Rosa spp. (Roses)

The genus Rosa includes over 100 species which are distributed throughout the temperate and subtropical regions of the Northern Hemisphere. Chromosome numbers range from 2n = 2x = 14, to 2n = 8x = 56. The DNA content of the rose genome is amongst the lowest recorded in the Angiospermae, the 4C value of Rosa wichuraiana (2n = 14) measuring only 0.45-0.48 pg. The chromosomes are correspondingly small.

Cultivars which are now most commonly grown in gardens as ornamental plants are classified as Hybrid Tea (large flowered), Floribunda (cluster flowered) and miniature roses. Hybrid Tea and miniature roses are also grown under glass for the sale of cut flowers and pot plants, respectively. Each of these classes of rose has the ability to produce flowers throughout the growing season. This “perpetual flowering” characteristic is determined by a recessive gene. The complex origins of modern roses from crosses between perpetual flowering roses which were introduced from China ca. 1800 A.D.and European cultivars with short flowering seasons, have been reconstructed by Hurst and Wylie. Wylie drew attention to the narrow genetic base of the modern garden roses, only eight species having contributed significantly to their gene pool. Attempts to introduce novel genes into modern garden roses by introgressive hybridization are constrained by F1 sterility, arising either through differences in ploidy level of the parents, or genomic incompatibility.

Cultivars of Rosa rugosa are widely used for amenity horticulture, particularly for roadside verges and urban landscapes. Although they do not possess the gene for perpetual flowering, they do have a longer flowering season than other “species” roses.

Most cultivars are highly heterozygous and do not breed true to type. They are, therefore, propagated vegetatively. Miniatures are often propagated from cuttings, but other classes are usually propagated by budding, or grafting onto root stocks of species which include R. canina ‘Inermis’, R. multiflora ‘Simplex’ and R. dumentorum ‘Laxa’. The advantages and disadvantages of “own-rooted” plants are important considerations in assessing the potential market of micropropagated roses.

Ornamental roses are grown for a variety of purposes, e.g., as cut flowers, pot plants, anemity and garden plants, and as many popular cultivars are not protected by patent, it is difficult to reliably assess production. It is quite clear, however, that throughout the world the rose is associated with beauty and special occasions and in many countries it is the premier ornamental plant. More than 200 million rose bushes are planted each year in gardens the world over, representing a retail market of about US$720 million. The importance of the rose as a cut flower is indicated by sales of more than 4 billion blooms with an appropriate annual retail value of US$ 11 billion. In the U.K., approximately 30 million field-grown plants and 0.5 million cut flowers are sold annually. In continental Europe, there is a greater demand for cut flowers and 900 million are sold annually in just one market at Aalsmere (Holland).

R. damascena is cultivated in Bulgaria, France, Italy, Turkey, Iran, the U.S.S.R, Morocco and the U.S.A. for the production of attar (otto) of rose or oil of roses. Attar is a highly regarded perfume which is widely used in the cosmetic industry and is one of the most highly prized essential oils. The estimated wholesale price of attar is US$3,000 per kg and the world retail market is US$12 million per annum. The oil of roses is extracted from the blooms of R. damascena and R. centifolia and about 1 kg can be distilled from 3000 kg of blooms. There are many chemical components in attar which include rhodinol, geraniol, nerol, linalool, citral, phenyl ethyl-alcohol, eugenol and carvone. All of these can be extracted from other flowers or manufactured synthetically; however, the demand for pure attar oil remains.

Rosa: Conclusions and Prospects

Rose cell suspension cultures have proved to be an excellent experimental system for investigating the mechanisms which regulate growth, cell division and the physiological and biochemical processes of cultured plant cells. It is likely that future studies will focus very much at the molecular level and this culture system has much potential for elucidating the molecular events which control protein synthesis and cell division activities in plant cells. This information may facilitate the manipulation and growth in culture of isolated plant cells and protoplasts.

The fundamental data obtained from rose cell cultures on the nutritional and hormonal requirements for plant growth are being exploited in the large-scale continuous culture of many plant cells. This is becoming increasingly important with the move away from synthetic food additives, products and fragrances towards nature-derived materials. There is much speculation as to whether cultured plant cells can be utilized as a controllable source of “natural products”. This approach has not fulfilled the early promise, and has been limited by the lack of appropriate source material and our inability to modify biochemical pathways to increase product yield. In the case of rose cell suspensions, future studies are likely to be directed to transformation studies, in which target genes from other species will be incorporated to produce, e.g., flavour, fragrances and enzymes.

Micropropagation of roses is proving to be commercially profitable and is now practised on a large scale in France, the U.K., the Netherlands and several other countries. A recent survey by the present authors, for example, showed that six companies in the U.K. are currently producing one million roses by micropropagation and that demand is unsaturated. Present outlets include specialist rose growers and garden centres. As evidence of quality and acceptability of micropropagated roses, such as that provided by Dubois et al., Martin et al. and Reist, is assimilated by the horticultural industry, demand for micropropagated roses may increase rapidly. Micro-propagation may be further enhanced by the transformation of roses by “shooty mutants” of Agrobacterium tumefaciens or by the root-inducing strains of Agrobacterium rhizogenes. Transformation by the former may result in precocious shoot production, whereas the latter could result in roses which produce more vigorous roots. The attractiveness of in vitro technology will be further enhanced by the prospect of establishing elite stocks of virus-free plants and the advantages associated with health certification of in vitro plants for export.

One major area of commercial importance which has not been exploited is that of marketing garden roses by mail order. If customers were able to transplant in vitro plantlets to soil, with acceptable chances of success, distribution of these plantlets by post or carrier could be achieved at a low cost per plant. Rooting of plants in agar does not lend itself to this concept, because of the predictably high levels of mortality. The rooting of plantlets in Sorbarod plugs results in a transplantation unit of plantlet plus plug, which is sufficiently resistant to damage and desiccation to justify careful evaluation of prospects for this material. It is anticipated that the in vitro acclimatization of plantlets will be further enhanced by chemical treatments, some of which are presently being investigated in the authors’ laboratories.

The use of in vitro methods in rose breeding offers both immediate and longer-term benefits. It is likely that existing methods could be used to great advantage for chromosome doubling and isolation of somaclones. Both processes are likely to benefit from a capacity to generate adventitious shoots, when this becomes applicable to a wider selection of roses. The potential for the production of new improved varieties of roses by genetic engineering techniques is now a real possibility. By protoplast fusion it may be possible to introduce drought resistance and a central red spot, which is characteristic of the flower of the Persian desert rose, R. persica, into important rose cultivars.

Possibly the most significant improvement which could be made to the rose relates to flower colour, and in the near future the techniques of molecular biology are likely to provide an exciting new range of colours. Meyer et al. have pointed the way by producing a new brick-red flowered petunia by transformation of a colorless petunia with a maize gene. Thus, a new flower pigmentation pathway has been established in petunia. By means of this approach it would seem possible that the delphinidin pigment could be introduced into the rose and so the search for the elusive blue rose may nearly be over.