Publication Date

2012-12-03

Availability

Embargoed

Embargo Period

2014-12-03

Degree Type

Dissertation

Degree Name

Doctor of Philosophy (PHD)

Department

Biology (Arts and Sciences)

Date of Defense

2012-11-15

First Committee Member

Leonel S. Sternberg

Second Committee Member

David P. Janos

Third Committee Member

Donald L. DeAngelis

Fourth Committee Member

William T. Anderson

Abstract

Cellulose is a very stable structural molecule integrated in the incremental growth of tree rings, often used for paleoclimate reconstruction. As cellulose is synthesized it incorporates the oxygen isotopic signature of source (stem) and leaf water, which provide valuable information about climate such as precipitation and relative humidity. However, source and leaf water isotopic signatures can be masked by plant physiological and biochemical processes during cellulose synthesis. The biochemical interference does not occur equally in all the oxygen of the cellulose molecule. In Chapter 2, I compared the observed versus the reconstructed oxygen isotope ratios of stem water (δ18OSW) using δ18O of cellulose and of an artificially modified cellulose molecule. The only difference between cellulose and modified cellulose molecules is that the latter had the oxygen attached to the second carbon of the cellulose glucose moieties (OC-2) removed. The modified cellulose molecule was a better predictor of δ18OSW than was the entire cellulose molecule. This difference suggests that the oxygen attached to C2 introduces variability in the δ18OCELL not related to the δ18OSW. In Chapter 3, I compared the δ18OCELL and δ18OSW from two transects encompassing salt to freshwater, under the same climatic conditions. The δ18OCELL did not record differences in 18OSW. In one of the transects the above modification of the cellulose molecule improved the relationship between δ18OCELL and δ18OSW. In the second transect, despite no differences in RH between mangrove and freshwater plants, oxygen isotopic fractionation associated with leaf physiology caused the δ18OCELL to be less enriched than what was expected based on its source water. The changes in leaf physiology were associated with a longer leaf water pathway from the xylem to the stomatal pore in mangrove than in freshwater plants. Considering that mangrove plants are salt tolerant, a longer L could be linked to ultrafiltration of salts in the leaves. Changes in leaf physiology introduced variability in the δ18OCELL that was neither related to δ18OSW nor RH. In Chapter 4, I investigated the salinity effect in overall plant biochemistry and how it interfered with the recording of δ18OSW in the δ18OCELL. A currently accepted tree ring model used in climate reconstruction assumes no variation in plant biochemistry, which is incorporated in the model as the proportion of oxygen isotope exchange between sucrose and stem water (pex = 42%) and biochemical fractionation (εbio = 27‰). However, salinity effects caused ~2x increase in pex likely by increasing the pool of soluble carbohydrates, allowing more time for oxygen isotope exchange between carbohydrate intermediates and stem water during cellulose synthesis. The increase pex in salt treated plants caused their δ18OCELL to be 1.5‰ less enriched than expected. Even though εbio values differed between salt and freshwater treatments, they were not significant. However, when investigating soluble carbohydrate excess in a starchless mutant Arabidopsis thaliana without a salinity effect, εbio variation was confirmed. Salinity impacts in plants are fundamental both at physiological and biochemical level and must be considered so that the δ18OCELL of plants exposed to salinity can only reflect δ18OSW and RH. This study brings to attention the chances of developing misleading conclusions about climate reconstruction if plant physiological and biochemical variation are not fully understood and taken into account.

Keywords

oxygen isotope fractionation during cellulose synthesis; tree ring cellulose oxygen isotope ratios; cellulose derivative (phenylglucosazone); salinity effects on the plant biochemistry and cellulose; salinity effects on the leaf oxygen isotope enrichment above source water and cellulose; cellulose as climate proxy

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