Pollen-Ovule Ratios: A Conservative Indicator of Breeding Systems in Flowering Plants

Pollen-ovule ratios (P/O's) of flowering plants, including grasses, reflect their breeding system and these can be divided into five classes: xenogamy, facultative xenogamy, facultative autogamy, obligate autogamy and cleistogamy. The evolutionary shift from class to class is accompanied by a significant decrease in the mean P/O. This pattern is found in anemophilous plants and their relatives as well as zoophilous plants and their relatives. Plants in Asclepiadaceae and Mimosaceae are exceptions in the xenogamous group. These plants invest minimal energy in pollen production and their fecundity is low, but when pollination is successful the reproductive return is relatively high. A comparison of P/O's with successional stage shows that P/O's increase significantly from disturbed habitats to late successional seres. In addition to suggesting that autogamy is adaptive in disturbed habitats and xenogamy in advanced successional stages, the data show that in intermediate habitats P/O's, hence breeding systems, also are intermediate, and display a balance between autogamy and xenogamy. P/O's may be minimal and if P/O's fall below a certain minimum, fecundity decreases. This is probably a consequence of insufficient pollen grains reaching the stigmas. Several examples are discussed showing that P/O's are a better indicator of a plant's breeding system than floral size and morphology.