Environmental factors, light nutrients

The possible effects of increased atmospheric CO2 on photosynthesis are reviewed by Goud-riaan and Ajtay (1979) and Rosenberg (1981). Increasing CO2 in a controlled environment (i.e., greenhouse) increases the assimilation rate of some plants, however, the anthropogenic fertilization of the atmosphere with CO2 is probably unable to induce much of this effect since most plants in natural ecosystems are growth limited by other environmental factors, notably light, temperature, water, and nutrients. 

C02 is not the only factor involved in photosynthesis, so that for its use, other factors must be at levels that do not limit the process. Light, temperature, amount of available nutrients and the relative humidity are other environmental factors affecting photosynthetic activity. 

Changes in environmental factors, including levels of grazing, salinity, nutrients, visible light, UV light and desiccation, can affect concentrations of chemical defenses in marine macroalgae. Work conducted on terrestrial and marine plants suggests that the direct effects of environmental factors on chemical defense concentrations are manifested in three ways ... 

Like many traits, concentrations of secondary metabolites in algae are determined by a combination of genetic and environmental factors. Secondary metabolites are known to respond to changes in a variety of environmental conditions, including light,14,49,70 herbivory,43" 6, nutrients,44,45,49,73,74 desiccation,75 and salinity.76 Most of these studies have looked at the effects of a single environmental stress at once. However, in nature, multiple stresses often occur simultaneously and may have synergistic effects. 

Cyanobacterial culture studies and reported observations of natnral blooms have shown that differences in cyanobacterial composition and toxicity are dependent on a number of environmental factors including, light, temperature, pH, and nutrients and physical factors such as buoyancy of the algae, prevailing weather conditions, and water currents (Rapala et al. 1993 Gupta et al. 2002). These variables, along with the age of the cells, their tendency to lyse and release toxins, decomposition and detoxification mechanisms (Rapala et al. 1994), help to explain why poisonons cyanobacterial blooms tend to occur in such an erratic and unpredictable fashion (Carmichael and Gorham 1980). 

Light is a key environmental factor having an impact on nearly aU phytoplankton physiological processes N uptake is no different. Nutrient uptake versus irradiance curves often show a hyperbolic relationship that have been mathematically fit by... 

To revisit our general recipe for NPP, environmental factors influence the rate at which light, CO2, nutrients and water are combined to form NPP. Any of these environmental factors or resources may constrain NPP, and it is ultimately a proper balance of these factors that is required for plant production. However, the importance of interacting controls in determining ecosystem processes demonstrates that NPP is not a simple function of the ratio of resources available and the environmental conditions. This simple stoichiometric approach would be valid only if plants responded passively to, and had no effect on, their environment. Plants, however, play an active role in their response to, and mediation of, resources and their environment. Within the constraints of their environment, they actively mediate the resource availability and environmental conditions that constrain NPP. Ultimately, biogeochemical cycling is driven by the interactions between organisms and their physical and chemical environment. NPP is therefore sensitive to... 

Although the influence of relevant biotic and abiotic variables on the fate and effects of chemicals can, to a certain extent, be explored through controlled experiments and observations of natural systems, the combinations of factors that can be tested in practice are very limited. Mesocosm and field studies are often expensive to perform, can be difficult to replicate sufficiently, and are frequently complicated to interpret. Because they typically represent one unique scenario (species composition and density, temperature, light, nutrient level, and timing of pesticide application in relation to the environmental conditions), questions are often raised about the generality and robustness of the results. Likewise, unexpected or uncontrollable events may occur (e.g., it may have been an unusually rainy, sunny, hot, or cool season), the influence of which on the estimate of risk can be difficult to assess. 

The main purpose of a mulch is to control the environmental factors that affect seedling survival e.g. moisture, temperature, light, chemical presence or absence, weeds and mechanical damage. The mulch also acts as a soil insulator to keep the soil warm in the early and late part of the growing season. It is also a vapour block to suppress evaporation and a weed suppressor so that saplings can make full use of light, moisture, and nutrients. 

The role of environmental factors in allelopathic inhibition. Moisture and nutrient stress, high temperature, light intensity, plant density, and the presence of pathogens can all alter the response of target plants to allelochemicals and also the production or release rates of allelochemi-cals by source plants. Chemicals that are not toxic at certain concentrations in a laboratory assay may, in fact, be significantly inhibitory under stressful field conditions. 

There are many factors that lower nutrient availability in packaged and stored foods. Environmental factors such as the presence of oxygen and light, or processing variables such as temperature, time, pH, and oxidizing and reducing agents affect the stability of nutrients like amino acids and vitamins (Borenstein et al. 1990, Harris 1977). Table 17.6 summarizes the susceptibility of hydrosoluble and liposoluble vitamins to these factors. 

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