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<br />
P.B. Tompsett<br />
Seed is the natural vehicle <strong>for</strong> gene movement and<br />
storage. It is the usual <strong>for</strong>m in which germplasm is<br />
collected. When procedures can be devised to transport<br />
and retain material in this <strong>for</strong>m, many <strong>of</strong> the technical<br />
problems associated with other methods can be avoided.<br />
This advantage renders seed especially appropriate <strong>for</strong><br />
users in tropical and subtropical countries. In general,<br />
seed is the most common <strong>for</strong>m <strong>of</strong> propagation <strong>for</strong><br />
af<strong>for</strong>estation and is the <strong>for</strong>m in which breeding stock is<br />
usually retained. There are, however, considerable<br />
problems remaining in the use <strong>of</strong> seed. Some <strong>of</strong> these<br />
are discussed below <strong>for</strong> dipterocarp species in relation<br />
to the underlying seed physiology processes.<br />
Much pioneering work on agricultural crop seed<br />
physiology was conducted over the last 20 years (see<br />
below <strong>for</strong> some references) and the principles<br />
discovered <strong>of</strong>ten apply to seed <strong>of</strong> woody species. These<br />
earlier results have been translated into technological<br />
principles. Thus, manuals have been published on the<br />
design <strong>of</strong> seed storage facilities (Cromarty et al. 1982),<br />
seed management techniques (Ellis et al. 1984) and a<br />
handbook on seed technology <strong>for</strong> genebanks (Ellis et al.<br />
1985). Knowledge <strong>of</strong> seed physiology has thus improved<br />
practical handling and management <strong>of</strong> crop seeds.<br />
Compared to crop species, relatively little research<br />
has been published on tropical and subtropical tree seed<br />
technology and physiology. Publications have been<br />
produced following IUFRO Seed Problems Group<br />
meetings, the most recent <strong>of</strong> which was held in Tanzania<br />
(Olsen 1996). Another source <strong>of</strong> in<strong>for</strong>mation is a<br />
summary <strong>of</strong> some relevant seed physiology projects<br />
which has recently been published in database <strong>for</strong>m<br />
(Tompsett and Kemp 1996a, b). Also, a seed compendium<br />
has been published which supplies succinct entries on<br />
many tropical trees (Hong et al. 1996).<br />
A considerable amount <strong>of</strong> empirical work on the<br />
storage <strong>of</strong> <strong>for</strong>est tree seed has been carried out; a<br />
sampling is given in Chapter 4. A more physiological<br />
research approach is relatively new. Many tree species<br />
have seed that is desiccation-sensitive (‘recalcitrant’),<br />
Chapter 3<br />
so that moisture physiology is especially important <strong>for</strong><br />
this group. However, in a recent <strong>review</strong> article on water<br />
in relation to seed storage, the section on desiccationsensitive<br />
seeds comprised only 4% <strong>of</strong> the article<br />
(Roberts and Ellis 1989). More attention is, however,<br />
now being given to recalcitrant seeds (see, <strong>for</strong> example,<br />
Berjak and Pammenter 1996).<br />
Framework <strong>of</strong> the Review<br />
Germination is basic to all aspects <strong>of</strong> seed studies; work<br />
on germination physiology, especially in relation to<br />
temperature, is thus considered first. Another important<br />
experimental consideration is the physiological<br />
condition <strong>of</strong> the seed at the time <strong>of</strong> harvest, moisture<br />
content being the single most important factor; this is<br />
considered next in the <strong>review</strong>. Thirdly, the effect <strong>of</strong><br />
desiccation is discussed; knowledge <strong>of</strong> this factor enables<br />
seed to be classed as orthodox (tolerant) or recalcitrant<br />
(intolerant). Finally, the effects <strong>of</strong> seed storage are<br />
considered.<br />
The Review<br />
Germination<br />
In general, dipterocarp seeds germinate quickly under<br />
moist, warm conditions.<br />
Early germination studies took the <strong>for</strong>m <strong>of</strong> nursery<br />
assessments leading to ecological conclusions. A major<br />
study <strong>of</strong> this type, in which 56 dipterocarp species were<br />
assessed <strong>for</strong> germination rate and final germination, is<br />
that <strong>of</strong> Ng (1980); no conclusions can be made<br />
concerning temperature effects. Conditions were more<br />
closely controlled in experiments on Shorea roxburghii,<br />
S. robusta and S. almon by Tompsett (1985); results<br />
showed optimum germination in the range 26-31°C <strong>for</strong><br />
these three species. Corbineau and Come (1986) found<br />
that final germination reached nearly 100% <strong>for</strong> both<br />
Hopea odorata and S. roxburghii over a broad range <strong>of</strong><br />
temperatures, but 30-35°C was deemed optimal because<br />
germination rates were faster. More recently, a standard