Copyright © Physiologia Plantarum 2001 PHYSIOLOGIA PLANTARUM 111: 17 – 22. 2001 Printed in Ireland —all rights resered ISSN 0031-9317 A hot water treatment induces resistance to Penicillium digitatum and promotes the accumulation of heat shock and pathogenesis-related proteins in grapefruit flavedo David Pavoncello, Susan Lurie, Samir Droby and Ron Porat* Department of Postharest Science of Fresh Produce, Agricultural Research Organization, The Volcani Center, P.O. Box 6, Bet Dagan 50250, Israel *Corresponding author, e -mail: rporat@olcani.agri.go.il Received 25 April 2000; revised 19 June 2000 43-kDa proteins that cross-reacted with citrus and tobacco A new postharvest hot water brushing (HWB) treatment, which sprays hot water on fruit as they move along a belt of  -1,3-glucanase antibodies. The induction of the 105-, 18- and 21-kDa HSPs by the HWB treatment indicates that it was brush-rollers, induced resistance against green mould decay caused by Penicillium digitatum (Pers. Fr.) Sacc in ‘Star sufficient to provide a heat stress and, thus, was able to induce biochemical changes in the fruit peel tissue. Nevertheless, the Ruby’ grapefruit. The HWB treatment (62°C for 20 s) was most effective in inducing disease resistance when the fruit accumulation of these HSPs was probably not related to the induction of fruit resistance against P. digitatum, since their were inoculated after 1 and 3 days, but was less effective when the fruit were inoculated on the same day or 7 days later. The accumulation could not be induced neither by direct inocula- HWB treatment induced the accumulation of a 105-kDa tion with the pathogen nor following exposure to other treat- ments, such as UV irradiation, which also induce disease protein that cross-reacted with an antibody raised against a bovine heat shock protein 70 (HSP70) and 18- and 21-kDa resistance. On the other hand, the increases in the accumula- tion of the 21-, 22- and 25-kDa chitinase proteins and of the proteins that cross-reacted with pea HSP18 and HSP21 38- and 43-kDa  -1,3-glucanases proteins, which were ob- antibodies. The accumulation of a grapefruit 17-kDa protein served 1 and 3 days after the HWB treatment when the fruit that cross-reacted with a pea HSP17 antibody was not af- fected by the HWB treatment. HWB also induced the accu- appeared to be more resistant to P. digitatum, may be part of mulation of 21-, 22- and 25-kDa proteins that cross-reacted the complex fruit disease resistance mechanisms induced by with citrus and tobacco chitinase antibodies and 38-, 42- and the heat treatment. against Helminthosporium sacchari (Byther and Steiner 1975). On the other hand, hot water dips increased the susceptibility of avocado fruit to Colletotrichum gloeospori - oides (Plumbley et al. 1993). In other cases, hot air treat- ments at 38°C for 3–4 days increased the resistance of tomatoes to Botrytis cinerea (Lurie et al. 1997) and reduced the development of Penicillium expansum in apple fruit (Fallik et al. 1995). In citrus fruit, hot water dips reduce postharvest decay development after storage (Wild and Hood 1989, Rodov et al. 1995, Schirra and D’hallewin 1997). Other heat treat- ments, such as curing (keeping the fruit for 1–3 days at 30 – 36°C in a water-saturated atmosphere) enhanced resis- Introduction Hot water dip treatments have been used for many years as non-chemical methods to control fungal rots in various fruit and vegetables (Couey 1989, Barkai-Golan and Phillips 1991, Lurie 1998). The primary mode of action of the hot water treatments is to disinfect the commodity from fungal spores and from latent infections that may have occurred in the outer cell layers of the fruit surface. However, in some plant species it has been reported that hot water dip treat- ments also induced resistance against inoculation with vari- ous pathogens. For example, hot water dips increased the resistance of barley against Erysiphe graminis (Schweizer et al. 1995), of cucumbers against Cladosporium cucumerinum (Stermer and Hammerschmidt 1984) and of sugarcane Abbreiations – HSP, heat shock proteins; HWB, hot water brushing. Physiol. Plant. 111, 2001 17