Manual Responses of Plants to UV-B Radiation (Advances in Vegetation Science)

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CO 2 concentration. There was no obvious downregulation of soybean leaf photosynthesis in response to elevated CO 2 ; in fact, photosynthetic capacity was increased. Leaf quantum yield increased from 0.

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Campbell et al. Furthermore, Campbell et al. They found that rubisco activity was almost constant at 1. Leaf soluble protein was nearly constant at about 2. Thus, soybean did not lose photo-synthetic capacity as did some other plant species see Allen, In other studies, however, both photosynthetic rate and rubisco activity of soybean declined during long-term CO 2 enrichment Thorne and Koller, ; Delucia et al. After cross-switching two of the treatment chambers at DAP 52, the subsequent canopy photosynthetic rates and respiration rates quickly adjusted to their new CO 2 exposure conditions.

Table 4.


Condensed from Valle et al. Thus, pre-dawn respiration rates were closely connected to the previous CO 2 fixation rates. Partitioning Growth of plants under elevated CO 2 results in changes in partitioning of photoassimilates to various plant organs over time Table 4. In soybean, elevated CO 2 generally promoted greater carbon dry matter partitioning to the supporting structure stems, petioles and roots than to the leaf laminae during vegetative stages of growth Allen et al.

During reproductive stages, there tended to be lower relative partitioning to reproductive growth pods by plants under elevated CO 2. Soybean plant components as a percentage of total dry matter grown at subambient and superambient concentrations of CO 2 in Condensed from Allen et al. Growth rates During the linear phase of vegetative growth after full ground cover is reached, the growth rates of plants exposed to a range of CO 2 concentrations varied from 5. The total final dry weight ranged from Carbohydrates Soybean accumulates non-structural carbohydrates, particularly starch, under CO 2 enrichment.

Elevated non-structural carbohydrates in CO 2 -enriched soybean plants were confirmed by Baker et al.

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The concentrations also varied across the life cycle of the plants. Yield Soybean seed yield was always increased by elevated CO 2 Allen et al. This model was used to project yields across several ranges of atmospheric CO 2 concentration increases Table 4. For a doubling of CO 2 this model predicted a The ratio of these two numbers, 1. Percentage increases of soybean midday photosynthetic rates, biomass yield, and seed yield predicted across selected carbon dioxide concentration [CO 2 ] ranges associated with relevant benchmark points in time.

Adapted from Allen et al. This CO 2 concentration is used as the basis for many CO 2 doubling studies. The CO 2 concentration is expected to double sometime within the 21st century. Carbon dioxide and temperature The CO 2 fertilization effect appears to be enhanced under elevated temperatures, at least up to a point. Idso et al. However, for soybean, Allen calculated a biomass growth modification ratio response to temperature of Elevated temperatures tended to shorten the grain-filling period of this crop.

The number of seed per plant increased slightly with increase of both CO 2 and temperature. Mass per seed decreased sharply with increasing temperature. Although CO 2 enrichment resulted in increased seed yield and above-ground biomass, harvest index was decreased with both CO 2 and temperature Baker et al.

The data of Table 4. The sensitivity of the growth modification factor data of Idso et al. Apparently, the growth vs. Of course, these other environmental factors are all part of the complex of plant responses to climatic conditions. However, a better test of temperature effects alone would be CO 2 enrichment throughout the season or life cycle of plants under natural conditions, but with consistent temperature differences cooler and warmer.

Seed yield, components of yield, total above-ground biomass and harvest index of soybean grown at two CO 2 concentrations and three temperatures in adapted from Baker et al. The fractional increase in WUE attributable to increased photosynthesis and decreased transpiration were about 0. There were essentially no differences in A between the two CO 2 treatments at each specific C.

Similar pairs of curves not shown were obtained for the other temperature treatments. Following canopy closure, the rice canopy net photosynthetic rate Pn vs. IR30 and high plant populations. Canopy Pn vs. Other studies of canopy Pn have shown only small differences across wide ranges of temperature; e. Evaporative cooling may lower foliage temperature below air temperature increasingly with increasing air temperature and increasing vapour pressure deficit Allen, ; Pickering et al. The summed response of the photosynthetic rates of leaves to temperature at all exposures of light may broaden the temperature response for the whole canopy photo-synthetic rates Pickering et al.

Figure 4. The parameters were The calculated percentage increase in response of whole-day canopy Pn at vs. Photosynthetic acclimation to CO 2 at the canopy level To test for acclimation of canopy photosynthetic capacity, Baker et al. Within each of these short-term CO 2 exposure comparisons, Baker et al. The relative effects of the short-term CO 2 exposure were greatest for the lowest short-term CO 2 concentration.

However, a large part of this apparent acclimation effect may be attributed to the greater respiration rates of the plants that had been grown under elevated CO 2 Boote et al. Rubisco protein percentage was used as evidence of leaf acclimation to a wide range of CO 2 concentrations Baker and Allen, ; Allen et al. Although Rowland-Bamford et al. One possibility would be that rubisco is not as limiting for photosynthesis under CO 2 enrichment as would be expected.

More work is needed, under a range of CO 2 treatments, to explore the interaction effects of sink capacity, nitrogen nutrition, and other internal CO 2 -fixation processes on photosynthetic behaviour and crop yield. Comparison of rice canopy net photosynthetic rate Pn vs. The Pn was estimated from linear regression equations of Pn vs.

Adapted from Baker et al. For soybean, leaf blade soluble protein expressed on a leaf blade area basis and percentage rubisco protein expressed on a leaf blade soluble protein basis for day-old soybean plants grown under a wide range of CO 2 concentrations. Adapted from Campbell et al. For rice, leaf nitrogen content expressed on a leaf area basis and percentage rubisco protein expressed on a leaf soluble protein basis for day-old rice plants grown under a wide range of CO 2 concentrations.

Adapted from Rowland-Bamford et al. Leaf appearance rates leaves per day are about twice as great in the vegetative phase as in the reproductive phase. Air temperature greatly influenced leaf appearance rate Figure 4. A Mainstem Haun scale growth units vs. B Phyllochron interval vs. A rectangular hyperbola equation 4. The calculated percentage increase in response at vs. Therefore, elevated CO 2 may slightly increase the maximum temperature at which rice plants can survive. Temperature effects on yield and yield components were highly significant. The number of panicles per plant increased while the number of filled grains per panicle decreased sharply with increasing temperature treatment.

Final above-ground biomass and harvest index were increased by CO 2 enrichment while harvest index declined sharply with increasing temperature. Notably, there were no significant CO 2 x temperature interaction effects on yield, yield components, or final above-ground biomass Table 4. At each CO 2 concentration, polynomial regression equations were fitted to the rice seed yield Y vs.

Figures 4. While future increases in atmospheric CO 2 should benefit rice yields, large negative effects are likely if temperatures also rise. The figures show that vegetative productivity is maintained at higher temperatures than is reproductive growth. Grain yield, components of yield, total above-ground biomass and harvest index of rice subambient and superambient CO 2 concentration experiments conducted in These data were not used in this analysis. Grain yield, components of yield, total above-ground biomass and harvest index for five separate rice experiments.

Adapted from Baker and Allen a and Baker et al. Rice seed yield vs. Second-degree polynomial fit of rice final biomass yield vs. The R d of the ambient and superambient CO 2 treatments reached a broad maximum around 30 to 50 DAP whereas the broad maximum of the subambient treatments occurred later around 50 to 70 DAP. Baker et al. Another explanation is that elevated CO 2 increased the amount of structural and non-structural carbohydrates in the plant tissues, so that a larger proportion of dry matter was sequestered in non-protein materials.

This study showed no respiration acclimation to long-term CO 2 enrichment indirect acclimation of rice that could not be explained by nitrogen concentration of the plant tissues. Evapotranspiration and water-use efficiency Both plant transpiration and direct evaporation from the floodwater surface contribute to water use evapotranspiration, ET of rice growing in the SPAR chambers. Diurnal trends of ET followed diurnal patterns of solar irradiance Baker et al.

The ET rates were similar when all chambers were exposed for one-half day to the same CO 2 concentration data not shown, Baker et al. Solar irradiance also has a large effect, probably through both the energy inputs to the canopy and through the stomatal opening response to light directly or indirectly. CO 2 treatment are shown in Table 4. Stomatal conductance decreases with increasing CO 2 concentration which can cause a reduction of both leaf and whole canopy transpiration. However, CO 2 enrichment may also increase canopy leaf surface area for transpiration, thereby offsetting some of the water savings Jones et al.

If WUE were based on grain yield, it would decrease drastically with increasing temperature not only because of increasing ET, but also because of sharply decreasing seed production. Rising atmospheric CO 2 is likely to benefit rice production by increasing photosynthesis, growth and grain yield while reducing water use and increasing WUE. In warm areas of the world, however, possible future global warming may result in substantial yield decreases because of the sensitivity of flowering and seed set to high temperatures and the possibility of water shortages that may result from increased evapotranspiration.

Summary of comprehensive reviews Several recent symposia proceedings and reviews leave little doubt that crop plants can respond well to elevated CO 2 Rozema et al. The distribution of weight ratios of CO 2 -enriched and control plants is shown in Figure 4. As a group, C 3 herbaceous crop plants responded more than wild herbaceous species 58 vs. Furthermore, the fast growing wild species responded more strongly than slow-growing wild species 54 vs. Poorter imposed two restrictions on his compilations that may have led to larger than expected responses to elevated CO 2.

Firstly, plants grown in competition were not included. Secondly, only vegetative stages of plants were compared since compiled data were selected prior to flowering. A number of studies have shown that vegetative growth responses may be greater than reproductive seed yield responses. Therefore, the compiled data of Poorter may give an impression of greater response than would be observed throughout the life cycle.

Secondly, crops in field conditions usually are grown in dense populations where they compete for space and light. Under more realistic field conditions, crop plants are likely to respond as a community rather than individual plants, wherein light solar radiation becomes a limiting factor for growth. Under these conditions, elevated CO 2 cannot promote horizontal expansion and greater light capture. Although the actual field responses may be less, the CO 2 fertilization effect is clearly well-established. The bar graph was created from averages of all the weight ratios of species selected from the literature.

Source: Poorter The CO 2 fertilization effect for forest species has also become firmly established. Wullschleger et al. Their frequency distribution of relative growth response of trees grown at elevated CO 2 vs. They also found a mean response ratio of 1. Of the observations, 51 showed a relative growth response less than 1, and 31 showed responses greater than 2.

However, under competitive conditions, tree response may be much less Bazzaz et al. Kimball et al. Their data are reproduced as Figure 4. Thus, the vegetative response to CO 2 should be enhanced at increased temperatures. As discussed before, this growth modification factor may not apply for reproductive growth responses seed yield of crops e.

Source: Wullschleger et al. Their simulations used 30 years of baseline weather data from 19 sites in 11 states. At each site, prevailing cropped soil types, planting dates, and cultivars were used in the simulations. Source: Kimball et al. Crop yield responses to climate change were simulated under four conditions: with or without direct CO 2 fertilization effects, and under rainfed or optimum irrigation culture.

The Penman-Monteith equation, which contains a term for canopy conductance, was used to compute the effects of elevated CO 2 on canopy transpiration. Simulations were run for doubled-CO 2 conditions with a crop photosynthetic enhancement factor of 1. No simulations were run with CO 2 fertilization effects only without climate change effects. Simulated soybean seed yields were averaged over 30 years and 19 sites Table 4.

UV Burns on vegetation

Yields under the GFDL scenario were severely impacted because of the rainfall reductions predicted by this model Table 4. Condensed from Peart et al. When CO 2 fertilization effects were included with climate change effects Table 4. Including the CO 2 fertilization effects with climatic change scenarios had little effect on the predicted yields of maize because it is a C 4 plant.

For the southern part of this region, yield reductions were greater for the GFDL scenario. Predicted yields increased for the northernmost stations because temperatures and growing season duration became more favourable. Yields decreased in the Southern Great Plains because higher temperatures shortened the life cycle of the crops. Where precipitation was predicted to decrease, irrigation requirements increased. The CO 2 fertilization effect offset the adverse effects of climate change at some locations of Ritchie et al. Irrigated 11 Adapted from Peart et al.

Production declined, in general, under the scenarios of climate change without CO 2 fertilization effects; however, commodity prices generally increased. Under the CO 2 fertilization plus climate change effects scenarios, predicted impacts on commodity prices were much less. The effect of temperature on the phenology of crop plants plays a critical role. One crucial need is for more detailed research on responses of plants to temperature, and temperature x CO 2 interactions, as inputs to crop models.

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More modelling studies are also needed on different planting dates as an adaptive strategy. Cultivars need to be designed for future climatic conditions Hall and Allen, Factors that should be considered are: extension of the grain-filling period and perhaps shortening the duration of vegetative growth which would also improve harvest index ; ability to flower and set seed at higher temperatures; photoperiod and thermoperiod adaptive interactions; selection for positive photosynthetic acclimation where negative photosynthetic acclimation has been observed; and capability of utilizing photoassimilates storage carbohydrates more effectively.

These factors need to be integrated into whole-plant physiology under real-world conditions. However, the climate change for an effective doubling of CO 2 may occur at CO 2 concentrations less than those used in this simulation, if radiatively active trace gases other than CO 2 play a large role in the greenhouse effect.

In that case, the direct CO 2 effects would be somewhat lower than shown in the example of Tables 4. All of the simulations assumed that climatic changes would occur simultaneously with increasing concentrations of CO 2 and other trace greenhouse-effect gases. If global warming lags the increases of atmospheric CO 2 , then some beneficial effects of CO 2 fertilization are likely to occur before the full impact of climate change is manifested.

However, Broecker and others caution that climate changes have not always been gradual during interglacial periods of the Pleistocene Era. There is clear evidence of relatively rapid climatic oscillations in the northern hemisphere during the previous interglacial period to years before present based on Greenland Ice-core Project GRIP records Anklin et al. These oscillations produced cold periods that were as severe as the proceeding glacial period.

Much of the reduction in soybean yields reported by Peart et al. Changes in management practices, such as changing planting dates or selection of other cultivars, may help to prevent some of the potential reductions in yield. In the future, plant breeders may need to adapt combinations of temperature tolerance and photoperiod responses into new germplasm.

In situations where non-structural carbohydrates accumulate as a CO 2 fertilization effect response, new germplasm needs to be developed that can make better use of the photoassimilate source. Irrigation is not likely to be a panacea for climate change. However, under the GFDL scenario, water resources would become scarce, and may not be readily available for crops. Some areas of the USA may have to adapt by irrigating less land area. Increasing temperatures and decreasing precipitation for the USA as predicted by the GFDL model would have a serious negative impact overall on agricultural productivity although producers in favourable regions may benefit from scarcity-mediated higher prices Adams et al.

Furthermore, progress has been made on predicting impacts of climate change using world trade models Rosenzweig and Parry, , The doubled-CO 2 climate change scenarios temperature, rainfall and evaporation changes were based on the climate change potential expected from increases of all greenhouse-effect gases. These climate changes are expected to occur well before CO 2 concentration has actually doubled. Thus, the CO 2 fertilization effect photosynthetic ratios for four crops soybean, wheat, rice and maize were taken as 1.

The impact of climate change scenarios was more severe in the tropical latitudes than in the mid- or high-latitudes. Simulations of soybean yields throughout the USA were a part of this international study Curry et al. They found that aggregated yields were 2. Temperatures were about 4.

Refinements and improvements in prediction methodology will continue, but these assessments provide the best currently available insights into the CO 2 fertilization and climate change effects on global crop productivity. Summary and conclusions Elevated CO 2 increases the size and dry weight of most C 3 plants and plant components.

Relatively more photoassimilate is partitioned into structural components stems and petioles during vegetative development in order to support the light-harvesting apparatus leaves. The harvest index tends to decrease with increasing CO 2 concentration and temperature. Selection of plants that could partition more photoassimilates to reproductive growth should be a goal for future research.

As more is learned about the effects of anticipated climate changes on crops, more effort should be directed to exploring biological adaptations and management systems for reducing these impacts on agriculture and humanity. Whether regional climates become drier or wetter with global warming remains to be seen.

This work was conducted in cooperation with the University of Florida at Gainesville. References Acock, B. Crop responses to elevated carbon dioxide concentration. Strain and J.

Cure eds. US Dept. Adams, R. Global climate change and US agriculture. Nature : Allen, L. Plant responses to rising carbon dioxide and potential interactions with air pollutants. Effects of increasing carbon dioxide levels and climate change on plant growth, evapotranspiration, and water resources.

Carbon dioxide increase: Direct impacts on crops and indirect effects mediated through anticipated climatic changes. In: Physiology and Determination of Crop Yield. Boote, J. Bennett, T. Sinclair and G. Paulsen eds.

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Woodwell and F. Mackenzie eds. Oxford University Press, New York. Allen, R. Appendix C, Agriculture, Vol. Smith and D. Tirpak eds. Rising atmospheric CO 2 and evapotranspiration. In: Advances in Evapotranspiration. ASAE Pub. American Society of Agricultural Engineers, St. Joseph, Michigan.

W, Jones, P. Response of vegetation to rising carbon dioxide: Photosynthesis, biomass, and seed yield of soybean. Global Biogeochemical Cycles 1 : Nonstructural carbohydrates and nitrogen of soybean grown under carbon dioxide enrichment. Crop Sci. W, Jones, J. Soybean leaf gas exchange responses to CO 2 enrichment. Soil Crop Sci. Soybean dry matter allocation under subambient and superambient levels of carbon dioxide. Carbon dioxide and temperature effects on rice.

In: Climate Change and Rice. Peng, K. Ingram, H. Neue, and L. Ziska eds. Springer-Verlag, Berlin, Heidelberg. Amthor, J. Respiration in a future, higher-CO 2 world. Plant Cell Environ. Plants have developed a number of ways to achieve this transport, such as vascular tissue , and the functioning of the various modes of transport is studied by plant physiologists.

Fourthly, plant physiologists study the ways that plants control or regulate internal functions. Like animals, plants produce chemicals called hormones which are produced in one part of the plant to signal cells in another part of the plant to respond. Many flowering plants bloom at the appropriate time because of light-sensitive compounds that respond to the length of the night, a phenomenon known as photoperiodism.

The ripening of fruit and loss of leaves in the winter are controlled in part by the production of the gas ethylene by the plant. Finally, plant physiology includes the study of plant response to environmental conditions and their variation, a field known as environmental physiology.

Stress from water loss, changes in air chemistry, or crowding by other plants can lead to changes in the way a plant functions. These changes may be affected by genetic, chemical, and physical factors. The chemical elements of which plants are constructed—principally carbon , oxygen , hydrogen , nitrogen , phosphorus , sulfur , etc. Only the details of the molecules into which they are assembled differs.

Despite this underlying similarity, plants produce a vast array of chemical compounds with unique properties which they use to cope with their environment. Pigments are used by plants to absorb or detect light, and are extracted by humans for use in dyes. Other plant products may be used for the manufacture of commercially important rubber or biofuel. Perhaps the most celebrated compounds from plants are those with pharmacological activity, such as salicylic acid from which aspirin is made, morphine , and digoxin.

Drug companies spend billions of dollars each year researching plant compounds for potential medicinal benefits. Plants require some nutrients , such as carbon and nitrogen , in large quantities to survive. Some nutrients are termed macronutrients , where the prefix macro- large refers to the quantity needed, not the size of the nutrient particles themselves. Other nutrients, called micronutrients , are required only in trace amounts for plants to remain healthy.

Such micronutrients are usually absorbed as ions dissolved in water taken from the soil, though carnivorous plants acquire some of their micronutrients from captured prey. The following tables list element nutrients essential to plants. Uses within plants are generalized. Among the most important molecules for plant function are the pigments.

Plant pigments include a variety of different kinds of molecules, including porphyrins , carotenoids , and anthocyanins. All biological pigments selectively absorb certain wavelengths of light while reflecting others. The light that is absorbed may be used by the plant to power chemical reactions , while the reflected wavelengths of light determine the color the pigment appears to the eye.

Chlorophyll is the primary pigment in plants; it is a porphyrin that absorbs red and blue wavelengths of light while reflecting green. It is the presence and relative abundance of chlorophyll that gives plants their green color. All land plants and green algae possess two forms of this pigment: chlorophyll a and chlorophyll b. Kelps , diatoms , and other photosynthetic heterokonts contain chlorophyll c instead of b , red algae possess chlorophyll a. All chlorophylls serve as the primary means plants use to intercept light to fuel photosynthesis.

Carotenoids are red, orange, or yellow tetraterpenoids. They function as accessory pigments in plants, helping to fuel photosynthesis by gathering wavelengths of light not readily absorbed by chlorophyll. The most familiar carotenoids are carotene an orange pigment found in carrots , lutein a yellow pigment found in fruits and vegetables , and lycopene the red pigment responsible for the color of tomatoes. Carotenoids have been shown to act as antioxidants and to promote healthy eyesight in humans. Anthocyanins literally "flower blue" are water-soluble flavonoid pigments that appear red to blue, according to pH.

They occur in all tissues of higher plants, providing color in leaves , stems , roots , flowers , and fruits , though not always in sufficient quantities to be noticeable. In these plants, the anthocyanin catches light that has passed through the leaf and reflects it back towards regions bearing chlorophyll, in order to maximize the use of available light.

Betalains are red or yellow pigments. Like anthocyanins they are water-soluble, but unlike anthocyanins they are indole -derived compounds synthesized from tyrosine. This class of pigments is found only in the Caryophyllales including cactus and amaranth , and never co-occur in plants with anthocyanins. Betalains are responsible for the deep red color of beets , and are used commercially as food-coloring agents. Plant physiologists are uncertain of the function that betalains have in plants which possess them, but there is some preliminary evidence that they may have fungicidal properties.

Plants produce hormones and other growth regulators which act to signal a physiological response in their tissues. They also produce compounds such as phytochrome that are sensitive to light and which serve to trigger growth or development in response to environmental signals. Plant hormones , known as plant growth regulators PGRs or phytohormones, are chemicals that regulate a plant's growth.

According to a standard animal definition, hormones are signal molecules produced at specific locations, that occur in very low concentrations, and cause altered processes in target cells at other locations. Unlike animals, plants lack specific hormone-producing tissues or organs. Plant hormones are often not transported to other parts of the plant and production is not limited to specific locations. Plant hormones are chemicals that in small amounts promote and influence the growth , development and differentiation of cells and tissues.

Hormones are vital to plant growth; affecting processes in plants from flowering to seed development, dormancy , and germination. They regulate which tissues grow upwards and which grow downwards, leaf formation and stem growth, fruit development and ripening, as well as leaf abscission and even plant death. The most important plant hormones are abscissic acid ABA , auxins , ethylene , gibberellins , and cytokinins , though there are many other substances that serve to regulate plant physiology.

While most people know that light is important for photosynthesis in plants, few realize that plant sensitivity to light plays a role in the control of plant structural development morphogenesis. The use of light to control structural development is called photomorphogenesis , and is dependent upon the presence of specialized photoreceptors , which are chemical pigments capable of absorbing specific wavelengths of light.

Plants use four kinds of photoreceptors: [1] phytochrome , cryptochrome , a UV-B photoreceptor, and protochlorophyllide a. The first two of these, phytochrome and cryptochrome, are photoreceptor proteins , complex molecular structures formed by joining a protein with a light-sensitive pigment. Cryptochrome is also known as the UV-A photoreceptor, because it absorbs ultraviolet light in the long wave "A" region. The UV-B receptor is one or more compounds not yet identified with certainty, though some evidence suggests carotene or riboflavin as candidates.

The most studied of the photoreceptors in plants is phytochrome. It is sensitive to light in the red and far-red region of the visible spectrum. Many flowering plants use it to regulate the time of flowering based on the length of day and night photoperiodism and to set circadian rhythms. It also regulates other responses including the germination of seeds, elongation of seedlings, the size, shape and number of leaves, the synthesis of chlorophyll, and the straightening of the epicotyl or hypocotyl hook of dicot seedlings.

Many flowering plants use the pigment phytochrome to sense seasonal changes in day length, which they take as signals to flower. This sensitivity to day length is termed photoperiodism. Broadly speaking, flowering plants can be classified as long day plants, short day plants, or day neutral plants, depending on their particular response to changes in day length. Long day plants require a certain minimum length of daylight to starts flowering, so these plants flower in the spring or summer.

Conversely, short day plants flower when the length of daylight falls below a certain critical level. Day neutral plants do not initiate flowering based on photoperiodism, though some may use temperature sensitivity vernalization instead. Although a short day plant cannot flower during the long days of summer, it is not actually the period of light exposure that limits flowering.

Rather, a short day plant requires a minimal length of uninterrupted darkness in each hour period a short daylength before floral development can begin. It has been determined experimentally that a short day plant long night does not flower if a flash of phytochrome activating light is used on the plant during the night. Plants make use of the phytochrome system to sense day length or photoperiod. This fact is utilized by florists and greenhouse gardeners to control and even induce flowering out of season, such as the Poinsettia. Paradoxically, the subdiscipline of environmental physiology is on the one hand a recent field of study in plant ecology and on the other hand one of the oldest.

It is roughly synonymous with ecophysiology , crop ecology, horticulture and agronomy. The particular name applied to the subdiscipline is specific to the viewpoint and goals of research. Whatever name is applied, it deals with the ways in which plants respond to their environment and so overlaps with the field of ecology. Environmental physiologists examine plant response to physical factors such as radiation including light and ultraviolet radiation , temperature , fire , and wind. Of particular importance are water relations which can be measured with the Pressure bomb and the stress of drought or inundation , exchange of gases with the atmosphere , as well as the cycling of nutrients such as nitrogen and carbon.

Environmental physiologists also examine plant response to biological factors. This includes not only negative interactions, such as competition , herbivory , disease and parasitism , but also positive interactions, such as mutualism and pollination. Plants may respond both to directional and non-directional stimuli. A response to a directional stimulus, such as gravity or sun light , is called a tropism. A response to a nondirectional stimulus, such as temperature or humidity , is a nastic movement. Tropisms in plants are the result of differential cell growth, in which the cells on one side of the plant elongates more than those on the other side, causing the part to bend toward the side with less growth.

Among the common tropisms seen in plants is phototropism , the bending of the plant toward a source of light. Phototropism allows the plant to maximize light exposure in plants which require additional light for photosynthesis, or to minimize it in plants subjected to intense light and heat. Geotropism allows the roots of a plant to determine the direction of gravity and grow downwards. Tropisms generally result from an interaction between the environment and production of one or more plant hormones.

Nastic movements results from differential cell growth e. A familiar example is thigmonasty response to touch in the Venus fly trap , a carnivorous plant. The traps consist of modified leaf blades which bear sensitive trigger hairs. When the hairs are touched by an insect or other animal, the leaf folds shut. This mechanism allows the plant to trap and digest small insects for additional nutrients.