Role of Underground Conditions in the Occurrence of Carnation Stunting and Proliferation Syndrome, and Relationship between the Symptoms and Endogenous Phytohormones

Abstract

Carnation stunting and shoot proliferation (CSP) syndrome typically shows symptoms of shoot shrinking and proliferation. Thus, it is believed that biosynthetic or metabolic problems of endogenous phytohormones are related to the developmental process. On the other hand, it is known that the rhizosphere environment greatly influences the incidence of this syndrome, although little abnormality is observed in the roots. In this syndrome, grafting tests were performed using a susceptible and non-susceptible cultivar to determine the relationship between the underground and aerial parts of the plant. The grafted plant showed symptoms when the stock was a susceptible cultivar, even if the scion was a non-susceptible cultivar. These findings confirm that the roots make an important contribution to the appearance of symptoms, and that typical symptoms of stunting and shoot proliferation appear in the aerial parts due to some signals from underground parts. These signaling substances may only occur in the roots, and may change before the stunting symptom appears as the first stage of the syndrome. Thus, the concentrations of endogenous phytohormones were quantified, as possible signaling substances. However, the concentrations of phytohormones measured in this test did not change in the roots, and it seems very likely that these phytohormones are not the signals. In contrast, in the shoot apices, indole acetic acid (IAA) concentration decreased at the same time as the stunting symptoms started to appear in the expanding leaves. The ratio of cytokinins (CKs) to IAA concentration increased because IAA concentration decreased and the concentration of CKs did not. It is clear from these findings that the appearance of symptoms in the shoots is influenced by a biosynthetic abnormality of IAA and a corresponding imbalance of CKs and IAA.