Effect of two different humic substances on seed germination of Pinus laricio

Two humic substances of high molecular weight, with different chemical characteristics, both showing auxin-like activity, were tested for their effects on seed germination of Pinus laricio, P. spp. calabrica. The biological activity of IAA and the humic substances was compared and the major enzymes involved in seed germination and IAA metabolism were analyzed. The data obtained with both humic substances showed an inhibition of seed germination and of α-amylase activity, while peroxidase and IAA oxidase activities were enhanced. These results confirm an IAA-like-activity of the humic substances and suggest a possible interaction of these substances with auxin receptors on the membranes of the seeds tested.