A review of dipterocarps - Center for International Forestry Research

Seed Handling 81
Most, but not all, studies concerning the effects on
viability of gases other than ambient air have been carried
out under poorly controlled conditions. In most cases
inflated polythene bags were used so that the gas under
test was liable to mixing over time with ambient air and,
in addition, respiration of seeds inside the bag altered
the gas environment.
Various gaseous environments have been assessed.
Song et al. (1984) was able to maintain 80% viability of
Hopea hainanensis seeds for up to 365 days by
maintaining oxygen levels above 10%. On the other hand,
Yap (1981) was able to store seeds of D. oblongifolius
in bags filled with nitrogen and reported a 60%
germination after a period of 60 days. However,
Tompsett (1983) reported a stepwise decrease in
longevity of the seed as oxygen was lowered
progressively from 10% to zero per cent for the
recalcitrant seed of the tropical tree species Araucaria
hunsteinii (Araucariaceae). Carefully controlled
conditions were employed; a continuous flow of the gas
under test was supplied to the seed at the correct relative
humidity. This study also highlighted two further points.
Firstly, increased concentrations of carbon dioxide and
ethylene had no beneficial effects (Araucaria
hunsteinii; Tompsett 1983) and, secondly, oxygen levels
above 21% did not enhance storage life (D. turbinatus;
Tompsett, unpublished). There appear to be no reports
that altering the gaseous environment from that of
ambient air can increase longevity for recalcitrant seeds.
Storage Using Germination Inhibitors
An alternative method to prevent germination during
storage is by incorporating germination inhibitors into
the storage system. Substances that have been used are
polyethylene glycol (PEG), sucrose, sodium chloride
and abscisic acid (ABA). Goldbach (1979) reported that
by treating seeds of Meliococcus (Sapindaceae) and
Eugenia (Myrtaceae) with 10 -4 molar ABA solution at
15°C it was possible to store seeds for four to six months
with at least 89% final viability. This general approach
for recalcitrant seed storage has subsequently not been
confirmed as successful; a problem encountered with
the ABA method is the speedy germination of seed during
storage.
Fungicide Treatment Followed by Partial Desiccation
and Storage at Controlled Temperatures
Partial desiccation was proposed as a favourable
approach by King and Roberts (1979). Furthermore,
several researchers have mixed fungicide with stored
seeds to protect against fungal growth. However, few have
conducted controlled experiments to test the effects of
applying combinations of fungicide treatments with
partial desiccation treatments. Nevertheless, Hor (1984)
treated cacao seeds with a 0.2% benlate/thiram mixture,
partially desiccated the seeds by air drying and then stored
them loosely packed in polythene bags at 21-24°C. The
viability of the seeds in his study was prolonged from
one week to about 24 weeks with a final 50% germination.
This approach needs to be further assessed with the factors
separately examined.
Partial Desiccation without Fungicide
Maury-Lechon et al. (1981) reported partial drying of
dipterocarp seeds but did not use fungicides. From their
results they recommended drying seeds to half the original
moisture content. This latter procedure prevents
pre-germination in storage. However, as their experiments
did not include undried controls, the overall benefit was
not established.
Storage at Harvest Moisture Content without
Fungicide Application or Partial Desiccation
The examples cited from Tompsett (1992) in Table 4
were not subjected to partial desiccation and were not
combined with fungicide. Further examples are given in
the Seed Physiology chapter and show a total of 13
species capable of storage for longer than 100 days.
The pre-germination problem associated with the
storage of moist seed is illustrated by results for S.
roxburghii; seeds of this species stored at 16°C with
approximately 40% moisture content had about 50% pregermination
after 44 weeks of storage (Tompsett 1985).
However, provided desiccation and mechanical damage
to the radicle are avoided, viable seedlings can still be
produced by a high proportion of the pre-germinated
seeds.
Research on Seedling Storage and
Cryopreservation
Despite the improvements in short to medium-term
storage, it is not feasible to store recalcitrant dipterocarp
seeds in the longer term. Complementary methods are
being sought to ensure a continuous supply of planting
material. Two approaches have been attempted at FRIM
in unpublished work of Sasaki and of Krishnapillay; these
comprise seedling storage and cryopreservation of seed
materials.