Fine Structure Aberrations in the Movement of11C and13N in the Stems of Plants

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Abstract
11CO2 was offered to leaves of sunflower, corn and ryegrass and 13N2 to root nodules of alfalfa and alder. Movement of the tracers out of the feed region was monitored along stems or petioles using geiger tubes. Fluctuations in radioactivity were not detected as statistically significant from random tracer decay in the ‘background’ section of the time-activity profiles before mass-flow commenced, but became highly significant in the mass-flow sections. These pulses of radioactivity could be followed from one detector to the next over 1–3 cm and were analysed for periodicity by cross-correlation and auto-correlation computer programmes. Periodicity was only rarely detected in 11C runs, but was evident in many 13N experiments. Speed of pulse movement (microfronts) were measured, both visually and by computer cross-correlations, and compared with rates found by the ‘moving intercept’ of mass flow. Microfront speeds were faster. Speeds of 11C movement were comparable with those reported for phloem, but 13N movements were often much higher, suggesting xylem movements. Fine structure pulses indicate that movements of 11C photosynthate or 13N compounds are rapid, erratic and far more complex than expected by a simple Münch pressure flow mechanism.