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Carbon distribution in the stem of 2-year-old cork oak plants was studied by  14 CO 2  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  14 C photoassimilated carbon was followed during two periods: the first 72 h (short-term study) and the first 52 weeks (long-term study) after the  14 CO 2  photosynthetic assimilation. The results showed that  14 C 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  14 C was found in the polar extractives. After 3 h, it started to be allocated to other stem fractions. In 1 day,  14 C was allocated mostly to vascular cambium and, to a lesser extent, to primary phloem; no presence of  14 C was recorded for the periderm. However, translocation of  14 C to phellem was observed from 1 week after  14 CO 2  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  14 C at the earliest sampling. The fraction of leaf-assimilated  14 C that was used by the stem peaked at 57% 1 week after  14 CO 2  plant exposure. The time lag between C photoassimilation and suberin accumulation was ∼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  14 C 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.

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