STATUS OF SELF-POLLEN IN BEE POLLINATION EFFICIENCY OF WHITE CLOVER (TRIFOLIUM-REPENS L.)

Flowers of white clover (Trifolium repens L.) are hermaphrodite and se lf-incompatible; their cross-pollination depends entirely on insect vi sitors, mainly bees (Apoidea). Because self-pollination of white clove r occurs before flower anthesis, we determined whether selfing affecte d the pollination efficiency of a honeybee visit. We compared pollen d eposition in emasculated and intact flowers following (1) a single hon eybee visit, (2) open-pollination for a day and (3) enclosure in a clo th bag to prevent insect visits. In emasculated flowers, open-pollinat ion resulted in more pollen deposited than after one visit (+ 30%) whi ch is consistent with flowers being visited more than once by pollinat ors during the course of a day. On intact flowers, saturation of the s tigma was achieved after the first visit of a honeybee (near 280 grain s) because of self-pollination. Additional visits did not increase pol len deposits, but they improved pollen efficiency in terms of numbers of pollen tubes reaching the ovules. In such a context of easily satur ated stigmas, self-pollen does not inhibit cross-pollen activity, but represents a constraint for pollination which demands multiple bee vis its to each flower to achieve maximum fertilization.