Journal
JOURNAL OF EXPERIMENTAL BOTANY
Volume 60, Issue 8, Pages 2271-2282Publisher
OXFORD UNIV PRESS
DOI: 10.1093/jxb/erp063
Keywords
Diffusion limits; drought; mid-day reduction of photosynthesis; model parameterization; photosynthesis model
Categories
Funding
- Estonian Ministry of Education and Science [SF1090065s07]
- Estonian Academy of Sciences
- Australian Research Council
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Mesophyll diffusion conductance to CO2 (g(m)) is an important leaf characteristic determining the drawdown of CO2 from substomatal cavities (C-i) to chloroplasts (C-C). Finite g(m) results in modifications in the shape of the net assimilation (A) versus C-i response curves, with the final outcome of reduced maximal carboxylase activity of Rubisco (V-cmax), and a greater ratio of the capacity for photosynthetic electron transport to V-cmax (J(max)/V-cmax) and alterations in mitochondrial respiration rate (R-d) when estimated from A/C-i responses without considering g(m). The influence of different Farquhar et al. model parameterizations on daily photosynthesis under non-stressed (C-i kept constant throughout the day) and stressed conditions (mid-day reduction in C-i) was compared. The model was parameterized on the basis of A/C-C curves and A/C-i curves using both the conventional fitting procedure (V-cmax and R-d fitted separately to the linear part of the response curve and J(max) to the saturating part) and a procedure that fitted all parameters simultaneously. The analyses demonstrated that A/C-i parameterizations overestimated daily assimilation by 6-8% for high g(m) values, while they underestimated if by up to 70% for low g(m) values. Qualitative differences between the A/C-i and A/C-C parameterizations were observed under stressed conditions, when underestimated V-cmax and overestimated R-d of A/C-i parameterizations resulted in excessive mid-day depression of photosynthesis. Comparison with measured diurnal assimilation rates in the Mediterranean sclerophyll species Quercus ilex under drought further supported this bias of A/C-i parameterizations. While A/C-i parameterization predicted negative carbon balance at mid-day, actual measurements and simulations with the A/C-C approach yielded positive carbon gain under these conditions. In addition, overall variation captured by the best A/C-i parameterization was poor compared with the A/C-C approach. This analysis strongly suggests that for correct parameterization of daily time-courses of photosynthesis under realistic field conditions, g(m) must be included in photosynthesis models.
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