It is possible to produce horticultural crops all year round in Korea, both in the open field and in greenhouses, even during the extremely cold winter. In recent years, protected horticulture during the off season has developed rapidly, with the aid of government support. By 1997, the area under greenhouse cultivation was 47,322 ha, including 300 ha of glasshouses and 47,000 ha of plastic greenhouses.
Horticultural production in greenhouses is intended to increase farmers' incomes. However, we should keep in mind that a higher investment is needed to set up a greenhouse, and that the investment cost varies for different kinds of protective structure. The more money that is invested, the more precise will be the environment control. However, the cost of an environmentally controlled greenhouse is much higher than that of an ordinary one. Nutrient culture in a glasshouse, using an artificial substrate, can generate higher income due to the higher quality and yield of harvested produce. However, it is impossible to produce high-quality horticultural produce without intensive technologies.
A series of experiments was carried out on the improvement of environmental conditions under structures of greenhouses for year-round cultivation of vegetables in Korea. This Bulletin summarizes the results of studies on the response of chili pepper growth and yield to different environments in various kinds of greenhouse.
Specification of Different Greenhouse Structures
Four types of greenhouse were used in the experiment: A glasshouse, and three houses covered in plastic film (one in polycarbonate (PC), one in polyethylene telephthalate (PET), and one in polyethylene (PE)). Specifications of the four houses are shown in Table 1. General building costs per square meter were US$126 for the glasshouse, US$88 for the PC house, US$63 for the PET house, and US$25 for the PE.
All four greenhouses included a heating system, an insulating curtain, a ventilation system with a fan, and a fertigation system. The glasshouse had a different heating system than the other greenhouses, and a different cultivation substrate was used. The glasshouse was heated by a water circulation system, while the other greenhouses used hot air heaters. The cultivation substrate in the glasshouse was perlite, while the other greenhouses used soil.
The amount of solar radiation transmitted into the greenhouses during the daytime was different in each type of structure, being affected mainly by the covering material. The transmittance of solar radiation into the glasshouse is shown in Fig. 1. The transmittance rate was higher in this type of structure (64.7%), than in the plastic houses. Of the plastic covering materials, the PC house transmitted a similar amount of solar radiation as the PET house (61.3% and 60.7%, respectively). The PE house had the lowest transmittance of solar radiation at 56.4%, because a double layer of film was used for heat insulation.
The level of solar radiation under protective structures changes according to the angle of the sun in different seasons, and the age of the covering material. Therefore, the result measured on Nov. 1, 1996 does not represent the solar radiation through the experimental period. It should also be noted that 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. The transmittance of light was lower near the gutter in the middle of the roof than on the side of the house, because of shading from the rolled-up or folded curtain under the gutter. In the PC greenhouse and the glasshouse, the folded curtain cast a wide shadow under the gutter, so solar transmittance at this part of the structure was lower than in the PE house.
One of the important factors in increasing solar transmittance in greenhouses is to minimize the area under the gutter shaded by the curtain. Not only is a high rate of solar transmittance important in winter vegetable production, but the solar spectral quality is also an important factor determining vegetable quality.
The ascending air temperature after sunrise (08:00 am and 11:00 am) in each greenhouse is shown in Fig. 3. Air temperature was highest in the glasshouse, followed by the PC house, the PET house and the PE house, in that order. These differences reflect the differences in the heat balance resulting from the short-wave radiation transmitted into the greenhouses and long- wave radiation transferred out (Godbey et al. 1979).
Control of air temperature in the greenhouses depends on the ventilation during the day. Air temperature in the PE house around noon was higher than in the other greenhouses, because ventilation was relatively poor. Gutter ventilation is generally not as effective as ridge ventilation. Minimum air temperatures showed a similar trend to maximum air temperatures in the different types of structure, and this tendency continued during the nighttime.
The diurnal variation in relative humidity in the different types of greenhouse is shown in Fig. 4. When the windows were closed at night, humidity was stable, while it fluctuated during the day when the windows were open.
During the period without ventilation, relative humidity was lowest in the glasshouse, followed by the PC house, the PET house, and PE house, in that order. Generally, the higher the relative humidity in the greenhouse, the more variation in humidity was recorded during the day. When the window was closed and ventilation ceased, the humidity tended to increase slowly for a few hours, and then remain stable.
Heat Accumulation and Loss
An important property of greenhouses in Korea during winter is high heat accumulation ability and low heat loss. The property of heat accumulation can be measured by recording the air temperature when the greenhouse is closed after sunrise, while heat loss can be measured by the heat loss coefficient during the night.
The results of our experiments are shown in Table 2. Of all the structures tested, the glasshouse had the highest air temperature at 11 am (28.5Â°C), followed by the PC house, the PET house, and the PE house, in that order. Similarly, the transfer of heat into the soil began about 1 hour earlier in the greenhouse than in the PE house. As a result, the glasshouse achieved the optimum air and soil temperature for photosynthesis more rapidly than the plastic greenhouses. The coefficient of the heat loss in the PE house and PET house was a little lower than that of the other structures, since they retained less heat during the night.
Water Condensation on Surface of Covering Materials
Generally, the air inside greenhouses is moist because of evaporation and plant transpiration. Water condensation on the inner surface of the greenhouse was observed when the air temperature inside the house began to fall around 15:00 hours during the cold season. The number and size of water drops, and the relative humidity inside various types of greenhouse, is shown in Table 3.
In the glasshouse, the water-drop ratio on the inner surface of the glass was low (only 4.6%) because of the hydrophilic (water-attracting) properties of the glass. The PE house, on the other hand, had a high water drop ratio (72.0%) because of the hydrophobic (water-repelling) properties of the surface of the PE film. Otherwise, the relative humidity in the PE house was high compared to the glasshouse and the other plastic film houses.
A lower water-drop ratio implies, not only a lower relative humidity, but also a higher rate of solar transmission into the greenhouse.
Growth and Yield of Chili Pepper in Different Types of Greenhouse
The growth rate of chili pepper grown in different types of greenhouse is shown in Table 4. Of all the structures tested, the glasshouse gave the best growth rate in terms of plant height, leaf area, and fresh weight, followed by the PC house, the PET house, and the PE house, in descending order.
The result suggests that the glasshouse was the most favorable environment, the result of a high transmittance of solar radiation, suitable temperatures for plant assimilation, and other environmental factors. However, appropriate cultivation techniques are also needed for the high yields, to minimize the adverse effects of climate and soil.
Relation between Leaf Area Index and Crop Growth Rate
Fig. 5 shows the correlation between leaf area index and crop growth rate (CGR) of chili pepper grown in four different types of greenhouse. The highest leaf area index (LAI) recorded on chili plants grown in the glasshouse was 6.35. That of the PC house was 4.32, that of the PET house was 3.62, while that of the PE house was 3.57. The highest LAI was calculated on the basis: x = 12a/b, when the regression equation, f'(x) = 0.
The correlation between crop growth rate and leaf area index was fairly high in all four types of greenhouse. The results suggest that chili pepper grown in glasshouses can sustain optimum growth, with a leaf area 1.8 times higher than plants grown in a PE house. The highest crop growth rate recorded in the PC house was higher than that of the PET or PE house.
Branching and Canopy Structure
Characteristically, after chili pepper bushes have reached the 8 - 12 leaf stage, they begin to develop two branches at every branch node on the main stem. It is possible to judge the growth stage of chili pepper by counting the number of branch nodes. Table 5 shows the branching and canopy of chili pepper at 30 days after transplanting (Apr. 26, 1996). The length of the main stem before branch divergence was longest in chili pepper grown in the glasshouse, because of the better environmental conditions.
No differences in canopy width or the rate of aborted branches were observed in the various types of greenhouse at an early stage of canopy formation. However, differences in canopy growth emerged over time. Three months after transplanting, there was a considerable difference between canopy growth in the glasshouse, with the highest growth rate, and that in the PE house, with the lowest ( Table 6). There was little difference between the PC house and the PET house.
Light interception by the canopy of chili pepper bushes is shown in Table 6. There was a significant difference between the different structures one month after trans-planting. Three months after transplanting, the differences were less marked, but still significant in some cases.
Fig. 6 shows an analysis of the source of assimilation. The distribution, based on the dry matter content of leaf, has a specific profile affected by environmental differences in the different types of greenhouse.
In the case of the glasshouse, the leaf dry matter as a source of assimilation increased as growth progressed until the plant reached a height of 100 cm height, the 9th node stage of a chili pepper plant. Afterward, it fell rapidly. The weight of branches and fruit as the sink of assimilation matter was a little higher than the assimilation part. The maximum weight was observed when the plant was at a height of 70 cm (6th node stage).
In the case of the PC and PET houses, the leaf weight as a source of assimilation showed a similar pattern to the glasshouse, increasing until the plant reached a height of 90 cm. Afterward, it fell more rapidly than in the glasshouse. The highest branch weight was observed at 70 cm, and the highest fruit weight was observed at 60 cm. Weights in the PC house were a little higher than those in the PET house.
The PE house had the lowest total weights of all the greenhouses. The maximum weight of the assimilation part was observed at 80 cm plant height, but the maximum weight of the non-assimilation part was observed at a plant height of only 50 cm.
These results suggest that the production of plant material in greenhouses depends on a build-up of assimilation parts, and that this is markedly affected by the growing environment. The more the leaves assimilate, the more the branches diverge, so that there is plenty of room for fruit set. Poor vegetative growth in chili pepper can induce competition between the source and the sink of assimilation matter, resulting in a smaller source (leaf weight), as in the case of the chili pepper grown in the PE house.
Yield and Quality
The characteristics of flowering and fruit set in chili pepper grown in different types of structure are shown in Table 7. Crops grown in the glasshouse needed fewer days to reach first flowering (73 days, 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 rate of fruit set, at 88 - 90%. The number of days until harvest after flowering had begun was 14 in the glasshouse and 17 in the PE house.
Table 8 shows the characteristics of chili peppers harvested in different types of greenhouse. The vitamin C content was highest in peppers grown in the glasshouse (97.7 mg/g). The vitamin C content of peppers grown in the PE house was only 65.3 mg/g. The general characteristics of chili peppers harvested from the glasshouse were: longer fruit, thick and soft fruit skin, and fewer seeds compared to the peppers produced in the other structures.
The yields of chili pepper were measured over the winter when the greenhouses were heated, and in the rest of the year when the heating was turned off. Peppers were harvested for five winter months with the heating on. The yield over this period in all greenhouses was low (only 10 - 27% of the quantity harvested in the un-heated period) due to adverse environmental conditions.
During the heated period, the glasshouse had the highest yields (15.6 mt/ha fresh weight) followed by the PC house, the PET house, and PE house, in that order. The major factors contributing to the higher yields in the glasshouse were probably solar transmittance and the use of a perlite substrate in the glasshouse. It is important to improve marketable yields over the winter when greenhouses are heated. 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 in hot pepper yield in the four types of greenhouse.
Difference between the total yields from the different greenhouses was not as significant as the yield difference during the heated period. However, the highest yield was observed in the glasshouse using perlite culture, followed by the PC house, the PET house, and the PE house, in that order.
The results suggest that the high cost of greenhouse construction could bring a higher return if supported by intensive production techniques. Profitable greenhouse production can be achieved by making best use of the environmental characteristics of each type of structure. Further research is needed to develop better crop production techniques in the greenhouse facilities during the off-season, when weather conditions are unfavorable.
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Index of Images
Figure 1 Diurnal Variation* in Transmission of Solar Energy in Greenhouses (Nov. 1, 1996)
Figure 2 Vertical Profile of Solar Transmittance by Types of Greenhouse Measured at 12:00, Nov. 5 in Suweon, Korea
Figure 3 Diurnal Variation in Air Temperature in Greenhouses Ventilated from 09:00 to 17:00 (Nov. 1 - Nov. 2, 1996)
Figure 4 Diurnal Variation in Relative Humidity in Greenhouses Ventilated from 10:00 to 17:00 (Nov. 1 - Nov. 2, 1996)
Figure 5 Correlation between Leaf Area and Crop Growth Rate of Chili Pepper Grown in Different Types of Greenhouse
Figure 6 Distribution of Non-Assimilation Part and Assimilation Part of Chili Pepper at 70 Days after Transplanting Table 1 Specifications of Four Greenhouses Table 2 Diurnal Heat Accumulation and Heat Loss in Different Types of Greenhouse, Korea (from Nov. 1 to April 6, 1997)
Table 1 Specifications of Four Greenhouses
Table 2 Diurnal Heat Accumulation and Heat Loss in Different Types of Greenhouse, Korea (from Nov. 1 to April 6, 1997)
Table 3 Comparison of the Water-Drop Ratio on the Surface of Covering Materials and Moist Air Properties in Various Types of Greenhouse in Korea (Measured at 15:00, Nov. 12, 1996)
Table 4 Growth of Chili Pepper Grown in Different Types of Greenhouse, Korea (Investigated at 30 Days after Transplanting)
Table 5 Characteristics of Branching and Canopy of Chili Pepper Plants 30 Days after Transplanting in Different Types of Greenhouse
Table 6 Comparison of Canopy and Light Interception at Monthly Intervals in Chili Pepper Crops Grown in Different Types of Greenhouse
Table 7 Characteristics of Flowering and Fruit Set in Chili Pepper Grown in Different Types of Greenhouse
Table 8 Characteristics of Chili Pepper Fruit Harvested from Different Greenhouses
Table 9 Yield of Chili Pepper during Heated (Winter) and Unheated Seasons in Different Greenhouses
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