Humic substance: Relationship between structure and activity. Deeper information suggests univocal findings

The complexity of humic substances (HS) and their remarkable properties in agricultural applications has attracted and continue to attain the attention of many investigators, bringing over the years new knowledge on their structure, physicochemical and biological properties. Nevertheless, the numerous studies produced controversial results because of the difficulty to identify a precise relationship between the structure and the activity of these substances. Evidences presented here showed that the effects of HS on plant growth depends on the source, concentration and molecular weight of humic fractions and mainly on different chemical compounds contained into them. Humic matter appears also to display a hormone-like activity in particular an auxin like activity. It is not clear if this activity is strictly linked to the chemical structure of HS or whether it depends on hormones of microbial origin such as indole acetic acid entrapped into them. In any case, HS exhibit stimulatory effects on plant cell growth and development. In this review, the relationship between humic substance structure, chemical composition and physiological effects on plant growth and metabolism are examined. In particular, the responses of plants to humic substances are described, as well as the regulatory circuits that allow plants to cope with humus. Furthermore, how the present findings can add new information to the humic substances issue is widely discussed.