Hydrogen-isotope composition of leaf water in C3 and C 4 plants: its relationship to the hydrogen-isotope composition of dry matter.

The natural abundance hydrogen-isotope composition of leaf water ([Formula: see text]) and leaf organic matter (δ D (org) ) was measured in leaves of C3 and C4 dicotyledons and monocotyledons. The [Formula: see text] value of leaf water showed a marked diurnal variation, greatest enrichment being observed about midday. However, this variation was greater in the more slowly transpiring C4 plants than in C3 plants under comparable environmental conditions. A model based on analogies with a constant feed pan of evaporating water was developed and the difference between C3 and C4 plants expressed in terms of either differences in kinetic enrichment or different leaf morphology. Microclimatic and morphological features of the leaves which may be associated with this factor are discussed. There was no daily excursion in the δ D (org) value in leaves of either C3 or C4 plants. When δ D (org) values were referenced to the mean [Formula: see text] values during the period of active photosynthesis, the discrimination against deuterium during photosynthetic metabolism (ΔD) was greater in C3 plants (-117 to -121‰) than in C4 plants (-86 to -109‰).These results show that the different water use "strategies" of C3 and C4 plants are responsible for the measured difference in deuterium-isotope composition of leaf water. However, it is unlikely that these physical processes account fully for the differences in hydrogen-isotope composition of the products of C3 and C4 photosynthetic metabolism.