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histodifferentiation

reserve transfer

desiccation

orthodox

moisture content

seed fresh weight

seed dry weight

seed fresh weight

recalcitrant

seed dry weight

moisture content

TIME

Figure. Relationship between seed fresh weight, dry weight and moisture content during the development of orthodox and recalcitrant seeds (Adapted from Berjak & Pammenter, 2000).

(30) Differences in the tolerance of recalcitrant species to water loss have been demonstrated over time. In this regard, there is a continuum of desiccation tolerance across species, varying from orthodox through less recalcitrant to highly recalcitrant as shown in Table 1. Where they fall on this scale may relate, in part, to the habitat to which they are adapted.

The less recalcitrant seeds can withstand more water being lost before viability is lost. In addition, the initial germination which continues despite the absence of additional water, usually proceeds very slowly. Thus, if not dehydrated to great extremes, these seeds can remain viable for fairy long periods. Those species are likely to have a subtropical habitat and, in some cases, a temperate distribution includes Quercus sp. and Araucaria sp.

Moderately recalcitrant species, for example Theobroma cacao or cocoa and Hevea brasiliensis or rubber tree do not tolerate as much water loss and germinate slightly faster than species in the previous category. However, in the absence of additional

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