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The low survival rates of in vitro-propagated plantlets under ex vitro conditions greatly inhibits the production of virus-free apple rootstock plantlets and necessitates tight control of ex vitro environments during plantlet acclimatization. Accordingly, this study investigated the effects of light intensity on the ex vitro acclimation of apple plantlets. In vitro-propagated ‘M9’ apple plantlets were acclimatized for 6 weeks under different 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 (L2M4) or 4 weeks (L4M2), 60 → 100 → 140 μmol m−2 s−1 (L2M2H2), and 60 → 140 μmol m−2 s−1 at 4 weeks (L4H2). Survival rate, maximum quantum yield of photosystem II (Fv/Fm), growth-related parameters, and photosynthetic rate were measured. The H and VH treatments yielded 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, whereas photosynthetic rate at 5 weeks after transplanting and stem diameter at 6 weeks after transplanting increased with increasing light intensity. Furthermore, the M treatment yielded greater relative growth rates than the other treatments at 2–4 weeks, and both the M and L2M2H2 treatments yielded significantly greater relative growth rates at 4–6 weeks. These results suggest that the M and L2M2H2 treatments are appropriate for the acclimatization of in vitro-propagated ‘M9’ apple plantlets.

The present study aimed to evaluate the effects of controlling the relative humidity (RH) and water content of the root-zone on the survival rate and growth of in vitro-propagated virus-free M9 apple plantlets in closed-type plant production systems. In the first experiment, three RH regimes were applied to pre-acclimated (PA) and non-PA apple plantlets for 6 weeks after transplantation. In the second experiment, the apple plantlets were transplanted into several growth media, including a mixture of peat moss and perlite (PP), rock wool (RW), and urethane sponge (SP), and in a deep flow technique (DFT) system for controlled root zone water content under controlled RH. In the first experiment, pre-acclimation improved the survival rate by preventing the loss of leaf water potential and promoting antioxidant capacity during the acclimation period. However, no clear difference was found among the three RH regimes. The antioxidant capacity was increased at 2 weeks after transplantation, followed by root initiation. The leaf water potential, which decreased continuously until 3 weeks after transplanting, tended to remain constant after root initiation. These results suggested that pre-acclimation is necessary for the survival of in vitro-propagated apple plantlets, and that the underdeveloped roots of apple plantlets have restricted water absorption under controlled RH. In the second experiment, the survival rate of plantlets grown in PP at 6 weeks after transplantation was only 70% accompanied by an increase in antioxidant capacity, whereas the survival rates of plantlets grown in RW, SP, DFT, and DFT-PP (replanted to PP from DFT 4 weeks after transplantation) were 98, 96, 93.8, and 93.8%, respectively. Most of the growth parameters of the plantlets grown in DFT were the highest among the growth media at 6 weeks after transplantation. The results of the second experiment implied that the application of DFT for in vitro-propagated apple plantlets can reduce the problems caused by poor root architecture during acclimation.

The objective of this study was to determine the effect of short-term low temperature on the concentration of phenolic antioxidant compounds in kale. For the low-temperature treatment, two kale cultivars (‘Manchoo Collard’ and ‘TBC’) grown for 3 weeks in a growth chamber were subjected to 4°C for 3 days, and subsequently allowed to recover for 2 days under normal growth conditions (20°C). Fresh and dry shoot and root weights, chlorophyll fluorescence (potential quantum yield in dark-adapted conditions), reactive oxygen species (O2 ·- and H2O2), total phenolic concentration, antioxidant capacity, individual phenolics, and phenylalanine ammonia-lyase (PAL) activity were measured before and after treatment. No significant difference was observed between the control and low-temperature treatments in the fresh or dry shoot or root weights of either cultivar. The Fv/Fm decreased during the low-temperature treatment in both cultivars, and O2 ·- and H2O2 were generated in ‘Manchoo Collard’ leaves treated with low temperature but not in ‘TBC’ leaves. ‘Manchoo Collard’ had a 15% higher total phenolic concentration than the control after 2 days of recovery, whereas that of ‘TBC’ was 16% lower than that of the control. Individual phenolic compounds, such as caffeic acid, ferulic acid, and kaempferol, exhibited a similar trend to the total phenolic concentration and antioxidant capacity. The increased PAL activity in ‘Manchoo Collard’ at low temperature was in accord with the total and individual phenolic content results. These results suggest that a short-term low temperature during cultivation of kale in a controlled environment is a potential strategy to increase the plant’s phenolic antioxidant compound content.