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Pulling pollen through a pore
The stylar pollination system of angiosperms is very familiar. Less widely known is the system in gymnosperms in which the pollen enters the micropyle directly in order to reach the ovule. In many gymnosperms this process is aided by secretion from the micropyle of a liquid drop, the pollination drop, as discussed by Mugnaini et al. (Siena and Rome, Italy, pp. 1475–1481) in relation to Juniperus communis. In this species, the pollination drop, approx. 0.2 mm3 in volume, lasts about 12 days in the absence of pollination. If the drop is removed artificially, the plant can secrete another one for up to 4 consecutive days. Analysis of the drop shows that it is very sugary: 45.5 mg mL-1 fructose, 8.30 mg mL-1 glucose and a trace of mannitol. However, the most remarkable features of the pollination drop are the responses to materials landing on it. It is already known that if compatible pollen lands on the drop it is withdrawn back into the ovule, facilitating the entry of the pollen. The authors have now extended our knowledge of this process by depositing a range of materials on the pollination drop. Viable J. communis pollen nearly always led to complete withdrawal of the drop; non-viable pollen or pollen from other species resulted mainly in partial withdrawal of the drop. This implies that the ovule is able to detect and respond to appropriate pollen, presumably via a biochemical signalling pathway. However, there is also a size component in the response: silica gel particles in the same size range as pollen elicited a full response in approx. 40 % of samples whereas larger particles mostly elicited no response at all. As the authors point out, it is thus possible that pollen-sized non-pollen particles may reduce the probability of successful pollination, a factor that could be significant in dusty habitats.
Professor J. A. Bryant
University of Exeter, UK
j.a.bryant{at}exeter.ac.uk
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