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