Growth of chlorophyllous explants (shoots and leafy nodal cuttings) of most woody plant species examined above was greater under photoautotrophic than under photomixotrophic conditions, when CO2 concentration in the culture vessel was raised by using the gas permeable filter discs and by increasing the CO2 concentration in the culture room. Increase in PPF was also required in most cases. Increase in CO2 concentration and PPF promoted photosynthesis and thus dry matter accumulation.4-6,17
In addition, an increase in the number of air exchanges of the culture vessel enhanced the air movement or air current speed around the plants in the culture vessel and promoted diffusion of CO2 and water vapor around the plants, resulting in the promotion of photosynthesis and transpiration of in vitro plants.18 The increase in the number of air exchanges decreased the relative humidity in the culture vessel from nearly 100% to 85-90%.4 This decrease in relative humidity, in combination with the enhancement of air movement in the culture vessel, increased the transpiration rate of in vitro plants significantly, and thus increased water and nutrient uptake of in vitro plants.4 The decrease in relative humidity also enhanced cuticular wax formation of leaves and stomatal functioning, and it improved the water stress tolerance of plants. 19 The decrease in relative humidity, in combination with the use of air porous supporting materials such as vermiculite and Florialite, improved root formation and functioning, especially formation of secondary roots and normal vascular systems.
Increase in the number of air exchanges also decreased ethylene concentration in the culture vessel. Ethylene is a gaseous phytohor-mone produced by plants, which, in turn, affects the differentiation, development, morphology and growth of plants. Plants in the culture vessel with high number of air exchanges did not show any physiological or morphological disorder such as hyperhydration, which is often considered to be caused by a combination of high ethylene concentration and high relative humidity in the culture vessel.
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