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The hormonal control of stem unit (foliar appendage and axillary structure, if present, plus subtending internode) number and length was investigated in shoots of Scots pine ( Pinus sylvestris  L.) and white spruce ( Picea glauca  (Moench) Voss). Seedlings were treated with six gibberellins (GA 1 , GA 3 , GA 4 , GA 5 , GA 9  and GA 20 ) and two auxins (indole-3-acetic acid (IAA) and naphthaleneacetic acid (NAA)) when either neoformed growth was occurring or the terminal vegetative bud was developing. Hormones were applied by drenching the shoot tip, injecting the stem or spraying the foliage. Combined results for all three application methods indicated that shoot elongation in first-year seedlings (i.e., neoformed growth) was promoted in both species by GA 1 , GA 3 , GA 4  and, less obviously, by GA 9 . This promotion was attributable to an increase in length, rather than number, of stem units. However, the number of stem units formed during terminal bud development, as reflected in the number of needles (white spruce) or cataphylls (Scots pine) present on the shoot resulting from the terminal bud, was stimulated by GA 1 , GA 3  and GA 4  in both species and by GA 9  in Scots pine. The GA-induced increase in the number of preformed stem units was associated with increased bud width in white spruce and increased bud length and resulting shoot length in Scots pine. In contrast, application of IAA or NAA either did not affect or inhibited both neoformed growth and terminal bud stem unit number, depending on the application method and concentration. We conclude that, in the Pinaceae, (1) GA stimulates the activity of both the subapical meristem during neoformed growth and the apical meristem during vegetative bud development, and (2) the early non-hydroxylation pathway, via GA 9 , is the major route of GA biosynthesis. The role of auxin in the control of stem unit number and length remains to be resolved.

Effects of exogenous gibberellin and auxin on shoot elongation and vegetative bud development in seedlings of Pinus sylvestris and Picea glauca