Changes in nuclear genome size and relative ribosomal RNA gene content in cambial region cells ofAbies balsameashoots during the development of dormancy

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Abstract
The relationship between nuclear genome size, measured cytophotometrically, and relative ribosomal RNA gene (rDNA) content, determined as the 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 of rest and quiescence. The dormancy status was manipulated by exposing potted trees for 13 weeks, starting August 14 when the cambium was still active, to one of the following environments: (i) the declining temperature and photoperiod of the natural environment (denoted N-N), (ii) simulated natural temperature and 15-h photoperiod (sN-15), (iii) simulated natural temperature and 8-h photoperiod (sN-8), and (iv) warm temperature and 15-h photoperiod (sN-15). On November 12, the trees were transferred to a greenhouse having environmental conditions favorable for growth to assess their ability to reactivate. The entire activity –rest–quiescence transition occurred in the N-N environment and was associated with an increase in nuclear genome size and a decrease in the relative rDNA content. The sN-8 environment also induced the transition but not completely, and the associated increase and decrease in nuclear genome size and relative rDNA content, respectively, were smaller than in the N-N environment. The sN-15 environment delayed the onset of rest, inhibited the changeover to quiescence, increased then decreased the nuclear genome size, and did not affect the relative rDNA content. The W-15 environment, which induced budbreak and the formation of a false ring, also prevented the rest–quiescence transition and transiently increased the nuclear genome size without altering the relative rDNA content. The data suggest that the normal rest–quiescence progression in shoot cambial cells is associated with an increase in nuclear genome size, which is caued by amplification of a fraction that is not rDNA but is recognized by our genomic probe. Keywords: dormancy, genome size, ribosomal RNA genes, vascular cambium.

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