P-33 translocation in the thallus of the mat-forming lichen Cladonia portentosa

The extent of vertical migration of P-33 in thalli of the heathland lichen Cladonia portentosa was investigated under field conditions. P-33-labelled orthophosphate was introduced into the bottom 25 mm of podetia cut to a len gth of 50 mm from the apices. The distribution of label was scanned using a molecular imager immediately after incubation, and after growing for 8 wk and 6 months. Differences in the relative distribution of label between pod etia harvested at the beginning and the end of the experiment showed that t here had been a significant migration of P-33 upwards out of the labelled 2 5 mm stratum towards the apex. This was confirmed by statistically signific ant changes in the median (md) and the 90 percentile of total relative dist ribution of P-33 label. In a control treatment in which label was introduce d into the apical 25 mm of podetia, which were then grown inverted (top dow n), no upward movement of label was detected. By contrast, a statistically significant reduction in the md of the distribution indicated migration dow nwards towards the thallus apex. The results are consistent with the hypoth esis that P is recycled within podetia of mat-forming lichens, migrating fr om degrading basal regions upwards to the growing apices following a source -sink relationship.