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Effects of greenhouse covers on seedless cucumber growth, productivity, and energy use
SCIENTIA HORTICULTURR Scientia Horticulturae 68 (1997) I 13- 123
ELSEVIER
Effects of greenhouse covers on seedless cucumber growth, productivity, and energy use Athanasios P. Papadopoulos *, Xiuming Hao Agriculture
and Agri-Food
Cunada, Greenhouse and Processing Crops Research Centre, Harrow,
Ont..
Canada NOR I GO
Accepted 16 August 1996
Abstract Effects of single-glass (glass), double inflated polyethylene film (D-poly), and rigid twin acrylic (acrylic) panels, as greenhouse covers, on seedless cucumber growth, productivity and energy use were investigated over three spring seasons (1988, 1990 and 1992). The early and final marketable yields of plants grown in D-poly houses were similar to or higher than those grown in glass houses. Also, plants grown in D-poly houses produced a similar or higher percentage of large and a lower percentage of small fruits than the plants grown in glass houses. The early and final marketable yield and percentages of fruit grades were similar in D-poly and acrylic houses; an exception was the early marketable yield in 1992 which was higher in the acrylic houses. Light transmission was the highest in glass houses, intermediate in acrylic houses and the lowest in D-poly houses. Relative humidity was highest in D-poly, intermediate in acrylic, and the lowest in glass houses. The plants in D-poly houses grew and developed faster (more leaves and flowers) than those in glass houses. The leaf size of plants in D-poly houses and glass houses was similar, but the dry matter content and specific leaf weight of plants grown in D-poly houses were significantly lower (40% less) than those in glass houses. Plants in D-poly houses might have acclimated to the low light conditions by reducing specific leaf weight and increasing their light interception efficiency. Plant growth and development in acrylic and D-poly houses was similar. For cucumber production in climatic conditions similar to South Western Ontario, the D-poly greenhouse is strongly recommended, because there is no loss of productivity in comparison with a glass house, while great savings on initial investment and energy use are achieved. 0 Elsevier Science B.V. Keywords: Cover material; Cucumber; Glass; Polyethylene; Acrylic; Yield; Energy
* Corresponding author. Tel: 5 19-738-2251; Fax: 5 19-738-2929. 0304-4238/97/$17.00 0 1997 Elsevier Science B.V. All rights reserved. PII SO304-4238(96)0096 l-2
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1. Introduction The high fuel costs during the 1970s and early 1980s led to a heightened interest in greenhouse cover materials which were more energy efficient than glass (Noble and Holder, 1989). A wide range of plastic cover materials including rigidly structured plastic panels, such as fibre-glass-reinforced polyester (FRP), acrylic (polymethylmethacrylate, PMMA), polycarbonate (PC), and polyvinyl chloride (PVC) panels, as well as, thin film coverings, such as, low-density polyethylene (LDPE), polyvinyl chloride (PVC), and ethylene vinyl acetate copolymer, have been tested as greenhouse covers. Double inflated polyethylene films (D-poly) are becoming the most popular greenhouse cover materials in North America because of the low initial investment required and the significant energy savings realized (Blom and Ingratta, 1985; Giacomelli and Roberts, 1993; Papadopoulos, 1994a). Twin wall acrylic panels (acrylics) are popular in garden centres, sale areas, and the side and end walls of large greenhouses with D-poly roofs (Giacomelli and Roberts, 1993). Some conversion of glasshouses to acrylic has taken place in Norway (Stromme et al., 1986). The economics of using energy-efficient cover materials depends not only on their cost and energy-saving potential but also on their effects on the greenhouse environment, which affects the yield and quality of greenhouse crops. A loss of 5-20% of visible light, or photosynthetically active radiation (PAR), in D-poly houses has been shown by many researchers (Blom and Ingratta, 1985; Bauerle, 1981). The light transmission of double acrylic may be lower (Stromme et al., 1986; Bjerre, 19811, similar (Ting and Giacomelli, 1987) or higher (Bredenbeck, 1985) than that of single glass depending on the composition, aging, or weathering of the acrylic panels. Greenhouse air temperature, humidity, and leaf temperature are in turn affected by the light transmissivity of the cover material. The improvements in insulation of doublelayered polyethylene and acrylic greenhouses have resulted in increased humidity in the greenhouse (Blom and Ingratta, 1985; Boulard et al., 1989). Changes in greenhouse microclimates may have significant effects on the growth, development and productivity of crops. Photosynthetic rates are reduced at low PAR and it is generally assumed that the loss of light will lead to a proportional loss in yield (Challa and Schapendonk, 1984). Photosynthetic rates may increase at high humidity (Acock et al., 1976; Bunce, 1984). Cucumber vegetative growth was enhanced by either high day or night humidity, and final total yield was closely related to daytime humidity (Bakker et al., 1987). The major concern regarding high humidity is that it may encourage plant diseases and cause physiological disorders such as calcium deficiency (Hand, 1988). The research results on the effects of double roofing on vegetable crops are far from conclusive. Experiments in The Netherlands indicated a lo-15 and 4-13% yield reduction for tomatoes grown under double-layered glasshouses, in comparison with single-layered glasshouses for spring and autumn, respectively; the corresponding yield decreases for cucumbers were 7-12 and 4-8% for spring and autumn, respectively (Winden et al., 1984). In Norway, the light level in acrylic and glass houses was 52 and 61% of the outside level, respectively, and the acrylic cover had a negative effect on the cucumber yield
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relative to glass (Reierson and Sebesta, 1981). However, the productivity of tomatoes was found to be similar under acrylic or glass cover (Stromme et al., 1986). In Ohio, spring tomato yields were significantly higher in a greenhouse with Exolite@ acrylic glazing than under single glass, although the percentage of PAR entering the greenhouse was 56 and 65% for the Exolite’ and glass cover, respectively (Bauerle, 1981). In this case the yield of the crop was not totally dependent on PAR and a reduction in PAR with some glazing materials may not indicate a reduction in productivity. In Ontario, tomato growth and development were better under glass than under twin-wall PVC during the light-deficient winter months but better under PVC during the bright months of the year (Papadopoulos and Jewett, 1984). Marketable yield in the early part of the spring season and total yield in autumn was higher under glass than under PVC but the total marketable yield in spring was higher under PVC than under glass although there was 18% less PAR in the PVC, compared with the glass house (Jewett and Papadopoulos, 1984). It appears that the effect of light reduction on the yield might be dependent on the varying crop requirement for light at different stages of plant development. Most of the experiments on the effects of cover materials on greenhouse crops have employed no replication for the cover materials because of the high cost of such facilities. This lack of replication has made it impossible to quantify the effects of the cover material on the greenhouse environment, energy use and crop productivity with any statistical certainty. The study reported in this article was conducted in nine mini-greenhouses covered with glass, D-poly or acrylic, in triplicate, to determine the effects of these three cover materials on cucumber growth, productivity and energy use.
2. Materials and methods The experiment was carried out in the spring of 1988, 1990 and 1992 in nine independent mini-greenhouses (South-North oriented) constructed in 1987 (Table 1). The nine greenhouses, all of the same size (6.2 X 7.2 X 3 m, LX W X H) and shape, were constructed to conform to a 3 X 3 Latin Square design. Twin-wall polycarbonate panels (8 mm thick) were used to glaze the north side wall for all greenhouses. The roofs
Table 1 Experimental treatments greenhouse covers
and timing of the three runs of the experiment
with seedless cucumbers
under three
YEU
1988
1990
1992
cu1tivars Growth Media a Planting Date First Harvest CO, Enrichment Final Harvest
Corona and Aramon Grodan@ and Pargro” Feb. 18, 1988 March 28.1988 1COOp11May 18, 1988
Corona Grodan’ and Pargro” Jan. 24.1990 March 1, 1990 IOOOpll-’ June 18, 1990
Corona and Aramon G&an@ and Pargro” Dec. 23, 1991 Jan. 23, 1992 lOOOpLL- ’ June 29, 1992
a G&an”;
Grodania
A/S,
Milton, Ontario, Canada. Pargro
, Pargro Inc., Orillia, Ontario, Canada. @’
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Horticulturae 68 (1997) 113-123
and other three side walls were covered with either double inflated polyethylene film (0.15 mm>, twin-wall acrylic panels (16 mm), or glass (3 mm), in accordance with the principles of the Latin Square. The roofs of the glass houses were covered with single layered glass but the side walls were covered with double layered glass. There were three greenhouses for each cover material, one in each row and column of the Latin Square. The rows and columns of the greenhouses were separated by 15m of open turfgrass land to avoid the greenhouses shading one another. The greenhouses were connected by 1.5 m wide enclosed walkways on their north sides. Heating was provided with forced-air, gas-fired heating units and ventilation with two stage, low-speed, high-volume fans and louvred air intakes. Fertigation, according to standard recommendations for rockwool grown cucumber crops (Papadopoulos, 1994b), was controlled by a Harrow Fertigation Manager@‘, a computerized multi-fertilizer injection system (Papadopoulos and Liburdi, 1989). Carbon dioxide enrichment at 1000 ~11~’ was implemented with pure CO, from a central supply of liquid carbon dioxide; the carbon dioxide enrichment was stopped whenever the greenhouses were ventilated. A PRIVA environmental computer controlled the climate in all greenhouses and recorded greenhouse climate data. The sensor box was placed just below the top of the cucumber canopy and was adjusted weekly following the cucumber growth. Cucumber seeds were sown in rockwool blocks (10 X 10 X 7.5 cm; Pargro@, Pargro Inc., Orillia, Ont., Canada), and after the appearance of the third true leaf, seedlings were transplanted into rockwool slabs (100 X 20 X 7.5 cm; Pargro@, Pargro Inc., Orillia, Ont., Canada; or G&an@, Grodania A/S, Milton, Ont., Canada), two plants per slab. Twenty-four rockwool slabs were arranged in three double rows with a total of 48 plants in each greenhouse, (24 test plants, plus 24 guard plants around the perimeter of the experiment). The final plant spacing in the greenhouse was at 0.6 m in the row, 0.6 m between the rows, and 1.8m between the double rows (walking paths); this spacing corresponded to an approximate plant density of 13 500 plants ha-‘. Within each greenhouse, treatments were set up as a split-plot with cultivars (Corona, Aramon) as the main plots and growth media (Grodan @, Pargro@) as the subplots; subplots were replicated twice and comprised 3 test plants each. The plants were trained according to the vertical cordon system. Fruit was harvested two or three times every week and graded according to commercial grading standards (Ontario Ministry of Agriculture, Food and Rural Affairs, Regulation 378/90). In 1992, plant height, leaf number and flower number of all test plants were recorded just before plants reached the overhead wires (end of January). Three representative leaves were excised from the top, middle and lower part of one plant in each plot, their fresh weight was determined and their leaf area was measured with a LI-COR 3100 leaf area meter (LI-COR Inc., Lincoln, NE 68504, USA); their dry weight was determined after drying at 65°C for a week; specific leaf weight was calculated as the dry weight per unit leaf area. On Feb. 6 1992, a sunny day, gas exchange was evaluated on the fifth youngest fully expanded leaf of one plant in each plot with a LI-COR 6200 portable photosynthesis system (LI-COR Inc., Lincoln, NE 68504, USA). In each house, four plants in south rows and four plants in north rows were measured. All measurements were made between 10:00 am to 2:00 pm by replicate to minimize the influence of
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117
irradiance variation in the day. The average PAR during the gas exchange measurements in glass, D-poly and acrylic houses was 610pmolm-* s-‘, 424pmolm-2 s-’ and 472 p_molmm2 s- ’ , respectively; the CO, concentration at the time of measurement was kept at 500 p.ll-‘. All data were analyzed using the General Linear Model Procedure in the SAS 6.09 package (SAS Institute Inc. Cary, N.C., USA). Since the interactions of cultivars and growth media with the cover materials were non-significant (P > 0.201, the analysis was conducted on the means of all 24 plants in each greenhouse. The air temperature average in each house was used as a covariate in the model to minimize the experimental error. The adjusted means were similar to the original means because of the counteracting effects of temperature variation (Snedecor and Cochran, 1991); therefore, unadjusted means are presented.
3. Results The average air temperature in D-poly and acrylic houses was higher than in glass houses although day and night temperature set points were the same (Table 2). The difference occurred mainly during the early part of the season (late Dec. to March); after March, there was little difference (Table 3). The average air temperature in D-poly and acrylic houses was similar. The average relative humidity was the highest in D-poly houses, and the lowest in glass houses (Table 2); the major difference in relative humidity occurred in January and March (Table 3). Glass houses had the highest PAR transmission, and D-poly the lowest. In comparison with glass houses, the acrylic and D-poly houses saved 30% of heating energy (Table 2). The photosynthetic rates of plants in acrylic and glass houses were similar, but higher than those of D-poly houses. There was no difference in stomata1 resistance caused by the greenhouse covers (Table 4).
Table 2 Light transmission, heating demand and microclimate Dec. 23, 1991 toMay I, 1992 Cover Material
PAR a Transmission
62.6 55.7 57.9
with different cover materials from
Heating b Demand
Average Air Temperature WI
(%)
o:OO8:00
8:0016:OO
16:0024:00
o:Oc8:OO
8:00I6:OO
16:Ot24:00
100.0
19.0 19.7 19.5
22.2 22.7 22.6
20.9 21.4 21.3
73.3 80.7 77.6
78.2 84.9 82.8
76.0 82.9 80.6
(o/o) Glass D-poly Acrylic
in mini-greenhouses
67.6 68.9
Average Relative Humidity (%)
a PAR was measured with a LI-COR 191SB line quantum sensor at the centre of each greenhouse from 1:oO to 2:00 p.m. The measurements were made at 50cm intervals from 0 to 2OOcm at four locations. Means presented are averages of 16 measurements in each of three greenhouses with the same cover. b The seasonal total of heating hours for each glass house was IO35 h, the output of each heater was 57 KJ s- ’ . The seasonal heating hours in spring of 1990 in D-poly and acrylic houses were 73 and 72.5% of that in glass houses, respectively.
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Table 3 Daily average air temperature and humidity in greenhouses covered with different cover materials. Spring of 1992 Month
December January Febmary March April May
Temperature CC)
Relative Humidity (%)
Glass
D-poly
Acrylic
GhSS
D-poJy
Acrylic
19.6 19.5 19.8 20.6 21.1 22.8
20.9 20.3 20.5 21.2 21.5 22.8
20.2 20.2 20.4 21.0 21.4 22.8
70.2 57.5 75.4 77.3 85.8 85.2
76.4 64.6 77.4 92.8 94.0 87.7
76.3 63.5 73.3 86.8 92.2 87.8
The plants in D-poly and acrylic houses developed faster than plants in the glass houses (Table 4). Plant height, leaf number per plant and flower number per plant in D-poly and acrylic houses were significantly larger than those in glass houses. The leaves of plants grown in glass houses had higher dry matter content and much higher specific leaf weight (over 40%) than the plants in D-poly and acrylic houses. Plant growth and development in D-poly and acrylic houses were similar (Table 4). The early marketable yield of plants grown in D-poly was similar to or higher than that in glass houses (Table 5). In 1990, plants in D-poly houses produced a higher early marketable yield than the plants in glass houses; in 1988 and 1992, the early marketable yield followed a similar trend although the difference was not significant. Early marketable yield in acrylic houses was higher than in glass or D-poly houses in 1992, higher than only in glass houses in 1990, and similar to glass or D-poly houses in 1988.
Table 4 Plant growth and development under glass, double polyethylene and acrylic greenhouse cover a Parameters
Glass
Double Polyethylene
Acrylic
Height (cm plant- ’ ) Leaf number per plant Fruit number per plant Leaf fresh weight (gleaf- ’ ) Leafarea (cm* leaf- ‘1 Leaf dry weight (g leaf- ’ ) Specific leaf weight (gm-*) Leaf dry mass content (%I Photosynthetic rate (mgm-’ s- ‘) Stomata1 resistance (scm- ’ 1
158.4 b 17.42 b 5.29 b 14.65 a 663.9 a 2.94 a 43.09 a 19.41 a 0.540 a 3.202 a
193.8 a 19.75 a 7.62 a 13.0 a 671.6 a 1.99 b 29.82 b 15.32 b 0.376 b 3.30 a
184.7 a 19.45 a 7.43 a 15.30 a 721.5 a 2.07 b 28.76 b 14.26 b 0.602 a 3.207 a
a Plant height, leaf number per plant and fruit number per plant were measured on Jan. 16, 1992; means presented are averages of 24 observations in each of three greenhouses with the same cover. The leaf fresh weight, leaf area, leaf dry weight, specific leaf weight and leaf dry mass contents were determined from samples collected on Jan. 22, 1992; means presented are the averages of 8 observations in each of three greenhouses with the same cover. The photosynthetic rate and stomatal resistance means am the averages of measurements from 24 plants. Different letters in the same row indicate significant difference (P < 0.051, according to the LSD test.
A.P. Pupadopoulos. X. Hao/ Scientia Horticulturae 68 (1997) 113-123 Table 5 Early yield of cucumber Cover material
Spring of 1988 Glass D-poty Acrylic Spring of 1990 Glass D-poly Acrylic Spring of 1992 Glass D-poly Acrylic
plants grown under three different
cover materials a (Fruit harvested
Grade#
119
until April 30)
1 fruit
Marketable
yield
Fruit perplant
kg per plant
Grade # 1 fruit b(%)
Extra large b (%)
Large b
Medium b
Small b
(o/o)
(o/o)
(o/o)
11.43 a 12.82 a 12.75 a
5.76 a 6.58 a 6.55 a
89.58 a 90.44 a 90.27 a
0.35 a 1.00a 0.76 a
17.14 b 20.58 a 20.38 a
56.39 a 57.65 a 56.53 a
15.69 a 11.21 a 12.61 a
31.00 b 35.49 a 35.04 a
14.31 b 16.18 a 16.18 a
90.97 a 91.55 a 91.18 a
2.07 a 1.83 a 2.09 a
14.67 a 14.90 a 15.23 a
47.51 a 46.18 a 48.81 a
26.73 a 28.64 a 25.05 a
41.65 b 43.14 b 47.13 a
16.93 b 18.18 b 19.91 a
89.21 b 93.13 a 92.20 a
1.24 a 2.03 a 2.20 a
9.45 b 14.12 a 14.44 a
35.57 b 41.63 a 40.21 a
42.95 a 35.35 b 35.35 b
a Means presented are averages from 24 plants in each of three greenhouses with the same cover. Means in the same column within years followed by different letters, are significantly different (P < 0.05), according to the LSD test. b Number of Grade #l fruit. as a % of total number of fruit.
higher than only in glass houses in 1990, and similar to glass or D-poly houses in 1988. The percentage of grade #l fruit in D-poly and acrylic houses was similar or higher than that in glass houses. In 1992, plants grown in D-poly and acrylic houses produced a higher percentage of large fruit and a lower percentage of small fruit in comparison with plants grown in glass houses; the experiment in 1988 showed similar trends. There was Table 6 Final yield of cucumber Cover material
Spring of 1988 Glass D-poty Acrylic Spring of 1990 Glass D-poly Acrylic Spring of 1992 Glass D-poly Acrylic
plants grown under three different cover materials a
Marketable
yield
Fruit plan-
kg plant-
’
Grade #I fruit
’
Grade # 1 fruit b (%)
Extra large b (o/o)
Large b
Medium b
Small b
(%I
(o/o)
(o/o)
15.82 a 16.04 a 16.84 a
7.71 a 7.92 a 8.32 a
83.5 a 85.9 a 85.4 a
0.48 a 0.90 a 0.76 a
13.37 b 18.17 a 18.28 a
49.58 a 52.57 a 50.36 a
20.10 a 14.26 b 16.04 b
46.04 a 52.21 a 50.96 a
22.33 a 24.53 a 23.30 a
89.2 a 90.4 a 89.3 a
2.77 a 2.99 a 3.02 a
20.52 a 19.90 a 18.95 a
42.12 a 43.25 a 46.44 a
23.82 a 24.20 a 20.89 a
64.29 c 72.57 b 77.92 a
26.98 b 31.78 a 34.18 a
85.9 b 90.6 a 89.9 a
1.76 a 3.24 b 3.33 b
12.23 b 17.46 a 17.11 a
35.24 b 40.24 a 38.84 a
36.67 a 29.65 b 30.56 b
a Means presented are averages from 24 plants in each of three greenhouses with the same cover. Means in the same column, followed by different letters within years, arc significantly different (P < 0.05), according to the LSD test. b Number of Grade #l fruit, as a % of total number of fruit.
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no difference in the percentage of different fruit grades between glass and D-poly houses in 1990. Fruit grades in D-poly and acrylic houses were similar in all three years. The effects of the three different cover materials on final marketable yield and grades (Table 6) were similar to those for early marketable yield. The total marketable yield and percentage of grade #l fruit in D-poly and acrylic houses were similar or higher than those in glass houses. There was no significant difference in the fruit yield and grades in 1990 among the three different cover materials. In 1988, the plants in D-poly and acrylic houses produced a higher percentage of large fruit and a lower percentage of small fruit than plants in glass houses, but there was no significant difference in the total marketable yield. In 1992, the total marketable fruit number and weight, and fruit quality under D-poly and acrylic were higher than those under glass. The plants in D-poly and acrylic houses produced a higher percentage of grade #l fruit, a higher percentage of large fruit and a lower percentage of small fruit, in comparison with glass houses. The total fruit number per plant in acrylic houses was larger than in D-poly houses. However, there was no significant difference in the total marketable fruit weight and grades between acrylic and D-poly houses.
4. Discussion The results obtained in this study were different from those of Reierson and Sebesta (1981) and Winden et al. (1984) who reported that double-layered cover materials (acrylic or glass) had negative effects on cucumber productivity, relative to single-layered glass houses. In this three-year study, no negative effects of double acrylic or polyethylene cover materials on the productivity of cucumber were found. Instead, the marketable yield and fruit quality produced in D-poly and acrylic houses, in some years, were better than those produced in glass houses. The geographic location, and climatic conditions of The Netherlands (52”N) and Norway (60”N), where Winden et al. (1984) and Reierson and Sebesta (1981) did their research, respectively, were different from those of Southwestern Ontario (42”N), where our research was done. The ambient PAR during the winter months is likely to be higher in Southwestern Ontario than in The Netherlands or Norway. In Ohio (4O”N), two spring crops of tomatoes produced a significantly higher yield in a greenhouse with Exolite@ acrylic glazing than in a glass house, and the weight of individual fruit was also larger in an Exolite@ acrylic greenhouse (Bauerle, 1981); the PAR transmission to glass houses and exolite acrylic houses was 65 and 56%, respectively. It appears that crop productivity under different cover materials varied with local climate conditions and light reduction due to’ cover material did not directly mean a reduction of crop productivity. Therefore, the PAR in D-poly and acrylic houses might be sufficient to support high crop productivity under the climates of Southwestern Ontario and Ohio. In a two-year study, Stromme et al. (1986) found that the productivity of tomatoes in acrylic houses was similar to that of single-layered glass houses, although there was a 9% reduction in light transmission in acrylic houses. Hurd (1983), in his review, has pointed that the negative effects of double-cladding houses obtained in experiments appear to contradict the results with double-cladded houses in practice; tomato yields
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have in some instances been higher in double-skinned than in single-skinned houses. Stromme et al. (1986), suggested that the air temperature in double-skinned houses might be higher than in a single-skinned house at the same temperature set point. The higher air temperature may stimulate plant growth and development in double-skinned greenhouses. This has been proven to be true in this study. The air temperature in D-poly and acrylic houses was up to 1 “C higher than in the glass houses and the largest difference occurred in the coldest month. This increase in air temperature might have compensated for the loss in light since a high temperature in young plants promotes the interception of radiation (Challa et al., 1995). The higher air temperature was part of the positive effect of double skinned houses. In this study, 30% savings in heating energy was achieved in D-poly and acrylic houses compared with glass, although the air temperature in the D-poly and acrylic houses was higher than in the glass houses. Evidence is accumulating that a rise in air humidity stimulates growth and photosynthesis. High humidity levels resulted in an increased photosynthetic rate (Acock et al., 1976; Bunce, 19841, fresh and dry weight, stem length and leaf area, and the final total cucumber yield was positively related to daytime humidity (Bakker et al., 1987). Although, in this study, no increase in the photosynthetic rate by higher humidity was observed, high humidity did affect the physiological status of plants in some way since the leaves of plants grown in D-poly and acrylic houses had a significantly higher water content. The higher percentage of large fruit and low percentage of small fruit produced in D-poly and acrylic houses indicate that the productivity of cucumber plants might be affected favourably by the high humidity that is normally found in closed plastic-covered greenhouses. The high water content in cucumber plants of plastic houses might have stimulated fruit expansion, An important finding in this study is that plants grown under D-poly cover adapted to the microclimate in D-poly houses. The plants in D-poly houses substantially reduced the specific leaf weight (40% less) and saved photoassimilates which could be used for new leaf and fruit growth. Because leaf size in D-poly houses was the same as in glass houses, but the plants under D-poly had more leaves, the plants in D-poly houses had an overall larger photosynthetic and PAR interception area which might have compensated for the lower availability of PAR in the D-poly houses. The difference in the photosynthetic rate (per unit area) between glass and D-poly houses (Table 4) was mainly caused by the difference in light n-radiance because the irradiance, reaching the measured leaves, was 40% higher under glass than under D-poly at the time of measurement. The photosynthetic efficiency per unit leaf area of plants in D-poly houses might be similar to that of plants in glass houses. Although photosynthesis is the ultimate determinant of biomass production, crop yield is related more to the total leaf area and photoassimilate distribution (Lawlor, 1995). The plants in D-poly houses achieved optimal usage of their resources through reduced specific leaf weight, and a greater leaf and fruit growth. Plants in acrylic houses showed similar acclimation and thus comparable yields to D-poly. The greenhouse PAR transmission and heating energy consumption are not only affected by the cover material, but also by many other factors, such as greenhouse structure, size, orientation and thermal screens (Edwards and Lake, 1964; Weimarm, 1984; Critten, 1990). Therefore, the microclimate and crop responses in mini-greenhouses
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might be slightly different than in large multispan greenhouses and, therefore, some caution should be exercised in the interpretation of our findings.
5. Conclusions The selection of the greenhouse cover material depends on many factors, such as initial investment, maintenance cost, its effects on greenhouse crop productivity, local climate and technological support and developments, Because of high energy costs, the heating demand has become a major concern in the last two decades. Some conversion of glass greenhouses to acrylic houses has taken place in Norway (Stromme et al., 1986). In North America, most of the newly built greenhouses are D-poly houses and acrylic panels have become a popular alternative for greenhouse side walls (Blom and Ingratta, 1985; Giacomelli and Roberts, 1993). The results of this study support this trend. For cucumber production in climatic conditions similar to Southwestern Ontario, the D-poly greenhouse is strongly recommended because there is no loss of productivity in comparison with a single-layered glass greenhouse while great savings on the initial investment and energy use are achieved. The development of anti-fog, UV-stabilized, IR barrier and long-lasting polyethylene films, which have improved PAR transmission and energy saving potential and low maintenance costs (Giacomelli and Roberts, 19931, promise a good future for D-poly greenhouses. Several studies have demonstrated the positive effects of these new improved polyethylene films on vegetable crop growth and production (Gilby, 1989; Kusnetsov and Karasev, 1989; Lagier, 1991). The selection of acrylic panels as a cover material needs to be cautious although no negative effects on cucumber productivity were detected in this study. The energy savings in acrylic greenhouses need to overcome the high initial investment. The recommendation of D-poly greenhouses in this study is for cucumber production under climatic conditions similar to Southwestern Ontario only. Studies under different climatic conditions are needed to determine the best greenhouse cover material for different areas.
Acknowledgements We thank J.L. Blackbum for invaluable technical support. Thanks are also offered to SK. Khosla, Dr. M.E.D. Graham and Dr. A. Mirza for their valued contributions to the study. The financial support of the Ontario Greenhouse Vegetable Producers Marketing Board is acknowledged.
References Acock, B., Charles-Edwards, D.A. and Hand, D.W. 1976. An analysis of some effects of humidity on photosynthesis by a tomato canopy under winter light conditions and a range of carbon dioxide concentrations. J. Exp. Bot., 27: 933-941. Bakker, J.C., Welles, G.W.H. and Ufflen, J.A.M. Van, 1987. The effects of day and night humidity on yield and quality of greenhouse cucumbers. J. Hort. Sci., 62: 363-370.
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Bauerle, W.L. 1981. Exolite acrylic for glazing greenhouses. Proc. of the National Agric. Plastics Congress: 225-234. Bjerre, H. 1981. Plant production in a greenhouse covered with double-Acryl. Acta Hortic., 115: 105-l 12. Blom, T.J. and Ingratta, F.J., 1985. The use of polyethylene film as greenhouse glazing in North America. Acta Hortic., 170: 69-80. Bredenbeck, H. 1985. Influence of different glazing materials on the light transmissivity of greenhouses. Acta Hortic., 170: 11 l-l 17. Boulard, T., Baille, H., Lagier, J., Mermier, M. and Vanderschmitt, E., 1989. Water vapour transfer in a plastic house equipped with a dehumidification heat pump. J. Agric. Eng. Res., 44: 191-204. Bunce, J.A. 1984. Effects of humidity on photosynthesis. J. Exp. Bot., 35: 1245-1251, Challa, H. and Schapendonk, A.H.C.M., 1984. Quantification of effects of light reduction in greenhouses on yield. Acta Hortic., 148: 501-5 IO. Challa, H., Heuvelink, E. and Meetemn, U. Van, 1995. Crop growth and development. In: J.C. Bakker, G.P.A. Bot, H. Challa, N. J. Van de Braak (Editors), Greenhouse Climate Control - an integrated approach. Wageningen Pers, Wageningen. pp. 62-84. Critten, D.L., 1990. A simplified model of light loss due to structural members in multispan greenhouse roofs. J. Agric. Eng. Res., 47: 197-205. Edwards, R.I. and Lake, J.V., 1964. Transmission of solar radiation in a large-span east-west glasshouse. J. Agric. Eng. Res., 9: 245-249. Giacomelli, G.A. and Roberts, W.J., 1993. Review: greenhouse covering systems. HortTec., 3(l): 50-58. Gilby, G.W., 1989. Greenhouse cladding film development. Plastic, 81: 19-28. Hand, D.W., 1988. Effects of atmospheric humidity on greenhouse crops. Acta Hortic., 229: 143-155. Hurd, R.G. 1983. Energy saving techniques in greenhouses and their effects on the tomato crop. Scientific Horticulture, Canterbury, 34: 84- 101. Jewett, T.J. and Papadapoulos, A.P., 1984. Comparative energy use and environmental aspects of a twin-wall PVC panel and single glass greenhouse. Acta Hortic., 148: 633-640. Kusneetsov, S.I. and Karasev, V.E., 1989. ‘Polisvetan’, a high performance material for cladding greenhouse. Plastic, 83: 13-40. Lagier, J. 1991. Choice of flexible cladding materials in relation to greenhouse and plant growth. Plastic, 90: 33-44. Lawlor, D.W., 1995. Photosynthesis, productivity and environment. J. Exp. Bot., 46: 1449-1461. Noble, R. and Holder, R., 1989. Pot plant production under various greenhouse cladding materials. J. Hort. Sci., 64485-493. Papadopoulos, A.P., 1994a. Maximize energy savings with poly, low temperatures. Amer. Veg. Grower, 42(l): 60-61. Papadopoulos, A.P., 1994b. Growing greenhouse seedless cucumbers in soil and in soilless media. Agric. and Agri-Food Canada Publ. 1902/E., pp. 126. Papadopoulos, A.P. and Jewett, T.J., 1984. Comparative tomato growth, development and yield in twin-wall PVC panel and single glass greenhouse. Acta Hortic., 148: 611-617. Papadopoulos, A.P. and Liburdi, N., 1989. The Harrow Fertigation Managero’, a computerized multifertilizer injector. Acta Hortic., 260:255-266. Reierson, D. and Sebesta, Z., 198 1. A comparison of the effects of single glass and double acrylic sheeting on plant growth and development. Acta Hortic., 115401-408. Snedecor, G.W. and Cochran, W.G., 1991. Statistical methods. Iowa State University, Ames, Iowa, 50010. pp: 393-397. Stromme, E., Z., Sebesta and Reiersen, D., 1986. Experience with double acrylic cladding for tomatoes iu Norway. Tag.-Ber., Akad. Landwirtsch.-Wiss. DDR, Berlin, 238: 167- 172. Ting, K.C. and Giacomelli, G.A., 1987. Solar photosynthetically active radiation transmission through greenhouse glazing. Energy in Agr., 6: 12 1- 132. Winden, C.M.M. van, Uffelen, J.A.M. van and Welles, G.W.H., 1984. Comparison of the effect of single and double-glass greenhouses on environmental factors and production of vegetables. Acta Hortic., 148: 567-573. Weimann, G., 1984. Development of energy saving greenhouse conceptions. Acta Hortic., 148: 683-690.
ELSEVIER
Effects of greenhouse covers on seedless cucumber growth, productivity, and energy use Athanasios P. Papadopoulos *, Xiuming Hao Agriculture
and Agri-Food
Cunada, Greenhouse and Processing Crops Research Centre, Harrow,
Ont..
Canada NOR I GO
Accepted 16 August 1996
Abstract Effects of single-glass (glass), double inflated polyethylene film (D-poly), and rigid twin acrylic (acrylic) panels, as greenhouse covers, on seedless cucumber growth, productivity and energy use were investigated over three spring seasons (1988, 1990 and 1992). The early and final marketable yields of plants grown in D-poly houses were similar to or higher than those grown in glass houses. Also, plants grown in D-poly houses produced a similar or higher percentage of large and a lower percentage of small fruits than the plants grown in glass houses. The early and final marketable yield and percentages of fruit grades were similar in D-poly and acrylic houses; an exception was the early marketable yield in 1992 which was higher in the acrylic houses. Light transmission was the highest in glass houses, intermediate in acrylic houses and the lowest in D-poly houses. Relative humidity was highest in D-poly, intermediate in acrylic, and the lowest in glass houses. The plants in D-poly houses grew and developed faster (more leaves and flowers) than those in glass houses. The leaf size of plants in D-poly houses and glass houses was similar, but the dry matter content and specific leaf weight of plants grown in D-poly houses were significantly lower (40% less) than those in glass houses. Plants in D-poly houses might have acclimated to the low light conditions by reducing specific leaf weight and increasing their light interception efficiency. Plant growth and development in acrylic and D-poly houses was similar. For cucumber production in climatic conditions similar to South Western Ontario, the D-poly greenhouse is strongly recommended, because there is no loss of productivity in comparison with a glass house, while great savings on initial investment and energy use are achieved. 0 Elsevier Science B.V. Keywords: Cover material; Cucumber; Glass; Polyethylene; Acrylic; Yield; Energy
* Corresponding author. Tel: 5 19-738-2251; Fax: 5 19-738-2929. 0304-4238/97/$17.00 0 1997 Elsevier Science B.V. All rights reserved. PII SO304-4238(96)0096 l-2
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1. Introduction The high fuel costs during the 1970s and early 1980s led to a heightened interest in greenhouse cover materials which were more energy efficient than glass (Noble and Holder, 1989). A wide range of plastic cover materials including rigidly structured plastic panels, such as fibre-glass-reinforced polyester (FRP), acrylic (polymethylmethacrylate, PMMA), polycarbonate (PC), and polyvinyl chloride (PVC) panels, as well as, thin film coverings, such as, low-density polyethylene (LDPE), polyvinyl chloride (PVC), and ethylene vinyl acetate copolymer, have been tested as greenhouse covers. Double inflated polyethylene films (D-poly) are becoming the most popular greenhouse cover materials in North America because of the low initial investment required and the significant energy savings realized (Blom and Ingratta, 1985; Giacomelli and Roberts, 1993; Papadopoulos, 1994a). Twin wall acrylic panels (acrylics) are popular in garden centres, sale areas, and the side and end walls of large greenhouses with D-poly roofs (Giacomelli and Roberts, 1993). Some conversion of glasshouses to acrylic has taken place in Norway (Stromme et al., 1986). The economics of using energy-efficient cover materials depends not only on their cost and energy-saving potential but also on their effects on the greenhouse environment, which affects the yield and quality of greenhouse crops. A loss of 5-20% of visible light, or photosynthetically active radiation (PAR), in D-poly houses has been shown by many researchers (Blom and Ingratta, 1985; Bauerle, 1981). The light transmission of double acrylic may be lower (Stromme et al., 1986; Bjerre, 19811, similar (Ting and Giacomelli, 1987) or higher (Bredenbeck, 1985) than that of single glass depending on the composition, aging, or weathering of the acrylic panels. Greenhouse air temperature, humidity, and leaf temperature are in turn affected by the light transmissivity of the cover material. The improvements in insulation of doublelayered polyethylene and acrylic greenhouses have resulted in increased humidity in the greenhouse (Blom and Ingratta, 1985; Boulard et al., 1989). Changes in greenhouse microclimates may have significant effects on the growth, development and productivity of crops. Photosynthetic rates are reduced at low PAR and it is generally assumed that the loss of light will lead to a proportional loss in yield (Challa and Schapendonk, 1984). Photosynthetic rates may increase at high humidity (Acock et al., 1976; Bunce, 1984). Cucumber vegetative growth was enhanced by either high day or night humidity, and final total yield was closely related to daytime humidity (Bakker et al., 1987). The major concern regarding high humidity is that it may encourage plant diseases and cause physiological disorders such as calcium deficiency (Hand, 1988). The research results on the effects of double roofing on vegetable crops are far from conclusive. Experiments in The Netherlands indicated a lo-15 and 4-13% yield reduction for tomatoes grown under double-layered glasshouses, in comparison with single-layered glasshouses for spring and autumn, respectively; the corresponding yield decreases for cucumbers were 7-12 and 4-8% for spring and autumn, respectively (Winden et al., 1984). In Norway, the light level in acrylic and glass houses was 52 and 61% of the outside level, respectively, and the acrylic cover had a negative effect on the cucumber yield
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relative to glass (Reierson and Sebesta, 1981). However, the productivity of tomatoes was found to be similar under acrylic or glass cover (Stromme et al., 1986). In Ohio, spring tomato yields were significantly higher in a greenhouse with Exolite@ acrylic glazing than under single glass, although the percentage of PAR entering the greenhouse was 56 and 65% for the Exolite’ and glass cover, respectively (Bauerle, 1981). In this case the yield of the crop was not totally dependent on PAR and a reduction in PAR with some glazing materials may not indicate a reduction in productivity. In Ontario, tomato growth and development were better under glass than under twin-wall PVC during the light-deficient winter months but better under PVC during the bright months of the year (Papadopoulos and Jewett, 1984). Marketable yield in the early part of the spring season and total yield in autumn was higher under glass than under PVC but the total marketable yield in spring was higher under PVC than under glass although there was 18% less PAR in the PVC, compared with the glass house (Jewett and Papadopoulos, 1984). It appears that the effect of light reduction on the yield might be dependent on the varying crop requirement for light at different stages of plant development. Most of the experiments on the effects of cover materials on greenhouse crops have employed no replication for the cover materials because of the high cost of such facilities. This lack of replication has made it impossible to quantify the effects of the cover material on the greenhouse environment, energy use and crop productivity with any statistical certainty. The study reported in this article was conducted in nine mini-greenhouses covered with glass, D-poly or acrylic, in triplicate, to determine the effects of these three cover materials on cucumber growth, productivity and energy use.
2. Materials and methods The experiment was carried out in the spring of 1988, 1990 and 1992 in nine independent mini-greenhouses (South-North oriented) constructed in 1987 (Table 1). The nine greenhouses, all of the same size (6.2 X 7.2 X 3 m, LX W X H) and shape, were constructed to conform to a 3 X 3 Latin Square design. Twin-wall polycarbonate panels (8 mm thick) were used to glaze the north side wall for all greenhouses. The roofs
Table 1 Experimental treatments greenhouse covers
and timing of the three runs of the experiment
with seedless cucumbers
under three
YEU
1988
1990
1992
cu1tivars Growth Media a Planting Date First Harvest CO, Enrichment Final Harvest
Corona and Aramon Grodan@ and Pargro” Feb. 18, 1988 March 28.1988 1COOp11May 18, 1988
Corona Grodan’ and Pargro” Jan. 24.1990 March 1, 1990 IOOOpll-’ June 18, 1990
Corona and Aramon G&an@ and Pargro” Dec. 23, 1991 Jan. 23, 1992 lOOOpLL- ’ June 29, 1992
a G&an”;
Grodania
A/S,
Milton, Ontario, Canada. Pargro
, Pargro Inc., Orillia, Ontario, Canada. @’
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Horticulturae 68 (1997) 113-123
and other three side walls were covered with either double inflated polyethylene film (0.15 mm>, twin-wall acrylic panels (16 mm), or glass (3 mm), in accordance with the principles of the Latin Square. The roofs of the glass houses were covered with single layered glass but the side walls were covered with double layered glass. There were three greenhouses for each cover material, one in each row and column of the Latin Square. The rows and columns of the greenhouses were separated by 15m of open turfgrass land to avoid the greenhouses shading one another. The greenhouses were connected by 1.5 m wide enclosed walkways on their north sides. Heating was provided with forced-air, gas-fired heating units and ventilation with two stage, low-speed, high-volume fans and louvred air intakes. Fertigation, according to standard recommendations for rockwool grown cucumber crops (Papadopoulos, 1994b), was controlled by a Harrow Fertigation Manager@‘, a computerized multi-fertilizer injection system (Papadopoulos and Liburdi, 1989). Carbon dioxide enrichment at 1000 ~11~’ was implemented with pure CO, from a central supply of liquid carbon dioxide; the carbon dioxide enrichment was stopped whenever the greenhouses were ventilated. A PRIVA environmental computer controlled the climate in all greenhouses and recorded greenhouse climate data. The sensor box was placed just below the top of the cucumber canopy and was adjusted weekly following the cucumber growth. Cucumber seeds were sown in rockwool blocks (10 X 10 X 7.5 cm; Pargro@, Pargro Inc., Orillia, Ont., Canada), and after the appearance of the third true leaf, seedlings were transplanted into rockwool slabs (100 X 20 X 7.5 cm; Pargro@, Pargro Inc., Orillia, Ont., Canada; or G&an@, Grodania A/S, Milton, Ont., Canada), two plants per slab. Twenty-four rockwool slabs were arranged in three double rows with a total of 48 plants in each greenhouse, (24 test plants, plus 24 guard plants around the perimeter of the experiment). The final plant spacing in the greenhouse was at 0.6 m in the row, 0.6 m between the rows, and 1.8m between the double rows (walking paths); this spacing corresponded to an approximate plant density of 13 500 plants ha-‘. Within each greenhouse, treatments were set up as a split-plot with cultivars (Corona, Aramon) as the main plots and growth media (Grodan @, Pargro@) as the subplots; subplots were replicated twice and comprised 3 test plants each. The plants were trained according to the vertical cordon system. Fruit was harvested two or three times every week and graded according to commercial grading standards (Ontario Ministry of Agriculture, Food and Rural Affairs, Regulation 378/90). In 1992, plant height, leaf number and flower number of all test plants were recorded just before plants reached the overhead wires (end of January). Three representative leaves were excised from the top, middle and lower part of one plant in each plot, their fresh weight was determined and their leaf area was measured with a LI-COR 3100 leaf area meter (LI-COR Inc., Lincoln, NE 68504, USA); their dry weight was determined after drying at 65°C for a week; specific leaf weight was calculated as the dry weight per unit leaf area. On Feb. 6 1992, a sunny day, gas exchange was evaluated on the fifth youngest fully expanded leaf of one plant in each plot with a LI-COR 6200 portable photosynthesis system (LI-COR Inc., Lincoln, NE 68504, USA). In each house, four plants in south rows and four plants in north rows were measured. All measurements were made between 10:00 am to 2:00 pm by replicate to minimize the influence of
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117
irradiance variation in the day. The average PAR during the gas exchange measurements in glass, D-poly and acrylic houses was 610pmolm-* s-‘, 424pmolm-2 s-’ and 472 p_molmm2 s- ’ , respectively; the CO, concentration at the time of measurement was kept at 500 p.ll-‘. All data were analyzed using the General Linear Model Procedure in the SAS 6.09 package (SAS Institute Inc. Cary, N.C., USA). Since the interactions of cultivars and growth media with the cover materials were non-significant (P > 0.201, the analysis was conducted on the means of all 24 plants in each greenhouse. The air temperature average in each house was used as a covariate in the model to minimize the experimental error. The adjusted means were similar to the original means because of the counteracting effects of temperature variation (Snedecor and Cochran, 1991); therefore, unadjusted means are presented.
3. Results The average air temperature in D-poly and acrylic houses was higher than in glass houses although day and night temperature set points were the same (Table 2). The difference occurred mainly during the early part of the season (late Dec. to March); after March, there was little difference (Table 3). The average air temperature in D-poly and acrylic houses was similar. The average relative humidity was the highest in D-poly houses, and the lowest in glass houses (Table 2); the major difference in relative humidity occurred in January and March (Table 3). Glass houses had the highest PAR transmission, and D-poly the lowest. In comparison with glass houses, the acrylic and D-poly houses saved 30% of heating energy (Table 2). The photosynthetic rates of plants in acrylic and glass houses were similar, but higher than those of D-poly houses. There was no difference in stomata1 resistance caused by the greenhouse covers (Table 4).
Table 2 Light transmission, heating demand and microclimate Dec. 23, 1991 toMay I, 1992 Cover Material
PAR a Transmission
62.6 55.7 57.9
with different cover materials from
Heating b Demand
Average Air Temperature WI
(%)
o:OO8:00
8:0016:OO
16:0024:00
o:Oc8:OO
8:00I6:OO
16:Ot24:00
100.0
19.0 19.7 19.5
22.2 22.7 22.6
20.9 21.4 21.3
73.3 80.7 77.6
78.2 84.9 82.8
76.0 82.9 80.6
(o/o) Glass D-poly Acrylic
in mini-greenhouses
67.6 68.9
Average Relative Humidity (%)
a PAR was measured with a LI-COR 191SB line quantum sensor at the centre of each greenhouse from 1:oO to 2:00 p.m. The measurements were made at 50cm intervals from 0 to 2OOcm at four locations. Means presented are averages of 16 measurements in each of three greenhouses with the same cover. b The seasonal total of heating hours for each glass house was IO35 h, the output of each heater was 57 KJ s- ’ . The seasonal heating hours in spring of 1990 in D-poly and acrylic houses were 73 and 72.5% of that in glass houses, respectively.
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Table 3 Daily average air temperature and humidity in greenhouses covered with different cover materials. Spring of 1992 Month
December January Febmary March April May
Temperature CC)
Relative Humidity (%)
Glass
D-poly
Acrylic
GhSS
D-poJy
Acrylic
19.6 19.5 19.8 20.6 21.1 22.8
20.9 20.3 20.5 21.2 21.5 22.8
20.2 20.2 20.4 21.0 21.4 22.8
70.2 57.5 75.4 77.3 85.8 85.2
76.4 64.6 77.4 92.8 94.0 87.7
76.3 63.5 73.3 86.8 92.2 87.8
The plants in D-poly and acrylic houses developed faster than plants in the glass houses (Table 4). Plant height, leaf number per plant and flower number per plant in D-poly and acrylic houses were significantly larger than those in glass houses. The leaves of plants grown in glass houses had higher dry matter content and much higher specific leaf weight (over 40%) than the plants in D-poly and acrylic houses. Plant growth and development in D-poly and acrylic houses were similar (Table 4). The early marketable yield of plants grown in D-poly was similar to or higher than that in glass houses (Table 5). In 1990, plants in D-poly houses produced a higher early marketable yield than the plants in glass houses; in 1988 and 1992, the early marketable yield followed a similar trend although the difference was not significant. Early marketable yield in acrylic houses was higher than in glass or D-poly houses in 1992, higher than only in glass houses in 1990, and similar to glass or D-poly houses in 1988.
Table 4 Plant growth and development under glass, double polyethylene and acrylic greenhouse cover a Parameters
Glass
Double Polyethylene
Acrylic
Height (cm plant- ’ ) Leaf number per plant Fruit number per plant Leaf fresh weight (gleaf- ’ ) Leafarea (cm* leaf- ‘1 Leaf dry weight (g leaf- ’ ) Specific leaf weight (gm-*) Leaf dry mass content (%I Photosynthetic rate (mgm-’ s- ‘) Stomata1 resistance (scm- ’ 1
158.4 b 17.42 b 5.29 b 14.65 a 663.9 a 2.94 a 43.09 a 19.41 a 0.540 a 3.202 a
193.8 a 19.75 a 7.62 a 13.0 a 671.6 a 1.99 b 29.82 b 15.32 b 0.376 b 3.30 a
184.7 a 19.45 a 7.43 a 15.30 a 721.5 a 2.07 b 28.76 b 14.26 b 0.602 a 3.207 a
a Plant height, leaf number per plant and fruit number per plant were measured on Jan. 16, 1992; means presented are averages of 24 observations in each of three greenhouses with the same cover. The leaf fresh weight, leaf area, leaf dry weight, specific leaf weight and leaf dry mass contents were determined from samples collected on Jan. 22, 1992; means presented are the averages of 8 observations in each of three greenhouses with the same cover. The photosynthetic rate and stomatal resistance means am the averages of measurements from 24 plants. Different letters in the same row indicate significant difference (P < 0.051, according to the LSD test.
A.P. Pupadopoulos. X. Hao/ Scientia Horticulturae 68 (1997) 113-123 Table 5 Early yield of cucumber Cover material
Spring of 1988 Glass D-poty Acrylic Spring of 1990 Glass D-poly Acrylic Spring of 1992 Glass D-poly Acrylic
plants grown under three different
cover materials a (Fruit harvested
Grade#
119
until April 30)
1 fruit
Marketable
yield
Fruit perplant
kg per plant
Grade # 1 fruit b(%)
Extra large b (%)
Large b
Medium b
Small b
(o/o)
(o/o)
(o/o)
11.43 a 12.82 a 12.75 a
5.76 a 6.58 a 6.55 a
89.58 a 90.44 a 90.27 a
0.35 a 1.00a 0.76 a
17.14 b 20.58 a 20.38 a
56.39 a 57.65 a 56.53 a
15.69 a 11.21 a 12.61 a
31.00 b 35.49 a 35.04 a
14.31 b 16.18 a 16.18 a
90.97 a 91.55 a 91.18 a
2.07 a 1.83 a 2.09 a
14.67 a 14.90 a 15.23 a
47.51 a 46.18 a 48.81 a
26.73 a 28.64 a 25.05 a
41.65 b 43.14 b 47.13 a
16.93 b 18.18 b 19.91 a
89.21 b 93.13 a 92.20 a
1.24 a 2.03 a 2.20 a
9.45 b 14.12 a 14.44 a
35.57 b 41.63 a 40.21 a
42.95 a 35.35 b 35.35 b
a Means presented are averages from 24 plants in each of three greenhouses with the same cover. Means in the same column within years followed by different letters, are significantly different (P < 0.05), according to the LSD test. b Number of Grade #l fruit. as a % of total number of fruit.
higher than only in glass houses in 1990, and similar to glass or D-poly houses in 1988. The percentage of grade #l fruit in D-poly and acrylic houses was similar or higher than that in glass houses. In 1992, plants grown in D-poly and acrylic houses produced a higher percentage of large fruit and a lower percentage of small fruit in comparison with plants grown in glass houses; the experiment in 1988 showed similar trends. There was Table 6 Final yield of cucumber Cover material
Spring of 1988 Glass D-poty Acrylic Spring of 1990 Glass D-poly Acrylic Spring of 1992 Glass D-poly Acrylic
plants grown under three different cover materials a
Marketable
yield
Fruit plan-
kg plant-
’
Grade #I fruit
’
Grade # 1 fruit b (%)
Extra large b (o/o)
Large b
Medium b
Small b
(%I
(o/o)
(o/o)
15.82 a 16.04 a 16.84 a
7.71 a 7.92 a 8.32 a
83.5 a 85.9 a 85.4 a
0.48 a 0.90 a 0.76 a
13.37 b 18.17 a 18.28 a
49.58 a 52.57 a 50.36 a
20.10 a 14.26 b 16.04 b
46.04 a 52.21 a 50.96 a
22.33 a 24.53 a 23.30 a
89.2 a 90.4 a 89.3 a
2.77 a 2.99 a 3.02 a
20.52 a 19.90 a 18.95 a
42.12 a 43.25 a 46.44 a
23.82 a 24.20 a 20.89 a
64.29 c 72.57 b 77.92 a
26.98 b 31.78 a 34.18 a
85.9 b 90.6 a 89.9 a
1.76 a 3.24 b 3.33 b
12.23 b 17.46 a 17.11 a
35.24 b 40.24 a 38.84 a
36.67 a 29.65 b 30.56 b
a Means presented are averages from 24 plants in each of three greenhouses with the same cover. Means in the same column, followed by different letters within years, arc significantly different (P < 0.05), according to the LSD test. b Number of Grade #l fruit, as a % of total number of fruit.
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no difference in the percentage of different fruit grades between glass and D-poly houses in 1990. Fruit grades in D-poly and acrylic houses were similar in all three years. The effects of the three different cover materials on final marketable yield and grades (Table 6) were similar to those for early marketable yield. The total marketable yield and percentage of grade #l fruit in D-poly and acrylic houses were similar or higher than those in glass houses. There was no significant difference in the fruit yield and grades in 1990 among the three different cover materials. In 1988, the plants in D-poly and acrylic houses produced a higher percentage of large fruit and a lower percentage of small fruit than plants in glass houses, but there was no significant difference in the total marketable yield. In 1992, the total marketable fruit number and weight, and fruit quality under D-poly and acrylic were higher than those under glass. The plants in D-poly and acrylic houses produced a higher percentage of grade #l fruit, a higher percentage of large fruit and a lower percentage of small fruit, in comparison with glass houses. The total fruit number per plant in acrylic houses was larger than in D-poly houses. However, there was no significant difference in the total marketable fruit weight and grades between acrylic and D-poly houses.
4. Discussion The results obtained in this study were different from those of Reierson and Sebesta (1981) and Winden et al. (1984) who reported that double-layered cover materials (acrylic or glass) had negative effects on cucumber productivity, relative to single-layered glass houses. In this three-year study, no negative effects of double acrylic or polyethylene cover materials on the productivity of cucumber were found. Instead, the marketable yield and fruit quality produced in D-poly and acrylic houses, in some years, were better than those produced in glass houses. The geographic location, and climatic conditions of The Netherlands (52”N) and Norway (60”N), where Winden et al. (1984) and Reierson and Sebesta (1981) did their research, respectively, were different from those of Southwestern Ontario (42”N), where our research was done. The ambient PAR during the winter months is likely to be higher in Southwestern Ontario than in The Netherlands or Norway. In Ohio (4O”N), two spring crops of tomatoes produced a significantly higher yield in a greenhouse with Exolite@ acrylic glazing than in a glass house, and the weight of individual fruit was also larger in an Exolite@ acrylic greenhouse (Bauerle, 1981); the PAR transmission to glass houses and exolite acrylic houses was 65 and 56%, respectively. It appears that crop productivity under different cover materials varied with local climate conditions and light reduction due to’ cover material did not directly mean a reduction of crop productivity. Therefore, the PAR in D-poly and acrylic houses might be sufficient to support high crop productivity under the climates of Southwestern Ontario and Ohio. In a two-year study, Stromme et al. (1986) found that the productivity of tomatoes in acrylic houses was similar to that of single-layered glass houses, although there was a 9% reduction in light transmission in acrylic houses. Hurd (1983), in his review, has pointed that the negative effects of double-cladding houses obtained in experiments appear to contradict the results with double-cladded houses in practice; tomato yields
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have in some instances been higher in double-skinned than in single-skinned houses. Stromme et al. (1986), suggested that the air temperature in double-skinned houses might be higher than in a single-skinned house at the same temperature set point. The higher air temperature may stimulate plant growth and development in double-skinned greenhouses. This has been proven to be true in this study. The air temperature in D-poly and acrylic houses was up to 1 “C higher than in the glass houses and the largest difference occurred in the coldest month. This increase in air temperature might have compensated for the loss in light since a high temperature in young plants promotes the interception of radiation (Challa et al., 1995). The higher air temperature was part of the positive effect of double skinned houses. In this study, 30% savings in heating energy was achieved in D-poly and acrylic houses compared with glass, although the air temperature in the D-poly and acrylic houses was higher than in the glass houses. Evidence is accumulating that a rise in air humidity stimulates growth and photosynthesis. High humidity levels resulted in an increased photosynthetic rate (Acock et al., 1976; Bunce, 19841, fresh and dry weight, stem length and leaf area, and the final total cucumber yield was positively related to daytime humidity (Bakker et al., 1987). Although, in this study, no increase in the photosynthetic rate by higher humidity was observed, high humidity did affect the physiological status of plants in some way since the leaves of plants grown in D-poly and acrylic houses had a significantly higher water content. The higher percentage of large fruit and low percentage of small fruit produced in D-poly and acrylic houses indicate that the productivity of cucumber plants might be affected favourably by the high humidity that is normally found in closed plastic-covered greenhouses. The high water content in cucumber plants of plastic houses might have stimulated fruit expansion, An important finding in this study is that plants grown under D-poly cover adapted to the microclimate in D-poly houses. The plants in D-poly houses substantially reduced the specific leaf weight (40% less) and saved photoassimilates which could be used for new leaf and fruit growth. Because leaf size in D-poly houses was the same as in glass houses, but the plants under D-poly had more leaves, the plants in D-poly houses had an overall larger photosynthetic and PAR interception area which might have compensated for the lower availability of PAR in the D-poly houses. The difference in the photosynthetic rate (per unit area) between glass and D-poly houses (Table 4) was mainly caused by the difference in light n-radiance because the irradiance, reaching the measured leaves, was 40% higher under glass than under D-poly at the time of measurement. The photosynthetic efficiency per unit leaf area of plants in D-poly houses might be similar to that of plants in glass houses. Although photosynthesis is the ultimate determinant of biomass production, crop yield is related more to the total leaf area and photoassimilate distribution (Lawlor, 1995). The plants in D-poly houses achieved optimal usage of their resources through reduced specific leaf weight, and a greater leaf and fruit growth. Plants in acrylic houses showed similar acclimation and thus comparable yields to D-poly. The greenhouse PAR transmission and heating energy consumption are not only affected by the cover material, but also by many other factors, such as greenhouse structure, size, orientation and thermal screens (Edwards and Lake, 1964; Weimarm, 1984; Critten, 1990). Therefore, the microclimate and crop responses in mini-greenhouses
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might be slightly different than in large multispan greenhouses and, therefore, some caution should be exercised in the interpretation of our findings.
5. Conclusions The selection of the greenhouse cover material depends on many factors, such as initial investment, maintenance cost, its effects on greenhouse crop productivity, local climate and technological support and developments, Because of high energy costs, the heating demand has become a major concern in the last two decades. Some conversion of glass greenhouses to acrylic houses has taken place in Norway (Stromme et al., 1986). In North America, most of the newly built greenhouses are D-poly houses and acrylic panels have become a popular alternative for greenhouse side walls (Blom and Ingratta, 1985; Giacomelli and Roberts, 1993). The results of this study support this trend. For cucumber production in climatic conditions similar to Southwestern Ontario, the D-poly greenhouse is strongly recommended because there is no loss of productivity in comparison with a single-layered glass greenhouse while great savings on the initial investment and energy use are achieved. The development of anti-fog, UV-stabilized, IR barrier and long-lasting polyethylene films, which have improved PAR transmission and energy saving potential and low maintenance costs (Giacomelli and Roberts, 19931, promise a good future for D-poly greenhouses. Several studies have demonstrated the positive effects of these new improved polyethylene films on vegetable crop growth and production (Gilby, 1989; Kusnetsov and Karasev, 1989; Lagier, 1991). The selection of acrylic panels as a cover material needs to be cautious although no negative effects on cucumber productivity were detected in this study. The energy savings in acrylic greenhouses need to overcome the high initial investment. The recommendation of D-poly greenhouses in this study is for cucumber production under climatic conditions similar to Southwestern Ontario only. Studies under different climatic conditions are needed to determine the best greenhouse cover material for different areas.
Acknowledgements We thank J.L. Blackbum for invaluable technical support. Thanks are also offered to SK. Khosla, Dr. M.E.D. Graham and Dr. A. Mirza for their valued contributions to the study. The financial support of the Ontario Greenhouse Vegetable Producers Marketing Board is acknowledged.
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