Distribution of (14)C-labeled photosynthate in loblolly pine (Pinus taeda) seedlings as affected by season and time after exposure

Distribution of (14)C-labeled photosynthate was determined in field-grown loblolly pine (Pinus taeda L.) seedlings on August 9 and October 15, 1984 and January 15 and March 12, 1985. Leaves on a lateral branch fixed (14)C photosynthetically and amounts of (14)C in seven biochemical fractions in each of six plant parts were determined 8, 24, and 72 h later. In all treatments, (14)C uptake was approximately 96% of that originally presented. Respiratory loss of (14)C ranged from 22 to 87% of uptake and increased sharply with increasing time after exposure and as the seedlings grew larger later in the study. Most (14)C was found in exposed leaves and very little occurred above the exposed branch. Amounts of (14)C decreased in the exposed leaves and increased in the roots with time after exposure and date. Sugars were generally the most heavily labeled fraction. Labeled sugar content of exposed leaves decreased by more than half between 8 and 72 h as sugars were metabolized and translocated to other parts, primarily the roots. In roots, the labeling of starch and residue (structural compounds) increased greatly with transport time and season. In all plant parts, proteins and amino acids contained very little (14)C regardless of date or time.