Growing peppers in plastic tunnels has gained interest among hydroponic growers in South Africa, although high temperatures during the summer season limit crop yield. Seed suppliers are continuously improving sweet pepper varieties and recommend varieties to overcome the low productivity and low quality of sweet peppers in South Africa. Lack of adaptive cultivars and poor fruit setting of existing varieties during the hot/dry season, when the demand for peppers is high, is a major challenge for farmers in South Africa (Maboko et al., 2012a). For good fruit set and better yield, pollination, germination of pollen grains, pollen tube growth, fertilization, and fruit initiation must take place successfully (Kinet and Peet, 1997). Pepper plants are more sensitive to temperature extremes than tomatoes. Temperatures in the range of 16 to 28 °C for 4 to 6 months meet the heat requirements for most peppers. Temperatures lower than 15 °C result in poor plant growth, whereas temperatures higher than 32 °C result in flower drop and poor fruit set (Niederwieser, 2001).
PGRs modify plant physiological processes. PGRs are used by growers to promote fruiting and fruit development when temperature inhibits fruit set. PGRs can affect rooting, flowering, fruiting and fruit growth, leaf or fruit abscission, senescence, regulation of some metabolic processes, and plant resistance to temperature or water stress (Gelmesa et al., 2010) and are normally active at very low concentrations in plants. Induction of artificial parthenocarpy through application of PGRs enables fertilization-independent fruit development that can reduce yield fluctuation in crops such as tomato and peppers (Heuvelink and Korner, 2001). Gemici et al. (2006) reported increased fruit size and fruit setting in tomato resulting from the application of PGRs such as 4-CPA and β-naphthoxyacetic acid (β-NAA). Tomato fruit setting was promoted by GA3 at low concentration (Sasaki et al., 2005). Alam and Khan (2002) reported reduced preharvest fruit drop with increased number of fruits per plant and increased yield as a result of the application of NAA or β-NAA spray. Repeated application of 27 mg·L−1 NAA at the beginning of flower initiation significantly increased marketable and total yield of greenhouse-grown peppers and reduced the number of unmarketable fruits (physiological damage, unsuitable color, or small size) (Belakbir et al., 1998). Gibberellin was reported to contribute in preventing flower and fruit abscission, which is a major determinant of production loss in peppers (Tiwari et al., 2012). Silveira and Taborda (1986) reported increased early pepper yield when gibberellic acid was applied at 10 mg·L–1.
Incidence of flower drop/abortion has been reported on sweet pepper grown under shadenets (Maboko et al., 2012b), whereas increasing temperatures during the summer season exacerbated the incidence of flower drop/abortion. GA3 is important for tomato production to boost yield and improve fruit quality under unfavorable climatic conditions of high temperature (Gelmesa et al., 2010). There is little or no information available regarding the use of PGRs, i.e., NAA, GA3, and 4-CPA, in sweet pepper production under conditions where production is negatively affected by high temperatures. Temperatures in South Africa are often higher than the optimal temperature, causing a reduction in quality and/or yield of vegetable crops such as tomatoes and peppers (Maboko et al., 2012b, 2013).
In South Africa, the majority of hydroponic farmers are growing peppers in non-temperature-controlled tunnels, which rely on natural ventilation by opening the doors and flaps. This often results in poor plant growth, low yield, and poor fruit quality during the hot summer season (Maboko et al., 2013). PGRs, especially those that promote fruit set and yield, might assist farmers in maximizing production under such conditions. As a result of the beneficial effects of PGRs on plant growth and yield, particularly under conditions of environmental stress, this study was carried out to identify PGRs and/or combinations thereof to promote fruit set and development of summer season sweet peppers grown in plastic tunnels.
Belakbir, A., Ruiz, J.M. & Romero, L. 1998 Yield and fruit quality of pepper (Capsicum annuum L.) in response to bioregulators HortScience 33 85 87
Chhonkar, V.S. & Ghufran, M.H. 1968 Effect of starters and NAA on growth and yield of Lycopersicon esculentum Indian J. Hort. 25 72 75
Choudhury, S., Islam, N., Sarkar, M.D. & Ali, M.A. 2013 Growth and yield of summer tomato as influenced plant growth regulators Intl. J. Sustain. Agr. 5 25 28
Emongor, V.E. 2007 Gibberellic acid (GA3) influence on vegetative growth, nodulation and yield of cowpea (Vigna unguiculata L. Walp) J. Agron. 6 509 517
Erickson, A.N. & Markhart, A.M. 2002 Flower developmental stage and organ sensitivity of bell pepper (Capsicum annuum L.) to elevated temperature Plant Cell Environ. 25 123 130
Gelmesa, D., Abebie, B. & Desalegn, L. 2010 Effects of gibberellic acid and 2,4-dichlorophenoxyacetic acid spray on fruit yield and quality of tomato (Lycopersicon esculentum Mill.) J. Plant Breed. Crop Sci. 2 316 324
Gemici, M., Güve, A. & Yürekli, A.K. 2000 Effect of some growth regulators and commercial preparations on the chlorophyll content and mineral nutrition of Lycopersicon esculentum Mill Turk. J. Bot. 24 215 219
Gemici, M., Türkyilmaz, B. & Tan, K. 2006 Effect of 2,4-D and 4-CPA on yield and quality of tomato (Lycopersicon esculentum Mill.) JFS 29 24 32
Heuvelink, E. & Korner, O. 2001 Parthenocarpic fruit growth reduces yield fluctuation and blossom-end rot in sweet pepper Ann. Bot. (Lond.) 88 69 74
Kinet, J.M. & Peet, M.M. 1997 Tomato, p. 207–248. In: Wien, H.C. (ed.). The physiology of vegetable crops. CAB International, Wallingford, UK
Maboko, M.M., Du Plooy, C.P. & Bertling, I. 2012a Comparison of performance of tomato cultivars in temperature vs. non-temperature controlled plastic tunnel Acta Hort. 927 405 411
Maboko, M.M., Du Plooy, C.P. & Chiloane, S. 2012b Effect of plant population, stem and flower pruning on hydroponically grown sweet pepper in a shadenet structure Afr. J. Agr. Res. 7 1742 1748
Maboko, M.M., Du Plooy, C.P. & Bertling, I. 2013 Effect of arbuscular mycorrhiza and temperature control on plant growth, yield and mineral content of tomato plants grown hydroponically HortScience 48 1470 1477
Niederwieser, J.G. 2001 Guide to hydroponic vegetable production. 2nd Ed. Agricultural Research Council, Roodeplaat, Vegetable and Ornamental Plant Institute, Pretoria, South Africa
Payne, R.W., Murray, D.A., Harding, S.A., Baird, D.B. & Soutar, D.M. 2008 GenStat for Windows®. 11th Ed. Introduction. VSN International, Hemel Hempstead, UK
Sasaki, H., Yano, T. & Yamasaki, A. 2005 Reduction of high temperature inhibition in tomato fruit set by plant growth regulators Jpn. Agr. Res. Q. 39 135 138
Silveira, H.L. & Taborda, M.L. 1986 Effects of growth regulators for fruit setting on pepper (Capsicum annuum L.) production Acta Hort. 191 189 197
Snedecor, G.W. & Cochran, W.G. 1980 Statistical methods. 7th Ed. Iowa State University Press, Ames, IA
Tiwari, A., Offringa, R. & Heuvelink, E. 2012 Auxin-induced fruit set in Capsicum annuum L. requires downstream gibberellin biosynthesis J. Plant Growth Regul. 31 570 578
Tonder, C.S.M. & Combrink, N.J.J. 2003 The effect of plant-growth regulators on the production of out-of-season greenhouse tomatoes (Lycopersicum esculentum) S. Afr. J. Plant Soil 20 165 168