Development of Nutrient Solution for in vitro Propagation of ‘M9’ Apple Rootstock Plantlets

This study was conducted to evaluate the performance of a newly-developed nutrient solution for apple plantlets (NAP) and to determine the proper electrical conductivity (EC) for the acclimation and growth of in vitro apple plantlets (IAP) in a closed-type plant production system to produce virus-free apple seedlings. IAP treated with pre-acclimation for one week were transplanted to a deep flow technique system supplied with several different concentrations (EC 0.5, 1.0, 2.0 and 3.0 dS·m-1) of Hoagland nutrient solution (HN) and NAP. Some of the NAP treatments with EC 0.5 dS·m-1 (NAP0.5) were increased to EC 2.0 dS·m-1 at two (NAPex2) and four weeks (NAPex4) of transplanting. IAP were acclimated gradually under reduced relative humidity (RH, 90%-80%-60% at two weeks interval), air temperature at 25°C, 60 μmol·m-2·s-1 of PPFD (fluorescent lamps), and a 16-hour light period for six weeks after transplanting. The survival rates tended to decrease with increasing EC levels in both HN and NAP. Although there was no significant difference between the shoot growth characteristics of HN and NAP, root growth characteristics of NAP wassignificantly higher at four weeks after transplanting. Shoot and root growth characteristics of the lowest EC (0.5 dS·m-1) were high at the initial acclimation stage and tended to increase with higher EC levels at the later acclimation stage. In both NAPex2 and NAPex4, the growth rate tended to increase after increasing the EC level, but the growth rate of NAPex2 decreased at four weeks after transplanting. The mineral contents of HN and NAP treatments were slightly different but the mineral absorption rate remained constant. These results suggest that if NAP is supplied at the proper EC level according to acclimation stage, it can alleviate the poor root development of IAP and contribute to successful acclimation.