EFFECTS OF PROHEXADIONE ON CAMBIAL AND LONGITUDINAL GROWTH AND THE LEVELS OF ENDOGENOUS GIBBERELLINS A(1), A(3), A(4), AND A(9) AND INDOLE-3-ACETIC-ACID IN PINUS-SYLVESTRIS SHOOTS

Prohexadione, a gibberellin (GA) biosynthesis inhibitor, was applied i n ethanol around the circumference at the midpoint of the previous yea r terminal shoot of dormant Pinus sylvestris seedlings. After cultivat ing the seedlings under environmental conditions favorable for growth for 10 weeks, longitudinal and cambial growth were measured, and the e ndogenous levels of GA(1), GA(3), GA(4), GA(9), and indole-3-acetic ac id (IAA) were determined by combined gas chromatography-mass spectrome try, using deuterated GAs and [C-13(6)]IAA as internal standards. Proh exadione application inhibited elongation and xylem and phloem product ion in the current year terminal shoot and xylem production in the pre vious year terminal shoots. Concomitantly, in both ages of shoots the cambial region contents of GA(1), GA(3), and GA(4) were decreased, whe reas the level of GA(9) was increased. However, the IAA content was no t altered in the terminal bud on the current year terminal shoot or in the cambial region of the current year or previous year terminal shoo ts. The results provide additional evidence that: (1) GAs are involved in the regulation of cambial growth, as well as longitudinal growth, in Pinus sylvestris shoots; (2) they act directly, rather than indirec tly, by altering the IAA level; and (3) the GA(9) --> GA(4) --> GA(1) pathway is a major route of GA biosynthesis in conifer species.