Germination of Eucalyptus sieberi L. Johnson seeds. II. Internal water relations

At 15-20 degrees C, seeds of Eucalyptus sieberi L. Johnson need to maintain a water content equal to 30% or more of dry weight for approximately 60 h in order to germinate. Several shorter periods of imbibition at 30% water content separated by between 6 and 120 h desiccation were as effective as an unintenupted imbibition in bringing seeds to the point of germination. Even after 8 months' desiccation an earlier imbibition shortened the time required for the completion of germination, although it did not hasten the onset of germination. The water potential of seeds sufficiently hydrated to germinate was about -0.6 MPa, whereas that of the embryo was only about -4.5 MPa. The difference was caused by the inhibition of water uptake by the inner integument of the seed coat. During germination, reserve protein in a small group of cells above the collet was hydrolyzed. Subsequent expansion of these cells caused rupture of the inner integument thereby allowing further water uptake by the embryo. The balance of water absorption and evaporative loss by the outer integument was such that most seeds on a moist substrate but exposed to dry air (6.5-8 x 10(-4) MPa) did not maintain the 30% water content necessary for germination. Thus although able to germinate on seedbeds subject to intermittent drying, E. sieberi seeds have the capacity to avoid germination except under humid conditions.