Effect of organic manures on the early seedling morphology of Irvingia wombolu Vermoesen in the tropical rainforest of Nigeria

: The study was aimed at evaluating the effects of four selected organic manures on the early seedling morphology of Irvingia wombolu . The experiment was undertaken in Asaba located in the tropical rainforest zone of Nigeria (60 0 12´ N and longitude 60 0 45´ E). The study consisted of four organic manure treatments, namely: Tithonia alone, Tithonia + pig manure, Tithonia + cow manure, Tithonia + poultry manure and control with no manure application. The five treatments were arranged in a randomized complete block design (RCBD) with three replicates. At 15 days after sowing (DAS) percentage of germinated seeds ranged from 95.45% (control) to 95.73 % ( Tithonia + poultry manure). A mix of Tithonia + pig manure, Tithonia + cow manure and Tithonia + poultry manure increased seedling height by 3.9, 9.7 and 11.5 % respectively compared to Tithonia alone. The highest leaf number (9.03) and LAI (2.90) were observed in Tithonia + poultry manure. Stem volume of seedlings increased significantly ( P <0.05) over the control with application of organic manures. Stem volume was found to be significantly ( P <0.01) correlated with plant height (r = 0.988), stem diameter (r = 0.980) and leaf area (r = 0.976). The study recommends the use of Tithonia + poultry manure mixture for better performance of Irvingia seedlings .


INTRODUCTION
Irvingia wombolu (Irvingiaceae) Vermoesen commonly called African mango tree or bush mango is a West and Central African indigenous wild tree species that is highly valued for its multiple uses such as soup thickening (i.e "Ogbono soup" in Southern Nigeria), as medicinal plant and timber production. The trade name of this species is Dika nut. Previous studies by Ngondi et al. (2005) have demonstrated that Irvingia seeds consumption can significantly reduce total cholesterol, LDL cholesterol and triglycerides in humans and as such relevant in management of obesity. The thickening attribute of the kernel has been associated with the presence of mucilaginous polysaccharides whose viscosity is more pronounced during cooking (Ndjouenkeu et al., 1997). About 90% of the total yearly crop of the kernel or fruit is harvested from wild forests, while the remaining 10% are obtained from the cultivated or domesticated trees (Ladipo et al., 1996). The kernel of Irvingia is currently an export product to the United States and European countries. Its popularity in the domestic and foreign markets necessitates the potential of this fruit crop for commercial production through cultivation outside the wild forests.
For any commercial plantation or reforestation programme based on nursery-produced seedlings, such seedlings must be of high quality, exhibiting vigorous growth at the early stage of establishment. The rate of growth of any plant seedling during the nursery is greatly dependent on several cultural practices, amongst which nutrient application remains prominent. Earlier studies (Onemli, 2004;Cernac et al., 2006) have reported the significant influence of soil nutrients on the rate of germination and emergence of plant seedlings. Several researchers (Igua and Huasi, 2009;Crespo et al., 2011;Hafifah et al., 2016) have demonstrated the green manure potential of Tithonia diversifolia in improving soil physical and chemical properties, increasing soil nutrients and enhancing microbial activity.
The use of agro-wastes such as animal manures as nutrients in raising of fruit tree crop nursery seedlings has extensively been documented. Ibe et al. (2014) observed that the indigenous potting medium Treculia africana (breadfruit) is commonly a mixture of poultry manure with farmland topsoil. Moyin-Jesu and Ani (2014) reported significant increase in growth parameters of seedlings of Cocos nucifera with application of poultry manure. Shodeke et al. (2017) observed highest growth of Irvingia wombolu in poultry manure mixed with NPK. There is limited information on the response of Irvingia wombolu to organic manures at the early stage of development. This study is therefore aimed evaluating the effects of four selected organic manures on the early seedling morphology of Irvingia wombolu.

MATERIALS AND METHODS
The experiment was undertaken in Asaba located in the tropical rainforest zone of Nigeria (60° 12´ N and longitude 60° 45´ E). Meteorological information of the experiment site includes: annual rainfall of 1800-3000 mm, annual temperature ranging from 27.5 to 39.9 0 C and relative humidity of 65-85 %. The soil is sandy loam and belongs to the Ultisol order (FDALR, 1985).
Mature fruits of Irvingia wombolu were purchased from the local market. The fruits were depulped mechanically and sun dried for three (3) days prior to planting before presoaking in water for four hours. The simple flotation technique was used to determine viability of seeds. Only viable seeds were sown in prepared nursery beds.
The dimension of the nursery site was 15 x 10 m. The site was cleared and raised nursery seed beds of 2 x 2 x 0.3 m were constructed. The pathway between seed beds and within seed beds on same block was 1 m.
The five treatments were arranged in a randomized complete block design (RCBD) with three replicates. Since the study was conducted on a field with raised seed beds, the blocking design is most appropriate to minimized variation in soil fertility between seed beds in the experimental plot. Composite soil sample of experimental site and dried forms of the organic manures were analyzed for chemical properties. Soil samples were analyzed for particle size distribution (Bouycous method), while sample of soil and dried forms of the organic manures were analyzed for organic carbon (wet dichromate oxidation method) and total N (Kjedahl method). Total K in samples were extracted with neutral ammonium acetate, and total K in extract was determined in flame photometer (IITA, 1979) Freshly harvested plants of Tithonia diversifolia (consisting of green tender stems and green leaves) and animal manures were properly air dried before use. Organic manures were applied at the rate of 6 t/ha. For mixed organic treatments, 1:1 ratio by dry weight was used. Hand trowel was used to incorporate materials into the soil and materials were left to undergo decomposition for 7 days before planting the seeds. Beds were irrigated to field capacity before planting of seeds. Seeds of Irvingia wombolu were sown on seed beds at spacing of 0.2 x 0.2 m.
Data collected were germination percentage at 15 days after sowing (15 DAS), plant height, number of leaves per plant, collar diameter (using vernier caliper) and leaf area (graphical method) of seedlings at three weeks interval from six weeks after sowing (WAS), leaf area index (LAI) for germinating seedlings was estimated using the formulae below: Where n= number of leaves; LA= leaf area; Ag = ground area.
Stem Volume (SV) of the seedlings was estimated using the formulae below: Where d = collar diameter of seedling; and h = seedling height.
All vegetative data collected were subjected to analysis of variance (ANOVA) appropriate for randomized complete block designs (RCBD) and significant treatment means separated using least significant difference at 5% level.

Physical and chemical properties of soil and organic manures
The soil of the experimental site is sandy loam ( Table 1). The nutrient sources are rich in organic matter ranging from 35.62 % (pig manure) to 52.38 % (Tithonia). These results are in consonance with the earlier studies of Onemli (2004) and Mathowa et al. (2014) which noted that amendment of the soil with the organic matter provided a pool of nutrients and enhanced the micro-environment through adequate water infiltration and diffusion of oxygen to the seeds for seedling emergence and growth.

Germination percentage
Germination percentage was not significantly different with manures applied (Figure 1). At 15 DAS percentage of germinated seeds ranged from 95.45% (control) to 95.73 % (Tithonia + poultry manure). The non-significant effect (P<0.05) of nutrients on germination percentage was confirmed by previous studies of Cernac et al. (2006) and Ibe et al. (2014) which noted that germination percentage of plants propagated by seeds was not dependent on the nutrient status of the soil, but rather the sprouting seeds derived their nourishment from the food reserves stored in storage organs like cotyledons,

Seedling height
Organic manure application significantly (P<0.05) influenced height of seedlings. Seedlings that received mix of Tithonia + poultry manure recorded the tallest  (Table 2). A mix of Tithonia + pig manure, Tithonia + cow manure and Tithonia + poultry manure increased the seedling height by 3.9, 9.7 and 11.5 % respectively when compared to Tithonia alone treatment at 15 WAS. Similar results were reported for seedlings of Cassia abbreviate (Sekepe et al., 2013) and Afzelia quanzensis (Mathowa et al., 2014). The higher organic matter content of soils containing animal manures could have resulted in increased water holding capacity and availability of nutrients which contributed to production of taller plants. Root and shoot elongation in plant seedlings have been linked to the substrate quality (Figueiredo et al., 2012;Mengel, 2013;Yerima et al., 2015).

Seedling foliage parameters
Significant (P<0.05) progressive increases were observed with time in all foliage parameters; number of leaves per plant, unit leaf area and LAI of Irvingia wombolu seedlings. The increase of these parameters was significantly higher in seedlings planted in soil with organic manures relative to the control (Tables 3, 4, 5). The highest leaf number per plant (9.03) and unit leaf area (128.25 cm 2 ) were observed in Tithonia + poultry manure. At 15 WAS, application of organic manures increased LAI by 10.9 % (Tithonia alone treatment), 15.8 % (Tithonia + pig manure treatment), 20.9 % (Tithonia + cow manure treatment) and 31.2 % (Tithonia + poultry manure treatment), when compared to control. Increased leaf number, leaf area and LAI in Tithonia + animal manures mixtures compared to control is confirmed by earlier results of Akanbi et al., (2000) and Ghosh et al., (2006) which noted that nutrient availability is a vital contributor to plant photosynthetic capacity.   Values followed by the same letter(s) in a column are not significantly different at 5% level using LSD

Seedling stem parameters
The application of organic manures resulted in significant variations in collar diameter of Irvingia wombolu seedlings from the ninth WAS (Table 6). Results from similar studies did not also show significant variations in collar diameter for seedlings of Plukenetia conophorum (Egharevba et al., 2005) and Afzelia quanzensis (Mathowa et al., 2014) prior to 9 WAS. Collar diameter ranged between 0.28 and 0.31 cm, 0.54 to 0.58 cm; 0.64 to 0.67 and 0.64 to 0.69 at 6, 9, 12 and 15 WAS respectively. Collar diameter as affected by different organic manure mixtures containing Tithonia and animal manures was not different among manure mixtures while it was significantly different (P<0.05) from that of control. Stem volume of seedlings increased significantly (P<0.05) over the control with the application of organic manures (Table 7). Percentage incremental stem volume between the 6 WAS and 15 WAS was 183.2 % in the control, 192.5 % in Tithonia alone, 175.7 % in Tithonia + pig manure, 161.3 % in Tithonia + cow manure and 163.7 % in Tithonia + poultry manure treatments. Irvingia wombolu seedlings that received Tithonia + poultry manure had the highest stem volume of 17.02 cm 3 while the control recorded the least value of 12.56 cm 3 . Soil amendments with adequate nutrients have the potential to determine biomass partitioning and allocation in seedlings, resulting in increased collar diameter and stem volume. Pinto et al., (2011) also observed incremental stem volume with  Table 8 shows values of simple linear correlation stem volume and other growth parameters studied. Plant height (r = 0.988), leaf number (r = 0.980), and unit leaf area (r = 0.976), showed a significant (P<0.01) positive correlation with stem volume. Stem volume was also found to be significantly (P<0.01) correlated with stem diameter (r = 0.980) and leaf area index (r = 0.976). The organic nutrients would have promoted the performance of these attributes which finally contributed to the increased stem volume. Plant height and foliage attributes have been reported to be major indicators of biomass yield in plants (Raun et al., 1999).

CONCLUSION
The findings from the present study showed that Irvingia wombolu seedlings responded to organic manure supplementation to nursery beds. That is explained by the increased plant height, leaf area index and stem volume and other growth parameter with organic manure application. The study recommends the use of Tithonia + poultry manure mixture for better performance of Irvingia seedlings for tree growers.