Distribution and Economic Importance
Hops (Humulus lupulus L., family: Cannabinaceae) are widely grown throughout the world, being indigenous in the northern hemisphere above 32 ° latitude; they have also been introduced into the southern hemisphere, including S.-America, S.-Africa, New Zealand, and Australia (Connell 1986). The hop is a dioecious plant, normally diploid (2n = 20), with a strong climbing habit, resulting in twining shoots reaching as high as 7-9 m in the growing season, the rootstock perennating from late autumn to the following spring. The stem (bine) and oppositely arranged leaves bear coarse hairs. The crop is cultivated for the resins and essential oils produced by the lupulin glands in the flowers (cones) of female plants). Scattered male plants may be grown amongst the stand of females in order to produce so-called seeded hops as traditionally produced in the UK, but brewers producing lighter, lager-type beers normally require seedless hops.
The growing of hops was brought to England from mainland Europe in the 15th century and they were used initially to preserve what had previously been termed ale, and which then became known as beer, characterized by its bitter taste and improved keeping quality. The peak period of production in England was reached during the last century, when almost 30000 ha were grown in 1878. The increasing competition from worldwide hop-growing countries has led to a marked drop both in price and in hop production in the UK and USA in recent years, the UK total being 4748 ha in 1985 and 4232 ha in 1986. World hop production levels are given in Table World hop production data (1986). Eighty percent of the crop has been produced by seven countries led by the FRG (29%), followed by the USA (19%), Czechoslovakia (10%), the USSR (10%), UK (5%), Yugoslavia (4%) and China (4%) – based on 1985 figures. Coincident with the fall in the world price for the crop have been changes in the market demand for particular types of hop. Thus, during the last 10 years there has been a trend towards the growing of cultivars producing a high content of bittering components, i.e. high α-acid cultivars. At the same time, demand for hops suitable for lager production has increased as a percentage of the total market. There is also a continuing demand for high aroma hops of the more traditional type in the UK for the brewing of the mis-named “real ale”, with its characteristic full flavour.
Compounding these market trends is the fact that a particularly destructive wilt disease of hops caused by the fungus Verticillium albo-atrum causes severe losses in the hop-growing areas of England, and that many of the traditional, high aroma hop cultivars are very susceptible to the more pathogenic progressive wilt strains of the pathogen. V. dahliae causes relatively mild outbreaks of the disease in the UK, USA and in Germany. Although V. albo-atrum does attack hops in Germany, outbreaks caused by severe defoliating strains of the pathogen have not been recorded there, or apparently otherwise outside the UK. A serious factor limiting production worldwide is that of virus infection in hops, but fortunately tissue culture methods have already proved commercially useful in the production of virus-free plants by meristem-tip propagation.
Table World hop production data (1986)a (English Hops Ltd.)
|Country||Area (ha)||Estimated production (zentners)b||Yield (alpha) kg/ha acid|
|Fed. Rep. Germany||19500||702000||100.26|
a Averages for 1978-1984 (alpha data for Japan, Poland, USSR, are also 1978-1984 averages).
b 1 zentner = 50 kg.
Humulus lupulus L. (Hop): Conclusions and Prospects
In vitro methods developed for propagation of virus-free hop plants are already of commercial use, and these techniques have been a starting point for the development of systems now available for establishment of callus, shake cell suspensions, immobilized cell lines and protoplast protocols from a variety of cell types and hop cvs. Unfortunately, there are no reports of haploid cell lines established by anther/pollen culture, which would be of considerable advantage for selection. Similarly, there are no mutational studies for hop combined with in vitro methods. Selection systems are also at present restricted by the lack of a successful method for the complete regeneration to the plantlet stage from a single hop cell or protoplast. However, it is possible to select at a green callus stage and achieve full regeneration to adult plants, and this has led to studies on the attempted production of novel disease resistance to Verticillium albo-atrum, a major fungal pathogen of the crop in the UK. Selection of cell lines regenerated from protoplasts has been reported, and is considered to be of potential value in the search for types with low degradative (peroxidase) ability, which at the high rates operating in vitro, apparently prevents accumulation of α-acids (bittering components). It is likely, however, that the successful production of bittering or flavour components by hop cells in culture will have to await further understanding of the complex regulatory/genetic systems which frequently prevent commercially applicable levels of secondary metabolite accumulation during in vitro culture of plant cells in general.
Undoubtedly, the application of a range of the techniques reported here has the potential to play an important role in modifying the traditional methods of hop breeding. This is likely to involve studies on somaclonal variation for selection of novel agronomic types with different flavour/bittering properties, yield/growth type (selection of dwarf, high yielding types is already a high priority), as well as resistance to a number of important hop fungal pathogens and possibly, insect pests, including aphids. The logistical advantage of the use of an in vitro toxin-screening system for detection of Verticillium resistance has considerable potential in breeding programmes, but more research is required to validate this approach. The newly developed techniques of electric field fusion present an opportunity for investigation of controlled somatic hybridization between cvs. with complementary agronomic features. Selectable markers at the in vitro stage to detect hybrids and recombinants include green chloroplast transmission, Verticillium toxin sensitivity, and drug resistance introduced by transformation involving Agrobacterium spp. plasmids. In the longer term, electroporation techniques will allow uptake of selected DNA from a donor cv. to protoplasts of the appropriate recipient cv.
Selections from the book: “Medicinal and Aromatic Plants II”, 1989.