Question

What is positive phototropism?

Answer 1

Hint: In plants any form of movement or growth in response to external stimuli is known as tropism or tropic movements. Apart from plants, members of Kingdom fungi also exhibit such types of movements. The external stimuli can be light or gravity.

Complete answer:
Growth in response to light is known as Phototropism. The process of phototropism is regulated by a Plant growth regulator called Auxin. Auxin reacts when light falls on the surface. In positive phototropism, the direction of plant growth is towards the direction of light. Here light acts as an external stimulus. Shoot system of most plants exhibit positive phototropism by bending and growing in the direction of the light source.

During this process, the plant parts rearrange their chloroplasts to the direction of light and maximize the effectiveness of photosynthesis and thereby promoting growth. This process allows green plants to be closer to the light energy source. Regulation of phototropism is done by molecules known as Phototropins which are proteins and are the photoreceptor molecules.

Phototropins activate the production and activation of Auxin molecules. Auxin gets concentrated on the shady part and elongation happens in the shady part causing the shoot to bend towards the light source thereby exhibiting positive phototropism.

Note: Negative phototropism is the phenomenon by which plant parts grow against the direction of the light source. This is a characteristic of most of the shoot systems which grows downwards away from the direction of light. Negative phototropism occurs in the plant parts where auxin concentrations are less or negligible.