Geotropism

In the syllabus and the exam

Geotropism is examined as the plant's directional growth response to gravity, and, like phototropism, it is explained through the uneven distribution of auxin, though the effect of auxin differs between roots and shoots, which is a distinction candidates must state clearly to earn full marks. In a shoot laid horizontally, auxin accumulates on the lower side and promotes greater cell elongation there, bending the shoot upward, showing negative geotropism; in a root laid horizontally, auxin also accumulates on the lower side, but at the higher concentration typical in root tissue, auxin actually inhibits cell elongation, so the upper side elongates faster and the root bends downward, showing positive geotropism. This opposite sensitivity of root and shoot tissue to the same hormone at similar concentrations is one of the more conceptually difficult ideas in the responses in plants topic and is frequently tested through diagrams of a potted seedling turned on its side. A common error is stating that auxin simply promotes growth in both organs without noting that its effect on roots is inhibitory at the concentrations that accumulate there, which is essential for correctly explaining why roots and shoots bend in opposite directions relative to gravity. Candidates should also be able to explain the adaptive value of this response, noting that positive geotropism in roots anchors the plant firmly and directs roots towards a reliable source of water and minerals in the soil, while negative geotropism in shoots directs growth towards sunlight above ground.

Example

Comparing root and shoot behaviour, a candidate writes: 'Roots show positive geotropism, growing downward in the direction of gravity, while shoots show negative geotropism, growing upward against gravity, even when a plant is placed on its side.'

Related terms

TropismAuxinPhototropism

Source:SRC-DSKP-EN