Environmental Control for Acclimatization of in vitro Cultured Plantlets

Abstract

The plantlets cultured under in vitro conditions of high relative humidity and low light intensity exhibit poor development of the epicuticular wax (Grout and Aston, 1977; Sutter and Langhans, 1979; Wardle et al, 1983) and photosynthetic system (Earle and Langhans, 1975; Grout and Aston, 1978). These plantlets are sensitive to environmental stress, especially water stress (Earle and Langhans, 1975; Hu and Wang, 1983). The transfer of in vitro-propagated plantlets to the external environment leads to damage or death for many crops, because of the dramatic changes in environmental conditions. Environmental control for acclimatizing plantlets is needed to overcome the problem. Therefore, the acclimatization unit was developed to control environment and to achieve high survival (low death) rates and rapid growth of plantlets. The description of this unit is as follows:(1) With the acclimatization unit, temperature, humidity, light intensity, CO2 concentration, air flow rate, and nutrient solution temperature etc. can be controlled (Figs. 1 and 2).(2) A micro-computer is used to control the acclimatization unit, and to record, analyze the data.The acclimatization unit can be used both for commercial purposes and for research purposes.(3) The computer program for controlling the acclimatization unit is written in the BASIC language. The control logic to operate actuators is not included in the program but is given as data to the program. The logic is given in the form of control expression (Hoshi and Kozai, 1984). Therefore, changing the control logic is relatively easy.(4) The environment inside the acclimatization unit is controlled on the basis of the acclimatization curves (Fig. 3). The curve for each environmental factor can be modified depending on the crop, and the season when acclimatized plantlets are transplanted to greenhouse or field conditions.(5) By using the acclimatization curve, the diurnal change in the environment could be modified gradually day by day (Fig. 7).(6) The trial run of the acclimatization unit showed that each environmental factor was controlled within a desired range (Fig. 4).The test cultivation was done to investigate survival rate and growth rate of in vitro-propagated plantlets. The taro (‘Ishikawawase’) and the strawberry (‘Houkouwase’) after transferred from in vitro culture were acclimatized with the acclimatization unit and with the traditional method (described as control hereafter). In the traditional method, the growing space was covered with transparent plastic film and shading material.(1) Death rate of plantlets of taro acclimatized with the acclimatization unit and with the traditional method were 8% and 23% respectively, and those of strawberry were 4% and 20% respectively.(2) Fresh weight, dry weight, plant height and the number of leaves of taro and strawberry acclimatized with the acclimatization unit were greater than those of the traditional method at the end of acclimatization, respectively (Figs. 5 and 8).(3) After being transferred from in vitro to the acclimatization unit or to the control, the fresh weight, dry weight, plant height and the number of leaves decreased (Figs. 5 and 8).A significant difference in death rate and growth rate of the plantlets between the two methods was shown. Further investigation for the optimum environments for each crop in acclimatization period is needed.