Phenolic compounds in developing kiwifruit: implication for calcium accumulation

Kiwifruit fruit quality is related to fruit calcium (Ca) concentration (Poovaiah et al., 1988). It has been shown that after an early rise, fruit calcium concentration actually decreases because Ca influx ceases by the mid-growth stage whereas volume growth continues till harvest (Xiloyannis et al., 2001). Recently, fruit Ca content has been investigated in response to several factors (e.g. light intensity, wind speed, position within the canopy) (Montanaro et al., 2006; Dichio et al., 2006; Thorp et al., 2003), however the underlying physiological mechanisms regulating fruit Ca inflow to the fruit are still not at all well understood. Calcium transport to the fruit is exclusively via the xylem - calcium is not phloem mobile (White, 2001), this suggest a possible causality between xylem formation (and/or functionality) and Ca accumulation. Lignin is a major structural component of the xylem tissues (Douglas, 1996) and its biosynthesis is derived from the phenylpropanoid pathway via hydroxycinnamic acids (Boerjan et al., 2003), however, in developing kiwifruit the biosynthesis of those phenols have not yet been investigated.We report the seasonal occurrence of phenolic compounds in fruits (fruitstalk and berry) grown under different light condition in South Italy. Calcium and phenols were weekly determined using atomic absorption spectrophotometer (HPLC techniques respectively. The higher light availability increased the concentration of certain phenols besides fruit Ca concentration. The predominant phenolic compounds were hydroxycinnamic acids, flavonols and the flavan 3-ol epicatechin. Results indicate that higher light exposure may increase the biosynthesis of some phenolic compounds which are involved in xylogenesis process and thus being beneficial to xylem-borne minerals transportation. The patterns of hydroxycinnamic acids suggest that in the berry a secondary growth of the xylem didn’t occur as in the pedicel, giving an explanation of the Ca import dynamic in fruit.