[Campbell Biology P.26] A botanist observes that a plant stem, when inverted, still directs... | Practice Question

A botanist observes that a plant stem, when inverted, still directs growth upwards, but at a significantly reduced rate. This observation is best explained by which characteristic of auxin transport in plants?

  • A: Auxin's movement is always against the concentration gradient.
  • B: Auxin's synthesis is inhibited when the stem is inverted.
  • C: Auxin exhibits polar transport, primarily moving unidirectionally from the morphological apex to the base.
  • D: Auxin is primarily transported through the xylem, which is gravity-dependent.

Explanation

Auxin's characteristic polar transport, which involves active transport systems within cell membranes, dictates its unidirectional movement from the shoot apex downwards to the base. When a stem is inverted, this polar transport system is still largely intact but becomes much less efficient in moving auxin 'up' the inverted stem, resulting in reduced but still observable upward growth. This is a fundamental aspect of how plants regulate their growth and development.