Effects of seed size on seedling vigor of okra (Abelmoschus esculentus L.) in Swaziland.
Soil temperature at 10 cm depth was negatively but not significantly correlated to some of the measured parameters: seedling emergence (r = 0.499; n = 18); petiole length (r = - 0.876; n = 18); plant height (r = - 0.934; n = 18); leaf count (r = 0.947; n = 18). Plants originating from large seeds had a not-significantly higher total dry mass (mean, 0.373 g) than plants originating from medium seeds (mean, 0.340 g) and small seeds (mean, 0.336 g). It was concluded that larger seeds produced not-significantly more vigorous seedlings than smaller seeds, although the differences diminished as the seedlings grew. It is recommended that farmers select large okra seeds for establishing their gardens.
Okra as a crop
Okra (Abelmoschus esculentus L.), also known as okro, or gumbo, or lady's finger, is a warm-season, annual vegetable crop. Okra is related to cotton (Gossypium spp.), and hibiscus (Hibiscus rosa-sinensis), two species that also belong to the same Malvaceae family to which okra belongs. It is valued for its young edible pods. Okra originated in Africa and is now cultivated tropical and sub-tropical warm temperate regions worldwide . Okra is among the most heat- and drought-tolerant vegetable species, but severe frost can damage the pods. Okra leaves might be cooked in a similar way to the greens of beet root (Beta vulgaris), or dandelions (Taraxacum officinale). To avoid sliminess, okra pods are often briefly stir-fried, or cooked with acidic ingredients such as citrus (Citrus spp.), tomatoes (Licopersicon esculentum L.), or vinegar. Some people prefer the sliminess and would cook okra and eat it for the sliminess.
An average temperature of 20-30[degrees]C is considered optimum for growth, flowering and fruiting. Producing a competitive and high-yielding crop begins with the seed . According to , water salinity retards growth, yield and physiological growth parameters of okra. Smith  reported that temperature, salinity and the hard seed coat of okra could interfere with water uptake, and might become constraints to germination, establishment and performance of seedlings.
Seed size and soil temperature
Some research has shown that for cereal crops, large seeds tend to produce more yield than small seeds . Large seeds have more food storage for embryo growth and development which lead to vigorous growth of the seedling before weeds can emerge and create competition. Soil temperature could have great influence on okra germination. At soil temperatures of about 15[degrees]C, germination takes about 27 days, whereas at about 23[degrees]C, germination might take 13 days, and under ideal conditions, germination may take about week .
Define Seedling vigor here Seedling vigor is a function of seed quality and genetics . Seedling vigor might be lost when the seed is exposed to frost or mechanical injury that cracks the seed coat allowing seed-rotting pathogens to enter . Helm and Spilde  noted that weak seedlings could not withstand adverse spring growing conditions, i.e., the weak seedlings could not recover as well as strong seedlings from spring frosts heavy enough to freeze top growth. This could have a considerate impact on final crop yield.
Anonymous  noted that for cereal crops, large seeds tended to produce more competitive, higher-yielding plants than small seeds. Other research indicated various relationships between seed size and agronomic performance factors such as seedling vigour and yield . Elliott et al.  noted that small seeds produced seedlings with much less vigour. Stobbe et al.  reported that crops grown from large kernels consistently yielded higher than crops grown from small kernels of the same cultivar, for both wheat (Triticum aestivum L.) and barley (Hordeum vulgare). Seed sizing offers an important low- or no-cost way of improving yield potential, that is, if only the largest kernels are used for seeding, yield potential could be maximized.
Upadhya and Cabello  observed that seed size had some significant effects on emergence and total marketable yields; they also noted that seedlings grown from large seeds had higher survival and total tuber in Irish potato (Solanum tuberosum). Helm and Spilde  reported that in corn (Zea mays L.), the size and shape of seeds were not related to genetic yield potential of the crop; the small seeds usually came from the tip of the cob; large and flat seeds usually came from the middle, while large and round seeds came from the butt. Genetically, maize seeds from the same cob are identical .
According to Nielsen , in practice, seed size and shape can influence yield of hybrids due to differences in quality, subsequently affecting final plant population. There is very little published information on the influence of seed size on growth and performance of okra in Swaziland, hence the need for the investigation. The specific objective of this investigation was to assess the effects of okra seed size on seedling vigor and growth.
Materials and methods
Location of experiment
The experiment was conducted at Malkerns Research Station outside the screenhouse of the Plant Pathology & Quarantine Department. Malkerns is 740 m above sea level; it is located at 26.34[degrees]S, 31.10[degrees]E, with an annual rainfall between 800 mm and 1200 mm, and has a mean annual temperature of 18[degrees]C .
Experimental design and treatments
The experimental design was a randomized complete block design with three seed-size treatments, each replicated six times. The treatments were: (1), Small size - mean mass, 4.2 g; (2), medium size - mean mass, 5.8 g; and (3), large size - mean mass, 6.8 g. Seed size determination was based on the mass of 100 seeds having the mean mass stated above. There were 18 plots; each plot comprised three 10-L black plastic bags.
Planting and watering
Each plastic bag was filled with 10 kg of soil that was watered to field capacity two days before planting. Planting was done on 30 September 2010 after soaking the seeds for two days at the biology laboratory of the University of Swaziland, Luyengo campus. From each type of seed (small, medium and large), 90 seeds were rolled-up in wet absorbent cotton towels for two days under laboratory ambient temperature. After two days, seven seeds from each batch were planted out in each pot. One week after emergence, the seedlings were thinned to four per bag, to reduce plant competition. Data were collected from these four seedlings per plot. After planting, the plots were watered to ensure there was enough water for germination. Irrigation was done using a watering can fitted with a spout. Before rains fell regularly, watering was done every two days; on each occasion, the soil was watered to field capacity.
Data collection and analysis
Data were collected on the following parameters: number of emerged seedlings per day - this was counted every day, starting from four days after planting (DAP) up to 13 DAP; soil temperature--recorded between 1400 hours and 1600 hours during a sunny day, without rain, and was measured at 10 cm away from the base of the plants at 5-cm depth, and at 10-cm depth. The temperature was taken in two pots per plot. Fisherbrand bi-metal dial soil thermometers  were used in measuring soil temperature; stem diameter was measured using a verrnier calliper at 4 weeks after planting, at 5 cm above the soil level; number of leaves per plant was taken once every week, starting from five days after emergence (DAE).
Other data collected were: plant height was measured with a ruler every four days, starting from day after emergence; dry mass of plants was determined at the end of the experiment. The plants were oven-dried at 100[degrees]C for 48 hours , and the plants were then weighed to 3 decimal places using a laboratory balance; leaf area per plant was calculated using the cork-borer method, whereby 10 leaf discs were punched using a 1-cm cork borer. The formula below  was used:
Area of 10 leaf discs ([cm.sup.2]) x leaf dry mass (g) of 4 plants Leaf area ([cm.sup.2]) = Dry mass of 10 leaf discs (g).
Data were analyzed using MSTAT-C statistical package, version 1.3; mean separation was done using the least significant difference test.
Number of emerged seedlings from 4 to 13 days after planting
Table I shows the numbers of emerged seedlings at 4-13 DAP. There were no significant (p > 0.05) differences in the numbers of emerged seedlings among the treatments.
Larger seeds showed a higher emergence initial emergence at four DAP (mean, 2.12 seedlings) than medium and small seeds (means, 1.70 seedlings), but the differences were not significant. Emergence showed a steady increase from day 4-6 after emergence. From day 7, the number of emerged seedlings showed a low rate and it reached a steady state at day 9-13 DAP.
Correlation data (Table II) showed that seedling emergence was positively and significantly (p < 0.05) correlated to: plant height (r = 0.569; n = 18); petiole length (r = 0.856; n = 18); number of leaves per plant (r = 1.000; n = 18); and stem diameter (r = 0.992; n = 18).
As seen in Table III, temperatures at 5-cm depth (small-size seeds, 31.5[degrees]C; medium-size seeds, 31.5[degrees]C; large-size seeds, 31.2[degrees]C) were higher than at 10-cm depth (small-size seeds, 30.6[degrees]C; medium-size seeds, 30.2[degrees]C; large-size seeds, 30.5[degrees]C), there were no significant (p> 0.05) differences among the treatments throughout the investigation.
Correlation data (Table II) show that soil temperature at 5-cm depth was negatively but not significantly (p > 0.05) correlated to: seedling emergence (r = - 0.277; n = 18); petiole length (r = - 0.733; n = 18); plant height (r = - 0.992; n = 18); and leaf count (r = - 0.795; n = 18). There was a general decline in soil temperatures from week 1 to 6 after planting at 10 cm depth. As seen in Table II, temperatures at 10-cm depth were negatively but not significantly (p > 0.05) correlated with some measured parameters: emergence (r = - 0.499; n = 18); petiole length (r = - 0.876; n = 18); plant height (r = - 0.934; n = 18); and leaf count (r = - 0.947; n = 18).
Stem diameter at 3-6 weeks after planting (WAP)
Table IV shows data on stem diameter measurements at 4-7 weeks after planting. There were no significant differences in stem diameter among the treatments. However, seedlings grown from large seeds developed the largest diameters (mean, 3.44 mm) than those grown from medium-size seeds (mean, 3.14 mm), and those from small-size seeds (mean, 3.19 mm) but the difference was not significant. Correlation data (Table II) indicated that stem diameter was positively and significantly (p < 0.05) correlated to petiole length (r = 0.783; n = 18); number of leaves (r = 0.992; n = 18); and plant dry matter (r = 0.995; n = 18). Stem diameter was positively but not significantly correlated to plant height (r = 0.459; n = 18). Stem diameter correlated negatively with soil temperature (r = - 0.153; n = 18 at 5-cm depth, and r = - 0.386; n = 18 at 10-cm depth).
Number of leaves at 1-4 weeks after planting
Table V shows the number of leaves of okra from 1-6 WAP. There were significant (p < 0.05) differences in the number of leaves among the treatments only at week 3 after planting. Seedlings developed from large seeds showed a significantly (p < 0.05) higher number (3.6 leaves per plant) in the third week than plants from small seeds (3.2 leaves per plant). Table II shows that the number of leaves per plant (leaf count) correlated positively and significantly (p < 0.05) with plant height (r = 0.569; n = 18) and petiole length (r = 0.856; n = 18).
Plant height (cm) of okra at 1-45 days after emergence
As shown in Table VI, plant height of okra at 1-45 DAE showed no significant (p < 0.05) differences among the seed sizes. However, plants that developed from large and medium-size seeds were significantly (p < 0.05) taller (mean height of seedlings from large and medium-size seeds, 6.5 cm) than plants grown from small-size seeds (mean height, 5.8 cm). As shown in Table II, plant height was positively and significantly (p < 0.05) correlated to: petiole length (r = 0.912; n = 18); number of leaves per plant (r = 0.569; n = 18); and leaf area (r = 0.999; n = 18). The resulting coefficient of correlation showed that petiole length ([R.sup.2] = 83.2%), number of leaves per plant ([R.sup.2] = 32.4%) and leaf area ([R.sup.2] = 99.8%) were contributed to, by the respective percentages to plant height. Plant height correlated negatively with soil temperature (r = - 0.948; n = 18 at 5-cm depth) and (r = - 0.997; n = 18 at 10-cm depth).
Plant dry mass
Fig. 1 shows the dry mass of okra plants at seven WAP. There were no significant differences in dry mass among the treatments at the end of the investigation. However, seedlings originating from large seeds had a higher total dry mass (mean, 0.373 g) than plants originating from medium-size seeds (mean, 0.340 g), and small-size seeds (mean, 0.336 g).
Correlation information (Table II) showed that dry mass of okra was positively and significantly (p < 0.05) correlated to some parameters: plant height (r = 0.647; n = 18); petiole length (r = 0.995; r = 18); stem diameter (r = 0.975; n = 18); seedling emergence (r = 0.995; n = 18) and leaf area (r = 0.618; n = 18). Plant dry mass correlated negatively with soil temperature (r = - 0.999; n = 18), (r = -0.583; n = 18) at 5-cm and 10-cm depths, respectively.
Fig. 2 shows leaf area of okra at seven WAP. Though there were no significant (p > 0.05) differences in leaf area among the different treatments, plants originating from large seeds had the largest leaf areas (51.6 [cm.sup.2]), whereas the lowest leaf area (mean, 45.7 [cm.sup.2]) was obtained in plants from small seeds.
Correlation studies showed that leaf area correlated positively and significantly (p < 0.05) with some parameters: plant height (r = 0.999, n = 18); petiole length (r = 0.896; n = 18); number of leaves (r = 0.537); and seedling emergence (r = 0.537; n = 18). Leaf area correlated negatively with soil temperature (r = - 0.959; n = 18) at 5-cm depth.
Mass of seeds
Germination, survival and growth of seedlings are influenced largely by the food reserves in the seed, which increase seed mass. Upadhya et al.  noted that at times germination might be independent of seed mass, and large seeds might have a higher germination percentage than smaller ones or small ones might have a higher germination percentage, depending on the plant species. It was pointed out  that seed size effects could be statistically significant, but not agronomically meaningful.
Number of emerged seedlings
Seed size did not significantly influence emergence in okra, which contrasted with the report of Upadhya and Cabello, that larger seed size improved the rate on emergence and marketable yield in Irish potatoes. Mandal et al.  noted that in Hypatis suaveolous, variation in seed size and mass influenced emergence; large seeds showed a higher emergence potential than smaller seeds. Larger seeds were capable of emerging from greater planting depths and showed an enhanced ability to penetrate ground cover and survive burial by litter . Differences in seedling emergence could be attributed to the fact that large seeds usually have higher food reserves, and as the plants grew, they then depended on the soil for nutrients, as reported by Stobbe et al. .
Temperature plays an important role in germination by regulating moisture absorption by the seed ; if soil temperature was to be too cool, germination would be delayed, which could result in seed damage and uneven seedling emergence .
That large seeds resulted in seedlings with the greatest diameters agreed with the observations of Chiamai et al.  who noted that large seeds of mung beans (Vigna radiate) produced larger sprouts including sprout mass and head diameter characters. This was attributed to the larger food and protein reserves in large seeds.
Number of leaves per plant
Seed size did not significantly affect the number of okra leaves in this experiment, in agreement with the report of Matatudi and Mariga , who observed no significant differences in the number of leaves of Virola surinamensis, although larger seeds emerged more rapidly than small seeds. Leaves are important in plant growth and survival; they are sites of photosynthesis, a process whereby plants manufacture their food, using simple chemical substances from the environment.
Our observation that plant height correlated with seed size was in agreement with the findings of Anon. , who noted that seed size was related to plant height. Large seeds produced thick roots and stems that could grow rapidly to great depth . Lima et al.  noted that crop growth rate at the beginning of the growth cycle was higher in plants originating from large seeds.
In our investigation, seed size variations did not significantly affect dry mass produced, consistent with earlier reports of Motatudi and Mariga , who noted that there were no significant influences in dry mass of leaves in Virola surinamensis. Chiamai et al.  noted that plants often depend on the soil instead of on the food stored in the reserves of the seed; such food reserves are depleted as the plant develops.
In this experiment, seed size did not significantly influence leaf area, contrary to the observations of Khurana and Singh , who noted that seed size variations affected leaf area, large seeds producing greater leaf area. Large seeds increased leaf area production of Abizia plants, particularly at the beginning of the growth cycle .
Conclusion and recommendation
The conclusion from this investigation is that larger seeds produced not-significantly more vigorous seedlings than small seeds. It is recommended that okra farmers select large seeds for establishing their gardens.
We are grateful to Crop Production Department, University of Swaziland, and Malkerns Research Station, Malkerns, for providing facilities to conduct this research.
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Ekpo Ossom, School of Agriculture & Food Technology, The University of the South Pacific, Private Mail Bag, Apia, Samoa E-mail: firstname.lastname@example.org.
(1) Phathizwe Magagula and (2) Ekpo Ossom
(1) Crop Production Department, Faculty of Agriculture, University of Swaziland, Private Bag Luyengo, Luyengo M205, Swaziland
(2) School of Agriculture & Food Technology, The University of the South Pacific, Private Mail Bag, Apia, Samoa
Phathizwe Magagula and Ekpo Ossom; Effects of seed size on seedling vigor of okra (Abelmoschus esculentus L.) in Swaziland
Table I: Number of emerged seedlings at 4-13 days after planting Seed size Numbers of emerged seedlings and days after emergence 4 5 6 7 Small 1.73a 4.62b 6.00c 6.21a Medium 1.70a 4.68b 5.53c 6.45a Large 2.12a 5.08b 5.90c 6.37a Means 1.85 4.79 5.81 6.34 Significance at 5% Ns Ns Ns Ns Seed size Numbers of emerged seedlings and days after emergence 8 9 10 11 Small 6.53b 6.53a 6.53a 6.58b Medium 6.50b 6.62a 6.62a 6.62b Large 6.47b 6.52a 6.52a 6.52b Means 6.50 6.56 6.56 6.57 Significance at 5% Ns Ns Ns Ns Seed size Numbers of emerged Means seedlings and days after emergence 12 13 Small 6.70b 6.70b 5.81 Medium 6.67b 6.67b 5.81 Large 6.52b 6.52b 5.86 Means 6.63 6.63b -- Significance at 5% Ns Ns -- Ns, not significant; Numbers followed by the same letters in the same column are not significantly different (p > 0.05), according to the least significant difference test. Table II: Correlation matrix showing relationships among some okra parameters measured Parameter Plant Petiole length length Petiole length 0.912 * Number. of leaves/plant 0.569 * 0.856 * Stem diameter 0.459ns 0.783 * Emergence count 0.569 * 0.856 * Soil temperature @ 5-cm depth -0.948ns -0.735ns Soil temperature @ 10-cm -0.997ns -0.876ns Leaf area 0.999 * 0.896 * Plant dry mass 0.647 * 0.995 * Parameter Number Stem of leaves/ diameter plant Petiole length Number. of leaves/plant Stem diameter 0.992 * Emergence count 1.000 * 0.992 * Soil temperature @ 5-cm depth -0.277ns -0.153ns Soil temperature @ 10-cm -0.500ns -0.386ns Leaf area 0.537 * 0.425ns Plant dry mass 0.995 * 0.975 * Parameter Emergence Soil count temp.@ 5-cm depth Petiole length Number. of leaves/plant Stem diameter Emergence count Soil temperature @ 5-cm depth -0.277ns Soil temperature @ 10-cm -0.500ns 0.971 * Leaf area 0.537 * -0.959ns Plant dry mass 0.995 * -0.999ns Parameter Soil Leaf temp. @ area 10-cm depth Petiole length Number. of leaves/plant Stem diameter Emergence count Soil temperature @ 5-cm depth Soil temperature @ 10-cm Leaf area 0.500 * Plant dry mass -0.583ns 0.618 * * Significant at p < 0.05; and ns, not significant Table III: Soil temperature at 1-6 weeks after planting. Seed size Soil depth Soil temperature ([degrees]C) (cm) and weeks after planting 1 2 3 Small 5 35.6a 34.7b 31.1d 10 36.7b 34.1c 30.2e Medium 5 35.3a 34.0b 30.9d 10 35.4b 32.0c 30.1e Large 5 34.7a 34.1b 31.5d 10 35.2b 32.6c 30.0e Means 5 35.2 34.3 31.2 10 35.7 32.9 30.1 Significance -- Ns Ns Ns at 5% Seed size Soil depth Soil temperature Means (cm) ([degrees]C) and weeks after planting 5 6 Small 5 31.2a 25.1a 31.5 10 30.2b 21.7c 30.6 Medium 5 31.8a 25.3a 31.5 10 30.7b 22.8c 30.2 Large 5 30.2a 25.5a 31.2 10 30.4b 24.2c 30.5 Means 5 31.1 25.3 31.4 10 30.4 22.9 30.4 Significance -- Ns Ns - at 5% Ns, not significant at 5% confidence level; Numbers followed by the same letters in the same column of the same soil depth are not significantly different (p > 0.05), according to the least significant difference test. Table IV: Stem diameter (mm) at 4-7 weeks after planting. Weeks after Small-size Medium-size Large-size Means planting seeds seeds seeds 4 2.80a 2.88a 3.22a 3.00 5 3.19b 3.07b 3.32b 3.19 6 3.30c 3.20c 3.50c 3.33 7 3.48d 3.37d 3.70d 3.52 Means 3.19 3.14 3.44 3.26 Significance at 5% Ns Ns Ns -- Ns, not significant at 5% confidence level; Numbers followed by the same letters in the same column are not significantly different (p > 0.05), according to the least significant difference test. Table V: Number of leaves per plant of seedlings Seed size Number of leaves per week Week 1 Week 2 Week 3 Week 4 Small 2.0a 3.0b 3.2b 4.2a Medium 2.0a 3.1b 3.4b 4.1a Large 2.0a 3.0b 3.6b 4.2a Mean 2.0 3.0 3.5 4.2 Significance Ns Ns 0 Ns Seed size Number of leaves Means per week Week 5 Week 6 Small 5.3b 6.8b 4.1 Medium 6.2b 7.5b 4.4 Large 6.0b 7.2b 4.3 Mean 5.8 7.2 4.3 Significance Ns Ns -- Ns, not significant at 5% level; *, significant at 5%; Numbers followed by the same letters in the same column are not significantly different (p > 0.05), according to the least significant difference test. Table VI: Plant height (cm) at 1-45 days after emergence. Days after Seed size Mean Significance emergence Small Medium Large 1 1.7a 1.9b 1.9b 1.8 * 5 2.6a 3.0b 2.8ab 2.8 * 9 3.4a 4.0b 3.7c 3.7 ** 13 4.1a 4.5a 4.6a 4.4 Ns 17 4.7a 5.7b 5.7b 5.4 ** 21 4.9a 6.0b 6.0b 5.6 ** 25 5.1a 6.1b 6.1b 5.8 ** 29 6.1a 7.1b 7.2b 6.8 ** 33 7.9a 9.2a 8.6a 8.6 Ns 37 9.1b 9.9b 10.1b 9.7 Ns 41 9.8c 10.1c 10.8c 10.2 Ns 45 10.3a 10.5a 11.0a 10.6 Ns Mean 5.8 6.5 6.5 5.5 -- *, significant only at 5% level; **, significant at 1% level Ns, not significant; Numbers followed by the same letters in the same rows are not significantly different (p > 0.05), according to the least significant difference test. Fig. 1: Dry mass (g) of okra per plant at seven weeks after planting. [LSD.sub.0.05] = 0.100 Seed size Small 0.336 Medium 0.34 Large 0.373 Note: Table made from bar graph. Fig. 2: Leaf area ([cm.sup.2]) of okra seedlings at seven weeks after planting and grown from different seed sizes. [LSD.sub.0.05] = 21.94 Seed size Small 45.682 Medium 51.318 Large 51.607 Note: Table made from bar graph.