Symbiotic mycorrhization can generally confer several benefits to host plants, such as supplying nutrients or conferring resistance to pathogens. Attempts have been made toward establishment of in vitro mycorrhization that could benefit culture growth. Arbuscular mycorrhizal (AM) fungi have been established in photoautotrophic strawberry cultures (Cassells et al., 1996). The inoculation and establishment of colonization was achieved more successfully under photoautotrophic conditions than under conventional photomixotrophic conditions, especially when the non-sterile AM fungi were used. The system employed by Cassells et al. (1996) included the use of polyurethane foam as a medium supporting material replacing agar gel. Importance of controlling PPF and CO2 concentration is recognized in the symbiotic effects of endomycorrhzation of plantlets (Louche-Tessandier et al., 1999). Further studies would be needed to identify ideal physical and environmental conditions that promote and maintain mycorrhization before the strategic use of symbiotic micorrhization in pathogen-free photoautotrophic micropropagation. Use of other beneficial microorganisms in vitro (Herman, 1996) also needs to be studied for use in photoautotrophic micropropagation.
Generally the tissue culture system is considered as a closed system and the concept of providing inputs to the system once it is closed is not widely regarded. In addition to medium ion availability, we should pay attention to the water availability or water usage in the closed system. The photoautotrophic micropropagation system developed by Zobayed et al. (1999) was designed to use 13.8 mL medium per plantlet during the 21 d culture of sweetpotato plantlets. The dry mass increase during the culture period was about 150 mg per plantlet. The volume of medium consumed per gram dry mass increase in the photoautotrophic system of Zobayed et al. (1999) is estimated roughly as one third of the volume of nutrient solution applied per gram dry mass increase of sweetpotato transplants grown ex vitro under artificial lighting conditions (estimated from values presented by Ohyama et al., 2000). The relatively low medium consumption per plantlet in the photoautotrophic micropropagation system suggests the possible accumulation and/or depletion of particular components in the system. A portion of water vapor in the headspace of the vessel leaves the vessel by ventilation and a significant portion of water vapor condenses on the sidewalls and is recycled (returned) to the medium. This may be one reason that such a small volume of medium used in photoautotrophic micropropagation as compared with an ex vitro transplant production system.
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