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 16 mg CO2 per 100 cm2 leaf area per hour. The measurements are taken over time so both photosynthesis and respiration are occurring during the measurement. 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 2 mg CO2 per 100 cm2 leaf area per hour. 1. What is the rate of net primary productivity for this leaf? Explain your answer. 2. What is the rate of gross primary productivity for this leaf? Explain your answer.
