Question

Using diagrams and words, describe the experiments performed by the Darwins, Boysen-Jensen & Went on phototropism which demonstrated the role of plant hormones in plant growth and response to the environment.

Answer 1

Darwin's experiments for Phototropism - Darwin illustrated that the factor responsible for the shoots bending towards sunlight, is present in the apex of the shoot not the elongating part. He tested this by:

• Covering the shoot apex with a foil.
• Covering the elongating region with the foil.

He concluded that the phototropic influence is exerted by the shoot apex not the elongating region.

Experimenti Experiment 2 Experiment 3 light light light 1-1 This shows that young shoots If an opaque foil cap is placed If an opaque foil collar is placed bend towards directional light. over the tip, bending does not below the tip, bending does occur as usual. occur.

^{Image adapted fro Pinterest}

Boysen-Jensen experiments for Phototropism - In an attempt to understand the nature of the chemical that diffuse from the shoot tip to the elongating part, boysen-jensen conducted a series of experiments.

• In one of the experiments, agar/gelatin was placed between the apex and the elongating part of the shoot
• in other experiment, mica was used instead of gelatin.
• he concluded the diffusing molecule is water soluble in nature as it passes through gelatin but not through mica.
• the  Darwin’s ‘influence’ was a hemical, capable of diffusing through the agar / gelatin from the tip.

• He also observed that placing a thin piece of mica part way into the dark side of the coleoptile restricted any phototropic response whereas placing it into the light side of the region didn't affect the response.

Boysen-Jensen (1913) İ mica barrier inserted unilaterally gelatin barrier inserted subapically

^{Image adapted from slidesharecdn.com}

Went's experiments for phototropism:

He cut the tips of the coleoptiles and placed them on agar blocks in dark, based on the prediction that the absorption of the water-soluble chemical in the agar block will occur. Then he placed it on the tip-less coleoptiles and saw if the effect was observed or not in response to light.

Allow time for hormone to diffuse into agar block. Offset blocks cause bending of shoots not exposed to light Cells on the shaded side elongate in response to the hormone (red dots)

^{Image adapted from https://www.pathwayz.org/Tree/Plain/PHOTOTROPISM+EXPERIMENTS.}