The effect of gravity on seed germination. why do plants grows towards light

because they prepare their food in presence of light

because they need sunlight to have photosynthesis

To understand the effect of gravity on seed germination and why plants grow towards light, we need to consider the processes of gravitropism and phototropism.

1. Effect of gravity on seed germination:
Gravity plays a crucial role in the orientation of plant growth. When a seed is initially planted in soil, it undergoes a process called geotropism or gravitropism, which is the growth response of plants to gravity. The main mechanisms involved in this process are:

a) Amyloplasts: Inside the cells of plant roots and shoots, there are specialized organelles called amyloplasts that contain starch grains. These amyloplasts act as statoliths, or gravity sensors. Due to gravity, these statoliths settle at the lowest part of the cell.

b) Auxin redistribution: Auxin is a hormone that regulates plant growth. When the plant is in a horizontal position, the statoliths settle at the base of the cells, causing auxin to accumulate in the lower side. This uneven distribution of auxin triggers enhanced cell elongation on the lower side of the seedling, resulting in downward growth of the root and upward growth of the shoot.

2. Why plants grow towards light (phototropism):
Phototropism is the growth response of plants towards or away from light. The main process involved in phototropism is:

a) Photoreceptors: Plants have photoreceptor molecules, such as phytochromes and phototropins, which are sensitive to light. These molecules are mainly located in the plant's stem and shoot tip.

b) Hormone regulation: When light hits the photoreceptors, it triggers a series of biochemical reactions that ultimately leads to the redistribution of auxin. In the presence of light, auxin accumulates on the shaded side of the plant (away from light), stimulating cells to elongate more rapidly on that side and causing the stem to bend towards the light source.

In summary, plants grow towards light due to the redistribution of auxin in response to light stimulation, while gravity influences seed germination and determines the orientation of root and shoot growth.