RHA Newsletter 6(4):6 (1975)
THE CHEMISTRY OF WHITE
Dr. Peter Hughes
Pigments in roses comprise (a) flavins and (b) cyanins, and in white roses, both must be absent.
If we consider biosynthesis of cyanins, presumably from carbohydrate, we have specific enzymes for each step: A → B → C → → Pelargonidin → Anthocyanin. This chain can be interrupted at any stage by loss of a specific enzyme, and as a general rule, the loss of enzyme is a recessive gene; e.g., loss of enzyme for hydroxylating pelargonidin to anthocyanin is recessive.
Our white garden roses fall into two groups: (1) Marcia Stanhope, Blanche Mallerin, Virgo, Message, etc. — all pure white but mildew susceptible, and (2) Mt. Shasta, Pascali, etc. — dirty, milky whites but not troubled much with mildew.
Crosses between these groups supply genes for both enzymes, so any colour may result. White is obtained by crosses within each group. Neither is likely to give much advance.
Two options remain open: (1) Cross say Virgo with a mildew resistant variety of any colour and treat the offspring with x-rays, alkylating agents, etc., to cause chromosome breakage and recombination. In this way the linkage between pure white and mildew may be broken. Repeated selfing of these seedlings could give the white needed. (2) Use another species. I have growing Rosa Fedtschenkoana Regel, from Turkestan. This has lovely glaucous foliage, never any blackspot or mildew, white flowers all summer, seeds readily, and is tetraploid naturally. This could be introduced into a breeding programs easily and selfing the seedlings would soon give four genes for pure white not linked to mildew.
RHA Newsletter 6(4):7 (1975)
E. B. LEGRICE ON WHITES
From 1972 New Zealand Rose Annual
I would say on the whole avoid white roses. They are only a nuisance, for how can you possibly tell, when you have five hundred white seedling roses, which are going to do better than the others. With all white, you can go maybe by the petals. If you make a lucky hit and say, "I have cut out all the full ones," the ones that result are too thin; if you cut out all the thin ones, those left will be too full to open. You cannot judge them very easily. All you can do in the seedling stage is to look at the quality of the petals, and if it has thin, papery petals, out it goes for that will be no good anyway. In the end, you will find yourself lumbered up — there is no other word for it — with white roses under test.
In one experience, I wanted to harden up the whites and used FRAU KARL DRUSCHKI. Those who say it is a hybrid tea have not tested it, for if you use it as a parent you will find that four-fifths of the seedlings will not flower the first year; up they go in the air, and no flowers. So out of maybe 200 to 300 plants, there would be no more than thirty with a chance of being any good. After two years, we finally found only two plants worth persevering with, not for release, but for future crossing.
Furthermore, there is an inhibitory factor in some crosses, and this can cause a lot of worry. This is common in yellow antirrhinums; they seed, germinate, and then produce a growth substance that kills the plants. This certainly does happen in some white rose seedlings, so keep off whites.
White roses consist of two types. Some are white in colour, and in others there is an absence of any colour factor. Those having white pigments will breed white, but those where the white is due to the absence of pigment, and the latter may result from crossing two vividly coloured varieties, may produce all colours in the next cross. The only way of identifying them is by looking at the stigmas and the anthers. Generally they will divide into two classes: (1) the greenish whites and (2) the fawny whites. They are quite distinct and breed quite differently. So if you want to breed whites, see that you get two parents on the same side.
I would advise amateurs to have a firm idea in mind and stick to that. If you should want a red rose, take those red roses, stick at red roses, and find out what's about those red roses. You are more likely to achieve something that way; it is a pretty uncertain dip in the lucky bag anyhow, but you make it very much worse if you diffuse your efforts.