Tropismi

Tropismi, or tropisms in English, are directional growth or movement responses of organisms toward or away from external stimuli. In plants, tropisms refer to growth on the organ level that results from differential cell elongation on the sides exposed to a stimulus. Tropisms help plants optimize light capture, water uptake, contact with substrates, and other environmental factors.

Major plant tropisms include phototropism (toward light), gravitropism or geotropism (direction relative to gravity), thigmotropism (response to touch or physical contact), hydrotropism (toward moisture), and chemotropism (toward or away from chemical gradients). Positive or negative tropisms describe whether growth is toward (positive) or away from (negative) the stimulus; for example, shoots show positive phototropism and negative gravitropism, while roots exhibit positive gravitropism.

Mechanisms involve perception of the stimulus and differential growth. Phototropism largely depends on light receptors (phototropins) and the redistribution of the growth hormone auxin toward the shaded side, promoting elongation and bending toward the light. Gravitropism involves gravity-sensing statoliths that influence auxin transport and result in curvature. Thigmotropism often arises from localized growth changes at contact points. Hydrotropism and chemotropism are guided by moisture or chemical gradients, with hormones such as auxin and others mediating the response. In pollen tubes, chemotropism guides growth toward ovules, guided by chemoattractants.

Tropisms describe directional growth in response to environmental cues, whereas other plant movements, such as nyctinasty, are non-directional and stimulus-driven rather than oriented by gravity or light. Tropisms are central to plant development and ecology.