Why some crosses fail

It is sometimes assumed that two plants won’t cross because of genetic incompatibility. That’s simple enough, so we just move on.

But there are cases where the failure is due to something that can be remedied. Maybe it’s pH, temperature or the absence of a key nutrient/element.

I happened across a paper that surprised me more than a little. Gudin and Arene (1991), working at Sélection Meilland, Antibes, France, observed the growth of pollen tubes from a rose designated ‘P 30’. They cultured the pollen tubes artificially, and used it on six cultivars. Three of the cultivars had stigmatic exudate (stigma goo) with a pH of 5, while the others had a pH of 9. I had no idea that the pH range in roses varied so widely. They found that ‘P 30’ pollen grew fastest and fertilized most successfully at pH 5. It grew slowly at pH 3 and 9.

I will venture to guess that a species with pH 9 stigma goo will also produce pollen that will grow in it.

The above two authors were joined by Bulard, in the same year, in a study of changes in pollen quality over the seasons. Success of fertilizations varied with the seasons. Temperature was likely involved in some of this variation.

Pollen quality can vary from specimen to specimen and from branch to branch. It may vary among flowers in the same cluster, and even on different anthers in the same flower. Yeamans (2014), studied pollen quality in Mimulus gutattus, and found that the raw protein content could vary from 15% to 45%. The higher protein content was positively associated with pollen viability.

And speaking of temperature, Končalová (1975) in Czechoslovakia, studied a specimen of Rosa hugonis that had been collected as a seedling in 1918, but had never produced hips until 1973. In that year the weather was unusually warm at the time the pollen was developing, and when fertilization occurred.

She also compared the growth of Hugonis pollen in various concentrations of sugar, and at different temperatures.

Končalová, Jičínská and Sýkorov (1976) expanded the earlier study to include six other Rosa species, and to observed the effect of calcium as well as sugar on the growth of their pollen tubes

“The stimulating effect of calcium was generally most pronounced in the pollen from roses of hybrid nature, such as R. jundzillii, R. canina, and especially in the case of the calciphilous species R. eglanteria.

This fact was established in vitro, but there might be some way to add a bit of calcium with the pollen to aid some crosses.

In other experiments, boron was the limiting factor in pollen tube growth.
Fang: Boron and apple pollen (2016)
Cheng: Boron and wheat pollen (1993)
Wang: Boron and spruce pollen (2003)
de Wet: Temp, Boron and Mango pollen (1989)
Peñaloza: Boron and pepper pollen (2018)
Muengkaew: Calcium-Boron and Mango pollen (2017)

Finally, the odd occurrence of “hybrid vigor” in pollen. This seems contradictory at first glance, because pollen tubes should not be able to express “heterosis” or “heterozygosity”. Apparently the hybrid plants pack more nutrients into their pollen. This seems to confirm the old idea that crossing with species is easier with plants that are “a little bit hybridized”. That is, crosses between closely allied species, or between different geographical selections of the same species. The “hybrid” pollen gains vitality, and in many cases is more tolerant of environmental conditions than that of the parents.
Jóhannsson and Stephenson: Pollen Vigor (1998)
Johnson & Mulcahy: Pollen vigor in inbred plants (1978)


In some hybrids, there is a problem with endosperm development. I don’t know that this occurs in roses, but I did find a paper with pics of Rosa roxburghii embryo and endosperm development.

Hi Karl,
This quite an article. It is going to take me some time to digest it.
Off hand, I can think of one rose that fits this bill. Therese Bugnet does not get very large in my garden.
The blooms are sparse, and it seldom sets hips with applied pollen or set OP seeds.
From what I have read on the Forum and other Social Media. Warren Millington is having very good success
with TB half a million years away in OZ.

Thanks Karl! I think that when I find time to peruse all the articles, this will address a number of questions I was trying to ask (but probably did poorly) in a prior thread.

I can’t say a lot about why crosses fail other than plant incompatibility but about Therese Bugnet there is something odd going on in my experience. AT my house it competes with roots a a neighbor’s maple tree and gets enough supplementary water to keep it growing. But it neve gets above 5 ft tall, and never, or almost never sets an OP hip in some 40 year of growing there.

Root suckers planted on an earth berm in full sun did well, tolerating repeated summer droughts and competition from honeysuckle, with one large flush of bloom and some after, depending on rains or occasional irrigation. Those same plants got relocated to an area of the same soil, a superior dark clay, and at the edge of a parking lot where they got the run-off from 100 ft uphill, the bushes topped out at 10 ft and bloomed all summer. No OP hips noticed.

Several years ago I tried pollinating the plant in my front yard with a triploid R pomifera I got from David Zlesak. Seeds set. and almost all grew into what look like ordinary rugosas. None have so far shown reliable repeat bloom, but neither do other rugosas mostly under my growing conditions. This year I tried pollinating again, counting out 25 pollinated flowers to tag. Eight produced hips from which I recently harvested over 100 seeds. Not a single OP hip despite many dozens of flowers. I have to conclude that TB has really strong self-incompatibility, and maybe will only cross with diploid equivalents for some reason.

The same R pom does well in crosses with several common tetraploids, seemingly in both directions. This is the mirror of “why do some crosses fail”.

I know others are probably less fond of the self-incompatibility thing but I like it a lot, it’s reassuring with liliums for example to know the cross you made is extremely likely to be what you get.

I got a TB sucker last season, it was tiny but is now a decent size so was already planning on throwing Calocarpa on it (and vice versa). Given both have (or theoretically could have) bred juvenile bloomers, could result in some interesting potential breeders to bridge between a bunch of classes.

The whole diploid pimp section falls into the consistently fails group for me. None of what I have wants to produce hips but they all do based on pictures on HMF. Being in Australia where lack of heat isn’t much of an issue it’s interesting.

Heslop-Harrison (1921):
I therefore got up earlier, at 4 a.m. (GMT), before any insects were at work, when I found that even then every newly expanded R. pimpinellifolia had its stigmas powdered with pollen from its own overarching stamens.

This suggests that the species is self-compatible, but he does not say so explicitly.

W. Paul, The Rose Garden: In Two Divisions. pt. 1, p. 84 (1848)
According to the statements of M. Boitard, there is scarcely any limit to the variation of Roses produced from seed. He affirms that M. Noisette, a French cultivator, has never sown seeds of the Chinese Roses (R. INDICA) without raising some Scotch Roses (R. SPINOSISSIMA) from them.

The point of these statements is that the Scotch roses are early risers, apparently receptive in the wee hours but with enough pollen left over for the somewhat-later China roses.

The only definite info I have on a diploid Pimp. is that R. hugonis needs heat as the pollen develops, and also when the pollen tube is making its way to the ovary. I don’t know that this will be true for all the diploid species in the section, but it’s something to consider.


Hi Karl,
We know and understand the effect of temperature,and soil on the growth and reproductive life of plants.
What role does altitude and latitude play in their flourishing?

Let’s not forget photoperiod, and the interaction between temperature and photoperiod. Then there is the alternation of daytime high and night tme low temps. In lilies, for example, height of the flowering stem is directly correlated with the day - night temp. difference. Other plants are also affected, but the lily bit came to mind first.

Latitude throws in some interesting complications because the connections between photoperiod and season change. Furthermore, the day-to-day change in photoperiod is more rapid, and the season-to-season change is more dramatic, the further from the equator that we travel.

Altitude is a further complication. High in the mountains, the temperature will likely be cooler than what we would expect from the latitude. Then there is the matter of air pressure.

Turesson (1922) reported on some dramatic changes that occurred when he transplanted plants from one habitat to another.
Lewontin (2000) discussed such matters, and included a drawing of seven Yarrow clones and how they varied at different elevations.
Hiesey (1940) quoted some older culture work. In some cases the low altitude and hight altitude species each became the other.

And I can forget Cole (1917). She thought that the low pollen fertility of some Rosa species she examined at the Arnold Arboretum must be due to hybridity.

But I noticed that some of the species with the worst pollen were alpine species that were being grown much nearer sea level.
As Darwin wrote, (‘Double flowers—their origin’, Gardeners’ Chronicle, no. 36, 9 September 1843, p. 628.):
“It is well known that plants (and indeed animals, as I could show by a series of facts) when placed out of their natural conditions, become, often from apparently slight and unintelligible causes, sterile. How many American plants fail in producing pollen in this country! the anthers of the Persian and Chinese Lilacs, as I observed this summer, are as destitute of good pollen as if they had been hybrids. Other plants produce good pollen, but are defective, as it appears, in their ovules, as their germen never swells. Linnaeus has remarked that most Alpine plants, when cultivated in the lowlands, are rendered quite sterile. In most of these cases, we see that sterility is compatible with long life and health.”


Collins (1905) The [Avocado] tree flourishes in many localities where it fails to bear fruit, and, as with the mango, this sterility is usually found in localities of almost continuous humidity. Under such conditions an artificial check, such as root pruning, has been found to induce flowering and the setting of fruit. This can easily be overdone, however, in which case the trees will bear one large crop and then die.

I think the use of binding, ringing, root pruning and such have not been sufficiently explored in rose breeding. Sometimes plants are so busy growing that they forget to fruit, even though they flower.

I finally remembered the items I wanted to mention, but forgot. They deal with roses.

Bull. Torr. Bot. Club 14(12): 253-256 (Dec 1887)
Remarks on the Group Carolinae of the Genus Rosa
G. N. Best, M.D.
There is no plant perhaps that reflects more its environments than the Rose. Growing at the foot of a hill, in damp rich soil, with stout stems, thick shining leaves, broad stipules and stout curved or reflexed spines, they present a very different appearance from those found at the hill top, where the soil is light and dry, exposed to the sun. Here the stems are slender, branches more diffuse, leaves thinner, stipules narrow and spines straight, or nearly so, often absent in stunted bushes. Seeing the extremes in an herbarium, knowing nothing of the situation in which they grew, one would be strongly inclined to think he had two well-marked species with which to deal. On the other hand, however, could he but see the intermediate forms, their perfect intergradation, he could come to no other conclusion than that he had before him but one.

This one caught my attention because I had just recently read the following.

Rosa mirifica, stellata, alcea (1910)
Edward L. Greene
The excellent specimens of R. stellata distributed by Mr. Wooton are from two separate and rather well isolated mountain ranges in southern New Mexico, and he has noted clearly enough some of their divergences in his paper on them (Bull. Torr. Club, XXV, 152). Nevertheless, I seem to see that the discoverer of the New Mexican shrubs, in his diagnosis, has been betrayed by those curious trichomes of this type into the making of a synthesis which, on the whole, can hardly meet with general approval among students of roses. In other words, the Rosa stellata of the Organ Mountains and that of the Sierra Blanca are so very different in characters of stem, spines, leaves and indument that, on principles well established among rhodographers, they must be held specifically distinct.

I will not dispute with Greene about the identity or not of Wooton’s two Rosa stellata identifications. But I would be interested to see how each performed (and maybe changed) when raised from seed in the home of the other.

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