Understanding sucrose metabolism and growth in a developing sweetgum plantation.
Stem diameter growth of 9-year-old sweetgum (Liquidambar styraciflua) trees was measured and related with the activity of sucrose synthase (SS), an enzyme that has been associated with carbon sink strength in agriculture crops and tree seedlings. In 1984, 10 sweetgum seedlings were transplanted to control plots and plots amended with sewage sludge or nitorgen and phosphorus fertilizer. After 8 growing seasons, trees on sludge contained four times as much volume as controls and twice as much volume as those on fertilized plots.In 1991, sludge treated soil had about 2 to 5 times as much in N and P as soils from other treatments.Beginning in 1992, sucrose metabolizing enzymes were assayed monthly in both phloem- and xylem-side cambial tissues. In late April when stem bark became easy to peel, there was SS activity in phloem-side but not in xylem-side stem cambial tissues. Phloem SS activity remained constant throughout the season. In less that 2 weeks, SS in xylem cambium exceeded that in phloem cambium by 20 fold and reached highest levels in June through August. For most of the growing season, there were no differences among treatments in the patterns for SS activity in xylem-side stem cambial tissues.However, SS activity in cambial tissues of trees from the sludge sites remained high in late September and October, when cambium of trees from the other treatments was dry and inactive and no SS activity was measured.Seasonal trends in activity of pyrophosphate-dependent phosphofructokinase (PPi- PFK) were similar to those of SS, but activity of its alternative enzyme, ATP-PFK, was low and constant throughout the growing season. We conclude that: (1) application of sewage sludge enhanced tree growth for at least 9 years, (2) sucrose metabolism is similar in 9-year-old sweetgm trees and in seedlings, and (3) sludge application increases the annual duration of high SS and PPi-PFK activities.