Superficial scald susceptibility and a-farnesene metabolism in ‘Bartlett’ pears grown in California and Washington

Authors: Whitaker, Bruce; VILLALOBOS, MAX - UNIV OF CALIFORNIA-DAVIS; MITCHAM, ELIZABETH - UNIV OF CALIFORNIA-DAVIS; Mattheis, James

Submitted to: Postharvest Biology and Technology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/3/2009
Publication Date: 7/5/2009
Citation: Whitaker, B.D., Villalobos, M., Mitcham, E.J., Mattheis, J.P. 2009. Superficial scald susceptibility and a-farnesene metabolism in ‘Bartlett’ pears grown in California and Washington. Postharvest Biology and Technology. 53:43-50.

Interpretive Summary: A storage disorder called superficial scald occurs in many varieties of apples and pears. Symptoms appear as sunken brown patches on the skin that make the fruit unmarketable. Scald often does not appear until after removal from cold storage, and thus can cause great loss of fruit after the expense of growing, harvesting, packing, storing and shipping. This study was conducted to find out why Bartlett pears grown in northern California are much more prone to scald than those grown in central Washington. Fruit harvested from the two regions were stored under the same conditions, and compared with respect to several key metabolic factors thought to play roles in scald development. Even when harvested at about the same time, California pears were softer and more advanced in maturity, with a more active metabolism. In particular, California fruit had higher levels of oxidized compounds known to generate damaging free radicals that cause tissue injury. This kind of information is needed by plant scientists working to understand and combat the biochemical processes involved scald development. It will also help fruit growers and distributors determine when pears are particularly prone to scald and require added control measures.

Technical Abstract: ‘Bartlett’ pears grown in northern California (CA) consistently show development of the physiological storage disorder superficial scald, particularly after prolonged storage of 4–5 months in air. In contrast, fruit of this cultivar grown in central Washington (WA) are typically less susceptible to scald, exhibiting mild or no symptoms. Conjugated triene (CT) oxidation products of the sesquiterpene a-farnesene are thought to play a key role in scald induction in apples and pears. This study compared accumulation of a-farnesene and its CT products in peel tissue of CA- and WA-grown ‘Bartlett’ pears during air storage at –1 °C in relation to subsequent scald development after transfer to 20 °C. Pears were harvested from commercial orchards in 2006 and 2007 and stored under nearly identical conditions for up to 24 weeks. Peel tissue samples taken at harvest and at 2 to 4 week intervals during storage were analyzed by HPLC to determine concentrations of a-farnesene and CTs. Measurements of flesh firmness, respiration, and ethylene production were also made from 1 to 5 days after removal from storage to 20 °C. Only WA fruit from the second harvest in 2006 developed light scald after 20 weeks of cold storage plus 5 days at 20 °C, whereas all CA fruit from both seasons showed light scald after 12–14 weeks, and moderate scald after 20–24 weeks, plus shelf life. Accordingly, a-farnesene and CTs accumulated more rapidly and on average reached about twofold higher concentrations in CA compared with WA fruit over the first 8–12 weeks of storage. CA fruit also had an earlier rise in ethylene production, higher respiratory rates, and lower flesh firmness at harvest than WA fruit. These suggest advanced maturity, which may have contributed to the increased rates of a-farnesene synthesis and oxidation, and higher incidence of scald.