A review of dipterocarps - Center for International Forestry Research

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