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Keywords: canopy structure, chili pepper, greenhouse structure, hot pepper, humidity, Korea, protected cultivation, solar radiation, temperature.

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This Bulletin discusses the growth and yield of chili pepper in four different kinds ofgreenhouse. One was a glasshouse, and three were plastic houses. Production of chili pepperin glasshouses was an effective way of increasing the yield per unit area and enhancing thequality. However, producing crops in a highly controlled environment needs a great deal oftechnical skill. If this is lacking, the productivity of a glasshouse may be lower than that of asimple polycarbonate or vinyl greenhouse.

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It is possible to produce horticultural crops allyear round in Korea, both in the open field and ingreenhouses, even during the extremely cold winter.In recent years, protected horticulture during the offseason has developed rapidly, with the aid of gov-ernment support. By 1997, the area under green-house cultivation was 47,322 ha, including 300 ha ofglasshouses and 47,000 ha of plastic greenhouses.

Horticultural production in greenhouses isintended to increase farmers’ incomes. However, weshould keep in mind that a higher investment isneeded to set up a greenhouse, and that the invest-ment cost varies for different kinds of protectivestructure. The more money that is invested, themore precise will be the environment control. How-ever, the cost of an environmentally controlled green-house is much higher than that of an ordinary one.Nutrient culture in a glasshouse, using an artificialsubstrate, can generate higher income due to thehigher quality and yield of harvested produce. How-ever, it is impossible to produce high-quality horti-cultural produce without intensive technologies.

A series of experiments was carried out on theimprovement of environmental conditions under

structures of greenhouses for year-round cultiva-tion of vegetables in Korea. This Bulletin summa-rizes the results of studies on the response of chilipepper growth and yield to different environmentsin various kinds of greenhouse.

Specification of Different GreenhouseStructures

Four types of greenhouse were used in theexperiment: A glasshouse, and three houses cov-ered in plastic film (one in polycarbonate (PC), one inpolyethylene telephthalate (PET), and one in poly-ethylene (PE)). Specifications of the four houses areshown in Table 1. General building costs per squaremeter were US$126 for the glasshouse, US$88 for thePC house, US$63 for the PET house, and US$25 forthe PE.

All four greenhouses included a heating sys-tem, an insulating curtain, a ventilation system witha fan, and a fertigation system. The glasshouse hada different heating system than the other green-houses, and a different cultivation substrate wasused. The glasshouse was heated by a water circu-lation system, while the other greenhouses used hotair heaters. The cultivation substrate in the glass-house was perlite, while the other greenhouses usedsoil.

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Solar Radiation

The amount of solar radiation transmitted intothe greenhouses during the daytime was different ineach type of structure, being affected mainly by thecovering material. The transmittance of solar radia-tion into the glasshouse is shown in Fig. 1. Thetransmittance rate was higher in this type of struc-ture (64.7%), than in the plastic houses. Of theplastic covering materials, the PC house transmitted

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a similar amount of solar radiation as the PET house(61.3% and 60.7%, respectively). The PE house hadthe lowest transmittance of solar radiation at 56.4%,because a double layer of film was used for heatinsulation.

The level of solar radiation under protectivestructures changes according to the angle of thesun in different seasons, and the age of the coveringmaterial. Therefore, the result measured on Nov. 1,1996 does not represent the solar radiation throughthe experimental period. It should also be noted that

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the covering materials tested were three years old.The vertical profile of solar transmittance in

the four types of greenhouse is shown in Fig. 2. Thetransmittance of light was lower near the gutter inthe middle of the roof than on the side of the house,because of shading from the rolled-up or foldedcurtain under the gutter. In the PC greenhouse andthe glasshouse, the folded curtain cast a wide shadowunder the gutter, so solar transmittance at this partof the structure was lower than in the PE house.

One of the important factors in increasingsolar transmittance in greenhouses is to minimizethe area under the gutter shaded by the curtain. Notonly is a high rate of solar transmittance important inwinter vegetable production, but the solar spectralquality is also an important factor determining veg-etable quality.

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The ascending air temperature after sunrise(08:00 am and 11:00 am) in each greenhouse is shownin Fig. 3. Air temperature was highest in the glass-house, followed by the PC house, the PET houseand the PE house, in that order. These differencesreflect the differences in the heat balance resultingfrom the short-wave radiation transmitted into thegreenhouses and long- wave radiation transferredout (Godbey et al. 1979).

Control of air temperature in the greenhousesdepends on the ventilation during the day. Airtemperature in the PE house around noon was higherthan in the other greenhouses, because ventilationwas relatively poor. Gutter ventilation is generallynot as effective as ridge ventilation. Minimum airtemperatures showed a similar trend to maximum airtemperatures in the different types of structure, andthis tendency continued during the nighttime.

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The diurnal variation in relative humidity inthe different types of greenhouse is shown in Fig. 4.When the windows were closed at night, humiditywas stable, while it fluctuated during the day whenthe windows were open.

During the period without ventilation, relativehumidity was lowest in the glasshouse, followed bythe PC house, the PET house, and PE house, in thatorder. Generally, the higher the relative humidity inthe greenhouse, the more variation in humidity was

recorded during the day. When the window wasclosed and ventilation ceased, the humidity tendedto increase slowly for a few hours, and then remainstable.

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An important property of greenhouses inKorea during winter is high heat accumulation abil-ity and low heat loss. The property of heat accumu-lation can be measured by recording the air tempera-ture when the greenhouse is closed after sunrise,while heat loss can be measured by the heat losscoefficient during the night.

The results of our experiments are shownin Table 2. Of all the structures tested, the glass-house had the highest air temperature at 11 am(28.5°C), followed by the PC house, the PET house,and the PE house, in that order. Similarly, the trans-fer of heat into the soil began about 1 hour earlier inthe greenhouse than in the PE house. As a result,the glasshouse achieved the optimum air and soiltemperature for photosynthesis more rapidly thanthe plastic greenhouses. The coefficient of the heatloss in the PE house and PET house was a little lowerthan that of the other structures, since they retainedless heat during the night.

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Generally, the air inside greenhouses ismoist because of evaporation and plant transpira-tion. Water condensation on the inner surface of thegreenhouse was observed when the air temperatureinside the house began to fall around 15:00 hoursduring the cold season. The number and size ofwater drops, and the relative humidity inside varioustypes of greenhouse, is shown in Table 3.

In the glasshouse, the water-drop ratio onthe inner surface of the glass was low (only 4.6%)because of the hydrophilic (water-attracting) prop-erties of the glass. The PE house, on the other hand,had a high water drop ratio (72.0%) because of thehydrophobic (water-repelling) properties of the sur-face of the PE film. Otherwise, the relative humidityin the PE house was high compared to the glass-house and the other plastic film houses.

A lower water-drop ratio implies, not only alower relative humidity, but also a higher rate of solartransmission into the greenhouse.

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Relation Between Leaf Area Indexand Crop Growth Rate

Fig. 5 shows the correlation between leafarea index and crop growth rate (CGR) of chili peppergrown in four different types of greenhouse. Thehighest leaf area index (LAI) recorded on chili plantsgrown in the glasshouse was 6.35. That of the PChouse was 4.32, that of the PET house was 3.62,while that of the PE house was 3.57. The highest LAIwas calculated on the basis: x = 12a/b, when theregression equation, f’(x) = 0.

The correlation between crop growth rateand leaf area index was fairly high in all four types ofgreenhouse. The results suggest that chili peppergrown in glasshouses can sustain optimum growth,with a leaf area 1.8 times higher than plants grown ina PE house. The highest crop growth rate recorded

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Growth Response

The growth rate of chili pepper grown indifferent types of greenhouse is shown in Table 4.Of all the structures tested, the glasshouse gave thebest growth rate in terms of plant height, leaf area,and fresh weight, followed by the PC house, the PEThouse, and the PE house, in descending order.

The result suggests that the glasshousewas the most favorable environment, the result of ahigh transmittance of solar radiation, suitable tem-peratures for plant assimilation, and other environ-mental factors. However, appropriate cultivationtechniques are also needed for the high yields, tominimize the adverse effects of climate and soil.

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in the PC house was higher than that of the PET orPE house.

Branching and Canopy Structure

Characteristically, after chili pepper busheshave reached the 8 - 12 leaf stage, they begin todevelop two branches at every branch node on themain stem. It is possible to judge the growth stageof chili pepper by counting the number of branchnodes. Table 5 shows the branching and canopy ofchili pepper at 30 days after transplanting (Apr. 26,1996). The length of the main stem before branchdivergence was longest in chili pepper grown in theglasshouse, because of the better environmentalconditions.

No differences in canopy width or the rate ofaborted branches were observed in the various typesof greenhouse at an early stage of canopy forma-tion. However, differences in canopy growth

emerged over time. Three months after transplant-ing, there was a considerable difference betweencanopy growth in the glasshouse, with the highestgrowth rate, and that in the PE house, with thelowest (Table 6). There was little difference betweenthe PC house and the PET house.

Light interception by the canopy of chili pep-per bushes is shown in Table 6. There was a signifi-cant difference between the different structures onemonth after trans-planting. Three months after trans-planting, the differences were less marked, but stillsignificant in some cases.

Fig. 6 shows an analysis of the source ofassimilation. The distribution, based on the drymatter content of leaf, has a specific profile affectedby environmental differences in the different typesof greenhouse.

In the case of the glasshouse, the leaf drymatter as a source of assimilation increased as growthprogressed until the plant reached a height of 100 cm

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Transplanting date of chili pepper: April 26, 1996

height, the 9th node stage of a chili pepper plant.Afterward, it fell rapidly. The weight of branchesand fruit as the sink of assimilation matter was a littlehigher than the assimilation part. The maximumweight was observed when the plant was at a heightof 70 cm (6th node stage).

In the case of the PC and PET houses, the leafweight as a source of assimilation showed a similarpattern to the glasshouse, increasing until the plantreached a height of 90 cm. Afterward, it fell morerapidly than in the glasshouse. The highest branchweight was observed at 70 cm, and the highest fruitweight was observed at 60 cm. Weights in the PChouse were a little higher than those in the PEThouse.

The PE house had the lowest total weights ofall the greenhouses. The maximum weight of theassimilation part was observed at 80 cm plant height,but the maximum weight of the non-assimilation partwas observed at a plant height of only 50 cm.

These results suggest that the production of

plant material in greenhouses depends on a build-upof assimilation parts, and that this is markedly af-fected by the growing environment. The more theleaves assimilate, the more the branches diverge, sothat there is plenty of room for fruit set. Poorvegetative growth in chili pepper can induce compe-tition between the source and the sink of assimila-tion matter, resulting in a smaller source (leaf weight),as in the case of the chili pepper grown in the PEhouse.

Yield and Quality

The characteristics of flowering and fruit setin chili pepper grown in different types of structureare shown in Table 7. Crops grown in the glass-house needed fewer days to reach first flowering (73days, compared to 76 days in the PE house).

The rate of fruit set in the PE house was low(81.3%). The other structures had a fairly high rateof fruit set, at 88 - 90%. The number of days until

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harvest after flowering had begun was 14 in theglasshouse and 17 in the PE house.

Table 8 shows the characteristics of chilipeppers harvested in different types of greenhouse.The vitamin C content was highest in peppers grownin the glasshouse (97.7 mg/g). The vitamin C con-tent of peppers grown in the PE house was only 65.3mg/g. The general characteristics of chili peppersharvested from the glasshouse were: longer fruit,thick and soft fruit skin, and fewer seeds comparedto the peppers produced in the other structures.

The yields of chili pepper were measuredover the winter when the greenhouses were heated,and in the rest of the year when the heating wasturned off. Peppers were harvested for five wintermonths with the heating on. The yield over thisperiod in all greenhouses was low (only 10 - 27% ofthe quantity harvested in the un-heated period) dueto adverse environmental conditions.

During the heated period, the glasshousehad the highest yields (15.6 mt/ha fresh weight)

followed by the PC house, the PET house, and PEhouse, in that order. The major factors contributingto the higher yields in the glasshouse were probablysolar transmittance and the use of a perlite substratein the glasshouse. It is important to improve market-able yields over the winter when greenhouses areheated. Prices paid for chili pepper at this time are 2- 4 times higher than at other seasons. In the non-heated period, no significant difference was seen inhot pepper yield in the four types of greenhouse.

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Difference between the total yields from thedifferent greenhouses was not as significant as theyield difference during the heated period. However,the highest yield was observed in the glasshouseusing perlite culture, followed by the PC house, thePET house, and the PE house, in that order.

The results suggest that the high cost of

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greenhouse construction could bring a higher re-turn if supported by intensive production tech-niques. Profitable greenhouse production can beachieved by making best use of the environmentalcharacteristics of each type of structure. Furtherresearch is needed to develop better crop produc-tion techniques in the greenhouse facilities duringthe off-season, when weather conditions are unfa-vorable.

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