ARS | Publication request: Photosynthetic Enzyme Level and Distribution through Canopies in Relation to Canopy Photosynthesis and its Acclimation to Light, Temperature and CO2

Submitted to: Annals Of Botany
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: June 18, 2010
Publication Date: September 23, 2010
Citation: Johnson, I., Thornley, J.H., Frantz, J., Bugbee, B. 2010. Photosynthetic Enzyme Level and Distribution through Canopies in Relation to Canopy Photosynthesis and its Acclimation to Light, Temperature and CO2. Annals Of Botany.doi: 10.1093/aob/mcq183.

Interpretive Summary: The distribution of canopy proteins involved in photosynthesis influences growth, quality, and nutrient (specifically, nitrogen) demand. Most canopy photosynthesis models assume a rapidly declining concentration of protein from top to the bottom of the canopy, since this mirrors light distribution, but it is rarely found in observations. A model of canopy photosynthesis is presented for C3 and C4 canopies that considers a balanced approach between photosynthesis and respiration as well as plant carbon partitioning. Protein distribution is related to light level in the canopy by a flexible equation, and can accomodate the common assumption previously mentioned. The model is designed to be simple to execute for unique crop, pasture, and ecosystem studies. The level and distribution of proteins is calculated in a way (optimized) such that canopy photosynthesis is maximized. The resulting output shows that the optimum distribution of proteins is not in fact exponential, but is quite linear near the top of the canopy (gradual or negligible decline), which is consistent with experimental observations. The overall protein concentration at any given point within the canopy is dependent on environmental conditions, including the distribution of direct and diffuse light. The present approach derives the optimum distribution of protein with characteristics that are consistent with observation. Although canopies may not always operate at an optimum, the analysis supports the theoretical approach. Enzyme distribution has implications for the prediction of carbon assimilation, plant quality and nitrogen demand. This approach should improve the accuracy of a wide range of plant growth models for a variety of plant growth systems in both managed and unmanaged environments.

Technical Abstract: The distribution of photosynthetic enzymes through the canopy affects canopy photosynthesis, as well as plant quality and nitrogen demand. Most canopy photosynthesis models assume an exponential distribution of photosynthetic enzymes through the canopy, although this is rarely consistent with experimental observation. Previous optimization schemes to derive the enzyme distribution through the canopy generally focus on the distribution of a fixed level of nitrogen through the canopy, which fails to account for the variation in both the actual nitrogen level in response to environmental conditions and the interaction of photosynthesis and respiration at similar levels of complexity. A model of canopy photosynthesis is presented for C3 and C4 canopies that considers a balanced approach between photosynthesis and respiration as well as plant carbon partitioning. Photosynthetic enzyme distribution is related to irradiance level in the canopy by a flexible equation for which the exponential distribution is a special case. The model is designed to be simple to parameterise for crop, pasture and ecosystem studies. The level and distribution of photosynthetic enzymes is calculated that maximizes canopy net photosynthesis. The optimum distribution of photosynthetic enzymes is not exponential, but is quite linear near the top of the canopy, which is consistent with experimental observations. The overall level within the canopy is dependent on environmental conditions, including the distribution of direct and diffuse components of irradiance. The widely used exponential distribution of photosynthetic enzymes through the canopy is generally inappropriate. The present scheme derives the optimum distribution with characteristics that are consistent with observation. Although canopies may not always operate at an optimum, the analysis supports the theoretical approach. Enzyme distribution has implications for the prediction of carbon assimilation, plant quality and nitrogen demand.