CHANGES IN NUCLEAR GENOME SIZE AND RELATIVE RIBOSOMAL-RNA GENE CONTENT IN CAMBIAL REGION CELLS OF ABIES-BALSAMEA SHOOTS DURING THE DEVELOPMENT OF DORMANCY

The relationship between nuclear genome size, measured cytophotometric ally, and relative ribosomal RNA gene (rDNA) content, determined as th e ratio of the hybridization signals from a 25S rRNA gene probe and a randomly labelled total genomic DNA probe, was investigated in cambial region cells of balsam fir (Abies balsamea (L.) Mill.) shoots during the onset of dormancy and the transition between the dormancy stages o f rest and quiescence. The dormancy status was manipulated by exposing potted trees for 13 weeks, starting August 14 when the cambium was st ill active, to one of the following environments: (i) the declining te mperature and photoperiod of the natural environment (denoted N-N), (i i) simulated natural temperature and 15-h photoperiod (sN-15), (iii) s imulated natural temperature and 8-h photoperiod (sN-8), and (iv) warm temperature and 15-h photoperiod (W-15). On November 12, the trees we re transferred to a greenhouse having environmental conditions favorab le for growth to assess their ability to reactivate. The entire activi ty-rest-quiescence transition occurred in the N-N environment and was associated with an increase in nuclear genome size and a decrease in t he relative rDNA content. The sN-8 environment also induced the transi tion but not completely, and the associated increase and decrease in n uclear genome size and relative rDNA content, respectively, were small er than in the N-N environment. The sN-15 environment delayed the onse t of rest, inhibited the changeover to quiescence, increased then decr eased the nuclear genome size, and did not affect the relative rDNA co ntent. The W-15 environment, which induced budbreak and the formation of a false ring, also prevented the rest-quiescence transition and tra nsiently increased the nuclear genome size without altering the relati ve rDNA content. The data suggest that the normal rest-quiescence prog ression in shoot cambial cells is associated with an increase in nucle ar genome size, which is caued by amplification of a fraction that is not rDNA but is recognized by our genomic probe.