Light Level and the Mean Speed of Translocation in Zea Mays Leaves

Abstract

The effect of light on the mean speed of translocation of carbon compounds in Zea mays leaves was investigated using the short-lived isotope carbon-11. This isotope was generated using a 3.0 MeV Van der Graaf accelerator and converted to ¹¹CO₂ before it was photosynthesised by a restricted, 2 cm wide, segment of a leaf attached to a plant. The plant was in a controlled environment chamber at 28 ± 1°C and 80% relative humidity. Carbon-11 transported along the leaf in the phloem was detected externally at several positions by NaI scintillation counters attached to photomultipliers. The pulses from the photomultipliers were passed through single channel analysers, amplified and counted, and the results corrected by computer for the half-life and background radiation.

The results established that the shape of the pulse was primarily determined by the loading phenomena in the region fed isotope. A mean speed of translocation determined from the time delay in arrival of the mid-point of the front of the pulse at successive counters was proportional to the light level, although a component accumulated in the light influenced the speeds in the dark. The maximum speed recorded was 11.0 cm min⁻¹.