Fruit and seed polymorphism and its relation to seedling growth in the genus Cakile

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Abstract
To ascertain the adaptive significance of dimorphism under unpredictable environments, the dimorphic fruit segments of Cakile edentula var. lacustris, Cakile edentula var. edentula, and Cakile maritima were tested for differences in size of propagules, dispersal ability, germination behaviour, and growth rate. The upper and lower fruit segments differed in some attributes but not in others. For example, the fruits, seeds, and shells of upper segments were significantly heavier than those of lower segments. However, the frequency distributions of mean mass per seed, shell, and fruit indicated a considerable zone of overlap beneath the histograms of upper and lower fruit segments. Under still conditions in a greenhouse, no differences were found in the floating ability of upper and lower fruit segments of C. edentula var. lacustris and C. maritima. Of the three taxa, C. maritima fruits had the highest shell to seed mass ratio and floated for the longest period of time. Morphological seed dimorphism was linked with a physiological seed dimorphism. The lower fruit segments of C. edentula var. lacustris germinated better than the upper fruit segments over a wider range of temperatures. Light did not alter the final germination percentage of upper or lower fruit segments but inhibited the rate of germination. The relative growth rate decreased in all three taxa with an increase in the age of a seedling, irrespective of its seed weight.
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