A review of dipterocarps - Center for International Forestry Research
A review of dipterocarps - Center for International Forestry Research
A review of dipterocarps - Center for International Forestry Research
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Seed Physiology 61<br />
§ orthodox; and<br />
§ recalcitrant.<br />
The orthodox type is capable <strong>of</strong> desiccation to a low<br />
moisture content (approximately 5%) and storage <strong>for</strong><br />
several years at -20 o C with little loss <strong>of</strong> viability (Roberts<br />
1973). By contrast, the recalcitrant type is not capable<br />
<strong>of</strong> desiccation to a low moisture content without loss <strong>of</strong><br />
germination capacity and cannot be stored <strong>for</strong> long<br />
periods <strong>of</strong> time (Roberts 1973).<br />
A third category <strong>of</strong> seed storage physiology has been<br />
described. It was first defined in 1984 in relation to<br />
Araucaria columnaris seed (Tompsett 1984) and was<br />
termed ‘orthodox with limited desiccation ability’<br />
(OLDA). A similar category was later defined <strong>for</strong> c<strong>of</strong>fee<br />
seed and termed ‘intermediate’; the name denotes its<br />
partial tolerance <strong>of</strong> desiccation (Ellis et al. 1990, 1991).<br />
Some recent evidence (Tompsett, unpublished), however,<br />
confirms there may be little physiological difference<br />
between this third category <strong>of</strong> seed and the orthodox<br />
type. There are, however, important practical handling<br />
difficulties associated with this third category. These<br />
problems justify its retention as a distinct storage type.<br />
Some tropical seed is additionally subject to lowtemperature<br />
damage when stored in the moist condition<br />
(chilling damage, see pages 58-59). As a result, further<br />
categories could have been included. However, it was<br />
considered preferable, <strong>for</strong> the sake <strong>of</strong> simplicity, to<br />
employ only the three desiccation-damage-based<br />
categories described above. To date, all dipterocarp<br />
species examined have been found to be subject to<br />
chilling damage when moist.<br />
Desiccation physiology<br />
Curves <strong>of</strong> germination percentage against moisture<br />
content percentage can be plotted <strong>for</strong> the results from<br />
controlled desiccation studies. These curves illustrate<br />
whether the seed is recalcitrant or not and give<br />
parameters <strong>for</strong> the way the seed responds when it is dried.<br />
One parameter is the lowest-safe moisture content<br />
(LSMC), defined as the value below which viability is<br />
immediately lost on drying. The value <strong>for</strong> this parameter<br />
provides a guide to the moisture content below which<br />
seed should not be held during handling procedures.<br />
LSMC values were assessed under standard drying<br />
conditions and were found to vary between 26% and 50%<br />
(Table 3). In Table 4 further LSMC data are given;<br />
although these were assessed using various desiccation<br />
methods, the results are in broad agreement with those<br />
in Table 3.<br />
Slope and intercept parameters are presented <strong>for</strong><br />
some species (Table 3); these define the relationship<br />
between germination and moisture content during<br />
desiccation.<br />
Desiccation rates<br />
It is possible that desiccation rate may influence viability;<br />
<strong>for</strong> example, seeds dried quickly might give lower<br />
germination than seeds dried more slowly and gently to<br />
the same moisture content. However, in the case <strong>of</strong> the<br />
‘recalcitrant’ seed <strong>of</strong> Araucaria hunsteinii<br />
(Araucariaceae) no such differences were observed<br />
(Tompsett 1982). No intensive study <strong>of</strong> this sort has been<br />
carried out on dipterocarp seed. However,<br />
Amata-Archachai and Hellum (unpublished) found that<br />
immature fruits <strong>of</strong> Dipterocarpus alatus clearly dried<br />
quicker than mature fruits; they suggested that the<br />
difference could be because <strong>of</strong> the death on drying <strong>of</strong><br />
the immature seeds. However, the faster loss <strong>of</strong> moisture<br />
by immature seeds could also be explained by their<br />
smaller size. Small seeds have a higher ratio <strong>of</strong> surface<br />
area to volume than large seeds, enabling quicker<br />
moisture loss. In this connection, Tamari (1976) found<br />
small seeds <strong>of</strong> S. parvifolia (0.3 g) gave low viability,<br />
whilst large seeds (0.5 g) gave higher viability. One<br />
explanation <strong>for</strong> the latter finding is that the smaller seeds<br />
had dried quicker and thus lost more viability than larger<br />
seeds prior to testing.<br />
Clear-cut differences in desiccation rates among<br />
seeds <strong>of</strong> species in the same genus have been reported<br />
by Tompsett (1986, 1987); rates <strong>for</strong> Dipterocarpus<br />
seeds varied greatly and depended on their size and<br />
structure. At the one extreme D. intricatus seed required<br />
only one week to dry to 7% moisture content; at the other<br />
extreme, seed <strong>of</strong> D. obtusifolius under identical<br />
conditions retained c. 30% moisture content even after<br />
5 weeks. Likewise, Yap (1986) found S. parvifolia seeds<br />
dried quicker than those <strong>of</strong> two larger-seeded species<br />
<strong>of</strong> Shorea; he believed the difference in rates to be<br />
related to pericarp thickness.<br />
Differences in desiccation rates <strong>of</strong> the type discussed<br />
above may possibly affect both the initial<br />
post-desiccation viability and the subsequent storage life<br />
<strong>of</strong> the seed. Further studies are needed to assess these<br />
effects.