The vigour of glasshouse roses : scion - rootstock relationships : effects of phenotypic and genotypic variation

Glasshouse roses commonly are combination plants, consisting of a scion variety and a rootstock of different genotypes. In this study, various environmental and genotypic factors have been investigated that influence the vigour of rootstocks and scion varieties, separately and in graft combination.In a field crop of Edelcanina (selections of <em>R.canina</em> L.) rootstock plants, significant phenotypic variation occurred for all plant characters investigated. Nurserymen partly disguise variation by grading the rootstock plants by standard classes for the diameter of the root collar. Even amongst plants of the same diameter (8-12 mm), both between and within batches of 'Inermis' of different provenance, significant variation occurred in branching of the roots, in the number of shoots, in the number of bottom-breaks and in the root collar diameter.The effect of the plant density, as a possible source of variation, was investigated by growing 'Inermis' rootstock seedlings at nine plant densities between 30 and 190 plants per m <sup><font size="-2">2</font></SUP>. As density increased, the root collar diameter of one-season-old stocks decreased; the distribution of plants over diameter classes shifted from a majority of thick (>10 mm) to one of thin (≤6 mm) stocks; interplant variation for root collar diameter increased; biomass increased to a maximum at a density of 170 plants per m <sup><font size="-2">2</font></SUP>. Density significantly affected the expression of plant characters. Hence, as density increased, shoot number per plant decreased, shoots became longer and lighter, the root system became lighter and less well-branched, and the shoot/root (fresh) weight ratio decreased. Density did not affect the percentage of plants with crooked root collars. Phenotypic variation of plant characters of 'Inermis' is mainly attributed to variation in plant age and in plant-to-plant distance in the seedling field.The diameter of the root collar of stocks at grafting had significant influence on the subsequent growth of 'Sonia' combination plants in the glasshouse. As the root collar diameter of the Edelcaninas 'Inermis'. 'Brögs Stachellose' and 'Pollmers' increased from 3 to about 15 min, the primary shoots of 'Sonia' were longer. bottom-breaks emerged earlier and yield measured in terms of the number. weight and length of shoots, increased. After 12 months. the absolute fresh weight of the roots of thick stocks was greatest. but the increase in weight of thin stocks was much larger (20 fold) than of thick ones (2-5 fold). The shoot/root (fresh) weight ratio in 12- month-old plants was not affected by the original stock diameter. but 'Brögs Stachellose' induced higher ratios than 'Inermis' or 'Pollmers'.Scion-rootstock relationships were studied by comparing the growth and development of own-rooted Hybrid Tea rose seedlings in the first year. with those of their clones grafted on 'Inermis' in the second year. Allowing for differences in level, the growth and development of Hybrid Tea seedlings generally agreed with those of their clones. The numbers of bottom-breaks and of harvested shoots of seedlings and clones were not affected by scionrootstock interaction. As the vigour of scion genotypes increased, the quiescent axillary buds of scions sprouted sooner after grafting. while more bottombreaks emerged in an earlier stage; over a 12-month period shoot yield was higher. and root weight of the stocks increased. The value set in practice on a high number of bottom- breaks as a basis for high flower production was confirmed. Breeders are recommended to use the number of bottom-breaks and the shoot yield in own-rooted seedlings as a predictor of the shoot yield of future cultivars.In clones of weak Hybrid Tea cut rose seedlings, the rootstock 'Inermis' promoted the vigour of combination plants but. as the original seedling had been more vigorous. increase of the vigour of combination plants on *Inermis' was smaller. In combination plants of various scion varieties grafted on one type of stock, the vigour is controlled by the scion variety rather than by the stock. The high cultural value attached to <em>R.canina</em> 'Inermis' as a rootstock. does not seem justified. A major component of the vigour of scion varieties is their branching capacity. Since branching capacity depends on degree of correlative inhibition, the ratio of auxins synthesized in the shoot and cytokinins produced in the root, is supposed to be the controlling mechanism of axillary bud-break. The lead in growth of vigorous over weak scion genotypes. is likely owing to a larger leaf area per plant as a result of earlier sprouting of more axillary buds after grafting or shoot harvest.To investigate the possibility of (undesired) hybridization in Edelcanina rootstocks, first the genotypic variation of 'Inermis' was studied as to characters and adventitious root formation of individual rootstock plants. After 6 months in the glasshouse, originally uniform (8-12 mm) 'Inermis' stocks appeared to differ in internode length, number of leaflets per leaf and the leaf area. In 23-day-old softwood cuttings of different genotypes, significant genotypic variation occurred for the time of axillary bud-break, axillary sprout length and weight of adventitious roots. Between rootstock clones, the root weight of cuttings was correlated with: number of leaflets per leaf, leaf area, and days to axillary bud-break.Subsequently, genotypic variation in <em>R.canina</em> 'Inermis' and 'Brögs Stachellose' was studied as variation in the growth and development of 'Sonia', grafted on clones of these stocks. Owing to different induction of vigour, 'Sonia' combination plants varied for: the date of emergence of the first and second bottom-break, the number of bottom-breaks, and the number of 'Sonia' shoots harvested after 12 months. Correlations between the root weight and the diameter of the graft union were significant for both 'Inermis' and 'Brögs Stachellose' clones. Graft unions of 'Brögs Stachellose' were always thicker than 'Inermis' of equivalent root weight. Genotypic variation of Edelcanina selections is attributed to hybridization with other Edelcaninas or other species. Finally, the effect of root clones of Hybrid Tea rose seedlings, pre-selected for vigour, was studied in 'Sonia' combination plants. For that purpose a complete Hybrid Tea rose seedling population was studied. In that population, the shoot yield of 8-month-old, own-rooted seedlings was negatively correlated with the time from seed germination to first flowering (juvenile period). The shoot yield of 12-month-old 'Sonia', grafted on root clones of low or high yielding Hybrid Tea seedlings, was positively correlated with both the yield of the original Hybrid Tea seedlings, and with the root weight of the clonal Hybrid Tea stocks.In all experiments the shoot yield of 'Sonia' combination plants was positively correlated with the number of bottom-breaks per plant per rootstock. It was generally found that as rootstock clones induced a first bottom-break earlier, the second bottom- break emerged earlier as well. Furthermore, 'early' rootstock clones induced more bottom- breaks and more shoots per 'Sonia' plant than 'late' clones. The diameter of bottom-breaks at pinching, was not-significantly correlated with the number of harvested shoots of a clone. The vigour of rose genotypes appears to be an inherent character. that is expressed in both the aerial and subsoil parts. In combination plants, the vigour of the rootstock genotype is transmitted to the scion variety.A major effect of vigorous rose rootstocks is an increased branching capacity of the scion variety, visible as earlier and more profuse axillary budbreak in all stages of plant development. Owing to induction of a larger branching capacity, scion varieties on vigorous stocks have a larger photosynthetically active area than those on weak stocks at an early stage of plant development. This results in a lead in growth. Main differences in the vigour of rootstocks are attributed to endogenous hormone action, a concept in which rootstocks are supposed to differ for cytokinin synthesis. A model of the vigour of combination plants, based on additive vigour of scion variety and the rootstock, derived from endogenous hormone action, is tentatively proposed.Edelcaninas generally induce mediocre vigour. show genotypic variation for yield and are too large plants for most artificial substrates. Because Edelcaninas generally are recalcitrant to vegetative propagation. they are unsuitable as clonal stocks. Also. because healthy Edelcanina stocks can only be produced in fields that are chemically disinfected, the important role of these stocks for the glasshouse culture seems to be finished. If. despite these disadvantages Edelcaninas are yet to be improved by breeding. at short notice genotypic variation may be stabilized by selecting good-looking mother bushes, to be planted in monoclonal seed gardens. At long term. such seed gardens may be established with mother bushes of which the (seedling) offspring has been thoroughly tested for a range of characters.For the modern cut rose culture in artificial substrates. clonal stocks should he given preference over seedlings. Populations for the selection of clonal stocks may arise from cross-breeding or genetic modification. Preliminary selection for rootstock vigour may be carried out on the basis of number of shoots and/or shoot length of individual genotypes. Vigorous genotypes are likely to occur among individuals with a short juvenile phase. Selection for rootstock genotypes that induce a high shoot/root (fresh) weight ratio at the same vigour of combination plants. should be an aim. The management of combination plants on vigorous stocks is discussed. In weighing pros and cons of the use of self-rooted cultivars and those on a vigorous rootstock. preference is given to the latter.