DISTRIBUTION OF C-14 IN PULSE-LABELED SPRING BARLEY - EFFECT OF LIGHT-INTENSITY AND LENGTH OF PHOTOPERIOD BEFORE LABELING

The distribution of photoassimilated C in spring barley plants was determined at different times after the onset of light and at different light intensities during assimilation. The plants were grown in pots in a green-house, and at late tillering and late elongation, 14CO2 pulse-labelings of 2 h duration were carried out 1.5, 4 or 8.5 h after the onset of light. At the labelling started after a 4 h photoperiod, two light intensities was included (80 and 170 W m-2). To analyse samples low in C-14, a 14CO2-trapping system interfaced with a Leco high-temperature induction furnace was developed. The 14CO2 was trapped directly in the scintillation vial in 5 mL of liquid Carbosorb, enabling subsequent liquid C-14-scintillation counting to take place without subsampling. The proportion of photosynthate translocated below ground tended to be higher early in the morning than later in the day. Labelling 1.5 h after the onset of light, 19.8 and 7.6% was translocated below ground at late tillering and late elongation, respectively. Corresponding values found at later labellings were 15.4-16.0 and 6.0-6.4%. Higher proportions tended to be translocated below ground when plants were exposed to low light intensity. Exposing plants to low light intensity caused below ground translocation to be 15.4 and 6.0% of the C-14 recovered at late tillering and late elongation, respectively, compared with 12.1 and 5.2% after exposure to a higher light intensity. Further experiments are needed to substantiate the observations of this study. The results suggest that the distribution of photoassimilates varies during the daytime and light intensity during the labelling.