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Puzzles about pistils, pollen and productivity
Olive (Olea europaea) is among those species that produce both hermaphrodite and single-sex flowers on the same plant. In O. europaea, the single-sex flowers are male and thus the species is described as andromonoecious. Olive trees are wind-pollinated and to some extent self-fertile, but previous research has suggested that more efficient fertilization is achieved by cross-pollination. What, then, are the roles of the two different flower types in pollen production? This question is at the heart of the research carried out by Cuevas and Polito (Almeria, Spain and Davis, California, pp. 547-553). Staminate (male-only) flowers arise by abortion of pistil development in flowers that start as hermaphrodites. There is earlier research indicating that staminate flowers are much more frequent when nitrogen nutrition is limiting; suggesting that this pattern of floral development may be a means of diverting resources (especially under stress) to pollen formation. This is especially important in wind-pollinated plants. However, the authors’ results indicate a more complex situation. Hermaphrodite and staminate flowers were compared in terms of time taken to anthesis, pollen production, pollen germinability and pollen potency. In none of these aspects were there any differences between the two types of flowers. Indeed, the only obvious difference in terms of pollen production was that the staminate flowers tended to open later than hermaphrodite flowers. This was based on the position within the flower panicle: apical flowers and flowers in primary pedicels were mostly hermaphrodite, whereas flowers in secondary pedicels were mostly staminate. But overall, at the level of the individual flower, the abortion of the pistils did not lead to a diversion of resources into pollen production. However, at the whole-plant level the occurrence of staminate flowers increased the ratio of male to female gametes at a later stage of floral development. This, the authors suggest, may be a way of enhancing male fitness.
j.a.bryant{at}exeter.ac.uk
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