change in the direction of growth of plant organs under the influence of illumination from one side. In positive phototropism, the stem bends toward the light source. In plagiotropism, or diatropism, the leaf blades move at an angle toward the falling light. In negative phototropism the plant organs bend in a direction opposite to the light source (for example, ivy stems and the apices of certain roots). The same organ may be positively phototropic to weak light, negatively phototropic to intense light, and not at all phototropic to light of medium intensity.
Plant species differ in their capacity for phototropism. Phototropic responses may change even in plants of the same species: in young individuals the responses are always stronger, given the same conditions, than in older plants. Phototropism may manifest itself only in the young organs of a plant. In stems and leaves the phenomenon leads to uniform leaf distribution, so that the leaves shade each other only slightly. Positive phototropism and negative geotropism enable the apices of sprouts to emerge to the soil surface even when the seeds have been planted very deeply.
The process of phototropism includes a series of successive reactions: perception of a light stimulus, excitation of cells and tissues, transmission of excitation to cells and tissues of the growth zone of the organ, and intensification or diminution of cell and tissue growth in the growth zone. Perception of the light stimulus is effected by a specific photoactive complex that includes carotenoids and flavin. Transmission of the excitation through the plant occurs with the participation of bioelectric currents and plant hormones known as auxins. (For a discussion of the mechanisms of these processes, see.)
The manifestations of phototropism depends on the spectral composition of the illumination. Maximum phototropic sensitivity in plants has been found in the absorption spectra of yellow and orange pigments—carotenoids and flavins. It is conjectured that photosensitive proteins containing the pigments perceive the light stimulus. Carotenoid “eyes” have also been found in certain unicellular algae displaying phototaxis and in the sporangiophores of fungi capable of phototropism.