Acetylcholine (ACh) is believed to improve plant growth. However, regulation at biochemical and molecular levels is largely unknown.• The present study investigated the impact of exogenously applied ACh (10 µM) on growth and chlorophyll metabolism in hydroponically grown Nicotiana benthamiana under salt stress (150 mM NaCl).• Salinity reduced root hydraulic conductivity while ACh-treated seedlings exhibited a significant increase, resulting in increased relative water content. Salinity induced a reduction in chlorophyll biosynthetic intermediates, such as protoporphyrin-IX, Mgphotoporphyrin-IX and protochlorophyllide, which were significantly ameliorated in the presence of ACh. This influence of ACh on chlorophyll synthesis was confirmed by up-regulation of HEMA1, CHLH, CAO and POR genes. Gas exchange parameters, i.e. stomatal conductance, internal CO 2 concentration and transpiration rate, increased with ACh, thereby alleviating the salinity effects on photosynthesis. In addition, the salinity-induced enhancement of lipid peroxidation declined after ACh treatment through modulation of the activity of the assayed antioxidant enzymes (superoxide dismutase and peroxidase). Importantly, ACh significantly reduced the uptake of Na and increased uptake of K, resulting in a decline in the Na/K ratio.• Results of the present study indicate that ACh can be effective in ameliorating NaClinduced osmotic stress, altering chlorophyll metabolism and thus photosynthesis by maintaining ion homeostasis, hydraulic conductivity and water balance.

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Experiments, both pot and field trial, were conducted to examine the effects of exogenously applied IAA and GA3 (at 0, 10, 20 and 30 days after topping) individually or in combination, on growth and C and N metabolisms of tobacco (Nicotiana tabacum L.). Application of IAA or GA3, at low concentrations had a promising effect in terms of promoting the yield and quality of tobacco. The key enzymes' activities were also enhanced by low PGRs concentrations, which is known to be involved in the C and N metabolisms. The combined treatment of the two PGRs was more effective than the PGRs applied individually in improving the activities of nitrate reductase (NR), invertase (INV) and amylase (AMY) finally resulting in improved soluble sugars, reducing sugars, starch, total C and N, soluble proteins and nicotine content. The optimum levels for improvement of C and N metabolites were found to be GA3 at 50 mg/L and IAA at 30 mg/L.

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