I am an academic with the Agriculture Department, PNG University of Technology. My research work covers crop improvement, resistance to drought stress, and wild crop relatives.
Ixora amplexifolia (Ixoroideae, Rubiaceae), belongs to subgenus Sathroclamys, section Hypsophyllu... more Ixora amplexifolia (Ixoroideae, Rubiaceae), belongs to subgenus Sathroclamys, section Hypsophyllum. It is endemic to the northern part of Papua New Guinea (PNG). This paper updates the morphological description of the species to incorporate new fruit data, and review its alignment with its cohorts of section Hypsophyllum and their distribution in PNG.
A study of the genetic variation existing within and between 7 countries of South East Asia and O... more A study of the genetic variation existing within and between 7 countries of South East Asia and Oceania was conducted. Overall, 2298 cultivars presently held in germplasm collections, have been characterised using 23 standardised morphological descriptors. 2081 cultivara were characterised using six enzyme systems (MDH, PGI, ICD, PGD, ME, SKDH) and 319 distinct zymotypes were identified in the region. Surprisingly, only 6 zymotypes represent more than 51% of the total number of accessions electrophoresed and only 21 zymotypes represent more than the two thirds (70%) of total accessions. Furthermore, the genetic variation existing between the major 6 zymotypes is rather limited. This indicates that the genetic base of most of the cultivara existing in this vast geographical region is narrow. At the isozyme level, there is limited variation in Vanuatu, Papua New Guinea and the Philippines and significant genetic variation in Indonesia, Thailand and Malaysia. South East Asia and Melane...
Brassica villosa subsp. bivoniana pictured with clip cage containing whiteflies in field trials f... more Brassica villosa subsp. bivoniana pictured with clip cage containing whiteflies in field trials for plant host resistance, Wageningen
Agro-morphological variation in the taro germplasm of Papua New Guinea was estimated using 18 pol... more Agro-morphological variation in the taro germplasm of Papua New Guinea was estimated using 18 polymorphic descriptor states to aid in the selection of a core sample for the formation of a regional core collection currently being assembled under the Taro Network for Southeast Asia and Oceania. A total of 276 accessions were stratified into five homogenous groups by using a hierarchical approach according to botanical variety (dasheen or eddoe), altitude (high or low) and stolon formation (present or absent). In selecting the core sample, the eddoe group were directly included because of their rarity in the germplasm collection. While, a ten per cent sample fraction within each group of the dasheen types were selected based on principal component scores. A total of 31 accessions were selected for the core sample. Multivariate analysis of the core sample revealed wide variation, which was mainly influenced by botanical variety, plant height, lamina colour and variegation, petiole colou...
ABSTRACT Twelve taro ( Colocasia esculenta) clones resistant to taro leaf blight ( Phytophthora c... more ABSTRACT Twelve taro ( Colocasia esculenta) clones resistant to taro leaf blight ( Phytophthora colocasiae) were selected from the 32 elite plants resulting from the second cycle of recurrent selection at the Wet-Lowlands Mainland Research Centre, Lae. These clones were evaluated for seven morphological characteristics, reaction to P. colocasiae inoculation, yield, and eating quality in comparison with the locally-preferred cultivar Numkowec. All twelve lines were resistant to the local race of the taro leaf blight pathogen, to which Numkowec was susceptible. However, only seven lines were acceptable to a taste-testing panel of local consumers. Of these seven lines, four (C2-E1, C2-E4, C2-E7 and C2-E10) significantly out-yielded Numkowec for corm and cormel production. These lines are recommended for regional evaluation in multi-location trials throughout Papua New Guinea.
The accumulation and perpetuation of viral pathogens over generations of clonal propagation in cr... more The accumulation and perpetuation of viral pathogens over generations of clonal propagation in crop species such as sweet potato, Ipomoea batatas,inevitably result in a reduction in crop yield and quality. This study was conducted at Bundaberg, Australia to compare the productivity of field-derived and pathogen-tested (PT)clones of 14 sweet potato cultivars and the yield benefits of using healthy planting materials. The field-derived clonal materials were exposed to the endemic viruses, while the PT clones were subjected to thermotherapy and meristem-tip culture to eliminate viral pathogens. The plants were indexed for viruses using nitrocellulose membrane-enzyme-linked immunosorbent assay and graft-inoculations onto Ipomoea setosa. A net benefit of 38% in storage root yield was realised from using PT materials in this study.Conversely, in a similar study previously conducted at Kerevat, Papua New Guinea (PNG), a net deficit of 36% was realised. This reinforced our finding that the ...
Cutnut, Barringtonia edulis , is one of six indigenous edible nut tree species found in Papua New... more Cutnut, Barringtonia edulis , is one of six indigenous edible nut tree species found in Papua New Guinea (PNG), Solomon Islands and Vanuatu, where it has market potential both for domestic consumption and as an export commodity (Wah, 1996). …
Ixora amplexifolia (Ixoroideae, Rubiaceae), belongs to subgenus Sathroclamys, section Hypsophyllu... more Ixora amplexifolia (Ixoroideae, Rubiaceae), belongs to subgenus Sathroclamys, section Hypsophyllum. It is endemic to the northern part of Papua New Guinea (PNG). This paper updates the morphological description of the species to incorporate new fruit data, and review its alignment with its cohorts of section Hypsophyllum and their distribution in PNG.
ABSTRACT Fresh seed weights of taro (Colocasia esculenta) from wild type (WT) and cv. Bangkok pla... more ABSTRACT Fresh seed weights of taro (Colocasia esculenta) from wild type (WT) and cv. Bangkok plants of Papua New Guinea provenance declined significantly (P< 0.05) by 50% after 24 h fan assisted air drying to seed moisture of 10-12% dry weight. Mean germination of WT and Bangkok seeds declined from 90% and 51% at commencement to zero after 8 and 9 months dry storage (over silica gel) at ambient room temperature (RT, mean 27.2°C, 15.6% RH) respectively. Mean germination of WT seed stored over silica gel at 5°C (16% RH) and −20°C (16% RH) remained at 80-90% for the first 12 months and was 88% and 55% respectively after 25 months. Bangkok seed remained viable after 21 months' storage at 5°C and −20°C and 47% germinated after 24 months' storage at −20°C. Seed moisture content of WT seed was reduced to 7.3% after 7.5 months' storage in open packets over silica gel at −20°C and RT and mean germination was 63%; reducing seed moisture to 7.3% extended viability at RT to 81% after 22 months' storage. Some germinating seeds (1-5%) were colonized by Curvularia sp., Fusarium sp., Penicillium sp., and Rhizoctonia sp. but these did not inhibit germination and zero infection occurred following surface sterilization. Taro germplasm can be conserved as seed for at least two years at constant 5°C and −20°C when seed moisture content is reduced to 10-12% and at RT (21.5-34.4°C, mean 27.2°C) when seed moisture content is reduced to 7.3%.
Ixora amplexifolia (Ixoroideae, Rubiaceae), belongs to subgenus Sathroclamys, section Hypsophyllu... more Ixora amplexifolia (Ixoroideae, Rubiaceae), belongs to subgenus Sathroclamys, section Hypsophyllum. It is endemic to the northern part of Papua New Guinea (PNG). This paper updates the morphological description of the species to incorporate new fruit data, and review its alignment with its cohorts of section Hypsophyllum and their distribution in PNG.
A study of the genetic variation existing within and between 7 countries of South East Asia and O... more A study of the genetic variation existing within and between 7 countries of South East Asia and Oceania was conducted. Overall, 2298 cultivars presently held in germplasm collections, have been characterised using 23 standardised morphological descriptors. 2081 cultivara were characterised using six enzyme systems (MDH, PGI, ICD, PGD, ME, SKDH) and 319 distinct zymotypes were identified in the region. Surprisingly, only 6 zymotypes represent more than 51% of the total number of accessions electrophoresed and only 21 zymotypes represent more than the two thirds (70%) of total accessions. Furthermore, the genetic variation existing between the major 6 zymotypes is rather limited. This indicates that the genetic base of most of the cultivara existing in this vast geographical region is narrow. At the isozyme level, there is limited variation in Vanuatu, Papua New Guinea and the Philippines and significant genetic variation in Indonesia, Thailand and Malaysia. South East Asia and Melane...
Brassica villosa subsp. bivoniana pictured with clip cage containing whiteflies in field trials f... more Brassica villosa subsp. bivoniana pictured with clip cage containing whiteflies in field trials for plant host resistance, Wageningen
Agro-morphological variation in the taro germplasm of Papua New Guinea was estimated using 18 pol... more Agro-morphological variation in the taro germplasm of Papua New Guinea was estimated using 18 polymorphic descriptor states to aid in the selection of a core sample for the formation of a regional core collection currently being assembled under the Taro Network for Southeast Asia and Oceania. A total of 276 accessions were stratified into five homogenous groups by using a hierarchical approach according to botanical variety (dasheen or eddoe), altitude (high or low) and stolon formation (present or absent). In selecting the core sample, the eddoe group were directly included because of their rarity in the germplasm collection. While, a ten per cent sample fraction within each group of the dasheen types were selected based on principal component scores. A total of 31 accessions were selected for the core sample. Multivariate analysis of the core sample revealed wide variation, which was mainly influenced by botanical variety, plant height, lamina colour and variegation, petiole colou...
ABSTRACT Twelve taro ( Colocasia esculenta) clones resistant to taro leaf blight ( Phytophthora c... more ABSTRACT Twelve taro ( Colocasia esculenta) clones resistant to taro leaf blight ( Phytophthora colocasiae) were selected from the 32 elite plants resulting from the second cycle of recurrent selection at the Wet-Lowlands Mainland Research Centre, Lae. These clones were evaluated for seven morphological characteristics, reaction to P. colocasiae inoculation, yield, and eating quality in comparison with the locally-preferred cultivar Numkowec. All twelve lines were resistant to the local race of the taro leaf blight pathogen, to which Numkowec was susceptible. However, only seven lines were acceptable to a taste-testing panel of local consumers. Of these seven lines, four (C2-E1, C2-E4, C2-E7 and C2-E10) significantly out-yielded Numkowec for corm and cormel production. These lines are recommended for regional evaluation in multi-location trials throughout Papua New Guinea.
The accumulation and perpetuation of viral pathogens over generations of clonal propagation in cr... more The accumulation and perpetuation of viral pathogens over generations of clonal propagation in crop species such as sweet potato, Ipomoea batatas,inevitably result in a reduction in crop yield and quality. This study was conducted at Bundaberg, Australia to compare the productivity of field-derived and pathogen-tested (PT)clones of 14 sweet potato cultivars and the yield benefits of using healthy planting materials. The field-derived clonal materials were exposed to the endemic viruses, while the PT clones were subjected to thermotherapy and meristem-tip culture to eliminate viral pathogens. The plants were indexed for viruses using nitrocellulose membrane-enzyme-linked immunosorbent assay and graft-inoculations onto Ipomoea setosa. A net benefit of 38% in storage root yield was realised from using PT materials in this study.Conversely, in a similar study previously conducted at Kerevat, Papua New Guinea (PNG), a net deficit of 36% was realised. This reinforced our finding that the ...
Cutnut, Barringtonia edulis , is one of six indigenous edible nut tree species found in Papua New... more Cutnut, Barringtonia edulis , is one of six indigenous edible nut tree species found in Papua New Guinea (PNG), Solomon Islands and Vanuatu, where it has market potential both for domestic consumption and as an export commodity (Wah, 1996). …
Ixora amplexifolia (Ixoroideae, Rubiaceae), belongs to subgenus Sathroclamys, section Hypsophyllu... more Ixora amplexifolia (Ixoroideae, Rubiaceae), belongs to subgenus Sathroclamys, section Hypsophyllum. It is endemic to the northern part of Papua New Guinea (PNG). This paper updates the morphological description of the species to incorporate new fruit data, and review its alignment with its cohorts of section Hypsophyllum and their distribution in PNG.
ABSTRACT Fresh seed weights of taro (Colocasia esculenta) from wild type (WT) and cv. Bangkok pla... more ABSTRACT Fresh seed weights of taro (Colocasia esculenta) from wild type (WT) and cv. Bangkok plants of Papua New Guinea provenance declined significantly (P< 0.05) by 50% after 24 h fan assisted air drying to seed moisture of 10-12% dry weight. Mean germination of WT and Bangkok seeds declined from 90% and 51% at commencement to zero after 8 and 9 months dry storage (over silica gel) at ambient room temperature (RT, mean 27.2°C, 15.6% RH) respectively. Mean germination of WT seed stored over silica gel at 5°C (16% RH) and −20°C (16% RH) remained at 80-90% for the first 12 months and was 88% and 55% respectively after 25 months. Bangkok seed remained viable after 21 months' storage at 5°C and −20°C and 47% germinated after 24 months' storage at −20°C. Seed moisture content of WT seed was reduced to 7.3% after 7.5 months' storage in open packets over silica gel at −20°C and RT and mean germination was 63%; reducing seed moisture to 7.3% extended viability at RT to 81% after 22 months' storage. Some germinating seeds (1-5%) were colonized by Curvularia sp., Fusarium sp., Penicillium sp., and Rhizoctonia sp. but these did not inhibit germination and zero infection occurred following surface sterilization. Taro germplasm can be conserved as seed for at least two years at constant 5°C and −20°C when seed moisture content is reduced to 10-12% and at RT (21.5-34.4°C, mean 27.2°C) when seed moisture content is reduced to 7.3%.
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Papers by Tom Okpul