The δ¹⁸O and δ²H of water in the leaf growth‐and‐differentiation zone of grasses is close to source water in both humid and dry atmospheres

Abstract

The oxygen and hydrogen isotope composition of water in the leaf growth-and-differentiation zone, LGDZ, (δ O , δ H ) of grasses influences the isotopic composition of leaf cellulose (oxygen) and wax (hydrogen) - important for understanding (paleo)environmental and physiological information contained in these biological archives - but is presently unknown. This work determined δ O and δ H , O- and H-enrichment of LGDZ (∆ O and ∆ H ), and the O- and H-enrichment of leaf blade water (∆ O ∆ H ) in two C and three C grasses grown at high and low vapor pressure deficit (VPD). The proportion of unenriched water (p ) in the LGDZ ranged from 0.9 to 1.0 for O and 1.0 to 1.2 for H. VPD had no effect on the proportion of O- and H-enriched water in the LGDZ, and species effects were small or nonsignificant. Deuterium discrimination caused depletion of H in LGDZ water, increasing (apparent) p -values > 1.0 in some cases. The isotopic composition of water in the LGDZ was close to that of source water, independent of VPD and much less enriched than previously supposed, but similar to reported xylem water in trees. The well-constrained p will be useful in future investigations of oxygen and hydrogen isotopic fractionation during cellulose and wax synthesis, respectively.

An oxygen isotope chronometer for cellulose deposition: the successive leaves formed by tillers of a C4 perennial grass

“…Furthermore, it was recently shown that VPD has no effect on the oxygen isotope composition of water in the leaf‐growth‐and‐differentiation zone (Liu et al . ) where leaf cellulose deposition occurs. These relationships indicate very strongly that the large change of δ 18 O Cel observed in the youngest leaves of transferred plants resulted from the VPD effect on ∆ 18 O LW in the new VPD environment, and the associated effect on the oxygen isotope composition of new photosynthetic products (i.e.…”

“…In addition, the δ 18 O of CO 2 in the growth chamberwhich differed between CO 2 sourceshad no effect on Δ 18 O Cel , as had already been established by DeNiro & Epstein (1979) and Liu et al (2016), as all oxygen in cellulose derives from water (DeNiro & Epstein 1979;Barbour 2007). Furthermore, it was recently shown that VPD has no effect on the oxygen isotope composition of water in the leaf-growth-anddifferentiation zone (Liu et al 2017) where leaf cellulose deposition occurs. These relationships indicate very strongly that the large change of δ 18 O Cel observed in the youngest leaves of transferred plants resulted from the VPD effect on Δ 18 O LW in the new VPD environment, and the associated effect on the oxygen isotope composition of new photosynthetic products (i.e.…”

“…In addition, a contribution of carbohydrate stores to cellulose deposition might have been detectable in transferred plants based on an increased ratio of f new O to f new C . In the theoretical situation, that metabolism of storage-derived substrate involves no carbonyl oxygen formation (which is unlikely; Hill et al 1995, Barbour 2007, Song et al 2014, or that such formation occurs only in unenriched medium water (note that water in the leaf growth-and-differentiation zone of grasses is at most slightly enriched; Liu et al 2017), the oxygen isotope composition of storage-derived metabolites should not be affected by a VPD change. However, if storage-derived metabolites exchange carbonyl oxygen with evaporatively enriched water (e.g.…”

“…The isotopic composition of water samples was measured by Cavity Ring-Down Spectroscopy (L2120-I, Picarro) as described by Liu et al (2017).…”

Further experiments comparing direct vapor equilibration and cryogenic vacuum distillation for plant water stable isotope analysis

“…The outer sheathes of older leaves were removed for all samples to mitigate contributions from that evaporatively enriched (in 2 H and 18 O) material. 13,14 This plant portion was selected for its lower MeOH and EtOH content. 1 We sought to utilize an analyte source that would inherently have limited spectral contamination issues.…”