CONTROL OF LONGITUDINAL AND CAMBIAL GROWTH BY GIBBERELLINS AND INDOLE-3-ACETIC-ACID IN CURRENT-YEAR SHOOTS OF PINUS-SYLVESTRIS

We investigated the involvement of gibberellins (GAs) and indole-3-ace tic acid (IAA) in the control of longitudinal and cambial growth in cu rrent-year shoots of Pinus syvestris L. Elongating terminal shoots, lo cated at the apex of previous-year (1-year-old) branches in the upperm ost whorl on the main stem, were variously decapitated (apical 5 to 10 mm removed), defoliated (all developing needle fascicles removed) and treated with exogenous GA(4/7) Or IAA, or both. Shoot length and the radial widths of xylem and phloem were measured, and the concentration s of GA(1), GA(3), GA(4), GA(9) and IAA in the stem were determined by combined gas chromatography-mass spectrometry with deuterated GAs and [C-13(6)]-IAA as internal standards. Decapitation decreased the produ ction of xylem and phloem and the IAA concentration, but did not alter either longitudinal growth or the concentrations of GAs. Defoliation markedly inhibited shoot elongation, as well as cambial growth, and re duced the concentrations of GA(1), GA(3), GA(4), GA(9) and IAA. Applic ation of GA(4/7) to defoliated shoots promoted longitudinal growth and phloem production, without affecting xylem production or IAA concentr ation. Application of GA(4/7) and IAA together to decapitated + defoli ated shoots increased shoot elongation, xylem and phloem production an d IAA concentration, whereas applying either substance alone had a sma ller effect or none at all. We conclude that, for elongating current-y ear shoots of Pinus sylvestris, (1) both the shoot apex and the develo ping needle fascicles are major sources of the IAA present in the stem , whereas stem GAs originate primarily in the needle fascicles, (2) GA s and IAA are required for both shoot elongation and cambial growth, a nd (3) GAs act directly in the control of shoot growth, rather than in directly through affecting the IAA concentration.