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Effects of Applied Auxin and Gibberellin on Phloem and Xylem Production in Needle Leaves of Pinus

Naphthaleneacetic acid (NAA) and gibberellic acid (GA3) were applied to needle leaves of Pinus brutia and P. strobus to elucidate the control mechanisms for vascular differentiation. In needles of P. brutia, phloem production was much greater and xylem production slightly greater with 15 mg/liter NAA, 150 mg/liter GA3, and combination treatments than in distilled water controls. The effects of growth regulators were greatest near the needle bases. In P. strobus, 0.1% NAA in lanolin paste stimulated phloem production but had no effect on the xylem. In both species the size and appearance of the cambial zone in the needles were not affected by the various treatments, indicating that the rates of cell differentiation were roughly equal to rates of cell division. Since mature needles of these species normally produce secondary phloem but little or no secondary xylem and since secondary growth is normally more extensive near the needle base, the applied growth regulators did not much alter the basic pattern of differentiation. We hypothesize that, in addition to endogenous auxin and GA3, there is an unidentified tracheid differentiation factor produced in young, but not in old, needles that limits tracheid differentiation both in the stem and in the needle itself.