Simple photometric auxanometers of high sensitivity.

The elongation phase of growth of plant parts, as 1-cm segments excised from wheat coleoptiles here, is very simply recorded photometrically. One or more aligned segments submerged in aerated buffer pushed an Al foil shutter over a slit of light incident on a photodetector such as a solar or photronic cell, connected directly to a recorder, or a photomultiplier tube in a commercial photometer. Convenient kinetic records were obtained at 5 mm per min chart speed when one segment was combined with a 1-mm long slit, or more segments and longer slits, for which a 1-mm chart unit usually exceeded noise and was equivalent to 4 mum growth per cm coleoptile. In the presence of 10 cm of coleoptile segments no replenishment of solution was necessary during kinetic measurements of response in a 50-ml reservoir of IAA more concentrated than 30 nm.