Minisymposium 21: Integrative Plant Biology

Abs # 35001: Linking carbon and nitrogen gain with respiration: soybeans fixing N or using nitrate

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): CRC for Greenhouse Accounting
Web Site:	http://www.rsbs.anu.edu.au

Plant growth requires the coupling of carbon gain through photosynthesis with N acquisition either from mineral sources or biological fixation of atmospheric N. Part of daily photosynthesis is consumed by respiration during the synthesis of new plant structural material, nutrient uptake, N reduction, phloem loading and maintenance. What remains is growth. A significant proportion of respiration is associated with symbiotic N fixation (7.9 mol C per mol N, equivalent to 0.4 mol C respired per mol C in biomass). To put this in context, we compared photosynthesis, respiration and growth of nodulated N fixing soybeans with non-nodulated soybeans supplied with three concentrations of nitrate. Leaf photosynthesis was measured and related to nitrogen content per unit leaf area. The relationship was independent of N source and was used to model daily photosynthesis from daily irradiance. Whole plant respiration rate was measured just prior to two destructive harvests for each treatment from which relative growth rate (RGR) was calculated. Whole plant respiration per unit plant mass increased with increasing RGR. At a given RGR, whole plant respiration was 35% greater for soybeans fixing N compared to those supplied with nitrate. However, because they produced thinner leaves, N fixing soybeans had a greater leaf area and hence photosynthesis per unit plant mass. As a result, the proportion of daily photosynthesis that was respired was only slightly greater for the N fixing compared to the nitrate treatments (0.50 and 0.46, respectively). This proportion was calculated in two ways by combining growth analysis with either whole plant respiration or single leaf photosynthesis measurements. The two independent methods yielded similar values. For this experiment, the similarity in the proportion of daily photosynthesis that was respired between N fixing and nitrate grown plants reflects the flexibility of plants in coordinating their carbon income and costs. In a separate experiment where nitrate was supplied to nodulated plants, there was no effect of nitrate addition on leaf thickness, and plants dependent upon N fixation had lower growth rates for a given rate of photosynthesis. This suggests that the impact of the respiratory costs associated with N fixation on plant growth rates will depend on the ability of the plant to change leaf thickness.