and 43.92% in the stomatal conductance, transpiration, and CO₂ assimilation, respectively. In contrast, it increased the antioxidant metabolism, mainly the catalase enzyme activity (+ 67.7%). The average weight of fruits was increased by 85% in the combination of 90/54 under 1.4 dS m⁻¹. Although the average number of fruits, in same combination (90/54) was 37% lower compared to high salinities. The threshold water salinity (1.12 dS m⁻¹) indicates that West Indian cherry is a salt sensitive crop.