Sucrose metabolism, growth and transplanting stress in sweetgum seedling taproots and stems
One-year-old nursery-grown bare-root sweetgum (Liquidambar styraciflua L.) seedlings were lifted and transplanted into a nearby nursery bed or a cleared forest field in January 1994. Seedlings remaining in the same bed for the second year were the nontransplanted controls. Seedlings growing in beds were watered regularly and those in field received only rain. Specific activities of sucrose synthase (SS), pyrophosphate-dependent phosphofructokinase (PPi-PFK), ATP-dependent PFK (ATP-PFK), fructokinase (FK), and glucokinase (GK) were determined in the xylem-side cambial tissues of control and transplanted seedling taproots and stems from April through November 1994. Stem and taproot SS activity of control seedlings exhibited similar seasonal patterns with stem SS higher than root most of the time. Low SS activity was observed during early leaf expansion in the spring with resumption of taproot SS activity occurring 2 weeks later than stem SS. Seedling SS activity peaked in June and September and decreased sharply near leaf abscission in fall. Seasonal patterns of PPi-PFK in control seedlings were similar to those of SS except that PPi-PFK peaked only in June. Taproots of nontransplanted seedlings had similar level of PPi-PFK to that of stems. From late April to mid-June, there were 35-75% and 20-45% decreases in SS and PPi-PFK activities in transplanted seedlings respectively, as compared with the controls. Moreover, seedlings transplanted into the forest field had much less growth and lower activities of SS and PPi-PFK than seedlings transplanted into the nursery beds. Activities of ATP-PKF and FK were generally lower in transplanted seedlings than controls. During spring and early summer, some transplanted seedling stems had tip dieback or black mottling and scars or both. These seedlings usually had small reddish leaves. little new root growth, and very low SS and PPi-PFK activities as compared with healthy transplanted seedlings of the same site. Some of the black mottling/scar areas on discolored seedlings were infected with Botryosphaeria dothidea (Moug.: Fr.) Ces. De Not. At the end of the first year, about 10% mortality was observed with seedlings transplanted into the nursery bed or the forest site. No further mortality occurred on the forest site for the following 3 years. It is concluded that transplanting stress, based on sucrose metabolism. lasted at least through late June for sweetgum seedlings planted in January.