Question

What is the benefit of a seed storing nutrients for the plant embryo, if the plant is already acquiring its food through photosynthesis?

Based on your knowledge of auxin production and how gravity and light affect plants, what would likely occur if a seed is planted upside down with a light source below the plant?

Answer 1

Answer. Seeds are formed as a result of fertilisation. Seeds themselves do not undergo photosynthesis, their cotyledons form primordial leaves that undergo photosynthesis later when seed germination occurs. Thus, it is necessary to have reserve food in seed till premordial leaf formation occurs.

Seed inside contains an embryo, that becomes metabolically active when seeds are provided with water. On getting water, the radicle part grows to form radicle and the plumule forms the shoot. The nutrients required for the growth of radicle and plumule are provided by cotyledons. As radicle and plumule grows, cotyledon comes out to for the primordial leaves which carry the function of photosynthesis. Now, at this stage the plantlet can synthesize its own food, does translocation of the food to increase its height and number of leaves, as primordial leaves wither on arrival of the main leaves. To summarise, cotyledons have only a limited amount of food reserve for the growing embryo.

The roots of the plants are mostly positively geotropic means growing in the direction of the gravity and the shoot is positively phototropic means always grows in the direction of the light, against gravity or negatively geotropic. So, in which direction we plant the seed, the shoot will always be in the direction of light and the roots will always be in the direction of gravity which is always pointing downwards. If the light source is below the plant, then the shoot will be in the direction of light, it will become positively geotropic in this case.