Allocation of 14C assimilated in late spring to tissue and biochemical stem components of cork oak (Quercus suber L.) over the seasons

Carbon distribution in the stem of 2-year-old cork oak plants was studied by (CO2)-C-14 pulse labeling in late spring in order to trace the allocation of photoassimilates to tissue and biochemical stem components of cork oak. The fate of C-14 photoassimilated carbon was followed during two periods: the first 72 h (short-term study) and the first 52 weeks (long-term study) after the (CO2)-C-14 photosynthetic assimilation. The results showed that C-14 allocation to stem tissues was dependent on the time passed since photoassimilation and on the season of the year. In the first 3 h all C-14 was found in the polar extractives. After 3 h, it started to be allocated to other stem fractions. In 1 day, C-14 was allocated mostly to vascular cambium and, to a lesser extent, to primary phloem; no presence of C-14 was recorded for the periderm. However, translocation of C-14 to phellem was observed from 1 week after (CO2)-C-14 pulse labeling. The phellogen was not completely active in its entire circumference at labeling, unlike the vascular cambium; this was the tissue that accumulated most photoassimilated C-14 at the earliest sampling. The fraction of leaf-assimilated C-14 that was used by the stem peaked at 57% 1 week after (CO2)-C-14 plant exposure. The time lag between C photoassimilation and suberin accumulation was similar to 8 h, but the most active period for suberin accumulation was between 3 and 7 days. Suberin, which represented only 1.77% of the stem weight, acted as a highly effective sink for the carbon photoassimilated in late spring since suberin specific radioactivity was much higher than for any other stem component as early as only 1 week after C-14 plant labeling. This trend was maintained throughout the whole experiment. The examination of microautoradiographs taken over 1 year provided a new method for quantifying xylem growth. Using this approach it was found that there was more secondary xylem growth in late spring than in other times of the year, because the calculated average cell division time was much shorter.