Successful pollination and fertilization are absolute requirements for sexual reproduction in higher plants. Pollen hydration, germination and penetration of the stigma by pollen tubes are influenced by the exudate on wet stigmas and by the pollen coat in species with dry stigmas. The exudate allows pollen tubes to grow directly into the stigma, whereas the pollen coat establishes the contact with the stigma. Pollen tubes then grow into the papillae, which are covered by a cuticle. The components of the exudate or pollen coat that are responsible for pollen tube penetration are not known. To discover the role of the exudate, we tested selected compounds for their ability to act as functional substitutes for exudate in the initial stages of pollen-tube growth on transgenic stigmaless tobacco plants that did not produce exudate. Here we show that lipids are the essential factor needed for pollen tubes to penetrate the stigma, and that, in the presence of these lipids, pollen tubes will also penetrate leaves. We propose that lipids direct pollen-tube growth by controlling the flow of water to pollen in species with dry and wet stigmas.

Morphological and physiological variation among annual bluegrass populations from the green, fairway, and rough were measured to determine the role annual bluegrass infestations outside the green play in maintaining the population in the green. Annual bluegrass populations from the fairway and the rough were similar to each other but differed from that of the green in habit, dry mass production, flowering, seed size, and germination. Seeds from each population established best in the type of turf from which they originated. It is concluded that, since few genotypes were common to the green and its surroundings, the populations in the fairway and rough played little or no role in maintaining the population of the green. The same is likely to be true wherever the management of greens and their surroundings differs sufficiently for marked population differentiation to occur. This finding does not preclude the possibility that some genotypes adapted to the green persist in the surroundings, and that these genotypes serve as sources of seeds for the colonization or reinfestation of annual bluegrass-free greens.

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Pollen tubes navigate the route from stigma to ovule with great accuracy, but the cues that guide them along this route are not known. We reproduced the environment on the stigma of Nicotiana alata by immersing pollen in stigma exudate or oil close to an interface with an aqueous medium. The growth of pollen in this culture system mimicked growth on stigmas: pollen grains hydrated and germinated, and pollen tubes grew toward the aqueous medium. The rate-limiting step in pollen germination was the movement of water through the surrounding exudate or oil. By elimination of other potential guidance cues, we conclude that the directional supply of water probably determined the axis of polarity of pollen tubes and resulted in growth toward the interface. We propose that a gradient of water in exudate is a guidance cue for pollen tubes on the stigma and that the composition of the exudate must be such that it is permeable enough for pollen hydration to occur but not so permeable that the supply of water becomes nondirectional. Pollen tube penetration of the stigma may be the most frequently occurring hydrotropic response of higher plants.Fertilization of the ovules of flowering plants occurs when desiccated pollen grains on the receptive surface of the female (the stigma) hydrate, germinate, and produce a tube that elongates directionally to penetrate the stigma (Knox, 1984;Dickinson and Elleman, 1994; Nasrallah et al., 1994). These events occur in the lipid-rich environment formed by the pollen coat or the stigma exudate, which is essential for successful fertilization (Pandey, 1963; Konar and Linskens, 1966b; Preuss et al., 1993;Goldman et al., 1994; Wolters-Arts et al., 1998). Cells of the "wet" stigmas of solanaceous plants release lipid droplets into the intercellular spaces of the stigma and directly onto its surface. The droplets accumulate and coalesce to form a transition layer of an oil-in-water emulsion between an aqueous phase within the stigma and a lipid phase on the surface (Konar and Linskens, 1966a;Herrero and Dickinson, 1979;Cresti et al., 1986; Kandasamy and Kristen, 1987). Pollen on solanaceous stigmas does not adhere to papillae but remains free within the exudate, where it hydrates despite the negligible amount of water present within the exudate itself (Konar and Linskens, 1966b) and the barrier to water movement that lipids are usually assumed to impose.Following germination the directional growth of pollen tubes (which elongate by tip growth) into the stigma suggests that some external cue establishes their polarity. Light, tactile, electrical, and chemical cues have been suggested as polarizing agents in pollen tubes and other tipgrowing cells (Heslop-Harrison and Heslop-Harrison, 1986; Reger et al., 1992;Cheung et al., 1995; Hü lskamp et al., 1995; Malho and Trewavas, 1996; Kropf, 1997); however, although potential external guidance cues have been identified for pollen tubes, their roles in guidance within the pistil remain unclear (Hepler, 1997; Sommer-Knudsen et al., 1998).By applying...

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