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Title: SOIL AND PLANT WATER RELATIONS, NOT PHOTOSYNTHETIC PATHWAY, PRIMARILY INFLUENCE PHOTOSYNTHETIC RESPONSES IN A SEMI-ARID ECOSYSTEM UNDER ELEVATED CO2

Author
item Lecain, Daniel
item Morgan, Jack
item Mosier, Arvin
item Nelson, Jim

Submitted to: Annals of Botany
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/12/2003
Publication Date: 5/9/2003
Citation: Lecain, D.R., Morgan, J.A., Mosier, A.R., Nelson, J.A. 2003. Soil and plant water relations, not photosynthetic pathway, primarily influence photosynthetic responses in a semi-arid ecosystem under elevated CO2. Annals of Botany. 92:41-52.

Interpretive Summary: The concentration of atmospheric CO2 has been increasing at historically unprecedented rates due to burning of fossil fuels and deforestation. Increasing CO2, along with other "greenhouse gases" are expected to cause an increase in global temperature. Since both CO2 and temperature have a big impact on plants, scientists have conducted research projects investigating the impact of increasing global CO2 and temperature on agricultural ecosystems. In this study we placed six large (15.5 m diameter) open-top-chambers over the native shortgrass prairie in Colorado, and injected three of the chambers with pure CO2 to a concentration expected to occur late in this century. As part of the study we investigated how plant photosynthesis and transpiration of two of the major grasses, Pascopyrum smithii (C3 type photosynthesis) and Bouteloua gracilis (C4 type photosynthesis), would adapt to the elevated CO2. Somewhat surprisingly (since CO2 is used in photosynthesis), there was very little direct benefit of elevated CO2 on photosynthesis of either grass species. However, we found that elevated CO2 reduced plant water use by reducing leaf transpiration rates. Therefore, the elevated CO2 chambers had improved soil water content. This allowed plants to maintain photosynthesis for longer periods during the typical dry summer months. This resulted in improved productivity (plant weight per m2) under elevated CO2. However, improved productivity was accompanied by reduced plant nitrogen concentration, which will result in reduced forage quality in an elevated CO2 world.

Technical Abstract: To model the effect of increasing atmospheric CO2 on semi-arid grasslands, the gas exchange responses of leaves to seasonal changes in soil water and how they are modified by CO2 must be understood for C3 and C4 species which grow in the same area. In this study, open-top-chambers were used to investigate the photosynthetic and stomatal responses of Pascopyrum smithii (C3) and Bouteloua gracilis (C4) grown at 360 and 720 µmol mol-1 CO2 in a semi-arid shortgrass steppe. Assimilation rate (A) and stomatal conductance (gs), at the treatment CO2 concentrations and at a range of intercellular CO2 concentrations, and leaf water potential (?leaf) were measured during four years with variable soil water content caused by season and CO2 treatment. Carboxylation efficiency of ribulose bisphosphate carboxylase/oxygenase (Vc,max), and ribulose bisphosphate regeneration capacity (Jmax) were reduced in P. smithii grown in elevated CO2, to the degree that A was similar in elevated and ambient CO2 (when soil moisture was adequate). Photosynthetic capacity was not reduced in B. gracilis under elevated CO2, but A was nearly saturated at ambient CO2. There were no stomatal adaptations independent of photosynthetic acclimation. Although photosynthetic capacity was reduced in P. smithii growing in elevated CO2, reduced gs and transpiration improved soil water content and ?leaf in the elevated CO2 chambers, thereby improving A of both species during dry periods. These results suggest that photosynthetic responses of C3 and C4 grasses in this semi-arid ecosystem will be driven primarily by the effect of elevated CO2 on plant and soil water relations.