INTRODUCTION
Roselle (Hibiscus sabdariffa L. Moench) is an indigenous, underutilized vegetable presently being introduced as a potential crop for the rainforest zone of Southern Nigeria. Prior to this, Giginyu et al. (1999) confirmed that it’s cultivation has been predominantly in the North East and North West part of Nigeria.
Roselle (H. sabdarifa L. Moench) belongs to the family of Malvaceae and according to Rehm and Espig (1994), the pattern of growth is that of an erect, bushy, herbaceous annual shrub which can grow up to a height of between 2.4 and 3.0 m. It has red or green main stem depending on landraces and distinct branches. Medlinger et al. (1992) revealed that it is probably native to Central or West Africa and it is a short day annual, while the flower buds are actually seed pods of roselle, enclosed in their fleshy calyces. Ogbe et al. (1998) described the red pods as the fruits and calyces being fused together and difficult to distinguish from one another.
Aliyu and Tanimu (1996) discovered that in a traditional set up, roselle is cultivated for its leaves, stem, seeds and calyx. They also found out the calyx called Zobo in Hausa dialet, is nationally used as a refreshing drink and medicinal preparation. It is presently receiving industrial attention in the country and Internationally. As documented by Small and Hoden (1991) found out that in many parts of the world, the leaves are consumed as a green vegetable and the stem as possible source of pulp for the paper industry.
Nutritionally, the calyces as reported by FAO (2004) have significant quantities of Vitamins A, C, phosphorous, iron and calcium but low in protein. The young leaves are also known to be rich in digestible protein and that the oil content of roselle seeds may vary from 25 to 30% and that it has similar properties with cotton seed oil.
Ginginyu et al. (1999) observed that fertilizer recommendation for roselle in Nigeria is lacking and farmers have attempted the fertilizer rate for Kenaf (Hibiscus cannabinus) a crop of the same family and also a fibre that is biochemically and botanically different.
According to Fagbayide (1997), the overall yield and nutritional quality of the crop should be taken into consideration, in any fertilizer programme. Unfortunately, little has been reported on the effects of mineral fertilizers on the yield and quality of the economic portion of roselle especially in the rainforest zone and in particular South Western Nigeria.
Consequently, Ginginyu et al. (1999) believed that the application of
nitrogen sources to roselle in the rainforest zone of Nigeria is almost a precondition
for optimum growth and yield of roselle. The objectives of this study were to
determine the fertilizer rate and the best nitrogen source for roselle and its
effect on growth, yield and yield components as well as a suitable variety for
the rainforest area of Nigeria.
MATERIALS AND METHODS
The field trial for 26 weeks from March to October 2004, was conducted at the Teaching and Research farm of the Faculty of Agriculture, University of Benin, Benin City (7°38’N, 5°52’E.) Nigeria. The seeds were first raised in seed trays filled with a 2:1:1 ratio of top soil, well cured poultry manure and river sand which were kept under shade in the screen house. The seedlings were transplanted to the field after four weeks of planting.
The experimental treatments consisted of two varieties: SD 2003 and DR 2003; two Nitrogen fertilizers: Urea and NPK 15:15:15 at 0, 100, 150 and 200 kg ha-1, respectively. The experiment was a 2x4x3 factorial laid out in a randomized complete block design with 3 replications. Soil samples were collected of various portions of the field site (23x10 m) within a 0-15 cm in depth and were analyzed for their physiochemical properties and the result is shown in Appendix I.
The seedlings were transplanted at four weeks old and spaced at 0.75x0.75 m on each bed. The beds were weeded at 2, 6 and 10 Weeks After Transplanting (WAT). Three weeks after transplanting, the treatments were allocated randomly and at 15 cm away from the plants at 0, 100, 150 and 200 kg ha-1, respectively. Podagrica beetles attacked the leaves often and these were controlled by use of Nuvacron 40EC at 3 in 500 mL of water.
The sampling of parameters of plant height, number of leaves and leaf area,
stem diameter and branch numbers were sampled forth nightly as Roselle grows
tall all the time. Number of fruits on the stems were counted at harvest at
the 10th, 11th and 12 WAT and were recorded.
Soon after harvest, at the end of the 12th week, the fresh and dry weights were carried out by means of destructive sampling using an oven for a period of 72 h at 80°C until constant weight was obtained (Anonymous, 1993).
RESULTS AND DISCUSSION
The favourable responses recorded as a result of fertilizer application is not surprising because as reported by Ehigiator (1998), fertilizer increases the soil fertility so that the yield of crops are no longer dependent on limited, available soil nutrients. Nitrogen plays an important role in the meristematic and physiological activities in plants while phosphorous promote root growth and so enhances nutrient uptake.
Plant height: The insignificant differences in plant height for some weeks (Table 1) has also been confirmed by Rhoden et al. (1993) in their investigation, that the increase in Nitrogen did not produce any increase in plant height of roselle. Significant differences observed due to varieties was due to environmental influence. The investigation of Chweya (1992) agrees with this view that it was possible for one vegetable cultivar of the same species to emerge and grow faster than the other in the same location if the environmental conditions are favourable.
Stem diameter: The insignificant differences for the varieties, kinds
of fertilizers and fertilizer levels. Observed in Table 2
were supported by Aliyu and Olanrewaju (1996) in their work on Capsicum annum
where they observed that the beneficial effects of N and P could be seen in
increasing the growth of stem diameter and thus interpreted as a cumulative
increase in growth.
| Table 1: |
The effect of different levels of NPK and urea on plant height
of roselle DR 2003 and SD 2003 varieties (means only) |
 |
| Means of the same treatment in a column with different letter(s)
are significantly different at 0.05% |
| Table 2: |
The effect of different levels of NPK and urea on stem diameter
of roselle, DR (2003) and SD (2003) variety (means only) |
 |
| Table 3: |
The effect of different levels of NPK and leaf area and number
leaves of roselle, DR (2003) and SD (2003) variety (Means only) |
 |
| Table 4: |
The effect of different levels of NPK and urea on number
of branches of roselle, DR (2003) and SD (2003) variety (means only) |
 |
| Table 5: |
Effect of NPK and urea on the number of fruits of roselle
at 10, 11 and 12 wat (kg/plant) (means only) |
 |
| Means of the same treatment in the same column and of same
letter(s) are not significantly different statistically at p = 0.05 |
| Table 6: |
The effect of NPK and urea on the fresh and dry weight of
two varieties of roselle, DR (2003) and SD (2003) after harvest (kg/plant)
(means only) |
 |
The DR 2003 variety and NPK 15:15:15 fertilizer influenced stem diameter to
a greater extent than Urea and SD 2003 variety of Roselle.
Number of leaves and leaf area index: The insignificant differences for number of leaves and leaf area index between the varieties and fertilizer levels at the beginning of the trial (Table 3) agrees with the results of Small and Hoden (1991) who worked on the effect of Nitrogen uptake of Roselle, on leaf area and number of leaves, respectively.
Also the non-significant differences observed in the interaction between NPK and Urea fertilizers on these parameters (Table 3) was supported by Ibrahim et al. (1998) where it was reported that in the interaction of N and P on Amaranthus, there was an indication that the application of either N or P without the other failed to have any effect on leaf yield.
Number of branches: The existence of non-significant differences in number of branches were observed for all the treatments (fertilizer levels and varieties) as in Table 4, especially in 100 kg ha-1 of NPK 15:15:15 and Urea (46 N) than the 150 or 200 kg ha-1 NPK 15:15:15, respectively, as the latter levels produced lower yields. This fact was also revealed by Sanni et al. (2000) who worked on garden eggs and observed that the flowers, branches and fruit numbers were also affected. The result also supported the previous findings of Bindinger et al. (1996) who observed that environmental factors have detrimental effects on plants hence low number of fruits for 150 and 200 kg ha-1 of NPK 15:15:15 and urea, respectively.
Number of fruits: The DR (2003) variety was observed to perform better
than the SD (2003) variety in terms of calyx production. There were significant
differences at 10, 11 and 12 WAT among the varieties (Table 5).
There was, however, no significant difference with the different levels of fertilizer
or type of fertilizer used, although the plants supplied with NPK did better
for both varieties in calyx production than all the control. This result corroborates
the findings of Tijani-Eniola et al. (2000) on leafy vegetables and confirmed
the views of Olasatan (1994) who reported the significant benefits derived from
adequate application of fertilizer (NPK) to vegetable crops.
The superiority of SD (2003) over DR (2003) is in consonance with the investigation of Joshua (1999) who confirmed that varieties of same crops may differ in growth rate, reproductive cycle and yield duration. This low fruit production of SD (2003) could also be attributable to genetic trait as well as to environmental conditions. This was substantiated by Kang (1998) who revealed that even if genetic traits are equal for crop varieties, environmental condition could reduce the number of fruits in vegetable crops.
Fresh weight and dry weight: The non-significant differences of both fresh and dry weight for all the parameters tested (Table 6) is in line with Rhoden et al. (1993) investigation on Roselle when he reported that the increase observed in dry matter production three weeks after additional nitrogen application,did not bring about any significant difference.
CONCLUSIONS
Based on the results of the trial, the growth and yield of DR (2003) and SD (2003) were both affected by type of fertilizers and levels. Most of the parameters studied performed better at 100 kg ha-1 for both kinds of nitrogen sources but the yield was more influenced by NPK compared to Urea at 100 kg ha-1. The main significance was from the varieties where DR (2003) out yielded SD (2003).
NPK is recommended for this ecological zone as it supported not only the growth
rate but yield of Roselle. Also, the DR (2003) variety is recommended for the
rainforest zone of Nigeria as it has a higher resistance to pests and diseases
which are prevalent in the rainy seasons especially as most crops in the south
are rain fed.
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