While specific responses to individual environmental factors will be discussed further in the final section, a comprehensive overview of common adjustments is presented here. Two major aspects of how a plant can optimize performance and survival under changing environmental conditions are adjustments in (1) physical form or (2) metabolic function and, of course, combinations of the two. Similar principal responses can be seen in response to a variety of different environmental factors. At the end of this section, adjustments (such as arrest) of the life cycle are also briefly discussed.
In contrast to the relatively narrow genetic constraints on form and size within a given animal species, plants exhibit a great deal of phenotypic plasticity. Many plant species exhibit indeterminate growth, that is, continued growth throughout the entire lifespan of the organism. Such growth is continually adjusted in response to multiple environmental factors via internal signaling networks that optimize the expression of the plant's genetic potential. Environmental factors contributing to the regulation of growth and development include light (quantity, quality, periodicity, and direction), temperature, water availability, nutrient availability, wind, pollution, soil compactness and available rooting volume, and gravity. The same factors also contribute to the modulation of plant function (see below). Each of these factors is perceived or measured by the plant, followed by transduction of that perception to a signal feeding into a network of regulatory pathways, and often involving one or more plant hormones as well as signals related to plant redox state (see below). Some of these factors will result in increased growth as a response to take advantage of available conditions or resources, whereas others will result in decreased growth as an acclimatory response to stress.
Each plant makes continual adjustments to maintain a balance between investment of resources into a root system that is sufficiently elaborate to supply the shoot with water and nutrients versus a shoot system that is sufficiently developed to provide the plant and its symbionts with an adequate supply of reduced carbon. For instance, in the understory of a canopy, where light is typically limiting for plant growth, plants generally have a lower root-to-shoot ratio, thereby placing a greater emphasis on light collection and carbon reduction than on the acquisition of water and nutrients. On the other hand, plants that develop in an open field in full sunlight generally have a higher root-to-shoot ratio, thereby placing a greater emphasis on water and nutrient acquisition under conditions where abundant light is available for photosynthesis in the shoot. Exposure to greater levels of wind also results in shorter and stockier plants that invest a greater proportion of resources into structural components (e.g., lignin) that provide resistance to mechanical strain. Similarly, a root system that develops in a more compact soil will be less extensive than one that develops in a looser soil.
This balance between investment into the root versus the shoot at the whole plant level is maintained through perception of the environmental conditions as well as the relative activities of the different portions of the plant. Plant hormones, and control over their synthesis, degradation, and inactivation through conjugation and transport, as well as control over the level and activity of hormone receptors, play a large role in this developmental flexibility. This is especially true of two hormones essential to plant viability (no mutants that lack either of these two hormones have been identified to date). The auxins are synthesized primarily in the shoot apical meristems and transported downward through the plant. On the other hand, the cytokinins are synthesized primarily in the root apical meristems and transported upward through the xylem of the plant. These two plant hormones act antagonistically with regard to differentiation of shoot versus root tissue, leading to appropriate adjustments in the development of each. When present at relatively higher levels, cyto-kinins stimulate shoot growth, whereas relatively higher levels of auxins stimulate root growth.
Within the plant, the two transport systems primarily responsible for the upward movement of water and nutrients (the xylem) and the bi-directional movement of sugars and remobilized nutrients (the phloem) are also subject to some acclimation depending on the conditions under which they develop. Under conditions where water is readily available, as well as under conditions where nutrients are limiting, the conducting cells of the xylem (tracheids and vessels) develop with a greater diameter to facilitate higher rates of water and nutrient delivery. On the other hand, tracheids and vessels develop with a smaller diameter under conditions where water is limiting or nutrients are plentiful. This is especially important to prevent the introduction of air bubbles into the water-conducting xylem cells (a process known as cavitation), a phenomenon that prevents continued water flow and is more likely to occur in larger cells under increasing water stress as well as during freeze-thaw cycles during the winter.
Acclimation of the vascular system can extend to the leaf level as well. In many species, leaves that grow and expand under high-light conditions develop a larger network of veins (have a higher vein density per leaf area) than those that acclimate to low-light conditions. In addition to supplying more water (important to support higher rates of transpirational water loss and cooling in the high-light environment) and nutrients to a leaf, a greater vein density can also provide for a more extensive pipeline for the export of sugars produced through the higher rates of photosynthesis in high-light-acclimated leaves. Other species (some of those that utilize an active, biochemical step to move sugars into the phloem; see below) do not exhibit any adjustments in the leaf vein density.
Many other anatomical and morphological aspects of leaves are subject to acclimation. Leaves that develop in the shade tend to be larger, thinner, and displayed horizontally (features emphasizing light capture), whereas leaves in the sun tend to be smaller, to have more layers of, and/or longer, palisade mesophyll cells, and can be displayed more vertically (lesser emphasis on light capture but a greater emphasis on reducing water loss). Other leaf-level features that may be adjusted (depending on species) to contribute to decreased water loss under higher-light conditions include an increased deposition of cuticular waxes, increased development of epidermal hairs (to increase reflection and decrease heat load, as well as providing a barrier to water loss), and increased deposition of salts on the epidermis or in leaf hairs (increased reflectance). Many of these adjustments are effective as features that contribute to the acclimation of plants to higher temperatures as well.
In contrast to the vast majority of animals, plants are, as noted above, able to self-amputate certain vulnerable portions of their structure as an acclimation strategy in response to stress or seasonal change. The most common is the annual shedding of leaves, but it is not unusual for entire branches and roots to be lost as well. Furthermore, plants can continue to thrive in the face of unanticipated loss or modification of their structure as a result of powerful winds or freeze-induced damage, or of the activity of organisms that rely on plants for shelter (birds, mammals, reptiles, insects) or resources (herbivores, parasites (e.g., mistletoes, dodder), and various pathogenic microorganisms (fungi, bacteria, and viruses)). Such losses often result in the compensatory growth of new foliage or roots.
Plant Function: Metabolism/Biochemistry
While the structural changes that plants undergo can be radical, the acclimatory adjustments at the molecular, biochemical, and physiological level are equally remarkable. For example, the primary pathways of energy metabolism, respiration and photosynthesis, are both subject to considerable regulation. When able to produce an abundance of carbohydrates, plants upregulate pathways for utilization (including respiration, and investment into additional growth and reproduction) and storage. If the demand for utilization and storage of carbon lags behind the production of sugars through photosynthesis, then the enzymes and electron-transport components of photosynthesis are downregulated. On the other hand, if the consumption of sugars exceeds their supply, then photosynthesis is upregulated in the source leaves to meet the demand of the plant for the carbohydrates. Also upregulated are enzymes responsible for converting photosynthetically produced sugars to the types of sugars transported throughout the plant, as well as transport proteins that move those sugars into the phloem (in those species that utilize such proteins).
Many other enzymes and pathways are also subject to regulation depending on their role in the plant, the developmental state of the plant, and the environmental conditions. For example, synthesis and accumulation of secondary plant compounds such as the phenolic flavo-noids (accumulating in the vacuoles of the epidermis, the cuticle layer, in epidermal hairs, and in epidermal cell walls as a screen against the damaging effects of ultraviolet radiation) is strongly upregulated in response to the blue and ultraviolet portions of sunlight. A subset of flavonoids, the red/blue/purple anthocyanins, is upregu-lated and accumulates in the epidermis of leaves under a variety of conditions, depending on the plant species. Irrespective of their specific roles (suggestions include functions as a screen against intense sunlight, as powerful antioxidants, as a sink for excess carbon, as a visual cue to attract pollinators or seed distributors), anthocyanins are highly responsive to environmental conditions and are expressed most strongly under high light. In some plant species, anthocyanins accumulate in leaves during the early phase of expansion (prior to developing photo-synthetic competence), in others during the flowering phase, and in many species they accumulate during the senescent phase of their lifespan prior to leaf fall. Anthocyanins also accumulate in the leaves of certain plant species during water stress, during high or low temperature stress, in response to insufficient or excessive nutrient levels (e.g., salinity), and in response to pollutants.
Signaling pathways that stimulate the synthesis of defense compounds are activated by biotic stress, such as attack by herbivores, nematodes, or any of a multitude of pathogenic microorganisms. Some of the signaling molecules produced in response to biotic stress are rather volatile, and such signals generated by one plant that has been attacked can be transmitted to neighboring plants, resulting in a greater defensive response on the part of the second plant if it should be attacked.
Although the lifespan of annual plant species is relatively fixed and short, that of many other plant species is relatively flexible. Even many biennial species, which normally die at the end of the second growing season after flowering and leaving seeds, will live for years (without flowering) if they do not receive the environmental cues that signal the normal progression through the seasons. The timing of progression through the life cycle in many other species is highly flexible, depending on the conditions under which a plant grows and develops. Reproduction is typically delayed when resources are limiting, and occurs sooner in plants that grow in high-light, nutrient-rich sites. Overall development, at both the organ (e.g., leaf) and whole plant level, is generally accelerated under resource-rich conditions. The longest-lived plants are found in relatively resource-poor environments, for example, the creosote bush of the hot deserts of North and South America, and bristlecone pine found in the upper montane and subalpine regions of the western United States.
Perhaps the most extreme mechanism for dealing with stress is to enter a state of dormancy until the stress is relieved. The multitude of different adaptations exhibited within the plant kingdom include seeds/spores that can desiccate fully (a state equally effective for persisting through a prolonged drought or subfreezing winter temperatures), whole plants that can desiccate fully, plants that allow their more sensitive portions to senesce (leaves, twigs, branches, shoots, or roots), or evergreen plants that downregulate photosynthesis and remain inactive until conditions and resources permit a resumption of metabolic activity. In fact, the persistence of desiccation-tolerant seeds, sometimes for decades, is the single adaptation that permits plant life to exist in the most arid habitats on Earth.
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