Biology · Plant Growth and Development · NEET
A bioassay means testing (assaying) a substance by its effect on a living thing (bio) instead of by chemistry. For auxin, we cannot easily see or weigh such a tiny amount, so we let it act on a living oat coleoptile and measure the curving it produces. More auxin makes a bigger bend, so the angle of curving becomes a way to measure the hormone amount. That is why the Avena curvature test is called the bioassay of auxin.
Auxin makes cells elongate (grow longer). When the agar block with auxin is placed on one side of the cut coleoptile, auxin moves down only that side. Those cells grow longer than the cells on the other side. When one side grows more than the other, the whole coleoptile bends towards the shorter (less auxin) side, which is away from the block. Many students wrongly think it bends towards the block. Remember: the side with more auxin grows more and pushes the tip to the opposite side.
The Darwins (Charles and Francis) worked on canary grass coleoptiles and showed that the TIP senses unilateral (one-side) light and that some influence moves down to cause bending (phototropism). They did not isolate the chemical. Went came later, used OAT (Avena) coleoptiles, cut off tips, placed them on agar blocks, and showed the influence was a diffusible chemical he called auxin. So Darwin discovered the tip effect; Went isolated auxin and gave the curvature bioassay.
Avena is the scientific name for oat, a monocot cereal. Its young shoot is covered by a straight, sensitive sheath called the coleoptile that grows fast by cell elongation and responds strongly to auxin. This makes it easy to place agar blocks on it and measure a clear angle of curving, so oat coleoptiles became the standard material for the auxin bioassay.
It is only for auxin (IAA). Auxin was isolated from oat coleoptile tips by Went, so its bioassay is the Avena curvature test. Gibberellins, ABA and ethylene are measured by other methods. In NEET, if the option list has ABA, GA3, IAA and Ethylene, the Avena curvature test always points to IAA.
The avena curvature is used for bioassay of:
Try the real previous-year questions from this chapter — each with the answer and a full solution.
F.W. Went. He isolated auxin from the tips of oat (Avena) coleoptiles using agar blocks and developed the curvature test as a bioassay for auxin.
The angle of bending (curvature) of the oat coleoptile. This angle is proportional to the amount of auxin in the agar block, so it measures the auxin concentration.
The intact tip is removed first because the tip is the natural auxin source. Then an agar block with a known or unknown auxin sample is placed off-centre on the cut stump, so any bending is due to the auxin you added, not the plant's own tip.
Charles Darwin and his son Francis observed that canary grass coleoptiles bend towards light (phototropism) and that the tip senses the light. They concluded a transmittable influence moves from the tip downward, which later led Went to isolate auxin.
NEET repeatedly asks which hormone the Avena curvature test measures and links it to the Darwin and Went discovery experiments. Knowing that the answer is auxin (IAA) and that bending is away from the block prevents easy trap mistakes.
