Online CO₂ and H₂O oxygen isotope fractionation allows estimation of mesophyll conductance in C₄ plants, and reveals that mesophyll conductance decreases as leaves age in both C₄ and C₃ plants

Abstract

SummaryMesophyll conductance significantly, and variably, limits photosynthesis but we currently have no reliable method of measurement for C 4 plants.An online oxygen isotope technique was developed to allow quantification of mesophyll conductance in C 4 plants and to provide an alternative estimate in C 3 plants. The technique is compared to an established carbon isotope method in three C 3 species.Mesophyll conductance of C 4 species was similar to that in the C 3 species measured, and declined in both C 4 and C 3 species as leaves aged from fully expanded to senescing. In cotton leaves, simultaneous measurement of carbon and oxygen isotope discrimination allowed the partitioning of total conductance to the chloroplasts into cell wall and plasma membrane versus chloroplast membrane components, if CO 2 was assumed to be isotopically equilibrated with cytosolic water, and the partitioning remained stable with leaf age.The oxygen isotope technique allowed estimation of mesophyll conductance in C 4 plants and, when combined with well-established carbon isotope techniques, may provide additional information on mesophyll conductance in C 3 plants.

Bundle-sheath leakiness and intrinsic water use efficiency of a perennial C4grass are increased at high vapour pressure deficit during growth

“…One commonly used assumption is that g m of C 4 plants is infinite, and thus C i = C m . However, a recent study indicated that g m of C 4 plants is finite, although close to the high-end values of g m of C 3 plants (Barbour et al , 2016). In our study, assuming a lower range of g m across C i levels (see Supplementary Fig.…”

“…C i = C m . However, a recent study using a new oxygen isotope approach found that g m of C 4 species ranged between 0.66 and 1.8 mol m −2 s −1 measured at a leaf temperature of about 32 °C (Barbour et al , 2016). We calculated the minimum g m as: g mmin = A / C i (von Caemmerer et al , 2014).…”

Acclimation of Biochemical and Diffusive Components of Photosynthesis in Rice, Wheat, and Maize to Heat and Water Deficit: Implications for Modeling Photosynthesis

“…In contrast, an increase in g m with measurement temperature was reported by von Caemmerer and Evans (2015) and supported by data from Xiong et al (2015) in rice. A recent reported showed a decline in g m as leaves aged from fully expanded to senescing (Barbour et al, 2016), supporting that the above discrepancies reflect the importance of the experimental conditions and leaf age.…”

Impact of Leaf Traits on Temporal Dynamics of Transpired Oxygen Isotope Signatures and Its Impact on Atmospheric Vapor

“…Oxygen isotopic signatures of transpired water (δ E ) were determined by mass balance (Dubbert et al, 2014a; Barbour et al, 2016): $\begin{array}{c}left{\delta }_E=\frac{u_{out}{w}_{out}{\delta }_{out}-u_{in}{w}_{in}{\delta }_{in}}{u_{out}{w}_{out}-u_{in}{w}_{in}}\\ =\frac{w_{out}{\delta }_{out}-w_{in}{\delta }_{in}}{w_{out}-w_{in}}-\frac{w_{in}{w}_{out}\left({\delta }_{out}-{\delta }_{in}\right)}{w_{out}-w_{in}}\end{array}$ where u is flow rate [mol (air) s −1 ], w is mole fraction [mol (H 2 O) mol (air) −1 ] and δ is isotope ratio of air, where subscripts denote the incoming ( in ) and outgoing ( out ) air stream of the chamber. δ, w and u were measured by alternately connecting the L2120-i with the sample and reference gas flow of the GFS-3000.…”