Submitted to: Tree Physiology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: February 29, 2000
Publication Date: N/A
Interpretive Summary: Carbon dioxide is fixed by plants during the process of photosynthesis. Stable carbon isotopes are carbon atoms stable in time (sub-12C and sub- 13C). About 1 percent of all carbon atoms are the heavier isotope (sub13-C)and this ratio of natural abundance is constant in atmospheric carbon dioxide. When plants that photosynthesize via a specific metabolic pathway (C-sub3) are subjected to salinity stress the ratio (sub13-C/ sub12-C) is altered in the plant tissue. This is because of a slower diffusion of heavy carbon dioxide through plant stomata and preferential fractionation for sub12-C over sub13-C during the enzymatic carboxylation reactions of photosynthesis. This is referred to as carbon isotope discrimination. In this study, salinity tolerance of eucalyptus was evaluated by the relative decrease in discrimination of sub13-C of plant leaves and wood tissue and yields the same salt tolerance assessment as would be obtained had the entire tree been destructively harvested as is normally performed in salt tolerance experiments. Additionally, the technique was shown to reflect boron injury also, but the effect of boron on carbon isotope discrimination is smaller for boron than for salinity stress.
A study on Eucalyptus camaldulensis Dehn., Clone 4544 demonstrated that saplings grown and irrigated with combinations of salinity (2 to 28 dS m-1) and boron (1 to 30 mg l-1) integrate the history of these stresses through the discrimination of stable isotopes of carbon in leaf and woody tissues. Carbon isotopic discrimination (delta) in Eucalyptus was reduced primarily by salinity. Decreases in discrimination due to boron stress were detectable in the absence of salinity stress and significant only in leaf tissues with significant boron injury. Sapwood core samples exhibited increased differences in discrimination due to salinity and boron with increasing age of the tree. Absolute values of delta varied with location of leaf or wood tissue, but relative effects of salinity on the relationship between (delta) and transpiration efficiency (W) were similar. Relative decreases in delta with increasing salinity stress were similar to relative decreases in biomass and both indices yielded similar salt tolerance model parameters. Strong correlations between delta, tree fresh weight, leaf area, and W suggest delta is useful in evaluating eucalyptus salt tolerance.