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.
The Expression of an Abscisic Acid-Responsive Glycine-Rich Protein Coincides with the Level of Seed Dormancy in Fagus sylvatica
