Uptake and metabolism of N6‐benzyladenine and 1‐naphthaleneacetic acid and dynamics of indole‐3‐acetic acid and cytokinins in two callus lines of Actinidia deliciosa differing in growth and shoot organogenesis

Abstract

Uptake, metabolism and accumulation of N6‐benzyladenine (BA) and 1‐naphthaleneacetic acid (NAA) as well as changes in endogenous indole‐3‐acetic acid (IAA) and several isoprenoid‐type cytokinins (Cks) were characterized in two callus lines of Actinidia deliciosa cv. Hayward showing different growth and shoot organogenic responses to exogenously applied 2.7 µM NAA and 4.4 µM BA. Studies were carried out in callus 0, 1, 12, 24 and 48 h after the onset of their fifth subculture on a medium containing [3H]NAA or 8‐[14C]BA. Kiwifruit callus of line A presented high caulogenic response and lower growth that was positively associated with faster BA uptake, with transient accumulation of BA and isoprenoid‐type Cks, mainly zeatin, exceeding three‐ and four‐fold that of the non‐caulogenic callus, and with values of the BA/NAA ratio exceeding 1, in fact higher than the BA/NAA ratio in the culture medium. The accumulation of BA took place in both callus lines during the first 24 h of subculture and before the re‐initiating of callus proliferation. The higher growth and the low or null caulogenic response shown by line B callus were correlated with faster NAA uptake, with endogenous NAA levels two‐fold higher than in A calli, with higher IAA amounts and with values of the BA/NAA ratio below 1. Moreover, at 48 h free NAA in both kinds of callus reached levels close to those found after 35 days of subculture. Results suggest that temporal accumulation of BA and endogenous Cks is involved in the initiation of cell division leading to callus growth, whereas the maintenance of high NAA and IAA levels are related to the support of long‐term callus development. It also appears that for callus cells to become committed to shoot organogenesis, they must have a concentration of active Cks higher than a threshold value during the first 2 days of culture on fresh medium, while at the same time the concentration of auxins must not exceed a certain maximum.