CYTOLOGY AND PHYSIOLOGY OF SILVERLEAF WHITEFLY-INDUCED SQUASH SILVERLEAF

Squash silverleaf is a disorder of certain cultivars of Cucurbita whic h results from feeding by the silverleaf whitefly Bemisia argentifolii (formerly, Bemisia tabaci, biotype B), but not by the sweetpotato whi tefly, Bemisia tabaci Biotype A (B. tabaci-A). Because squash silverle af aetiology is unknown, we compared the symptomology, cytology and ph ysiology of Cucurbita pepo plants Fed upon by B. argentifolii and B. t abaci-A to find the mechanism of squash silverleaf induction. B. argen tifolii induced rapid-onset, 100% (of upper leaf surface) expression o f squash silverleaf regardless of the season, whereas B. tabaci-a indu ced some intervenal chlorosis but did not induce full expression of sq uash silverleaf. Cytology of silvered tissue revealed large intercellu lar air spaces between deformed mesophyll palisade cells and the adaxi al epidermis. The chloroplast in the palisade cells and the plasmalemm a around some of the vascular cells showed minor ultrastructural damag e. Cellular autolysis, similar to other homopteran-induced phytotoxemi as, was observed in tissue where the whitefly nymphs had fed and this damage was not due to direct stylet penetration. Ultrastructural exami nation of squash silverleaf-affected plants did not reveal any viral p articles or inclusion bodies. Both whiteflies caused reductions in chl orophyll content, but the loss was greater with B. argentifolii. Two n ew intercellular fluid proteins were induced in silvered leaf tissue a nd the constitutive expression of another protein was suppressed. Enzy me assays performed on the intercellular fluid proteins indicated it e xhibited reduced activities of chitinase and peroxidase. Similarities between this insect-induced host response and pathogen-induced respons es are discussed.