Longitudinal Translocation of ¹⁴C-Labelled Assimilates in Leaf Blades of Lolium temulentum

Abstract

The distribution of ¹⁴C activity in leaf blades of L. temulentum following application of ¹⁴CO₂ to a small area near the middle of the blades was studied over periods of up to 6 h. The profiles of ¹⁴C activity towards the leaf base comprised several regions. The very steep logarithmic profiles within a few centimetres of the area exposed to ¹⁴CO₂, and symmetrical about it, were shown to be probably due to diffusion of gaseous ¹⁴CO₂ within the leaves prior to its photosynthetic fixation.

The major profile region represented the movement of a pulse of ¹⁴C activity, in the hour following exposure to ¹⁴CO₂, down the leaf at speeds of 40-84 cm/h. The activity in this pulse was attenuated along the pathway, the attenuation decreasing with increase in distance moved. In leaves harvested more than 1 h after exposure to ¹⁴CO₂, there was an irregular transition zone between these two logarithmic profile regions which may represent a later or slower-moving component of translocation. In leaves harvested less than 30 min after exposure to ¹⁴CO₂, there was an increase in slope at the profile front which was not attributable to changes in phloem cross-section relative to leaf catchment area.

No steady acropetal movement of ¹⁴C activity was detected, but some activity was found in the distal areas of even the earliest-harvested leaves. Long, parallel-veined leaves of grasses may well provide suitable systems for more detailed analysis of translocation profiles.