Key message
Applying paclobutrazol at dormancy induction inhibited shoot apical meristem mitotic activity, thereby decreasing rate and duration of needle-primordium initiation and thus needle-primordium number in terminal buds ofPicea marianaseedlings.
Abstract
The effect of applying various rates of paclobutrazol (an inhibitor of gibberellic acid biosynthesis) at dormancy induction on shoot apical meristem activity during terminal bud development in first-year Picea mariana Mill. (B.S.P.) seedlings was investigated. During needle-primordium initiation, mitotic activity was reduced in shoot apical meristems of treated seedlings compared with control seedlings. The reduction was most evident within the peripheral zone where primordia arose but also occurred within the apical zone, which provides cells to the peripheral zone. The reduced mitotic activity within the peripheral zone coincided with a decrease in both rate and duration of needle-primordium initiation on nascent embryonic shoots within terminal buds in treated seedlings compared with control seedlings. Moreover, meristems of treated seedlings were smaller, shorter, and narrower (determined by cell counts as another measure of mitotic activity) compared with control seedlings. Thus, these meristems had less available space for needle-primordium initiation. As a result, embryonic shoots in treated seedlings had fewer needle primordia compared with control seedlings. Furthermore, onset of bud endodormancy (delimited by an absence of mitotic activity within the shoot apical meristem after completion of needle-primordium initiation) was realized in treated seedlings before the last sampling date, whereas it was not realized in control seedlings by the last sampling date.