Question

Which of the following bioassays is used to detect the presence of auxin?
A. Tobacco pith culture and Avena curvature test
B. Split pea stem curvature test and Avena curvature test
C. Split pea stem curvature test and tobacco pith culture
D. Only tobacco pith culture

Answer 1

Hint:
Bioassay is an analytical method used to detect the concentration of a substance by observing its effect on living tissues or cells. A bioassay can be used to detect the biological activity in terms of the growth response of auxin by taking plant parts. Auxin bioassay is a quantitative method because it measures the concentration of auxin to produce the effect.

Complete step by step answer:
Tobacco pith culture is used for the detection of cytokinins rather than auxin. Tobacco pith will not grow in culture media with auxin alone. It requires a supply of cytokinins. So tobacco pith culture is not used for auxin bioassay.
Avena curvature test can be done by producing a 10° curvature by auxin of 150 micrograms per litre. Auxin present in the shoot tip or any other organ is diffused into agar blocks. Oat coleoptile of about 15-30 mm long is grown in dark and is held vertically over water. The tip of the coleoptile for about 1 mm is removed without injuring the primary leaf. After 3 hours, another decapitation is carried out for a distance of 4 mm. After 90-110 min, we can see the coleoptile has bent. Curvature should be measured.
Split pea stem curvature test can be done based on the differential growth response of the epidermal cells and the cortical cells of the plant stem to auxin. Germinated pea seeds are grown in dark for 8 days. These seedlings should be exposed to 3 h of red light per day. The removed section is soaked in distilled water for an hour. The stem section is split and placed in a Petri dish containing auxin. After incubation, the curvature can be observed.

Option ‘B’ is correct

Note:
The Avena curvature test is based on the polar transport of the auxin in the coleoptile of Avena. Auxin applied on one side of the coleoptile will diffuse down on that side only and allow that side of the coleoptile to grow more with curvature.