Poster: Photosynthesis
Abs #
333: What proportion of daily photosynthesis is consumed in respiration by soybeans fixing N or given nitrate?
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Presenter: |
Evans, John R, evans@rsbs.anu.edu.au |
Authors | Evans, John R (A) (B) Box, Sally (A) (B) | | Affiliations: |
(A): Environmental Biology Group, Research School of Biological Sciences, Australian National University
(B): Cooperative Research Centre for Greenhouse Accounting
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| Web Site: | http://www.rsbs.anu.edu.au | |
Daily photosynthesis can be divided into two parts, one used for respiration and the other being growth. Daily photosynthesis depends on the leaf area, nitrogen content and daily irradiance, in the absence of water and temperature limitations. Respiration rate depends on the rate of growth, nutrient uptake, N reduction pathway, phloem loading and maintenance, as well as being influenced by substrate supply. Given the significant respiratory cost associated with symbiotic N fixation (7.9 mol C per mol N, equivalent to 0.4 mol C respired per mol C in biomass), we compared photosynthesis, respiration and growth of nodulated N fixing soybeans with non-nodulated soybeans supplied with three concentrations of nitrate. The curvilinear relationship between photosynthesis and nitrogen content per unit leaf area was independent of N source. Therefore, knowing the average leaf nitrogen content and daily irradiance enabled daily photosynthesis to be calculated. Whole plant respiration rate was measured just prior to two destructive harvests for each treatment from which relative growth rate (RGR) was calculated. The RGR of N fixing soybeans was intermediate between the 1.5 and 15 mM nitrate treatments. Whole plant respiration per unit plant mass increased with RGR. At a given RGR, whole plant respiration was 35% greater for N fixing soybean compared to those supplied with nitrate. However, due to greater rates of photosynthesis, the proportion of daily photosynthesis respired was only slightly greater, being 0.50 and 0.46 for the N fixing and nitrate treatments, respectively. This calculation was made in two ways, combining growth analysis data with either whole plant respiration or single leaf photosynthesis measurements. The two independent methods yielded similar values.
