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Effect of Plant Density and Harvesting Frequency on Yield Components of Hydroponically Grown Mustard Spinach (Brassica juncea) M.M. Mabokoa Agricultural Research Council-Roodeplaat Vegetable and Ornamental Plant Institute Private Bag x293, Pretoria, 0001 South Africa Keywords: bolting, closed hydroponic system, gravel-film technique, inflorescence, leaf

fresh mass, leaf number, plant density Abstract

Ideal plant population can lead to optimum yields, whereas too high or too low plant densities can result in relatively lower yields and quality. The objective of this study was to determine the combined effect of plant densities and leaf harvesting frequencies on yield of hydroponically grown mustard spinach. Mustard spinach plantlets were transplanted 21 days after seeding, utilizing a gravel-film technique hydroponic system. Eight treatment combinations were used, namely four plant densities (10, 16, 20 and 25 plants/m2) combined with two leaf harvesting frequencies (after every 7 and 14 days). Leaf area, fresh and dry mass, and number of leaves were measured from four weeks after transplanting. Total number of inflorescences and bolted plants were also recorded. The results show that plant densities and harvesting frequencies, both, affected leaf area, leaf fresh mass and leaf dry mass per m2 planted. Harvesting frequency of 14 days significantly improved leaf area, leaf fresh and dry mass as well as number of inflorescences and bolted plants compared to 7 day frequencies. Conversely, leaf number was higher at a harvesting frequency of 7 days. Leaf fresh mass, area, dry mass and number of leaves at a plant density of 25 plants/m2 improved significantly compared to a plant density of 20, 16 and 10 plants/m2. An increase in plant population resulted in an increase in the number of inflorescences and bolted plants. Plant density of 25 plants/m2 combined with a harvesting frequency of 14 days improved yield of mustard spinach significantly.

INTRODUCTION

Brassica juncea (mustard spinach) is a leafy vegetable grown in southern Africa under the name leaf mustard (B. juncea ssp. ‘Rugosa’) or rape. The latter name is rather confusing since it is also refers to the leafy equivalent of the oilseed crops B. rapa and B. napus (Schippers, 2002). According to Schippers (2002), B. juncea is not generally considered indigenous to Africa and it is a more important crop in China and South-East Asia, where it is known in a variety of forms. In Africa and many parts of Asia, the leaves are eaten as a vegetable; they are often shredded, cooked and served as a side dish with the staple food (Grubben and Denton, 2004). In South Africa, mustard spinach, commonly known as “Motshaina” in Limpopo province, is mainly grown as a vegetable for its leaves and/or for swollen stem petioles.

Mustard spinach is a cool season and fast growing crop, green-coloured with broad leaves oval in shape and reaches maturity at 35-65 days after transplanting. It has many uses such as seed oil, crushed seed used in the production of mustard and a variety of vegetable uses. It is also used as forage and for medicinal purposes. Mustard greens are a good source of dietary fiber, provitamin A, vitamin C, vitamin K, thiamine, riboflavin, vitamin B6, folate and mineral nutrients (van Wyk, 2005).

Soilless production of vegetables is known to improve yield and quality as compared to conventional production, and arguably represents the most efficient crop production system in terms of nutrient and water use efficiency in the world. Vegetable a mmaboko@arc.agric.za

Proc. 2nd All Africa Horticulture Congress Eds.: K. Hannweg and M. Penter Acta Hort. 1007, ISHS 2013

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production in a soilless culture (hydroponics) is highly productive, conserves water and land, and is more environmentally friendly compared to field production (Resh, 1996). Soilless cultivation recently tended towards closed hydroponic systems to avoid nutrient losses and thereby protect the environment (Schwarz et al., 2009).

Ideal plant population can lead to optimum yields, whereas too high or too low plant population can result in relatively lower yields and quality. Authors reported conflicting plant spacing for field production. According to Schippers (2002), mustard spinach can be planted at a spacing of 50 cm between rows and 30-45 cm in the row. Grubben and Denton (2004), however, suggested 30-50 cm between rows and 20-40 cm in the row. In spite of the high nutritional value of mustard spinach, lack of information on the soilless cultivation practices is a major limitation for hydroponic farmers. The objective of this study was to determine the combined effect of plant density and leaf harvesting frequencies on yield of hydroponically grown mustard spinach. The study was conducted in a shade-net structure in order to enhance the effect of plant density and harvesting frequency of mustard spinach when grown in a closed hydroponic system (gravel-film hydroponic systems).

MATERIALS AND METHODS

A trial was conducted from 2 May to 7 August 2011 in a 40% black and white shade-net structure at the Agricultural Research Council-Roodeplaat Vegetable and Ornamental Plant Institute (ARC-Roodeplaat VOPI), Pretoria, South Africa (25°59’S; 28°35’E at an altitute of 1200 m a.s.l.). Mustard spinach seeds (Cultivar ‘Florida Broadleaf’, Starke Ayres seed Pty. Ltd., South Africa) were sown on 2 May 2011 in 200 cavity polystyrene trays filled with a commercial growth medium, Hygromix® (Hygrotech Seed Pty. Ltd., South Africa) and covered with a thin layer of vermiculite after sowing. Seedlings were transplanted 28 days after sowing into a gravel-film technique hydroponic system as described by Maboko et al. (2011). The gravel-film technique is based on the nutrient-film technique system, where the nutrient solution flows down gullies by gravitation. The nutrient solution was pumped to the top of the gullies where a thin layer of nutrient solution flowed by gravitation into the reservoir at the bottom of the gullies, from where it was then pumped back to the top of the gullies (re-circulating system). At the top of the gullies, four tubes released the nutrient solution at a rate of 700 ml/min per tube (2800 ml/min and re-circulated), on a continuous basis. The pH of the nutrient solution was measured using a pH meter (HANNA instrument, Mauritius) and maintained within a range of 5.8 to 6.1. The composition and chemical concentration of fertilizers used for mustard spinach were: Hygroponic (Hygrotech Seed Pty. Ltd., South Africa) comprising of N (68 mg/kg), P (42 mg/kg), K (208 mg/kg), Mg (30 mg/kg), S (64 mg/kg), Fe (1.254 mg/kg), Cu (0.022 mg/kg), Zn (0.149 mg/kg), Mn (0.299 mg/kg), B (0.373 mg/kg) and Mo (0.037 mg/kg) and calcium nitrate (CaNO3) comprising of N (117 mg/kg) and Ca (166 mg/kg). An amount of 1000 g Hygroponic and 1000 g CaNO3 were applied in 1000 L water and the nutrient solution was renewed with fresh nutrient solution on a weekly basis. Data-loggers (Gemini Data Loggers, United Kingdom), placed in the Stevenson type screen (ACS-5050) at a height of 1.5 m, were used to record temperature (Table 1).

Eight treatment combinations were used, i.e., two leaf harvesting frequencies (after every 7 and 14 days) combined with four plant densities (10, 16, 20 and 25 plants/m2 at plant spacing of 25×40, 25×25, 20×25 and 20×20 cm, respectively). Each plot size was 2×1 m. Harvesting was done by removing the outer matured leaves and leaving four small inner leaves. A randomised complete block design with four replicates was used in this experiment. Flowers were removed by pinching inflorescences off on weekly basis.

During harvesting at 30 days after transplanting, leaf area, and leaf fresh and dry mass were measured on harvested yield on ten data plants per replicate. The leaf area (cm2) was measured using a leaf area meter (LI-3100 area meter, Nebraska, USA). Leaves were dried in an oven at 70°C for 48 hours for leaf dry mass determination. The

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total number of flower inflorescences and bolted plants (per m2) were also recorded. Data was subjected to analysis of variance (ANOVA) using the statistical program

GenStat® version 11.1 (Payne et al., 2008). Treatment means were separated using Fisher’s protected T-test least significant difference (LSD) at the 5% level of significance (Snedecor and Cochran, 1980).

RESULTS AND DISCUSSION

There was no significant interaction effect between the plant population and harvesting frequencies on mustard spinach growth and yield, therefore only the main factors are discussed below. Effect of Plant Population

Plant density resulted in significant differences in the yield parameters of mustard spinach (Figs. 1-4). Total leaf fresh mass was high at plant population of 25 plants/m2, while 16 and 20 plants/m2 did not differ significantly and the least was recorded for 16 plants/m2. Generally, leaf yield per unit area increased due to increased plant density, while leaf yield per plant decreased with increasing plant density (Figs. 1-4). This finding is concurrence with the results reported by Maboko and Du Plooy (2009) and Mwai et al. (2009). An increase in plant population resulted in an increase in leaf area (Fig. 2), number of leaves (Fig. 3) and leaf dry mass (Fig. 4). Even though the lower plant population (10 plants/m2) gave more leaves, larger leaf area, higher leaf fresh and dry mass per plant as compared to high plant population (Figs. 1-4), it still resulted in lower values per unit area. The results also agree with those of Badi et al. (2004) and Maboko and Du Plooy (2009, 2012), where the closer spacing produced a significant increase in leaf area and leaf number m-2 which in turn produced significantly higher fresh and dry mass m-2.

The number of flower inflorescence were significantly higher at a plant population of 25 plants/m2 compared to 20, 16 and 10 plants/m2, whereas the total number of bolted plants were high at plant populations of 25 and 20 plants/m2 compared to 16 and 10 plants/m2 (Fig. 5). Bolting normally reduces leaf yield due to assimilates channelled to flower and seed formation instead of leaf growth. However, in this study flowers were removed while still small in order to allow continuous growth of leaves. Effect of Harvesting Frequency

Total leaf fresh mass, area and dry mass were significantly higher at a harvesting frequency of 14 days (d) compared to a harvesting frequency of 7 d (Table 2). A harvesting frequency of 14 d resulted in 38% more fresh mass due to the longer period given for leaf development. Conversely, plants harvested on a weekly basis generated a higher number of leaves compared to harvesting bi-weekly (Table 2), but the leaves of a harvesting frequency of 7 d were not larger in size or higher in mass. The reduced leaf area, leaf fresh and dry mass for the harvesting frequency of 7 d might be because plants are not given enough time to recover from the previous harvest (harvesting shock).

The harvesting frequency of 14 d produced a higher number of flower inflorescences and bolted plants compared to the harvesting frequency of 7 d (Table 2). The lower number of bolted plants at a harvesting frequency of 7 d might be because plants were recovering the loss of leaves or leaf area responsible for photosynthesis rather than flowering. Flowers were removed on a weekly basis in order to continue harvesting the vegetative leaves. Removing the flowers at an early stage allowed the plant to continue channelling the energy to vegetative growth rather than to flower and seed development. The development of inflorescences and seed is known to result in poor eating quality of the leaves as the leaves tend to be bitter (Maboko and Du Plooy, 2012).

CONCLUSION

Plant population of 25 plants/m2 (20×20 cm) in a closed hydroponic system significantly improved yield of mustard spinach while a harvesting frequency of 14 days

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significantly improved yield as well as increased bolting compared to 7 days during the winter season.

ACKNOWLEDGEMENTS

The Agricultural Research Council for financial support of the study and Ms. Liesl Morey from the ARC-Biometry Unit for statistical analysis. Ms. Gumane Radeneba and Salome Lebelo for assistance in data collection.

Literature Cited Badi, H.N., Yazdani, D., Ali, S.M. and Nazari, F. 2004. Effects of spacing and harvesting

time on herbage yield and quality/quantity oil thyme, Thymus vulgaris L. Ind. Crop Prod. 19:231-236.

Grubben, G.J.H. and Denton, O.A. 2004. Plant Resources of Tropical Africa 2. Vegetables. PROTA Foundation, Wageningen, Netherlands/Backhuys Publishers, Leiden, Netherlands/CTA, Wageningen, Netherlands.

Maboko, M.M. and Du Plooy, C.P. 2009. Effect of plant spacing on growth and yield of lettuce (Lactuca sativa L.) in a soilless production system. S. Afr. J. Plant Soil 26:199-201.

Maboko, M.M., Du Plooy, C.P. and Bertling, I. 2011. Comparative performance of tomato cultivars cultivated in two hydroponic production systems. S. Afr. J. Plant Soil 28:97-102.

Maboko, M.M. and Du Plooy, C.P. 2012. Effect of plant density and harvesting method on yield components of hydroponically grown amaranth. Acta Hort. 947:415-421.

Mwai, G.N., Onyango, M.O.A., Onyango, J.C., Chadha, M.L. and Oluoch, M.O. 2009. Effect of plant density on yield components of nightshade. Acta Hort. 806:733-739.

Payne, R.W., Murray, D.A., Harding, S.A., Baird, D.B. and Soutar, D.M. 2008. GenStat for Windows® (11th Edition) Introduction. VSN International, Hemel Hempstead, UK.

Resh, H.M. 1996. Hydroponic food production (5th edn), Woodridge Press Publ. Co., Santa Barbara, California.

Schippers, R.R. 2002. African Indigenous Vegetables, An Overview of the Cultivated Species, 2002-Revised version on CD-ROM. Natural Resources International Limited, Aylesford, UK.

Schwarz, D., Franken, P., Krumbein, A., Kläring, H.P. and Bar-Yosef, B. 2009. Nutrient management in soilless culture in the conflict of plant, microorganism, consumer and environmental demands. Acta Hort. 843:27-34.

Snedecor, G.W. and Cochran, W.G. 1980. Statistical methods, 7th edn., Iowa State University Press.

van Wyk, B.-E. 2005. Food plants of the world, identification, culinary uses and nutritional value (1st edn). Briza publication, Pretoria, South Africa.

Tables Table 1. Monthly average temperature during the trial period in a 40% shade-net structure

at the experimental site. Month Max. Average Min. June 25.5 9.9 -5.5 July 26.6 9.5 -5.0 August 25.2 10.6 -1.8

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Table 2. Effect of harvesting frequency on yield parameters.

Harvesting frequency (days)

Leaf fresh mass

(g/m2)

Leaf area (cm2/m2)

Leaf dry mass

(g/m2)

Number of leaves/m2

Total number of

inflorescence/m2

Total number

of bolted plants/m2

Harvesting frequency7 7297b 87503b 719b 441.6a 16.9 b 8.1b 14 12273a 109018a 873a 368.3b 40.8 a 16.7a LSD 0.05 809.4 9658.4 65.6 35.41 5.77 4.08 Figures in a column followed by the same letter are not significantly different (P>0.05), using Fisher’s protected t-test Figures

Fig. 1. Effect of plant population on leaf fresh mass.

Fig. 2. Effect of plant population on leaf area.

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Fig. 3. Effect of plant population on leaf number.

Fig. 4. Effect of plant population on leaf dry mass.

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Fig. 5. Effect of plant population on number flower inflorescence and bolted plants per m2.

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