Sink to Source Transition in Tendrils of a Semileafless Mutant, Pisum sativum cv Curly

Sink to source transition parallels loss of thigmotropic capacity in tendrils of a semileafless mutant, Pisum sativum cv Curly. Macroscopic tendril development is subdivided based on thigmotropic capacity. Stage I is the elongation stage and, although the rate of photosynthesis is similar to that of stage II and III tendrils, dark respiration rates are higher in stage I. During stage II, tendrils are thigmotropic and act as a sink. Even though stage II tendrils have CO₂ exchange characteristics similar to those of stage III tendrils, which are coiled, our fluorescein, ¹⁴C-partitioning, and ¹¹C-translocation experiments suggest that stage I and II tendrils do not export carbon. Only stage III tendrils act as sources of newly fixed carbon. Export from them is blocked by cold, heat girdling of the petiole, or anoxia treatment of the tendrils. A late stage II tendril complex, in which coiling is occurring, may be exporting photoassimilates; however, this phenomenon can be attributed to the fact that the pea leaf is a compound structure and there may be one or more stage III tendrils, no longer thigmotropic, within the tendril complex. Photosynthetic maturity in pea tendrils occurs at stage III and is characterized by the ability of these tendrils to export photoassimilates.