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Summary Photosynthetic capacity per unit irradiance is greater, and the marginal carbon revenue of water (∂A/∂E) is smaller, in shaded leaves than sunlit leaves, apparently contradicting optimization theory. I tested the hypothesis that these patterns arise from optimal carbon partitioning subject to biophysical constraints on leaf water potential.In a whole plant model with two canopy modules, I adjusted carbon partitioning, nitrogen partitioning and leaf water potential to maximize carbon profit or canopy photosynthesis, and recorded how gas exchange parameters compared between shaded and sunlit modules in the optimum.The model predicted that photosynthetic capacity per unit irradiance should be larger, and ∂A/∂Esmaller, in shaded modules compared to sunlit modules. This was attributable partly to radiation‐driven differences in evaporative demand, and partly to differences in hydraulic conductance arising from the need to balance marginal returns on stem carbon investment between modules. The model verified, however, that invariance in the marginal carbon revenue of N (∂A/∂N) is in fact optimal.The Cowan–Farquhar optimality solution (invariance of ∂A/∂E) does not apply to spatial variation within a canopy. The resulting variation in carbon–water economy explains differences in capacity per unit irradiance, reconciling optimization theory with observations.
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A Question-Based Approach to Teaching Photosynthesis, Carbohydrate Partitioning, and Energy Flow
To help high school students develop a deeper understanding of energy and matter flow in biological systems, a key goal of the Next Generation Science Standards (NGSS), we have designed a series of inquiry-driven modules that experimentally explore photosynthesis in the context of carbohydrate partitioning. Carbohydrate partitioning refers to the distribution of photosynthesis products from source organs, such as leaves, to growing or storage tissues. These cost- and time-effective modules help students develop a more integrated understanding of how energy flows from light into leaves before moving outward to other parts of the plant and ultimately into other organisms. Our approach of teaching carbohydrate partitioning along with photosynthesis greatly expands the number of NGSS core ideas and cross-cutting concepts in an integrated manner, empowers students to see how photosynthesis and carbohydrate partitioning are central to their existence, and provides a rich platform for experimentally addressing questions related to photosynthesis, crop production, global climate change, plant physiology, and the carbon cycle.
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- Award ID(s):
- 1748105
- PAR ID:
- 10461778
- Date Published:
- Journal Name:
- The American Biology Teacher
- Volume:
- 79
- Issue:
- 8
- ISSN:
- 0002-7685
- Page Range / eLocation ID:
- 655 to 660
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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- Free Publicly Accessible Full Text
-
Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1525/abt.2017.79.8.655
