Temporal and spatial aspects of root and stem sucrose metabolism in loblolly pine trees.

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.