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Plants face a variety of biotic and abiotic stresses throughout their life cycle that adversely affect their growth and production. These stresses decrease the annual crop yield up to 70% globally and had created a problem of food shortage as well as the malnutrition for ever-increasing world population. Different strategies are being practiced to overcome these problems and found successful up to some extent. Exogenous use of plant growth promoters through different modes is one such among them that found helpful to improve the plant growth and production under stressful conditions and is being employed at commercial level to obtain better production. However, most of them are the inorganic chemicals and are found toxic for organism and are not environmental friendly. Trend is increasing toward the use of organic and environmental-friendly biostimulants with the ability to improve production and stress tolerance induction. Studies have revealed that exogenous use of these biochemicals through different modes (seed soaking, foliar spray, or as soil application) is found helpful in improving the plant growth and production under stressful conditions. These biostimulants include the chelation of macro- or micro-nutrients with different organic compounds, plant growth regulators of microbial origin, and plant-based biostimulants that are found helpful in improving the plant growth and production under stressful conditions. These include humic acids (humic and fulvic acid), chitosan, phosphites, bio-char, and plant-based extracts, including algal extracts and of different plant parts. These biostimulants along with the improvement of plant growth and production also found helpful to improve the yield and yield nutritional quality when applied exogenously through different modes. Improvements in the attribute due to the exogenous use of biostimulants under stressful condition are associated with their role in different physio-biochemical attributes, including plant antioxidative defense mechanism, plant photosynthetic efficiency, and plant water relation through the accumulation of different osmotica (osmolytes and osmoprotectants) that help to maintain cellular osmotic potential and better nutrient uptake and mobility. Plant-based biostimulants improve the plant growth due to the presence of different phytohormones and other secondary metabolites, vitamins, antioxidants, and the inorganic nutrients in the extract that directly affect the plant growth and production through improving the plant tolerance against abiotic stresses. Exogenous use of biostimulants through different modes also improves the plant growth, yield as well as the yield nutritional quality under stressful conditions. However, studies revealed that the effective dose for the induction of stress tolerance of these biostimulants is plant species, growth stage, and stress type specific. Moreover, the use of biostimulants for the effective improvement is also dependent on the mode of application. Although large work has been done to study the role of biostimulants in improving plant growth and production, still more is required for final recommendations. More attention is required in this regard and in future due to their non-toxic and environmental-friendly nature.

Due to sessile nature of plants, they face a variety of biotic and abiotic stresses during their life cycle. These stresses are responsible for disturbed cellular processes that adversely affect their growth and yield. To cope with these stresses, plants have developed various physiological mechanisms at cellular level that result in a change in morphology and help them to tolerate these environmental changes and respond to these changes with an optimal response. These responses of plants toward environmental stresses are both dynamic and complex that help them complete their life cycle rapidly under these stressful environmental conditions. The defense response of the plants to these stresses starts through variations in different molecular events with the involvement of different signaling molecules including the phytohormones. Phytohormones are small low-molecular-weight endogenous molecules that play important roles in different defense responses of plants against biotic as well as abiotic stresses. Along with their role in defense signaling, they also regulate various physiological, growth, and developmental processes in plants. These phytohormones include the auxins, cytokinins (CKs), gibberellins (GAs), ethylene (ET), jasmonic acid (JA), abscisic acid (ABA), salicylic acid (SA), and brassinosteroids (BRs) tocotrienols, triacontanols, and polyamines which are important ones that have ability to help plants to respond to different environmental stresses through their specific signaling properties. These signaling defense responses are the results of the interaction of various genes, helping the phytohormones to be involved in almost all cellular metabolic processes due to their specific modulations in the activities of these genes. All these phytohormones have their specific roles, e.g., auxin is involved in the regulation of differentiation and plant growth, cytokinin is responsible for cell division, gibberellin is responsible for stem elongation, seed germination, dormancy, senescence, and flower development, ethylene is involved in fruit ripening, and ABA has stress tolerance ability. The stress tolerance mechanism of plant is much complex that also has the involvement of other phytohormones, including the brassinosteroids (BRs), jasmonic acid (JA), salicylic acid (SA), polyamines (spermine, spermidine, putrescine, and thermospermine), tocotrienols, triacontanols (TRAI) as newly discovered ones. All the phytohormones help plants to survive under adverse environmental conditions with their specific roles in various growth, developmental, and physiological processes either through their endogenous accumulation or by exogenous application (foliar spray or seed priming) where the optimal concentration for the stress response is not sufficient. The exogenous use of these phytohormones has been increasing in crop plants with their economic value for obtaining the desired characters along with better production. The information given in the chapter will be helpful for plant growers and researchers to understand the mechanism of action of these phytohormones for better growth and production under changing environmental conditions.