Anatomical and Histochemical Factors Associated with Bronzing Development in Strawberry Fruit

Bronzing of strawberry (Fragaria ×ananassa Duchesne) fruit that is not the result of arthropod feeding or chemical spray application occurs frequently in California’s central coast strawberry production region from late spring through midsummer, a period characterized by relatively high temperature, low relative humidity, and high solar irradiance. The cause of this phenomenon is not known, but in preliminary trials, intermittent, midday misting of plants and increased drip irrigation rate resulted in reduced incidence of fruit bronzing. To characterize the bronzing phenomenon and its development in strawberry fruit tissues, we conducted an anatomical and histochemical examination of bronzed fruit. Bronzed and nonbronzed fruit were sampled over a range of fruit maturities. Results show that bronzing derives from a lesion at the cortical surface early in the fruit’s development. Epidermal cells become radially compressed and the cell contents coalesce into a densely staining mass. The cuticular layer becomes disrupted and discontinuous. As the fruit develops, densely staining materials, possibly phenolic precipitates, accumulate within subepidermal cells of bronzed fruit, subepidermal cell walls thicken, and intercellular spaces fill with pectic substances and other densely staining materials. Results are consistent with reports of sunscald injury from other fruit species, and raise the possibility that strawberry bronzing occurs in response to heat or solar radiation injury. to warm the soil and enhance plant growth, also reduced Type III bronzing (Larson and Sjulin, 2001). These observations suggest that environmental and plant physiological factors may be important in development of Type III fruit bronzing. Little is known about how Type III bronzing progresses in the strawberry fruit. In an attempt to better understand the phenomenon, we initiated the present research to characterize strawberry bronzing and its development. Materials and Methods ‘Commander’ strawberry fruit exhibiting Type III bronzing symptoms and control samples that appeared to be developing normally were collected in the Watsonville, Calif., area on 2 May 2000. Collections included young fruit (<7 d postanthesis) that were sampled as early as incipient bronzing damage could be identified, midmature fruit (2 to 3 weeks postanthesis) with incipient bronzing symptoms evident, and full-size, unripe fruit with clear bronzing symptoms. Receptacle tissue was excised from the fruit and placed in a fixative solution of 3% glutaraldehyde and 4% sucrose in 0.025 M phosphate buffer at pH 6.8 and stored at 4 °C. The material was dehydrated in a tert-butyl alcohol series, embedded in Paraplast Plus (Sherwood Medical, St. Louis, Mo.), sectioned at a nominal thickness of 8 μm, and mounted in water on chrome alum-coated slides. The sections were stained with toluidine blue O (Sekai, 1973) for general anatomical features. Three fluorochromes were used to determine cytoand histochemical characteristics of cells and tissues that showed bronzing damage: coriphosphine (Ueda and Yoshioka, 1976; Weis et al., 1988) for cell-wall pectic materials, calcofluor white (Hughes and McCully, 1975) for cellulose, and nile red (Greenspan et al., 1985) for lipoidal materials. Photomicrographs were taken using a digital camera (model 750; Optronics Inc., Goleta, Calif.) on a microscope (model E600; Nikon USA, Stamford, Conn.).