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.) pla ntation. 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 quickl y, whereas taproots exhibited SS activity even in January. Pyrophospha te-dependent phosphofructokinase (PPi-PFK) activity tracked SS activit y. The activities of ATP-dependent PFK and several other glycolytic en zymes (e.g., phosphoglucomutase and phosphoglucoisomerase) remained re latively constant in cambial tissues of stem, taproot, and all first-o rder lateral roots (FOLRs) throughout the year. However, during the gr owing season, individual FOLRs exhibited variable sucrose metabolic ac tivities that were independent of root diameter or position on the tap root. The FOLRs with low or no SS activity also had low PPI-PFK activi ty. We propose that when intense competition for sucrose occurs among different organs of a tree, the variable activities of the sucrose met abolic enzymes in FOLRs ensure that enough sucrose is allocated to the stem and taproot for growth. For a tree's long-term survival and grow th, second or higher-order roots can be sacrificed, whereas FOLRs, ste m and taproot are essential.