The Fractionation of ²H and ¹⁸O in Leaf Water of Barley

Abstract

Small changes in the natural abundance of hydrogen and oxygen isotopes in water occur as a result of transpiration. We considered whether these may be sufficient to study the differences in water relations of barley cultivars having different water-use efficiency (W). A range of cultivars of varying W were grown in the glasshouse and in the field in breeders' plots. Foliage water samples were taken from a specific leaf stage over the course of a day. Certain field-grown cultivars had consistently higher ²H and ¹⁸O compositions in foliage water than others at almost all sampling times, resulting in a consistent ranking of the cultivars. Some evidence of a consistent ranking of isotopic compositions was also present in the glasshouse experiment.

Concurrent variations in ²H and ¹⁸O compositions of the stem and leaf water reflect the conditions under which transpiration is occurring. The composition of stem water remains steady over the course of such experiments. The slope of the line connecting the isotopic compositions of the stem water and leaf water varied with changes in leaf water composition. This variation in slope was more pronounced under the more humid conditions of the glasshouse experiment, as is predicted from theory.

The ²H and ¹⁸O compositions of water in the other plant parts, atmosphere and soil were also examined and porometry measurements were made. These indicated that daily variations in the contributions of soil and leaf water to the atmosphere occurred. This involves a maximum in the proportion of soil evaporation in the middle of the day, and a converse maximum in flag leaf evaporation early and late in the solar day.