The intrinsic quality of plants produced in vitro is one of the key factors governing the percentage of survival during the acclimatization to greenhouse or field conditions. The condition of a plant at anytime is the summation of the effects of all the environmental conditions it has experienced up to that time. It is therefore of the utmost importance that as many factors as possible should be under control at all stages of growth. Nevertheless, environmental conditions of the plant microclimate cannot be controlled directly in the conventional system of plant tissue culture as it can be in photoautotrophic system. In conventional photomixotrophic system the microclimate exposes the culture to unnaturally high relative humidity, low exchange of gases between the culture vessel and the culture room, especially the low PPF coupled with the presence of sugar in the medium suppresses the need and opportunity for photosynthesis. As a result morphological and physiological abnormalities such as malfunctioning of stomata, thin and unorganized palisade and mesophyll layer in the leaf, less or no wax deposition on the leaf surface etc. become obvious. The abnormalities and malfunctioning of plants in the conventional airtight system emphasize the need for the optimization of the in vitro culture condition. To overcome these biologically damaging side effects, the recommendation is deduction of sugar from the growing medium, increment of light, CO2 and ventilation of the culture vessel microclimate. The elimination of the ex vitro acclimatization stage, the enhancement of the latent photosynthetic ability of plantlets and simplification of the culture medium are the major advantages of photoautotrophic micropropagation (Figure 13). Photoautotrophic micropropagation of plantlets is rather a new technique and thus our knowledge of sugar-free medium micropropagation owes much to the researches done so far to unravel and better understand the underlying factors which regulate the growth and development of plantlets in the absence of exogenous source of carbohydrate.
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