Pollination ecology and seed-set in the rare annual species Melampyrum arvense L. (Scrophulariaceae)

Pollination of Melampyrum arvense L., a large-flowered hemiparasitic annual, was investigated in populations in The Netherlands, Germany, France and Switzerland. Pollination is achieved by bumblebees, with varying tongue lengths, collecting pollen and/or nectar. Long- and medium-tongued workers pollinate the flower nototribically (headthorax pollination) and pollen-collecting short-tongued bumblebees sternotribically (venter pollination), as analysed by the actual transfer of fluorescent powder. Short-tongued bumblebees collect nectar by robbing the flowers, and sometimes also destroying the pistil. Nectar was present in a volume of 0-26 pi, with a sugar concentration of 30% and a ratio of fructose: glucose: sucrose = l:0-67:0-73. M. arvense is self-fertile, but caged flowers do not produce seeds by the absence of auto-deposition. Artificial pollination showed that increasing pollination intensity increased fruiting, although after three pollinations fruiting did not increase and the number of seeds per fruit did. Observations in plots with variable plant density showed that during a low visitation intensity, flowers were visited at least 6-7 times during the flower’s life span (7 days), and at a high visitation intensity 39-3 times. The additional effect on the seed-set of one-hand pollination during the flower’s lifetime was low. Plants in low-density plots are highly branched in contrast to plants in high density. Branching of plants, resulting in more flowers per plant, and the fact that flowers on branches produce more seeds per flower, influence the total seed production per plant. The M. arvense population in The Netherlands had a rather high fruiting percentage and seed production per plant compared to three French populations.