C. Egea-Gilabert and J.A. Fernández. Universidad Politécnica de Cartagena. E-30203, Cartagena. Spain. Keywords: Capsicum annuum L., solar radiation, high ...
Effect of Shade on Yield, Quality and Photosynthesis-Related Parameters of Sweet Pepper Plants J. López-Marín, A. Gálvez and A. González Dpt. Hortofruticultura, IMIDA, La Alberca E-30150, Murcia Spain
C. Egea-Gilabert and J.A. Fernández Universidad Politécnica de Cartagena E-30203, Cartagena Spain
Keywords: Capsicum annuum L., solar radiation, high temperature, shading, chlorophyll fluorescence Abstract To avoid the problem of too high temperature and high radiation during late spring and summer period, growers reduce the incident radiation with several methods, like with the use of shading screens and whitening. To determine the effects of shade, simultaneous comparisons were carried out among greenhouses that were either not shaded (control treatment) or shaded with reflective aluminized shade cloth positioned below the roof, which attenuated 40 (T40) or 60% (T60) of direct sunlight. The shade was applied at the beginning of hot weather in early May. The shading screens were kept until the end of the crop cycle and fruit was picked until August. Leaf CO2 assimilation rate, relative (SPAD) and absolute chlorophyll content, Fv/Fm, transpiration rate, stomatal conductance, internal CO2 concentration and water use efficiency were measured. Plants cultivated under 40 and 60% of shading significantly decreased the net CO2 assimilation rate, stomatal conductance, and transpiration. Plants cultivated under 60% of shading had higher contents of chlorophyll a, b. Under 40% of shading, plants yielded 1.26 kg·m2 more than under control. However, the yields of T60 and control treatment were similar (8.9 kg·m2). The use of shading decreased the unmarketable yield. INTRODUCTION Some sweet pepper crops in SE Spain are grown from late autumn to late summer, and their the production is distributed between early spring and late summer (LópezMarín et al., 2008). During summer, the greenhouse producers face problems of high temperatures, diminishing the quality of vegetables and flowers, and having negative effects on plant growth and yield. High solar radiation during the summer results in excessive light and heat load on leaves and fruits (López-Marín et al., 2011). These environmental variables can severely limit the productivity and nutritional quality of a crop (Adams et al., 2001). To prevent high temperatures or to reduce the cooling load in greenhouses during high temperature periods has been one of the most important problems to be solved in protected cultivation in most countries worldwide (Heming et al., 2006). To solve the problem related with high temperature and high radiation, growers reduce the incident radiation by several methods: ventilation (Bartzanas et al., 2004; Sase, 2006), increasing humidity levels by means of water sprays and creating moist air currents (Arbel et al., 2003; Kittas et al., 2003) and/or using thermal screens or mesh shades. The use of screens can reduce incoming radiation during the day, reducing the heat load in the greenhouse and assisting in maintaining humidity around the plants, so reducing plant stress, although the system has the disadvantage of limiting the exchange of air and vapour inside the greenhouse. The installation of an aluminised shading screen to reflect the strong radiation received by Mediterranean greenhouses is not cheap, but it does solve some of the problems of whitewash (Callejón-Ferre et al., 2009). The purpose of this study was to identify the effects of using aluminised screens offering different degrees of shading. Simultaneous comparisons were made among greenhouses that were either not shaded or shaded with reflective aluminized shade cloth that attenuated 40 or 60% of direct sunlight Proc. 7th IS on Light in Horticultural Systems Eds.: S. Hemming and E. Heuvelink Acta Hort. 956, ISHS 2012
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MATERIALS AND METHODS Plant Material and Greenhouse Conditions Sweet pepper plants ‘Herminio’ (Syngenta Seeds, USA) were transplanted on 22 December 2009 in an unheated arch-shaped multispan greenhouse covered with thermal polyethylene, located at the Experimental Farm of IMIDA ‘Torreblanca’, in Murcia, SE Spain (37°45’N; 0°59’W). The crop ended on 14 August 2010 and the number of harvests was six. The experiment was carried out in three units of a multispan greenhouse separated by polycarbonate. The unit 1 had a thermal screen of Aluminet®, 40% shading (T40), the unit 2 had a thermal screen of Aluminet®, 60% shading (T60) and the unit 3 was kept without thermal screen (non shaded condition: T0), as a control. The thermal screens were placed below the roof of the greenhouse. The thermal screen was unfolded on 15 May 2011 and was kept until the end of the crop. The PAR radiation and air temperature in each unit were monitored during the growing cycle using a quantum sensor (LI-COR Inc., Lincoln, Nebraska) and a Hobo U12 temperature data logger (Onset, USA), respectively. Chlorophyll Content, Chlorophyll Fluorescence and Gas Exchange Measurements of gas-exchange, chlorophyll fluorescence and chlorophyll content were monitored in fully expanded leaves. The closest leaf to the fruit recently set was used to measure these parameters. Gas exchange and chlorophyll fluorescence measurements were performed from 9:00 to 11:00 am (GMT). Net CO2 fixation rate (A, µmol CO2 m-2 s-1), stomatal conductance to water vapour (gs, mmol H2O m-2 s-1), transpiration rate (E, mmol H2O m-2 s-1), and substomatal CO2 concentration (Ci, µmol CO2 mol air-1) were measured at steady-state under conditions of saturating light (800 µmol m-2 s-1) and 400 ppm CO2 with a LI-6400 (LICOR, Nebraska, USA). Water use efficiency (WUE) was calculated as A/E. Chlorophyll fluorescence was determined with a pulse-modulated fluorometer ADC Fim 1500 (Analytical Development Company Ltd., Hoddesdon, UK). Minimal fluorescence values in the dark-adapted state (Fo) were obtained by application of a lowintensity red measuring light source (630 nm), whereas maximal fluorescence values (Fm) were measured after applying a saturating light pulse of 8000 µmol-2 s-1, and thus maximum quantum use efficiency of PSII in the dark-adapted state was calculated, Fv/Fm=(Fm−Fo)/Fm. The leaf area assayed was dark adapted for at least 30 min prior to Fv/Fm measurements. Chlorophylls a and b were extracted from leaf samples with N, Ndimethylformamide, for 72 h in darkness at 4°C. Subsequently, the absorbance was measured with a spectrophotometer at 750, 664 and 647 nm (Porra et al., 1989). The relative chlorophyll content of leaves was determined with a Minolta SPAD-502 meter. Experimental Design and Statistical Analysis Plants were grown in three units of a multispan greenhouse, two of them shaded with a mobile thermal screen. In each unit the experimental design was a randomized block design, three blocks and five plants each. The Statgraphics statistical package was used to calculate significant differences by ANOVA and means were compared at probability P≤0.05 according to Duncan’s multiple range test. RESULTS AND DISCUSSION A large difference in air temperature was noted between the unshaded control greenhouse and the greenhouses shaded by screen. This difference in air temperature was as high as 5.2°C under T60 and 4.1°C under T40 (Fig. 1b). Previous work has shown different reductions in the greenhouse air temperature by shading. As demonstrated by Kittas et al. (2001), the real temperature reduction with shade screens was not proportional to the percentage of shading. Beppu and Kataoka (2000) reported that 546
shading levels of 53 and 78% reduced the daily maximum air temperature by 1.9 and 3.3°C, respectively, compared with the unshaded control. The maximum daily PAR radiation in the shaded units decreased to 580 and 220 µmol m-2 s-1 with respect to the non-shaded unit (Fig. 1). The maximum value in the non-shaded unit reached 1180 µmol m-2 s-1 at 12:00 am (GMT). In general, all the gas-exchange related parameters increased in plants with incoming radiation. A similar trend was observed for the rest of photosynthesis-related parameters (stomatal conductance, transpiration and WUE), with the plants registering the highest values under non-shaded conditions, except on internal CO2 concentration (Fig. 2). Plants showed a strong reduction in the treatment shading, which can be a regulation factor of stomata closure, resulting in an increase of stomata diffusive resistance and decrease of the respiratory rate. The researchers interpreted the change of the leaf development as an adaptive phenomenon to capture irradiance more efficiently, although the effects of irradiance on stomatal development and associated photosynthetic capacity of Capsicum are still insufficient (Fu et al., 2010). The chlorophyll a fluorescence provides important information on the photochemical process in photosynthesis, and the Fv/Fm ratio, related to the maximum quantum yield of PSII photochemistry, has become an important and easily measurable parameter of the physiological state of the photosynthetic apparatus in intact plant leaves (Krause and Weis, 1991). The Fv/Fm ratio has been found to stay in a narrow range among leaves of many different species and ecotypes (Björkman and Demmig, 1987). In this work, Fv/Fm was reduced by 10% under non-shaded conditions compared to the shaded plants (Table 1). The increased chlorophyll content related to lower radiation levels is being widely reported in literature, mainly in studies with forestry species such as Croton urucurana (Alvarenga et al., 2003), Bombacopsis glabra (Pasq.) (Scalon et al., 2003), and Acacia mangium Willd. (Almeida et al., 2005). On the other hand, chlorophyll a was reduced about 5% in the non-shaded compared with shaded conditions (Table 1). A similar trend was observed in SPAD readings. However, no significant differences were found in chlorophyll b. The increase in the chlorophyll b relative proportion is an important characteristic of shaded environments because it catches the photons energy in longer wavelengths, therefore, with less energy and transfers it to chlorophyll a which acts effectively in the photosynthesis photochemical reactions (Whatey and Whatey, 1982). Under T40, the plants yielded 1.26 kg m2 more than under control (Fig. 2A). These results agree with those obtained by Rylski and Spigelman (1986), where a reduction in radiation during summer increased production in Capsicum annuum, compared with exposure to full sunlight, because of the adverse effect of high temperatures on fruit set. However, the yields of T60 and control treatment were similar (8.9 kg per plant) (Fig. 3). There was a decrease in yield attributable to shade in the interval after the shading was applied. This response may be due to adaptation to shading of plants. According to Steingber and Rabinowitch (1991), peppers at the mature-green and breaker stages of development seem to be more susceptible to sunscald, and it has been suggested that chlorophyll is essential for the development of this physiological disorder, especially if these fruit stages are characterized by chlorophyll disintegration, which made them highly susceptible to thermo-photooxidative processes. In this experiment the use of shading decreased the unmarketable yield mainly due to sunscald (Fig. 4). CONCLUSIONS The use of shading screen alleviated thermal and light stress in sweet pepper in a winter-summer crop cycle. This alleviation was reflected in a higher marketable fruit yield in treatment T40 but it was not observed in T60. The use of 40% shading seems to be an efficient alternative to improve yield reduce the thermal stress in sweet pepper.
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ACKNOWLEDGEMENTS This study was supported by Feder-IMIDA project POI-07-004 “Mejora de la productividad y la calidad de la producción de los cultivos hortícolas protegidos inducidas por la multifuncionalidad de los materiales plásticos”. Literature Cited Adams, S.R., Cockshull, K.E. and Cave, C.R.J. 2001. Effect of temperature on the growth and development of tomato fruits. Ann. Bot. 88:869-877. Almeida, S.M.Z., Angela Maria Soares, A.M., de Castro, F.M., Vieira, C.V. and Gajego, E.B. 2005. Morphologic alterations and biomass allocation in young plants of forestry species under different conditions of shading. Ciencia Rural. 35(1):62-68. Alvarenga, A.A., Castro, E.M., Castro, É. and Magalhães M.M. 2003. Effects of different light levels on the initial growth and photosynthesis of Croton urucurana baill. in southeastern Brazil. Revista Árvore 27(1):53-57. Arbel, A., Barak, M. and Shklyar, A. 2003. Combination of forced ventilation and fogging systems for cooling greenhouses. Biosyst Eng. 84:45-55. Bartzanas, T., Boulard, T. and Kittas, C. 2004. Effect of vent arrangement on windward ventilation of a tunnel greenhouse. Biosyst. Eng. 88:479-490. Beppu, K. and Kataoka, I. 2000. Artificial shading reduces the occurrence of double pistils in ‘Satohnishiki’ sweet cherry. Sci. Hort. 83:241-247. Björkman, O. and Demmig, B. 1987. Photon yield of O2 evolution and chlorophyll fluorescence characteristics at 77K among vascular plants of diverse origin. Planta 170:489-504. Callejón-Ferre, A.J., Manzano-Agugliaro, F., Díaz-Pérez, M., Carreño-Ortega, A. and Pérez-Alonso, J. 2009. Effect of shading with aluminised screens on fruit production and quality in tomato (Solanum lycopersicum L.) under greenhouse conditions. Span. J. Agric. Res. 7(1):41-49. Fu, Q.S., Zhao, B., Wang, Y.J., Ren, S. and Guo, Y.D. 2010. Stomatal development and associated photosynthetic performance of Capsicum in response to differential light availabilities. Photosynthetica 48(2):189-198. Hemming, S., Kempkes, F., van der Braak, N., Dueck, T. and Marissen, N. 2006. Greenhouse cooling by NIR-reflection. Acta Hort.719:97-106. Kittas, C., Katsoulas, N. and Baille, A. 2001. Influence of greenhouse ventilation regime on the microclimate and energy partitioning of a rose canopy during summer conditions. J. Agr. Eng. Res. 79:349-360. Kittas, C., Bartzanas, T. and Jaffrin, A. 2003. Temperature gradients in a partially shaded greenhouse equipped with evaporative cooling pads. Biosyst. Eng. 85(1):87-94. Krause, G. and Weis, H. 1991. Chlorophyll fluorescence and photosynthesis: the basic. Annu. Rev. Plant Physiol. Plant Mol. Biol. 42:313-349. López-Marin, J., Gálvez, A. and González, A. 2011. Effect of shade on quality of greenhouse peppers. Acta Hort. 893:895-900. López-Marín, J., González, A., García-Alonso, Y., Espí, E., Salmerón, A., Fontecha, A. and Real, A.I. 2008. Use of cool plastic films for greenhouse covering in southern Spain. Acta Hort. 801:181-186. Porra, R.J., Thompson, W.A. and Kriedmann, P.E. 1989. Determination of accurate extinction coefficients and simultaneous equations for assaying chlorophylls a and b extracted with four different solvents; verification of the concentration of chlorophyll standard by atomic spectroscopy. Biochim. et Biophys. Acta 975:384-394. Rylski, I. and Spigelman, M. 1986. Effect of shading on plant development, yield and fruit quality of sweet pepper grown under conditions of high temperature and radiation. Sci. Hort. 29:31-35. Sase, S. 2006. Air movement and climate uniformity in ventilated greenhouse. Acta Hort. 719:313-323. Scalon, S.P.Q., Mussury, R.M., Rigoni, M.R. and Scalon, H. 2003. Initial growth of Bombacopsis glabra (Pasq.) A. Robyns seedlings under shading conditions. Revista 548
Árvore 27(6):753-758. Steinberg, M. and Rabinowitch, H.D. 1991. The role of oxygen in thermo-photodynamic processes leading to sunscald-like damages in green tissues. Free Radic. Res. Commun. 2:809-817. Whatley, F.H. and Whatley, F.R. 1982. A luz e a vida das plantas: temas de biologia. São Paulo: EDUSP. vol.30, 101p. Tables Table 1. Effect of shade on the maximal efficiency of PSII (Fv/Fm), SPAD readings and the content of chlorophylls a and b. Columns with the same letter are not significantly different at P≤0.05 (Duncan test). Values are the means of five replicate samples. T0 T40 T60
Fv/Fm 0.75 a 0.82 b 0.83 b
SPAD 54.32 a 57.71 b 58.44 b
Chlorophyll a 4.40 a 4.88 ab 5.05 b
Chlorophyll b 2.09 a 2.11 a 2.28 a
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Figures
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Fig. 1. Time course of PAR radiation (A) and weekly average temperature (B) starting from transplanting.
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Fig. 2. Linear correlations between PAR radiation (A), leaf net CO2 assimilation rate (µmol CO2 m-2 s-1) (B), stomatal conductance (mmol H2O m-2 s-1) (C), intercellular CO2 concentration (µmol CO2 mol air-1) (D), transpiration E (mmol H2O m-2 s-1) (E) and water use efficiency WUE(A/E) (F) and % of shading of sweet pepper plants. Values are the means of five replicate samples.
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Fig.3. Effect of the shade on cumulative marketable fruit yield of sweet pepper plants at 126, 146, 165, 194, 217 and 229 days after transplanting. Values are the means of 15 plants ± SE.
1.2 Control
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Fig. 4. Effect of the shade on cumulative unmarketable fruit yield of sweet pepper plants at 126, 146, 165, 194, 217 and 229 days after transplanting. Values are the means of 15 plants ± SE.
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