Performance of Okra(Abelmoschus esculentus L. Moench) under Different Irrigation Frequencies

Introduction Okra (Abelmoschus esculentus L. Moench) is a perennial vegetable of African origin, with production areas expanding throughout the tropical, sub-tropical and warm temperate regions of the world (Benchasri, 2012). Okra is a versatile crop produced forits pods, leaves, seedoil and protein, gums, and fiber in different parts of the world (Lamont,1999). It is produced in Vilanculos primarily for the immature pods, though leaves are consumed to a limited extent in rural communities. Important attributes of okra pods are shape and color, earliness, and totalmarketable yield. Since okra is handharvested, plant height and architecture,as well as the absence of spines are important tofacilitate harvesting (Simonne, et al, 2012).Okra is not only reach in nutrients (fats, proteins, carbohydrates, minerals and vitamins), but it is also believed to have medicinal properties. The climate in much of Mozambique presents favorable conditions for the production of Okra (INE, 2011) and because of the growing preference for Okra among consumers in Mozambique, its area of production had rapidly expanded to every part of the country, mainly in the provinces of Tete, Manica, Sofala and Inhambane. Okra provides a good alternative or supplemental income for smallholder farmers (COSTA et al., 1981) and its production in the district of Vilanculos requires the application of considerable amount of irrigation water to improve productivity. Small farmers-led irrigation in Mozambique takes place in diverse forms, including bucket irrigation, sprinkler and drip systems, furrow and small pumped irrigation systems. However, Mozambique has limited access to raw water supplies and the country as a whole is extremely vulnerable (ranks 3rd amongst African countries) to extreme weather patterns (i.e. recurring droughts and flood events), which contribute to crop instability, food insecurity and malnutrition (USAID, 2010). However, information on watering (irrigation) frequencies for optimal growth and development of this valuable crop under the edaphic-climatic conditions in the district of Vilanculos is lacking. Climate-smart strategies to increase irrigation efficiency at small-farm level are essential in helping rural smallholders optimize production of okra and other vegetable crops under climate change. Materials and Methods 1. Geographical location The field experiment was carried out between October and December 2016 on the campus of the Universidade Eduardo Mondlane-Escola Superior de Desenvolvimento Rural (UEM-ESUDER) located in the coastal town of Vilanculo within the district of Vilanculos, Province of Inhambane in Mozambique at coordinates 21º59’30.6” S, 035º16’14.8” E. and an average altitude of 49m. The district of Vilanculosoccupies the northern part of the province of Inhambane, bordering the district of Inhassoro to the north, the district of Massinga to the south, the districts of Mabote and Funhalouro to the west and the Indian Ocean to the east (MAE, 2005). 2. Climatic condition The climate of the study area is Aw according to the Köeppen e Geiger, which corresponds to a dry tropical climate with two distinct seasons. A wet season spans from October to March, with an average annual precipitation of 1300 mm; and a dry season from April to September, with an average annual precipitation of 700 to 900 mm (MICOA, 2009 and MAE, 2005).Figure-1 illustrates the average temperature and precipitation patterns in Vilanculos. The driest month is July, which averages 17 mm of rainfall, and the month of February is the wettest with an average precipitation of 166 mm. On average, the temperatures are always high, with an annual average of 24oC. 3. Soils of the study area The soils in the district of Vilanculos are sandy and permeable in the coastal areas, and sandy-loam to loamy-clay in the interior. The study site was approximatively 10 km from the coastal line and its soils are predominantly sandy and permeable with low organic content (8.55%) and low water holding capacity (less than 5cm/m) (MAE, 2005; Maite, 2014). The growing season averages 120 to 149 days, and because of low precipitation and recurring drought periods during the growing seasons, the area’s potential for rain-fed agriculture is marginal (Mafalacusser, 2013.) Figure 1. Monthly temperature and precipitation patterns in the district of Vilanculo, Mozambique. Adapted from Climograma Vilanculos: https://pt.climate-data.org/location/52395/ 4. Experimental design The experiment was laid out on an area of 83.375m2 within which 3 blocks of equal sizes were setup, each block divided into 4 parcels of 4 m2 (2 m x 2 m) for a total of 12 parcels. The blocks were setup at 1m apart and the parcels within each block were spaced at 50cm. Treatments were then randomly assigned to each parcel, resulting in each parcel representing a unique treatment within each block. Table-1 describes the different treatments (irrigation frequencies) used in the experiment. Table 1. Treatments details: Four (4) treatments randomly assigned to the experimental blocks. IF1 Irrigate twice a day in the mornings (5-6 AM), and afternoons (4-5 PM) IF2 Irrigate once a day in the afternoons (4-6PM) IF3 Irrigate once a day in the mornings (5-6 AM) IF4 Irrigate twice a day in the mornings (5-6 AM), and afternoons (4-5 PM) at 1- day intervals. 5. Soil preparation and planting Soil preparation consisted of removing natural vegetation from the experimental site with a manual hoe without revolving the top soil, and the soil surface was raked and leveled prior to subdivision of the experimental area into parcels, blocks and replications for the study.Seeds of okra (Abelmoschus esculentus L. Moench) variety Clemson Spineless, were soaked in water for 24 hours to stimulate germination, and then sown by placing two (2) seeds in each planting hole at an approximate depth of 5cm in each parcel. Fifteen (15) days following the emergence, the seedlings were thinned by removing the least developed plant in each planting hole, leaving only one (1) okra seedling per hole in each treatment. The seedling selection criteria was based on height, and the number of developed leaves. 6. Soil fertilization Organic fertilizer in the form of bovine manure was applied seven (7) days prior to sowing and 15 days after seedling emergence, at the rate of 15 tons/ha each. All plants received the same fertilizer application. 7. Irrigation A hand-held irrigation bucket was used for applying water to the experimental plots, a watering scheme consistent with the most popular irrigation system among smallholders in the study area. All parcels were uniformly irrigated with 13 liters of water, starting from one (1) day prior to seeding until fifteen (15) days after seeding when the okra plants developed up to three (3) fully expanded leaves. Starting from day-15 through the end of the experiment, plants in each parcel were irrigated according to the established watering schedule (Table-1.) 8. Pests/Weed Control Weed control was manual and continuous throughout the study, by hand plucking at the onset of their appearance to avoid perturbation of soil and plants. The most common weeds were amaranthus, commelina benghalensis and cyperus esculentus species, with amaranthusandcyperus esculentus being the most dominants.Observed pests were acarids and caterpillars, and these were treated with pesticides Cipermetrina 25% E.C. and Fords. The occurrence of fungi during the experiment was sparse, but fungi was treated with the fungicide Bravo. Data Collection 1. Pre-harvest Data Pre-harvest plant parameters consisted of plant height and stem diameter. Plant height was determined by measuring the height of the primary stem of random sample of plants in each treatment from the base at soil surface to the apex, using a metric ruler. At the same time, stem diameter was measured with a digital Caliper, within 300mm at 5 cm above the soil surface. Growth Rate of okra plants was determined from height measurements and expressed as Absolute Growth Rate (AGR), based upon increments of measured plant heights between sampling dates (i.e. Height at day2- Height at day1) over the time-period (number of days) between measurement dates. 2. Post-harvest Data Post-harvest data included Fruit Length, Fruit Diameter, and Fruit Weight. Okra fruits were harvested twice; the first harvest took place 90 days after seeding, and the second harvest 8 days later, or 98 days after seeding. Harvest data were from random fruit samples under each treatment for both harvests. A graduated metric rulerwas used to measure fruit length, while fruit diameter was measured with a digital Caliper within at 5 cm from thebase of each fruit, and fruit weight was determined by weighing sample fruits per treatment from each harvest, with a digital scale equipped with a high precision strain gauge sensor system. Data Analyses All data collected from the experiment were analyzed with the statistical software SPSS(v.20, IBM SPSS Chicago). Analyses of Variance (ANOVA) were performed to determine differences between treatments, followed by mean comparisons based on Tukey HSD (α = 0.05). Homogeneity of the variances was verified through the test of Levene (α = 0.05), and graphical illustrations were produced with the Microsoft Excel program. Result and Discussion 1. Plant Height The different irrigation treatments had no significant effect on plant height at each sampling dates. Okra plants grown in this experiment averaged 4.73cm at 7 days after sowing (DAS), and increased through the study to an average of 49.39 cm at 63 DAS (Fig.2.) In contrast with other studies, okra plant height at the end of this experiment was relatively shorter than heights observed elsewhere. For example, Saifullah and Rabbani (2009) reported from evaluation of different genotypes that okra plant height at final harvest ranged from 81.80cm to 196.17cm. It is probable that the conditions under which the present study was carried out did not favor greater shoot elongation of the okra plants. However, significant differences in plant heights were observed between sampling dates according to the growth progression of plants through the study period, and between treatments across sampling dates. As illustrated in Figure 2, plant heights during the first three (3) sampling dates (7-21 DAS) were significantly lower than plant heights at other sampling dates. Figure 2. Average plant height per treatment per sampling date Furthermore, plant heights at 28-35 DAS was significantly lower than heights at 63 DAS. Overall, plant heights continuously increased during the study period, except under IF2 (watering once a day in the afternoons), where plant height leveled off between 28 and 35 DAS, before increasing again through the end of the study period (Fig.2.) Starting from 35 DAS to 63 DAS, okra plants height under IF2 increased rapidly to match heights of plants under IF1 and surpass heights of plants under IF3 and IF4 at 63 DAS. Figure-3 further shows okra plant height averaged across treatments at each sampling date. Okra plant height followed a rather exponential pattern from 7 DAS until 35 DAS, and then the growth pattern became linear thereafter. Significant differences in plant heights were observed during the first phases of plant growth between 7 DAS and 21 DAS, and between the set of 7 DAS - 21 DAS and the remainder DAS. However, no significant differences in plant height were observed between sampling dates from 28 DAS through 63 DAS (Fig.3.) Figure 3.Average plant height per sampling date across the treatments Figure 4 illustrates the average plant height per treatment across sampling dates. Plants grown under IF3 (watering once a day in the mornings) were significantly taller (P<0.05) than those under IF2 and IF4. Although watering plants twice a day (IF1) resulted in plant heights similar to those under IF3, these results suggest that in terms of okra plant growth, watering once a day in the mornings (IF3) was more efficient in promoting plant growth with less use of irrigation water, compared with IF1 (watering twice a day.) Figure 4.Average plant height per treatment across sampling dates Moreover, plants grown under IF4 which received irrigation water twice a day but at one-day intervals, significantly outperformed those under IF2 (watering once a day in the afternoons) in terms of plant height. Given equal amounts of irrigation water applied but at different intervals in or IF2 and IF4, the treatment IF4 appeared to be more efficient in promoting okra plant growth as compared with IF2 (Fig.4.) 2. Plant Growth Rate Growth Rate of okra plants was determined from height measurements and expressed as Absolute Growth Rate (AGR), based upon increments of measured plant heights between sampling dates (i.e. Height at day2- Height at day1) over the time-period (number of days) between measurement dates. Figure 5 illustrates the AGR pattern of okra plants under each treatment at each sampling date, with dates expressed in number of days after sowing (DAS). Figure 5. Average Plant Growth Rate (cm/day) per sampling date across the treatments As illustrated in Fig.5, okra plants grown under IF1 and IF3 exhibited relatively similar AGR pattern during much of the study period, while those under IF2 and IF4 constitute the other pair of similar AGR. Averaged across treatments, the AGR of okra plants under this study was 0.568 cm/day at 14 DAS and then increased significantly (P<0.05) to a maximum rate of 2.346 cm/day at 28 DAS. After 28 days or so of growth have passed, the AGR dropped to a low of 0.309 cm/day at 35 DAS, before leveling off to an average rate 0.536 cm/day for the remainder of the growth period (Fig.6.) Figure 6. Average Growth Rate per sampling date across treatments Not many studies appear to have been carried out on okra growth rate, but the AGR pattern exhibited under this study was similar to those observed in previous experiments (Hunt, 1978; Tarassoum and Lovane, 2019.) Similar to previous studies, the reduction in AGR after 28 DAS have passed was attributable to Phenology and/or resource allocation to functions other than plant height.Comparisons of AGR between treatments across sampling dates showed that the average AGR was significantly faster (P<0.05) under IF2 and IF4 when compared with IF1 and IF3 (Fig.7), with no significant differences observed between the respective pairs of treatments.Strikingly, the two treatments (IF1 & IF3) which produced taller plants (Fig.4) resulted in relatively lower overall AGR during the study period, compared with the pair of IF2 and IF4, which resulted in shorter plants. While plant height under IF2 and IF4 were lower during the first 21 days and after 35 DAS have passed (Fig. 3), the significantly rapid increase in their heights between 21 and 28 DAS resulted in an overall greater AGR compared with the other treatments. Figure 7. Average Growth Rate per treatment across sampling date 3. Plant Diameter Table 5 presents the average plant diameter under each treatment. The different irrigation frequencies had no significant effects on okra plant diameter, which averaged from 7.83 cm to 7.86 cm across the treatments. Table 5. Average plant diameter per treatment across sampling dates Treatments (Irrigation Frequency) Average Plant Diameter (cm) IF1 7.83ª IF2 7.83ª IF3 7.86ª IF4 7.83ª Average Treatments 7.85 Means followed by the same letters are not significantly different (P>=0.05) The average stem diameter of okra plants under IF3 was slightly above the diameter under other treatments, but the difference was not significant (P>0.05.) However, like plant height, plant diameter also increased exponentially from an average of 2.5cm on the first sampling date (7 DAS), to about 10.0cm on the fourth sampling date (28 DAS) and remained constant for the remainder of the study period (Fig.8.) Figure 8: Average plant diameter per sampling date across treatments 4. Fruit Characteristics Summarized in Table-6 are the okra fruit characteristics under the different irrigation frequencies. Fruit diameter, length and weight under this experiment averaged 0.1852 cm, 11.83 cm and 0.04625 kg per pod across the treatments. Mateus (2011) reported that fruit characteristics of Okra (Abelmoschus Esculentus) Clemson variety, averaged 1.7 cm in diameter, 7.5 cm in length and 10g in weight. Other studies reported fruit diameter from 1.26 to 2.86cm, fruit length from 5.46 to 17.25 cm, and individual fruit weights from 0.01528 to 0.02615kg (Owolarafe and Shotonde, 2004; Saifullah and Rabbani, 2009.) Considering data from these previous studies, it appeared that okra plants under the present experiment were shorter but produced heavier and taller fruits compared with the above referenced study results. The variability in okra fruit characteristics is attributable to genotypes, experimental designs or climate variances amongst study areas. Table 6. Average fruit length, diameter and weight under different irrigation frequencies. IRRIGATION FREQUENCIES VARIABLES Fruit Length (cm) Fruit Diameter (cm) Fruit Weight (kg) IF1 13.0208a 0.2000a 0.060a IF2 11.3542b 0.1913ab 0.041ab IF3 11.2417b 0.1704b 0.046ab IF4 11.7083 b 0. 1813b 0.038b AVERAGE 11.83125b 0.1852ab 0.04625ab Means followed by the same letters in a column are not significantly different (P>=0.05) As shown on the Table-6, irrigating plants twice a day (IF1) resulted in significantly longer pods (P< 0.05) compared with all the other treatments. IF4 resulted in a slightly longer fruits when compared with plants grown under IF2 and IF3, but the differences were not statistically significant. Fruit diameter was similar under IF1 and IF2, with no significant differences observed between IF2, IF3 and IF4, although IF4 resulted in a slightly larger fruit diameter than IF3. As for fruit weight, plants grown under IF1, IF2 & IF3 produced fruits of similar weights. The only statistical difference observed was between IF1 and IF4, with the later treatment resulting in significantly lower fruit weight. The results from this study suggest that even though IF1 resulted in longer fruits, the non-significant differences between IF1 & IF2 in terms of fruit diameterand between IF1, IF2 & IF3 in terms of fruit weight, irrigating okra plants once a day (IF2 or IF3) could yield marketable fruits while at the same time reduce the amount of irrigation water. Conclusions Okra is produced in the district of Vilanculos primarily for its immature pods, which are either sold in local market or consumed by the producing households. Smallholders comprise the vast majority of okra producers in the study area, growing small plots of various sizes under bucket or furrow irrigation systems to improve growth and yield. Differences in Okra plant characteristics (height and growth rate) under this study appeared to show variation between pairs of treatments, with IF1 & IF3 constituting one pair, and IF2 & IF4 constituting the other pair. Interestingly, the taller plants exhibited lower average growth rate when compared with the shorter plants (Fig.3 & 6.) Differences in fruit characteristics were also observed between plants grown under different watering frequencies. While IF1 resulted in longer okra pods compared with the other treatments, fruit diameter was similar under IF1 and IF2, and fruit weight similar under IF1, IF2 &IF3. These results seemed to suggest that watering okra plants once a day in the mornings (IF3) offer better alternatives for irrigation water-saving strategy for optimal plant height and fruit weight. Additional studies will further assess the once-a-day irrigation schedules at different day-intervals to determine optimal water-saving irrigation schedule for okra production in the district of Vilanculos