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The predictions of their hypothesis were met in both the laboratory and the field. Net photosynthesis increased much more rapidly with light at low salinity than at high salinity. They measured the rate of stomatal conductance of CO2, a measure of the rate of transpiration. The results confirmed that high salinity limits leaf transpiration. Similar results were obtained in the field. Both survival and growth were affected by the combination of light and salinity. The response to increased light depended on the salinity: at low salinity, both growth and survival increased rapidly as light intensity increased; at high salinity, the positive effect of light intensity was significantly reduced. Together the greenhouse and field studies supported the hypothesis that CO2 limits photosynthesis when light is abundant but salinity reduces stomatal conductance. These results confirm the logical prediction that physical factors do not operate independently of one another. Instead, the impact of each is affected by others. Of course light and salinity are not the only important physical factors in this system. There is also variation in nutrient availability as a function of tidal influx, storm damage, drought, and soil moisture. The structure and dynamics of mangrove ecosystems are driven by the interactions among these factors.
Mangroves, a collection of trees found in low-latitude coastal environments, are ideal organisms to address this question. Mangroves occur in discrete bands from the shore inland, each species occupying a specific region of soil moisture and salinity. López-Hoffman et al. (2007) used the strong environmental gradient experienced by mangroves to examine the interaction of salinity and light. These factors constitute a logical pairing because each potentially affects the flow of CO2 into the plant through the stomata. Saline conditions limit the plant’s water uptake, which in turn leads the plant to close the stomata to reduce water loss. However, this also reduces the uptake of CO2 for photosynthesis and hence the plant response to variation in light intensity.
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The researchers chose the black mangrove (Avicennia germinans) for their study because in their system it is the most salt-tolerant species. In the greenhouse they subjected seedlings to three salinity levels (20 percent, 70 percent, 120 percent full seawater) and four light levels (6 percent, 12 percent, 25 percent, 50 percent of photosynthetically active radiation) for a total of 12 possible combinations. The net rate of photosynthetic assimilation was measured on sample plants after four months of treatment. Plants were harvested after 197 or 276 days and a series of morphological measurements taken that allowed the researchers to calculate the relative growth rate, net assimilation rate, and leaf area. In the field they established plots of 12 light levels under each of two salinity conditions. Seedlings were transplanted into each plot; nine months later the plants were harvested to measure growth and survival.
Supply-side pests reduce the photosynthetic rate by attacking root, leaves and stem in a manner that reduces the photosynthetic rate. Important supply-side pests are defoliators, sapsuckers, spidermites, nematodes, diseases, and others. Defoliation attacks leaves and may cause wound healing losses, but the effects on yield depend on the age of leaves attacked, the loss rate, and compensation due to increased light penetration. In contrast, spidermites kill leaf cells, reducing photosynthesis in damaged leaves that are not shed, reducing light penetration to lower leaves. Stem borers and vascular plant diseases may slow the photosynthetic rate by reducing the translocation of water and nutrients, and some may kill whole plants. Thrips and armyworms may damage the terminal, inducing developmental delays and reducing yield.
And Photosynthesis In Aquatic Ecosystems…
The researchers reasoned that at low light, photosynthesis is limited primarily by light, but when light is plentiful, leaf CO2 becomes the most limiting factor.
Prediction 1: When subjected to a range of salinities and light regimes in the laboratory, mangroves should achieve the highest rate of net photosynthesis at low salinity and the lowest rate of net photosynthesis when salinity is high.
Prediction 2: Similar results should occur in the field when seedlings are transplanted into an array of light-salinity levels.
All of the sugar produced in the photosynthetic cells of plants and other organisms is derived from the initial chemical combining of carbon dioxide and water with sunlight.
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Glossary of Terms: P - Physical Geography
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