A review of dipterocarps - Center for International Forestry Research

Seed Physiology 68
which some germination loss occurs on desiccation, is
associated with various properties of the seed and its
parent tree. Seed size, seed desiccation rate, seed
longevity and the habitat of the parent species have all
been found to be related to storage physiology.
Storage physiology and seed size
For three Shorea species a relationship has been noted
between seed size and desiccation tolerance; lowest-safe
moisture content values increase as size increases from
the small, desiccation-tolerant seed of S. roxburghii to
the larger, desiccation-intolerant seed of S. almon
(Tompsett 1985). A similar relationship was found in the
Dipterocarpus genus (Tompsett 1987), but in this case
it is the size of the embryo that appears more important.
Thus, two relatively small-embryoed species (D.
intricatus and D. tuberculatus) were shown to be OLDA
in their storage physiology (and can therefore be dried
with relatively little damage); on the other hand, two
species with large embryos (D. obtusifolius and D.
turbinatus) were shown to have high LSMC values and
recalcitrant physiology. There are other species that fit
this pattern (Tompsett 1986).
A further association which has been observed is that
recalcitrant seeds tend to be smooth surfaced (globular),
whilst OLDA seeds have tubercles or other projections
from the calyx. These projections may enhance
desiccation rate, leading to better storage on the forest
floor (Tompsett 1987), as explained below.
Storage physiology in relation to habitat and longevity
Seeds of three recalcitrant Shorea species from
different habitats have been found to have different
desiccation tolerances. The low-rainfall area species S.
roxburghii has seed which can be dried safely down to
35%, whereas the two monsoon or rain forest species S.
almon and S. robusta cannot be safely dried below 40%
moisture content (Tompsett 1985). Interestingly, the seed
with the greatest desiccation tolerance (S. roxburghii)
is also the seed with the greatest longevity.
A more extreme example is found in the genus
Dipterocarpus. Two dry-zone, deciduous species (D.
intricatus and D. tuberculatus) have OLDA-physiology
seeds, whilst two other species with distributions
extending into the relatively wet, evergreen areas (D.
turbinatus and D. obtusifolius) have recalcitrant seeds
(Tompsett 1987). The longevity of dry OLDA seeds is
relatively great (Table 5), whilst recalcitrant seeds cannot
be stored in the long term at present. As with other
factors, these patterns have been found to extend to seeds
of other species; trees from low-rainfall and sandy-soiled
areas tend to have greater longevity and lower LSMC
values (Tompsett 1986).
Storage physiology in relation to seed desiccation
rate
The OLDA seeds of Dipterocarpus intricatus and D.
tuberculatus can dry to below 10% in 2 weeks, whereas
the recalcitrant seed of D. obtusifolius remains above
28% moisture content even after 5 weeks in the same
drying conditions (Tompsett 1987). This situation may
have evolved because OLDA species benefit from
desiccation in terms of enhanced storage life as follows.
If the wet season arrives late, so that the seeds lie on the
ground for several weeks, viability is nonetheless
preserved by their low moisture content under natural
conditions. Conversely, the slow desiccation rate
characteristic of recalcitrant seeds is protective against
desiccation damage. The differences in desiccation rates
observed are generally associated with seed size (small
seeds dry faster) and probably also to seed anatomy.
Induction of Flowering and Seeding
Little work has been done on the artificial induction of
flowering and seeding. However, Tompsett,
Tangmitcharoen, Ngamkhajornwiwat and
Sornsathapornkul (unpublished) have found a positive
effect of the growth inhibitor paclobutrazol in promoting
the flowering of Dipterocarpus intricatus in north-east
Thailand. The best effect was found by applying the
substance at 20 g/l to buds between late September and
early November. The ability to control flowering would
aid breeding programmes and may enhance seed
production in years when it is otherwise poor.
Future Research
More work is needed to assess the seed storage
physiology categories of dipterocarp species, exploring
desiccation tolerance to assess whether the currently
known species with OLDA seed are the only ones in
existence. There are currently three such species known.
A broad range of species should be included to enable a
steady flow of material, despite the infrequent fruiting
and the logistical problems of locating, collecting and
transporting materials.