A review of dipterocarps - Center for International Forestry Research

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Some recent evidence (Tompsett, unpublished), however,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