Effect of plant population and nitrogen rates on growth and yield of okra [ Abelmoscus esculentus ( L ) . Moench ] in Gambella region , Western Ethiopia

Okra is one of the most important crops next to maize and sorghum production in Gambella Regional States. Okra production and yield maximization has not been attained due to lack of appropriate production practices such as optimum plant spacing and fertilizer use. Therefore, the research was conducted to assess the effect of plant population, and nitrogen rate on growth and yield components of Okra (Abelmoscus esculentus (L). Moench). The treatments were factorial combinations of five spacings (45 cm × 20 cm, 45 cm × 30 cm, 60 cm × 20 cm, 60 cm × 30 cm and 60 cm × 40 cm, and four nitrogen rates (0, 23, 46 and 69 kg N ha -1 ). The experiment was laid out in randomized block design factorial arrangement with three replications. Farmers’ local variety of okra, ‘Amula’, was obtained from the same institute was used as a test crop. Results indicated that, plant population and nitrogen rate had significance influence on growth and yield components of okra. Maximum number of branches (2.93), number of leaves (15.95) and pod length (29.01 cm) was obtained from the interaction of 60 cm × 30 cm spacing, 60 cm × 40 cm and at 46 kg N ha -1 . The highest fresh pod yield (46.14 t ha -1 ) and above ground biomass yield (119.34 t ha -1 ) was obtained from 45 cm × 30 cm spacing and at rate of 46 kg N ha -


INTRODUCTION
Okra (Abelmoscus esculentus (L).Moench) is one of the most well-known and utilized species of the family Malvaceae.It is also a chief vegetable crop grown for its immature pods that can be consumed as a fried or boiled vegetable or may be added to salads, soups and stews (Kashif et al., 2008).
There is no available record on production area and productivity of the crop under Ethiopian condition; however, it has high diversity in some parts of the country particularly in the South Western low lands (550 to 650 m above sea level) of the country.The production and productivity okra is seriously affected due to the use of low yielder local varieties, suboptimal plant density, inappropriate planting date, and decline in soil fertility and a decreased use of organic amendments, heavy attack of various insect pests and weeds (Akanbi et al., 2010).
Maintaining optimum spacing or plant population and nitrogen fertilization dose are most important elements in improving productivity of okra.Optimum plant density is the key element for higher yield of okra, as plant growth, yield and quality are affected by inter and intra-row spacing (Amjad et al., 2002;Paththinige et al., 2008).With increasing plant population, yield per unit area increases until a certain limit, beyond which yield decreases due to limitation of environmental resources required for plant growth (Amjad et al., 2002).
The lack of nitrogen in soil may lead to poor plant growth due to a decline in soil productive potential and fertility status.Nitrogen is the most essential element of plant nutrition as plants take it up in significant amounts.Sufficient nitrogen supply improves cell division, foliage production, and photosynthetic activity of the plant, thus producing higher numbers of flowers and fruits (Sharma and Yadav, 1996).Optimal use of nitrogen improves dry matter, especially the economic parts of the plant (that is, flowers and fruits).However, nitrogen availability to plants depends on the source, soil type, and environmental conditions, which may affect crop performance (Salazar et al., 2011).
Many farmers are currently cultivating okra in Gambella Region at various spacings which are inappropriate for obtaining maximum yields.These inappropriate plant spacings often lead to poor plant growth, fruit quality, and low yields which are insufficient to offset production costs which results in substantial losses of yield.The farmer's low yield problem is further compounded by the utilization of low or no nitrogen fertilizer use.
The agro ecological condition of Ethiopia is favourable for home garden and commercial production of okra, however, the overall national production and consumption is neglected and only known and grown in a few part of the country like Gambella, Humera and Benshangul Gumuz.People grow okra conventionally although no any clear record on production area and productivity of the crop under Ethiopian condition.No attention has so far been given for the development of improved agronomic management practices like spacing and nitrogen fertilizer application to increase the productivity of the crop in the region.The hypothesis was to find out optimum plant population and nitrogen rate that could give maximum pod yield of okra.
Understanding the economic importance of okra in Gambella Region, research was carried out to evaluate the effect of plant population and nitrogen rates on growth and yield components of okra.

Description of the study area
The experiment was conducted at Gambella Research Institute (GARI), Pignkew station, which is located 18 km from Gambella town, during the period from July to November 2013.The experimental site is located at an altitude of 445 m above sea level.Gambella is situated in the sub-humid hot to warm agro ecological zone that is well known for okra cultivation by small holder farmers.The region's mean annual rainfall is 1245.8mm with a uni-modal rainfall pattern which occurs from early April and extends to the end of November.The experiment was conducted on vertisol having a pH of 5.5.The average annual maximum and minimum temperatures are 36.7 and 22.3°C, respectively (Gambella Meteorological Agency, 2014, unpublished data).

Experimental material, treatments and experimental design
Local cultivar f okra ('Amula') was used as a planting material.The cultivar has been used by farmers in the region and it takes 55 to 75 days to mature under Gambella condition.It is short/ dwarf type of okra which is known to be high-yielding as compared to the other available cultivars.
The treatments consisted of five inter and intra row spacing (45 cm × 20 cm, 45 cm × 30 cm, 60 cm × 20 cm, 60 cm ×30 cm, and 60 cm × 40 cm) and 4 nitrogen rates (0, 23, 46 and 69 kg N ha -1 ) having a total of 20 treatments.The experiment was laid out as a Randomized Block Design (RBD) in factorial arrangement of 5 × 4 with three replications.The gross plot size was 3.6 m × 3.0 m (10.8 m 2 ).The experimental blocks and plots were spaced 1.5 m and 1 m apart, respectively.Each plot consisted of 6 and 8 rows for 60 and 45 cm inter row spacing, respectively, the number of plants per row was 15, 10 and 7 for the intra-row spacings, 20, 30 and 40 cm respectively.The net plot consisted of the central 4 and 6 rows for 60 cm and 45 cm row spacing, respectively.Plants in a 0.6 m length at both ends of the row were considered as border plants, and were not harvested to avoid border effects.Thus, the harvestable plot area was 5.28 m 2 (2.4 m × 2.2 m).

Experimental procedures and cultural practices
Okra seeds were sown on 24 August 2013 by dibbling 2-3 seeds per hole on the prepared plots according to the treatment combination.Triple super phosphate was applied at sowing time to the control plots.Nitrogen at treatment levels were applied in the form of urea in three splits: emergency of plants, at active growth stage and the remaining at flowering.Nitrogen in the form of urea (4.86 kg) was applied to the plots accordingly.Nitrogen at the rate of 23 kg ha -1 (0.27 kg urea), 46 kg ha -1 (0.54 kg urea) and 69 kg ha - 1 (0.81 kg urea) was applied to the experimental plots within blocks after crop emergency at 15 days interval.No potassium fertilizer applied.
Thinning operation was done after emergence of seedlings: one vigorous plant was maintained per hole.Weeding and other management practices like hoeing, pest and disease control were applied uniformly as required.

Soil sampling and analysis
Soil samples were collected from the entire experimental site in zigzag pattern from 0 to 30 cm depth of soil using an auger.Then all samples were mixed together in order to get one composite sample weighing 1 kg for determination of selected soil properties.The composite soil sample was dried and crushed to pass through a 2 mm size sieve for the analysis of pH, texture.For the determination of total nitrogen and organic carbon, the soil sample was made to pass through 1 mm pore size sieve.The soil samples were analyzed for some parameters at Melkassa Research Center Soil Laboratory.The soil was analyzed for texture, pH, organic carbon, total N and available P. Soil pH was measured from a suspension 1:2.5 soil-water ratio using an electrodes pH meter (Motsara and Roy, 2008).The organic carbon content of the soil was determined by the volumetric method (Walkley and Black, 1934).Total nitrogen was estimated by the Micro-Kjeldahl method with sulphuri acid (Jackson, 1962).Available phosphorous was estimated by the Olsen method (Olsen et al., 1982).
The soil analysis results indicated that the experimental site had soil PH of 5.5 with 0.13% total nitrogen, 1.92 and 2.64% organic carbon, phosphorous and a texture of vertisol, respectively.The total nitrogen was low as it was below 0.15% as described by Bruce and Rayment (1982).

Data collection and measurement
Data were collected on crop phenology, growth, yield and yield related traits.

Number of days to 50% flowering:
The number of days to 50% flowering was determined from each plot by counting the number of days required for 50% of plants in a plot to reach flowering.

Days to pod set:
The number of days taken for 50% of plants in each experimental plot set at least one pod per plant.
Plant height (cm): Plant height was measured at the last picking of the fruit from ten randomly selected plants in each plot from the central rows using a measuring tape from the soil surface to the tip of plant.

Length of pod bearing zone (cm):
This refers to the length of stem from the point of first pod set to the point where pod formation ends.

Number of branches per plant:
Total number of branches was counted from 10 randomly selected plants from the central rows and then mean was calculated at the final harvest.

Number of leaves per plant:
The total number of leaves was recorded by counting the number of leaves per plant until the final harvest.Leaves were counted from the same ten randomly selected plants, which were selected for the measurement of plant height.

Number of green pods per plant:
The number of green pods per plant was counted at every picking day from ten randomly selected and tagged plants in each plot.The total number of pods obtained from the selected plants was divided to get the average number of pods per plant.

Length of green pod (cm):
The length of 10 green marketable pods collected from sample plants was measured and averaged.
Pod diameter (cm): Pod diameter was measured using a vernier calliper and averaged.
Total green pod yield (g/plant and t ha -1 ): Yield obtained at each harvest from the net plot area was summed up as marketable and unmarketable yield and converted to a hectare basis.
Pod fresh weight (g plant -1 and t ha -1 ): Average fresh pod weight from 10 randomly taken pods from each net plot area was measured by using a digital balance.
Pod dry weight (g plant -1 and t ha -1) : This refers to the dry weight from 10 pods selected randomly from the net plot area, and dried in an oven to a constant weight.
Above ground dry biomass yield (g plant -1 and t ha -1 ): The sum of above ground parts of 10 selected plants along the pods was weighed and averaged after oven-drying to a constant weight.

Crop stand:
The number of plant in the net area of each plot was counted fifteen days after emergence and at the time of the last harvest.

Data analysis
The data were subjected to analysis of variance (ANOVA) appropriate to randomized complete block design technique using the SAS computer software programme, version 9.2 (SAS, 2008).Least significant difference (LSD) at 5% probability level was carried out for means separation.

Phenology and growth parameters
The results of analysis of variance indicated that main effect nitrogen showed significant (P < 0.001) difference on 50% flowering; pod setting, plant height and length of pod bearing zone (Table 1).However, the main effect of plant population and nitrogen fertilizer application rate significantly (P < 0.05) influenced the number of leaves produced per plant.The interaction effect of plant population and nitrogen fertilizer application significantly resulted in increased number of branches produced per plant (Table 1).
Application of nitrogen at the rate of 46 and 69 kg N ha - 1 led to the longest days (51.40) to 50% flowering as compared to the control treatment, which gave least days to flowering (46.33).The application of the 69 kg N ha -1 increased flowering date by 10.94% as compared to the control (Table 3).
Nitrogen application from nil to the highest level (69 kg N ha -1 ) prolonged the number of days to pod setting in a similar way that it prolonged the number of days required for 50% flowering.Increasing, the application rate of nitrogen from 0 to 46 and 0 to 69 kg N ha -1 prolonged the duration of 50% pod set by 7.6 and 8.8%, respectively (Table 3).At the control level (0 kg N ha -1 ), days to pod setting was 49.93 while at 69 kg N ha -1 it was 54.33 days.The delay in number of days to flowering and fruit formation in response to increasing the rate of N may be attributed to the prominent role nitrogen plays in promoting cell division and vegetative growth, which delays flowering and fruiting in crop plants.
Maximum plant height was recorded from the application of nitrogen fertilizer level of 23 kg N ha -1 , whereas the minimum plant height was recorded from the rest of nitrogen fertilizer levels.Application of 23 kg N ha -1 significantly increased plant height by 13.56% as compared to the control plots which received no nitrogen (Table 4).Results are in line with the finding of Bin-Ishaq (2009) who reported that increasing the rate of N up to 45 kg N ha -1 was associated with significant progressive increases in plant height of okra.The increase in plant height in response to increased application of nitrogen could be attributed to enhanced synthesis of protein in the plant, which is, fundamental building material of cells and a constituent of all enzymes.
The application of nitrogen rates at 23 kg N ha -1 rate resulted in maximum length of pod bearing zone.Application of 23 kg N ha -1 significantly increased length of pod bearing zone by 18.22% compared to the control plots.However, increase in the rate of nitrogen beyond 23 to 46 and 69 kg N ha -1 resulted in a significant reduction of length of pod bearing zone by 8.9 and 13.28%, respectively, compared to 23 kg N ha -1 application (Table 4).
The results of this study in consistent with Firoz (2009) who reported that inter-node length of okra plant was significantly influenced by the application of nitrogen fertilizer.Consistent with this result, Malik and Mondal (1996) revealed that okra pods are continually produced on new nodes of developing stems, which results in several weeks of harvesting.
The plant spacing of 60 cm × 40 cm and 60 cm × 30 cm, resulted in maximum number of leaves per plant (Table 4).This result is supported by that of Bin-Ishaq (2009), who reported that number of leaves per plant was significantly increased as the plant density decreased and number of leaves decreased as plant density increased.This is because at wider spacing, there are favourable growth conditions which enhance vegetative growths such as increase in number of branches which results in number of leaves per plant.
The application of nitrogen fertilizer rate of 46 kg N ha -1 significantly increased the number of leaves per plant by 16.81% compared to the number of leaves per plant recorded to control plots (Table 4).Increased application of nitrogen rate was associated with increase in vegetative growth components of plants and resulted in new leaf formation.This result is in line with the finding of Bin-Ishaq ( 2009) who reported that increasing N applied rate up to 45 kg N ha -1 was associated with significant progressive increases in number of leaves per plant.Similar results were reported by Ghoneim (2000) who reported that application of 60 kg N ha -1 to okra plants increased leaves per plant.
The highest number of branches was recorded from plant populations grown at the spacing of 45 cm × 30 cm and 60 cm × 30 cm plant and application of 46 kg N ha -1 , which was in statistical parity with the number of branches obtained from the spacing of 60 cm × 40 cm spacing and N application of 46 kg N ha -1 and 60 cm × 30 cm combined with N rate of 69 kg ha -1 .The results of the study is in line with the finding of Bin-Ishaq (2009) who reported that increasing N application rate up to 45 kg N ha -1 was associated with significant progressive increases in number of branches per plant.Application of more nitrogen beyond 46 kg ha -1 did not favour the production of more number of branches (Table 5).Consistent with the results of this study, Ekwu et al. (2010) reported that the application of nitrogen at the rate of 140 kg ha -1 produced the highest number of branches and fruits as compared to 0 and 70 kg ha -1 .Khan et al. (2010) also reported that the highest number of fruits per plant and of number of branches per plant of sweet pepper in response to increased application of nitrogen fertilizer up to 150 kg N ha -1 .

Yield and yield components of okra
The interaction effect of plant population and nitrogen  fertilizer rate showed significant differences on length of green pods(cm), pod diameter(cm), pod fresh and dry weight yield(t/ha) and above ground biomass yield(t/ha).
The main effect of nitrogen rate significantly affected green pod yield of okra per plot (kg) (P<0.001)(Tables 1  and 2).Highest number of green pods per plant was recorded from plant population (45 cm × 30 cm and 60 cm × 30 cm) which was not significantly different from numbers of green pods produced in response to the spacing of 60 cm × 40 cm.Plant population spaced at 45 × 30 and 60 × 30 cm significantly increased number of pods by 19.78% and 19.71%, respectively as compared to the highest plant population spaced at 45 cm × 20 cm (Table 4).This result is supported by the finding of Amjad et al. (2001) who reported that the lowest planting density (37,000 plants ha -1 ) resulted in maximum number of matured pods per plant in okra.Similarly, Ekwu and Nwokwu (2012) reported that the number of fruit of okra significantly increased with decrease in population density.The maximization of green pods in response to lowering plant population could be due to the fact that plants grown under low population density have good growth performance since competition for available resources are limited as compared to plants grown under high plant population density.
Maximum length of green pods was recorded from the interaction of low plant population (60 cm × 40 cm) and nitrogen application rate of 46 kg N ha -1 (Table 6).However, minimum length of green pods was recorded from high plant population spaced at 45 × 20 cm and from the control plots.Increased in length of green pods in relation to the interaction of the two factors could be due to the fact that low plant population responded well to the applied nitrogen rate which favours metabolic changes in plant resulting in the production of lengthy pods.It is obvious that wider spacing results in better growth performances of plant as a result of low competition for resources which can significantly result in increased pod length per plant.
Pod diameter was higher from the low plant population spaced at 60 × 30 cm and from the nitrogen application rate of 46 kg N ha -1 (Table 7).However, the minimum pod diameter was recorded from the control plots and from the high plant population spaced at 45 cm × 20 cm and 60 cm × 20 cm, respectively.Table 5. Interaction effect of plant population and N-rates on number of branches of okra.
Means followed by or sharing the same letters are not significantly different at 5% level of significance; CV = Coefficient of variation; LSD = Least significant difference at 5%; SE = Standard error of the means.The maximum pod fresh weight was recorded from low plant population from 60 cm × 30 cm plant spacing and was not significantly different from the pod fresh weight yield which was obtained from spacing of 60 cm × 40 cm (Table 8).However, minimum pod fresh weight was recorded from higher plant population of 45 cm × 20 cm  and 60 cm × 20 cm plant spacing, respectively.The result is in line with findings of Ali (1999) who reported that wider spacing leads to heavier individual pod weight in okra.Similarly, Amjad et al. (2001) reported that the weight of pods was highest at the closest plant spacing (50 cm × 25 cm).The maximum pod fresh weight per plant at low plant population might have been resulted from efficient utilization of growth nutrients favouring optimum vegetative growth of okra resulting in increased pod fresh weight as a result of limited or very less competition for resources.Maximum green pod yield per plant was recorded from the application of nitrogen fertilizer rate of 46 and 69 kg N ha -1 while, minimum green pod yield was recorded from the control plots (Table 8).This result is in agreement with the finding of Yih-Chi Tan et al. (2009) who reported that nitrogen had highly significant effect on the yield of okra.Likewise, Akanbi et al. (2010) reported that application of N led to significant influence on fresh fruit yield of okra.The author stated that fruit yield increased with increases in N level reaching peak with the highest N level.
The main effect of nitrogen fertilizer application showed significance increase on pod fresh weight per plant (Table 8).The highest pod fresh weight per plant was recorded from the application of nitrogen fertilizer rate of 46 and 69 kg N ha -1 respectively.However, minimum pod fresh weight was recorded from the control plots and low N rate (23 kg ha -1 ).The result is in line with the finding of Firoz (2009) who reported that increasing the rates of N from 0 to 40 or 40 to 80 kg N ha -1 , significantly increased total fresh pod yield and the mean fresh pod yield of okra.Pod fresh weight yield showed significant differences by the interaction effects of plant population and nitrogen rates (Table 9).The maximum fresh pod weight yield was recorded from the application of nitrogen fertilizer rate of  46 kg N ha -1 and from plant population spaced at 45 cm × 30 cm plant spacing respectively.Increase in pod fresh yield at this spacing level and N-rate could have been resulted from efficient utilization of resources leading to optimum morphological growth characters which favours pod fresh yield increase of okra (Onyegbule et al., 2012).. Table 10 showed that the maximum pod dry weight yield was recorded from the application of nitrogen rate of 69 and 46 kg ha -1 and plant spacing (45 cm × 30 cm).However, the minimum dry pod yield was recorded from plant spacing (60 cm × 40 cm) and from all the applied nitrogen fertilizer rates This result is in consistent with the finding of Frezgi (2007) who reported that haulm dry matter yield significantly increased with increased nitrogen and high planting density; the highest yield of haulm dry matter was recorded for 75 cm inter row spacing, 20 cm intra row spacing and 150 kg N ha - 1 treatment combination.
Significantly maximum above ground biomass yield per plant was recorded from low plant population spaced at 60 cm × 40 cm (Table 8).The maximum above ground biomass yield per plant was recorded from the application of nitrogen fertilizer of 46 kg N ha -1 .Application of nitrogen rate of 46 kg N ha -1 significantly increased yield by 31.06% over the control plots.Increase in nitrogen fertilizer rate beyond 46 kg N ha -1 resulted in yield reduction by about 23.70%.
The interaction effects of plant population and nitrogen fertilizer rate resulted in significant yield increase of the above ground biomass yield.Maximum above ground biomass yield was recorded from plant spacing (45 cm × 30 cm) and from the application of nitrogen at rate of 46 kg ha -1 .However, the minimum above ground biomass yield was recorded from low plant population (Table 11).The above ground biomass yield significantly increased relatively at narrow spacing because of favourable conditions for morphological growth characters which resulted from less competition between plants, that is, number of pods, number of leaves, number of branches, pod diameter and pod length which contributes for above ground biomass yield increase.Nitrogen at 46 kg ha -1 gave the highest yield due to efficient utilization of the resources and beyond that biomass yield showed a reduction trend.

Conclusion
This investigation conducted on growth and yield components of okra indicated that plant population and nitrogen rate played a significant role in increasing the yield of the crop.Growing okra at the spacing 45 cm between rows and 30 cm between plants with plant population of 74,047 plants ha -1 and at 46 kg N ha -1 resulted in the optimum growth and highest fruit yield of okra in Gambella Region.

Table 1 .
Mean squares values of crop phenology and yield components of okra.Significant at P≤ 0.05; ** significant P≤ 0.01; *** significant at P≤ 0.001; NS = non significance at P≥0.05.N x PS, Nitrogen x plant spacing; DF, Degree of freedom; PH, Plant height; LPBZ, length of pod bearing zone; NBP, Number of branches per plant; NL= Number of leaves per plant; NPP, Number of pods per plant; PL, Pod length; PD, Pod diameter *

Table 2 .
Mean squares values of yield components of okra.

Table 3 .
Main effect of nitrogen rate on crop number of days to 50% flowering and number of days to 50% pod set of okra.
Means followed by or sharing the same letters within a column are not significantly different at 5% level of significance; CV = Coefficient of variation; LSD = Least significant difference at 5%; SE = Standard error of the means.

Table 4 .
Main effects of plant population and nitrogen rate on some growth parameters and number of pods per plant in okra.
*Length of pod bearing zone; **Number of leaves per plant; *** Number of pods per plant.

Table 6 .
Interaction effect of plant population and nitrogen rate on length of green pods of okra.

Table 7 .
Interaction effect of plant population and nitrogen rate on pod diameter of okra.

Table 8 .
Main effects of plant population and nitrogen rate on yield attributes of okra.

Table 9 .
Interaction effect of plant population and nitrogen rate on pod fresh weight of okra (t/ha).Means followed by or sharing the same letters are not significantly different at 5% level of significance; CV = Coefficient of variation; LSD = Least significant difference at 5%; SE = Standard error of the means.

Table 10 .
Interaction effects of plant population and nitrogen rate on pod dry weight of okra (t ha -1 ).]

Table 11 .
Interaction effects of plant population and nitrogen rate on above ground biomass yield of okra (t ha -1 ).Means followed by or the same letters are not significantly different at 5% level of significance; CV = Coefficient of variation; LSD = Least significant difference at 5%; SE = Standard error of the means.