**3. Future challenges for plant scientists and growers**

This chapter deals about the current state of knowledge on the main factors affecting the flavonoid content in onions, as well as different approaches that can be applied to increase the accumulation of these compounds. For example, red cultivars contain the highest flavonoid levels; in this sense, also resistant onions present higher flavonoid levels than those that are susceptible. The nonedible dry skin is richer in flavonoids than the flesh, promoting the nonedible portions as a source of natural antioxidants. Within the edible bulb, a decrease across the onion from the outer onion scales to the inners is also found. With regard to soil management factors, the nitrogen fertilizer levels should be minimized to favor flavonoid levels. It was also found that organically grown onions present higher levels of flavonoids and antioxidant activity than conventional. Late lifting of onions generally results in higher concentrations of quercetin glucosides than early lifting.

Phenolic compounds can affect sensory attributes such as color, flavor, bitterness, and texture affecting the consumer assessment. The identification of specific compounds in different onion cultivars and agronomic practices would lead to a better understanding of the physiological responses to onion consumption [17–20, 23–25]. This would aid the development of onion production systems that provide an increased health benefit [56] and the development of guidelines for the consumption of these compounds. An interesting and challenging aspect for future research is to clarify the interactions between genotype and agro-environmental factors on the flavonoid composition in onions [129].

The production of fresh "functional food" with defined health claims may be favorable for a premium market segment. In the future, the minimum quality of plant foods could be defined on the base of their content of bioactive components [130]. One of the projects that have been awarded over the years is given below as an example of the fruit and vegetables research community [131] to generate successful applications in the calls published by the EU Commission: FLAVO is the project for "flavonoids in fruits and vegetables: their impact on food quality, nutrition and human health." The project is centered on fruits widely available to Europeans—apple, grape, and strawberry—together with their derivatives. FLAVO aimed to monitor the flavonoids in fruits and vegetables and to optimize their beneficial effects. This action was promoted by the European Fruit Research Institutes Network (EUFRIN), and a similar project would be desirable for the vegetable sector with the support of the European Vegetable Research Institutes Network (EUVRIN) to cover areas such as (a) the study of consumer behavior about new products, (b) selection of improved plant foods by breeding, (c) the choice of agronomic techniques to maximize flavonoids, (d) knowledge on the appropriate dose of flavonoids for beneficial effects, and (e) the dissemination of the results to consumers and other stakeholders.
