Thrips and Tomato spotted wilt.

Thrip numbers were determined by collecting five flowers per plot 4 weeks after transplanting (fall season). The number of thrips per flower was determined with a dissecting microscope. Individual plants were monitored visually for TSW symptoms for the entire season (in both spring and fall). The presence of the virus that causes TSW in all symptomatic plants was confirmed by enzyme-linked immunosorbent assay (ELISA) on pepper leaves using a commercially available kit (Agdia Inc., Elkhart, IN). Asymptomatic plants were not subjected to ELISA.

Soil water status.

Soil water status was measured as either soil water content (fall) or soil water potential (spring). Soil water content (volumetric) over the season was measured once every 2 to 3 d (three readings per experimental plot) with a portable time domain reflectometry (TDR) sensor (CS-620; Campbell Scientific, Logan, UT). The two metallic 12-cm rods of the TDR sensor were inserted vertically within the row between two plants.

Soil water potential was measured manually (every 2 to 3 d) with granular matrix sensors (Water Mark Soil Moisture Sensor, Model 200SS; Irrometer Co., Riverside, CA) at 10-cm depth within the row in between two plants using one sensor per experimental plot.

Root zone temperature and photosynthetically active radiation.

RZT was measured by determining soil temperature midway between the plants at 10 cm below the mulched soil surface. RZT over the growing season was measured with copper-constantan thermocouples (Model 107; Campbell Scientific) connected to a data logger (CR10X; Campbell Scientific) and an AM416 Relay Multiplexer (Campbell Scientific). The data logger was programmed to record readings every 10 min and store hourly averages for each plot. Air temperature data were obtained from a University of Georgia weather station located within 300 m of the plots.

Incoming and reflected photosynthetically active radiation (PAR) was determined with a ceptometer (Sunfleck PAR ceptometer; Delta-T devices) in the fall season. Incoming and reflected PAR was measured at 14 and 21 d after transplanting (DAT) on clear days at 1300 to 1400 hr. Reflected PAR was measured by placing the ceptometer rod in between two rows of plants facing the mulch surface and at 30 cm above the mulch. Two measurements of each incoming and reflected PAR were taken for each experimental plot. The amount of PAR reflected by the mulches was expressed as a percentage of the incoming PAR.

Gas exchange.

Leaf gas exchange measurements (net photosynthesis) were determined with a portable photosynthesis system (LI-6400; LI-COR Inc., Lincoln, NE). Gas exchange measurements were conducted under natural temperature (mean = 33.4 °C) and air humidity conditions (mean 17.0 mmol H₂O/mol); CO₂ concentration was set at 400 μmol·mol⁻¹ by means of a CO₂ mixer and a CO₂ tank and PAR at 2000 μmol·m⁻²·s⁻¹ using a red/blue light source (LI-6400-02B; LI-COR, Inc.). Measurements were conducted in clear days (PAR greater than 1900 μmol·m⁻²·s⁻¹), at 1200 to 1400 hr Eastern Standard Time, using three developed and fully exposed leaves per experimental plot. Each measurement lasted ≈45 to 60 s.

Plant growth.

On Day 28 after transplanting, two randomly selected vegetative tops (above-ground portion of the plant) from each replication were removed from the soil. Plant tops were dried at 70 °C and dry weights (DWs) were determined.

Leaf and fruit mineral nutrients.

Leaf samples (25 fully developed leaves from new growth) were collected 28, 42, 56, and 70 DAT. Fruit samples (five marketable fruit per treatment) were taken from each harvest. Leaf and fruit samples were dried at 70 °C for 2 d and analyzed for mineral nutrient concentration. Concentrations of P, K, calcium, magnesium, iron, manganese, zinc, and copper were determined by atomic absorption spectrophotometry. Tissue N concentration was determined by the copper catalyst method (AOAC, 1990).

Harvest.

For the fall crop, harvests were conducted on 18 and 23 Oct. and 13 Nov., whereas for the spring crop, harvests were on 19 and 30 June. At the last harvest, plants were excised at the soil level, enclosed individually in plastic bags, and kept at 10 °C until their vegetative fresh weight (FW) and fruit yield (including both developing and fully developed fruit) were determined within 24 h. Fruit were graded as marketable and culls according to the U.S. grade standards (USDA, 2005).

Statistical analysis.

Data were analyzed using the Analysis of the Mixed Procedure of SAS (SAS Institute Inc., Cary, NC). Average, maximal, and minimal RZTs for the season were calculated from daily values of mean, maximal, and minimal RZTs for each of the mulches during the entire season. The means for RZT, plant growth attributes, and fruit yields were separated by the Fisher's protected least significant difference test. Relationships of plant growth attributes with RZT were constructed using the means of the interaction of season, mulch, and cultivar. Various models were evaluated to determine whether there were differences in the relationship of plant growth and yield to RZT among seasons and mulch treatments.

Root zone temperature and reflected photosynthetically active radiation.

Fall mean air temperatures [27 °C (first 42 DAT) and 26.2 °C (entire season)] and RZTs [28.5 °C (first 42 DAT) and 27.8 °C (entire season)] were higher than spring air temperatures [22.7 °C (first 42 DAT) and 24.0 °C (entire season)] and RZTs [25.7 °C (first 42 DAT) and 27.0 °C (entire season)]. The values of mean RZT for each season represent a composite for all plastic mulches.

During the growing season, air and RZTs showed a decreasing trend in the fall and an increasing trend in the spring. Plastic film mulches differed in their soil-warming ability with RZTs in both spring and fall seasons being highest in black mulches and lowest in silver mulches (Table 1). The degree of thermal fluctuation varied among mulches with silver mulches displaying the smallest diurnal fluctuation.

Table 1. Mean seasonal root zone temperature (RZT), reflected photosynthetically active radiation (PAR), and number of thrips per flower in bell pepper grown on colored plastic film mulches, Tifton, GA, Fall of 2002 and Spring of 2003.^z

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The percentage of PAR reflected from the plastic film mulch surface varied among mulches. Reflected PAR was highest in silver mulches and lowest in black mulches (Table 1). The mean RZT under the plastic mulch decreased with increasing percentages of reflected PAR (Fig. 1). The presence of a silver strip in the black mulch significantly increased light reflection and reduced the heat captured by the plastic mulch, resulting in differences in mean RZT of 0.21 °C (fall) and 0.32 °C (spring) (Table 1). The presence of a black strip on silver mulch reduced RZT by 0.34 and 0.24 °C in the fall and spring, respectively, whereas the presence of a silver strip on the white mulch did not consistently modify RZT under the white mulch.

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Thrips and Tomato spotted wilt.

The number of thrips per flower was not significantly affected by plastic mulch treatment (Table 1). The number of thrips per flower had no relationship with the percentage of PAR reflected from the mulches (r² = 0.059) or with RZT (r² = 0.251). In both seasons, the incidence of TSW in mature plants was not significantly different among plastic mulch treatments. The mean incidences were 54% and 2% in the fall and spring, respectively. The incidence of TSW was not related to the number of thrips per flower.

Soil water status.

Silver2 mulch had the highest values of soil water content (fall) and soil water potential (spring) among plastic mulches (Table 2). However, there were not consistent values of soil water content and soil water potential associated with mulch color. Neither soil water content nor soil water potential had a relationship with RZT.

Table 2. Soil water status of bell pepper crop grown on colored plastic film mulches, Tifton, GA, Fall of 2002 and Spring of 2003.^z

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Plant growth.

In the fall, growth attributes during the first 28 DAT were among the highest in silver mulches and the lowest in black mulches (Table 3). Top DW, leaf DW, stem DW, and stem diameter decreased with increasing values of mean RZT during the first 28 DAT (Fig. 2). Plant height also decreased with increasing RZTs (R² = 0.695; P = 0.051).

Table 3. Growth variables of bell pepper plants grown on colored plastic film mulches 28 d after transplanting, Tifton, GA, Fall of 2002.^z

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Stem diameter was highly related to leaf DW (R² = 0.951; P < 0.0001), stem DW (R² = 0.925; P < 0.0001), and top DW (R² = 0.947; P < 0.0001). Top DW was more related to stem diameter than to plant height (R² = 0.776; P = 0.004). Bell pepper plant growth during plant establishment had a large effect on subsequent plant growth and fruit yield. Both final vegetative top FW and total fruit yield increased with increasing top DW during the first 28 DAT (Fig. 3). Final vegetative top FW did not respond to changes in RZT among mulches in the spring but declined quadratically with increasing RZTs in the fall (R² = 0.80; P = 0.017).

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Gas exchange.

There were few differences in net photosynthesis (P_n) resulting from plastic mulches. In the fall, 28 DAT, P_n was highest in silver mulch with a black strip (25.0 μmol·m⁻²·s⁻¹) and lowest in silver1 (21.7 μmol·m⁻²·s⁻¹), whereas it was intermediate for the other plastic mulches. In the spring, P_n was similar among mulch treatments 40 DAT (mean = 26.2 μmol·m⁻²·s⁻¹) and 43 DAT (mean = 27.3 μmol·m⁻²·s⁻¹). There was no relationship between P_n and RZT.

Leaf and fruit mineral nutrients.

Leaf and fruit mineral nutrient concentrations showed no response to mulch in the fall (Tables 4 and 5), whereas in the spring, some nutrients varied among mulch treatments (Tables 6 and 7). The concentrations of the majority of nutrients (except calcium and manganese) declined as the season progressed in both fall and spring. Leaf nutrient concentrations 28 DAT in the fall were not related with the mean RZT (daily mean from planting to 28 DAT) or with growth attributes. The foliar concentrations of N, P, K, calcium, and magnesium in mature plants were not correlated with their corresponding fruit concentrations.

Table 4. Leaf nutrient concentrations in bell pepper as affected by plastic mulch color and days after transplanting. Tifton, GA, Fall of 2002.^z

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Table 5. Fruit nutrient concentrations in bell pepper grown on colored plastic film mulches. Tifton, GA, Fall of 2002.^z

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Table 6. Leaf nutrient concentrations in bell pepper as affected by plastic mulch color and days after transplanting. Tifton, GA, Spring of 2003.^z

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Table 7. Fruit nutrient concentrations in bell pepper grown on colored plastic film mulches, Tifton, GA, Spring of 2003.^z

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Fruit yield.

Fruit yields in fall and spring seasons are shown in Table 8. In the fall, when soils were warmer, primarily early in the season, both marketable and total yields were higher on silver mulches and lowest on black mulch. In the spring, when soils were cooler early in the season, there were few yield differences among mulch treatments. The range in fruit yield among treatments was 0.36 kg/plant (77% of the mulch treatment with the lowest yield) in the fall and 0.23 kg/plant (27% of the mulch treatment with the lowest yield). In both seasons, the total number of fruit was much larger than the number of marketable fruit because bell pepper plants had a large number of cull fruit, which included immature green fruit as well as fruit with decay and physiological disorders.

Table 8. Fruit yields of bell pepper plants grown on colored plastic film mulches, Tifton, GA, Fall of 2002 and Spring of 2003.^z

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The relationships of both fall and spring fruit yield data as a function of mean RZTs for the season and for the first 42 DAT are shown in Figure 4. Fruit yield was little affected below 27 °C but decreased above ≈27 °C. From the quadratic equations in Figures 4A and 4B, optimal RZTs for fruit yield were calculated to be 25.9 °C (mean RZT for the first 42 DAT) and 26.3 °C (mean seasonal RZT). However, curves in Figures 4A and 4B suggest that the relationships of fruit yield with RZT may be better described by an optimal range in RZT rather than an optimal RZT point value (as derived from the quadratic equations). The optimal ranges were calculated as the range in RZTs in which fruit yield was 90% or higher compared with the yield at the optimal RZT point value. The RZT data from both trials were not low enough to allow to calculate the lower point of the optimal RZT range, i.e., RZTs were not low enough to reduce maximum fruit yield by greater than10%. Thus, 25 °C or less was chosen as a lower point of the optimal range, which corresponds to the lowest mean RZT (seasonal or for the first 28 DAT). Then, the optimal ranges of RZT were 26 to 27.6 °C or less (calculated from the mean seasonal RZTs) and 25 to 27.3 °C or less (calculated from the mean RZTs for the first 42 DAT). Using an average of the two estimates, the optimal range of RZT for bell pepper fruit yield was computed to be 25 to 27.5 °C or less.

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Fruit number did not respond to changes in mean RZT for the season in the spring but declined quadratically with increasing RZTs in the fall (y = –3.29 x² + 184 × –2579.3; R² = 0.68; P = 0.059).

In both seasons, total number of fruit was much larger than the number of marketable fruit because bell pepper plants had a large number of cull fruit, which included immature green fruit as well as fruit with decay and physiological disorders.

Root zone temperature and reflected photosynthetically active radiation.

Plastic film mulches alter the energy balance around the crop and thus modify root zone temperature (Ham et al., 1993; Lamont, 2005; Liakatas et al., 1986; Tarara, 2000). The effect of plastic mulches on energy balance, RZT, and light reflected from the mulch is associated with mulch optical properties of transmittance, absorbance, and reflectance of both short wave (0.3 to 1.1 μm, ultraviolet and visible light) and long wave (2.5 to 25 μm, infrared). Results of this study are consistent with reports indicating that black mulches have higher RZTs compared with white or silver mulches (Lamont, 2005; Tarara, 2000). The finding that silver mulches showed reduced RZTs and the smallest diurnal fluctuation is also consistent with previous reports in tomato (Díaz-Pérez and Batal, 2002) and tomatillo (Díaz-Pérez et al., 2005).

The percentage of PAR reflected from the surface of the film mulch was negatively related with RZT under mulch (Fig. 1), consistent with previous reports showing high RZTs under mulches (e.g., black mulch) with reduced percentage of reflection of PAR (Díaz-Pérez and Batal, 2002; Díaz-Pérez et al., 2005).

The combination of two different mulch colors on the same mulch surface may affect the optical or thermal properties of the film as shown by the increased PAR reflection and reduced RZT in black mulch having a silver strip in the central portion of the film. Interestingly, the presence of a black strip on silver mulch resulted in decreased RZT, rather than increased, as would be expected by the higher ability of black mulch of absorbing more incoming infrared radiation compared with silver mulch. Possibly, the black strip allowed for a higher loss of infrared radiation during the night. Previous studies have shown that silver mulches retain more heat during the night resulting in higher nighttime values of RZT and less diurnal fluctuations in RZT compared with other plastic film mulches (Díaz-Pérez, 2009; Díaz-Pérez et al., 2005).

Thrips and Tomato spotted wilt.

Thrips is an important insect pest in bell pepper because of its ability to transmit the virus that causes TSW, a disease that causes serious damage to bell pepper and tomato in several areas of the world, including the southeast United States (Zitter et al., 1991). The strategies to control TSW have included the use of silver mulches intended to reduce thrips activity (Abou-Jawdah et al., 2000; Greenough and Black, 1990; Momol et al., 2004). In this study, the finding that the number of thrips per flower was unaffected by mulch treatment and not related to the amount of light reflected from the mulches contrasts with other studies showing that silver reflective mulches, by reflecting ultraviolet light, reduce the populations of flower thrips and other insect vectors (Csizinszky et al., 1995; Olson et al., 2000; Schalk et al., 1979; Schalk and Robbins, 1987). Thus, the data suggest that the mechanism through which silver mulches influence thrips behavior and transmission of TSW may be not fully understood. In tomato, use of plastic mulches that created conditions of high RZT stress resulted in reduced plant growth and yield and predisposed the plants to earlier expression of TSW symptoms compared with plants grown at RZTs more favorable to tomato plant growth (Díaz-Pérez et al., 2007).

Soil water status.

Over the last decades, vegetable growers have used plastic film mulches to warm the soil, reduce weeds, and conserve soil moisture (Lamont, 1993; Zhang et al., 2008). Because of the high heat capacity of water, soil moisture affects soil temperature with soil temperature decreasing with increasing soil water status. However, the effect of plastic film mulch color on soil water status is far from understood. In this study, mulch color was not consistently related to soil water status (soil water content and soil water potential), and there was no relationship between soil water status and RZT under mulch. This may be because all plastic mulches used had similar thicknesses and were made of low-density polyethylene, thus probably having similar water vapor transfer properties.

Plant growth, gas exchange, and plant mineral nutrients.

Root zone temperature affects the growth and the physiological activities in plants. During bell pepper plant establishment, occurrence of low RZTs (infraoptimal RZTs) may limit plant growth, whereas presence of high RZTs (supraoptimal RZTs) may also inhibit plant growth. Numerous studies report on the beneficial effect of mulches on soil warming resulting in increased plant growth early in the spring season (Farias-Larios and Orozco-Santos, 1997; Lamont, 1993, 2005; Rubeiz and Freiwat, 1995; Tarara, 2000). In this study, plant growth was not determined in the spring. However, in the fall, vegetative growth decreased with increasing RZTs, suggesting that heat stress was the cause of growth reduction in bell pepper plants. Similarly, in tomato and tomatillo grown in Tifton, GA, like in this study, the use of dark mulches produced high RZTs resulting in reduced plant vegetative growth during the fall season.

Despite the effects of RZT on plant growth, gas exchange and leaf and fruit mineral nutrient concentrations were not associated with plastic mulches and RZT. These results contrast with studies in other crops that show that RZT influences both gas exchange and the concentration of foliar mineral nutrients. In tomato grown in solution culture at various RZTs, nutrient uptake for essential elements peaked at 26.7 °C (Tindall et al., 1990), whereas in potato grown on colored mulches, differences in RZT were associated with differences in the concentrations of copper and zinc in the leaves and the tubers (Baghour et al., 2002). In tomato (Lycopersicon esculentum Mill. cv. Vendor), an increase in RZT from 12 to 24 °C increased P, K, magnesium, calcium, iron, and manganese content of leaves but had the opposite effect on Na (Gosselin and Trudel, 1983).

Plastic mulches may affect bell pepper plant growth and fruit yield by factors other than RZT. In a study in South Carolina, mulch surface color was reported to affect the growth of bell pepper plants and the amount and quality of upwardly reflected light. The authors proposed that bell pepper plant growth is affected by relatively small changes in light environment (far-red:red ratio) induced by the surface color of the mulch (Decoteau et al., 1990).

The effect of light quality on bell pepper plants was not evaluated in this study. It is then not possible to know whether the different plant responses to plastic mulches in the spring and the fall are related to the percentage of light reflected from each the mulches. The percentage of light reflection from the mulch is primarily determined by the optical properties of the plastic mulch (Ham et al., 1993). The total irradiance differed among seasons [14.5 MJ·m⁻² (fall) and 22.2 MJ·m⁻² (spring)] and may explain, at least partially, the occurrence of lower yields in the fall than in the spring.

Soil moisture is another factor that significantly affects plant growth. However, soil moisture probably played no significant role to explain plant growth in this study because there was no relationship (P ≤ 0.05) between plant growth with either soil water content or soil water potential.

Fruit yield.

Fruit yields were low in both fall and spring seasons. Marketable and total yields were 5,227 and 9,992 kg·ha⁻¹ in the fall and 6,026 and 13,794 kg·ha⁻¹ in the spring. In Georgia, the average bell pepper marketable yield in 2002 and 2003 was 22,400 kg·ha⁻¹ (USDA, 2009). Fruit yields were low probably because of high insect and disease pressures and low temperatures late in the fall season and a short harvesting season in both seasons.

RZT is an important factor in plant growth and yield of vegetable crops (Cooper, 1973). In this study, plastic mulch influenced bell pepper fruit yield, although yield responses in the fall were different from those in the spring. In several crops grown in areas with cool spring conditions, the use of plastic mulches (e.g., black) with a high soil-warming ability has been found to increase yield (Ibarra et al., 2001; Lamont, 1993). In a study conducted in Maine, bell pepper plants on either black mulch with a white strip or on silver mulch produced significantly greater yields than plants on black plastic mulch with no white strip (Hutton and Handley, 2007).

The literature is often conflicting with respect to the effects of plastic mulch color on plant growth and yield of vegetable crops. This lack of consistency in the responses is probably the result of environmental differences caused by geographical location or season. Additionally, many reports do not include information about the environmental conditions (e.g., air temperature and RZT) during the study, making the comparison of results among studies difficult. There are few reports evaluating the same plastic mulches in different seasons (e.g., spring and fall) and thus under contrasting environmental conditions (Díaz-Pérez, 2009; Díaz-Pérez and Batal, 2002), as we did in this study. The evaluation of the impacts of plastic mulches under a wider range of air and soil temperatures provides more complete information on the crop responses to RZT and other environmental factors.

In the fall, both marketable and total yields decreased with increasing mean seasonal RZT, whereas in the spring, the yield differences were not related to RZT. Our results showed that the optimal range in RZT of bell pepper fruit yield (25 to 27.5 °C or less) is similar to the optimal RZT of tomato (26.3 °C) and tomatillo (Physalis ixocarpa Brot. ex Hornem) (27.3 °C), another solanaceous species (Díaz-Pérez and Batal, 2002; Díaz-Pérez et al., 2005). The similarity in the thermal responses of bell pepper, tomato, and tomatillo is not surprising considering that these crops are often grown under analogous climatic conditions. The calculated optimal range in RZT of this study for bell pepper yield is consistent with the optimal air temperature (20 to 25 °C) for pepper plants (Wien, 1997), although it differs from the optimal RZT (30 °C) for bell pepper yield in a study under greenhouse conditions (Gosselin and Trudel, 1986).

Fruit yield was more related to RZT during the first 42 DAT (r² = 0.854) than to seasonal RZT (r² = 0.509). This is probably because RZTs tend to be similar among plastic mulches once the plants reach full canopy cover, which under the conditions of this study occurred at ≈42 DAT. In tomatillo with full canopy cover, it was found that RZTs are similar among colored mulches and that RZTs depend on air temperature (Díaz-Pérez et al., 2005).

In conclusion, plastic film mulches significantly influenced the microenvironment, plant growth, and fruit yield of bell pepper plants, although these effects varied between seasons. The differences in plant growth and yield were significantly related to RZT. The reduced plant growth and fruit yields in black mulches during the fall were probably the result of the increased RZTs, and thus higher heat accumulation, that resulted in higher plant heat stress conditions compared with silver and white mulches. Fruit yield decreased with mean seasonal RZTs above 27.5 °C. The optimal range of RZT for bell pepper fruit yield was computed to be 25 to 27.5 °C or less.