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Review
. 2016 May;67(10):3027-39.
doi: 10.1093/jxb/erw139. Epub 2016 Apr 19.

Why small fluxes matter: the case and approaches for improving measurements of photosynthesis and (photo)respiration

David T Hanson  1 , Samantha S Stutz  2 , John S Boyer  3
Affiliations
Review

Why small fluxes matter: the case and approaches for improving measurements of photosynthesis and (photo)respiration

David T Hanson et al. J Exp Bot. 2016 May.

Abstract

Since its inception, the Farquhar et al. (1980) model of photosynthesis has been a mainstay for relating biochemistry to environmental conditions from chloroplast to global levels in terrestrial plants. Many variables could be assigned from basic enzyme kinetics, but the model also required measurements of maximum rates of photosynthetic electron transport (J max ), carbon assimilation (Vcmax ), conductance of CO2 into (g s ) and through (g m ) the leaf, and the rate of respiration during the day (R d ). This review focuses on improving the accuracy of these measurements, especially fluxes from photorespiratory CO2, CO2 in the transpiration stream, and through the leaf epidermis and cuticle. These fluxes, though small, affect the accuracy of all methods of estimating mesophyll conductance and several other photosynthetic parameters because they all require knowledge of CO2 concentrations in the intercellular spaces. This review highlights modified methods that may help to reduce some of the uncertainties. The approaches are increasingly important when leaves are stressed or when fluxes are inferred at scales larger than the leaf.

Keywords: Diffusion; internal leaf CO2; mesophyll conductance; photosynthesis; respiration; stomatal conductance; xylem CO2..

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Figures

Fig. 1.
Fig. 1.
Leaf cross section showing small fluxes and conductances in leaves. CO2 fluxes are labelled with solid black arrows between concentrations at each location (c a = above the leaf, c i = intercellular spaces, c c = chloroplast, c m = mesophyll cytosol, c x = xylem sap) along with stomatal (g s) and mesophyll (g m) conductances. Water fluxes are shown with blue arrows, along with transpiration through stomata (E s) and the cuticle/epidermis (E c). The dashed circular arrow represents photorespiration and R d is respiration in the absence of photorespiration. Organelles in the exaggerated palisade layer cell (upper right) are coloured and labelled with a bold letter (C = chloroplast, M = mitochondrion, P = peroxisome), blue circles represent xylem in a leaf vein.
Fig 2.
Fig 2.
Common approach for determining respiration in the day (R d), the compensation point within the cell in the absence of respiration (Γ *), and the internal leaf CO2 concentration at which photosynthesis is balanced by photorespiration (c i*). The method of Laisk (1977) as modified by von Caemmerer et al. (1994) uses the intersection of the linear portion (low CO2) of multiple photosynthetic CO2 response curves, each measured at a different light intensity (low light is favoured for at least one, here Populus deltoides was measured at 500, 250, and 100 µmol m−2 s−1). The usual interpretation is that the intersection of the lines occurs at c i* and R d, and Γ * is at a higher CO2 concentration due to the effects of mesophyll conductance and respiration. This approach has several assumptions that are not always valid and may cause significant errors (von Caemmerer, 2013).
Fig. 3.
Fig. 3.
(A) Gas exchange of a sunflower leaf fed ABA to close the stomata. The leaf was excised and initially fed water, to which 10–4 M ABA was added at the arrow. Shown are the assimilation rate (A s), CO2 concentration in the bulk air (c a), CO2 concentration directly measured inside the leaf by sealing a cup to the abaxial surface (c i), and the CO2 concentration calculated for the same leaf [c i(calc)), round grey points] according to Boyer (2015a). Note that A s and c i decrease when the stomata close but c i(calc) increases. (B) Transpiration [E (s+c)] and A s for the leaf in (A). Transpiration is shown as E (s+c) because water vapour moves through both stomata and the cuticle/epidermis while CO2 moves mostly through stomata. The axes in (B) have been adjusted so the diffusion of water vapour and CO2 are superimposed before feeding ABA. This demonstrates the overestimation of water vapour diffusion after feeding with ABA. Data in (A) are redrawn from Boyer (2015a).
Fig. 4.
Fig. 4.
(A) Gas exchange of a sunflower leaf darkened at the arrow. Shown are the assimilation rate (A s), CO2 concentration in the bulk air (c a), CO2 concentration directly measured inside the leaf by sealing a cup to the abaxial surface (c i), and the CO2 concentration calculated for the same leaf [c i(calc), round grey points] according to Boyer (2015a). Note that A s becomes negative in the dark and the leaf produces CO2. (B) Conductances for CO2 (g c) and water vapour (g w) for the leaf in (A). The axes in (B) have been adjusted so the conductances in the light are superimposed in order to highlight the differences after darkening.
Fig. 5.
Fig. 5.
(A) A s–c i relationship for sunflower leaf with open stomata. (B) A s–c i relationship after closing the stomata with 10–4 M ABA. The large data point indicates multiple data on top of each other at higher c i because stomata closed more tightly as c i increased. Redrawn from Tominaga and Kawamitsu (2015a, b).
Fig. 6.
Fig. 6.
Light response of photosynthetic discrimination, Δobs, in Nicotiana tabacum measured at 400ppm CO2 with δe values of −4‰ and +148‰. The same leaf area was measured for each δe by starting with −4‰, transiently switching to +148‰ for 5–10min, and then returning to −4‰ until Δobs matched the prior Δobs at −4‰ (usually less than 10min) before proceeding to the next light intensity. This demonstrates that CO2 efflux from leaves has large effects on Δobs when leaves are provided air with 13C-enriched CO2, and that the effect is not constant across light intensities. N = 2.
Fig. 7.
Fig. 7.
Oxygen sensitivity of dΔobs (Δ-4–Δ+148) in Populus deltoides measured at a PAR of 200 µmol m−2 s−1 and 400ppm CO2 with δe values of −4‰ and +148‰. The same method was used as described for Fig. 6. This demonstrates the utility of this method for detecting CO2 efflux from photorespiration. N = 3.
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