Carbon Partitioning

Partitioning of 14C
as a function of time

Changes in partitioning of 14C

Carbon partitioning among chemical fractions in leaves of northern red oak seedlings changes over time and with stage of plant development. This result indicates that both stem and taproot accumulate reserves during the lag period when more assimilate is available than needed for growth. This might help explain high root shoot ratios often observed in seedling sprouts and thus the ability of oaks to persist for long periods of time in the understory.

When first-flush leaves at 1-Lg at 1-Lg (see Quercus Morphological Index, QMI) are fed ₁₄C -CO2, the percentage of ₁₄C in sugar decreases with time after treatment as sugar is metabolized or translocated from the leaf. In contrast, the percentage of ₁₄C in the residue (i.e., structural carbohydrate) fraction increases with time, indicating continuing vascular development. The percentage of ₁₄C in starch increases during the light period (i.e., the first 12 hours), then decreases during the subsequent dark period. Starch storage in leaves during 1-Lg is primarily associated with the diurnal cycle of carbon storage. The starch accumulated during the light period is degraded in the dark to maintain sugar transport out of the leaf. The percentage of ₁₄C found in protein increases during the first 6 hours after treatment, then remains constant (Isebrands et al., 1994).

Partitioning patterns in the stem and roots after ₁₄C labeling of mature leaves are similar to those found in leaves (Dickson et al 1990). In both stem and root tissue, the percentage of ₁₄C in sugars decreases and that in structural carbohydrates (residue) increases with time. In stems, ₁₄C in starch increases for 12 hours in the light, decreases during the dark period, then increases for the remaining period. This pattern indicates that stems have both diurnal and long-term starch storage pools. In roots, the percentage of ₁₄C in starch increases for 12 to 24 hours, then remains constant, indicating long-term storage. Taproots contain about three times as much ₁₄C starch as lateral roots do (data not shown). The ₁₄C content in protein of stem and root tissues increases during the first 12 hours, then remains constant for the rest of the chase period. Lateral roots contain almost twice as much ₁₄C protein as the taproot does (data not shown) (Isebrands et al., 1994).

Changes in 14C Fixation

Like carbon fixation, carbon partitioning patterns in northern red oak are episodic and related to leaf growth and subsequent plant development (Dickson 1986, 1991; Dickson and Tomlinson 1988). The partitioning of recently fixed ₁₄C among different chemical fractions first-flush source leaves changes dramatically when leaves are exposed to ₁₄C -CO₂ and analyzed at different QMI stages. These changes reflect both maturation processes with the leaf and metabolic changes in carbon flow due to changing sink demands in the seedling. For example, almost 50 percent of the ₁₄C recovered in first flush source leaves at 1-Lg is found in residue. Although ₁₄C partitioning into residue decreases during each subsequent QMI stage through 2-Lg, more than 15 percent ₁₄C is still incorporated into residue at 2-Lg and later QMI stages, indicating continued vascular development well after full leaf expansion. The percentage of ₁₄C incorporated into protein increases from 1-Lg to 2-Bd, decreases to 2-Lg, then increases again at the beginning of the third flush. This pattern of protein synthesis indicates that physiological development continues well after full leaf expansion and shows a response (similar to that observed for CER) to increased sink demand during flushing episodes, perhaps indicating cyclic synthesis of Rubisco (Isebrands et al., 1994).

Carbon Exchange Rate

Transport of 14C-Photosynthate

The percentage of ₁₄C recovered in both sugar and starch also changes with QMI. The ₁₄C remaining in the sugar fraction after 48 hours increases almost linearly from 1-Lg to 3-Lg, although there may be a slight cycle from 1- to 2-Lg and from 2- to 3-Lg. This sugar is probably a storage pool that increases with leaf age and does not respond to sink demand as do CER and translocation. ₁₄C incorporation into starch in first-flush source leaves at 1-Lg is primarily into the diurnal storage pool because less than 10 percent of ₁₄C is present in starch after 48 hours. In contrast, the percentage of 14C found starch increases from 1-Lg to 2-Lg, then decreases to 3-SL, then increases again. This partitioning pattern indicates that, in addition to diurnal storage, there is a long-term starch storage pool in northern red oak leaves that changes in size over development. 14C incorporated into this starch storage pool may also reflect both the increasing physiological maturity of the source leaf ( 1-Lg to 2-Lg) and changing sink demand for assimilate (after 2-Lg) (Isebrands et al., 1994).

The incorporation of ₁₄C into residue, starch, and sugars of both stem and taproot is also cyclic and varies with QMI. Although more total ₁₄C is recovered in each of these fractions during 2-Lg and 3-Bd when most transport is downward from source leaves, the percentage of ₁₄C decreases in residue and increases in sugar and starch (data not shown).  (Isebrands et al., 1994).