Photosynthesis: During photosynthesis, plants both absorb and emit carbon dioxide. The net exchange (output or input) depends on many factors. Two of these factors are light and temperature. Figure 1.44 on the following page shows25 the net carbon dioxide exchange rates for 5-week-old rice plants at 40 degrees, 60 degrees, and 80 degrees Fahrenheit. The vertical axis shows the net exchange, in milligrams, of carbon dioxide per hour per plant. Values above the horizontal axis indicate that the plant is taking in more carbon dioxide than it releases. Values below the horizontal axis indicate that the plant is releasing more carbon dioxidethan it takes in. The horizontal axis shows amount of light, measured in thousands of foot-candles.26 The following questions pertain to the 5-week-old rice plants at the indicated temperatures.
a. At a temperature of 80 degrees, about how many foot-candles of light will cause a plant to emit the same amount of carbon dioxide that it absorbs?
b. At about how many foot-candles will plants at 80 degrees and 40 degrees show the same net exchange of carbon dioxide?
c. At 0 foot-candles—that is, in the dark—which temperature (80 degrees, 60 degrees, or 40 degrees) will result in the largest emission of carbon dioxide?
d. At which temperature (80 degrees, 60 degrees, or 40 degrees) is the net exchange of carbon dioxide least sensitive to light? Explain your reasoning.