Ecophysiology
Ecophysiology, environmental physiology or physiological ecology is a biological discipline that studies the response of an organism's physiology to environmental conditions. It is closely related to comparative physiology and evolutionary physiology. Ernst Haeckel's coinage bionomy is sometimes employed as a synonym.
Plants
Plant ecophysiology is concerned largely with two topics: mechanisms and scaling or integration, and how their collective effect on plant growth and gas exchange can be understood on this basis.In many cases, animals are able to escape unfavourable and changing environmental factors such as heat, cold, drought or floods, while plants are unable to move away and therefore must endure the adverse conditions or perish. Plants are therefore phenotypically plastic and have an impressive array of genes that aid in acclimating to changing conditions. It is hypothesized that this large number of genes can be partly explained by plant species' need to live in a wider range of conditions.
Light
is the food of plants, i.e. the form of energy that plants use to build themselves and reproduce. The organs harvesting light in plants are leaves and the process through which light is converted into biomass is photosynthesis. The response of photosynthesis to light is called light response curve of net photosynthesis. The shape is typically described by a non-rectangular hyperbola. Three quantities of the light response curve are particularly useful in characterising a plant's response to light intensities. The inclined asymptote has a positive slope representing the efficiency of light use, and is called quantum efficiency; the x-intercept is the light intensity at which biochemical assimilation balances leaf respiration so that the net CO2 exchange of the leaf is zero, called light compensation point; and a horizontal asymptote representing the maximum assimilation rate. Sometimes after reaching the maximum assimilation declines for processes collectively known as photoinhibition.As with most abiotic factors, light intensity can be both suboptimal and excessive. Suboptimal light typically occurs at the base of a plant canopy or in an understory environment. Shade tolerant plants have a range of adaptations to help them survive the altered quantity and quality of light typical of shade environments.
Excess light occurs at the top of canopies and on open ground when cloud cover is low and the sun's zenith angle is low, typically this occurs in the tropics and at high altitudes. Excess light incident on a leaf can result in photoinhibition and photodestruction. Plants adapted to high light environments have a range of adaptations to avoid or dissipate the excess light energy, as well as mechanisms that reduce the amount of injury caused.
Light intensity is also an important component in determining the temperature of plant organs.
Temperature
In response to extremes of temperature, plants can produce various proteins. These protect them from the damaging effects of ice formation and falling rates of enzyme catalysis at low temperatures, and from enzyme denaturation and increased photorespiration at high temperatures. As temperatures fall, production of antifreeze proteins and dehydrins increases. As temperatures rise, production of heat shock proteins increases. Metabolic imbalances associated with temperature extremes result in the build-up of reactive oxygen species, which can be countered by antioxidant systems. Cell membranes are also affected by changes in temperature and can cause the membrane to lose its fluid properties and become a gel in cold conditions or to become leaky in hot conditions. This can affect the movement of compounds across the membrane. To prevent these changes, plants can change the composition of their membranes. In cold conditions, more unsaturated fatty acids are placed in the membrane and in hot conditions, more saturated fatty acids are inserted.Plants can avoid overheating by minimising the amount of sunlight absorbed and by enhancing the cooling effects of wind and transpiration. Plants can reduce light absorption using reflective leaf hairs, scales, and waxes. These features are so common in warm dry regions that these habitats can be seen to form a 'silvery landscape' as the light scatters off the canopies. Some species, such as Macroptilium purpureum, can move their leaves throughout the day so that they are always orientated to avoid the sun. Knowledge of these mechanisms has been key to breeding for heat stress tolerance in agricultural plants.
Plants can avoid the full impact of low temperatures by altering their microclimate. For example, Raoulia plants found in the uplands of New Zealand are said to resemble 'vegetable sheep' as they form tight cushion-like clumps to insulate the most vulnerable plant parts and shield them from cooling winds. The same principle has been applied in agriculture by using plastic mulch to insulate the growing points of crops in cool climates in order to boost plant growth.
Water
Too much or too little water can damage plants. If there is too little water then tissues will dehydrate and the plant may die. If the soil becomes waterlogged then the soil will become anoxic, which can kill the roots of the plant.The ability of plants to access water depends on the structure of their roots and on the water potential of the root cells. When soil water content is low, plants can alter their water potential to maintain a flow of water into the roots and up to the leaves. This remarkable mechanism allows plants to lift water as high as 120 m by harnessing the gradient created by transpiration from the leaves.
In very dry soil, plants close their stomata to reduce transpiration and prevent water loss. The closing of the stomata is often mediated by chemical signals from the root. In irrigated fields, the fact that plants close their stomata in response to drying of the roots can be exploited to 'trick' plants into using less water without reducing yields. The use of this technique was largely developed by Dr Peter Dry and colleagues in Australia
If drought continues, the plant tissues will dehydrate, resulting in a loss of turgor pressure that is visible as wilting. As well as closing their stomata, most plants can also respond to drought by altering their water potential and increasing root growth. Plants that are adapted to dry environments have a range of more specialized mechanisms to maintain water and/or protect tissues when desiccation occurs.
Waterlogging reduces the supply of oxygen to the roots and can kill a plant within days. Plants cannot avoid waterlogging, but many species overcome the lack of oxygen in the soil by transporting oxygen to the root from tissues that are not submerged. Species that are tolerant of waterlogging develop specialised roots near the soil surface and aerenchyma to allow the diffusion of oxygen from the shoot to the root. Roots that are not killed outright may also switch to less oxygen-hungry forms of cellular respiration. Species that are frequently submerged have evolved more elaborate mechanisms that maintain root oxygen levels, such as the aerial roots seen in mangrove forests.
However, for many terminally overwatered houseplants, the initial symptoms of waterlogging can resemble those due to drought. This is particularly true for flood-sensitive plants that show drooping of their leaves due to epinasty.
concentration
is vital for plant growth, as it is the substrate for photosynthesis. Plants take in through stomatal pores on their leaves. At the same time as enters the stomata, moisture escapes. This trade-off between gain and water loss is central to plant productivity. The trade-off is all the more critical as Rubisco, the enzyme used to capture, is efficient only when there is a high concentration of in the leaf. Some plants overcome this difficulty by concentrating within their leaves using carbon fixation or Crassulacean acid metabolism. However, most species used carbon fixation and must open their stomata to take in whenever photosynthesis is taking place.The concentration of in the atmosphere is rising due to deforestation and the combustion of fossil fuels. This would be expected to increase the efficiency of photosynthesis and possibly increase the overall rate of plant growth. This possibility has attracted considerable interest in recent years, as an increased rate of plant growth could absorb some of the excess and reduce the rate of global warming. Extensive experiments growing plants under elevated using Free-Air Concentration Enrichment have shown that photosynthetic efficiency does indeed increase. Plant growth rates also increase, by an average of 17% for above-ground tissue and 30% for below-ground tissue. However, detrimental impacts of global warming, such as increased instances of heat and drought stress, mean that the overall effect is likely to be a reduction in plant productivity. Reduced plant productivity would be expected to accelerate the rate of global warming. Overall, these observations point to the importance of avoiding further increases in atmospheric rather than risking runaway climate change.
Wind
Wind has three very different effects on plants.- It affects the exchanges of mass and of energy between the plant and the atmosphere by renewing the air at the contact with the leaves.
- It is sensed as a signal driving a wind-acclimation syndrome by the plant known as thigmomorphogenesis, leading to modified growth and development and eventually to wind hardening.
- Its drag force can damage the plant.
Exchange of mass and energy