USE OF TRANSFER-FUNCTION AND COMPARTMENTAL ANALYSIS TO QUANTIFY C-11 LABELED PHOTOASSIMILATE EXPORT FROM WHEAT LEAVES

Compartmental analysis of tracer loss from a leaf after pulse-labelling with carbon isotopes has often been used to infer the flow of photosynthate through the leaf. Recently, a more general approach has been suggested based upon estimation of the transfer function using data from pulse-labelling as well as continuous-labelling experiments. A comparison of these two approaches shows that with the same data set they give equivalent physiological interpretations. The measured decline of C-11 activity from a wheat leaf after (CO2)-C-11 pulse-labelling was extrapolated by compartmental as well as transfer function analysis. Both methods estimated a 66.4% loss of the initially fixed C-11 due to export and respiration. The advantage of transfer function analysis, however, is its applicability to continuous-labelling experiments. The model allows the use of the net photosynthetic rate as the reference (100%) value. Data from continuous-labelling experiments with wheat plants indicate diurnal Variations in the export of freshly labelled assimilate of between 32.7% and 43.6% of net photosynthesis.