Comparative anatomical and hormonal analyses between kohlrabi and broccoli seedlings: Relevance to kohlrabi stem tuber initiation

Abstract

To unveil the mechanism of tuberous stem formation at early growth stages in kohlrabi, the comparative stem morphology, anatomy, and endogenous plant hormone profiles were studied at different days after germination (10 to 30 DAG). Stem anatomy revealed that the notable feature of the kohlrabi stem during the transition from primary to secondary growth was the formation of vascular bundle circles, with many phloems formed separately and arranged in a circular pattern along with the cambium layer, resulting in an expanded ring of primary and secondary phloem. In addition, medullary supplemental vascular bundles were differentiated in the central region of pith and successively supplied small-sized meristematic parenchymatous cells which contribute to the tuber initiation in kohlrabi. Six phytohormones were quantified in the stems and leaves of both genotypes. The absence of Gibberellin A4 (GA4) in kohlrabi stem might be the cause of the stunted stem of kohlrabi. The total endogenous cytokinin in stem of kohlrabi was more than twice of that of broccoli, where trans-zeatin was the most abundant bioactive form. This can be explained by the meristematic nature of the cambium of kohlrabi stem. Profiles for Indole-3-acetic acid, Indole-3-acetylaspartic acid, and Abscisic acid of the stems and leaves of kohlrabi and broccoli were significantly different at several time points tested. Thus, the key features of kohlrabi during the initiation of tuberous stem were the formation of the phloem ring along with the cambium and the supplemental vascular bundles. This is the first paper showing the hormonal profile in the early seedling stage of kohlrabi. These findings provide useful information to elucidate the mechanism of tuberous stem formation of kohlrabi and other tuber crops.