MODIFYING SEXUAL EXPRESSION OF CONTAINERIZED JACK PINE BY TOPPING, ALTERING SOIL-NITROGEN AND WATER, AND APPLYING GIBBERELLINS

The numbers of seed strobili and pollen strobilus clusters and the ext ent of branch terminal grow were determine on 6-year-old containerized jack pine (Pinus banksiana Lamb.) trees following modification of the soil nitrogen (N) supply (NH4NO3 at 3, 100, or 300 mg N/L; NO3- at 10 0 mg N/L; or NH4+ at 100 mg N/L in a nutrient solution), soil water su pply (soil water potentials above -20 kPa compared with potentials nea r -70 kPa), and tree crown size (intact trees outside polythene shelte rs and lightly versus severely topped trees under polythene shelters). These factors were tested with or without biweekly foliar application s of spray solutions containing 400 mg/L of GA4/7. Intact trees outsid e polythene shelters did not display sexual or growth responses to N o r GA4/7 treatments. Seed strobilus production on topped trees under sh elters was not influenced by the level of topping or N supply, but it was depressed by low soil moisture potentials and stimulated by GA4/7 with high or low soil water potentials. Pollen strobilus production wa s depressed by severe topping and by low soil water potential; it was stimulated by GA4/7 on lightly topped trees but not on severely topped trees and by a low (3 mg N/L) N supply. In the year after treatment, terminal growth of a branch from the 2-year-old nodal whorl was not in fluenced by nitrogen supply or by light topping but it was increased b y severe topping; it was increased by GA4/7 treatment if soil water po tential was high but not with low water potential; it was depressed by low soil water potential.