**1. Introduction**

Phenolic compounds are responsible for the major organoleptic characteristics of plantderived foods and beverages, particularly color and taste properties, and they also contribute to the nutritional qualities of fruits and vegetables [1, 2].

Plants present diverse defense mechanisms, including physical and chemical barriers. Phenolic compounds are particularly abundant and play an important role in both strategies, as monomers for the synthesis of lignin and as chemical agents. Flavonoids are one of the most relevant secondary compounds in plants and currently more than 9000 being identified [3]. A most significant function of the flavonoids, especially the anthocyanins, together with

flavones and flavonols as copigments, is their contribution to flower and fruit colors. This is important for attracting pollinators and seed-dispersing animals. Phenolics may influence the competition among plants "allelopathy." They act in plant defense mechanisms against herbivores or pathogens, contributing to the disease resistance mechanisms in plants, and act as supporting materials of cell walls as photoprotectors against UV radiation and plant-microbe symbiosis and involved in the repair of wounds and contribute to healing by lignifications of damaged areas. Stress conditions such as excessive UV light, wounding, or infection induce the biosynthesis of phenolic compounds [66, 67].

Plants composition can be affected by pre-harvest factors, including genotype (cultivar and variety), maturity at harvest and tissue distribution, and exogenous factors, including climate, soil micro-environment, and pest and disease attack [4, 5]. Environmental factors have a major effect on polyphenol content. These factors may be pedoclimatic (soil type, sun exposure, and rainfall) or agronomic (culture in greenhouses or fields, biological culture, hydroponic culture, fruit yield per tree, etc.). With the current state of knowledge, it is difficult to determine for each family of plant products the key variables that are responsible for the polyphenol variability. A huge amount of analysis would be required to obtain this information [10].

This paper reviews recent literature on the main factors affecting the flavonoid content in onion, as well as different approaches aiming to increase the accumulation of these compounds in onions, which provide an added functional value.
