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Relationships between physical and chemical characteristics of dipterocarp seeds by Michiko Nakagawa and Tohru Nakashizuka

Abstract The physical and chemical seed characteristics of 11 dipterocarp species (four Dipterocarpus, two Dryobalanops and five Shorea species) were quantified to examine variations among species. We measured dry weight, pericarp thickness and concentrations of nitrogen, total phenol, condensed tannin, lignin, lipid, starch and total sugar. Although there was low intraspecific variation among parent trees, each species had unique characteristics. We found significant correlations between chemical defensive compounds (i.e. lignin and total phenol) and nitrogen concentration. However, these relationships were no longer significant when an outlier was removed. Fruiting frequency was not significantly correlated with any chemical compound.

Results

Chemical characteristics of dipterocarp seeds

All chemical compounds found in seeds differed significantly among species. The highest concentration of condensed tannin was measured in Dryobalanops lanceolata. Dipterocarpus palembanicus had the highest total phenol, lignin and nitrogen concentrations. Shorea beccariana and S. laxa seeds contained the highest concentrations of total sugar and starch, respectively, among the species studied, while S. parvifolia had the highest lipid concentration. Four species (D. globosus, D. pachyphyllus, D. tempehes and S. laxa) did not contain condensed tannin.


Correlation between concentration of nitrogen and degrees of defence

We found a significant positive correlation between nitrogen concentration and lignin or total phenol concentrations, whereas nitrogen concentration was not significantly correlated with pericarp thickness. However, when the values for an outlier, D. palembanicus, were removed, the differences were no longer significant. There was no significant separation of species by genus in terms of nitrogen, total phenol and lignin concentrations, whereas Dipterocarpus species, with the exception of D. palembanicus, showed a tendency to have a lower concentration of total phenols than the Shorea species; the two Dryobalanops species were intermediate

Correlations between fruiting frequency and chemical compounds

With the exception of starch, all chemical compounds tended to increase in concentration with fruiting frequency, although none of these correlations was significant. Starch concentration was negatively, though not significantly, correlated with fruiting frequency.

Physical and chemical properties of dipterocarp seeds

Our first hypothesis, regarding nitrogen concentration and physical or chemical defences, was not supported when the outlier, D. palembanicus, was excluded. This result suggests that defensive systems in dipterocarp seeds by means of physical characteristics (pericarp thickness) and chemical compounds (total phenol and lignin) may not be well developed, and that there may be another means in place for preventing seed predation. One possible system for avoiding seed predation could be synchronous fruiting among various species, such as GF, in order to satiate seed predators; this hypothesis has been discussed as one of the most likely explanations with respect to the ultimate impact of GF (Janzen, 1974; Sakai, 2002). However, our second hypothesis was not supported, as neither defensive substances nor storage compounds were correlated with fruiting frequency. Thus, the characteristics of dipterocarp seeds may be regulated not only by protection from seed predators, but also by substitute benefits of seed dispersal or seedling establishment.

Across all species, seed characteristics changed continuously, and we did not find any apparent association between physical and chemical seed properties and genus. However, the composition of chemical compounds in dipterocarp seeds differed significantly among species. In particular, D. palembanicus had unique characteristics, with a high concentration of both defensive substances and nitrogen. Ecological study on seeds and seedlings of D. palembanicus will permit an understanding of the significance of these unique properties.

Among the three genera studied, Dipterocarpus species, with the exception of D. palembanicus, had properties that differed somewhat from those of Dryobalanops and Shorea. A significant difference between Dipterocarpus and Shorea was detected only in lipid concentrations, in that Shorea seeds had a higher lipid concentration than those of Dipterocarpus. In this study, while seeds of Dryobalanops were excluded from inter-genus examination because of the small sample size, their characteristics were similar, relatively speaking, to those of Shorea. A trend emerged, in that Dipterocarpus seeds had a lower total phenol concentration and a higher starch concentration, while the other genera had a higher total phenol concentration with a higher lipid concentration. Given the many reports on the preferences of seed predators for lipid-rich seeds (Smith and Follmer, 1972; Smallwood and Peters, 1986), the selection for protection by total phenol in seeds with a higher lipid concentration may have been precipitated by increased predation pressure.

Further studies of the physical and chemical characteristics of other dipterocarp species or genera, as well as other families, are necessary for a comprehensive understanding of the significance of seed content. We also need to investigate seed predation rates by vertebrates, and the survivorship and initial growth of seedlings, to elucidate the ecological implications of variations in seed characteristics.