Abstract
The response of carbon fixation in C3 plants to elevated CO2 is relatively larger when photosynthesis is limited by carboxylation capacity (VC) than when limited by electron transport (J). Recent experiments under controlled, steady-state conditions have shown that photosynthesis at elevated CO2 may be limited by VC even at limiting PPFD. These experiments were designed to test whether this also occurs in dynamic field environments. Leaf gas exchange was recorded every 5 min using two identical instruments both attached to the same leaf. The CO2 concentration in one instrument was controlled at 400 μmol mol−1 and one at 600 μmol mol−1. Leaves were exposed to ambient sunlight outdoors, and cuvette air temperatures tracked ambient outside air temperature. The water content of air in the leaf cuvettes was kept close to that of the ambient air. These measurements were conducted on multiple, mostly clear days for each of three species, Glycine max, Lablab purpureus, and Hemerocallis fulva. The results indicated that in all species, photosynthesis was limited by VC rather than J at both ambient and elevated CO2 both at high midday PPFDs and also at limiting PPFDs in the early morning and late afternoon. During brief reductions in PPFD due to midday clouds, photosynthesis became limited by J. The net result of the apparent deactivation of Rubisco at low PPFD was that the relative stimulation of diurnal carbon fixation at elevated CO2 was larger than would be predicted when assuming limitation of photosynthesis by J at low PPFD.
Highlights
Photosynthesis by terrestrial vegetation is a large component of the annual global carbon balance, and predicting how the photosynthetic CO2 assimilation (A) of terrestrial vegetation will respond to increased concentrations of CO2 in the atmosphere is vital to understanding the future global carbon cycle [1,2,3]
The results indicate that for all three species, when photosynthetic photon flux density (PPFD) was stable, A at 600 μmol mol−1 CO2 was accurately modeled by assuming limitation of A at both 400 and 600 μmol mol−1 CO2 by Vc rather than J (Table 1)
The results of this study indicated that for gradual changes in PPFD over the course of a day, caused by changes in the solar angle, leaf photosynthesis was always limited by Vc rather than by J in all three of these C3 species, both at the approximate current ambient CO2 concentration and at 1.5 times the current concentration
Summary
Photosynthesis by terrestrial vegetation is a large component of the annual global carbon balance, and predicting how the photosynthetic CO2 assimilation (A) of terrestrial vegetation will respond to increased concentrations of CO2 in the atmosphere is vital to understanding the future global carbon cycle [1,2,3]. Plants with C3 photosynthetic metabolism are of predominant importance in terms of numbers of species, total CO2 fixation, use as food for humans, and the responsiveness of photosynthesis to projected changes in atmospheric CO2. CO2 concentrations often use the Farquhar–von Caemmerer–Berry biochemical model of photosynthesis [4] and its recent modifications [5]. In the Farquhar–von Caemmerer–Berry model of C3 photosynthesis [4], A at high photosynthetic photon flux density (PPFD) is limited by the maximum carboxylation capacity of Rubisco (VCmax ) at low CO2 concentrations, by maximum electron transport capacity (Jmax ) at higher CO2 , and sometimes by the rate of utilization of triose phosphates (TPU) at the highest CO2 concentrations [5], with all of these rate-limiting parameters having different temperature dependencies. The external CO2 at the transition between limitation by VCmax and Jmax at high PPFD varies among species, and with implementations of the FvCB model, from about 450 to 700 μmol mol−1 [3], so the issue is highly relevant to global change issues
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