Oxygen-18 Content of Atmospheric Oxygen Does Not Affect the Oxygen Isotope Relationship between Environmental Water and Cellulose in a Submerged Aquatic Plant, Egeria densa Planch

Abstract

We determined that the oxygen isotopic composition of cellulose synthesized by a submerged plant, Egeria densa Planch., is related to the isotopic composition of environmental water by a linear function, P80 cellulose = 0.48 6180 water + 24.1%o. The observation of a slope of less than 1 indicates that a portion of cellulose oxygen is derived from an isotopically constant source other than water. We tested whether this source might be molecular oxygen by growing plants in the presence of high concentrations of 180 in the form of 02 bubbled into the bottom of an aquarium. Cellulose synthesized during this experiment did not have significantly different oxygen isotope ratios than that synthesized by control plants exposed to 02 of normal '80 abundance. We propose that oxygen in organic matter recycled from senescent portions of the plant is incorporated into cellulose. Our findings indicate that paleoclimatic models linking the oxygen isotope composition of environmental water to cellulose from fossil plants will have to be modified to account for contributions of oxygen from this or other sources besides water.Based upon experiments with '80-labeled water, Ruben et al. (21) Empirical models have been developed that account for the relationship between 0 isotope ratios of environmental water and cellulose (3,8). However, one of the major assumptions made, that the only source of 0 that affects the 180/1'60 ratios of cellulose is leaf water, may not be true. The evidence concerning the validity of this assumption comes from submerged aquatic plants, which can be used advantageously in studies of the relationship between 0 isotope composition of water and cellulose. Due to the lack oftranspiration, the '80/ 160 ratio of tissue water in submerged plants is close to that of the water in which the plant grows (2). This is not the case for terrestrial plants or emergent aquatic plants, which can have leaf water significantly enriched in the heavy isotopes of 0 and H relative to source water due to isotope effects during evapotranspiration (2,11,12,17,19,29

Comparison of measured and modeled variations in piñon pine leaf water isotopic enrichment across a summer moisture gradient

“…The threshold relationship we observed between s and h was likely related to reduced transpiration rates at low h. When h was greater than about 25%, s ranged from 1 h to 18 h, which is comparable with h of 20-50% and s of 2-12.5 h in semi-arid Eucalyptus mallee (Walker and Brunel 1990), but longer than 1-3 h for wheat in growth chambers (Walker et al 1989) and for tropical rainforest trees (Zundel et al 1978). Values of s>1 day in succulent plants in southern California were associated with low diurnal variability in enrichment (Cooper and DeNiro 1989). Yakir et al (1994b) observed temporal variations in leaf water isotopic composition related to water stress in Peperomia congesta.…”

“…Enrichment of 18 O in phloem sucrose supports the importance of the Pe´clet effect and transpiration rate on the difference between measured and predicted d lw (Barbour et al 2000a), and this explanation for lower than expected d lw values may apply to many situations (Barbour et al 2004). Less commonly, measured d lw values may be higher than predicted steady-state values; this has been explained by decreasing leaf water volume and nonsteady-state conditions (e.g., Farris and Strain 1978;Wang and Yakir 1995;Cernusak et al 2002), or by repeated cycles of enrichment in a partially closed pool (Cooper and DeNiro 1989) or along a series of pools (Helliker and Ehleringer 2000). These different explanations for unexpectedly high d lw values have so far been applied to a limited number of data sets.…”

Variation in oxygen isotope fractionation during cellulose synthesis: intramolecular and biosynthetic effects

“…Autotrophically synthesized cellulose should have its oxygen isotope ratios completely determined by the oxygen isotope ratios of the in situ water. Previous studies of autotrophically synthesized cellulose usually measured slopes between 0.48 and 0.88 (Cooper & DeNiro 1989; Yakir & DeNiro 1990; Sauer, Miller & Overpeck 2001). Previous explanations proposed that a change in the isotopic composition of the in situ water available during the growth of the plant material caused the slope to be less than expected (Yakir & DeNiro 1990).…”

Oxygen isotope ratios of cellulose-derived phenylglucosazone: An improved paleoclimate indicator of environmental water and relative humidity

“…In the above case it is expected that F should be very close to 1 (=1.027) but investigators usually observe a slope less than 1 ranging from 0.48 to 0.88 (Cooper and DeNiro, 1989;Yakir and DeNiro, 1990;Sauer et al, 2001). However, if we consider that the autotrophic fractionation Also shown are the values of the parameters used in the calculation of the model equations and the average residuals between equations and observations.…”