In measuring photosynthesis in crop species you find that two species respond di

Question

In measuring photosynthesis in crop species you find that two species respond differently to a decrease in the concentration of O2 in the gas surrounding the leaf. Species A maintains a constant rate of photosynthesis in both high and low O2 concentration while Species B exhibits a significant increase in photosynthesis in the low O2 environment.
   a.   Propose a hypothesis to explain this observation.
   b.   Propose an experiment to test your hypothesis (assume you have access to any necessary tracers and lab facilities).

Thank you!

Explanation / Answer

One simple way to get an estimate of the level of phototsynthetic activity in a green plant is to place the plant in a sealed container and measure the rate at which oxygen is produced (photosynthetically).

When such an experiment is actually performed you can find that by increasing the intensity of light increases the rate of photosynthesis, but only up to a certain point, beyond which increasing the brightenss of the light has little or absolutely no effect on the rate of photosynthesis.

Conversely, reducing the brightness of the light causes a decrease in photosynthetic activity.

The light intensity at which the net amount of oxygen produced is exactly zero, is called the compensation point for light. At this point the consumption of oxygen by the plant due to cellular respiration is equal to the rate at which oxygen is produced by photosynthesis.

The compensation point for light intensity varies according to the type of plant, but it is typically 40 to 60 W/m2 for sunlight. The compensation point for light can be reduced (somewhat) by increasing the amount of carbon dioxide available to the plant, allowing the plant to grow under conditions of lower illumination.

b) Under conditions of constant and uniform illumination the rate of photosynthesis can be increased by simply increasing the amount of carbon dioxide (i.e. increasing the atmospheric partial pressure) available to plants.

As before, one can measure the rate of photosysthesis as a function of carbon dioxide pressure by placing a green plant in a sealed container and measuring the rate at which oxygen is produced.

As the partial pressure of carbon dioxide increases there is an almost linear increase in the rate of oxygen production, which implies an identical increase in the rate of phtotosythesis.

This increase eventually levels off, and further increases in the concentration of carbon dioxide have no further effect.

Conversely, reducing the carbon dioxide concentration reduces the rate of photosynthetic activity. The level at which the oxygen production rate drops to zero is called the compensation point for carbon dioxide.

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