ABSENCE OF FRUCTAN DEGRADATION DURING FRUCTAN ACCUMULATION IN WHEAT STEMS

Turnover of fructan accumulated in the penultimate internode of wheat (Triticum aestivum L.) was studied after anthesis. Two genotypes differing in the rate of accumulation and remobilization of water-soluble carbohydrates in the stem were used in the study; plants were grown in growth chambers at ambient CO2. After anthesis the set of plants was divided into two groups and each group was maintained at 1000 or 300 µL CO2 L-1 in order to alter the supply of photosynthates to the stem. Between 7–15 days after anthesis plants were labelled with 14CO2 during a whole photoperiod. Plants were harvested before labelling and at one to two-day intervals up to 6 days after 14C application in the 300 µL CO2 L-1 treatment and up to 12 days in the 1000 µL CO2 L-1 treatment, respectively. Water-soluble carbohydrates (WSC) were extracted and the concentrations and 14C content of their components determined. The WSC concentration increased at a linear rate during the observation period and was exclusively due to accumulation of fructan. The rate of fructan synthesis differed between genotypes and was higher at 1000 than at 300µL CO2 L-1. 14C-activity determined in the penultimate internodes was highest one day after labelling and mainly present in sucrose and already in fructan as well. The amount of label then decreased to about two-thirds of its initial value. A strong incorporation of 14C in fructan occurred only during the first two days after labelling. For the following 10 or 5 days, 14C-activity in fructan remained constant in the penultimate internodes of both genotypes growing at 1000 or at 300 µL CO2 L-1, respectively, whereas 14C-activity in the remaining WSC components was low or decreased drastically. The results indicate that no degradation of recently synthesized fructan occurred during the phase of net fructan accumulation in wheat stems which means that there was no continuous turnover of the fructan pool in both genotypes at either 1000 or 300 µL CO2 L-1. Differences in fructan accumulation observed between genotypes or CO2 concentrations were therefore due exclusively to differences in fructan synthesis. The lack of turnover of the fructan pool during WSC accumulation in wheat internodes contrasts with findings for gramineous leaves where fructan seems to be turning over rapidly, indicating that fructan metabolism in wheat stems is regulated differently and that this carbohydrate fraction in wheat stems is a longterm storage pool. © 1990, Gustav Fischer Verlag, Stuttgart. All rights reserved.