Submitted to: Planta
Publication Type: Peer reviewed journal
Publication Acceptance Date: 5/28/2004
Publication Date: 7/7/2004
Citation: Bernacchi, C.J., Ort, D.R., Morgan, P.B., Long, S.P. 2004. The Growth of Soybean Under Free Air Concentration Enrichment (FACE) Stimulates Photosynthesis While Decreasing In Vivo Rubisco Capacity [abstract]. Planta (2005) 220:434-446. Interpretive Summary: Human induced activities are driving changes in the atmosphere; most noticeable is the increase in concentrations of carbon dioxide ([CO2]). While the pollutant carbon dioxide is known to have many negative effects on the climate, this increase is often demonstrated to increase the growth of plants. Knowing how crop species alter their growth in elevated [CO2] is a fundamental question as these responses may help to slow the impact of climate change and result in higher crop yields. In this study photosynthesis, the process in which plants use energy light energy and to produce sugars from CO2, is studied for soybean grown in current and future predicted [CO2]. Soybean was chosen because it is one of the most important crop species for both the United States and the world. In particular, we determined whether the primary enzymes responsible for driving photosynthesis were altered with changes in [CO2]. The results indicate that plants optimize the amounts of enzymes in the leaf to account for the change in the atmosphere. That is, the amount of the photosynthetic enzyme responsible for fixing CO2 was reduced but there was no loss in photosynthesis. Rates of carbon assimilation were increased throughout the life of the plants. We conclude that optimization of photosynthesis for soybean will occur in future environments.
Technical Abstract: As the world's most important seed legume, the responses of net leaf level carbon assimilation (Asat) of soybean (Glycine max Merr.) to elevated atmospheric concentrations of CO2 ([CO2]) have been investigated extensively. Down-regulation of Asat at elevated [CO2] has been demonstrated for many C3 species and often associated with inability to utilize additional photosynthate and/or nitrogen limitation. In soybean, a nitrogen fixing species, both limitations are less likely than in crops lacking an N-fixing symbiont. Other studies have suggested that down-regulation may be a growth condition artifact, caused by limited rooting volume. Here Free Air Concentration Enrichment (FACE) is used to investigate whether, under field conditions, down-regulation occurs at elevated [CO2]. The maximum velocities of Rubisco carboxylation (Vc,max) and electron transport through photosystem II (Jmax), were determined from the responses of Asat to intercellular [CO2] (Ci) at biweekly intervals over two growing seasons. Mesophyll conductance to CO2 (gm) was determined from the responses of Asat and whole chain electron transport (J) to light. Elevated [CO2] increased Asat by 15-20% even though stomatal conductance was reduced. There was a small, yet statistically significant decrease in Vc,max and in Vc,max /Jmax inferring a shift in resource investment away from Rubisco. This raised the Ci at which the transition from Rubisco- to RuBP regeneration-limited photosynthesis, derived from the A/Ci response, occurred. The decrease in Vc,max/Jmax was not the result of a shift in gm, which was unchanged by elevated [CO2]. The results suggest that down-regulation occurs even when a plant with large sinks for photosynthate is grown with no artificial restriction of rooting volume.