© 1996 Heron Publishing—Victoria, Canada
Temporal and spatial aspects of root and stem sucrose metabolism in loblolly pine trees
Shi-Jean S. Sung (1), Paul P. Kormanik (1) and Clanton C. Black (2)
1. Institute of Tree Root Biology, Southern Research Station, USDA-Forest Service, 320 Green Street, Athens, GA 30602, USA / 2. Department of Biochemistry and Molecular Biology, Life Sciences Building, University of Georgia, Athens, GA 30602, USA / Received October 25, 1995
Summary
We studied root and stem sucrose metabolism in trees excavated from a 9-year-old artificially regenerated loblolly pine (Pinus taeda L.) plantation. Sucrose synthase (SS) activities in stem and taproot vascular cambial tissues followed similar seasonal patterns
until they peaked during September. After September, stem SS activity disappeared quickly, whereas taproots exhibited SS activity
even in January. Pyrophosphate-dependent phosphofructokinase (PPi-PFK) activity tracked SS activity. The activities of ATP-dependent
PFK and several other glycolytic enzymes (e.g., phosphoglucomutase and phosphoglucoisomerase) remained relatively constant
in cambial tissues of stem, taproot, and all first-order lateral roots (FOLRs) throughout the year. However, during the growing
season, individual FOLRs exhibited variable sucrose metabolic activities that were independent of root diameter or position
on the taproot. The FOLRs with low or no SS activity also had low PPi-PFK activity. We propose that when intense competition
for sucrose occurs among different organs of a tree, the variable activities of the sucrose metabolic enzymes in FOLRs ensure
that enough sucrose is allocated to the stem and taproot for growth. For a tree’s long-term survival and growth, second or
higher-order roots can be sacrificed, whereas FOLRs, stem and taproot are essential.
Keywords:
first-order lateral root, glycolysis, phosphofructokinase, Pinus taeda, sucrolysis, sucrose synthase, taproot, vascular cambium enzymes.
