Kagoshima University Research Center for the Pacific Islands Occasional Papers No.39, 55-58, 2003
Section 2, Report 4. The Progress Report of the 2000 and 2001 Survey of the Research Project“Social Homeostasis of Small Islands in an Island-zone”

55

COMPARISON OF TUBER PROTEINS OF YAM INTRODUCED FROMTHE POHNPEI ISLAND AND YAP PROPER,THE FEDERATED STATES OF MICRONESIAONJO Michio＊ ), HAYASHI Mitsuru＊ ), and MATSUO Tomoaki ＊)Abstract
Yam belongs to the genus of Dioscorea, in the Pacific Islands, mainly four species of Dioscorea
are cultivated. We collected some tubers of yam from the Pacific Islands, especially Pohnpei Islandand Yap Proper. Most yam tubers have viscosity that is composed of proteins. We examined the differences of tuber proteins of yam by using polyacrylamide gel electrophoresis method.
Keywords: Dioscorea spp., polyacrylamide gel electrophoresis, Pohnpei Island, tuber protein, YapProper

Introduction
Yam is the generic name of plants belonging to the genus of Dioscorea, Dioscoreaseae. In
the Pacific Islands, yams (Dioscorea alata, D. esculenta, and D. nummularia) and aerial yam(D. bulbifera) are cultivated (Society of the Pacific 1989). In the Pohnpei Islands and in Yap
Proper, the Federated States of Micronesia, D. alata and D. esculenta are the primary species
cultivated and used for food. Because the alata species is used for various ceremonies in thePohnpei Islands, inhabitants sometimes compete to grow the largest tubers. Therefore, while thenormal tuber weight per plant of the alata species reaches a maximum of more than 500 kilograms, sometimes weights 20 times as large can be seen (STANLEY 1992).
During our 1994 survey of the Pohnpei Islands, we were able to identify 20 or more different strains of the alata species, so we estimated that many different strains exist on this island.
However, there are few examples of positive cultivation of the nummularia or bulbifera
species because they include some poisonous strains and at present are positioned as emergency
crops.
The authors have been participating in the Overseas Academic Survey by the Kagoshima
University Research Center for the Pacific Islands since 1994. During this period, we collected
five species of yam from both islands and, by storing them in the Kagoshima University
Experimental Farm Ibusuki Botanical Garden, have continued cultivating them.
These species of yam have been studied mainly from the aspects of yield and earliness, butrelatively few studies have been conducted from the aspect of nutrition. Generally speaking,many yam tubers are viscous, and the main component producing viscosity is protein. Thoughprotein is an important nutritive element, there are few studies of yam protein and many points
have not yet been clarified. In this study, therefore, we investigated the differences in tuber pro＊)

Faculty of Agriculture, Kagoshima University. Kagoshima 890-0065, Japan.

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ONJO Michio, HAYASHI Mitsuru, and MATSUO Tomoaki

tein as the first phase for attempting an analysis of the species of yam collected on both islands
from the aspect of nutrition.

Materials and Methods
In this study, we tested the 11 strains of the three species of yam we collected in thePohnpei Islands, Federated States of Micronesia, where we implemented several on-site surveys
from 1994 to 2001 (see Table 1). Because the strains were not clearly named, we assigned anumber to each strain. Of the tubers we collected on Yap Proper in November 2000, we directlyanalyzed only Yp4, tubers harvested from the plants cultivated continuously in the Ibusuki
Botanical Garden after the collection was used for analyzing other strains.
Table 1. Characteristics of tubers in tested strains of Dioscorea spp.
Strain’s No.＊Species

P 1
P 2
P 9
P 14
P 17
P 20
Yp 1
Yp 2
Yp 3
Yp 4
Yp 5
Solo yam

alata
alata
Unknow
Unknowalatabulbifera
bulbiferaalata
esculenta
esculenta
alata
alata

Shape
Round
CylinderRound
CylinderCylinder
Round
Glove
GloveRound
Round
Round
Round

Characteristics of tubers
Change ofColorcolor
WhiteNon
Light purple
NonWhiteNon
WhiteNon
Light purple
NonLight purple
Brown
Light purple
Brown
WhiteBrown
CreamBrown
CreamBrown
WhiteNonWhite
Non

ViscosityMiddle
SoftMiddle
Middlesoft
Very soft
Very soft
MiddleStrongStrongMiddleStrong＊: P: Pohnpei island in F.S.M., Yp: Yap Proper in F.S.M.,
Solo yam: Indonesia

The “Solo yam” strain was introduced from Indonesia by Kagoshima University (ISHIHATA
et al. 1984). It is currently cultivated on Yaku Island and has been used by the authors as the indicator plant in the studies of yam, so the Solo yam strain was also used in this study for comparison. First, we investigated the form and color of the tubers harvested in December 2001. We
then placed a tuber into a mortar cooled to -20 °C, poured acetone cooled in the same manner
into the mortar, and crushed the tuber immediately. We extracted and filtered the juice and thenfully dried it to produce acetone powder. Using this acetone powder, we investigated differencesin protein by the NON-SDS polyacrylamide gel electrophoresis method.
After grating the tuber with a household plastic grater, we compared viscosities by visualinspection using the viscosity of Solo yam as the reference (strong).

Results and Discussion
Table 1 shows the characteristics of the strains subjected to this study. Six strains of thealata species, two of the esculenta species, two of the bulbifera species, and two unknown species were tested. The unknown species resemble the alata species in form, but the cross section

COMPARISON OF TUBER PROTEINS OF YAM INTRODUCED FROM THE POHNPEI ISLAND AND YAP PROPER, THE FEDERATED STATES OF MICRONESIA

57of the stem of the alata species is rectangular and that of the unknown species is round.
Furthermore, wings of stem are not observed in the unknown species, and its form differs inmany points from the esculenta species and others reported to be cultivated in the Pacific islands.
Therefore, though we plan to analyze that species on the DNA level, we treated it as an unknown
in this study. From the morphological observation of the leaves and stems, we conjecture thatthese two strains belong to the same species.
First, we investigated the form and color of the tuber, presence or absence of discoloration,and viscosity. The forms of tubers varied from round to cylindrical in the same species. Colorvaried from white and cream to light violet, and we found no correlation between the form andcolor. We observed discoloration after grating in all strains of the esculenta species and bulbifera
species and one strain of the alata species.
The viscosity examination results revealed that the two strains of the esculenta species hada viscosity almost equal to that of Solo yam, which has the highest viscosity among the strains
so far preserved. Furthermore, strains belonging to the same species of alata species had varying
viscosities ranging from strong to weak. Though the bulbifera species is seldom used for food,many of its plants can be seen in mountains and fields, and it is sometimes used as emergency
food during a famine. The viscosity of the bulbifera species was found to be very weak.
The results of NON-SDS polyacrylamide gel electrophoresis are shown in Fig. 1. We examined the detected bands and observed numerous bands with similar patterns among different
strains belonging to the same species. In the alata species, therefore, even strains that have extremely different viscosities exhibit similar band patterns. We thus determined that viscous protein cannot always be detected by the NON-SDS polyacrylamide gel electrophoresis method
used in this study. However, band patterns largely differ between species, so we considered thatthe types of protein existing in the tuber are somewhat similar within the same species. In the future, we are planning to conduct further studies of yam from the aspect of nutrition by implementing other types of electrophoresis methods and investigating the volumes of protein andstarch components.

Fig. 1. Different band pattern of tuber protein of yam collected from Pohnpei island
and Yap Proper by NON- SDS polyacrylamide gel electrophoresis

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ONJO Michio, HAYASHI Mitsuru, and MATSUO Tomoaki

References
ISHIHATA, K., F UKUMURA K., and N AKAZAKI A. 1984. Bull. Exp. Farm Fac. Agr. Kagoshima
Univ. 9: 13–17.
STANLEY, D. 1992. State of Pohnpei. In Micronesia handbook, 110–132. Chico: Moon
Publications Inc.
太平洋学会編 . 1989 ．太平洋諸島百科事典 , pp. 560–562 ．東京：原書房 [Society of thePacific. 1989. Encyclopedia of the Pacific Islands, 560–562. Tokyo: Hara-Shobo].