Seasonal Variation in the Yield and Polyphenol Content of Sweet Potato (Ipomoea batatas L.) Foliage

Abstract

Despite increasing demand for sweet potato foliage, which is rich in functional components, efficient methods to maximize yield are still needed. In this study, cultivation tests for sweet potato (line Kyukei05303-3) were conducted over three consecutive years at a greenhouse to characterize seasonal changes in the foliage yield (leaves and stem-petioles), as well as the polyphenol content. The sweet potato foliage was harvested from May to November every week, and the average leaf yield was 855.3 g·m⁻²·year⁻¹ on a dry weight (DW) basis. The yield and polyphenol content of the leaves were negatively correlated. The yield increased from spring to summer but decreased after mid-August. In contrast, the polyphenol content was highest in May, lower during the summer (June to August), and increased again after September. The average polyphenol content in the leaves was 6.9 g·100 g⁻¹ DW and the total annual polyphenol yield was 59.0 g·m⁻². The major component of polyphenols was caffeoylquinic acids. The seasonal changes in caffeoylquinic acids were highly correlated with the changes in total polyphenols. The polyphenol content was significantly correlated with air temperature, but not with sunshine duration, suggesting that air temperature is an important determinant of the polyphenol content during cultivation. These results provide a basis for the rapid cultivation of sweet potato for foliage production.