Growth and acclimation of in vitro-propagated ‘M9’ apple rootstock plantlets according to light intensity

The low survival rates of in vitro-propagated plantlets under ex vitro conditions greatly inhibits the production of virus-freeapple rootstock plantlets and necessitates tight control of ex vitro environments during plantlet acclimatization. Accordingly,this study investigated the eff ects of light intensity on the ex vitro acclimation of apple plantlets. In vitro-propagated‘M9’ apple plantlets were acclimatized for 6 weeks under diff erent light treatments: 60 μmol m −2 s −1 (L), 100 μmol m −2 s −1(M), 140 μmol m −2 s −1 (H), 180 μmol m −2 s −1 (VH), 60 → 100 μmol m −2 s −1 at 2 weeks (L 2 M 4 ) or 4 weeks (L 4 M 2 ),60 → 100 → 140 μmol m −2 s −1 (L 2 M 2 H 2 ), and 60 → 140 μmol m −2 s −1 at 4 weeks (L 4 H 2 ). Survival rate, maximum quantumyield of photosystem II (Fv/Fm), growth-related parameters, and photosynthetic rate were measured. The H and VH treatmentsyielded the lowest survival rates (78 and 71%, respectively), whereas the M treatment yielded the highest (95%). Meanwhile, the Fv/Fm ratio at 6 weeks after transplanting decreased with increasing light intensity at 4 and 5 weeks, whereasphotosynthetic rate at 5 weeks after transplanting and stem diameter at 6 weeks after transplanting increased with increasinglight intensity. Furthermore, the M treatment yielded greater relative growth rates than the other treatments at 2–4 weeks,and both the M and L 2 M 2 H 2 treatments yielded signifi cantly greater relative growth rates at 4–6 weeks. These results suggestthat the M and L 2 M 2 H 2 treatments are appropriate for the acclimatization of in vitro-propagated ‘M9’ apple plantlets.