Most of the potential seed crop in oaks is lost when pistillate flowers abort between pollination and fertilization. Several causes of flower loss have been proposed: weather, late spring frosts, poor pollination, insects, "physiolological drop", and competition for photosynthates. In the present study the relationship between pollen tube (PT) growth, fertilization, and flower retention was examined. Pistillate flowers of northern red oak (Quercus rubra L.), black oak (Q. velutina Lam.), and white oak (Q. alba L.) were collected weekly from mid-April to midSeptember and processed for light and fluorescence microscopy.
Northern red oak (NRO) and black oak (BO), members of the Erythrobalanus subgenus, are pollinated in central Missouri during late April and early May, respectively, and require two growing seasons for their flowers to develop into acorns. At anthesis, the pistillate flowers of both species have small locules with rudimentary ovules. By the end of the first summer, the ovules were still rudimentary, although the locules had enlarged slightly. In both species, soon after the pollen grain adheres to the stigmatic surface, the PT elongates from the aperture and penetrates the epidermis of the stigma. A callose plug is quickly synthesized in the PT, isolating the discharged contents from the pollen grain that soon falls off the stigma. Many callose plugs, seen within PTs in the solid transmitting tissue of the style, may be synthesized to maintain turgor in the tip of the PT or they may be an incompatibility reaction. During the first growing season, the pollen tubes stop elongating by mid May before they reach the junction of the three styles at about the level of the visible portion of the perianth. In the spring of the second growing season, PTs resume elongation after the ovules differentiate into a nucellus and integuments. The PTs of NRO entered the compitum and locules about May 21 and those of BO on June 4. In both species, the megaspore mother cell was undergoing meiosis before the PTs approached the ovules in the locules. Fertilization was estimated to occur during the weeks of June 11 for NRO and June 25 for BO. The pistillate flowers of white oak, a member of the Lepidobalanus subgenus, mature into acorns in one growing season. The course of the PTs is similar to that of the other two species, except that after the PTs cease growth in mid May, they resume elongation in early June and fertilization occurs during the week of June 11.
Key words: pollen tube, callose, fertilization.
Correspondence: Robert A. Cecich, USDA-Forest Service, 1-26 Agriculture Building, Columbia, Missouri 65211, USA