Apical mitotic activity and growth in clones of Norway spruce in relation to cold hardiness

Seasonal development of apical mitotic activity and growth in three clones of Norway spruce (Picea abies (L.) Karst.) of the same age and origin but w ith differences in accumulated height growth and cold hardiness were invest igated. The clones showed no consistent difference ill mitotic index (NI), either in period or in general levels. The response of MI to temperature di ffered in spring and fall. Differences in cold hardiness between the clones was not directly coupled to differences in MI. Diameter growth ended earli er in one clone than in the other two clones, and this clone also produced lower numbers of stem units in both lateral and leader shoots. Cessation of diameter growth showed no relation to the duration and level of apical MI. The tallest clone had, as a combined effect of both size and number of ste m units, significantly longer leader shoots than the other two clones. The greater leader shoot growth of the tallest clone relative to the other two clones during 1987-1996 was also most prominent after years with sudden dro ps in fall minimum temperatures to below ca. -12 degrees C (median; interva l: -11 to -13 degrees C), following several weeks with mean temperatures ab ove 3 degrees C.