Location of study site

The study was located in the municipality of Sabinas Hidalgo, Nuevo León, whose geographical coordinates are: 26°30’ 3” north latitude, 100° 8’ 36” west longitude at an altitude of 290 m. The average anual temperature is 28 °C, the prevailing winds are Northeast and East with an annual average precipitación of 700 mm. The climate of the region is dry esteppe (^{INAFED, 2016}). For the development of the experiment, four oval-shaped tunnels of 4 m wide, 2.5 m high and 12 m long with a 6 m tunnel spacing were established. The tunnels were oriented North-South with the access door on the south side. The cover of the three tunnels was polycarbonate film each of different color (blue, red and transparent), the other was covered with high density polyethylene (180 μm thick). Polycarbonate is a thermoplastic polymer in alveolar plates consisting of two or three parallel walls transversely joined by walls of the same material. Plates were 12.2 m length and 1.83 m wide and 6 mm thick with an air space in the alveoli for thermal insulation to reduce night cooling.

Establishment and management of the crop

The established crop was zucchini (Cucurbita pepo L.), variety Zuchinni. Two beds in each tunnel were established of 0.6 m wide, separated at 1.2 m. Direct seeding was performed on November 12, 2015, placing one seed at a depth of 25 mm and 0.5 m between plants in the two beds of each of the four tunnels. The seeds germinated six days after sowing. The soil of each tunnel was fertilizer with vermicompost (3 kg m^-2) in one application before planting. Irrigation was applied with a system of drip irrigation using a 16 mm diameter hose with integrated droppers at a separation of 0.4 m. With an expence of 2 LPH with daily watering times of 25 min, corresponding to a daily sheet of 3.47 mm. Weeding was done manually.

Instrumentation and performed measurements

At the center of each tunnel, on the surface of a fixed pole at a 0.7 m height a silicone pyranometer was intalled (LI 200 x model, LI-COR, Inc., Lincon, Nebraska, USA) and a quantum sensor (apoge instruments, Logan, Utah, USA) for measuring solar radiation (RSW) and photosynthetically active (PAR) respectively. Those sensors were connected to two CR1000 datalogger (Campbell Scientific, Logan, Utah, USA, UU). Measurements were performed at a frequency of 1 s and continuous averages of 30-min were generated throughout the crop growing cycle (November 27, 2015 to February 19, 2016).

In addition to continuously digitized measurements at intervals of 15 days and on each of the four tunnels, between 12:00 and 14:00 h the relative chlorophyll content of leaves was determined (SPAD 502 Plus, Spectrum, Technologies, Inc.). The transmittance of polycarbonate covers and polyethylene to solar radiation in the PAR band was determined on a day with completely clear sky at the time of higher incidence of solar radiation (13:00 to 14:00), placing a spectrum radiometer (PS-100, Apogee Inst., Logan, Utah, USA), above and below each cover.

Growth and yield of the crop

Through the development cycle of the crop and at a frequency of 15 days the number of leaves per plant and its area (with a tape measure and a digital vernier) were measured. During the harvest the number of fruits per plant was quantified. For each fruit harvested from each cut, the weight (with an analytical balance), the diameter and length (with a digital vernier) were measured. In each section, only fruits of a length greater than 12 cm were harvested.

For determining the leaf area a factor proposed by ^{Rouphael and Colla factor et al. (2005)} was used for the leaves of zucchini plants.

Where: AF= leaf area (cm2); A= product of greater width (a) by the greatest length (l) of the sheet.

Statistical evaluation

For the analysis of variance and comparison of meanss of leaf growth (number and leaf area), chlorophyll content of leaves and fruit quality (weight, diameter and length), the non-parametric Kruskal-Wallis test (α≤ 0.05) was used (because the data were not normally distributed), considering four treatments (four tunnels) and four replications per evaluated parameter. For the number and leaf area, the experimental unit was a plant. For chlorophyll content it was the average of 12 measurements (3 readings per leave of 4 leaves from different plants), while for fruit quality, statistical evaluation was performed with the fruit harvested in all the cuts.

Spectral properties of the plastic covers

Figure 1 shows the transmittance of the polycarbonate sheets and high density polyethylene to solar radiation in the band of photosynthetically active radiation (PAR) (400-700 nm). Fort he whole band of the PAR, the transparent polycarbonate showed more transmittance and was approximately uniform over the entire range of wave length. The blue polycarbonate showed high transmittance from 400 to 500 nm, decreases from 500 to 600 nm and increases again from 600 to 700 nm. The red polycarbonate does not transmit radiation from 400 to 575 nm, but showed high transmission from 600 to 700 nm. High density polyethylene had a PAR radiation transmittance pattern similar to transparent polycarbonate, but of smaller magnitude. For the entire band of PAR radiation, the red polycarbonate had a lower transmittance (23.51%), followed by blue (57.46%), high density polyethylene (60.51%) and the light color polycarbonate showed higher transmittance (82.57%) (Table 1).

Table 1 Photosynthetically active radiation (PAR) that is transmitted through each cover of four macro tunnels, and the percentage ratio relative to PAR radiation inciding on covers (1 339.035 μmol m^-2 s^-1). 

Figure 1 Incident solar radiation in a range of 400 to 700 nm wavelength, and that transmitted in three tunnels with polycarbonate covers of clear color, blue and red and one tunnel with high density polyethylene.  

Lower PAR radiation impinging on plants also has direct effect on the photosynthesis rate and growth and yield of plants, so plants under red polycarbonate can show lower growth and yield. In this regard, studies carried out by ^{Sandri et al. (2003)} in greenhouses showed a yield decrease in tomato (Solanum lycopersicum) due to a low PAR incidence by shading effect. Similarly, ^{Bagdonavičienė et al. (2015)} reported a lower CO₂ assimilation in cucumber (Cucumis sativus) and tomato when plants received 200 μmol m^-2 s^-1 of PAR radiation, compared to the plants with a PAR radiation of 400 μmol m^-2 s^-1.

It has also been widely documented the morphogenic effect associated with variations in the light environment of the plants. ^{Smith (2000)} showed the role of phytochromes in controlling various aspects of plant development regarding to the relative amount of light within the spectrum of red (600-700 nm) and far red (700-800 nm); ^{Lin (2000)} also found that the relative amount of light within the spectrum of blue (400-500 nm) regarding to that found in the spectrum of red has various morphological effects.

The red polycarbonate showed no radiation transmittance from 400 to 575 nm (Figure 1). This can affect the plant in two main ways: through the reduction of the efficiency with which photosynthesis can be carried out, as there is no radiation in one of the absorption peaks determined by ^{McCree (1971)} and through modifying the balance between red and far red and between red and blue (^{Smith, 2000}; ^{Lin, 2000}), although the transmittance in the spectrum of blue, which is the highest plant uptake (^{Hogewoning et al. 2012}) is very high. Instead, blue polycarbonate showed higher transmittance in the range of 400 to 500 nm, but shows a transmittance drop in the range of 550 to 650 nm. Based on the same criteria used to analyzed the red polycarbonate, the blue in principle has better properties for use as roofing material. ^{Grbic et al. (2016)} conducted tests on tunnels with green and lavender plastic covers in Perilla frutensces and found a significant effect of the tunnels in the vegetative development of the crop, finding an inverse relationship between luminosity and vegetative growth.

Total incident and transmitted solar radiation on tunnels covers

During the development cycle of the crop, plants within each tunnel, received a different magnitude of total solar radiation (Rsw) and lower than that received on the tunnel cover (Figure 2). The lower Rsw transmittance was observed in red polycarbonate and the highest in the one of light color. The higher incidence and penetration of radiation at this location and time of year is observed between 12:00 and 14:00 h.

Figure 2 Incident total solar radiation (Rsw) and that transmitted through clear, blue, red color polycarbonatecovers and high density polyethylene, between 7:00 am and 7:00 pm on a clear day of the months of growth of the zucchini crop. 

Table 2 shows the total amount of solar radiation (integrated daily values from 7:00 am to 7:00 pm) received by plants during the months of crop growth (November 28, 2015 to February 18, 2016). Again, the plants under the red polycarbonate received less total amount of solar radiation during the entire growing period (1 084.02 MJ m^-2), the highest incidence of Rsw was in light polycarbonate (1 344.23 MJ m^-2), followed by high density polyethylene (1 160.84 MJ m^-2) and blue polycarbonate (1 149.86 MJ m^-2). In tunnels with white, green, yellow and red plastic polyethylene covers Ombódi et al. (2015) observed a decrease in the transmittance of PAR radiation between 23 and 39%, and 32-46% in PAR radiation in all colors and that the highest retention was shown by the yellow and green color in the ranges of 450 to 450 nm and 550 to 670 nm respectively. Similarly, ^{Retamal-Salgado et al. (2015)} reported a reduction in PAR radiation of 25% using a high density polyethylene greenhouse cover.

Table 2 Integrated daily total solar radiation (7:00 am a 7:00 pm) during the months of the growth cycle of the zucchini crop, under tunnels of different colors and materials. 

Incident photosynthetically active radiation and transmitted in the tunnels covers

The transmittance difference of the covers between tunnels of different colors and materials to photosynthetically active radiation (PAR) was more evident than that observed for incident solar radiation (Figure 3). This was because the different covers are more selective to PAR radiation than to near and far infrared radiation. It was clearly observed that the plants under the red polycarbonate received the least amount of PAR radiation, much lower than that received by the polycarbonates of other colors and the one that incides on the tunnels (Figure 3). The lower PAR radiation transmitted on the red polycarbonate will have a marked effect on the plants physiology and on the growth and yield of plants.

Figure 3 Photosynthetically active radiation (PAR), incident and transmitted through the light, blue, red colored polycarbonateband high density polyethylene covers between 7:00 am and 7:00 p.m. on a clear day of the development months of the zucchini crop. 

^{Oliviera et al. (2016)} observed that, for a melissa crop (Melissa officinalis L.), using a blue shadow mesh increased plant height, leaf area and chlorophyll content, contrary to the results obtained with red and black meses. After the red, the cover with lower PAR radiation transmittance was blue polyethylene, followed by high density polyethylene and the cover with the highest transmittance was clear polyethylene (Figure 3). Based on these data, it can be inferred that plants under the tunnel of clear polycarbonate would have greater growth and yield, however, in addition to the intensity of radiation, it should be considered radiation quality, since the degree of radiation diffusivity is higher in the high density polyethylene, which can result in higher yield (^{Jongschaap et al., 2006}). Similarly, for the Rsw radiation, from 12:00 to 14:00 h, the highest incidence of PAR radiation was observed.

The differences in total integrated PAR (7:00 to 19:00 h) during the months of crop development under the different colors covers was very pronounced (Table 3). Clearly it was observed that the plants under red polycarbonate received the least amount of radiation (516.1 mol m^-2), followed by the blue polycarbonate (1 026.92 mol m^-2), high density polyethylene (1274.49) and the one with the highest transmittance was clear polycarbonate (1 757.87 mol m^-2). In terms of percentage, the plants under red cover received only 29.36% of the PAR radiation received under the light color cover. The low amount of PAR radiation received by the plants under the red polycarbonate should have a strong impact on growth and yield of plants. In this regard, ^{Martínez et al. (2016)} evaluated in basil crop (Ocimum basilicum) red, blue, pearl and black color polyethylene and a control with 50% shading, their research showed a PAR radiation reduction in the colors treatments, and in the red polyethylene plants showed greater height but less dry weight, stem diameter and number of buds.

Table 3 Photosynthetically active radiation (PAR) integrated daily (7:00 to 19:00 h) during the months of crop growth cycle of zucchini, under tunnels of different colors and matterials. 

Chlorophyll content in leaves

Plants grown under the tunnel with light-colored polycarbonate, had the highest chlorophyll content (Kruskal-Wallis, α≤ 0.05) through the crop growth cycle (Table 4). This was directly related to higher incidence of both total solar radiation and photosynthetically active radiation on plants (Tables 2 and 3). From the beginning and during the growth cycle of the crop, plants under red polycarbonate showed less chlorophyll content (Table 4) (Kruskal-Wallis, α≤ 0.05), this was due to the zero radiation received by plants in the blue spectrum (400-570 nm) (Figure 1), and also lower amount of PAR radiation received by plants daily (Figure 3) and during the growth cycle (Table 3), which had a strong impact on chlorophyll content.

Table 4 Chlorophyll relative content (SPAD units) of a zucchini crop (cv Zucchini) under tunnels with different color and cover material. 

Medias con letra diferente dentro de las columnas son estadísticamente diferentes (Kruskal-Wallis α≤ 0.05).

For example, studies by ^{Trouwborst (2016)}, found that the cucumber plants had lower chlorophyll content when grown under conditions of red light (638 nm). Similar results were obtained by ^{Su et al. (2014)} in a study also conducted on cucumber seedlings where they observed a lower chlorophyll content under red light (658 nm) compared with seedlings exposed to blue light (460 nm). The lower content of chlorophyll in the plants under the red polyethylene, can be manifested as a lower growth and yield of fruits by the plants.

Leaf growth

The plants grown under the tunnel with light colored polycarbonate cover, received more radiation PAR (Table 3) and showed the highest development of leaf number and leaf area (Tables 5 and 6) (Kruskal-Wallis, α≤ 0.05), indicating a direct relationship between these variables. On the contrary, plants under red polycarbonate, showed the lowest PAR radiation incidence (Table 3), with zero radiation of 400 to 575 nm band, resulting in fewer leaves and leaf área (Kruskal-Wallis, α≤ 0.05). The leaf area of the plants in the tunnel with a high density polyethylene cover was the same as that of the plants in the blue polycarbonate tunnel at the different sampling dates through the crop growth cycle (Table 6) (Kruskal-Wallis, α≤ 0.05). Studies conducted by ^{Casierra et al. (2012)} showed that in a strawberry crop (Fragaria sp.) the use of red, green and blue polypropylene covers increased the leaf area compared to that observed in the uncovered plants. They also observed that of the plants that grew under the polypropylene covers, those of the red color had the lowest development.

Table 5 Number of leaves of plants of a zucchini crop (Cucurbita pepo L.), variety Zuchinni, growing in tunnels of different materials and colors. 

Medias con letra diferente dentro de las hileras son estadísticamente diferentes (Kruskal-Wallis, α≤ 0.05).

Fruit yield

Since the maturation for cutting the fruits and the yield was very uneven between tunnels of different colors, the data showed are the result of the sum of 10 cuts. Plants grown in the tunnel with clear polyethylene cover received the highest quantity of PAR radiation (Table 3) and had higher chlorophyll content during the growth cycle of the plants (Table 4), showed greater precocity and yield (total number of fruits per tunnel) (Figure 4). On the contrary, the plants under the red polycarbonate tunnel received less PAR radiation (Table 3), and showed the lowest chlorophyll content in most of the growth cycle (Table 4), fruit yield was null. The diameter, length and average fruit weight was equal (Kruskal-Wallis, α≤ 0.05) between tunnels of clear polycarbonate, high density polyethylene and blue polycarbonate (Figure 4). Similar results were reported by ^{Kitta et al. (2014)} in a chili pepper crop (Capsicum annuum L.) under different color meshes (pearl, green and white) with 78, 62 and 59% of PAR radiation transmittance respectively. The pearl colored mesh that showed the highest transmittance was the one of greater fruit yield, and dry matter. While the white mesh, of lower transmittance showed smaller yields.

Figure 4 Yield and fruit quality of a zucchini crop (Cucurbita pepo L.), variety Zuchinni growing under tunnels of polycarbonate (PC) of different colors and high density polyethylene (PEHDD).