STRUCTURE AND FUNCTION OF THE GREEN STEM TISSUE IN OCOTILLO (FOUQUIERIA SPLENDENS)

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Abstract
Fouquieria splendens, a desert plant native to the southwestern United States, was studied to determine the capacity for photosynthesis of green stem tissue. The plant is leafless most of the year because of drought, so the capacity for gas exchange by the stems is essential for their photosynthetic function. With secondary growth, sclerified leaf bases which cover the stem become separated, and a transparent cork forms in the furrows between them. A well‐developed chlorenchyma occurs beneath this cork as well as beneath the leaf bases. Chloroplasts of the stem have an unusually high degree of granal stacking, but are mostly typical. Light is transmitted through the leaf base on the young primary shoot and the furrow cork, but not through the older leaf base. Chlorophyll fluorescence studies indicated that the chloroplasts were fully competent and indeed stem tissue is capable of fixing 14CO2 if supplied to cut sections. Despite competent chloroplasts, no exogenous CO2 uptake occurs because the cork is impermeable to CO2, and presumably water. The functional significance of competent chloroplasts in stems that do not transfer gas may be the production of high energy compounds for metabolism, the recycling of internally generated respiratory CO2, or it may simply be a relictual feature in this species of the Fouquieriaceae.
Funding Information
  • National Science Foundation (PCM 8021645)