RESUMO
We assessed the effects of nitrogen, drought and gibberellin A(4/7) on sexual expression of 2- and 3-year-old jack pine (Pinus banksiana L.) seedlings grown in either a 1/3 peat/sand mix or a 3/1 peat/perlite mix. The seedlings were either watered daily (well-watered treatment) or droughted by withholding water from July 13 to August 24 (drought treatment). Half of the plants in each irrigation treatment were sprayed with 200 mg l(-1)gibberellin A(4/7) at weekly intervals from June 28 to August 24; the remaining plants were not sprayed. Each gibberellin treatment was split into three subplots, and each subplot was supplied with nitrogen in the nutrient solution at 3, 100 or 300 mg l(-1) from June 27 to September 11. The drought treatment increased pollen strobilus production, whereas seed strobilus production was higher in well-watered trees than in drought-treated trees in the first year after treatment, but not in the subsequent year. Gibberellin A(4/7) promoted seed strobilus production but not pollen strobilus production in the first year after treatment but had no effect in the second year. Among the nitrogen treatments, the largest proportion of trees bearing pollen strobili was in the 3 mg l(-1) N treatment. Nitrogen supply did not affect seed strobilus production in the first year after treatment, but in the following year, the proportion of trees with seed strobili was higher in the 3 mg l(-1) N treatment than in the 100 and 300 mg l(-1) N treatments. Growing medium had no detectable effect on pollen strobilus production, but in the year following treatment, more trees produced seed strobili in peat/sand than in peat/perlite.
