DIRECTIONAL GUIDANCE OF NICOTIANA-ALATA POLLEN TUBES IN-VITRO AND ON THE STIGMA

Pollen tubes navigate the route from stigma to ovule with great accura cy, but the cues that guide them along this route are not known. We re produced 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 o n stigmas: pollen grains hydrated and germinated, and pollen tubes gre w toward the aqueous medium. The rate-limiting step in pollen germinat ion 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 o f pollen tubes and resulted in growth toward the interface. We propose that a gradient of water in exudate is a guidance cue for pollen tube s on the stigma and that the composition of the exudate must be such t hat it is permeable enough for pollen hydration to occur but not so pe rmeable that the supply of water becomes nondirectional. Pollen tube p enetration of the stigma may be the most frequently occurring hydrotro pic response of higher plants.