Schneeschirm (Floribunda) [Johanna Tantau x (Karen Poulsen x Stämmler)] (SJH)
September 5, 2009
According to Wulff:
"The ancestry of Schneeschirm, an ornamental rose, is not quite clear. It blooms twice a year. The first flowering period lasts from June to August. The second begins after a short interval, and ends with the first frosts.
It is a very remarkable fact that the flowers of both periods differ in their meiotic behavior; only those produced during the first period are able to produce hips and viable achenes. Their reduction division is characterized by the occurrence of only 0-3 trivalents, with pairing to 7 bivalents and 7 univalents or 1 trivalent, 6 bivalents and 6 univalents. This reduction division proceeds rather regularly. The univalents, splitting twice, are mostly taken up into the young tetrad nuclei. Chromosome elimination is low, and so viable pollen grains and egg cells will contain 14 chromosomes.
The offspring of about 60 plants were tetraploid, each seedling having 28 chromosomes in the root tips.
We also studied the reduction division of these tetraploid seedlings. In most pollen mother cells (63%), 14 bivalents were formed. The remaining pollen mother cells showed 10-13 bivalents and the corresponding numbers of uni-, tri- or quadri-valents.
Growth habit and leaf shape are highly similar in the tetraploid seedling plants and their triploid mother. Differences chiefly exist in some floral characters. The diameter of the tetraploid flower is smaller (4-9 cm) than in the triploid (8-12 cm), but the former have a tendency to produce double flowers. The number of anthers and ovaries is lower in the tetraploids, and their pollen grains are larger and show a higher degree of fertility. The production of hips and achenes is far better in the tetraploids than in the triploid mother plant.
Reduction division in the second period Schneeschirm flowers shows a remarkable increase of trivalent formation. Here only 5% of the pollen mother cells have 0-1 trivalents. 3-5 trivalents are found in 76% of the pollen mother cells. The general course of meiosis is very irregular, chromosome elimination is high and there is a clear tendency to form pollen grains with 7-9 chromosomes (instead of 14 as in the flowers of the first period).
What we are learning from the differences in the meiotic behavior of the flowers of both periods is that the reduction division of the triploid Schneeschirm can be rather easily affected by environmental conditions. It seems reasonable to suppose that the warm temperature from June to the end of August causes the formation of no or few trivalents. This prevents chromosome elimination, and favors the production of hips and achenes. On the other hand, the colder temperatures of September and later months seem to be responsible for the high meiotic irregularities and the failure of fruit-setting."