Gravitropism (Geotropism)

Positive and negative geotropism Gravitropism is most significant during germination. Why?

This is an adaptive advantage. Seeds are randomly oriented where they land. The shoots (plumules) need to grow upwards to the light in order to photosynthesis, so they are negavetively gravitropic (grow against gravity). The roots (radicles) need to grow into the soil to enable them to get to water and nutrients, and to anchor the plant, so they are positively gravitropic (grow with gravity.

Do roots remain gravitropic? After branching out, most roots lose their gravitropism and can grow horizontally.

How is gravity detected? Animals Involves statholiths (small grains of mineral material). These are dense so sink downwards in cells and stimulate receptors. Plants Involves starch-storing plastids called amyloplasts. Are also dense so sink down onto the cytoplasmic membrane which triggers the gravitropic response.

Amyloplasts Present in all gravity-sensitive plant organs

What is the role of an Amyloplast? An amyloplast’s primary functions are the degradation and synthesis of starch. Amyloplasts are non-pigmented or colorless organelles known as plastids that convert glucose into starch.

Transmission of the signal When amyloplasts sink against the lower side of the cell, some kind of signal must be generated, which them passes to the zone of cell elongation. As In phototropism, the messenger is auxin. Auxin acts as a growth-stimulator in shoots, but a growth-inhibitor in roots.