Plants and Conservation

Plant fibres are obtained from the lignified cells of sclerenchyma and xylem in plants such as flax, hemp and jute. They are described as sustainable because they come from a renewable resource: the plants can be regrown each season, so the supply is not used up in the way that finite oil-based materials are.

Plant fibres are also biodegradable, so once discarded they break down naturally rather than persisting as pollution, and producing them generally uses less energy than synthetic fibres.

One use of plant fibres is to make fabric or textiles — for example, flax fibres are processed into linen. (Rope, paper and composite materials are also acceptable uses.)

(i) Magnesium ions are needed to make chlorophyll. A deficiency causes chlorosis — yellowing of the leaves (typically between the veins) — which reduces photosynthesis.

(ii) Nitrate ions (providing nitrogen) are needed to make amino acids and therefore proteins. A deficiency causes stunted growth and yellowing of the older leaves.

(iii) Calcium ions are needed to build the middle lamella (calcium pectate) that cements adjacent cell walls together. A deficiency causes weak, distorted growth and the death of growing points such as buds and root tips.

The seeds are dried to a low water content (about 5%). A low water content slows the seeds' metabolism and prevents them germinating or decaying, so they stay dormant.

The dried seeds are also stored at a low temperature (around −20 °C). The cold slows metabolism further, so the seeds use up their stored food very slowly and remain viable for many years.

Plants are a source of food, medicines (drugs), fibres and other materials, and many useful plants have not yet been discovered, so conserving species protects resources that humans may need in the future.

Maintaining genetic diversity — the variety of alleles within a species — is important because a large gene pool provides alleles that can be used in crop breeding, for example to introduce disease resistance, drought tolerance or higher yield into commercial crops.

A diverse gene pool also allows a species to adapt to changing conditions, such as new pests or climate change; a genetically uniform population is much more likely to be wiped out by a single disease.

Finally, extinction is irreversible: once a species is lost, its unique combination of alleles and chemicals is gone forever, removing any chance of using it for future drug discovery or crop improvement.

Seeds are first collected from a wide range of plants and populations so that the store captures as much of the species' genetic diversity as possible.

The seeds are then dried, reducing their water content to about 5%. A low water content greatly slows down the seeds' metabolism and prevents them from germinating or decaying while in storage.

Next, the dried seeds are cooled and stored at a low temperature, typically around −20 °C. The cold slows metabolism even further, so the seeds use up their stored food very slowly and remain alive but dormant for many years.

Because seeds gradually lose viability over time, a sample is taken out and germinated periodically to check what proportion still grows. If viability falls too low, the remaining seeds are grown into mature plants and fresh seed is collected to replenish the store.

Together, drying and cooling keep the seeds dormant but viable, allowing a species to be conserved and later replanted or used for crop breeding.

Yellowing of the leaf tissue between the veins, while the veins stay green, is a classic symptom of magnesium deficiency (chlorosis).

Magnesium ions are a component of the chlorophyll molecule. When magnesium is in short supply, the plant cannot make enough chlorophyll, so the affected areas of the leaf lose their green colour and appear yellow. The veins stay green because magnesium tends to be moved preferentially to where it is needed.

Chlorophyll is the pigment that absorbs light for photosynthesis. With less chlorophyll, the leaf absorbs less light, so the rate of photosynthesis falls.

Because photosynthesis produces the glucose the plant needs for respiration and for growth, a reduced rate means the plant grows more slowly and may become stunted.

To correct the symptom, the gardener could add a magnesium-containing fertiliser to the soil, allowing the plant to make chlorophyll again.

The data show that the three plant fibres differ in tensile strength: hemp has the greatest mean breaking force at 35.0 N, followed by flax at 28.0 N, with cotton the weakest at 14.0 N. In other words, hemp and flax can withstand a greater pulling force before breaking than cotton can.

To compare hemp and cotton:

ratio = breaking force of hemp ÷ breaking force of cotton = 35.0 ÷ 14.0 = 2.5.

So hemp fibres are 2.5 times stronger (have 2.5 times the mean breaking force) than cotton fibres.

These fibres are useful sustainable materials because they have a high tensile strength: the cells they come from (sclerenchyma and xylem) are lignified, so they resist pulling forces and can be used for ropes, textiles and composites. They are also renewable, because the plants can be regrown each year, and biodegradable, so they cause less long-lasting pollution than oil-based synthetic fibres such as nylon.

It should be noted that, to make the comparison fair, the fibres should have the same length and diameter, since a thicker fibre would have a greater breaking force regardless of the plant it came from.

There are several environmental advantages to plant-based plastics. First, they are made from a renewable resource: starch and cellulose come from crops that can be regrown each year, whereas crude oil is a finite, non-renewable resource that will eventually run out. Second, many plant-based plastics are biodegradable or compostable, so they break down naturally and cause much less long-lasting pollution than oil-based plastics, which can persist in the environment for hundreds of years. Third, they can have a lower carbon footprint, because the carbon dioxide released when the bioplastic decays was recently absorbed from the air by the crop during photosynthesis, rather than being fossil carbon that has been locked away for millions of years.

However, the claim should not be accepted uncritically. Growing the crops needed for bioplastics uses land, water and fertiliser, and may compete with food production or lead to the clearing of natural habitats, reducing biodiversity. The manufacturing process, the production of nitrogen fertiliser and the transport of the crop may all use fossil fuels, which reduces the saving in carbon. In addition, some plant-based plastics only break down fully under industrial composting conditions, so in an ordinary landfill they may not biodegrade much faster than oil-based plastic.

In conclusion, switching to plant-based plastics is likely to be better for the environment provided that the crops are grown sustainably and the plastic is disposed of correctly. The benefit is real but not automatic: a full life-cycle assessment is needed to confirm a net environmental gain in each case, so the claim is broadly justified but oversimplified.

Seed banks have several important advantages. Because seeds are small, an enormous number of them — representing a wide range of species and a large amount of genetic diversity — can be stored in a small and relatively inexpensive space; storing seeds is far cheaper than maintaining the equivalent number of living adult plants in a botanic garden. The seeds are also kept away from the threats they face in the wild, such as habitat destruction, disease, climate change and natural disasters, so a species can be replanted even if it becomes extinct in its natural habitat. In addition, the stored seeds act as a resource for the future: they provide alleles that plant breeders can use to develop improved crop varieties — for example with disease resistance, drought tolerance or higher yield — and they may be a source of chemicals for future drug discovery.

There are, however, significant limitations. Not all seeds can be stored in this way: some species produce 'recalcitrant' seeds (for instance many tropical and large-seeded plants) that are killed by the drying and freezing process, so these species cannot be conserved in a conventional seed bank. Seed banks are also expensive to establish and run, because they require controlled low-temperature storage, electricity, trained staff and collection expeditions. Seeds gradually lose their viability over time, so a sample must be germination-tested periodically and, when viability falls, the remaining seeds must be grown into plants and fresh seed collected — a labour-intensive process that may inadvertently select plants that are not fully representative of the wild population. Finally, the seeds are conserved out of their natural ecosystem, so the species' interactions with pollinators, soil organisms and the wider habitat are not preserved.

In conclusion, seed banks are a valuable conservation method because they store a great deal of genetic diversity cheaply and protect species from extinction, but they are not a complete solution: they cannot conserve recalcitrant-seeded species, are costly to maintain and do not preserve a species within its ecosystem. They are therefore best used alongside in-situ conservation, such as protected habitats, rather than as a replacement for it.

Wild plants are a vast reservoir of different chemicals and of different alleles, much of which has not yet been studied. Many existing medicines were originally discovered in plants — for example, the painkiller aspirin was developed from a chemical in willow bark, and several anti-cancer drugs come from plants such as the rosy periwinkle and the yew tree. Because only a small fraction of plant species has been screened for medicinal chemicals, undiscovered species may contain compounds that could treat diseases in the future. If those species become extinct before they are studied, their potential medicines are lost forever, because extinction is irreversible.

Wild varieties are equally important for crop breeding. Commercial crops are often grown as monocultures and are genetically very uniform, which makes them vulnerable to being wiped out by a single new pest or disease. The wild relatives of these crops still carry a wide variety of alleles, including ones for useful traits such as disease resistance, drought or salt tolerance, and higher yield. Plant breeders can cross commercial crops with these wild varieties (or transfer their genes) to introduce the beneficial alleles, producing improved varieties that are more resistant or productive.

In short, conserving wild plant varieties keeps a diverse pool of genes and chemicals available. This protects food security by allowing the continued improvement of crops, and it preserves the opportunity to discover new medicines — opportunities that would be permanently lost if the species became extinct.