Why does spraying the hip 7-14 days after pollination help the hip set?
The following is the abstract of a scientific literature study comparing gibberellic and cytokinin sprays on another member of the Rosaceae family:
Title: Apple shape changing possible with cytokinin and gibberellin sprays.
Authors: Martin, George C.; Brown, Dillon Sidney; Nelson, M. M.
Authors affiliation: Univ. of California, Davis, CA, USA.
Published in: California Agriculture, volumn 24, page 14, (1970).
Abstract: "Sprays of cytokinin were more effective than gibberellins A3 or
A4/7 in improving fruit set and elongating apples. Applications of cytokinin
(270 or 500 ppm) at petal fall plus 3 days were the most effective in increasing
fruit set and elongation.
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There have been published studies on the application of gibberellic acid to roses but I have not been able to find one concerning cytokinins.
Authors: OGILVIE I; CLOUTIER D; ARNOLD N; JUI P Y
Authors affiliation: AGRIC CAN, RES BRANCH, EXP FARM, PO BOX 3398, L'ASSOMPTION, QUEBEC, CAN J0K 1G0.
Title: THE EFFECT OF GIBBERELLIC ACID ON FRUIT AND SEED SET IN CROSSES OF GARDEN AND WINTER HARDY ROSA ACCESSIONS
Published in: Euphytica, volumn 52, pages 119-124, (1991)..
Abstract: "The effect of gibberellic acid (GA3) was studied on a wide range of crosses between various garden roses and two hardly breeding lines. Although there were fewer seeds per fruit, fruit set was higher in most crosses when GA3 was applied to the stigma at the rate of 250 ppm ten days after pollination. However, higher fruit set did not result in more seeds per pollination for many crosses and seemed to be related to the degree of male parentage. Both parents used had a major effect on pollination success. Recently developed Rosa cultivars have a narrow genetic base and the use of distantly related cultivars as parents would possibly give a high seed set and greater numbers of progeny."
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Title: "Gibberellins-New Tools for Rose Breeders
Author: E. F. ALLEN, M.A.
Author affiliation: Dip.Agric.(Cantab.), A.I.C.T.A.
Published in: British Rose Annual, pages 123-127, (1967).
The article does not contain an abstract; the following is the summary presented at the end of the article::
"Summary
1. Treatment of tetraploid rose flowers, ten days after pollination, with three drops of a 250 ppm. solution of gibberellin A.3 in 25 per cent isopropyl alcohol resulted in both an improved fruit set and also an increased "seed" yield.
2. Where diploid pollen was used, from a tetraploid male parent, the resulting fruit set was 100 percent.
3. Treated flowers produced larger and earlier ripening heps than untreated ones.
4. It is suggested that the mechanism of the beneficial effect is to hold on the flower long enough for the slower growing pollen tubes to reach the ovules before abscission.
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A very interesting paper is available and can be purchased to download for a very nominal amount at: http://www.actahort.org/books/226/226_26.htm
The abstract is:
Title: FACTORS AFFECTING FRUIT AND SEED SET IN THE HYBRID TEA-ROSE 'SONIA'
Authors: d.P. de Vries and Lidwien A. M. Dubois
Authors affiliation: Institute for Horticultural Plant Breeding (IVT), P. O. Box 16, 6700AA Wageningen, The Netherlands
Published in: Acta Horticulturae, volumn 226, pages 223-230, (1988).
Authors: D.P. de VRIES, L. A.M. DUBOIS
Abstract: "In various years the effects of temperature, repeated pollination of the same flower, the time interval between emasculation and pollination and of the application of GA 4+7 on the fruit set, seed set and seed germination of roses were investigated in the large-flowered Hybrid Tea cv. 'Sonia', using 'Hadley' or 'Ilona' as pollinators.
Optimum conditions for breeding with 'Sonia' are: greenhouse temperatures between 20 and 25°C, and pollinating at least twice between 1 and 4 days after emasculation. To improve mainly fruit set, GA may be applied 7 or 14 days after emasculation, in the concentrations 10, 50 or 250 ppm.
Generalization of present results and the penetration of pollen tubes into the 'Sonia' ovary to explain maximum seed set are discussed."
The graph below ( based on the data in de Vries and Dubois paper cited above) suggests that applying the gibberellic acid between the 7th and the 14th day in a concentration range of 10 ppm to 250 ppm will give satisfactory results. The number of seeds per pollinated, but untreated, flower was 3.7. Thus, gibberellic acid treatment gave about twice the number of seeds.
Since in the natural process of the fertilized hip (see http://koning.ecsu.ctstateu.edu/fruit/fruit.html and http://www.actahort.org/books/134/134_17.htm ), the cytokinin is produced before the gibberellic acid; one may want to try a shorter time period than 7 days if you are applying a cytokinin (suggested by Chris Mauchline). Please keep in mind that too soon an application will result in a perfectly formed hip with few or zero seeds.
In the literature there are examples of applying more than one chemical; unfortunately, none of the examples are with roses. Since both cytokines and gibberellic acid are produced naturally in a fertilized hip ), there does not appear to be any reason not to apply both. One could try applying both at the same time or applying first the cytokinin and then a few days later the gibberellic acid.
Both the cytokinin and gibberellic acid are expected to help seed set by keeping the hip in a receptive state for a longer period of time which allows slower growing pollen to reach and fertilize the ovules. There are also literature examples of injecting the flower stem at the base of the hip with 2, 4 - D in order to speed up the pollen travel. I am planning on trying this procedure on hard to pollinate roses this summer (2004).