Chapter 8, Problem 1ITD

Photosynthesis directly opposes respiration in determining how plants influence atmospheric CO₂ concentrations. When a leaf is in the light, both photosynthesis and respiration are occurring simultaneously. The data in the Table were collected from the leaf of a sagebrush plant that was enclosed in a chamber that measures the rate of CO₂ exchange. The same leaf was used to collect the data in Interpret the Data in Chapter 7. Respiration is shown as a negative and photosynthesis as a positive rate of CO₂ exchange. The net photosynthesis rate is the amount of CO₂ (in micromoles per square meter per second) assimilated by the leaf while respiration is occurring; a positive value indicates more photosynthesis is occurring than respiration. The light exposed to the leaf is quantified as the number of photons in the 400 to 700 nm wavelength, the photosynthetic photon flux density (PPFD); 2,000 μmol/m²/s is equivalent to the amount of light occurring at midday in full Sun.

Observation	Photosynthetic Photon Flux Density (PPFD) (μmol/m2/s)	Net Photosynthesis (μmol/m2/s)
1	2,000	9.1
2	1,500	8.4
3	1,250	8.2
4	1,000	7.4
5	750	6.3
6	500	4.8
7	250	2.2
8	0	-2.0

Why is net photosynthesis negative when PPFD is zero? Looking at the respiration data from Interpret the Data in Chapter 7, at what temperature do you think these data were collected?

Source: Data based on unpublished research by Brent Ewers, University of Wyoming.