Distribution and metabolism of current photosynthate by single-flush northern red oak seedlings

Carbon distribution and metabolism by northern red oak seedlings (Quercus rubra L.) were followed for 72 h after a 30-min photosynthetic exposure to (14)CO(2). Approximately 50% of the assimilated carbon was lost during this time, presumably through respiration. Most of the (14)C recovered in the plant remained in the source leaf. Most of the exported (14)C was found in stems and especially roots. Position of the source leaf did not affect distribution of translocated (14)C; however, upper source leaves retained slightly more (14)C than lower source leaves. Most (14)C in all tissues was found initially in sugars. Labeled carbon in this fraction decreased rapidly and increased in other chemical fractions, particularly residue (structural carbohydrates and lignin). More (14)C was incorporated into residue than into any other chemical fraction, indicating continued growth by each of the tissues during the lag stage of seedling development. Labeled carbon increased in proteins for 6 to 12 h after treatment, then remained essentially constant for the remainder of the transport period, indicating both rapid incorporation of the pulsed (14)C into, and slow turnover of, the labeled protein components. In source leaves, (14)C incorporated into starch in the light was lost during the dark period, a typical diurnal storage pattern. In stems, (14)C in starch increased for 12 h, then decreased during the dark period, and then increased during the rest of the transport period. This pattern indicates that stems have both diurnal and long-term storage pools of starch. In roots, (14)C in starch increased rapidly for 12 h, then remained constant for the rest of the transport period, indicating primarily long-term storage in this tissue. Peak (14)C in amino acids in taproot preceded that in stem tissue, indicating recycling of (14)C from the roots to the shoot in amino acids.