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

We examined diurnal fluctuations in acquisition and partitioning of recentl y assimilated (CO2)-C-14 and in subsequent allocation and partitioning to r oots of loblolly pine (Pinus taeda L.) seedlings. Nonmycorrhizal seedlings were grown under optimal nutrient conditions in continuously flowing soluti on culture. Shoots of 15-week-old loblolly pine seedlings were labeled with (CO2)-C-14 for 30 min at four separate labeling times: 1000, 1200, 1400 an d 1600 h. Six whole plant harvests were conducted during a 45 h chase perio d. i.e. 0, 4, 8 12, 24 and 48 h after the end of the labeling and evacuatio n periods. Although assimilation of (CO2)-C-14 was constant between 1000 an d 1400 hi there were significant differences in partitioning of C-14-labele d assimilate in needles of all age classes. The highest percentage of recen tly assimilated (CO2)-C-14 in the ethanol-soluble Fraction of photosynthesi zing tissue was observed near the beginning and end of the photoperiod. Par titioning of C-14 in the ethanol-soluble fraction declined between the 1000 and 1400 h labeling periods, and was accompanied by an increase in partiti oning of recently assimilated (CO2)-C-14 toward starch and a decrease in re spiratory losses. These data suggest that most of the (CO2)-C-14 assimilate d at 1000 h was used to support shoot metabolic activities and possibly res tore soluble sugar reserves. Peak starch accumulation in needles during the 1400 h labeling period, concomitant with minimal respiratory loss, indicat ed;hat photosynthate production exceeded demand and export out of source le aves. A possible feedback regulation of photosynthesis by starch and/or sug ar accumulation may be responsible for the observed decline in assimilation of (CO2)-C-14 during the 1600 h labeling period. Net accumulation of recen tly assimilated (CO2)-C-14 in roots was correlated with assimilation rate o f (CO2)-C-14, but independent of partitioning of recently assimilated carbo n in photosynthetic tissue. However, the percentage of total seedling C-14 allocated to roots was essentially the same throughout the 45 h chase, rega rdless 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 independe nt of assimilation rate. Allocation and transport of recently assimilated c arbon to roots of loblolly pine seedlings were not subject to short-term fl uctuations in supply and demand.