Distribution and Importance of the Plant
The genus Dianthus comprises a relatively large group of some 300 species, which have attracted attention because of spectacular flower color combinations ranging from white to yellow, red, and deep purple. Particular morphological traits and pigmentation distinguish Dianthus from other genera within the family Caryophyllaceae, although the evolutionary phyllogenetic background and the subdivision of the genus have remained controversial. The genus was thought to have originated in the Mediterranean hillsites, but is now believed to have inherited traits from various nontropical locations of Europe and Asia. Some species spread along the southeast African continent into South Africa and even into the Far East, which is recognized in their taxonomic designation, D. chinensis being one example. Dianthus generally prefers moderately dry and warm conditions, high light intensities, and mineral-rich soils, but a few species such as D. alpinus and D. glacialis have managed to colonize rock soils in the Austrian and Italian Alps at 2000 to 2800 m altitude. Crosses between species may occur spontaneously, and numerous hybrids are known to exist in their natural habitat as well as under cultivation. The plants show perennial growth, but there is a trend towards annual cultivars in breeding for ornamental varieties.
Conventional Practices for Propagation, Potential and Demand on the World Market
Since the 16th century D. caryophyllus, D. barbatus, D. plumarius, D. carthusianorum, and D. chinensis have been chosen in particular as ornamental plants because of their appealing coloration and growth characteristics. Nevertheless, the existence of a truly wild form of D. caryophyllus has remained questionable, and the commercial hexaploid cultural variants subsumed nowadays under D. caryophyllus presumably trace back to the Mediterranean D. sylvestris and a broadleaved relative such as Dianthus rupicola. The latter is also responsible for delayed and repeated blossoms. Similarly, the shorter growing D. plumarius used today in rock gardens most likely descended from hybrids of D. plumarius and D. gratianopolitanus ().
Carnation cuttings, mostly of Dianthus caryophyllus, make up a considerable share of the world’s cut flower market, which, in economic terms, has steadily increased over recent years. Large-scale cultivation and breeding for worldwide sales are being pursued in countries such as Columbia, France, Holland, Israel, Italy, and Korea. Therefore, the generation and use of cultivars with enhanced natural pest resistance is definitely needed. Field-grown Dianthus caryophyllus blooms during the summer, the plant dries down in autumn to the root that stores “lactosin”-type galactanes as carbohydrate reserve rather than starch, and new shoots emerge in the following spring. Commercial growers, however, desire high prices during the wintertime and, therefore, explant side shoots which are harvested from the autumn roots. These shoots root within a few weeks in the greenhouse and grow into plants that yield flowers from December through March. Shoot initiation from cryopreserved shoot apices would provide for storage of germplasm, but has rarely been pursued in commercial nurseries.
Carnation breeding has focused on the ornamental characteristics, and major goals are the “filling” of the flower head by petaloid transformation of stamen and anthers but, in particular, modifications in pigmentation as well as an extended vase life of the cuttings. Chalcones, flavonoids, and anthocyanins are the natural pigments in the petals, and it is noteworthy in this context that the Caryophyllaceae represents the only family in the Caryophyllales ( = Centrospermae) which lack the otherwise typical betaine pigments. Accordingly, the genetics of pigmentation of carnation was investigated. Natural mutation appears to occur frequently, and the selection of spontaneous carnation mutants as well as irradiation mutation have been employed in addition to conventional breeding techniques in the search for new cultivars. The durability of carnation cuttings, on the other hand, depends immensely on the amount of ethylene released by the petals, which causes rapid senescence of the flowers. Indeed, the cut carnation has become a standard model system for the investigation of the role of plant hormones in flower senescence, and numerous reports have been published on the enzymes involved in ethylene biosynthesis, on ethylene-regulated flower genes, on the degradation of phospholipids and on changes in ultrastructure and selected enzyme activities.
Although Dianthus flowers were occasionally used as a medical stimulant, only a few components from this plant other than pigments have been studied in detail. Among these were benzoic and salicylic esters that give rise to the flower fragrance, triterpene saponins, pyran-type glycosides, seed oil, and protein contents, etc., none of which has aroused any commercial interest. A few years ago, however, the accumulation of unusual and novel anthranilates was reported from carnation plants infected by fungi. These compounds, which will be dealt with below, show great bioactive potential, and, as suggested previously, the re-investigation of carnation for alkaloid contents appears necessary.
Commercial Aspects and Future Prospects
The major importance of carnation will remain with the cut flower market. Consumer preferences regarding pigmentation and growth characteristics is changing so rapidly, however, that breeders are forced to reduce the time for release of new cultivars. The improvement of natural resistance, which would require time-consuming field testing, is neglected under these conditions, and, consequently, commercial cultivation of carnation in nurseries is accompanied by heavy preventative pesticide treatments. Molecular markers for horizontal resistance would greatly speed up the selection of valuable shoots for propagation and strengthen the natural resistance of commercial carnation cultivars. Phytoalexin accumulation is considered as a major factor in horizontal resistance and antisera specific for benzoyl-CoA: anthranilate N-benzoyltransferase and anthranilate synthase or the corresponding DNA probes would be sensitive tools to determine the resistance quality of plants from single leaves and at a very early stage of growth by Western or Northern blotting techniques. Provided that homologous enzymes catalyze phytoalexin biosynthesis in oat plants, the probes might also be employed for the selection of oat plants.
Elicited carnation cells may serve as a simple model system to study the regulation of anthranilate metabolism at the molecular level. Such studies would provide the basis for further investigations on acridone or pharmacologically important indole alkaloids. The enzymes from carnation and from Catharanthus () are the first anthranilate synthases purified from plants. Although these studies are in their very beginning, an improved knowledge of anthranilate synthase characteristics may pave the way for the development of selective inhibitors of tryptophan biosynthesis. Synthetic inhibitors of amino acid biosynthesis account for a multi-million dollar share of the world’s herbicide market and are being used, for example, for postharvest field cleaning. None so far have been developed for interference with the tryptophan pathway.
The successful infection of carnation plants with Agrobacterium tumefaciens and the ease with which carnation plants can be regenerated from callus tissue suggest that the stable transformation with useful genes will be accomplished soon. This reveals new ways to improve the ornamental, disease resistance, and metabolite qualities of carnation plants for commercial use. It is to be expected that antisense constructs will be introduced first for inhibition of ethylene biosynthesis and delayed senescence of carnation cuttings. The enhancement of resistance to fungal diseases might be accomplished by the introduction of foreign phytoalexin synthesis genes, as has been demonstrated recently with stilbene synthase in tobacco. The modification of carnation N-aroylanthranilates by introduction of, for example, the N-aroylanthranilate 5-hydroxylase from oat may yield new metabolites upon elicitation. This may further attract interest to use carnation as a source of valuable medicinal metabolites, thus giving the plant a new role in addition to its ornamental value. Carnation phytoalexins are composed of the anthranilate and benzoyl moieties, both of which derive from the shikimate pathway. Whereas the biosynthesis of anthranilate branches off early and drains away chorismate as the substrate, the benzoyl moiety is formed later in the sequence from cinnamate. Provided that all reactions proceed in the same cellular compartment, the scenario requires that the flow of metabolites through the shikimate pathway and particularly the percentage of chorismate channeled into anthranilate is tightly controlled so that sufficient amounts of both anthranilate and benzoate are produced during the elicitation of cells. The investigation of branch-point enzymes may reveal further information on the modes of control of anthranilate-indole and benzoate metabolism, which might eventually contribute to an understanding of alkaloid biosynthesis regulation.
Selections from the book: Medicinal and Aromatic Plants VII (1994).