Time-course Study of Translocation of Products of Photosynthesis in Soybean Plants

Abstract

In a previous report (7) we have shown that soybean plants, in response to age and culture conditions, selectively translocate varying proportions of C14labeled sucrose, serine-glycine, and malic acid from among the products of photosynthesis in a primary leaf exposed to C1402 for 10 minutes. Regardless of age or culture conditions or the amount or type of compound exported, total radioactivity in the stem decreased logarithmically with distance from the input node to the root. Moreover, the concentration of radioactivity at the input node did not vary significantly in spite of some relatively large changes in the total export in the 10 minutes interval of these experiments. Further information on these observations drawn from short-term experiments required data which could be obtained only by extending the duration of the experiments. Logarithmic gradients of translocated radioactivity in stems have been noted by others (1, 9, 10) and attempts have been made to interpret them in relation to velocity or rate of translocation or to concurrent losses from the sieve tubes to the surrounding tissues (1, 2, 3, 4). In all cases but one (9), these gradients have been observed in short-term experiments. Only 2 reports (1, 10) have noted the changes in gradient characteristics that occur when the duration of C14-assimilation and translocation is increased, and in these the experiments were limited to periods from 5 to 30 minutes. Kursanov and Pristupa (5, 6, 8) have recently reported evidence in support of the classical concept of a circulation of carbon compounds from the leaves to the roots and back to the tops with metabolism in the roots playing an important role in the overall process. They have found that bean, pumpkin, and maize translocate mainly sucrose from the leaves to the roots. There it is metabolized to organic acids and amino acids in varying degree, (depending on the status of the mineral nutrition) and these acids are then translocated to the tops in the xylem sap. If such a criculation exists in the soybean, it was reasoned that, with increasing translocation time, the initial logarithmic decrease in C14 concentration with distance down the stem could change to an increasing gradient. This alteration might result from the combined effects of the decrease in export from the leaf and the increase in return flow from the root. The influence of increasing time on the translocation of the products of photosynthesis wvas, therefore, followed in young soybean plants maintained under defined conditions for intervals of 20 minutes to 70 hours after C1402 was assimilated for 10 minutes by one of the primary leaves. At each time interval the total amount of C14 exported by the leaf, its distribution within the plant and the identity of the labeled compounds were determined. The results of these experiments are interpreted in terms of the significance of the C14-concentration gradients in the stem as a measure of the rate of downward translocation and as an indicator for return flow from the root.