Single plant cells have amazing capacities for regenerating into entire plants. This property is known as ‘totipotency’ discovered in the 1920s. Linking this with increasing understanding of growth control by plant hormones resulted in the development of the sterile, in vitro, culture. Tiny groups of cells, explants, are cut from the rapidly growing tips of shoots in controlled environments and washed in sterilising agents. These are cultured sterile jars containing a layer of agar supplemented with nutrients and hormones.
Green plantlets growing on sterile agar
The process is known as ‘tissue culture’ or micropropagation. As the cells divide and multiply, they are transferred through a series of sterile conditions which encourage root formation.
Roots growing from newly developing plantlets
Ultimately numerous new whole plants are generated. At that point they are removed from sterile conditions and weaned by planting into clean compost in high humidity environments. High humidity is essential as these transplants lack the protective coating of leaf and stem waxes which prevent desiccation. Ultimately when fully weaned the plants are grown under normal nursery conditions into saleable products.
Why bother with this processes which requires expensive facilities and highly skilled staff? A prime advantage is that micropropagated plants have genotypes very closely similar to those of the original parent, essentially they are clones. As a result vast numbers of progeny can be generated from a few parents preserving their characteristics. That is particularly important as a means of bulking-up newly bred varieties of many ornamental and fruit producing plants which otherwise would be reproduced vegetatively from cuttings or by grafting and budding onto rootstocks. Micropropagation is therefore a means for safeguarding the intellectual property of plant breeding companies.
Explants cut from parent plants before culturing can be heat-treated as a means of removing virus infections. The resultant end-products of rooted plants are therefore disease-free or more accurately disease-tested. These plants are usually more vigorous and produce bigger yields of flowers and fruit. Orchids are one of the crops where the impact of micropropagation is most obvious in florists’ shops and supermarkets.
Orchids have benefitted greatly from micropropagation
Large numbers of highly attractive orchids are now readily available. Previously orchids were very expensive and available in sparse numbers.
The world is not perfect and there are disadvantages with micropropagation. Because the progeny are genetically similar they are uniformly susceptible to pests and pathogens. Crops of clonal plants can be and have been rapidly devasted by existing and new strains of insects and diseases to which they have no resistance.
Elderflowers are in full bloom this month, both in hedgerows as well as gardens across the country. Whether they are the wild Sambucus nigra or a cultivated variety with green or black leaves they are all beautiful and useful plants.
The black leaved cultivar growing in the SCIence Garden has pink blooms, whereas the wild species has white flowers. It was purchased as ‘Black Beauty’, but is also sold as ‘Gerda’.
Sambucus nigra f. porphyrophylla ‘Black Beauty’ growing in the SCIence Garden
This cultivar, along with ‘Black Lace’ (Eva) was developed by Ken Tobutt and Jacqui Prevette at the Horticulture Research International research station at East Malling in Kent and released for sale in the horticulture trade in 2000. The leaves stay a dark purple throughout the year and the flowers have a good fragrance.
The shrub will tolerate hard pruning so is useful for smaller spaces and provides a long season of interest. The plant is also a forager’s delight, both in early summer (for the flowers) and in the autumn (for the berries).
Most commonly one may think of elderflower cordial, or perhaps even elderflower champagne, but an excellent alternative to the rose flavoured traditional “Turkish Delight” can be made - https://www.rivercottage.net/recipes/elderflower-delight. I can highly recommend it!
The chemistry of the elderflower aroma is complex. Analyses such as that in the reference below* have identified many different terpene and terpenoid components including nerol oxide, hotrienol and nonanal.
* Olfactory and Quantitative Analysis of Aroma Compounds in Elder Flower (Sambucus nigra L.) Drink Processed from Five Cultivars. Ulla Jørgensen, Merete Hansen, Lars P. Christensen, Karina Jensen, and Karl Kaack. Journal of Agricultural and Food Chemistry 2000 48 (6), 2376-2383. DOI: 10.1021/jf000005f
Some plants such as lettuce require cool conditions for germination (<10 oC), a condition known as thermo-dormancy. This reflects the evolution of the wild parent species in cooler environments and growth cycles limited by higher summer temperatures. Transforming live but dormant seed into new healthy self-sufficient plants requires care and planning. The conditions in which seed is stored before use greatly affect the vigour and quality of plants post-germination. Seed which is stored too long or in unsuitable environments deteriorates resulting in unthrifty seedlings.
Seed is either sown directly into soil or into compost designed especially as an aid for germination. These composts contain carefully balanced nutrient formulae which provide larger proportions of potassium and phosphorus compounds which promote rooting and shoot growth. The amounts of nitrogen needed at and immediately post-germination are limited. Excess nitrogen immediately post-germination will cause over-rapid growth which is susceptible to pest and pathogen damage.
Minor nutrients will also be included in composts which ensures the establishment of efficient metabolic activities free from deficiency disorders. Composts require pH values at ~ 7.0 for the majority of seedlings unless they are of calicifuge (unsuited for calcareous soils) species where lime requirement is limited and the compost pH will be formulated at 6.0. Additionally, the pC will be carefully tuned ensuring correctly balanced ionic content avoiding root burning disorders. Finally, the compost should be water retentive but offering a rooting environment with at least 50 percent of the pore spaces filled with air. Active root respiration is essential while at the same time water is needed as the carrier for nutrient ions.
Seedlings encountering beneficial environments delivering suitable temperatures will germinate into healthy and productive plants.
Some plants such as lettuce require cool conditions for germination (<10 oC), a condition known as thermo-dormancy. This reflects the evolution of the wild parent species in cooler environments and growth cycles limited by higher summer temperatures.
Careful husbandry under protection such as in greenhouses provides plants which can be successfully transplanted into the garden. The soil receiving these should be carefully cultivated, providing an open crumb structure which permits swift and easy rooting into the new environment. It is essential that in the establishment phase plants are free from water stress. Measures which avoid predation from birds such as pigeons may also be required.
Netting or the placing of cotton threads above plants helps as a protection measure. Weeds must be removed otherwise competition will reduce crop growth and encourage pests and diseases, particularly slug browsing. Finally, the gardener will be rewarded for his/her work with a fruitful and enjoyable crop!
Another month starts in the SCIence Garden with no visitors to appreciate the burgeoning growth of fresh new leaves and spring flowers, but that doesn’t mean we should forget about it!
Hopefully in our absence the Laburnum tree in the garden, Laburnum x watereri ‘Vossii’ will be flowering beautifully, its long racemes of golden yellow flowers looking stunning in the spring sunshine!
Laburnum x watereri ‘Vossii’ in the SCIence Garden
This particular cultivar originated in the late 19th century in the Netherlands, selected from the hybrid species which itself is a cross between Laburnum alpinum and L. anagyroides. This hybrid species was named for the Waterers nursery in Knaphill, Surrey and was formally named in a German publication of 1893 (Handbuch der Laubholzkunde, Berlin 3:673 (1893)
The laburnum tree is found very commonly in gardens in the UK, and is noticeable at this time of year for its long chains of golden yellow flowers. However, the beautiful flowers hide a dark side to this plant. The seeds (and indeed all parts) of the tree are poisonous to humans and many animals. They are poisonous due to the presence of a very toxic alkaloid called cytisine (not to be confused with cytosine, a component of DNA). Cytisine has a similar structure to nicotine (another plant natural product), and has similar pharmacological effects. It has been used as a smoking cessation therapy, as has varenicline, which has a structure based on that of cytisine. These molecules are partial agonists at the nicotinic receptor (compared to nicotine which is a full agonist) and reduce the cravings and “pleasurable” effects associated with nicotine.
Cytisine is found in several other plants in the legume family, including Thermopsis lanceolata, which also looks stunning in early summer and Baptisia species, also growing in the SCIence Garden and flowering later in the year.
In 2018 there were 9.6 million deaths from cancer and 33% of these were linked to exposure to tobacco smoke.* Since the link between smoking and lung cancer was established in 1950, the market for smoking cessation therapies has increased enormously. In 2018 it was worth over 18 billion dollars annually worldwide and is projected to increase to 64 billion dollars by 2026.** Staggering! Varenicline, sold under the brand names Champix and Chantix, is one of the most significant smoking cessation therapies apart from nicotine replacement products.
If you see a laburnum tree whilst out on your daily allowed exercise this month, have a thought for its use as a smoking cessation therapy!
* Data from the Cancer Research UK website https://www.cancerresearchuk.org/health-professional/cancer-statistics/worldwide-cancer#heading-Zero accessed May 2020.
Seed is one of Nature’s tiny miracles upon which the human race relies for its food and pleasure.
Each grain contains the genetic information for growth, development, flowering and fruiting for the preponderant plant life living on this planet. And when provided with adequate oxygen, moisture, warmth, light, physical support and nutrients germination will result in a new generation of a species. These vary from tiny short-lived alpines to the monumental redwood trees growing for centuries on the Pacific west coast of America.
Humankind has tamed and selected a few plant species for food and decorative purposes.
Seed head of beetroot, the seeds are in clusters.
Seed of these, especially food plants, is an internationally traded commodity. Strict criteria governed by legal treaties apply for the quality and health of agricultural and many horticultural seeds. This ensures that resultant crops are true to type and capable of producing high grade products as claimed by the companies who sell the seed.
Companies involved in the seed industry place considerable emphasis on ensuring that their products are capable of growing into profitable crops for farmers and growers. Parental seed crops are grown in isolation from farm crops thereby avoiding the potential for genetic cross-contamination. With some very high value seed the parent plants may be grown under protection and pollinated by hand.
Samples of seed are tested under laboratory conditions by qualified seed analysts. Quality tests identify levels of physical contamination, damage which may have resulted in harvesting and cleaning the seed and the proportion of capable of satisfactory germination. There may also be molecular tests which can identify trueness to type. Identifying the healthiness of seed is especially important. The seed coat can carry fungal and bacterial spores which could result in diseased crops. Similarly, some pathogens, including viruses, may be carried internally within seed.
Septoria apicola – seed borne pathogen causing late blight of celery
Pests, especially insects, find seed attractive food sources and may be carried with it. Careful analytical testing will identify the presence of these problems in batches of seed.
The capabilities of seed for producing vigorous plants is particularly important with very high value vegetable and salad crops. Vigour testing is a refined analytical process which tracks the uniformity and speed of germination supplemented with chemical tests determining the robustness of plant cells. Producers rely on the quality, uniformity and maturity rates of crops such as lettuce, green broccoli or cauliflower so they meet the strict delivery schedules set by supermarkets. Financial penalties are imposed for failures in the supply chain.
Biology’s seemingly inert tiny seed grains are essential ingredients of humankind’s existence!
Growing in just about the most challenging of locations in the SCIence Garden are a small group of Helleborus niger. They are planted in a very dry and shady location underneath a large tree sized Escallonia and although they struggled to establish when they were first planted (in May 2017) they are now flowering and growing well.
This plant was first featured as a Horticulture Group Medicinal Plant of the Month in December 2011 and as it is now in the SCIence garden I thought a reprise was in order.
Helleborus is a genus of 15 species of evergreen perennials in the buttercup family, Ranunculaceae. In common with most members of the family, the flowers are radially symmetric, bisexual and have numerous stamen.
Helleborus is the Latin name for the lent hellebore, and niger means black – referring in this species to the roots.
This species is native to the Alps and Appenines. Helleborus niger has pure white flowers, with the showy white parts being sepals (the calyx) and the petals (corolla) reduced to nectaries. As with other hellebores, the sepals persist long after the nectaries (petals) have dropped.
All members of the Ranunculaceae contain ranunculin, an unstable glucoside, which when the plant is wounded is enzymatically broken down into glucose and protoanemonin. This unsaturated lactone is toxic to both humans and animals, causing skin irritation and nausea, vomiting, dizziness and worse if ingested.
Protoanemonin dimerises to form anemonin when it comes into contact with air and this is then hydrolysed, with a concomitant ring-opening to give a non-toxic dicarboxylic acid.
Many hellebores have been found to contain hellebrin, a cardiac glycoside. The early chemical literature suggests that this species also contains the substance but later studies did not find it suggesting that either mis-identified or adulterated material was used in the early studies.
It is reported to contain many other specialized metabolites including steroidal saponins.
This plant has long been used in traditional medicine – in European, Ayurvedic and Unani systems and recent research has been aimed at elucidating what constituents are responsible for the medicinal benefit.
Extract of black hellebore is used sometimes in Germany as an adjuvant treatment for some types of tumour.
A recent paper* reports the results of a safety and efficacy investigation. The Helleborus niger extract tested was shown to exhibit neither genotoxic nor haemolytic effects but it was shown to have anti-angiogenetic effects on human umbilical vein endothelial cells (HUVEC), anti-proliferative effects and migration-inhibiting properties on tumour cells thus supporting its use in cancer treatment.
* Felenda, J.E., Turek, C., Mörbt, N. et al. Preclinical evaluation of safety and potential of black hellebore extracts for cancer treatment. BMC Complement Altern Med 19, 105 (2019) doi:10.1186/s12906-019-2517-5