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

The natural abundance hydrogen-isotope composition of leaf water (5~ 2~ and leaf organic matter (5~rg) was measured in leaves of C a and C 4 dicotyledons and monocotyledons. The 6~20 value of leaf water showed a marked diurnal varia- tion, greatest enrichment being observed about midday. However, this variation was greater in the more slowly transpiring C 4 plants than in C 3 plants under comparable environmental conditions. A model based on analogies with a constant feed pan of evaporating water was developed and the differ- ence between C 3 and C4 plants expressed in terms of either differences in kinetic enrichment or differ- ent leaf morphology. Microclimatic and morpho- logical features of the leaves which may be asso- ciated with this factor are discussed. There was no daily excursion in lhe u, value in leaves of either C 3 or C4 plants. When 3~rg values were refer- enced to the mean 6o n~~ values during the period of active photosynthesis, the discrimination against deuterium during photosynthetic metabolism (A D) was greater in C 3 plants (-117 to -121% o) than in C~ plants (-86 to -109~ These results show that the different water use strategies of C 3 and C 4 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 C 3 and C 4 photosynthetic metabolism.