Annual 1955: 37-40. (1955)
Hulthemosas—New Hope for the Rose Breeder
G. D. Rowley
A search for "new breaks" in rose-breeding has led to increased attention being paid to the wild species, of which the great majority (perhaps as many as 95 per cent) have played little or no part yet in the evolution of garden roses. Several of the features we take for granted in modern roses owe their origin to selection of one particular wild ancestor — often the only species known to have that character. Without the China Rose (still unknown as a species in European gardens) there would be no reliable autumn display of blooms; without the Austrian Briar (Rosa foetida) no flame and bicolor effects. It has truly been said:
"There are few greater mistakes a plant breeder can make than to assume that a species can be passed over as a parent because it has little decorative value. We may have a species which in itself is quite useless as a garden plant but which possesses one very good feature that is lacking in its more ornamental relatives. Such a plant used in a cross may be the starting point for striking improvements.1”
Nor do we have to confine ourselves to seeking new characters within the genus Rosa. Breeding techniques have advanced sufficiently for crosses once thought impossible to be made experimentally in the glasshouse or laboratory. Thus the crossing of apples and pears at Bayfordbury has been achieved by the use of β-naphthoxy-acetic acid to delay fruit fall and enable slow-growing pollen tubes to reach the ovary. Further, sterility barriers so often encountered in the past can be avoided or broken down.
In Rosa, then, we need not rule out as fantastic the possibility of introducing novel features from allied genera. Indeed, the first pointer in this direction was given us over 118 years ago by one of those stokes of sheer luck that so often opens the way to the undreamed-of novelties in Garden flowers. It was the birth of Hulthemosa, an intergeneric hybrid whose history is worth recalling in some detail.
The closest ally of Rosa is Hulthemia, a curious monotypic genus whose single species, H. persica, comes from the semi-deserts of Persia and Afghanistan. It is the "Rosa persica" of more conservative botanists, but recent rhodologists such as Hurst and Boulenger maintain it as a separate genus, and genetical evidence supports their decision.
Hulthemia persica is a low, wiry, xerophytic shrub which covers large areas by means of subterranean rhizomes. It is at once distinct from Rosa by the simple, grey, glaucous leaves without stipules, and by the small Cistus-like blooms, which are a clear, bright yellow with a conspicuous crimson blotch at the base of each petal (Fig. 1).2 It is this last character that makes it so alluring a proposition to the breeder. It has been in cultivation since 1790, but always as a great rarity. The reason, given by Lindley in 1829, applies no less today than then:
"It resists cultivation in a remarkable manner, submitting permanently neither to budding, nor grafting, nor laying, nor striking from cuttings; nor, in short, to any of those operations, one or other of which succeed with other plants. Drought does not suit it, it does not thrive in wet; heat has no beneficial effect, cold no prejudicial influence; care does not improve it, neglect does not injure it."
About 1836, M. Hardy of Luxembourg raised a number of seedlings, of which one proved strikingly different from the others. It had greener, pinnate leaves and much larger flowers, but still with the attractive central eyespots (Fig. 2).3 Undoubtedly it was a chance hybrid with Rosa, the species being deduced as R. clinophylla, which was flowering nearby. It received the name Rosa X hardii Cels, but being an intergeneric hybrid must now be called X Hulthemosa hardii (Cels) Rowl. Not the least remarkable part of our story is the survival of this hybrid to the present day, for it is little less intractable to cultivation than Hulthemia itself. It can be budded on a variety of stocks, but mildews badly and is apt to die off suddenly, especially after a hard winter. The healthiest plants are those grown in pots and given frost protection in frames in winter. When well-grown it is a dainty and most beautiful plant, and was a favourite of William Paul, who once had over 400 plants of it.4
The practical issue facing the rose-breeder is: can the eyespot character of Hulthemia be transmitted to the garden roses to give a new race of bicolors? I feel certain that it can, and that it is only a matter of time before this is done. It is no secret, I think, that breeders in Germany and America are no less aware than ourselves of the potentialities of eye-spotted roses, and are now labouring to introduce the new character. On the credit side we know that Hulthemia persica is a diploid with fourteen chromosomes similar to those of the diploid roses, and that from its one unquestionable hybrid the desired eyespot character is behaving as a simple dominant. The main barrier so far to further progress from H. hardii, apart from cultural difficulties in raising sufficient plants, has been its sterility; Hurst found a complete abortion of megasporangial tissue and no good pollen at all. Several courses are open to overcome this sterility, and have been the subject of study at the John Innes Institution over the past five years. Ideally an amphidiploid hardii is needed which would not only be expected to be an improvement on the barren diploid, but should produce good diploid pollen for direct crossing with the tetraploid H.T.'s and the majority of other garden roses. However, all attempts so far to double the chromosome number have failed, partly on account of the way in which the meristems are heavily protected by the imbricating stipules, so that none of the drugs tried reaches the growing cells. Colchicine, Acenaphthene and Hexachlorobenzene were equally unsuccessful, as have been shock treatments using high and low temperatures. Recently it has been suggested that nitrous oxide under ten atmospheres pressure is a valuable means of inducing tetraploidy in difficult subjects, and given a suitable pressure chamber it would be worth trying on hardii.
More successful has been the effect of X-rays on young flower buds in an attempt to produce diploid pollen. At sixteen days after treatment with 500 r up to 4.3 per cent apparently normal pollen resulted and 1.5 per cent giant (presumed diploid) grains. Since the hybridizations using X-rayed pollen took place only this summer, it will be some years before the results are seen. New hybrids of Hulthemia with various roses have been attempted, so far without success. Here again breeders in England are heavily handicapped as there is, to my knowledge, only one plant of H. persica in the country that can be depended upon to flower each year! Having failed so far to produce any Hulthemia hybrids with diploid and regular polyploid species of Rosa, attention is being paid to the Caninae as possible female parents, where Fagerlind5 has shown that, because of their peculiar breeding mechanism there is less of a barrier to wide crossing due to dissimilar chromosome ratios in the endosperm and embryo.
A useful approach would be a search for natural hybrids of Hulthemia in regions where it grows in company with roses. The genus Hulthemosa was, in fact, founded on two supposed wild hybrids, and, if this is so, every effort should be made to bring them into cultivation as potentially valuable breeding stock.
How the public would react to ocellated roses in various combinations of buttercup yellow and richest crimson cannot be said, any more than one can predict the type of colour schemes which the new gene or genes can unleash. But of one thing I am certain: we are in no danger of the garden roses stagnating through lack of novelty.
The genus X Hulthemosa was founded in 1941 on two alleged wild hybrids between Hulthemia and Rosa, and published in Komarov's Flora of the USSR, x (1941), 507-8 and 638-9. These were christened X H. guzarka Juz., the type (Hulthemia persica Bornm. X Rosa sp. Sect. Cinnamomeae?), and X H. kopetdaghensis (Meffert) Juz., said to be Hulthemia persica Bornm. X Rosa rapinii Boiss. [Syn. R. bungeana Boiss. & Buhse.] Neither of these is yet in cultivation, nor do their descriptions tally in any way with the one unquestionable Hulthemosa (H. hardii). Further, since H. guzarica is known only from a herbarium specimen lacking petals and fruits, how do we know it is a member of the Rosoideae at all? The position, therefore, of these two alleged wild Hulthemosas is at present very vague, and more material—preferably living—is needed to check their status.
The combination X Hulthemosa hardii is made here, it is believed, for the first time. The full citation should read:
x Hulthemosa hardii (Cels) Rowley comb. nov.
= Rosa hardii Cels in Ann. de Flore et de Pomone IV (1835-6) 372-3. Hulthemia persica (Michx.) Bornm. x Rosa clinophylla Thory [Syn. R. involucrata Roxb.].