Establishing a grassland signature in veins:¹⁸O in the leaf water of C₃and C₄grasses

Abstract

We show that 18 O evaporative enrichment of bulk leaf water in grass species can be significantly more enriched than predicted by the Craig-Gordon model, with C 4 grasses considerably more enriched than C3 grasses. Our results suggest that the unanticipated 18 O in surface waters (such as lakes) was described by Craig and Gordon (1) more than three decades ago and was later applied to leaves, which also undergo an isotopic enrichment during transpiration (2-5). Application of the Craig-Gordon model to leaves from trees and herbs has been shown experimentally to be robust over a wide range of environmental conditions (4, 6-7). Capitalizing on this established relationship, Farquhar and colleagues (8) showed that the equilibration of 18 O between CO 2 and leaf water could be used to partition the gross fluxes of CO 2 from vegetation on a global basis.Previous studies showed that there were no differences in bulk leaf water 18 O enrichment of Dicotyledonous plants possessing either the C 3 or C 4 photosynthetic pathways and that leaf water behaved similarly to predictions of the Craig-Gordon model (4,6 O retrodiffusing from terrestrial ecosystems to the atmosphere (8-11). However, the Craig-Gordon model of evaporative leafwater enrichment has remained untested on grasses and other Monocotyledonous species. A fundamental difference between monocots and dicots that hinted that ␦ 18O wl might be different is the veinal structure of leaves, with monocot leaves having parallel veins whereas dicot leaves primarily have a reticulate veinal structure. We hypothesized that grass leaves will exhibit a leaf-water 18 O evaporative enrichment pattern that is not consistent with the commonly used single-water-source version of the Craig-Gordon model, because the nature of water movement along parallel veins in a grass leaf is more analogous to movement of water through a sequence of pools in series.We investigated possible 18 O differences in leaf water enrichment with a variety of C 3 and C 4 grasses, because grasses with different photosynthetic pathway are known to have different interveinal differences (12, 13). Our hypotheses were (i) leaf water 18 O enrichment of grasses should be greater than observed in dicots and (ii) leaf water 18 O enrichment of C 4 grasses (narrower interveinal distances) should be greater than leaf water 18

18O Spatial Patterns of Vein Xylem Water, Leaf Water, and Dry Matter in Cotton Leaves

“…Although we expect the weighted mean of ⌬ lw from leaf punches to be larger than ⌬ lw, bulk because of the exclusion of veins in leaf discs sampling, the weighted mean of ⌬ lw being larger than the Craig-Gordon prediction clearly diverged from the expectations of all leaf water models. A higher than expected ⌬ lw has previously been encountered Helliker and Ehleringer, 2000), and the latter group explained their observations by applying the string-of-lakes model, with water pools having variable transpiration rates across the length of the leaf. In our experiment, the sampling of the leaf discs was biased given that the leaf punch could not sample within 1 mm of the major veins without rupturing the veins.…”

“…The tritiated water moved down a concentration gradient and was mostly absorbed in the cells and cell walls around the xylem vessels. The idea of progressive enrichment of vein water by tissue water was first mooted by Yakir (1992) and elaborated by Wang and Yakir (1995) and Helliker and Ehleringer (2000) to explain the spatial heterogeneity of leaf water. This phenomenon is strongly supported by our observations that the isotopic composition of xylem water in veins (␦ vw ) is responsive to changes in environmental conditions such as humidity (Table I).…”

“…The first quantitative application to leaves of the Gat-Bowser formulation was by Helliker and Ehleringer (2000). This approach requires the leaf blade to be divided into a discrete, finite number of evaporating elements.…”

“…Although the Craig-Gordon model basically describes water enrichment at the sites of evaporation compared with locally transpired water, it cannot adequately account for other aspects of leaf water enrichment, particularly the spatial variation of leaf water 18 O and/or D contents (Luo and Sternberg, 1992; Bariac et al, 1994;Wang and Yakir, 1995;Helliker and Ehleringer, 2000). Also, the Craig-Gordon model has often been found to overestimate the isotopic enrichment of bulk leaf water (Allison et al, 1985; Bariac et al, 1989;Walker et al, 1989;Walker and Brunel, 1990;Yakir et al, 1990;Flanagan et al, 1991aFlanagan et al, , 1991bFlanagan et al, , 1994Wang et al, 1998).…”

Oxygen isotope content of CO2 in nocturnal ecosystem respiration: 1. Observations in forests along a precipitation transect in Oregon, USA

“…Several studies have focused on the mechanistic details of leaf water enrichment through transpiration [Flanagan et al, 1991;Wang and Yakir,1995;Wang et al, 1998;Roden and Ehleringer, 1999;Helliker and Ehleringer, 2000], photosynthetic discrimination against C 18 O 16 O [Farquhar et al, 1993;Flanagan et al, 1994;Yakir et al, 1994;Gillon and Yakir, 2000a, 2000b, soil water enrichment through evaporation Mathieu and Bariac, 1996;Melayah et al, 1996a], and the processes by which soil-respired CO 2 is isotopically altered by interactions with soil water [Amundson et al, 1998;Tans, 1998;Miller et al, 1999;Stern et al, 2001]. While many of these processes are now well understood and can be fairly accurately modeled, such models required detailed accompanying measurements which were impossible to obtain at all of our sites for all sampling periods.…”