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Pollination mechanisms and pollen-ovule ratios in some Genisteae (Fabaceae) from Southwestern Europe

Abstract:

We studied the biology and floral rewards of 34 taxa of Genisteae from the SW of Europe. Most of the floral attributes show a statistically significant direct relationship. Anther biomass of the lower whorl (1W) is significantly lower than that of the upper whorl (uW), and the ratio of the two (Rv) differs among the taxa. All taxa are polliniferous, and Retama sphaerocarpa also secretes nectar. They can be considered xenogamous or facultative xenogams on the basis of their high pollen/ovule (P/O) ratios. Three principal mechanisms of pollen releasing are identified in this tribe: valvular, pump and explosive; the latter comprises specialized and nonspecialized subtypes. Pollination is sternotribic except in the specialized explosive mechanism, in which it is noto-sternotribic. Thus some pollen serves as food (pollen from the uW, which adheres to the ventral surface of the insect) and part of the pollen fulfils a polliniferous function (pollen from the uW, which adheres to the dorsal surface). Species that use a pump mechanism have very low Rv values (Rv=0.08-0.26); species with valvular or nonspecialized explosive mechanisms have Rv values between 0.24 and 0.58; those with a specialized mechanism of pollen presentation have high Rv values (0.6-0.76). In contrast to expectations, the highest P/O ratios appear in the specialized explosive system, which allows a single visit from the insect.