The protective effect of fruits and vegetables has generally been attributed to their antioxidant constituents, including vitamins C and E, carotenoids, glutathione, flavonoids, and phenolic acids, as well as other unidentified compounds. Dietary antioxidant compounds are thought to protect against chronic diseases through mitigation of free radical damage to proteins, lipids, and DNA in humans.

Because it is impractical to quantify all of the compounds in plants that exhibit antioxidant activity, assays have been developed to quantify total antioxidant capacity of plant extracts. Total antioxidant capacity of many fruits and vegetables has been determined by the oxygen radical absorbance capacity (ORAC) assay, which measures the ability of plant extracts to scavenge peroxyl radicals. These studies show that fruits and vegetables vary greatly in their total antioxidant capacity, and extracts with high levels of total phenolics typically have high ORAC values.

Phenolic and ORAC levels in fruits and vegetables can be influenced by genetics, environmental growing conditions, maturation, and postharvest storage conditions. Although most work regarding these factors has been done on anthocyanin rich blueberries, blackberries, and strawberries, vegetables such as spinach, which also possess high ORACs, have received little attention.

Spinach (Spinacia oleracea) is an important dietary vegetable that is consumed fresh, in salad mixes, or after cooking in boiling water. The total flavonoid content of spinach is high (1000 mg/kg), compared with those of other fruits and vegetables.

Plant-derived polyphenolic flavonoids exhibit numerous biological and pharmacological properties that could potentially afford protection against chronic diseases. Spinach flavonols, in particular, have been shown to possess antioxidant, antiinflammatory, antimutagenic, and anticarcinogenic properties. The 10 predominant flavonoids in spinach include glucuronides and acylated di- and triglycosides of methylated and methylene-dioxiderivatives of 6-oxygenated flavonols. Although the effect of storage and processing on flavonoid and antioxidant changes in fresh-cut spinach has been studied, no information is available concerning the effect of genetics and growing season on these important bioactive constituents.

This study was undertaken at the University of Arkansas to determine how commercial cultivars (CC) and advanced breeding lines (ABL) of spinach vary in flavonoid content, and to determine how different growing seasons known to vary in biotic and abiotic stresses influence phenolic content and antioxidant capacity.

Total phenolic content and antioxidant capacity of 11 commercial cultivars and 15 advanced breeding lines of spinach were determined over two growing seasons known to vary in biotic and abiotic stresses. Flavonoid composition and content of fall-grown commercial cultivars and advanced breeding lines were also determined.

Over-winter spinach, which was planted in late fall and harvested in the spring, had much higher levels of total phenolics and antioxidant capacity than spinach planted in early fall and harvested in late fall, indicating that growing conditions, as well as biotic and abiotic stresses, influenced phenolic metabolism.

Genotype also appeared to play an important role in affecting phenolic metabolism and antioxidant capacity in spinach. University of Arkansas advanced breeding lines of spinach, which show increased disease resistance, had higher levels of total phenolics, individual and total flavonoids, and antioxidant capacity than commercial cultivars.

Our results indicate that plant breeders can select for increased phenolic content to increase antioxidant capacity in spinach cultivars, or the crops can be grown in different seasons or under certain stress conditions to elevate levels of antioxidants.