Goals of medicinal plant breeding- compared to other horticultural crops-are characteristically different to some extent. These characteristic features crops-are summarized in the following paragraphs.
Breeders are always straining for an enhanced yield. Here, the enhancement is not focused on the total biomass but especially on the organs which provide the medicinally useful drug. Thus, increase of root mass and
optimising root morphology are the goals in the breeding of valerian (Valeriana officinalis) today. Increase of shoot mass and improvement of leaf proportion compared to total shoot mass is the goal –among others – in breeding of mint species (Mentha spp.), basil (Ocimum basilicum) or lemon balm (Melissa officinalis). Yield of upper flowering parts –the so-called flowering horizon of the shoots- is of primary importance inside the total biomass in breeding of some species providing herbal drugs like St. John’s Worth (Hypericum perforatum) or motherworth (Chrysanthemum parthenium). Enhancement of regeneration capacity would increase the yield of several species through the possibility of multiple harvests in the same year. This may contribute to the economics of the crop production in cultures of thyme (Thymus vulgaris), marjoram (Majorana hortensis) and similar species.
Increase of the productivity of the propagation organs has been a goal in breeding of some vegetatively propagated species. Thus, abundant and good quality stolon development (Figure 2.5.) in peppermint (Mentha piperita) production or ample growth of adventitious shoots in breeding of tarragon (Artemisia dracunculus) is important features.
Figure 2.5. Stolons (modified shoots developing in the soil) of peppermint
Another very frequent goal of breeding is the changing the accumulation of active ingredients of the drug. Breeders strain for assuring an elevated level of the most important chemical compounds or optimizing the composition and proportion of ingredients. Thus, enhancement of essential oil content has been the main focus of selection of hyssop (Hyssopus officinalis) or lavender (Lavandula spp.) plant breeding. Increase of the level of flavonoids has been fixed as breeding aim in improvement of gene stocks of St. John’s Worth (Hypericum perforatum) and milk thistle (Silybum marianum) but an elevated level of fatty acids is necessary in seeds of oil pumpkin (Cucurbita pepo) or evening primrose (Oenothera erythrosepala). In some cases a complex of biologically active materials are present in the drug which may slow down the breeding process. In several species not only the total concentration but its composition is also important, e.g. proportion of carvone in essential oil of caraway, proportion of chamazulene in the essential oil of chamomile or the proportion of morphine among the alkaloids of poppy capsules.
In other cases breeders face the task to decrease or cease the presence of certain components of adverse effects. A special example in this context is poppy
(Papaver somniferum), because a high alkaloid level may be the goal of breeding industrial varieties, however it should be avoided and minimized while breeding food varieties. Selection activity has been carried out in order to decrease the level of beta-thujone (A monoterpene ketone of thujone skeleton, frequent constituent of volatile oils. It has an adverse effect on the neural GABA receptors therefore its level is maximized in the food industry). (e.g. Salvia officinalis) or the toxic pyrrolizidine alkaloids (Derivatives of the bicyclic alkaloid molecule necin. Numerous adverse effects from stomach disorders till the carcinogenous, mutagenous reactions have been proven). (e.g. Symphytum officinale).
Concentration of heavy metals in the drug is becoming more and more important as a drug quality parameter. It has been described, that there is a difference in the accumulation potential among species and also among intraspecific genotypes in this respect. Therefore breeding for low accumulation potential of heavy metals is going to be a further goal of certain species, too (e.g. Hypericum perforatum, Linum usitatissimum).
In several species, visual performance of the drug is part of drug quality.
Therefore improvement of morphological characteristics is frequently among the breeding goals of these taxa. Breeding of lemon balm (Melissa officinalis) or that of marigold (Calendula officinalis) should not decline of keeping an attractive green colour of the leaves and a deep orange colouration of the petals, respectively. Similarly, the characteristic spicy aroma of marjoram has been fixed as a breeding aim in several countries.
For the agricultural practice, plant characteristics enabling the application of intensive cultivation and/or postharvest methods, mechanisation, large scale technologies may be of special interest and therefore also frequent goals of breeding activity. These plant characteristics might stay in connection with growth dynamics, competition ability (e.g. earliness, weed suppression potential, feasibility for mechanical harvest) but may be in relation with propagation-biological features (e.g. closed capsule, non-flaking seeds, prompt germination, uniform flowering time), (Figure 2.6.), or morphological traits (e.g. size of seeds, form of the roots, location of leaves).
Figure 2.6. Annual variety of mullein (background) with the biennial “wild”
type (foreground)
Development of resistant or tolerant genotypes has a great significance in breeding of medicinal plants, similarly to other crops. Unfortunately, this aspect has been marginally handled till now. Successful examples are the establishment of resistant clones of peppermint against Verticillium species. Breeding is going on for generating resistance in caraway (Carum carvi) and St. John’s Worth (Hypericum perforatum) against diseases caused by different fungi. An important goal should be the enhancement of tolerance against abiotic factors facilitating acclimation of plants to unfavourable climatic or edafic conditions. Thus, efforts are known for widening the spectrum of winter poppy (Papaver somniferum) varieties with enhanced frost tolerance and increasing the salt tolerance of safflower (Carthamus tinctorius) in special areas of the Middle East. Cultivation of arnica (Arnica montana) has only been successful after the development of its new variety (’Arbo’) of elevated tolerance against calcareous soils.
2.3. BREEDING METHODS IN GENETIC IMPROVEMENT OF MEDICINAL PLANTS