A biochemical assessment of the value of top clipping nursery-grown loblolly pine seedlings
Seasonal sucrose metabolism (sucrolysis) was studied in taproot cambial tissues of nursery-grown loblolly pine seedlings to assess the value of top clipping. In sucrose-importing taproots of nonclipped seedlings, sucrose synthase (SS) was the dominant enzyme for sucrose cleavage, and its activity exhibited a distinct seasonal activity. Both root SS activity and growth were most active during fall. Sucrose synthase activity decreased to the lowest level in mid-January and resumed after that.Neither root acid invertase (AI) nor neutral invertase (NI) changed activity appreciably the seasons. Both August and September top clipping treatments decreased seedling top weight by 20% to 45% whereas total root weight was slightly decreased by August clipping only. Top clipping did not change the basic seasonal pattern of sucrolytic pathway in taproot cambial tissues.However, 2 to 3 months after top clipping, losses of root SS activity in clipped seedlings were observed. The largest decreases in SS activity occurred from November through early January followed by another decrease during active shoot elongation.Generally, August clipping decreased more root SS activity than September clipping. Neither root AI nor NI activity was affected by the clippi9ng treatment. It was concluded that: (1) sucrose synthase was the dominant sycrolytic enzyme in cambial tissues of pine seedling taproots; (2) sucrose synthase activity can be used as an indicator for the physiological status of tissues; and (3) top clipping, especially in August, imposes stress on nursery seedlings based on biochemical analysis and growth measurements.