An investigation into the sapstaining intrinsic properties of selected rubberwood (Hevea brasiliensis) clones
The general physical and chemical properties of rubberwood have been studied, as have the general relative levels of tolerance of sapstain fungi. New rubberwood clones have been recommended for planting since 1967. It is therefore timely to examine clonal differences in sapstaining properties of rubberwood clones. The purpose of this study was to investigate the intrinsic properties of 3 selected rubberwood clones vis-à-vis their relationship to susceptibility to sapstain fungi. Two clones (PB217 and RRIM600) were obtained from the Bukit Pilah Estate, Negeri Sembilan, Malaysia and the third clone (GT1) came from the Tangkah Estate, Johor, Malaysia. The nitrogen contents were determined by the micro- Kjedahl method (Kjeldahl, 1883), the total organic carbon contents were determined by the Walkley-Black procedure (Walkley and Black, 1934), the sugar contents in the selected rubberwood clones were determined by High Performance Liquid Chromatography (HPLC), while starch content was determined by the method adapted from Humphreys and Kelly (1961). Quantitatively, biocontrol activity of several moulds and nonstaining fungi were examined using a range of agar and wood wafer techniques. The results indicated that the nitrogen content of clone RRIM600 (1.06 mg/g) was significantly higher (p<0.05) than those of clones GT1 (0.73 mg/g), and clone PB217 (0.78 mg/g). After drying (45°C), clone PB217 contained significantly (p<0.05) higher amounts of fructose (5.55 mg/g), glucose (2.30 mg/g) and total sugars (13.15 mg/g) as compared to those of clone GT1 and RRIM600. Clone GT1 had the lowest fructose (0.23 mg/g), glucose (0.14 mg/g), sucrose (2.20 mg/g) and total free sugar (2.56 mg/g) contents. B. theobromae Pat. spreads significantly (p<0.05) faster on RRIM600 (4.3 days) than on PB217 (5.6 days) or on GT1 (5.8 days) which was associated with the overall higher nutrient availability in clone RRIM600. Starch content was found to be highest in clone RRIM600 (5.81%), followed by GT1 (4.30%) and PB217 (3.63%). A new tool attempted in the current investigation has been offered by the use of colorimeter, which has been used to conduct a quantitative assessment of the dark stain caused by the fungus B. theobromae. Results of the quantitative assessment of sapstain development (caused by B. theobromae, Aureobasidium pullulans and Aspergillus niger) in three rubberwood clones are reported here. Rubberwood blocks from three clones (GT1, PB217 and RRIM600) were inoculated with the test fungi and incubated for four weeks, after which the test samples were air dried and sanded (removing approximately 0.5 mm of the surface layer), and then quantitatively assessed for sapstain severity by spectrophotometry. Blue stain from B. theobromae was still present and pronounced after sanding, and was significantly (p<0.05) darker (absorbance: 56.5%) compared with wood infected by A. niger (38.24%) or A. pullulans (35.88%). Clone RRIM600 had significantly (p<0.05) higher mean percent darkness (41.85%) than clone PB217 (38.49%). The mean percent darkness of clone GT1 (39.67%) was not significantly different (p>0.05) from either clone RRIM600 or clone PB217. Single or mixed inocula of A. pullulans and A. niger showed biocontrol activities on clones RRIM600 and PB217.
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