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12 18 24

Time (hour)

12 10 24

Time (hour)

Figure 8. Daily changes of relative humidities, air temperatures and absolute humidities inside and outside the vessel with and without gas permeable film, and without and with in vitro potato plantlets 2 days and 24 days after transplanted in the vessel (Kozai et al., 1995b). A1: with filter and plantlets, A2: with filter and without plantlets, B1: without filter and with plantlets, B2: without filter and plantlets.

2.3. Light

Light environment needs to be considered with respect to photon flux, spectral distribution, lighting cycle (photoperiod/dark period), lighting direction (downward and sideward lighting), etc. Light environment for in vitro plantlets is affected by the optical characteristics of light source, vessels and their surroundings, and by the geometrical relationships among light source, vessels and their surroundings. Effects of these light-related environmental factors on the growth and development of in vitro plantlets are discussed by Fujiwara and Kozai (1995b).

2.3.1. Increase in PPF by reflective sheet placed above the lamps

In micropropagation, straight-line fluorescent tubes, 120 cm in length and 25 or 32 mm in diameter (35-40 W), are used as the light source in most cases, which are the most cost-effective among many types of light sources. These straight-line fluorescent tubes emit light outward along the axis of each tube to all directions uniformly from each point of tube surface. Thus, 50% of light is emitted upward and the rest is emitted downward. Since the tubes are usually placed 30-40 cm above the culture shelf and the culture shelf is about 60 cm wide, about 50% of downward light or only about 25% of light emitted by the tube reaches the culture shelf directly (Figure 9). The rest of downward light goes to the outside of the culture shelf.

shelf. Less than 25% of light energy emitted from the fluorescent (FL) lamps reaches the culture shelf directly. Half of light energy emitted from the FL lamps goes upward. About 80% of upward light can be reflected downward at the inner surface above the lamps if the inner surface is white. About half of the reflected light energy reaches the culture shelf directly. About 25% of light energy that is emitted from the FL lamps and reaches neither the culture shelf nor the inner surface above the lamps directly can be partially reflected downward by the sides of the culture shelf and can reach the culture shelf if the sides of culture shelf are covered partially or totally with white material.

shelf. Less than 25% of light energy emitted from the fluorescent (FL) lamps reaches the culture shelf directly. Half of light energy emitted from the FL lamps goes upward. About 80% of upward light can be reflected downward at the inner surface above the lamps if the inner surface is white. About half of the reflected light energy reaches the culture shelf directly. About 25% of light energy that is emitted from the FL lamps and reaches neither the culture shelf nor the inner surface above the lamps directly can be partially reflected downward by the sides of the culture shelf and can reach the culture shelf if the sides of culture shelf are covered partially or totally with white material.

In order to increase the percentage of light received by the culture shelf, it is recommended to use a reflective sheet (white paper or aluminium foil sheet with reflectivity of 80-90%) above the lamps (below the upper culture shelf) to reflect the upward light downward. The PPF on the culture shelf is thereby increased by 4050%. This is because 80% of the upward light (40% of emitted light = 50 x 80/100) is reflected back downward and 50% of the reflected light (20% of emitted light = 50 x 40/100) reaches the culture shelf. Thus, the total light received by the culture shelf is 45 (= 25 + 20) % of the emitted light. Overall, a 44% (= 100 x 20/45) increase in PPF on the culture shelf is achieved. The PPF on the culture shelf increases with decreasing distance between shelves and increasing reflectivity of reflective sheets. The use of a slightly inclined reflective sheet at the upper part of each shelf edge is also effective to reflect down the outgoing light to the culture shelf and, thus, to increase the PPF on the culture shelf (Figure 9).

Without the use of reflective sheets, the upward light is mostly absorbed by the bottom surface of opaque culture shelf above the fluorescent tubes. This light is converted into heat which raises the surface temperature of the culture shelf and thus the air temperature around the fluorescent tubes. In a case where a transparent glass sheet is used as shelf board, the upward light transmitted through the glass sheet above the lamps is absorbed by the culture medium on the upper shelf, resulting in a relatively high medium temperature. Therefore, the use of glass sheets should be avoided unless there is a definite reason for it.

2.3.2. Reduction in PPF due to vessels with closures

PPF values given in the literature are mostly measured on an empty shelf or at the top of a vessel placed on the shelf. Thus, the PPF at plantlet level in a culture vessel is significantly lower than the PPF at a top of vessel. Figure 10 shows the PPF measured in glass test tubes closed with different types of closures and placed upright in the stainless stand. Four hundred and fifty glass test tubes with and without closures were set on a shelf (90 cm by 120 cm) and PPF in each test tube was measured on its quarter section, using a small PPF sensor. The light source was one cool white fluorescent lamp (40 W) of 120 cm in length installed 33 cm above the shelf and above the line AD as drawn in Figure 10. The mean PPF in the test tubes without caps (38 ^mol m-2s-1) is 64% of the PPF on the empty shelf (59 ^mol m-2s-1). The PPF transmissivity of the polypropylene cap itself was 85%. The mean PPF in the test tubes with the polypropylene caps (33 ^mol m-2s-1) is 2.4 times that of the test tubes with the aluminium foil caps (14 ^mol m-2 s-1), and is 3.8 times that of the test tubes with silicon foam rubber plug caps (9 ^mol m-2 s-1). The mean PPF in polycarbonate box type (Magenta type) vessels with polycarbonate formed lids was 38 ^mol m-2 s-1 (64% of the PPF on the empty shelf: 59 ^mol m-2 s-1).

A large culture vessel with a lid having a high light transmissivity is essential to increase the PPF in the vessel with minimum number of fluorescent lamps and minimum electricity consumption.

As described above, PPF in the empty vessel with a transparent lid is about 65%. Consequently, 16% (= 25 x 65/100) of light emitted from the tube reaches the culture medium surface of an empty vessel. Plantlets/explants in vitro absorb some percentage of the light transmitted into the vessel (probably less than 10% for explants, 40% and 80% for plantlets at the middle and end of multiplication stage, respectively). After all, only a small percentage of light energy emitted from the tubes is received by cultures in the vessels. The rest of the light energy is uselessly converted into heat energy and increases the cooling load of air conditioners and, thus, electricity consumption for lighting and cooling. Increasing the percentage of light energy received by the cultures is a key to reducing electricity consumption.

CLCSURE:-
Figure 10. PPFs in the test tube type vessels as affected by types of vessel caps (Fujiwara et al., 1989). (a) aluminium foil caps, (b) polypropylene formed caps, (c) silicon rubber plugs, and (d) no caps. MAX, MIN and MEAN denote maximum, minimum and mean PPF, respectively.

2.4. Air movement

2.4.1. Air movement above the culture shelf

Warm air tends to move up due to its buoyancy. Thus, air temperature tends to be higher at the upper part of each culture shelf unit than at its lower part, and it tends to be higher inside the shelf than outside. To avoid this uneven distribution of air temperature within the culture shelf and culture room, smooth air movement across the culture shelf needs to be enhanced. This enhancement can be realized by creating space gap at upper part of each shelf to let the warm air escape to the outside of the shelf or to the upper shelf. Fluorescent lamps are manufactured to give a maximum light output at a tube surface temperature of about 40 C. This tube surface temperature can be obtained when the air temperature is about 25 C and air current speed around the tube is around 5-10 cm s-1.

2.4.2. Air movement in the vessel with natural ventilation

Air current speed in the culture vessel during the photoperiod is about 1-20 mm s-1 in most cases, which is low compared with that in the greenhouse (10-1000 mm s-1) (Table 2). This low air current speed restricts the diffusion of Co2, for photosynthesis, and of water vapor, for transpiration in the vessel. Air current speed in the vessel increases with the increase in PAR (or light intensity) at the top of the vessel (Figure 11) and/or with the increase in absorptance of transmitted radiation on the culture medium (Omura et al., 1995). This is because higher absorption of radiation at the medium surface raises the medium surface temperature and, thus, enhances free convection of air in the vessel. The addition of activated charcoal to the culture medium changes the medium color from white-translucent to black, increasing the air current speed in the vessel (Figure 11). Furthermore, air current speed increases when the plantlets are small, because smaller plantlets are less restrictive to air movement in the vessel. of course, air movement is enhanced with an increased number of air exchanges of the vessel and the air current speed around the vessel. Shape of the vessel also affects the air current pattern in the vessel considerably. The smaller and thinner the vessel, the more restricted the air movement is in the vessel.

2.4.3. Air movement in large vessels with forced ventilation

Figure 12 shows the two dimensional patterns of air currents in a culture vessel with forced ventilation using air distribution pipes on the plug tray with sweetpotato plantlets 6 days after transplanting. Air moves up at the central part of the vessel and moves down along the sidewalls of the vessel. Air current over the plantlets ranged between 5 to 10 mm s-1, which is approximately the same as the air current speed in the naturally ventilated vessel with plantlets. The forced ventilation rate was 13 ml s-1, which corresponds to the number of forced air exchanges of the vessel being 3.9 h-1. This number of air exchanges is difficult to obtain in the case of such a large culture vessel, if it is limited to natural ventilation using gas permeable films. Air current speed needs to be increased as the plantlets grow to reduce uneven spatial distribution of CO2 concentration in the vessel.

Short-wave radiation flux 20 VV m'! 30 W m'2
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