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.
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.