NUCLEOLAR ACTIVITY IN THE FUSIFORM CAMBIAL CELLS OF ABIES-BALSAMEA (PINACEAE) - EFFECT OF SEASON AND AGE

Nucleolar involvement in the regulation of the activity-rest-quiescenc e cycle of the vascular cambium was assessed by determining the season al variation in number, diameter, and volume of nucleoli in fusiform c ells of Abies balsamea (L.) Mill. The cells were isolated from 1- and 19-yr-old cambia and stained with either silver nitrate or Feulgen + n aphthol yellow-S. The ability of fusiform cells to incorporate [5-H-3] -uridine into nuclei and nucleoli was also determined. In the 1-yr-old cambium, the activity of the nucleoli, as evidenced by their diameter , total volume per cell, and intensity of staining with silver nitrate , exhibited two maxima during the year-a large one during cambial reac tivation in April-May and a small one during the rest-quiescence trans ition in October. Incorporation of radiolabeled uridine at 20 C was lo w at the end of the active period and increased during the rest-quiesc ence transition, suggesting that the quiescent, but not the resting, c ambium can rapidly resume nucleolar activity when the temperature is p ermissive. The number of nucleoli per cell varied between two and eigh t, and was higher during the dormant than the active period. The incre ase in number took place during the autumnal activity-rest-quiescence transition, when cambial cells were arrested in the G, phase of the ce ll cycle. Similar seasonal changes in nucleolar morphology were observ ed in the 19-yr-old cambium. Nucleolar diameter and total nucleolar vo lume were larger in the 19-yr-old cambium than in the 1-yr-old cambium , whereas nucleolar number was lower. The results suggest that repress ion of rRNA genes underlies the development of rest when the cambium w ill not produce new cells.