POLLEN FLOW IN LYTHRUM JUNCEUM, A TRISTYLOUS SPECIES

Abstract

SUMMARYStudies of the pollen loads present on stigmas of flowers of the self‐incompatible tristylous Lythrum junceum in Morocco indicate that the position of the stigma has an important effect on the proportion of pollen from long‐level anthers that is deposited on the stigmas. The greatest amount of large pollen from the longest set of stamens is found on stigmas of long‐styled flowers. Because of the inability to discriminate between small pollen grains from mid‐level anthers and those from short‐level anthers it is not clear how stigma position may influence the distribution of these small pollen grains. In the Moroccan population studied, plants with long‐, mid‐, and short‐styled flowers are present in a 1:1:1 ratio. The chief pollinator during the period of study was the honey bee, Apis mellifera, which preferentially removes large pollen grains from flowers. Longs and Mids produce approximately equal numbers of pollen grains per flower, and Shorts produce c. 77% of the pollen of each of these two forms. Acetolysis of stigmas provides more accurate estimates of stigmatic pollen loads than does microscopic examination of intact stigmas, although the proportional representation of the two size classes of pollen is similar with either method. This study represents the first published analysis of pollen flow in a tristylous species within its native range and suggests that tristyly serves to promote compatible pollinations, thus supporting the classical interpretation of the biological significance of tristyly.