ROOTING OF APPLE STEM SLICES IN-VITRO IS AFFECTED BY INDOLEACETIC-ACID DEPLETION OF THE MEDIUM

We have studied rooting of l-mm thick apple stem slices in relation to indoleacetic acid (IAA) depletion of the medium. When slices were cul tured in the dark close to one another on a 3-mm thick medium (20 mi i n a 9-cm Petri dish), the IAA concentration in the medium beneath the slices dropped to 20% of the initial concentration during the first da y of culture. This was due to uptake, enzymatic oxidation and slow rep lenishment from medium in other areas of the Petri dish. Because of de pletion, the rate of IAA uptake decreased with time. When medium was r enewed daily, the rate of IAA uptake remained the same. When the slice s were cultured scattered over the Petri dish or on a thick layer of m edium (6-mm instead of 3-mm thick), they took up more IAA. As could be expected, differences in uptake amongst the various conditions (slice s cultured scattered or close to one another; 20 or 40 mi of medium) w ere small during the first day of culture but large after that. Becaus e the rhizogenic action of IAA occurs not immediately at the start of culture but later (viz., from 24 to 72 h after the start of culture; D E KLERK et al., 1995; J. Exp. Bet. 46, 965-972), rooting at the variou s conditions corresponded closely to the amount of IAA taken up from 2 4 to 72h. Since the various incubation conditions resulted in the same maximal root number (but at different IAA concentrations), there was only depletion of the medium with respect to IAA.