COLORED SHADE NET IN HORTICULTURE: PHOTOSELECTIVE NETS
Benefits of colored shade net for agriculture
Colored shade net is a great secret ally for growing produce. Higher plants respond to the amount, quality, direction, and frequency of light. There are a number of photoreceptors in plants. Among them can be found chlorophyll, phytochrome, cryptochrome, phototropins, and photoreceptors that react to green light.1 Light, together with other environmental factors, allows plants to adapt to environmental conditions. For decades attempts have been made to manipulate plant morphology and physiology. This has been done using photoselective filters, especially in greenhouses.2 Recently colored shade net has been developed in order to manipulate the growth and development of plants. This shade cloth can be used outdoors or in a shade house, as well as in a greenhouse. It can provide physical protection (from birds, hail, insects, excessive radiation), affect environmental modification (humidity, shade, temperature),3 and increase the relative proportion of scattered light. In addition, it absorbs various spectrum bands, thereby modifying light quality.
These effects can influence crops as well as the organisms associated with them.
MICROCLIMATE EFFECTS under shade net
Solar radiation under a shade net
Shade Cloth, no matter the color, reduces the amount of radiation that reaches the crops under it. Obviously, the higher the shade factor, the greater the amount of radiation that will be blocked. The reduction in radiation as a result of the application of the shade net will affect the temperature and relative humidity.4 Besides affecting the amount of radiation, the net can also influence the direction of the radiation.
Scattered radiation with netting
It has been shown that scattered light increases the efficacy of radiation and improves yield (at both plant and ecosystem level). It even turns out to be an important influence in the flowering of plants (timing and quantity).5 Any shade cloth is capable of scattering radiation, especially ultraviolet rays, because shade cloth is normally made from plastic which is resistant to ultraviolet light.6 It has been shown that shade net, which increases the scattering of light but does not affect the spectrum of light, increases branching, compactness of the plant, and the number of flowers per plant.7 Colored shade net can also increase the scattering of light by 50% or more, and this by itself can influence plant growth and development.
Photoselectivity with Shade Net
Colored shade net has been intensively tested primarily due to its capacity for manipulating the spectra of radiation reaching the crops below the net. It can be used to change red to far-red light ratios that are detected by phytochromes, to change the amounts of radiation in order to activate the blue/ultraviolet photoreceptors, to change the blue light involved in phototropic responses mediated by the phototropins, and to change radiation at other wavelengths that can influence plant growth and development.
Air flow under shade net
Netting can also reduce wind velocity and its intensity,4 factors that can alter temperature, relative humidity, and concentrations of gas that result from reduced aeration.9 These changes can affect transpiration, photosynthesis, respiration, and other processes. The effects on air flow depend on the porosity and physical location of the net in relation to the plants and can vary according to the time of day, the season, and other factors.
Temperature of the shaded crop
Shade net is often spread out over crops in order to reduce heat stress.10 Even so, in structures enclosed with shade net, inside daytime temperatures are typically higher than on the outside.11 Inside, nighttime temperatures can be lower, at least during radiation freezes.21
Relative humidity for shaded crops
Relative humidities are often higher under shade net than outside as a result of the water vapor that gets transpired by the crop and limited interchange with drier air outside the net.12 This is the case even when temperatures under the net are higher than outside.4
Blueberries and shading
Black, gray, red, and white shade cloth with nominal shading percentages of 35 and 50 for each color were compared with a control treatment without shade to see the effects on Berkeley blueberries that grow in central Chile.13 The treatments with black netting reduced photosynthetically active radiation (PAR) most (by 47% and 54%) and were what affected vegetative growth most–increasing internode, leaf, and shoot lengths and leaf width in comparison with the control without netting. The other colored nettings (gray, red, white) reduced PAR by 29% to 41% and had no effect on internode and shoot length and less effect on the other vegetative parameters (increased leaf length—gray 50% shading, increased leaf width—all three at 50% shading and also gray at 35% shading).
Oddly the gray shade net at 35% shading, the red shade net at 50% shading, and the white shade net at 50% shading, increased yield of the Berkeley blueberry from 60% to 91% the first year and from 26% to 45% the second year. However, the 50% gray netting and the 35% white netting, did not increase yield even though they transmitted the same amount of PAR, in spite of their nominal shading percentages.13 Analyses of the spectrum under the nettings were not conducted. Increased yield was more the result of the increase in the number of fruits than increase in size of the fruit, according to the authors, which is a very relevant factor for easily justifying the installation costs of netting.
Cast iron plant in a shade house
The weight of fresh leaves harvested from variegated cast iron plant (Aspidis-tra elatior‘Variegata’) was higher when a black shade net was used than when blue, gray, or red was used and the total number of harvestable leaves was higher with black netting than with blue or red.14 The production of variegated leaves and the percentage of uniformly green leaves was the same under black, blue, gray, and red netting.14 The color of the netting had no significant effect on vase life for the leaves, a critical factor for any cut foliage.
Kiwifruit with shading
An experiment in southern Italy evaluated the effects of blue, gray, red, white, and no shade net over Hayward kiwifruit.15 The weight of fresh prunings during winter was higher with blue netting than with red or no netting. It is not clear if this was attributable, in part, to the fact that the blue netting decreased PAR the most–by 26.9% compared with 0% and 22.8% for no netting and red netting, respectively. Control of plant vigor could be beneficial because pruning represents a major production cost. The authors also calculated the effects of the nets for summer pruning and concluded that the costs of summer pruning could be greater using red and gray netting compared with the control with no netting. In a study conducted in southern Italy that used white, red, blue, and gray nets with shading factors from 20.4% to 26.9%, the number of flowers and buds per shoot was lower with netting than without.15 The yield of fruit (number of fruits and weight) was reduced with all treatments with colored shade net (blue, gray, red, white with PAR shading percentages of about 20% to 27%). Even so, the increase in the size of the fruit under netting (except for gray) compensated for lower yield to the extent that their values were the same as the values for the control with no netting.15
Orchids grown with shade
Research in Brazil about a number of cultivars and hybrids of Phalaenopsis showed a rather consistent pattern of increased foliage biomass production (fresh and dry weights) under blue netting, in spite of reduced PAR transmission compared with black and red netting.16 The red netting, compared with the black and blue (all three with nominal 30% shading), induced earlier flowering in nine out of 10 crops of Phalaenopsis and hybrids.
Peaches and the effects of shade
Considering the weight of pruned material in all cases–blue, gray, pearl, red, and yellow netting at 30%, and white at 12%–vegetative growth of Hermosa peach trees was recorded.17 After two years under shade net, flowering of the Hermosa peaches increased, under five of the six nets (white with 12% shading; blue, pearl, red, and yellow with 30% shading) compared with the control with no netting.17 Only the gray net at 30% shading did not affect flowering compared with the control. The fruits of the Hermosa variety were increased in two treatments with netting compared with the control with no netting.17 Red netting at 30% had the biggest effect on fruit quality compared to the control and three other net treatments also had improved fruit quality. Fruit size was greater under netting except for blue and red netting. However, the larger fruits had less total soluble solids and were not as firm.
Pittosporum under light scattering netting
It has been reported that red netting increased and blue decreased branch length in the case of variegated pittosporum.8 It also was the case that gray and red netting and reflective and light-scattering aluminum netting increased branching compared with black netting. The authors, however, did not do a statistical evaluation of the results and an examination of the bars for standard error in their graph suggests that there might not have been an increase in branching as a result of the aluminet and use of the red net. The production of commercial-value branches (used by florists) was reduced under blue netting. In the same study, a lower percentage of small leaves (less than 10 cm2) and a higher percentage of large leaves (15 to 30 cm2) was produced under the green and red netting compared with the conventional black. Leaf photosynthesis rates generally were in line with the parameters of plant growth. In a three year study that also included data for the yield of Pittosporum tobira ‘Variegata,’ production was highest under red netting, intermediate under gray, and lowest under black and blue.14 Oren-Shamir et al. show that the gray and blue netting increased leaf color diversity compared to black netting.8 Increased color diversity is a desirable characteristic for this crop of cut foliage plants. However, no control treatment was conducted by the authors. In another study in which netting of similar colors was used but with high shading percentages (≈70%), the treatments did not affect color diversity of Pittosporum tobira ‘Variegatum.’14 In the first year of the second study, vase life of the stems was the longest with red netting, intermediate with gray, and shortest in the case of using black netting and blue netting. In the following two years, use of shade net did not have any effect on vase life.
Vegetative quality of shaded crops
Vase life of Xanadu leaves was not affected by the color of the netting that was used.14 In a two year experiment that used black, blue, gray, and red netting with nominal shading of 70%, leaf mass of Xanadu Philodendron was not altered, but the number of leaves was highest under red netting and lowest under blue.14
Fruit quaility and yield with colored netting
Apples grown with shade
In the first year of a study, Shahak Et al. (2004) found that a red/white net that reduced PAR by 18%, increased fruit number of the Smoothie Golden Delicious (SGD) variety compared to the control with no netting.17 Although none of the other colored shade net treatments increased fruit number of the SGD and none, including the red/white treatment, affected the number of fruits for ‘Topred Red Delicious’(TRD), all the treatments with nets reduced midday stem water potentials (less negative) and prevented thermal damage from sun/heat.
The reduced stress was the result of the shading factors of the nets which varied from ≈10% to 36% for PAR and 19% to 42% for ultraviolet-A/ultraviolet-B and resulted in larger SGD fruit size. However, TRD fruit diameters were unaffected by netting. In contrast, all the shade nets except white improved TRD midseason red coloration. Subsequently distinct responses were recorded according to the season.18 Every year the nets were erected after bloom (early to mid-May) and taken down the beginning of November.
In the case of SGD, fruit size and yield increased with pearl, red, and white but not with blue, gray, or black nets. In the case of TRD, only white net increased fruit size, which the authors attributed to the lower productivity, a larger inherent fruit size, and greater plant vigor compared with SGD.
Grapes shaded with black or red produce more
Some studies about table grapes have been reported from Israel.18 Cluster and berry weights of the Superior cultivar were increased most under a yellow net with 30% shading at three locations, but the 30% gray net reduced yield compared with the control with no netting. Yield increased under the 30% black net, 30% red, 22% white. In other trials using the Red Globe cultivar, the authors report an increase in berry size under 30% yellow shade net compared to five other colored nets and an increase in berry weight under black, red, and white nets compared to no net. It was also shown that colored netting affects the rate of fruit maturation with light-scattering nets (pearl and white) increasing the rate of maturation of a number of cultivars and black and red nets delaying the maturation of ‘Red Globe.’ With netting maturity was advanced for Superior but delayed for Perlette.
Larger pears with pearl netting
Preliminary results indicate that colored shade nets can influence pear fruit size and russeting.18 30% pearl nets increased large fruit yield and 26% red nets reduced fruit russeting.
Bell peppers need 35% shading
Elad Et al. (2007) have reported an increase in yield for two Capsicum annuum cultivars when grown under black (nominal 25% and 40% shade), blue (40% shade), blue–silver (40% shade), silver (40%), and white (25%) shade nets as compared with the control with no net. However, there was no difference in total yield when comparing the black and white nets (both 25% nominal shade).12 The only increase in yield using a net with 40% shade occurred for one cultivar (Louisiana) for which the yield under silver netting was greater than under black or blue. Data from another experiment included in the same paper showed the actual shade factors can differ considerably from the nominal values and that shade factors change over time. Shahak (2008) reported that the production for three cultivars of bell peppers was increased by 16% to 32% under pearl and red netting compared with black netting.18 Unfortunately, the author did not report actual or nominal shade factors for that study.
There are many factors that should be considered with respect to the use of colored shade net in addition to the direct effects on the crops.
Use of colored shade net in Horticulture
Radiation quality can have effects on plants as well as on the microorganisms associated with them. For example, although pepper yields were increased under colored shade nets, powdery mildew (Leveillula taurica) leaf coverage and leaf shedding resulting from disease were more severe in the shade and had to be controlled by spraying.12
Poor control of insects and mites with yellow netting
Colored netting may affect mites and insect plagues. It has also been reported that although whiteflies preferred landing on yellow compared with black netting, fewer adult whiteflies were caught on yellow sticky traps inside the yellow-netted chambers than under black netting.19 However, the number of immature whiteflies counted on cotton leaves inside the two chambers was not different.
Effect on humans working in the shade
Colored shade net can affect workers that care for the crops20 and also their ability to judge the degree of maturity when harvesting.
Colored shade net factors such as relative costs, color fastness, and durability should also be considered.
Colored shade net is a relatively new tool. It can be used for a wide variety of purposes by horticulturists. The effects are varied, and plant responses can differ even among cultivars of the same plant. For this reason, much additional investigation is needed in order to demonstrate and elucidate the effects of colored shade net. Since colored shade net has numerous effects in addition to photoselective ones, it is very important that investigators give prudent and complete descriptions of experimental conditions. Radiation quality and quantity values and microclimate parameters should be measured and recorded in order to determine the factors that may lead to a result.
- Batschauer, 1999; Folta Y Maruhnich , 2007.
- Cerny Et al., 2003; Ilias Y Rajapakse , 2005; Kambalapally Y Rajapakse , 1998; Li et al., 2000; Mortensen y Stromme , 1987; Rajapakse Y Kelly, 1991, 1992, 1995; Rajapakse Et al., 1999; Wilson y Rajapakse , 2001un,2001b.
- Pérez et al., 2006.
- Sellos, 1994.
- Gu Et al., 2002; Guenter Et al., 2008; Healey et al., 1998; Ortiz et al., 2006; Sinclair et al., 1992.
- Wong, 1994.
- Nissim-Levi et al., 2008.
- Oren-Shamir et al., 2001.
- Rosenberg et al., 1983.
- 10.Elad Et al., 2007; Retamales Et al., 2008; Shahak Et al., 2004.
- 11.Pérez et al., 2006; Sellos, 1994.
- 12.Elad Et al., 2007.
- 13.Retamales Et al., 2008.
- 14.Sellos, 2008.
- 15.Basile et al., 2008.
- 16.Leite Et al., 2008.
- 17.Shahak Et al., 2004.
- 18.Shahak Et al., 2008.
- 19.Ben-Yakir et al. 2008.
- 20.Crane et al., 2008;Elliot and Niesta, 2008; Lin et al., 2008
- 21.Stamps, 1994.
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