26 Adv. Hort. Sci., 2011 25(1): 26-31 *Corresponding author efallik@volcani.agri.gov.il Received for publication 10 February 2011. Accepted for publication 18 February 2011. Photoselective shade nets reduce postharvest decay development in pepper fruits A. Goren, S. Alkalia-Tuvia, Y. Perzelan, Z. Aharon, E. Fallik* Department of Postharvest Science of Fresh Produce, ARO, The Volcani Center, P.O. Box 6, 50250 Bet Dagan, Israel. Key words: Capsicum annuum, postharvest, shelf life, storage. Abstract: During two-year studies, we evaluated the influence of photoselective coloured shade nets on the quality of fresh harvested pepper fruits (Capsicum annuum) after prolonged storage and shelf life simulation. Pepper cultivar ‘Romans’ grown in a semi arid region under 35% pearl and yellow shade nets significantly main- tained better pepper fruit quality after 16 days at 7°C plus three days at 20°C, mainly by reducing decay inci- dence during two consecutive years (2008 and 2009), compared to commercial black and red nets. No significant differences were observed in percentage of weight loss, firmness and total soluble solids in fruit harvested under the different coloured shade nets. The skin colour of fruit harvested under Pearl net was significantly lighter than that of fruit harvested under red and black shade nets and this fact can be associated with inhibition of fruit ripening during growth. After storability and shelf life simulation however skin colour was red to dark red under all shade nets. Pearl and yellow shade nets significantly reduced Alternaria spp. population in the field, which was evaluated with Alternaria-selective growing medium. The highest Alternaria population was found under red shade net. The significant low decay incidence in fruit harvested under pearl and yellow shade nets can be explained by the low inoculum level of Alternaria spp. in the field, and inhibition of fungal sporulation, and/or by a slowing of fruit ripening during its growth, reducing fruit susceptibility to fungal infection in the field due to the scattered light, its quality and the ratio between the light spectrum under the two shade nets. 1. Introduction Sweet bell pepper (Capsicum annuum L.) is an important export commodity in Israel with more than 120,000 tons per year. Fruits may be green (unripe), red, yellow, orange, or brown when ripe. Peppers are rich in vitamins, minerals and dietary fibres, and are low in calories (Kevers et al., 2007) and they have become popular decorative items (Frank et al., 2001). However, to extend the export season with high pepper quality during the summer season, shade netting is nec- essary to protect agricultural crops from excessive solar radiation and pests (Shahak, 2008). In recent years coloured shade netting (photoselec- tive shade nets), designed specifically to manipulate plant development and growth, has become available. These nets can be used outdoors as well as in green- houses. They can provide physical protection (from birds, hail, insects, excessive radiation), affect environ- mental modification (humidity, shade, temperature) (Perez et al., 2006), and increase the relative proportion of diffuse (scattered) light as well as absorb various spectral bands, thereby affecting light quality (Shahak et al., 2004). These effects can influence crops as well as the organisms associated with them. The effect of colour shade nets on plant development in crops, foliage crops, fruit trees and vegetables has been stud- ied in recent years (Nissim-Levi et al., 2008; Shahak et al., 2008). Bell peppers are grown commercially in semi-arid regions of Israel under black shade nets. Netting is fre- quently used to protect pepper plants from excessive solar radiation (shade-nets), environmental hazards (e.g. hail-nets), or pests (bird, or insect-proof nets) (Shahak et al., 2004). Shahak (2008) reported that pro- ductions of three cultivars of bell pepper were increased by 16 to 32% under pearl and red netting compared with equivalent black shade netting, or with open field production. In addition, Elad et al. (2007) reported that blue-silver, green and red nets were asso- ciated with lower levels of powdery mildew disease, caused by Leveillula taurica in pepper field experi- ments. Other studies have suggested differential effects of photoselective screens and shade nets on pest infes- tation and vector-borne viral diseases (Ben-Yakir et al., 2008). In a preliminary research conducted in 2007, 27 Fallik et al. (2009) demonstrated the potential use of coloured shade nets to maintain the postharvest quality better of sweet pepper grown under red and yellow nets, during prolonged storage and shelf life. Therefore, we undertook the two-year study pre- sented in this work to evaluate, on a commercial growth scale, the influence of different coloured shade nets on the quality of fresh harvested bell pepper after prolonged storage and shelf life in order to understand, in part, the mode-of-action of those shade nets on har- vested fresh produce. 2. Materials and Methods Plant materials and coloured net shades Red sweet bell pepper (Capsicum annuum L.) cv. ‘Romans’ was grown at the B’sor Experimental Station in the south-west of Israel (31.271° N; 34.389° E), using commercial cultivation practices (planting seedlings directly into the soil [~ 90% sandy regosol and ~ 10% arid brown soil], automatic drip fertigation), under four different coloured shade-nets, as follows: red, yellow, pearl and black (commercial shade net) with 35% relative shading (in PAR) (Polysack Plastics Industries, Nir-Yitzhak, Israel under the trade mark ChromaticNet), in four random replicates of 18 x 18 m each, all within one large horizontal net-house, 2.5 m high. Plants were planted during the third week of May, both in 2008 and 2009, and harvested as described below. Light measurements under the nets Measurements of light spectra, light scattering and microclimate parameters under the nets were taken as described previously by Shahak et al. (2008). Spectra of solar radiation outside and under the nets were mea- sured by a spectroradiometer PS-100 (Apogee Instru- ments Inc. Logan UT, USA) which employs a light dif- fuser of 4 cm diameter above the 300 µm fiber optics. The diffuser was oriented along the sun beams. A round opaque plate of 6 cm diameter was held at 40 cm above the diffuser to block the direct light to measure the scat- tered light. Direct radiation was measured through a 4 x 45 cm tube placed on top of the diffuser. The radia- tion was measured three times at noon in the middle of August 2009 (Fig. 1). Quality parameters Fruit samples for storability and shelf-life were har- vested five times each year, every three weeks, between the beginning of September and the end of November 2008 and 2009. Fruits (185±10 g) without defects or diseases were harvested without a calyx, at ~85% light red/red colour (~16% green ‘cheek’), rinsed and brushed in hot water as described by Fallik et al. (1999) and packed in four 6.5 kg corrugated car- tons. Fruit quality parameters were evaluated immedi- ately after each harvest and at the end of 16 days stor- age at 7°C and relative humidity (RH) of ~ 94%, plus three days at 20°C (RH ~ 70-75%) as follows: Weight loss was expressed as percentage of weight loss from the initial weight of ten fruits. Fruit firmness was mea- sured on ten fruit as described by Ben-Yehoshua et al. (1983). Each fruit was placed horizontally between two flat surfaces and a 2 kg weight loaded on top of the flat surface. A dial fixed to a graduated plate recorded the deformation of the fruit in millimetres. Full deforma- tion was measured 16 s after exerting the force on the fruit, then the weight was removed and the residual deformation was measured 16 s later. Fruit was consid- ered very firm with 0-1.5 mm deformation; firm = 1.6- 3 mm deformation; soft = 3.1-4.5 mm deformation; very soft = above 4.6 mm deformation. Total soluble solids (TSS) were measured on the same ten fruits test- ed for firmness, by squeezing out juice from fruits onto an Atago digital refractometer (Atago, Tokyo, Japan) and taking readings. The development of fruit colour was expressed as colour index (CINX) – on a scale of 1 to 4 with 1 = light red with 10-16% green peel; 2 = light red; 3 = red; and 4 = dark red. The index was cal- culated as follows: (number of fruits at light red colour X 1 + number of fruits at light red X 2 + number of fruit at red colour X 3 + number of fruits at dark red X 4)/total number of fruits. Decay incidence was consid- ered once fungal mycelia appeared on fruit pericarp and/or calyx. Decay was expressed as a percentage of the total initial fruit number. Evaluation of total microorganism population levels and Alternaria spp. levels under coloured shade nets Population levels of conidia in the greenhouse envi- Fig. 1 - Influence of coloured shade nets on the scattered light under the different nets. Measurements were taken at noon in August 2009. Scattering relate to PAR (in µmol m-2 s-1). The values represent integrals of the spectra over the following wavelength ranges: PAR: 400-700 nm; UV: 300-380 nm; Blue: 410-470 nm; Red: 640-680 nm; Far Red: 690-750 nm (Means of 3 replicates ± SE). 28 ronment were evaluated according to Elad (1997) by exposing 90 mm Petri dishes, under each net, contain- ing PDA (Potato Dextrose Agar, Difco) supplemented with chloramphenicol (Sigma chemicals, 50 µg/ml) at a height of 60 cm above the ground, in five different places (five plates) under each coloured shade net for 60 min. Plates were then incubated for four days at ~22°C and colonies were counted. Based on prelimi- nary results obtained in 2007, the main decay-causing agent on pepper fruits after harvest was Alternaria alternata. Population levels of Alternaria spp. were evaluated by exposing 90 mm Petri dishes containing selective medium for Alternaria (Soon Gyu and Pryor, 2004), as described above. Colonies were counted after 10 days incubation at ~22°C. The plates were exposed horizontally 60 cm above the ground. Each evaluation was repeated three times from the beginning of Octo- ber to the end of November. Statistical analysis All the results were analyzed using a one- or two- way Anova statistical analysis at P=0.05 using JMP 6 Statistical Analysis Software Program (SAS Institute Inc. Cary, NC., USA) (Sall et al., 2001). 3. Results Light quality under different shade nets Figure 1 shows the spectra of the scattered light under the different nets. The pearl net significantly reduced UV, while it significantly increased the blue light, thus significantly increasing the ratio between blue/UV compared to all other shade nets. The yellow net significantly increased the scattered red light com- pared to all other shade nets, while the blue light under this net was significant lower compared to red and pearl nets (Fig. 1). The three shade nets significantly increased the far-red scattered light compared to the commercial black net. Quality evaluation After 16 days storage at 7°C plus three days at 20°C, the major differences in fruit quality were found between 2008 and 2009 in weight loss and fruit firm- ness. However, TSS was similar in the two years (Table 1). In 2008, fruits lost less weight under the red net (2.9%), while under the yellow net weight loss was higher (3.5%). In 2009, percentage of weight loss under the four shade nets was similar (between 3.4 to 3.6%). Immediately after harvest, fruits picked from the pearl net treatment were significantly lighter in their red colour index (2.33) than fruit picked from the com- mercial black or red shades (2.60 and 2.64, respective- ly) (Table 2). No significant differences were observed in fruit colour index between the yellow and pearl treatments. After 16 days at 7°C plus three additional days at 20°C, all fruits turned almost dark red, howev- er fruits picked under the red shade were significantly darker (3.86) than fruits picked under yellow and pearl shades (3.76 and 3.75, respectively) (Table 2). Decay incidence In both years, decay incidence under yellow and pearl shade nets were significantly lower compared to fruit picked under the commercial black or red net (Fig. 2). In 2009, the average decay incidence was higher than in 2008. In 2008, the highest decay incidence was observed on fruit picked under black and red shade nets, which was significantly higher than on fruit picked under the yellow and pearl shade nets. In 2009, Black Red Yellow Pearl Weight loss (%) z 3.2 ABb w 2.9 Bb 3.5 Aa 3.3 Ab Firmness (mm) y TSS (%) x 3.6 Aa 3.4 Aa 3.5 Aa 3.6 Aa 2.1 Ab 2.3 Ab 2.3 Ab 2.0 Ab 3.0 Aa 3.0 Aa 2.7 Ba 2.9 ABa 6.7 Aa 6.4 Aa 6.6 Aa 6.5 Aa 6.6 Aa 6.4 Aa 6.6 Aa 6.4 Aa Colored shade net 2008 2009 2008 2009 2008 2009 Table 1 - The influence of coloured shade nets on weight loss, firmness and TSS after 16 days at 7°C plus three days at 20°C (means of five exper- iments each year, with four 6.5 kg boxes per treatment) z Percent weight loss from initial. y Firmness - millimeter deformation (flexibility). x Percent total soluble solids. w Values followed by the same upper-case letter do not significantly differ between the treatments, while values followed by the same lower-case letter, do not significantly differ between the harvest seasons at p=0.05 according to Duncan’s multiple range test. Colored shade net Immediately after harvest (1-4) z After storage and shelf life (1-4) Black Red Yellow Pearl 2.60 a y 2.64 a 2.46 ab 2.33 b 3.80 ab 3.86 a 3.76 b 3.75 b Table 2 - The influence of coloured shade net on fruit colour index (CINX) development immediately after harvest and after 16 days at 7°C plus three days at 20°C (means of five experi- ments each year, with four 6.5 kg boxes per treatment) z Colour Index: 1= light red with 10-16% green peel; 2= light red; 3= red; and 4= dark red. y Values within a column followed by the same letter do not signif- icantly differ at P= 0.05 according to Duncan’s multiple range test. 29 Fig. 2 - Influence of coloured shade nets on decay incidence in fruit of cv. Romans, in 2008 and 2009 after 16 days at 7°C plus three days at 20°C. (Means of five experiments with four 6.5 kg boxes per treatment ± SE). Fig. 3 - Influence of coloured shade nets on the epiphytic population of fungi detected under the nets (CFU/plate – means of 3 experiments conducted in 2009 ± SE). a significantly lower decay incidence was observed on fruit picked under the pearl shade net, whereas the highest one was observed on fruit picked under the red shade net (Fig. 2). Fig. 4 - Influence of coloured shade nets on the Alternaria spp. popu- lation detected under the nets using selective medium (CFU/plate – means of 3 experiments conducted in 2009 ± SE). Evaluation of total microorganisms population levels and Alternaria spp. levels under coloured shade nets The epiphytic population of fungi evaluated on a regular PDA medium was lower under the pearl net, compared to the population that was counted under the red or black shade nets. However, no significant differ- ence between treatments was found (Fig. 3). Using a selective medium for Alternaria spp., a sig- nificant reduction in its population was observed under pearl and ellow shade nets (Fig. 4). The highest Alternaria population was found under the commercial black shade net (Fig. 4). 4. Discussion Photoselective coloured shade nets (Chro- matiNets™) have been developed over the last decade to filter selected regions of the spectrum of sunlight, concomitantly with inducing light scattering, and are designed specifically to modify the incident radiation (spectrum, scattering and thermal components) (Sha- hak et al., 2004). Depending on the pigmentation of the plastic and the knitting design, the nets provide varying mixtures of natural, unmodified light together with spectrally modified scattered light. They are aimed at optimizing desirable physiological responses, in addi- tion to providing physical protection to the crop, thus improving plant growth, flowering and yield and fruit quality (Rajapakse and Shahak, 2007; Shahak, 2008; Shahak et al., 2008). The most prominent effect of the coloured shade nets was found on decay incidence in fruit harvested under pearl and yellow nets, without affecting fruit quality after prolonged storage and shelf life. It is well known that light plays an important role in plant growth. Red light induced resistance in broad bean against Botrytis cinerea (Islam et al., 1998) and Alternaria tenuissima (Rahman et al., 2003), and in rice against Magnaporthe grisea (Arase et al., 2000). Tabira et al. (1989) reported that continuous irradia- tion of visible light of 570-680 nm protected apple leaves by inducing insensitivity to Alternaria alternata apple pathotype. Disease suppression under red light was also observed in glasshouse-grown Corynespore cassiicola-inoculated cucumbers, and indicated that delay and suppression of Corynespora leaf spot of cucumber were due to induction of resistance in cucumber, and not to differences in environmental con- ditions or fungal populations between the two green- houses (Rahman et al., 2009). We therefore speculate 30 that under yellow net, significantly more scattered red light penetrates into the plant and fruit (Fig. 1), which in turn inhibits fruit ripening as shown by fruit colour index (Table 2) and/or, indirectly, induces resistance against Alternaria alternata infection after harvest. Preliminary results have revealed high amounts of chlorophyll in fruit harvested under pearl and yellow nets and relatively low amounts of carotenoids, which might indicate slow fruit ripening (Fallik and Goren, unpublished). In parallel, it was found that under the pearl and yel- low shade nets the population of Alternaria alternata, the main decay-causing agent after harvest, was signif- icantly low compared to commercial black nets. Light has profound effects on fungal biology. Growth and development of many fungal species are intricately regulated by light (Springer, 1993; Purschwitz et al., 2006). Fungal responsiveness to different wavelengths of light has been well documented, and blue light/UV and red/far-red light are two types of photoresponses primarily observed (Mooney and Yager, 1990; Griffith et al., 1994; Purschwitz et al., 2006, 2008; Olmedo et al., 2010). Blue light or a high ratio of blue to UV radi- ation was inhibitory to the sporulation of many impor- tant phytopathogens (Raviv and Antignus, 2004; Paul et al., 2005). Increases in the spectral ratios between transmitted light:Blue/near-UV was found to inhibit the sporulation of several isolates of the fungal pathogen Botrytis cinerea in tomato (Kotzabasis et al., 2008). Hence, the ability of the pearl and yellow shade nets to filter UV light and enrich the blue spectrum (Shahak et al., 2008) and the blue/UV (Fig. 1) may be involved in Alternaria alternata inhibition in the field, thus reduc- ing inoculum levels and fruit infection during its growth and thereafter decay development during pro- longed storage and shelf life (Barkai-Golan, 2001). In conclusion, based on our results from two con- secutive years of study which have shown a significant reduction in decay development on pepper fruit har- vested under pearl and yellow shade nets, it seems that these two coloured shade nets influence both the pathogen in the field and the fruit ripening and its sus- ceptibility to pathological deterioration after harvest. 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