Introduction

Watermelon Citrullus lanatus L. is a member of the family Cucurbitaceae. This family includes cantaloupes, cucumbers, pumpkins, squash, zucchini and butternut. Watermelon is a vine-like (scrambler and trailer) flowering plant originally from Southern Africa. Its fruit which is called watermelon is a special kind referred to by Botanists as pepo, a berry that has a thick exocarp and fleshy mesocarp and endocarp.

Watermelon is a highly relished fruit salad all over the world. This is because watermelon is low in calories but highly nutritious and thirst-quenching. The fruit also contains vitamin C and A in form of the disease fighting beta-carotene, Lycopene and beta-carotene compounds which are not found in vitamin/mineral supplements ((Maynard, 2001; FAO, 2010). Potassium is also available which is believed to help in the control of blood pressure and prevention of heart attack. Watermelon fruit contains about 6% sugar and 91% water by weight. As with many other fruits, it is a source of vitamin C and amino acid called citrulline (Mandel et al., 2005). Watermelon is highly diuretic and contains large amounts of carotenoids (The Associated Press, 2008; Ibrahim, 2011).

Watermelon is cultivated all over tropical climate the largest producers are China, Turkey, Iran, Brazil and USA in that order. It is widely cultivated in both the rainforest and the humid and dry savanna agroecologies of Nigeria (Edoma, 2013). Watermelon has been found to the Lycopene leader among fresh produce. When it became clear to farmers that watermelon is highly beneficial, many have diversified from its subsistence cultivation to commercial scale. Watermelon requires warm climate and relatively long growing season (Maynard, 2001; Charles, 2005; Anon, 2006). On wet soils, the sugar content of the fruit is diluted and further internal pressure due to excess absorption of water could lead to cracking and spoilage of the fruit thus edibility and market yield significantly reduced (Boyhan and Masiunas, 1999; Lacier and Plummer, 2003). Worldwide, the yield of watermelon ranges between 5-60 t/ha and averages about 25 t/ha depending on cultivar and cultural practices. Seed yields of watermelon are 150-400 kg/ha for most cultivars (FAO, 2010).

The high demand of this fruit is not met in the southern part of Nigeria because of the high rainfall covering many months of the year. However, Okonmah et al. (2011) reported that Sugar baby as adaptable variety for rainforest agroecology of southern Nigeria. Five watermelon varieties were examined for adaptation to the early and late rainy season cropping opportunities in addition to their responses to fertilizer types in terms of growth, yield and nutritional quality in a rainforest ecological zone of Nigeria. The specific objectives are to: examine Varietal and manurial effects on biomass accumulation, fruit yield and nutritional quality of watermelon varieties and identify attributes associated with seasonal adaptation and productivity among the tested varieties.

1 Materials and Methods

1.1 Experimental Site and Conditions

Attempts Five watermelon varieties were selected on the basis of contrasting growth duration and fruit yield and were evaluated for seasonal adaptation and productivity during the rainy and late cropping season of 2012. The study was conducted at the Teaching and Research Farm of the Federal University of Technology Akure, located in the rainforest vegetation zone of Nigeria.

The experiments were carried out between June and August, and September to December 2012 for rainy and late season crop respectively. Pre-planting soil analysis was carried out, and soil samples were collected randomly from the experimental site, bulked and taken to laboratory for routine chemical analysis. The experimental site was cleared, ploughed and harrowed before planting, followed by herbicide spraying to control weeds before planting was done. The experiment was laid out in a Randomized Complete Block Design with three replications.  The experimental site was a field plot measuring 74 by 10 m with an alley way of 2 and 1 m between blocks and plots respectively. Seeds were sown at a spacing of 1x2 m on 45 plots each measuring 2 by 4 m.

1.2 Source of Experimental Materials and Treatments

The five varieties tested were (Crimson sweet, Charleston Grey, Kaolac, Anderson and Sugarbaby) while the manures were organomineral fertilizer (OMF) and NPK fertilizer, and the unmanured which is the control. For each season (rainy and late), treatment consisted of factorial combina tions of five watermelon varieties by 3 manure types in three replications. The Seeds of three of varieties were procured from National Institute of Horticulture (NIHORT) Kano while two were obtained from Akure. The manures (OMF & NPK) were applied at 5 t/ha and 200 kg/ha respectively at 3 weeks after planting as spot application. The composition of the manure used is as follows; NPK 15:15:15 and Sunshine organomineral fertilizer OMF, a product of natural compost fortified with additional amounts of nitrogen and phosphorus in the combination N 3.5%, P2O 2.5% and K2O 1.5%.

1.3 Weed and Pest Control

The Weed control was by both manual weeding at 3, 7 and 10 weeks after planting and chemical control which was done three times during the period of experimentation. The herbicide used was glyphosate and applied at the rate of 3 l/ha. Pests were controlled through application of cypermethrin (500 l/ha) and spraying commenced at the onset of flowering and at weekly interval till the end of the experiment.

1.4 Data Collection

Data collection on growth and yield parameters commenced from the third week after planting 3(WAP) and was terminated at the 12WAP when fruits were harvested. Data were taken on the number of leaves, vine branches, flowers and fruits per plant; vine length per plant. Also observed were the number of cracked and rotten fruits per plant; fruit girth and length and fruit weight per plant. The number of leaves, vine branches, flowers and fruits per plant were determined by a visual counting of leaves, vine branches, flowers and fruits on the watermelon plant. The vine lengh per plant was determined using a measuring tape to measure the length of the main vine from its attachment to the root on the soil to the tip of the vine of the watermelon plant. The number of fruit rots and cracked fruit per plant were determined by visual counting of rotten fruits and cracked fruits that were on the watermelon plant before harvesting of fruits. Fruit length and diameter was measured using a measuring tape around the outer part of the fruit when it has been detached from the main plant. The fruit weight was determined as the fruit was placed on a measuring scale to give the weight of each fruit in kilogrammes. Samples of harvested fruit from each treatment were taken to the laboratory for analysis of chemical and proximate composition. The chemical and proximate constituents measured include N, P, K Ca and Mg, moisture content, ash, crude fibre and ascorbic acid were determined using standard laboratory procedures and methods as described by AOAC (2002).

1.5 Data Analysis

The statistical significance of the data collected on growth and yield were subjected to analysis of variance (ANOVA) to estimate the variance components due to manure types and watermelon varieties and their interactions on the variables examined (Steel and Torrie, 1997). Treatment means were separated using Duncan’s New Multiple Range Test (DMRT).

2 Results

2.1 Initial Soil Physical and Chemical Properties

The data on the initial physico-chemical properties of the soil of the site of experiment are presented in Table 1.  The particle size fraction showed that the soils are sandy clay loam in texture and low in fertility as reflected by the low content of organic matter (3.87g kg _^-1) and total nitrogen (0.94%). The soil pH (5.52) shows that the soil is acidic. The available phosphorus (P) with mean value of 19.60mg kg^_-1 was adequate. However, soil potassium (K) content with mean value of (0.25cmol/kg) was low (FMANR, 1990).

2.2 Effect of variety on growth and yield characters of watermelon

For both rainy and late season crops, the measurements which were made on vegetative growth characters of the tested varieties of watermelon are presented in Table 2 and Table 3. There were significant (p<0.05) differences in the number of leaves and vine length produced per plant among the varieties in the rainy season crop (Table 2). Charleston Grey produced the highest, but was not significantly different from those of Kaolac. However, these two varieties produced significantly more leaves and longer vine length than Crimson sweet, Anderson and Sugarbaby. Anderson had the lowest number of leaves and vine length respectively. In the late season crop, variety had no significant effect on both the number of leaves and vine length per plant of watermelon (Table 3), and Crimson sweet had the highest values for these. In the rainy season crop, the effect of variety on number of vine branches per plant was not significant (P>0.05) (Table 2). Charleston Grey produced the highest number of branches. Varietal differences were also not significant for number of vine branches per plant in the late season crop, and Crimson sweet produced the highest number while Sugarbaby produced the least (Table 3). In the rainy season crop, flower production per plant was highest in Charleston Grey although it was not significantly (P =0. 05) different from those of Kaolac and Crimson sweet (Table 2). Anderson and Sugarbaby produced significantly lower number of flowers per plant compared with the former two varieties (Figure 1). No significant varietal difference occurred for number of flowers per plant in the late season crop (Table 3). The number of fruits produced per plant in the rainy season was similar in Anderson and Kaolac and significantly (P=0.05) more than those of other varieties, particularly Crimson Sweet (Table 2). The reverse occurred in the dry season crop where Anderson produced significantly lower number of fruits per plant relative to the other varieties which had similar values (Table 3). There were significant differences in fruits rots per plant among varieties. Fruit rot occurred among the varieties in a descending order of Charleston Grey > Kaolac >Anderson > Crimson sweet > Sugarbaby in the rainy season crop (Table 2). Incidence of cracked fruits was obtained only in Kaolac and Charleston Grey, and it was more pronounced in the former variety. In the late season crop, cracked fruit occurred only in Crimson Sweet, Kaolac and Sugarbaby and it was more pronounced in the latter variety (Table 3). Crimson sweet produced fruits with the shortest mean length in the rainy season crop while Charleston Grey produced fruit which were significantly (P=0.05) longer than all the other varieties except Kaolac (Table 2). Fruit lengths in all the varieties were not significantly different in the late season (Table 3). In the rainy season crop, mean fruit diameter was similar in Kaolac, Anderson and Sugarbaby, and significantly (P=0.05) lower values were obtained in Crimson Sweet and Charleston Grey respectively (Table 2). In contrast, Crimson Sweet had the widest fruit diameter in the late season crop and differences with Kaolac, which had the least fruit diameter, was pronounced (Table 3). Although Anderson had the heaviest mean fruit weight in the rainy season crop, this was not significantly different from values for  Charleston Grey and Kaolac, while those for Crimson sweet and Sugarbaby were significantly lower (P=0.05)(Table 2).  In the late season crop, the highest mean fruit weights was obtained in Charleston Grey and these differed significantly from values for Anderson, Kaolac and Sugarbaby varieties (Table 3).

2.3 Effects of manuring on growth and yield characters of watermelon

In the rainy season, manure types had no significant (P=0.05) effect on the number of leaves and flowers per plant produced respectively by watermelon (Table 4). However NPK fertilizer significantly enhanced vine length per plant, number of vine branches per plant, number of flowers per plant, number of fruits per plant, number of cracked fruit per plant, mean fruit length, mean fruit diameter, and mean fruit weight respectively. In the late season, manorial type had no significant (P=0.05) effect on most growth and yield characters measured (Table 5). However organomineral fertilizer enhanced (P=0.05) both the number of fruit rots per plant and cracked fruit per plant, NPK fertilizer enhanced mean fruit diameter while the control treatment (unmanured) enhanced mean fruit weight.

2.4 Effects of variety and manuring on growth and fruit yield characters of watermelon

Table 6 and Table 7 show the combined effect of variety and manuring on growth and yield characters of watermelon for both rainy and late season crops. The results show that treatment effects were not significant on growth parameters (number of leaves per plant, vine length per plant and number of vine branches per plant) except for unmanured Anderson where significantly lower values were obtained. Charleston Grey in combination of NPK fertilizer gave the highest values for rainy season crop excepting Kaolac in combination of NPK which produced the highest number of vine branches in the rainy season crop. In the late season there was no significant difference in the number of leaves per plant, vine length per plant and number of vine branches per plant in terms of response to manuring. Unmanured Crimson Sweet and Anderson produced the highest and lowest values of number of vine branches per plant respectively. The measured fruit yield parameters, which included number of fruits, fruit rots and cracked fruits per plant, mean fruit weight and mean fruit diameters, differed among varieties in both rainy and late season sowing.

In the rainy season crop, the highest number of fruits was obtained for Crimson Sweet under NPK fertilizer application, Anderson under both organo-mineral fertilizer and unmanured control and Sugarbaby under organo-mineral fertilizer application. The highest number of fruit rots is recorded for Charleston Grey for both manured and unmanured treatments. Sugarbaby shows no record of fruit rot for manure types tested. There was no significant effect of manuring on number of cracked fruits except for unmanured Kaolac. Mean fruit length, mean fruit diameter and mean fruit weight respectively showed significant response to organo-mineral and NPK fertilizers with the highest values recorded in Crimson Sweet, Charleston Grey and Anderson.

For late season crop, result showed that Charleston Grey recorded the highest number of fruits per plant for NPK/ fertilized plot. The effect of manure application on fruit rot was similar among the varieties. Cracked fruits did not occur in the manured treatments except for Crimson Sweet (unmanured), Kaolac (organo-mineral) and Sugarbaby (unmanured and organomineral). The widest fruit diameter was obtained for Anderson (NPK fertilizer) while the biggest fruit weight was obtained for Charleston Grey (unmanured).

2.5 Effects of variety and manuring on chemical and proximate (nutritional) composition of watermelon

The results of chemical and proximate (nutritional) quality of watermelon for both rainy and late season crops as affected by variety and manure types are presented in Table 8 and Table 9.

For the rainy season crop, NPK application produces increase in nitrogen content in Crimson Sweet and Charleston Grey, compared to control, while Anderson, Kaolac and Sugarbaby highly responded to organo-mineral fertilizer application and (Table 8). In the late season, nitrogen concentrations in fruit tissues were mostly not affected by manuring except in Sugarbaby which has enhanced nitrogen content on application of NPK fertilizer.

About 76% increase in phosphorus content due to use of NPK fertilizer was obtained in Charleston Grey compared to OMF and control, while OMF enhanced P content in Kaolac in the rainy season crop. In the late season crop, application of NPK fertilizer increased P content in Charleston Grey, Anderson and Sugarbaby, and the highest % increase of 185.42% increase were obtained for Anderson compared to OMF and control. Crimson sweet and Kaolac responded to OMF application which leads to higher percentage increase in phosphorus.

Charleston Grey and Sugarbaby responded to NPK application compared to control and OMF. In the late season crop, the effect of manurial treatment in most of the varieties with respect to potassium content was not enhanced except in Anderson with NPK application giving a very great increase.

Application of NPK enhanced Ca and Mg content in Sugarbaby compared to control and OMF for rainy season crop, while OMF enhanced Ca and Mg content in Anderson compared to control.   In late season crop, OMF application increased Ca in Charleston Grey, Kaolac and Anderson compared to control and NPK, while NPK application increased Ca in Crimson sweet, Charleston grey, Kaolac and Anderson. In the rainy season crop slight increases in moisture content in Crimson sweet, Kaolac and Charleston Grey were obtained for NPK application and in Charleston Grey in the late season. Higher moisture content was obtained in Kaolac under NPK application and Sugarbaby with OMF and NPK application (Table 9). In the rainy season crop, manure application did not give increase in the ash content in all the tested varieties. However in the late season, the varieties responded as regards ash content and Sugarbaby in particular had highest ash content when NPK was applied. The results show that crude fiber content was not enhanced  reduce in Crimson Sweet, Charleston Grey, Kaolac and Sugarbaby  when NPK fertilizer was applied in rainy season, likewise application of OMF  except for Anderson and Sugarbaby where 0.76 and 0.86% crude fibre were obtained with OMF application.  In late season crop, NPK enhanced crude fibre in Sugarbaby.

Total Solid in fruits of Charleston Grey and Sugarbaby were enhanced under NPK and OMF application respectively. In the late season, increases in total solids were obtained in Crimson sweet, Charleston Grey and Anderson with manuring, where Kaolac and Sugarbaby shows no response to manure application. Unmanured Sugarbaby had the highest vitamin C content (92.31 mg/100 g), followed by Crimson Sweet and Anderson. These varieties show no or little response to manure application (Table 8). The highest increase in vitamin C was observed in Kaolac with NPK application. The vitamin C content in Charleston Grey responded to both OMF and NPK application. In late season crop, unmanured Anderson had the highest vitamin C while Crimson sweet, Kaolac and Sugarbaby responded to manure treatment compared to control. Charleston Grey and Anderson show no response to manurial treatments in terms of vitamin C content in fruits. 

3 Discussion

The effects of variety and manure types were significant on growth, yield and nutritional qualities of watermelon in both rainy and late season crops. In general, water melon varieties responded differently with respect to their growth and yield characters to the application of manure compared to control. These varied responses can be attributed to differences in the genetic makeup or potentials of the tested varieties.

In the late season, variety had no significant effects on the number of leaves per plant, vine length per plant and number of vine branches per plant in watermelon. However, these parameters were outstanding for Crimson Sweet during the late season, and this could be related to the ability of Crimson sweet to better adapt to the environmental condition of the late season. It may also indicate the differing ability of the variety for water uptake and use efficiency of nutrient and light (Wakindiki et al., 2011; Sensoy et al., 2012). In rainy season, Charleston Grey covered the ground faster, while Crimson Sweet grew very rapidly in the late season compared with other varieties. This could have increased their ability and competitive ability and tolerance of season weather conditions (Chen et al., 2004; Agele et al., 2008).

The highest number of flowers was also obtained in Charleston Grey which was not significantly different from Kaolac and Crimson Sweet, while Anderson still had the lowest number of flowers and was not significantly different from those produced by Sugarbaby and Crimson Sweet. The numbers of flowers produced by the tested varieties were not significantly different for rainy season crop. Crimson sweet produced the highest number of flowers in the late season and this response further supported the better tolerance ability of adverse conditions of the late season by this variety (Sensoy et al., 2012). Non consistent trend were found for fruit yield components which include number of fruits, number of fruit rots and cracked fruits, fruit weight, fruit length and diameter among the tested varieties. Kaolac and Anderson recorded the highest number of fruits while Crimson Sweet produced significantly low fruits compared to other varieties in the rainy season. The number of fruits produced in Anderson contrasted with its low prior vigour of growth. This trend shows that the fewer numbers of flowers produced in Anderson were set to fruit. Other varieties had higher number of flowers but poor fruit setting which may be due to high fruit abortion and other physiological dysfunction during rainy season. In both rainy and late season, Kaolac produced the highest number of fruits, presumably due to better adaptation to season growing environmental conditions. Charleston Grey which recorded highest number of flowers but low fruit setting had high number of cracked and rotten fruits, Although Charleston Grey is more vigorous in growth, it produced poor fruit yield in the rainy season. The fruit sizes are depicted by the fruit length and fruit diameter in all the tested varieties.  In the rainy season the fruits of Charleston Grey were the biggest in length, while Anderson has the widest fruit diameters but not significantly different from Kaolac and Sugarbaby. Crimson Sweet produced highest fruit diameter in the late season crop in contrast to its lowest number of fruits produced. Fruit weight was highest in Anderson for rainy season crop, while Charleston Grey is highest in the late season crop. Hence, Anderson can well tolerate the rainy season conditions of site of study which can be related to its ability to produce larger/heavy fruits which are preferred qualities for farmers in terms of income. Fruit yield is a function of genes expression and in response to the growing environmental conditions.  Since all varieties in this study were grown under the same environmental conditions, fruit yield depended on the genetic potential of a variety.

There were varied responses in growth component of the varieties to manure application for both rainy and late season crops. There were varied responses in growth component of the varieties to manure application for both rainy and late season crops. For rainy season crops, the result showed that the effect of NPK fertilizer was significant over organomineral (OMF) and control on growth characters of watermelon. Agele and Olabomi (2010) and Agele et al. (2011) observed similar increase in growth parameters of tomato and pepper in Akure, Nigeria on application of NPK fertilizer and farm yard manure. The vigour of growth of Anderson in the unmanured control was better while NPK fertilizer enhanced growth of Charleston Grey and Anderson in the rainy season Sabo et al. (2013). Although, the growth components of Anderson improved when manured in rainy season but this was accompanied by lower fruit yield. This result supports the claim of Silva et al. (2007), Dauda et al. (2008) and Audi et al. (2013) that inorganic fertilizer promoted vegetative growth in watermelon but caused reduction in number of fruits, delayed and reduced fruit setting. The effect of NPK and OMF application was profound on number of fruit rots, cracked fruits and fruit diameter. NPK gave larger fruit compared to control but recorded higher number of fruit disorders in both seasons. High fruit rot due to NPK may be due to excessive vegetative growth due to NPK fertilizer application. In Capsicum spp, excessive growth aggravates blossom-end -rot disorder because the huge demand for N may shut Calcium away from the fruit. Calcium is not translocated within the plant from older to younger tissues; therefore young fruits are especially sensitive to a lack of calcium (Bar et al., 2001; Agele and Olabomi, 2010). However, in other to produce bigger fruits, NPK fertilizer is required for watermelon.  Ogunremi (1990) reported increase in the yield of melon fruits due to fertilizer application. The effect of OMF and NPK fertilizers was not significant on most yield characters measured. OMF application enhanced number of fruit rot over the control. This may still be due to presence N in OMF. The control produced significantly higher fruit weight over the NPK and OMF. The higher fruit weight in the control was accompanied with higher moisture content of the watermelon fruit. In the rainy season crop, application of NPK and organo-mineral fertilizer enhanced performance of watermelon varieties in terms of fruit yield components. Highest number of fruits, fruit length, and fruit diameter and fruit weight were produced in Crimson sweet when treated with NPK and the same was recorded in Anderson, and Sugarbaby under combination of organo-mineral fertilizer. Organo-mineral fertilizer appears advantageous for low input system for improving nutrient status of tropical soils: Adeoye et al. (2008) and Agele et al. (2008).

However, among the varieties, Sugarbaby showed no record of fruit rot for all fertilizer types tested despite the high number of fruits produced.  This implies that Sugarbaby performed best in both seasons and there is possibility of producing larger and heavier fruits when treated with mineral fertilizers. Charleston Grey has been noted for its fewer but larger and heavier fruits but highest number of fruit rots when manured or unmanured in the rainy season. This implies that this variety is not adequately adapted to rainy season environmental conditions.

The number of fruits produced by Anderson in unmanured plot was still reasonably high and Crimson sweet will perform well when manured in the rainy season. The late season enhanced  response  of Charleston grey as it produces highest number and  highest fruit weight and no fruit rot when manured or not. Hence this variety may thrive well and is better adapted to the late season. This shows that Anderson when manured can perform well as both rainy and late season crop. Sugarbaby which was observed as the best variety in the rainy season (no record of fruit rot) across manure types did not follow this same trend as late season crop, when it had large number of cracked fruit. Meanwhile cracked fruit may be due to attacks by animal pest and not varietal dependent, therefore if properly managed, this variety may still perform well in late season crop.

The chemical and proximate composition of watermelon fruits showed that the nutritional quality differed among the varieties and manure types. In particular, N, P, K, Ca, Mg, moisture content, crude fiber, ash content and vitamin C contents of tested varieties differed significantly. In the rainy season crop, NPK enhanced the content of N, P, K, moisture content, total solids and vitamin C in Charleston grey. Nitrogen content and moisture content in Crimson sweet were enhanced by NPK application in addition to increases in K, Ca and Mg content in Sugarbaby and moisture content and vitamin C in Kaolac. However, organo-mineral fertilizer (OMF) produced increases in N, Ca, Mg and crude fiber in Anderson, P content in Kaolac, N and crude fiber in Sugarbaby and vitamin C only in Charleston grey. The effects of  NPK and organo-mineral fertilizer application on fruit nutritional (proximate) quality of  the varieties mentioned above may be probably due to the role of  nitrogen in crop maturation, flowering and fruiting (Indira, 2005, Agele et al., 2008). In the late season crop, NPK application enhanced the nutritional content of Sugarbaby and Anderson and total solids. In the same vein, NPK enhanced P, K and Mg and total solids in Anderson unlike the rainy season crop. This shows that N mineralization and availability for root uptake may be hindered by heavy rainfall, hence the yield of late season crop.  NPK application in the late season increased P, Mg, total solids content in Charleston Grey, Ca and Mg, total solids and vitamin C in Crimson Sweet and Mg, moisture content and vitamin C in Kaolac respectively. The fruit contents of P, Ca, crude fiber and vitamin C content in Kaolac, P, total solids and vitamin C in Crimson Sweet, Ca and total solids in Charleston Grey and Anderson and moisture content and vitamin C in Sugarbaby were enhanced by OMF application. In rainy season crop, ash and crude fibre content were not enhanced by NPK application in the varieties. Vitamin C content shows no response to manuring in Sugarbaby, Crimson Sweet and Anderson. Unmanured Anderson had highest vitamin C content, which shows the non- response to manuring in the late season. It is noteworthy to say that Anderson is recommended as best in terms of vitamin C content irrespective of manuring in both seasons. This variety seems more nutritious without enhancement by manuring especially in the rainy season. The study shows that the tested varieties of watermelon are high in mineral contents, which play important roles in man and animals.

4 Conclusion

The The results of this study show that the water melon varieties tested responded differently in terms of growth, yield and nutritional qualities to application of manure in both the rainy and late season. Charleston Grey variety performed well in terms of growth (biomass) but produced poor fruit yield in the rainy season.  Other varieties like Kaolac and Anderson recorded high fruit yield components in the rainy season. Late season favored growth and yield in Crimson Sweet variety and it is better adapted to the environmental conditions then. Varieties like Kaolac adapted well in terms of growth and yield to both rainy and late season. For the rainy season crop, NPK fertilizer significantly enhanced growth and yield characters of watermelon compared to organomineral (OMF) fertilizer. NPK enhanced fruit size compared to control but recorded higher number of fruit disorders in both seasons. Organo-mineral fertilizer application enhanced number of fruit rot over the control. However in some cases the control produced significantly higher fruit weight over NPK and OMF. This result confirms that watermelon at times does not have large fertilizer requirements and can be grown with little or more fertilizer. In the rainy season, application of NPK and organo-mineral fertilizer enhanced performance of watermelon varieties in terms of fruit yield and its components in varieties like Charleston grey, Anderson, Crimson Sweet and Sugarbaby. Sugarbaby showed no record of fruit rot for all fertilizer types tested, which implies that Sugarbaby performs best in both seasons and there is possibility of producing larger and heavier fruits when treated with mineral fertilizers. The chemical and proximate composition of watermelon fruits showed that the nutritional quality differed among the varieties and manure types. In particular N, P, K, Ca, Mg, moisture content, crude fibre, ash and vitamin C contents of tested varieties were highly enhanced. In the rainy season, NPK enhanced the contents of N, P, K, moisture, total solids and vitamin C in most of the varieties except in Anderson. In the same vein, organo-mineral fertilizer (OMF) produced increases in N, Ca, Mg and crude fibre in Anderson, P content in Kaolac, N and % crude fibre in Sugarbaby and vitamin C only in Charleston Grey. NPK application in late season enhances the nutritional (total solid content in particular) in Sugarbaby and Anderson. OMF application also enhanced fruit contents of P, Ca, crude fibre and vitamin C, and total solids in some of the tested varieties. Among the tested varieties Kaolac, Anderson and Sugarbaby will perform well as both rainy and late season crops, the application of either NPK or OMF enhanced fruit yield in these varieties. 

Charleston Grey will perform as a late season crop and therefore recommended as a source of high economic returns in terms of fruit weight and yield. Unmanured Anderson had highest vitamin C content in the late season, hence this variety is recommended as best in terms of vitamin C content irrespective of manuring in both seasons. Chemical and organomineral fertilizers be applied for enhancing the growth, yield and chemical and nutritional qualities of the tested varieties of watermelon in the rainforest agro-ecology of Nigeria. It is recommended that the rate of NPK and OMF to be applied in these tested varieties should be further studied because the rates (5 t/ha OMF and 200 kg/ha NPK) was found to promote fruit disorders.

Acknowledgement

The authors gratefully acknowledge Institutions and Persons from where the water melon varieties were sourced and the Technical and Field Staff of the Federal University of Technology, Akure, Nigeria, for their invaluable assistance in the conduct, data collection and laboratory analyses activities with respect to this study.

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