Effects of Fall Fertilizer Applications on Mitotic Index and Bud Dormancy of Loblolly Pine Seedlings

Abstract

Abstract A series of studies examined the effects of fall fertilization with diammonium phosphate (DAP) on mitotic index and bud dormancy [as measured by mean days to budbreak (DBB)] of two half-sib seed sources of loblolly pine. The first study tested different rates of DAP (0, 67, and 202 kg N/ha), the second study compared DAP with ammonium nitrate, and the third study examined the effect of different application dates (September 28, October 19, and November 9). An increase in mitotic index of unfertilized seedlings was observed during October and was due to developmental activity which follows initial budset. Differences in mitotic index were observed between families in all three studies. Overall, the Georgia family has a higher mitotic index, but in one study, the Virginia family had higher values in the spring. Both families tended to reach a minimum level of mitotic index at the same time (mid- to late December). However, the Virginia family reached maximum rest (as measured by days to budbreak) about 1 to 2 weeks prior to the Georgia family. Fertilization with DAP in the fall (after budset in September) did not delay the progression of the bud dormancy cycle as measured by days to budbreak in a greenhouse. The overall effect of fall fertilization on increasing the mitotic index was temporary and only lasted for about three weeks after fertilization. These findings indicated that a direct relationship may not exist between the bud dormancy cycle and mitotic index. For. Sci. 38(2):336-349.