Tritiated water vapor exchange in sunflowers.

Water vapor exchange between sunflowers (Helianthus annuus L.) and the atmosphere was studied in darkness and in light using THO as a tracer. In the presence of tritiated water vapor, the exchange of THO measured as tritiated water in the leaves was slower in darkness than under illumination. This is explained by the variation of stomatal conductance. The authors show that a single curve is capable of summarizing the tritium exchange under any environmental condition, provided adequate units are used; this curve characterizes the behaviour of sunflowers with regard to tritiated water exchange. Using a numerical simulation model, we showed that the tritiated water vapor exchange is well represented by Fick's equation and affects only 80% of the total leaf water (lamina plus veins).