Submit Manuscript | http://medcraveonline.com Introduction Under global warming conditions, there are different challenges facing horticulture production, and it is expected to get worse shortly. Therefore, focusing on improving horticulture crop productivity considered the main target for many researchers to achieved food security. 1 Climate changes have affected the rain precipitation and reduce the growth and productivity of various crops, 2 the water shortage is one of the severe problems in horticulture production, it affects various metabolisms, impaired photosynthetic activity, reduce vegetative growth, decrease total yield, and produce poor fruits, the water shortage is one of the severe problems in horticulture resulting in yield loss and affects negatively fruit quality. 3 In recent decades, due to the rapid increase in the world population, there is a more growing demand for food worldwide, it is necessary to increase crop yield, even under stressful conditions, like water defcit, salinity, and rising temperature. Currently, there is more attention to understanding tree behavior under this condition to sustain crop productivity. Researchers looking for new ingredients to sustain the production systems for horticultural crops and reducing the negative effects of abiotic stress particularly drought and salinity, application of biostimulants is more effcient in promoting the recovery of different crops after subjecting to water shortage stress. 4 Biostimulants play important role in controlling the adverse effects of abiotic stresses like water defcit and have an anti-stress role on plant metabolism, 3 consequently improve the growth and productivity of different horticulture crops, also, plant stimulants that contain seaweed and fulvic acid had antioxidant effects and protect plant through reducing produce free radical. 5 Plant stimulants are used to increase agricultural production under serious challenges facing humanity, which represented by feeding a growing population worldwide and reducing the impact of agricultural agrochemicals on human health and the environment, 6 Many biostimulants improve plant nutrition, for instance, biofertilizers as a subcategory of biostimulants, improve nutrient use effciency, and increase nutrients acquisition uptake by plants. 5 This work discusses the role of plant stimulants in the horticultural sector, their effects on nutrient absorption, both primary and secondary metabolisms. Materials and methods This review was explaining the importance of plant stimulants and their role in improving horticulture crop productivity, also classifcation of plant stimulants, and the mechanism of different biostimulants like Protein hydrolysates (PHS), Humate substances, Seaweed extracts, and chitosan. A systematic literature review was conducted, searching the platforms Research Gate, Academia and Egyptian Knowledge Bank, Web of Science, Google Scholar, AGRIS, for a combination of the following keywords: “Plant stimulants”, “horticulture crops”, “Protein hydrolysates”, “Humate Substances” and “Seaweed extracts”. The most recent papers dealing with plant stimulants were selected for this review. Plant stimulants Plant stimulants are defned according to European Biostimulant Industry Council (www.biostimulants.eu), as follow: plant biostimulants contain substance (s) and/or micro-organisms whose function when applied to plants or the rhizosphere is to stimulate natural processes to increase the utilization of nutrients, increase their effciency, and raising the plants’ tolerance to abiotic stress, and to enhancing fruit quality. 7 Furthermore, plant biostimulants including any materials or microorganisms used to improve nutrition effciency, enhance tolerance of abiotic stress, and improve crop quality. 4 Classifcation of plant stimulants Biostimulants categorize according to their nature, modes of action, and types of effects on crops. There are different classifcations of plant biostimulants, for example, Kauffman et al. 8 divided biostimulants into three basic groups according to their source and their content of organic ingredients as follows: a. Humate substances, MOJ Eco Environ Sci. 2020;5(6):261‒265. 261 ©2020 Abobatta. This is an open access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and build upon your work non-commercially. Plant stimulants and horticultural production Volume 5 Issue 6 - 2020 Waleed Fouad Abobatta Department of Citrus Research, Horticulture Research Institute, Agriculture research center, Egypt Correspondence: Waleed Fouad Abobatta, Department of Citrus Research, Horticulture Research Institute, Agriculture research center, Giza, Egypt, Email Received: November 24, 2020 | Published: December 15, 2020 Abstract Plant stimulants is an organic substance and micro-organisms, used by small quantities, Biostimulants categorize according to their nature, modes of action, and types of effects on crops, there are main groups of plant stimulants include Protein hydrolysates, Humate substances, Seaweed extracts, Biopolymers (Chitosan and other polymers), and Microbial biostimulants like mycorrhizal, non-mycorrhizal fungi, Rhizobium, and Trichoderma. Horticulture crop production facing several challenges particularly abiotic stresses and malnutrition resulting in yield loss and affects negatively fruit quality. The main effects of plant stimulants due to its working as the auxin-like effect, enhancing Nitrogen uptake, and stimulate plant growth. There is various stimulation effects on horticulture crops including promote plant growth, increase plant tolerance for biotic and abiotic stresses. Applying plant stimulants to plants or the rhizosphere stimulating plant metabolic processes, increase the effciency of the nutrients, and increase plant tolerance to abiotic stress, consequently, improving plant growth increases yield, and enhancing fruit quality. Keywords: horticulture crops, humate substances, plant stimulants, protein hydrolysates, seaweed extracts MOJ Ecology & Environmental Sciences Research Article Open Access