GENE EXCHANGE IN LOBLOLLY PINE: THE RELATION BETWEEN POLLINATION MECHANISM,FEMALE RECEPTIVITY AND POLLEN AVAILABILITY

The pollination process in loblolly pine has been examined over several years, both in the field (seed orchards) and experimentally on greenhouse-grown material. Female strobili are receptive to pollination for periods of a wk or more. Initially, background pollen from outside the seed orchard is the main source of pollen but as peak receptivity approaches, pollen from the stand itself predominates especially in older orchards. Consequently, strobili can receive pollen both from outside the orchard as well as from within. The pollen lands on the micropylar horns where it is transferred through the micropyle onto the nucellus by either rainfall or the pollen drop, whichever comes first. Since the pollen drop does not occur until the latter part of the receptive period, rainfall is the most likely transfer agent and pollen flotation is vital if rain occurs. Early arriving pollen does not appear to have an advantage over later arriving pollen for uptake onto the nucellus, even if rain follows the first pollination immediately. Therefore, total pollination of the strobilus can result from both distant and nearby pollen sources.