Foliage Orientation and Carbon Gain in Two Tussock Grasses as Assessed with a New Whole-plant Gas Exchange Model

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Functional Ecology

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1. The effect of foliage orientation on light interception and photosynthesis within two tussock grasses, Agropyron desertorum and Pseudoroegneria spicata, was assessed with a new three-dimensional simulation model. The model can account for spatial differences in foliage distribution within the tussock as well as differences in physiological properties of the foliage elements. 2. Light interception and net photosynthesis are calculated for sunlit and shaded foliage. Model predictions of whole-plant net photosynthesis (Atuss), transpiration (Etuss), water-use efficiency, expressed as the ratio Atuss to Etuss, and average intercellular CO2 compared very well with measurements in the field taken with a whole-plant gas-exchange system. 3. Results of simulations indicate that for well-developed tussocks of Agropyron desertorum, steep foliage orientation provides a slight advantage in daily net carbon gain over plants with more horizontally oriented foliage, but these relationships change considerably with time of day. Foliage orientation had little effect on daily net carbon gain of Pseudoroegneria spicata. The differences in carbon gain related to foliage orientation between these species are probably due to differences in tussock foliage density. 4. Substantially lower midday incident fluxes in tussocks with steep foliage orientation could greatly reduce the potential for photoinhibition over plants with more horizontally oriented foliage.

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