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 �g) or ABA (20 �g) plus GA4 (2 �g) 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 �g) of carrier GA4, [3H]GA4 metabolism was rapid, and ABA (20 �g) 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, GAB and GA4 glucosyl ester. At high levels (22 �g) 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.