Rapid acclimatization of in-vitro eucalyptus plantlets by controlling relative humidity ex vitro

Eucalyptus camaldulensis shoots (initial shoot length, 2.1+/-0.1 cm) were cultured photoautotrophically in vitro for 21 days. In vitro plantlets were transferred for 1-day acclimatization ex vitro, with treatments of 70+/-2, 80+/-2, or 90+/-2% relative humidity. Then, plantlets from each acclimatization condition were cultured ex vitro for 6 days at 60+/-2% relative humidity, 35 degrees C air temperature and 1200 mu mol m(-2) s(-1) PPFD. One-day acclimatization decreased the transpiration rates of plantlets during the first two days of subsequent ex vitro culture with decreased transpiration rates directly related to lower relative humidity from the 1-day treatment. This contributed to higher net photosynthetic rates and higher percent survival of plantlets after ex vitro culture. After seven days of ex vitro culture, the net photosynthetic rate, percent survival, dry weight and leaf area of plantlets were the highest in the plantlets treated one day for acclimatization with 70+/-2% relative humidity.