Promotion of seed production in Douglas-fir grafts by girdling + gibberellin A_4/7 stem injection, and effect of retreatment

Abstract

Twelve- to 17-year-old Douglas-fir grafts, which in 1985 received no treatment, stem girdles only (G), or girdles plus stem injection of gibberellin A_4/7 (G + GA) at vegetative bud burst, were in 1987 retreated with G + GA or left untreated. Tree were untreated in 1986 and cone production the following year was very sparse. G + GA treatment in 1987 increased the 1988 production of seed cones from 465 to 1600 per tree, with a comparable increase in frequency of grafts producing a heavy crop of pollen cones. However, induction treatments applied in 1985 adversely affected seed- and pollen-cone production in 1988, independent of treatment in 1987. Cone size, total and filled seed per cone, and seed germination were little affected by treatment either year. Treatment with G + GA in 1987 only maximized the production of filled seeds (48 100/tree), although G + GA in both 1985 and 1987 (31 200/tree) was still highly effective relative to no treatment in either year (14 700/tree). The G + GA treatment adversely affected tree condition more severely than previously (or since) experienced, probably as a consequence of grafts also being subjected to prolonged late-summer droughts during each of the treatment years.