Red light does not affect stem length. Blue light Blue light working through carotenoid and flavin pigments cause stem growth to be inhibited in growth in length. The brighter the blue light the shorter the stem. Full sunlight has a large amount of blue light, and stems grown in full sunlight are shorter than plants grown in more shaded areas. Light actually inhibits the growth of stems if there is much blue light present.
Stems grow the longest in total darkness cause there is no blue light to inhibit their growth.
Tropisms Tropisms are growth responses plants make to environmental factors that affect plants from one side more than another or unidirectionally. The factors that may come from one side more than the other may include light, gravity, touch, even water. If it light is the uni-directional stimuli the tropism that is caused is referred to as phototropism. If the factor is gravity then the plants have a gravitropic response. If one side of a stem is touched more than another then the plant will grow towards the touch.