Rosa palustris and Other Diploids

Do I understand correctly that Rosa palustris, and diploids in general, do not generally self-pollinate? Therefore, if using Rosa palustris one would only need to cover the blooms to prevent an insect from bringing in stray pollen from elsewhere, but there is no need to emasculate the bloom?
Tom D.

Hi Tom,

It varies. Roses have a kind of self incompatibility called gametophytic self incompatibility. This means the pollen does germinate and starts growing, but if it is an incompatible combo, it gets stuck in the style. If the temperature is extra hot or some other factors are at play they can get through to the egg. In my colder climate most diploids are relatively strongly self incompatible I grow, but in other areas of the country where it is warmer, that is not so much the case. There are different degrees or strengths I think to between genotypes for how strong the SI mechanism is. If you spend your time mass pollinating your R. palustris, you’ll hopefully be able to tell which are at least obvious hybrids based on how they look.

Years ago I had some polyanthas in a hot greenhouse and had lots of self seed set. I could tell as the few in there had different traits easy to follow. The thornless one (recessive trait) had all thornless seedlings from those hips, etc. Outside though here they seem to much more self incompatible not setting hips in isolation and lots of thorny seedlings from the few thornless ones I have, etc.

Please let us know what you do and the results.

The self-incompatibility system does have some “holes” that may be exploited. Henslow (1877) wrote, “Lobelia fulgens was self-sterile in Germany, but Mr. Darwin found it self-fertile. This, like the Brazilian Eschscholtzia, appeared to become self-fertile in consequence of the lower temperature of England; for the latter became again self-sterile in Germany.”

In addition, the various S-alleles may vary in effectiveness. A plant carrying S1 may block every pollen tube carrying that allele, but a plant carrying S5 may allow some percentage of S5 pollen tubes to slip through. And the two alleles may differ in their response to environmental conditions.

Compatibility, whether self- or cross-, may depend as much on the specimen as on the species. And environmental conditions may play a role in both.

And on a related matter, Traub (1935) found that an assortment of self-sterile Amaryllids could be persuaded to bear seeds by removing the flower-bearing scape from the bulb and standing it in water.

Jeffrey (1915) reported similarly:

Physiological sterility is frequently due to entirely different causes than genetical lack of harmony, as for example in the horseradish or the potato (Solanum). In the former it has been found possible to bring about the formation of fertile seed by simply girdling the top of the subterranean storage region of the plant, so as to prevent the undue descent of assimilates. The common white lily, > Lilium candidum, > presents a similar condition, for here the setting of seed takes place only when the leafy flowering axis is severed from its bulb and kept in water. So far as I am aware, there have been no experiments as to the result of severing the continuity of the phloem (girdling), in relation to the restoration of seed production in the potato.

This “girdling” (done carefully) might be tried on roses that seem reluctant to hold onto their hips following certain crossings or selfings. Binding is another technique that has been used on other plants.