Hydrogen-isotope composition of leaf water in C₃ and C₄ plants: its relationship to the hydrogen-isotope composition of dry matter

Abstract

The natural abundance hydrogen-isotope composition of leaf water ($\delta _{\text{D}}^{{\text{H}}_{\text{2}} {\text{O}}}$) and leaf organic matter (δ ^org_D ) was measured in leaves of C₃ and C₄ dicotyledons and monocotyledons. The $\delta _{\text{D}}^{{\text{H}}_{\text{2}} {\text{O}}}$ 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 ^org_D value in leaves of either C₃ or C₄ plants. When δ ^org_D values were referenced to the mean $\delta _{\text{D}}^{{\text{H}}_{\text{2}} {\text{O}}}$ 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.