Tropisms

Plants cannot move from place to place. Usually they are held in position firmly by their roots. They can, however, orientate their stems and roots to obtain favourable conditions. They can move parts of the plant to where there is more light or water for example. Tropisms can be positive (towards a stimulus) or negative (against or away from a stimulus).

Tropisms Phototropism – growth of stem towards greater light intensity. Hydrotropism – growth of roots towards higher concentrations of water in soil. Geotropism – Positively seen in roots to obtain more water deep down in soil but negatively seen in stems in order for leaves to obtain more light for PHOTOSYNTHESIS.

Plant Growth Factors Plants have no nervous system to control the movement to or away from a stimulus. Therefore they need an alternative means of coordination This comes from the release of growth factors (hormones). An example of one of these is indoleacetic acid (IAA)

IAA (AKA AUXIN) Affects growth They usually affect cells in close proximity to where they have been released from. Causes cells to elongate.

Control of Tropism by IAA Cells in shoot tip produce IAA This then diffuses down stem If IAA distributed evenly, stem grows straight up as all cells grow at same rate/amount. BUT.. If IAA distributed unevenly (IAA accumulates in side of shoot in shade) Cells in shade receive more IAA – so grow more. Side in shade elongate more, causing plant to bend over towards light PHOTOTROPISM.

IAA in roots vs shoots IAA promotes cell elongation in shoots but inhibits cell elongation in roots. This allows plants to respond to gravity as well as light. Shoots grows against gravity (negative geotropism) Roots grow with gravity (positive geotropism).