Relationships between plant hormones, carbohydrate metabolism and excision-induced adventitious root formation of two contrasting hydrangea cultivars

Abstract Main conclusion Cytokinins and abscisic acid act as auxin antagonists during adventitious root induction, thereby limiting AR development depending on plant genotype and the distance to the wounding site. Abstract This study investigated the role of plant hormones in the lower rooting capacity of cuttings of the Hydrangea macrophylla cultivar ‘Clarissa’ compared to ‘Caipirinha’, further considering interrelations to carbohydrates. At first, the rooting response to 1 min pulse treatments of the stem base with indole-3-acetic acid (IAA) was analyzed. Without IAA, rooting of ‘Clarissa’ lagged behind that of ‘Caipirinha’. Increasing IAA doses up to 50 mM particularly enhanced rooting in ‘Clarissa’ to the same level as ‘Caipirinha’. The number of adventitious roots was positively correlated with the length of the rooting zone, which was greater in ‘Caipirinha’ and was further enlarged by IAA application. Carbohydrate analysis indicated higher carbohydrate utilization in the 0–1 cm basal stem section compared to the 1–2.5 cm section above, which was further stimulated by IAA. Phytohormone profiling by LC-MS/MS in both stem base sections revealed similar dynamics for jasmonic acid, jasmonoyl-isoleucine, and IAA in both cultivars, but significantly higher cytokinin and abscisic acid (ABA) levels in ‘Clarissa’ compared to ‘Caipirinha’. ‘Clarissa’ revealed lower IAA/cytokinin and IAA/(cytokinin + ABA) ratios during the first 48 h after cutting excision compared to ‘Caipirinha’, while these ratios were higher in the basal 0–1 cm compared to the 1–2.5 cm stem section above. These findings provide new perspectives on hormonal crosstalk in adventitious rooting and support the conclusion that the lower rooting capacity and shorter rooting zone of ‘Clarissa’ is likely based on locally higher cytokinin and ABA levels that antagonize IAA during the induction phase.