Gardens and parks provide visual evidence of climate change. Regular observation shows us that our flowering bulbous plants are emerging, growing and flowering. Great Britain is particularly rich in long term recordings of dates of budbreak, growth and flowering of trees, shrubs and perennial herbaceous plants. Until recently, this was dismissed as ‘stamp collecting by Victorian ladies and clerics’.
The science of phenology now provides vital evidence that quantifies the scale and rapidity of climate change. Serious scientific evidence of the impact of climate change comes, for example, from an analysis of 29,500 phenological datasets. This research shows that plants and animals are responding consistently to temperature change with earlier blooming, leaf unfurling, flowering and migration. This scale of change has not been seen on Earth for the past three quarters of a million years. And this time it is happening with increased rapidity and is caused by the activities of a single species – US – humans!
Iris unguicularis (stylosa).
Changing seasonal cycles seriously affects our gardens. Fruit trees bloom earlier than previously and are potentially out of synchrony with pollinators. That results in irregular, poor fruit set and low yields. Climate change is causing increased variability in weather events. This is particularly damaging when short, very sharp periods of freezing weather coincide with precious bud bursts and shoot growth. Many early flowering trees and shrubs are incapable of replacing damaged buds, as a result a whole season’s worth of growth is lost. Damaged buds and shoots are more easily invaded by fungi which cause diseases such as dieback and rotting. Eventually valuable feature plants fail, damaging the garden’s benefits for enjoyment and relaxation.
Plant diseases caused by fungi and bacteria benefit from our increasingly milder, damper winters. Previously, cooling temperatures in the autumn and winter frosts prevented these microbes from over-wintering. Now they are surviving and thriving in the warmer conditions. This is especially the case with soil borne microbes such as those which cause clubroot of brassicas and white rot, which affects a wide range of garden crops.
Hazel (Coryllus spp.) typical wind-pollinated yellow male catkins, which produce pollen.
Can gardeners help mitigate climate change? Of course! Grow flowering plants which are bee friendly; minimise using chemical controls; ban bonfires – which are excellent sources of CO2; establish wildlife-friendly areas filled with native plants and pieces of rotting wood, and it is amazing how quickly beneficial insects, slow worms and voles will populate your garden.
Professor Geoff Dixon is the author of Garden Practices and their Science, published by Routledge 2019.
Understanding organisms’ capabilities of sensing environmental changes such as increasing or declining temperature is becomes ever more important. Deciduous woody trees and shrubs growing in cool temperate and sub-arctic regions enter quiescent or dormant states as protection against freezing temperatures.
These plants pass through a two-stage process. Firstly, they gradually acclimatise (or 'acclimate', in the USA) where lowering temperatures encourage capacities for withstanding cold. This is a reversible process and if there is a spell of milder weather the acclimatisation state is lost. This can happen, for instance, with a fine spell of 'Indian summer' in October or even early November.
Winter weather and dormant trees. All images by Geoff Dixon
Where acclimation is broken, plants become susceptible to cold-induced damage again. If acclimation continues, however, plants eventually become fully dormant. This is not a reversible state and only ends after substantial periods of warming weather and increasing day-length. Some plants will require an accumulation of 'cold-units' – ie, temperatures below a specific level before dormancy is broken.
Detailed research information is accumulating to describe how acclimatisation develops. Changes take place that strengthen cell membranes, possibly by increasing the bonding in lipid molecules, and causing alterations in respiration rates, enzyme activities and hormone levels.
Non-acclimatised azalea (front), acclimatised azalea (back).
Leaves in a non-acclimated state will leak cellular fluids when they are chilled, whereas acclimated leaves are undamaged. These processes result from an interaction between genotype and the environment. Cascades of genes come into play during acclimation and dormancy.
The genus Rhododendron offers a model for studies of these states. Some species originate from alpine environments, such as R. hirsutum coming from the European Alps and one of the first English garden 'rhodos'. By contrast, plants of R. vireya come from tropical areas such as the East Indies.
Comparing the leakage of cellular fluids in acclimatised and non-acclimatised rhododendron leaves subjected to -7°C
Practical outcomes from studies of acclimation and dormancy are twofold. Firstly, are there substances that could be sprayed onto cold susceptible crops, eg potatoes or cauliflowers, that prevent damage? This is so-called 'anti-freeze chemistry'. Some studies suggest that spraying seaweed extracts will dimmish damage. The downside of this approach is that rain washes off the application. Secondly, identifying genes which increase cold hardiness offers possibilities for their transfer into susceptible crops. Gene-editing techniques may offer means of tweaking existing cold-hardiness genes in susceptible crops.
Professor Geoff Dixon is the author of Garden Practices and their Science, published by Routledge 2019.
Fertile soils teem with life of all shapes and sizes, from badgers and moles to insects and the most minute microbes, forming an intricate web of life. Each plays its part – earthworms, for example, burrow through soils opening out channels that improve aeration and water percolation. They are, in Charles Darwin’s words, ‘nature’s ploughs’.
Microbes are quite probably the largest biomass, certainly numerically. The great majority form beneficial relationships with plants, relatively few are pathogens capable of causing crop diseases. Some of the most beneficial are nitrogen-fixing bacteria, which form symbiotic associations with the roots of legumes (clovers, peas and beans).
Their nitrogenase enzymes are capable of combining atmospheric nitrogen with hydrogen-forming ammonia. Followed by conversion into nitrites and nitrates which are made available for the host plant in exchange for carbohydrates, sources of energy for the microbes. The presence of these bacteria is indicated by white nodules on the roots of legumes.
The white nodules on the roots of legumes indicate the presence of nitrogen-fixing bacteria, which provide nourishment to microbes in the soil.
The fungi mycorrhizae also form associations with plant roots. These may form sheaths wrapping round the root, ecto-mycorhizea, or penetrate into the root cortex as endo-mycorrhizea, working in close association with host cells. Mycorrhizae solubilise soil deposits of phosphates and other minerals, making them available for the host. They also provide protection from root-invading plant pathogens.
These fungi utilise carbohydrates supplied by their hosts as energy sources in a similar manner to nitrogen fixing bacteria. Mutualistic mycorrhizal associations are found across most higher plant families with the key exception of the brassicas. This exception quite probably relates to production of the iso-thiocyanate mustard oil, which is fungi-toxic, in brassica roots.
Farmyard manure and compost stimulate soil health by introducing beneficial microbes.
Benefits from nitrogen-fixing bacteria and mycorrhizal fungi were recognised by 19th century agronomists. Much more recently, science has begun uncovering the biological capital of myriad microbes present in healthy soils. Research is being stimulated by recognition of the need for sustainable forms of crop husbandry that utilise ecologically sound techniques in integrated management.
Soil health can be stimulated by incorporation of farmyard manure or well composted green wastes, both containing huge populations of beneficial microbes. The critical importance of building and maintaining healthy soils cannot be over-emphasised. Quite simply, our food supplies depend upon it.Interested in soil health? Why not register for free to attend the 2020 Bright SCIdea Challenge final? One of the teams in this year’s final are pitching their method to restore the fertility of heavy metal ion rich farmland and increase crop yields.
All organisms are fitted for the habitat in which they live. Some are sufficiently flexible in their requirements that they can withstand small shifts in their environment. Others are so well fitted that they cannot withstand habitat change and will eventually fail. The extent of seasonal changes varies with latitude. Plants in temperate and sub-arctic are fitted for changing weather patterns from hot and dry to cold and wet as the calendar moves from summer into winter. Deciduous plants start growing in spring with varying degrees of rapidity and move through flowering and fruiting in summer and early autumn. Finally, some produce a magnificent display of autumn colour, but all senesce and shut down with the return of winter. Evergreen plants frequently inhabit the higher latitudes and retain their foliage. This is an energy conservation measure as they can respond more quickly when winter ends and growth restarts.
Plants respond to seasonal change by sensing alterations in daylength, spectral composition and most importantly temperature. It is known as acclimatisation (acclimation in the American literature). Falling temperatures are the most potent triggers in preparation for winter dormancy. Cold and ultimately freezing weather will seriously damage plant growth where acclimatisation has not been completed. Without preparation freezing ruptures cell membranes in leaves and stems disrupting their normal functions. These effects are measurable and used as means of quantifying plant hardiness. Membrane leakiness correlates with increased ionic concentrations when damaged leaves are placed in water and the resultant pC measured. Changes in chlorophyll fluorescent indicated damaged photosynthetic apparatus and measurable. Similarly, in some species bonding in lipid molecules alters and can be traced by mass spectroscopy. Understanding these processes and their ultimate goal which is protective dormancy underpins more accurate understanding of the natural world. It also provides information useful for breeding cold tolerant crops and garden plants.
Cold Damaged Plant
The rapidity of climate change is such that the protective mechanisms of plants and other organisms cannot respond with sufficient speed. Autumn in cool temperate regions, for example, is now extending as an increasingly warm period. This means that plants are not receiving the triggers necessary for acclimatisation in preparation for severe cold. Buds are commencing growth earlier in spring and now frequently are badly damaged by short bursts of deep cold. These buds cannot be replaced and as a consequence deciduous trees and shrubs in particular are losing capacities for survival.
Soil is a very precious asset whether it be in your garden or an allotment. Soil has physical and chemical properties that support its biological life. Like any asset understanding its properties is fundamental for its effective use and conservation.
Soils will contain, depending on their origin four constituents: sand, clay, silt and organic matter. Mineral soils, those derived by the weathering of rocks contain varying proportions of all four. But their organic matter content will be less than 5 percent. Above that figure and the soil is classed as organic and is derived from the deposition of decaying plants under very wet conditions forming bogs.
Essentially this anaerobic deposition produces peat which if drained yields highly fertile soils such as the Fenlands of East Anglia. Peat’s disadvantage is oxidation, steadily the organic matter breaks down, releases carbon dioxide and is lost revealing the subsoil which is probably a layer of clay.
Cracked clay soil
Mineral soils with a high sand content are free draining, warm quickly in spring and are ‘light’ land. This latter term originates from the small number of horses required for their cultivation. Consequently, sandy soils encourage early spring growth and the first crops. Their disadvantage is limited water retention and hence crops need regular watering in warm weather.
Clay soils are water retentive to the extent that they will become waterlogged during rainy periods. They are ‘heavy’ soils meaning that large teams of horses were required for their cultivation. These soils produce main season crops, especially those which are deeply rooting such as maize. But in dry weather they crack open rupturing root systems and reducing yields.
Silt soils contain very fine particles and may have originated in geological time by sedimentation in lakes and river systems. They can be highly fertile and are particularly useful for high quality field vegetable and salad crops. Because of their preponderance of fine particles silt soils ‘cap’ easily in dry weather. The sealed surface is not easily penetrated by germinating seedlings causing erratic and patchy emergence.
Soil finger test
Soil composition can be determined by two very simple tests. A finger test will identify the relative content of sand, clay and silt. Roll a small sample of moist soil between your thumb and fingers and feel the sharpness of sand particles and the relative slipperiness of clay or the very fine almost imperceptible particles of silt. For a floatation test, place a small soil sample onto the top of a jam jar filled with water. Over 24 to 48 hours the particles will sediment with the heavier sand forming the lower layer with clay and silt deposited on top. Organic matter will float on the surface of the water.
Soil floatation test
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.
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!
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!
Transferring plants between countries was a profitable source for novel commercial and garden plants until quite recently.
Potato crop: Geoff Dixon
Potatoes and tomatoes are classic examples arriving in Europe from South America during the 16th century. Substantial numbers of new plants fuelled empire expansion founding new industries such as rubber and coffee. One of the earliest functions of European botanic gardens was finding potentially valuable new crops for colonial businesses. At home selecting orchids and other exotics from imported plants brought fame and fortune for head gardeners managing the large 19th century estates such as Chatsworth. Commercially seed merchants selected by eye and feel new and improved vegetables, fruit and flowers.
The rediscovery of Mendel’s laws of inheritance brought systematic science and formalised breeding new crops and garden plants. Analysing the effects of transferring physical, chemical and biological characters identified gene numbers and their functions.
Colour range in Gladioli: Geoff Dixon
As a result, varieties with improved colourfulness, fruitfulness, yield and pest and pathogen tolerance fill seedsmen’s catalogues. Breeding increased food supplies and added colour into the gardens springing up in suburban areas as affluence increased.
Greater plant reliability and uniformity arrived with the discovery of F1 hybrids.
Hybrid Sunflowers: Geoff Dixon
Selected parental lines each with very desirable characters such as fruit colour are in-breed for several generations. Then they are crossed bringing an explosion of vigour, uniformity and reliability (known as heterosis). Saving seed from the hybrid lines does not however, perpetuate these characters; new generations come only from remaking the original cross. That is a major boon for the breeder as competitors cannot pirate their intellectual property.
Knowledge at the molecular level has unravelled still further gene structure and functioning. Tagging or marking specific genes with known properties shortens the breeding cycle adding reliability and accuracy for the breeder. Simplifying the volume of genetic material used in crosses by halving the number of chromosomes involved adds further precision and control (known as haploidisation).
Opportunities for breeding new plants increases many-fold when advantageous genes are transferred between species. Recent developments of gene-editing where tailored enzymes very precisely snip out unwanted characters and insert advantageous ones is now offering huge opportunities as a non-transgenic technology. Breeding science makes possible mitigation of climate change, reducing for example the impact of soil degradation brought about by flooding.
Flood degraded land: Geoff Dixon
Every garden centre will currently bombard you with colourful displays of seed packets (figure 1). Each contains tiny grains of dormant life. Provided with water, warmth, suitable soil or compost and eventually light (figure 2) that resting grain will transform into the roots and shoots of a new plant.
Image 1: Racks of seed packets
Inside that seed cascades of genes trigger enzymes which release energy from stored starch and in some cases lipids. As a result, the seed coat opens and a root emerges which takes in supplies of water and nutrients. Shoots follow which grow upwards towards the light. They turn green as chlorophyll is manufactured and photosynthesis commences. At that point the seemingly inert grain becomes a self-sustaining living plant. Root and shoot growth result from active cell divisions with genetic controls determining the form and functions of each organ.
Image 2: Germinating seeds and the correct conditions
Each seed’s compliment of genes will determine what type of plant develops. But it is the environment provided by the gardener which determines the plant’s success. Careful and accurate husbandry results in succulent, health-promoting vegetables or colourful, vigorous flowers. Seedlings of some plants may be given nursery treatment before being placed into the garden’s big wide world. Providing protection in the early stages either in a green house or under cloches for many annual flowers and most vegetables boosts growth (figure 3) and eventually the quality of the produce.
Image 3: Legumes grown under protection
This does require time, skill and investment by the gardener. An alternative is purchasing seedlings from garden centres (figure 4). But an element of caution is required. These plants will have been raised under protection. Hence planting directly into the garden means still need care and attention. Frost protection and watering are essential, otherwise poor results may follow.
Image 4: Garden centre seedlings
Direct sowing seeds into garden soil is another alternative. Hardy vegetables and annual flowers may be cultured in this way. The requirements for success are a fertile soil with a fine tilth, that means it is free from stones and consists of uniform, aggregated particles allowing unimpeded movement of air and water.
Vegetables such as beetroot, carrots and parsnips will grow vigorously given these conditions. Hardy annuals such as African daisy, larkspur, love-in-the-mist, marigold and nasturtium will also thrive from direct sowings. Success in both garden departments depends on watering during dry spells and supplementary nutrition. Avoid nitrogenous fertilizers as these will encourage leaf growth whereas phosphate (P) and potassium (K) will promote root and flower formation.
Holly berries are emblematic of Christmas. Decorative wreaths containing sprays of holly boughs, bright red with berries, or sprigs set on cakes and puddings help bring seasonal cheer.
Holly is a problem for horticulturists! Male and female flowers develop separately requiring cross-pollination before fertilised berries develop. Dutch nurserymen got around this by selecting a self-fertile variety ‘J. C Van Tol’ which sets copious berries. Adding further colour in the winter garden is the variety ‘Golden King’ producing mixtures of creamy-white and green foliage. Most hollies in Great Britain are Ilex aquifolium which is a native of Northern Europe and is still found wild in the Welsh Marches. It is a flexible and valuable garden evergreen, very suitable for hedges as they form tough, prickly, impenetrable barriers.
Why plants use considerable energy to produce brightly coloured fruits is a puzzle for botanists. Co-evolution is an explanation. Bright berries attract birds which eat them, digesting the flesh and excreting the seeds. Wide seed distribution accompanied by a package of manure helps spread these plants increasing their geographical range.
Which came first, bright berries or vectoring birds? A combination is the answer. Plants with brighter berries attracted more birds spreading their seed more widely. Brighter berries are more nutritious and hence those birds which ate them were stronger and better fitted for the rigours of winter. Garden residents such as blackbirds and thrushes now thrive and survive on such natural food. Migratory species such as fieldfares travel from Scandinavia, attracted particularly by other berried treasures such as Cotoneaster.
Fleshy fruits such as those of holly or Cotoneaster are examples of some of the last energy sinks formed in the gardening year.
They draw products of photosynthesis from the manufacturing centres in leaves and accumulate sugars plus nutrients drawn up from the soil via root systems. That provides a rich diet for birds.
While digestive acids in the vector’s gut starts degrading the hard shell which surrounds the seed at the centre of the berry. Botanically that term is a misnomer since true berries, such as gooseberry fruits contain several seeds. Holly has one seed contained within a hard case encased in flesh and should be a drupe! Not a term which fits well for Christmas carols, decorations or cards!
Merry Christmas and a Prosperous New Year.
Gooseberries- true berry
Springtime colour is one of gardening’s greatest joys. Colourful bursts dispel the long darkness of winter with its depressing wetness and cold. Social research is clearly showing the physical and mental benefits obtained from the emergence in spring of bright garden colours linked with lengthening daylight. As with most gardening pleasures, this requires advanced financial outlay and an understanding of the rhythms of plant growth.
Planting bulbs such as daffodils, tulips and hyacinths in autumn is the necessary investment. In return, plant breeders now provide a huge array of colours, shapes, sizes and seasonal sequencing with bulbous plants.
Geoff Dixon: February Gold daffodils
Bulbs are large pieces of vegetative tissue which come pre-loaded with immature leaves and flowers, safely wrapped inside a dry coating of protective scales. Essentially, bulbs are large flower buds which are stimulated into growth by planting in warm, moist soil or compost. These conditions trigger the emergence of roots from the base of each bulb. Because bulbs are nascent plants, they require careful handling and are safest once planted.
Many bulbous species originate from higher altitude mountainous pastures and are naturally evolved for dealing with fluctuating periods of heat, cold and drought. Once safely planted at depths which should equal twice the length of each bulb, they will survive the freezing, thawing and fluctuating soil water- content delivered by winter weather.
Geoff Dixon: Bulb structure showing the flower bud embedded in the bulb
Warming soils of spring encourage growth and emergence of the leaves and flower buds contained within each bulb. Speed of emergence is governed by interaction between the genetic complement of bulbs and an interaction with their environment. Identifying and understanding the impact of this interaction formed the basis for Charles Darwin and Alfred Wallaces’ theory of natural selection. For springtime gardeners it is expressed in the multiplicity of bulbs on offer. Choosing a range of daffodil varieties for example, provides colourful gardens from February through to late May.
Geoff Dixon: Technique for planting bulbs using hand trowel and some sand for drainage under the bulb
Conserving the joys of spring pleasure over years can be achieved by naturalising bulbs. This means planting them in grass swards. This works effectively for daffodils, provided the foliage is allowed 8 to 10 weeks of uninterrupted growth and senescence after flowering. During this period, photosynthesis produces the chemical energy needed for replacement growth, which provides bulb multiplication and flower bud development for the following year. Tulips are much less easily naturalised in British gardens. This is because the leaves mature and senesce much more quickly after flowering, hence, less energy is produced, therefore, regrowth is less, and replacement flower buds are not formed.
For most gardeners the policy should be one of enjoying each springtime’s show and replacing bulbs with new ones every autumn for a relatively modest outlay.
Aldrin, Armstrong and Collins, Apollo 11’s brave astronauts were the first humans with the privilege of viewing Earth from another celestial body. These men uniquely wondered “what makes Earth special?” Certainly, within our Solar System, planet Earth is very special. Its environment has permitted the evolution of a panoply of life.
Green plants containing the pigment, chlorophyll either in the oceans as algae or on land as a multitude of trees, shrubs and herbs harvest energy from sunshine. Using a series of chemical reactions, known as photosynthesis, light energy is harvested and attached onto compounds containing phosphorus.
Captured energy then drives a series of reactions in which atmospheric carbon dioxide and water are combined forming simple sugars while releasing oxygen. These sugars are used further by plants in the manufacture of larger carbohydrates, amino acids and proteins, oils and fats.
The release of oxygen during photosynthesis forms the basis of life’s second vital process, respiration. Almost all plants and animals utilise oxygen in this energy releasing process during which sugars are broken down.
Released energy then drives all subsequent growth, development and reproduction. These body-building processes in plants are reliant on the transfer of the products of photosynthesis from a point of manufacture, the source, to the place of use, a sink.
Leaves and shoots are the principle sources of energy harvesting while flowers and fruits are major sinks with high levels of respiration.
Figure 1: Photosynthesis vs respiration, drawn by James Hadley
Transfer between sources and sinks occurs in a central system of pipes, the vascular system, using water as the carrier. Water is obtained by land plants from the soils in which they grow. Without water there would be no transfer and subsequent growth. Earth’s environment is built around a ‘water-cycle’ supplying the land and oceans with rain or snow and recycles water back into the atmosphere in a sustainable manner.
Early in Earth’s evolution, very primitive marine organisms initiated photosynthetic processes, capturing sunlight’s energy. As a result, in our atmosphere oxygen became a major component. That encouraged the development of the vast array of land plants which utilise rain water as the key element in their transport systems.
Subsequently, plants formed the diets of all animals either by direct consumption as herbivores or at second-hand as carnivores. As a result, evolution produced balanced ecosystems and humanity has inherited what those astronauts saw, “the Green Planet”.
Earth will only retain this status if humanity individually and collectively defeats our biggest challenge – climate change. Burning rain forests in South America, Africa and Arctic tundra will disbalance these ecosystems and quicken climate change.
Controlling when and how vigorously plants flower is a major discovery in horticultural science. Its use has spawned vast industries worldwide supplying flowers and potted plants out-of-season. The control mechanism was uncovered by two American physiologists in the 1920s. Temperate plants inhabit zones where seasonal daylength varies between extending light periods in spring and decreasing ones in autumn.
Those environmental changes result in plants which flower in long-days and those which flower in short-days. ‘Photoperiodism’ was coined as the term describing these events. Extensive subsequent research demonstrated that it is the period of darkness which is crucially important. Short-day plants flower when darkness exceeds a crucial minimum, usually about 12 hours which is typical of autumn. Long-day plants flower when the dark period is shorter than the crucial minimum.
Irises are long day flowers. Image: Geoffery R Dixon
A third group of plants usually coming from tropical zones are day-neutral; flowering is unaffected by day-length. Long-day plants include clover, hollyhock, iris, lettuce, spinach and radish. Gardeners will be familiar with the way lettuce and radish “bolt” in early summer. Short-day plants include: chrysanthemum, goldenrod, poinsettia, soybean and many annual weed species. Day-neutral types include peas, runner and green beans, sweet corn (maize) and sunflower.
Immense research efforts identified a plant pigment, phytochrome as the trigger molecule. This exists in two states, active and inactive and they are converted by receiving red or far-red wavelengths of light.
Sunflowers are day neutral flowers. Image: Geoffery R Dixon
In short-day plants, for example, the active form suppresses flowering but decays into the inactive form with increasing periods of darkness. But a brief flash of light restores the active form and stops flowering. That knowledge underpins businesses supplying cut-flowered chrysanthemums and potted-plants and supplies of poinsettias for Christmas markets. Identifying precise demands of individual cultivars of these crops means that growers can schedule production volumes gearing very precisely for peak markets.
Providing the appropriate photoperiods requires very substantial capital investment. Consequently, there has been a century-long quest for the ‘Holy Grail of Flowering’, a molecule which when sprayed onto crops initiates the flowering process.
Chrysanthemums are short day flowers. Image: Geoffery R Dixon
In 2006 the hormone, florigen, was finally identified and characterised. Biochemists and molecular biologists are now working furiously looking for pathways by which it can be used effectively and provide more efficient flower production in a wider range of species.
A lavender field near Provence, France.
Flowering is the process by which higher plants transfer male gametes to female organs thereby uniting two sets of chromosomes and increasing natural diversity. During the formation of male and female gametes, slight changes take place in chromosome structure. Consequently, the resultant next generation differs slightly from its parents. That is the stuff on which natural selection operates.
Useful variations increase the survival fitness of some offspring, while individuals with disadvantages wither and die. Charles Darwin recognised the power of natural selection for the environmentally fittest individuals and how that leads eventually to species evolution. Succeeding generations of scientists have discovered details of the processes involved and how these may result in more useful plants for humankind by plant breeding.
Transferring the male gametes (i.e. pollination) happens by a variety of mechanisms which are suited for the environment in which particular plants grow. At its simplest, pollen which consists of cells containing male gametes is transferred within the same flower. That is suitable for plants growing in for example, alpine environments where few other options exist.
Pollen grains contain both reproductive and non-reproductive cells.
Cross-transfer of pollen from one flower to another is achieved either by physical means such as wind or water, or by partnerships with animals – particularly insects and especially bees. Wind transfer is suitable for trees such as hazel, birch and willow, which flower ahead of leaf formation in the early spring when it is too cold for insect flight. Biologically, it is a wasteful mechanism because much of the pollen does not reach its target.
Cross-pollination by insects produces by far the most colourful and exuberant flowers. These have evolved brilliantly colourful displays and intricate mechanisms suitable for either general interaction with insects or as means for partnership. These relationships have co-evolved and converged over numerous generations meeting the needs of both parties.
Sexual reproduction in plants. Video: FuseSchool - Global Education
Plant scientists are presented with intriguing questions in understanding how these relationships could have developed. On the practical side, plant breeders are presented with enormous opportunities for developing massive arrays of new varieties, particularly with ornamentals such as the garden favourites like dahlias, chrysanthemums, lilies and roses.
Enormous international trade has developed over the last hundred years exploiting increasingly colourful flowering plants.
An estimated 24% of Europe’s bumblebees are threatened with extinction.
Cross-pollination is absolutely vital for many field vegetable crops, especially peas and beans and the top and soft fruits. A reduction in beneficial insect populations now presents dire threats for natural biodiversity, our food supplies and the enjoyment of ornamentals.