Okra (Albelmoschus esculentus L. Moench) species belong to the Malvaceae family and it is grown in tropical and subtropical regions (Tindall, 1983). In Africa, tender young leaves are edible, and the seeds are used as the material for extracting oil (Lamont, 1999). Althought variatal differece is observed, it is usually harvested after 7 days of florescence, and the length becomes tallest after 13 days after florescence, and after 7 ~ 10 days of florescence, the green pods are marketable (Kim et al., 1994). Okra is a vegetable with abundant protein, vitamins and minerals (Olasantan and Bello, 2004). Presently it is a widely planted vegetable crop not only in tropical areas but also in Europe, the U.S., China, and Japan (Kim et al., 1995). The demand for okra is steadily increasing in Korea and it is increasingly recognized as a health food (Ahn et al., 2011). With this in consideratrion, cultivation strategies for stable yield production of okra in Korea have been reported, such as the characteristics of adaptive varieties in Korea(Kim et al., 1994), fertilizing and planting density (Lee et al., 1990), seeding time (Ahn et al., 2012), optimal harvest time of green pod(Kim et al., 1994), and tunnel coverage (Son et al., 2014). Shanthakumar et al. (2011) tried to assess genetic diversity among popular hybrids and to isolate the superior hybrids from the segregated population. However, there are no reports on methods for stable standard cultivation produce systems in Korea.
Generally, mulching lessens water evaporation on the soil surface which facilitates the advantage of increased moisture usage by the crop and increases the yield. In addition, mulching cultivation is beneficial in maintaining moisture in the soil, increasing soil temperature, prevention of weeds, and prevention of soil erosion(Kim and Hong, 1986; Park et al., 2004; Roh and Pyon, 2004; Willis, 1962; Willis et al., 1963). In this view of highly advantageous mulching, it is essential to study the effect of mulching on okra cultivation, which is a tropical and subtropical crop.
Consequently, the present study shows the establishment of stable and standard cultivation methods to increase the yield and self-sufficiency of okra by observing the effect of mulching material on the yield of okra in a Korean environment.
MATERIALS AND METHODS
Testing material
This study was conducted in 2014, in the experimental field (N 35° 49' 24", E 128° 74' 25") of the Agricultural Research Institute for Climate Change, National Institute of Horticultural ? Herbal Science located in Jeju, south Korea, to analyze the characteristics of growth and yield characteristics of okra grown in various mulching materials. The okra cultivar used in this study was Marumijjang breed, purchased from Takii seed company of Japan and okra seed was stored in room temperature until the experiment was performed.
Treatment method
Mulching materials were, three kinds of polyethylene vinyl which was P.E. material: black (Black), green (Green), white and black in both sides (White), and black non-woven fabric (Fabric) and barley straw (Straw), and the control plot (Control). These experimental plots were treated with mulch in 6 different treatments, in a randomized block design (RBD), with 3 replications. Three plants were planted per each hole in two rows, and the space was 60 × 45cm. Seeds were soaked in the water for 12 hours, then sowed on May 7th. Soil temperature for each treatment was checked by HOBO (Onset technology support, Ho8-006-04), which is shown in Fig. 1. Each experimental plot was 15 m2 per each mulching, and the fertilizers used were urea, fused phosphate, and calcium chloride in a composition of nitrogen 10 kg, phosphoric acid 7 kg, and potassium 8 kg per 10a. All plots were basally fertilized before sowing. The Control plot without mulch was maintained without weeds by performing hand weeding to keep the soil exposed.
Plant Growth Development and Yield Characteristics
Plant growth and development was monitored at 70 days after sowing on May 7th, and 4 times at approximately10 days intervals. Plant height, and node characteristics and leaves were tested. Yield was monitored from July 17th to September 30th, at the time of fruit formation. The length of each edible fruit in the treatment was more than 10 cm and less than 18 cm.
Weather data during the testing period is shown in Fig. 1. The average temperature marked from mid-June till the period of growth was lower that year than average, and rainfall was present in July through mid-August to the beginning of September. Also, there were 5 typhoons during growth duration, and exceptionally heavy rainfall in August and September because of ‘HALONG’, 'FUNGWONG' and others. The duration of sunshine from July was shorter than the average years because of frequent rainfall, during the growth period between May and September, which was 84.2% compared to the average years, marking 146 hours lesser.
Collected data was compared in average value 5% significance with least significant difference (LSD), and Duncan- s multiple rage test, DMRT by SAS program (V. 9.2, Cary, NC, USA).
RESULTS
Growth characteristics of Okra
Variation in soil temperature in different mulching materials:
The variation in the average temperature and change of soil temperature after using mulching materials is shown in Fig. 2. Green colored mulching material showed the highest average temperature of 24.4°C, followed by Black > Fabric > Control > White > Straw, with 23.1, 22.8, 22.7, 22.3, 21.5°C respectively. Average temperature during research was 22.6°C, which was higher than White and Straw, and the difference in the maximum temperature between Green and Straw was 5.5°C.
Plant growth and development characteristics in different mulching materials:
Plant height, weight of aerial portion of plant and root weight of okra in various mulching materials is shown in Fig. 3. Plant height was highest in Green with 66.2 ~152.2 cm during testing period (omit during testing period), whereas in Straw it was lowest with a height of 32.2 ~ 109.7 cm. Similarly, aerial part and root weight of okra showed the same pattern as plant height in the order of Green> Black > White > Fabric > Control > Straw. Green plant and root weight was the heaviest compared to the other mulching materials and had a weight range of 209 ~ 408 g, indicating 20.0 ~ 35.0 g heavier than observed in the barley straw plot. Node number, leaf number, leaf length and leaf width in various mulching materials were documented as shown in Fig. 4.
Node number in each mulching material was 6.2 ~ 19.9 nodes per plant during the test period, which was least in Control and Straw. Green was observed with the most average nodes per plant. Leaf number was highest in White and Green with 9.6 ~ 19.9, leaves per plant, and the least leaf number was observed in Straw and Control. There was no significant difference in leaf length and leaf width.
Number of okra fruits in various mulching materials:
Okra fruit length and numbers were the yield characteristics tested in various mulching materials and shown in Table 1. During the initial harvest period in July, longer fruit length was observed in Green and Black with 15.0 ~ 14.9 cm whereas in Fabric, White, and Straw the fruit length was 14.0 ~ 13.7cm, which was short. In early August, average fruit length in White was as short as 13.2 cm, but fruit length was similar in different mulching materials with 13.9 ~ 12.3 cm till the end of September, the last period for harvest. Similarly, fruit weight showed a similar tendency with fruit length, with a significant difference in various mulching materials in the end of July and the beginning of August, whereas there was no significant difference later from the middle of August till the end of September. According to the observations Taken at the end of July, fruit weight was 16.9 g in Black which was the highest for that period. However, it was observed in early August that the average fruit weight was highest in Green, with a mass of 16.8 g.
Number of harvested fruits is shown in Fig. 5. A significant difference was observed in fruit mulching materials. Observations were made from the end of July to the end of September. In the early part of harvest the highest yield was observed in Green with the fruit number of 2,577 ~ 5,044, whereas during the middle of September, White and Black marked the highest yield with a fruit number of 5,933 to 5,711. Throughout the harvest period, the least yield was observed in Straw. The yield of Straw was 64% less than the yield of the control plot.
The weight of harvested fruit in each month was tested and observed to be correlated with harvested fruit number (Fig. 6). In Green, it was 72.6 ~ 42.5 kg, which was the heaviest from the end of July to the middle of September. Straw had the lowest overall yields during the harvest period with a range of 60.5 ~ 19.5 kg.
The results of total yield in terms of fruit number and weight in different mulching materials are shown in Fig. 7. Total fruit number was highest in Green, with 281,000 fruits. The least number of fruits were observed in Straw, which was 62.3% less than Green. Similarly, the total fruit weight was highest in Green with 385kg and Straw was the least with 226 kg. Hence the total yield followed the order of Green > Black = White > Fabric = Control.
DISCUSSION
Mulching materials usually comprised of polyethylene films are popular because they cause the soil temperature to change. In this study, transparent P.E. mulching showed the highest soil temperature, followed by black, green, and white. These results partially contradict with the results reported earlier that barley straw mulching showed a lower temperature compared to no mulching (Kasahara & Nishi, 1964; Park, 1978), Soil temperature change results were in the order of Green P.E. > Black P.E. > black non-woven fabric > control plot > white and black in both sides P.E. > barley straw mulching plot. The variation in soil temperature between green and black might be due to the variation in polyethylene materials. In this study, transparent P.E. mulching plot was ruled out due to the occurrence of weeds which grew because of light penetration (Lee & Yoon, 1975; Park, 1978).
Our results showed a clear variation not only in plant growth and development characteristics but also in yield in different mulching materials. This might be due to the raise in soil temperature, thereby affecting plant growth, development, leaves and roots in different mulching materials. In this view, the above mentioned factors showed better plant growth and development influencing the later part of plant development, inducing the higher assimilation quotient during fruit enlargement and maturation period and hence increasing the total yield. These results correlated with the results of Soltani and others (1995) in watermelon cultivation.
Previous research has reported that there is a relation in P.E. mulching with augmentation of photsynthesis caused by accumulation of CO2 . CO2 is released from roots or soil by organic matter decomposition and accumulates under the plastic. CO2 is escapes from under the mulch from the hole in vinyl mulch on the soil surface, and provides a high level of CO2 to plant leaves. This is called ‘Chimney Effect’ (Lamont, 1993). The concentration of CO2 around crops planted in holes of mulching plot is almost double that of concentration of CO2 in the outside atmosphere, whereas tunnel and mulching block CO2 release from mulching holes into the atmosphere. This method of mulching provides a higher assimilation quotient of CO2. It is reported that the higher assimilation quotient brings high growth and development including leaf area, and therefore there is an increase of the total yield and the amount of initial yield (Soltani et al., 1995).
In addition, a large difference was observed in the migration of fertilizer caused by water migration and change in eluviation leading to the change in nutrient content of soil and nutrient uptake by crop in PE mulching, barley straw mulching, black non-woven fabric and no mulching (Shimada, 1983), according to Lee and others (1990), NH4-N and NO3-N in soil under P.E mulching were higher in topsoil. Also, NO3-N in mulchless plots had migrated significantly to subsoil, the eluviation quotient caused by migration of K in P.E. mulching testing plot was decreased, the exchangeable amount of K absorbed into the soil was high. It was also reported that P, lime, and absorbed amount of magnesia increased by16 ~ 30%. According to Clarkson (1960), P.E mulching increases accumulated amount of NO3-N in soil and gives a huge effect on yield. Also, according to Bhella (1988), black P.E. mulching in tomato has more accumulation amount of NH4-N, NO3-N and Mg than no mulching and therefore it lead to the increase of crop growth and development and also the amount of dry matter production.
In conclusion, distinctive growth, development and yield of okra were observed in different mulching materials, each of which created a variation in the cultivation environment. Especially, there was large variation found between P.E. mulching, barley mulching, black non-woven fabric and control plot (no mulching). Therefore, in okra cultivation, accurate choice of mulching materials is highly important for growth, development and high yield. Further studies on the characteristics of edible okra fruits in various mulching materials would facilitate the establishment of high quality okra cultivation and production.
