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Hydathodes of young, folded strawberry (Fragaria × ananassa Duch.) leaves had unoccluded water pores With various sized apertures, as observed by low-temperatu re scanning electron microscopy. Hydathodes of fully expanded leaves were brownish and the water pores within the hydathodes were covered with a solid material, presumably comprised of epicuticular waxes and substances excreted through the hydathodes. The entire water pore area of the hydathode was occasionally covered with a shield-like plate. The shield-like plate over the hydathode water pores impeded water flow even with an induced positive pressure. Mechanical scraping of the hydathode area elim- inated impedance to water conduction. These observations suggest that external occlusion of water pores in the hydathodes is the resistance component associated with the absence of guttation in older strawberry leaves. Strawberry leaves can exude xylem fluid through the water pores of hydathodes (guttation). The function of hydathodes and the relationship of guttation to plant water relations and growth are not fully understood. Kramer (1969) considered guttation of only incidental importance, but Klepper and Kaufmann (1966) suggested that guttation serves for mineral nutrition when tran- spiration is suppressed, solutes being absorbed from the gutta- tion water as it moves upward in the plant. Tipburn in emerging strawberry leaves, caused by a localized deficiency of Ca, oc- curs least in plants grown under conditions, that favor guttation (Bradfield and Guttridge, 1979). In several species of plants, an exudate is deposited over the water pores and a gummy material accumulates in the intercellular spaces of the epithem as leaves mature (Fahn, 1979; Maeda and Maeda, 1987; Mort- lock, 1952). Such processes in Fragaria sp. or in other rosa- ceous plants have not been described. Strawberry plants guttate under conditions of high humidity, low transpiration, and adequate soil moisture. Under field con- ditions, guttation is an indicator of high plant water status and soil water availability (Fahn, 1979; Faiz, 1983; Glenn and Take- da, 1989). Guttation occurred when predawn leaf water poten- tial (PLWP) of strawberry was above - 0.07 MPa and absent when PLWP was below - 0.11 MPa (Glenn and Takeda, 1989). Only the young expanding leaves and not the older leaves con- sistently guttated with high PLWP conditions. However, PLWP or solute potential was not significantly different between leaves of varying ages within a plant, suggesting that the absence of guttation in older leaves was associated with an increased re- sistance to water flow through the hydathodes or a loss of vas- cular functions in the older leaves. Donnelly and Skelton (1987) described the hydathode struc- ture of micropropagated plantlets and relatively young leaves of greenhouse-grown strawberry plants. However, we found no information on the anatomy of hydathodes in the leaves of field- grown strawberry plants. Tissue preparation for light micros-

Occlusion of Water Pores Prevents Guttation in Older Strawberry Leaves