Avoidance Strategies
Hardiness Strategy
Hardiness of plants is a term used to describe their ability to survive adverse growing conditions. It is usually limited to discussions of climatic adversity. Thus a plant's ability to tolerate cold, heat, drought, flooding or wind are typically considered measurements of hardiness. The hardiness of plants is defined by their native extent's geographic location: longitude, latitude and elevation. These attributes are often simplified to define a hardiness zone. In temperate latitudes, the term is most often used to describe resistance to cold, or 'cold-hardiness' and is generally measured by the lowest temperatures that a plant can withstand. The hardiness of a plant is usually divided into three categories; tender, half-hardy and hardy.
Plants vary a lot in their tolerance of growing conditions. The selective breeding of varieties capable of withstanding particular climates forms an important part of agriculture and horticulture. Plants can adapt to some extent to changes in climate. Part of the work of nursery growers of plants consists of cold hardening, or hardening off, their plants, to prepare them for likely conditions in their later life.
US Hardiness Zones:
Shade Tolerance
Shade tolerance is a relative term, and its use and meaning depends on context. One can compare large trees to each other, but when comparing under story trees and shrubs, or non-woody plants, the term takes on a different meaning. Even in a specific context, shade tolerance is not a single variable or simple continuum, but rather a complex, multi-faceted property of plants, since different plants exhibit different adaptations to shade.
Except for some parasitic plants, all plants need sunlight to survive. However, in general, more sunlight does not always make it easier for plants to survive. Where water is scarce, life can actually be easier in the shade. In direct sunlight, plants face desiccation and exposure to UV rays, and must expend energy producing pigments to block UV light, and waxy coatings to prevent water loss.
Plants in shade have the ability to absorb far-red light (730nm). Red light gets absorbed by the shade intolerant plants, but the far-red penetrate the canopy, reaching the under story. The shade tolerant plants found here have the ability to absorb light at this wavelength. On the other hand, when less light is available, less energy is available to the plant. Whereas in sunny and dry environments water can be a limiting factor in growth and survival, in shade, energy is usually the limiting factor. The situation with respect to nutrients is often different in shade and sun. Most shade is due to the presence of a canopy of other plants, and this is usually associated with a completely different environment richer in soil nutrients than sunny areas. Shade tolerant plants are thus adapted to be efficient energy-users. In simple terms, shade-tolerant plants grow broader, thinner leaves, to catch more sunlight relative to the cost of producing the leaf. Shade tolerant plants are also usually adapted to make more use of soil nutrients than shade intolerant plants.
List of avoidance strategies
Mesophytes
In temperate zones, many wildflowers and non-woody plants persist in the closed canopy of a forest by leafing out early in the spring, before the trees leaf out. This is partly possibly because the ground tends to be more sheltered and thus the plants are less susceptible to frost, during the period of time when it would still be hazardous for trees to leaf out. As an extreme example of this, winter annuals sprout in the fall, grow through the winter and flower and die in the spring.
Just like with trees, shade-tolerance in herbaceous plants is diverse. Some early-leafing out plants will persist after the canopy leafs out, whereas others rapidly die back. In many species, whether or not this happens depends on the environment, such as water supply and sunlight levels.
Although most plants grow towards light, many tropical vines, such as Monstera deliciosa (and a number of other members of the Philodendron genus) initially grow away from light; this helps them locate a tree trunk, which they then climb to regions of brighter light.
Xerophytes
Drought tolerance refers to the degree to which a plant is adapted to arid or drought conditions. Desiccation tolerance is an extreme degree of drought tolerance. Plants naturally adapted to dry conditions are called xerophytes.
Drought tolerant plants typically make use of either C4 carbon fixation or crassulacean acid metabolism (CAM) to fix carbon during photosynthesis. Both are improvements over the more common but more basal C3 pathway in that they are more energy efficient. CAM is particularly good for arid conditions because carbon dioxide can be taken up at night, allowing the stomata to stay closed during the heat of day and thus reducing water loss.
Many adaptations for dry conditions are structural, including the following:
Adaptations of the stomata to reduce water loss, such as reduced numbers or waxy surfaces.
Water storage in succulent above-ground parts or water-filled tubers.
Adaptations in the root system to increase water absorption.
Trichomes (small hairs) on the leaves to absorb atmospheric water.
Halophytes
A halophyte is a plant that naturally grows where it is affected by salinity in the root area or by salt spray, such as in saline semi-deserts, mangrove swamps, marshes and sloughs, and seashores. An example of a halophyte is the salt marsh grass Spartina alterniflora. Relatively few plant species are halophytes - perhaps only 2% of all plant species. The large majority of plant species are "glycophytes" and are damaged fairly easily by salinity.
One quantitative measure of salt tolerance is the "total dissolved solids" in irrigation water that a plant can tolerate. Sea water typically contains 40 grams per litre of (g/l) dissolved salts. Beans and rice can tolerate about 1-3 g/l, and are considered glycophytes (as are most crop plants). At the other extreme, Salicornia bigelovii (dwarf glasswort) grows well at 70 g/l of dissolved solids, and is a promising halophyte for use as a crop. Plants such as barley (Hordeum vulgare) and the date palm (Phoenix dactylifera) can tolerate about 5 g/l, and can be considered as marginal halophytes.
Adaptation to saline environments by halophytes may take the form of salt tolerance or salt avoidance. Plants that avoid the effects of high salt even though they live in a saline environment may be referred to as facultative halophytes rather than 'true', or obligatory, halophytes.
For example, a short-lived plant species that completes its reproductive life cycle during periods when the salt concentration is low would be avoiding salt rather than tolerating it. Or a plant species may maintain a 'normal' internal salt concentration by excreting excess salts through its leaves or by concentrating salts in leaves that later die and drop off.
Carbon Gaining and Water Loss
Carbon dioxide, a key reactant in photosynthesis, is present in the atmosphere at a concentration of about 384 ppm. Most plants require the stomata to be open during daytime. The problem is that the air spaces in the leaf are saturated with water vapour, which exits the leaf through the stomata. Therefore, plants cannot gain carbon dioxide without simultaneously losing water vapour.
Alternative approaches
Ordinarily, carbon dioxide is fixed to ribulose-1, 5-bisphosphate (BTAC) by the enzyme RuBisCO in mesophyll cells exposed directly to the air spaces inside the leaf. This exacerbates the carbon/water tradeoff for two reasons: first, Rubisco has a relatively low affinity for carbon dioxide and second, it fixes oxygen to RuBP, wasting energy and carbon in a process called photorespiration. For both of these reasons, Rubisco needs high carbon dioxide concentrations, which means high stomata apertures and consequently high water loss.
However, plants possess another enzyme that can also fix carbon dioxide: PEP carboxylase or BTAC. This enzyme has high carbon dioxide affinity, so a given rate of carbon dioxide fixation can be achieved with less stomata opening, and hence less water loss. The catch is that the products of carbon fixation by PEP Case must be converted in an energy-intensive process to continue through the carbon reactions of photosynthesis. As a result, the PEP Case alternative is only preferable where water is more limiting but light — which provides the energy in this case — is plentiful, and/or where high temperatures increase the solubility of oxygen relative to that of carbon dioxide, magnifying Rubisco's oxygenation problem.
Crassulacean acid metabolism plants
A group of mostly desert plants called Crassulacean acid metabolism or CAM plants open their stomata at night, use PEPcarboxylase to fix carbon dioxide and store the products in large vacuoles. The following day, they close their stomata and release the carbon dioxide fixed the previous night into the presence of RuBisCO. This saturates RuBisCO with carbon dioxide, allowing minimal photorespiration. This approach, however, is severely limited by the capacity to store fixed carbon in the vacuoles, so it is preferable only when water is severely limiting.
However, most plants do not have the aforementioned facility and must therefore open and close their stomata during the daytime in response to changing conditions, such as light intensity, humidity, and carbon dioxide concentration. It is not entirely certain how these responses work. However, the basic mechanism involves regulation of osmotic pressure.
When conditions are conducive to stomata opening, a proton pump drives protons (H+) from the guard cells. This means that the cells' electrical potential becomes increasingly negative. The negative potential opens potassium voltage - gated channels and so an uptake of potassium ions (K+) occurs. To maintain this internal negative voltage so that entry of potassium ions does not stop, negative ions balance the influx of potassium. In some cases chloride ions enter, while in other plants the organic ion malate is produced in guard cells. This in turn increases the osmotic pressure inside the cell, drawing in water through osmosis. This increases the cell's volume and turgor pressure. Then, because of rings of cellulose microfilbrils that prevent the width of the guard cells from swelling, and thus only allow the extra turgor pressure to elongate the guard cells, whose ends are held firmly in place by surrounding epidermal cells, the two guard cells lengthen by bowing apart from one another, creating an open pore through which gas can move.
When the roots begin to sense a water shortage in the soil, abscisic acid (ABA) is released. ABA binds to receptor proteins in the guard cells' plasma membrane and cytosol, which first raises the pH of the cytosol of the cells and causes the concentration of free Ca2+ to increase in the cytosol due to influx from outside the cell and release of Ca2+ from internal stores such as the endoplasmic reticulum and vacuole. This causes the chloride (Cl-) and inorganic ions to exit the cells. Secondly, this stops the uptake of any further K+ into the cells and subsequently the loss of K+. The loss of these solutes causes a reduction in osmotic pressure, thus making the cell flaccid and so closing the stomata pores. Interestingly, guard cells have more chloroplasts than the other epidermal cells from which guard cells are derived. Their function is controversial.
Inferring stomata behaviour from gas exchange
One way to determine the degree of stomata opening in a leaf is by measuring leaf gas exchange. A leaf is enclosed in a sealed chamber and air is driven through the chamber. By measuring the concentrations of carbon dioxide and water vapour in the air before and after it flows through the chamber, one can calculate the rate of carbon gain and water loss by the leaf.
However, because water loss occurs by diffusion, the transpiration rate depends on two things: the gradient in humidity from the leaf's internal air spaces to the outside air, and the diffusion resistance provided by the stomata pores. Stomata resistance can therefore be calculated from the transpiration rate and humidity gradient. The humidity gradient is the humidity inside the leaf, determined from leaf temperature based on the assumption that the leaf's air spaces are saturated with vapor, minus the humidity of the ambient air, which is measured directly. This allows scientists to learn how stomata respond to changes in environmental conditions, such as light intensity and concentrations of gases such as water vapor, carbon dioxide and ozone.
The fossil record has little to say about the evolution of stomata. They may have evolved by the modification of conceptacles from plants' alga-like ancestors. It is clear, however, that the evolution of stomata must have happened at the same time as the waxy cuticle was evolving - these two traits together constituted a major advantage for primitive terrestrial plants.
There are three major epidermal cell types which all ultimately derive from the L1 tissue layer of the shoot apical meristem, called protodermal cells: trichomes, pavement cells and guard cells, all of which are arranged in a nonrandom fashion. An asymmetrical cell division occurs in protodermal cells resulting in one large cell that is fated to become a pavement cell and a smaller cell called a meristemoid that will eventually differentiate into the guard cells that surround a stoma. This meristemoid then divides asymmetrically one to three times before differentiating into a guard mother cell. The guard mother cell then makes one symmetrical division, which forms a pair of guard cell. Stomata are an obvious hole in the leaf by which, as was presumed for a while, pathogens can enter unchallenged. However, it has been recently shown that stomata do in fact sense the presence of some, if not all, pathogens. However, with the virulent bacteria applied to Arabidopsis plant leaves in the experiment, the bacteria released the chemical coronatine, which forced the stomata open again within a few hours
Acydophobes and Calcicophobes
Plants are known to be well-defined with respect to their pH tolerance, and only a small number of species thrive well under a broad range of acidity. Therefore the categorisation acidophile/acidophobe is well-defined. Sometimes a complementary classification is used (calcicole, with calcicoles being 'lime-loving' plants). In gardening, soil pH is a measure of acidity or alkalinity of soil, with pH=7 indicating the neutral soil. Therefore acydophobes would prefer pH above 7. Acid intolerance of plants may be mitigated by lime addition and by calcium and nitrogen fertilisers.
Acidophilic species are used as a natural instrument of monitoring the degree of acidifying contamination of soil and watercourses. For example, when monitoring vegetation, a decrease of acidophilic species would be indicative of acid rain increase in the area. A similar approach is used with aquatic species.
White worms (Enchytraeus albidus), a popular live food for aquarists, are acidophobes.
Acidophobic compounds are the ones which are unstable in acidic media.
Acidophobic crops: alfalfa, clover
Disease, Disorder and Pest Tolerance
This is crucial to the reliable production of food, and it provides significant reductions in agricultural use of fuel, land, water and other inputs. There are numerous examples of devastating plant disease impacts, as well as recurrent severe plant disease issues. However, disease control measures are reasonably successful for most crops. Across large regions and many crop species, it is estimated that diseases typically reduce plant yields by 10% every year in more developed settings, but yield loss to diseases often exceeds 20% in less developed settings.
Plant disease resistance derives both from pre-formed defences and from infection-induced responses mediated by the plant immune system. Relative to a disease-susceptible plant, disease resistance is often defined as reduction of pathogen growth on or in the plant, while the term disease tolerance describes plants that exhibit less disease damage despite similar levels of pathogen growth. Disease outcome is determined by the three-way interaction of the pathogen, the plant, and the environmental conditions (an interaction known as the disease triangle). Defence-activating compounds can move cell-to-cell and systemically through the plant vascular system, but plants do not have circulating immune cells so most cell types in plants retain the capacity to express a broad suite of antimicrobial defences. Although obvious qualitative differences in disease resistance can be observed when some plants are compared (allowing classification as 'resistant' or 'susceptible' after infection by the same pathogen strain at similar pathogen inoculum levels in similar environments), a gradation of quantitative differences in disease resistance is more typically observed between plant lines or genotypes. Plants are almost always resistant to certain pathogens but susceptible to other pathogens; resistance is usually pathogen species-specific or pathogen strain-specific.
Common Mechanisms of Disease Resistance
Structures and compounds that contribute to resistance
Plant cuticle/surface
Plant cell walls
Antimicrobial chemicals (for example: glucosides and saponins)
Antimicrobial proteins
Enzyme inhibitors
Detoxifying enzymes that break down pathogen-derived toxins
Receptors that perceive pathogen presence and activate inducible plant defences
Inducible plant defences that are generated after infection
Cell wall reinforcement (callose, lignin, suberin, cell wall proteins)
Antimicrobial chemicals (including reactive oxygen species such as hydrogen peroxide, or peroxynitrite, or more complex phytoalexins such as genistein or camalexin)
Antimicrobial proteins such as defensins, thionins or PR-1
Antimicrobial enzymes such as chitinases, beta-glucanases, or peroxidases
Hypersensitive response - a rapid host cell death response associated with defence mediated by resistance genes.
An example of this adaptation occurs in lower forms of plant forms including mosses and ferns. The complete organism is genetically programmed to exist only during the most favourable periods of the year. The most taxing issue with avoidance theory is the accomplishment of both vegetative growth and reproduction, within the short lifespan. A good example of a niche that uses this strategy is the dessert dwelling plants.