The population in tropical countries has been increasing at an annual rate of 2.8% during the last ten years, and it will continue to increase at a similar rate for the next 10-20 years. On the other hand, the total area under forest in those countries has been decreasing at an annual rate of 0.8% according to a survey by FAO. It was 1,910 million hectares in 1981, but it decreased to 1,756 million hectares in 1990, i.e., it decreased by 15.4 million hectares per year. An annual afforestation and reafforestation area in those countries is estimated to be 1.8 million hectares during the period 1981-1990, resulting in a yearly net decrease in forest area of 13.6 (15.4 - 1.8) million hectares.
The forest area in temperate and cold climate regions has also been decreasing. In addition, the decrease in biomass of woody plants due to desertification in arid regions is significant. Such local and global decrease of the forest areas, and consequently the plant biomass, are the factors causing recent climate changes on different geographical scales.
It has been predicted that, in future decades, demands for woody transplants will rise considerably in the pulp, paper, timber, plantation, horticulture and furniture industries, for re-afforestation, afforestation and desert rehabilitation, for environment conservation and for energy/food production.1-3 Use of plant biomass in the industries mentioned above is essential to reduce the consumption of fossil fuels and to lower atmospheric CO2 concentration, and to stabilize local and global climate changes.
Transplant production based upon micropropagation has advantages over transplant production using seeds or cuttings, with respect to genetic and phenotypic uniformity and scheduled year round production of disease-free or pathogen-free transplants.4 We have been developing a system for producing a large number of quality transplants at low production cost, based on the idea of photo-autotrophic (no sugar in the culture medium) or photosynthetic micropropagation under high CO2 concentration or CO2 enrichment.5-7 In this chapter we demonstrate experimental results of photoautotrophic microprop agation for woody plant species such as eucalyptus (Eucalyptus camaldulensis), coffee (Coffea arabusta), acacia (Acacia mangium) and man-gosteen (Garcinia Mangostana), and we discuss the advantages and disadvantages of photoautotrophic micropropagation over the conventional, heterotrophic and photomixo-trophic micropropagation (with use of culture medium containing sugar). In addition, we discuss advantages of air porous supporting materials such as vermiculite or cellulose fibers and large culture vessels (5-1000 liter in air volume) over gelled supporting materials such as agar and small or conventional culture vessels (0.1-1 liter in air volume), respectively. We also emphasize the importance of in vitro acclimatization over ex vitro acclimatization. Finally, we discuss possibilities of factory style micropropagation systems.
Reasons for High Production Costs and Their Reduction by Photoautotrophic Micropropagation4-7
Micropropagation is an advanced vegetative propagation technology for producing a large number of genetically superior and pathogen-free transplants in a limited time and space. However, widespread use of micropropagation for woody plant species is still limited, mainly due to its high production costs.4 High production costs of micropropagated plants are mostly attributed to their low growth rate, a significant loss of plants in vitro by microbial contamination, poor rooting, low percent survival at the ex vitro acclimatization stage and high labor costs due to intensive manual operations of small plants and small culture vessels. Most of the above factors, which bring about the high production costs, are directly or indirectly related to the heterotrophic or photomixotrophic nature of plant growth in conventional micropropagation, where sugar is supplied to the culture medium as a sole or a main carbon and energy source for plant growth in vitro.
Our recent research achievements5-7 have revealed that most chlorophyllous plants in vitro have photosynthetic ability to grow photoautotrophically, and that the low CO2 concentration in the airtight culture vessel during the photoperiod is a main cause of the low net photosynthetic rate of plants in vitro. Also, the net photosynthetic rate of plants in vitro is considerably lower when cultured on sugar-containing medium than when cultured on sugar-free medium. Furthermore, we have shown that the photoautotrophic growth of chlorophyllous plants in vitro can be significantly promoted by increasing CO2 concentration and light intensity or photo-synthetic photon flux (PPF), by decreasing the relative humidity in the culture vessel, and by the use of fibrous and/or porous supporting materials with high air porosity.
By using a culture medium containing no sugar, the loss of plants in vitro due to microbial contamination can be significantly reduced. When a culture vessel with high ventilation rate or high number of air exchanges is used, the relative humidity in the vessel is reduced. This reduction in relative humidity results in enhanced rooting and high percent survival at the ex vitro acclimatization stage, especially when the porous supporting materials are used in vitro. In the following sections, we show the beneficial effects of in vitro environment control under photoautotrophic and photomixotrophic conditions on growth promotion of some woody plant species in small culture vessels with natural ventilation, and large culture vessels with forced ventilation.
Growth Promotion and Quality Improvement Using Small Culture Vessels
Experimental results on photoautotrophic micropropagation using relatively small (or conventional) culture vessels like Magenta culture vessels with an air volume of 300-400 ml are shown below. For photoautotrophic micropropagation, gas permeable filter discs are attached to the lid or sides of the culture vessel. These filter discs are effective for enhancing the natural ventilation of the culture vessel, and thus keeping the CO2 concentration in the culture vessel during the photoperiod higher to increase the net photosynthetic rate, and keeping the relative humidity comparatively lower to increase the transpiration rate, than those in the culture vessels without the gas permeable filter discs.
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