Factors affecting intrafruit pattern of ovule abortion and seed production in Hormathophylla spinosa (Cruciferae)

Abstract.

We investigated the pattern of intrafruit seed production in Hormathophylla spinosa (Cruciferae) over a 7-yr period. H. spinosa fruit have two chambers, each chamber containing two ovules, and usually situate perpendicular to the infructescence axis with one chamber above the other. The percentage of ovules ripening to seeds is usually lower than 50%. In addition, we found a consistent position-dependence in seed ripening probability, since more than 90% of the ripe seeds are arranged in the lower chamber of the fruit. This pattern of seed production is not produced by the direct or indirect effect of seed predators, by pollen limitation or by nonrandom fertilizations. By contrast, fruit removal experiments showed that sibling rivalry can partially explain the seed production pattern, be the cause of the reduction in the seed number from the uppermost threshold of 50%. Moreover, experimental manipulations of fruit orientation suggest that the position-dependence in seed production is the cause but not the effect of seed ripening. We presume that some architectural effect is producing a significant increase in the ripening probability of the ovules arranged in the lower chamber, causing thus the observed pattern in the intrafruit seed production in H. spinosa and severely constraining the maximum number of seeds per fruit to two.