The growth of chlorophyllous explants (shoots and leafy nodal cuttings) of the woody plant species examined above is greater under photoautotrophic conditions than under photomixotrophic conditions, in the presence of increased CO2 concentration in the culture vessel during the photoperiod (which is achieved by the use of gas permeable filters in combination with CO2 enrichment in the culture room). The increase in PPF is also required in most cases. Increases in CO2 concentration and PPF promoted photosynthesis and thus carbohydrate accumulation of plants (Aitken-Christie et al., 1995; Jeong et al., 1995; Kozai et al., 1995). The use of fibrous or porous supporting materials in place of agar has been shown to be beneficial under high CO2 concentration and PPF conditions for enhancing root initiation and function. These materials are especially effective in promoting the formation of lateral roots and normal vascular systems, which thereby benefits the overall growth and quality of the woody plants.
In addition, an increase in the number of air exchanges of the culture vessel enhances the air movement around the plants growing in the culture vessel. Then, it, in turn, promotes the diffusion of CO2 and water vapor around plants, resulting in the promotion of photosynthesis and transpiration of plants in vitro (Kitaya et al., 1996).
The increase in the number of air exchanges decreases the relative humidity in the culture vessel from nearly 100% to 85-90% (Aitken-Christie et al., 1995). The decrease in relative humidity in combination with the enhanced air movement in the culture vessel increases the transpiration rate of in vitro plants significantly, and thus increases water and nutrient uptake of plants in vitro (Aitken-Christie et al., 1995).
It should be also noted that calli are not formed at the base of the shoot when grown on sugar-free medium with porous supporting materials, which is an advantage of woody plant micropropagation. Physiological mechanism of this phenomenon needs to be studied in the future.
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