Submitted to: Plant Science
Publication Type: Peer reviewed journal
Publication Acceptance Date: 6/20/2014
Publication Date: 6/27/2014
Citation: Sun, J., Feng, Z., Leakey, A., Zhu, X., Bernacchi, C.J., Ort, D.R. 2014. Inconsistency of mesophyll conductance estimate causes the inconsistency for the estimates of maximum rate of Rubisco carboxylation among the linear, rectangular, and non-rectangular hyperbola biochemical models of leaf.... Plant Science. DOI: 10.1016/j.plantsci.2014.06.015. Interpretive Summary: Carbon dioxide in the world’s atmosphere is on the rise. In the last century it has risen 20% and this geologically unprecedented pace will continue resulting in an atmosphere in 2050 containing 50% more CO2 than it did in 1900. While this enrichment of the atmosphere CO2 portends certain hazards of which global warming is the most publicized, it also offers the potential to increase plant production as carbon dioxide is in effect a fertilizer. Using technology that enables us to mimic the atmosphere of 2050 under natural field conditions, we are able to directly investigate the expected effects of rising atmosphere CO2 soybean photosynthesis. However, interpreting the underlying mechanisms of the responses requires appropriate fitting of the response function. This study compared and optimized alternative fitting methods to determine that the down-regulation of soybean bean photosynthetic capacity by elevated CO2 and leaf aging was due to a combination of decreased carboxylation and decrease conductance of CO2 the interior of the leaf.
Technical Abstract: The responses of CO2 assimilation to [CO2] (A/Ci) were investigated at two developmental stages (R5 and R6) and in several soybean cultivars grown under two levels of [CO2], the ambient level of 370 µbar versus the elevated level of 550 µbar. The A/Ci data were analyzed and compared using various curve-fitting methods by either the combined iterations or the separated iterations of the Rubisco-limited photosynthesis (Ac) and/or the RuBP-limited photosynthesis (Aj) using various curve-fitting methods: the linear 2-segment model; the non-rectangular hyperbola model; the rectangular hyperbola model; the constant J method and the variable J method. Inconsistency was found among the various methods for the estimation of the maximum rate of carboxylation (Vcmax), the mitochondrial respiration rate in the light (Rd) and mesophyll conductance (gm). The analysis showed that the inconsistency was due to inconsistent estimates of gm values that decreased with an instantaneous increase in [CO2], and varied with the transition Ci cut-off between Rubisco-limited photosynthesis and RuBP-regeneration-limited photosynthesis, and due to over-parameters for non-linear curve-fitting with gm included. We proposed an alternate solution to A/Ci curve-fitting for estimates of Vcmax, Rd, Jmax and gm with the various A/Ci curve-fitting methods. The study indicated that down-regulation of photosynthetic capacity by elevated [CO2] and leaf aging was due to partially the decrease in the maximum rates of carboxylation and partially the decrease in gm. Mesophyll conductance lowered photosynthetic capacity by 18% on average for the case of soybean plants.