1. You are an ecologist interested in energetics of ecological systems. You begi
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Question
1. You are an ecologist interested in energetics of ecological systems. You begin your current research program with a study of primary productivity of corn plants growing in a field in Iowa during mid-summer. At noon on a sunny day, you enclose a single leaf (still attached to the plant) in a transparent chamber. The chamber is connected to a measurement device called an infrared gas analyzer, which is capable of measuring minute changes in the concentration of carbon dioxide (CO2) of the atmosphere within the chamber. The chamber is also maintained at a constant 30°C (same as the outside air temperature) during your observations. You first expose the corn leaf to full sunlight and observe that CO2 is being depleted by the leaf from the chamber at a rate of 18 mg CO2 per 100 cm2 leaf area per hour. You then cover the chamber with aluminum foil to exclude all light and observe that CO2 is being released by the leaf into the chamber at a rate of 3 mg CO2 per 100 cm2 leaf area per hour.
Questions
What is the rate of net primary productivity for this leaf during the measurement period (In mg CO2 per 100 cm2 leaf area per hour)? What is the rate of respiration for this leaf (same units as above)? What is the rate of gross primary productivity for this leaf (same units as above)? At this site, corn is planted at a density that results in 75% of the surface being covered by leaves. What is the total net primary productivity for one hectare of this corn field (in g C per hectare per year)? Assume that day length averages 12 hours per day over the course of a 180 day growing season and that your measurement is representative of the NPP for the entire growing season. Show your calculations.
Explanation / Answer
The usage and release of CO2 are dependent on the rate of photosynthesis of the plant. During the daylight the plant uses the sunlight to convert the CO2 present in the atmosphere to produce carbohydrates for its metabolism, in turn it releases oxygen out into the atmosphere. This is why when the corn leaf exposed to sunlight, you observe the depletion of CO2 from the chamber. When you cover the chamber with aluminium foil to restrict the light, the plant uses the carbohydrates for its respiration where plant releases CO2 into the chamber. That is why there is an increase in CO2 release in the chamber but less compared to the depletion during photosynthesis.
Net primary productivity (NPP) is the amount of primary production after the cost of plant respiration (R). Gross primary productivity (GPP) is total amount of CO2 fixed during photosynthesis.
Rate of respiration (R)= 3mg CO2 per 100cm2 leaf per hour.
Gross primary productivity (GPP)= 18mg CO2 per 100cm2 leaf per hour.
NPP= GPP-R
NPP= 18-3= 15mg CO2 per 100cm2 leaf per hour.
The total net productivity for the one day is 15mg x 24 hrs= 180mg of CO2
For the whole 180 day it is 64800mg of CO2
For 1 hectare it is 64.8 x 108 g of CO2 per hectare per year.
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