Recent investigations on the pattern of pollen portioning in Ruta graveolens(Rutaceae)

Pollen portioning in Ruta graveolensis realized by successive dehiscence of the anthers, accompanied by successive movements of the (ten or eight) stamens. Pentamerous flowers show more differences in their pattern than tetramerous ones. In tetramerous flowers a zigzag-pattern can be found first in the antesepalous stamen whorl and later in the antepetalous one (Fig. 2a–b). The zigzag-pattern is predetermined ontogenetically, i.e., primordial initiation has the same pattern.Humidity and temperature have no effect on the movement pattern in the androecium. The movement process, however, shows a different period of time dependent on the temperature: Within the range from 8 °C to 40 °C, the movement is faster at higher temperatures. Under moderate temperature conditions (24 °C) the decrease of the germination rates of pollen is relatively low after several hours (after 20h50min the pollen germination rate of an individual anther amounts still 67 %). The influence of water drops on dehisced anthers of Ruta graveolenscauses a loss in the rate of pollen germination from 78 % to 69 % after 30min, to 34 % after 1h 5min, to 6 % after 2h 35min, and to 3 % after 5h 5min (25 °C). After short rain showers the reduction in the pollen germination rate of a whole flower may be moderate because of its portioned pollen release. At high temperature, filament movement is accelerated, and the duration of pollen presentation shortened. This might reflect an optimized pollen dispersal strategy favouring outcrossing (and thus inter-individual recombination rate) in view of increased pollinator visits but lowered pollen germination ability at high temperatures.