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 59<br />
(Sasaki 1980, Yap 1981, Tompsett 1985, Corbineau and<br />
Come 1986).<br />
Sasaki (1980) considered seeds <strong>of</strong> (i) Shorea species<br />
in the ‘yellow and white meranti’ groups, (ii) Hopea, (iii)<br />
Dipterocarpus, (iv) Vatica, (v) Dryobalanops, (vi)<br />
Balanocarpus and (vii) Parashorea to be tolerant down<br />
to 4°C. By contrast, he believed that seeds <strong>of</strong> Shorea<br />
species in the ‘red meranti and balau’ groups were<br />
intolerant <strong>of</strong> temperatures below 15°C. He classified<br />
Anisoptera as a tolerant genus in a separate publication<br />
(Sasaki 1979). Yap (1981) later proposed a three-group<br />
classification: firstly, seed <strong>of</strong> species in the<br />
Dipterocarpus, Dryobalanops, Neobalanocarpus and<br />
Vatica genera were said to be intolerant <strong>of</strong> temperatures<br />
below 14°C; secondly, seed <strong>of</strong> Shorea species in the<br />
sections Mutica, Pachycarpae and Brachypterae were<br />
considered intolerant <strong>of</strong> temperatures below 22°-28°C;<br />
and, finally, seed <strong>of</strong> Shorea species in the Anthoshorea<br />
section and seed <strong>of</strong> Hopea and Parashorea could be<br />
cooled to 4°C (but were recommended to be stored at<br />
14°C). Further details relating taxonomic classification<br />
to chilling damage are given elsewhere (Tompsett 1992).<br />
A possible explanation <strong>for</strong> the above inconsistencies is<br />
that different authors have studied the effects <strong>of</strong> chilling<br />
<strong>for</strong> different periods <strong>of</strong> time, leading to different<br />
conclusions; exposure <strong>of</strong> seed to longer periods <strong>of</strong><br />
chilling can show up chilling damage which might<br />
otherwise have ben missed in the case <strong>of</strong> relatively chillresistant<br />
species.<br />
The processes behind the chilling physiology<br />
phenomenon have not been adequately studied. However,<br />
differences among species in susceptibility to chilling<br />
damage are confirmed by the base temperature data in<br />
Table 1. In particular, Hopea species appear the most<br />
resistant to chilling damage, since they have the lowest<br />
base temperatures. A low value <strong>for</strong> the base temperature<br />
is expected if germination ability decreases relatively<br />
slowly as germination temperature is reduced. It should<br />
be emphasised, however, that these results apply<br />
exclusively to moist seeds. Storage <strong>of</strong> dry orthodox<br />
dipterocarp seeds at low temperatures is described in<br />
the storage section below.<br />
The differences in chilling tolerance <strong>of</strong> seeds among<br />
dipterocarp species are quantitative rather than qualitative.<br />
Seed <strong>of</strong> the ‘tolerant’ species S. roxburghii eventually<br />
suffers damage at 2°C -5°C relative to seed kept at<br />
warmer temperatures (Purohit et al. 1982, Tompsett<br />
1985). Another example <strong>of</strong> chilling damage which<br />
occurred over a lengthy period <strong>of</strong> time is that to H.<br />
hainanensis. For this species, seed at 5°C almost all<br />
died after 6 months; by contrast, at 15°C -20 °C no loss<br />
<strong>of</strong> viability occurred (Song et al. 1984).<br />
Harvest and Maturity<br />
The condition <strong>of</strong> seed at harvest is <strong>of</strong> primary concern<br />
in the planning <strong>of</strong> all physiological experiments.<br />
Moisture contents at or near harvest are given <strong>for</strong> 25<br />
species in Table 2, including examples from both seasonal<br />
and aseasonal dipterocarp <strong>for</strong>ests. Seeds <strong>of</strong> the three<br />
species with the lowest moisture contents, which are<br />
found in seasonal <strong>for</strong>ests, were collected from the<br />
ground after natural desiccation. For these seeds, drying<br />
occurs very swiftly after abscission because the open<br />
canopy exposes them to direct sunlight. Of the remaining<br />
species listed, some are derived from the dry <strong>for</strong>est and<br />
others from moist areas; they possessed a relatively high<br />
range <strong>of</strong> post-processing moisture contents between 29<br />
and 56% (usually, seeds were just de-winged).<br />
It has been realised <strong>for</strong> some time that there can be a<br />
considerable difference between whole-seed moisture<br />
content and moisture content <strong>of</strong> the embryo or embryo<br />
axis (Grout et al.,1983). Since axis or embryo moisture<br />
content is more closely related to basic physiological<br />
processes than whole seed moisture content, it is a<br />
preferable measure to use herein. Axis values have been<br />
determined <strong>for</strong> dipterocarp species (Table 2) and range<br />
from 51 to 74%, except in the case <strong>of</strong> the much lower<br />
value <strong>for</strong> the dry-zone species Dipterocarpus<br />
tuberculatus, which was collected after natural drying.<br />
Seed maturation<br />
A few developmental studies have been carried out on<br />
dipterocarp species; whole-seed moisture content has<br />
been employed in most <strong>of</strong> these as the main physiological<br />
criterion. Sasaki (1980) reported that the moisture<br />
content (wet basis) <strong>of</strong> Shorea roxburghii declined from<br />
60 to 50% in the final 3 weeks <strong>of</strong> maturation on the tree.<br />
Panochit et al. (1986) reported a comparable decline<br />
from 40 to 30% <strong>for</strong> the same species, whilst a reduction<br />
from 59 to 49% was reported <strong>for</strong> S. siamensis (Panochit<br />
et al. 1984).<br />
Nautiyal and Purohit (1985a) assessed changes<br />
during maturation <strong>of</strong> S. robusta seed; they described<br />
these changes as biphasic. Over the 60 days from anthesis<br />
to maturity, concentrations <strong>of</strong> soluble carbohydrates,<br />
starch, soluble protein and acid phosphatase were