The Mechanism of Photosynthetic Product Secretion by Carrot-Tissue 2

Green carrot-callus cultures exposed for 3 h in liquid mediato light and ¹⁴CO₂ secreted a constant 6–8 per cent oftheir total ¹⁴C-photosynthate into the medium over a wide rangeof ¹⁴CO₂-fixation rates, obtained by varying ¹⁴CO₂ levels. Themajor secreted products (sucrose and glutamine) or analoguesof these compounds did not affect secretion when included inthe bathing medium. Dual-labelling studies with [6-T] glucoseand ¹⁴CO₂ demonstrated that secretion occurred from a metabolicrather than a storage compartment. The effects of long preincubationperiods in liquid media and of osmotic shock showed that secretiontook place from specific sites on the boundary of the metaboliccompartment.