Non-climatic variations in the oxygen isotopic compositions of plants

摘要

The O-18 content of leaf water strongly influences the O-18 contents of atmospheric CO2 and O-2 The O-18 signatures of these atmospheric gases, in turn, emerge as important indicators of large-scale gas exchange processes. Better understanding of the factors that influence the isotopic composition of leaf water is still required, however, for the quantitative utilization of these tracers. The O-18 enrichment of leaf water relative to local meteoric water, is known to reflect climatic conditions. Less is known about the extent variations in the O-18 content of leaf water are influenced by nonclimatic, species-specific characteristics. In a collection of 90 plant species from all continents grown under the same climatic conditions in the Jerusalem Botanical Garden we observed variations of about 9 parts per thousand in the delta(18)O values of stem water, delta(s), and of about 14 parts per thousand in the mid-day delta(18)O enrichment of bulk leaf water, delta(LW) - delta(s). Differences between delta(18)O values predicted by a conventional evaporation model, delta(M), and delta(LW) ranged between - 3.3 parts per thousand and + 11.8 parts per thousand. The delta(18)O values of water in the chloroplasts (delta(ch)) in leaves of 10 selected plants were estimated from on-line CO2 discrimination measurements. Although much uncertainty is still involved in these estimates, the results indicated that delta(ch) can significantly deviate from delta(M) in species with high leaf peclet number. The delta(18)O values of bulk leaf water significantly correlated with delta(18)O values of leaf cellulose (directly) and with instantaneous water use efficiency (A/E, inversely). Differences in isotopic characteristics among conventionally defined vegetation types were not significant, except for conifers that significantly differed from shrubs in delta(18)O and delta(13)C values of cellulose and in their peclet numbers, and from deciduous woodland species in their delta(18)O and delta(13)C values of cellulose. The results indicated that predictions of the delta(18)O values of leaf water (delta(LW), delta(M) and delta(ch)) could be improved by considering plant species-specific characteristics. [References: 48]