Diurnal changes in allocation and partitioning of recently assimilated carbon in loblolly pine seedlings

We examined diurnal fluctuations in acquisition and partitioning of recently assimilated (14)CO(2) and in subsequent allocation and partitioning to roots of loblolly pine (Pinus taeda L.) seedlings. Nonmycorrhizal seedlings were grown under optimal nutrient conditions in continuously flowing solution culture. Shoots of 15-week-old loblolly pine seedlings were labeled with (14)CO(2) for 30 min at four separate labeling times: 1000, 1200, 1400 and 1600 h. Six whole plant harvests were conducted during a 45 h chase period. i.e. 0, 4, 8 12, 24 and 48 h after the end of the labeling and evacuation periods. Although assimilation of (14)CO(2) was constant between 1000 and 1400 hi there were significant differences in partitioning of (14)C-labeled assimilate in needles of all age classes. The highest percentage of recently assimilated (14)CO(2) in the ethanol-soluble Fraction of photosynthesizing tissue was observed near the beginning and end of the photoperiod. Partitioning of (14)C in the ethanol-soluble fraction declined between the 1000 and 1400 h labeling periods, and was accompanied by an increase in partitioning of recently assimilated (14)CO(2) toward starch and a decrease in respiratory losses. These data suggest that most of the (14)CO(2) assimilated at 1000 h was used to support shoot metabolic activities and possibly restore soluble sugar reserves. Peak starch accumulation in needles during the 1400 h labeling period, concomitant with minimal respiratory loss, indicated;hat photosynthate production exceeded demand and export out of source leaves. A possible feedback regulation of photosynthesis by starch and/or sugar accumulation may be responsible for the observed decline in assimilation of (14)CO(2) during the 1600 h labeling period. Net accumulation of recently assimilated (14)CO(2) in roots was correlated with assimilation rate of (14)CO(2), but independent of partitioning of recently assimilated carbon in photosynthetic tissue. However, the percentage of total seedling (14)C allocated to roots was essentially the same throughout the 45 h chase, regardless of time of labeling and assimilation rate. The data suggest a strong diurnal regulation of starch and soluble sugars synthesized from recently assimilated carbon in needles of loblolly pine seedlings that was independent of assimilation rate. Allocation and transport of recently assimilated carbon to roots of loblolly pine seedlings were not subject to short-term fluctuations in supply and demand.