Stable isotope studies of hydrogen and oxygen stable isotope ratios of water within plants are providing new information on water sources, competitive interactions and water use patterns under natural conditions. Variation in the utilization of summer rain by aridland species and limited use of stream water by mature riparian trees are two examples of how stable isotope studies have modified our understanding of plant water relations. Analyses of xylem sap and tree rings have the potential of providing both short-term and long-term information on plant water use patterns.

Water uptake by plants: perspectives from stable isotope composition

In order for plants to grow, they must fix carbon. Carbon usually enters the leaves as carbon dioxide, diffusing through pores in the epidermis called stomata. Increased stomatal conductance, g, of leaves causes an increase in the partial pressure of CO2 inside the leaves, p i . This usually causes an increase in the rate of CO2 assimilation, A, but also allows a greater rate of transpirational water loss, E. Such an action by a plant is a gamble, because while it increases the likelihood of growth and reproductive success, it also increases the probability of desiccation and death (Cowan 1986).

Carbon Isotope Fractionation and Plant Water-Use Efficiency

The vapor pressure difference between H2
18O and H2
16O is the reason for the accumulation of the heavy molecule in transpiring leaves. Since photosynthesis on land is the main source of atmospheric oxygen, this mechanism is important for the remarkable enrichment of18O in atmospheric O2 (Dole effect). Using a simple box model for transpiring leaves a quantitative understanding of the isotope fractionation is possible which is well confirmed by the results of model experiments as well as by measurements on trees. Maximum enrichment of H2
18O in the water of leaves (relative to soil water) is 25 ‰ (theoretically, for dry air) and was found under natural conditions to be 21 ‰ (for 28 % relative humidity); minimum theoretical enrichment is zero (observed 2.5 ‰).

On the enrichment of H2 18-O in the leaves of transpiring plants.

Predicting animal δ18O: Accounting for diet and physiological adaptation

http://digital.csic.es/bitstream/10261/60376/4/Mesophyll_diffusion_conductance.pdf

Mesophyll diffusion conductance to CO2: an unappreciated central player in photosynthesis

ATMOSPHERIC oxygen is not in equilibrium with sea water with respect to the isotope exchange illustration but has an 18O excess of about 22‰ compared to sea water1. This could be due to isotope fractionation during respiration2. Another large contribution to the effect has been overlooked up to now. Photosynthesis on land takes place in transpiring leaves, where the difference in the vapour pressure of 16OH2 and 18OH2 concentrates the heavy molecules in their stationary water content. Since the free oxygen stems from the water in which photosynthesis takes place3–8 (with only a very small shift in isotopic composition9), photosynthesis on land is an 18O source for atmospheric O2. We have begun to study this effect quantitatively.
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H. Förstel

Papers

On the enrichment of H2 18-O in the leaves of transpiring plants.

18 O-rich Oxygen from Land Photosynthesis