Hormonal Physiology of Wood Growth in Pinus radiata D. Don: Effects of Gibberellin A₄ and the Influence of Abscisic Acid Upon (³H) Gibberellin A₄ Metabolism

Abstract

Xylem cell radial growth of 1-year-old seedlings of P. radiata was significantly increased by application of gibberellin A₄ (GA₄), or of abscisic acid (ABA) plus a smaller, and non-significant, dosage of GA₄. In 10 out of 15 additional tests throughout the growing season, ABA tended to increase radial growth, and only once (in mid-summer) did ABA significantly reduce xylem cell radial growth. Seedlings were grown under well watered, warm, long-day conditions. Where GA₄ (22 µg) or ABA (20 µg) plus GA₄ (2 µg) increased xylem cell radial growth, the metabolism of [³H]GA₄ was followed. Although not characterized from P. radiata, GA₄ is native to P. attenuata, a species which will hybridize with P. radiata. At low levels (2 µg) of carrier GA₄, [³H]GA₄ metabolism was rapid, and ABA (20 µg) enhanced metabolism, especially into acidic metabolites. One metabolite was definitively characterized as GA₃₄ and other metabolites eluted in regions of a SiO₂ partition column coincidental with GA₁, GA₂, GA_B and GA₄ glucosyl ester. At high levels (22 µg) of carrier GA₄, [³H]GA₄ metabolism was dampened, and ABA reduced the metabolism of [³H]GA₄ even further.

Enhanced metabolism of [³H]GA₄ in P. radiata by ABA is analogous to an earlier report [Nadeau et al. (1972). Planta 107, 315-24] where ABA enhanced the metabolism of [³H]GA₁ in barley aleurone layers while reducing their GA₁-stimulated amylase production. However, in P. radiata, ABA increased xylem cell radial growth while enhancing [³H]GA₄ metabolism. If the metabolites of GA₄ that were enhanced by ABA were highly active, a similar effect might also be exerted on endogenous GAs, thus explaining the modest tendency of ABA to promote radial growth through much of the year.