Relations between Light Level, Sucrose Concentration, and Translocation of Carbon 11 in Zea mays Leaves

Abstract

The mechanism of C transport in Z. mays leaves was investigated using 11C which is a short lived (half-life 20.4 min) positron-emitting isotope. The .gamma.-radiation produced on annihilation allows in vivo or nondestructive measurement of the isotope and the short half-life allows many measurements of translocation to be made on the same leaf within the same day. 11C produced by the 10B (d,n)11C nuclear reaction was converted to 11CO2, fed to a leaf as a short pulse, and assimilated during photosynthesis. The progress of the radioactive pulse along the leaf in the phloem was monitored in several positions simultaneously with counters. The counters were NaI crystals with photomultipliers and the output was amplified, passed to single channel analyzers, and the counts accumulated for 20 s every 30 s. Corrections were made for the half-life and background radiation by computer and the results were displayed on a high speed plotter. Information derived from the corrected data included the speed of translocation, the shape of the radioactive C pulse, and the influence of light and distance along the leaf of these parameters. The plants were kept under controlled environment conditions during all measurements. A speed was derived from the time displacement of the midpoint of the front of the pulse, measured at 2 positions along the leaf. This was an apparent mean speed of translocation because it averaged a variation in speed with distance, variation in speed between or within sieve tubes, and it averaged the mean speed of all of the particles in the pulse. A wide range of speeds of translocation from 0.25-11 cm min-1 was observed but most of the variability was due to the variation in light available to the leaf; e.g., the speed of translocation was proportional to the light level on either the whole plant or individual leaf. Shading of the leaf established that the light effect was not localized in either the feeding area or in the portion of the leaf on which the measurements were made. Apparently the speed was dependent on the proportion of the leaf in the light upstream from the last counter. The speed of translocation was relatively independent of the stage of growth of the plant, age of the leaf and the time during the diurnal light cycle. Data obtained on the level of the reducing sugars, starch and sucrose in the leaf were related to the speed of translocation. A biphasic relationship between speed and sucrose concentration in the leaf was established and the high speeds measured during experiments only occurred when sucrose concentrations in the leaf exceeded 8% of the dry weight. The shape of the pulse loaded into and translocated in the phloem was estimated from the half-width of the pulse. The half-width was primarily determined by loading phenomena which resulted in an increase in the half-width from 2 min when fed to the leaf to more than 40 min in the phloem. The pulse continued to broaden with distance along the leaf from the fed region. The half-width was independent of the speed but highly dependent on the light level.