Bioassay means the testing of substance for its activity in causing a growth response in a
living plant or its parts. The test measured the concentration required to produce the effect, and
thus it is quantitative. For auxins, bioassay is usually performed by Avena coleoptile curvature test and
cross root inhibition test.
Avena coleoptile curvature test : (a) It involves extraction of auxins from the coleoptile tip in an
agar block of placing the cut tip on the latter, (b) The auxin containing agar block is now placed on
one side of the decapitated stump of the coleoptile.
(c) An agar block without auxin is placed on another
decapitated coleoptile to serve as control.
(d) Within few hours the coleoptile with auxin agar
block bends on to the side opposite to which agar block is placed.
(e) However, no curvature is observed in the control coleoptile.
(f) The curvature occurs due to rapid growth of the side of coleoptile on which agar block is placed.
(g) This curvature can be measured by exposing the curved coleoptile to a photographic plate.
(g) The amount of auxin in the agar block is directly proportional to the degree of curvature