Wechselwirkung zwischen Spross und Wurzel zweier Sojabohnensorten bei unterschiedlichem Phosphaternährungszustand : $^{14}C Assimilatverteilung und Kohlenhydratstatus

The influence of the phosphate nutritional status on assimilate distribution between shoot and root was studied for two soya bean varieties (Century and Woodworth). Plants at a full nutritional level ($\textbf{+ P}$ variants) and those in a condition of moderate P stress ($\textbf{- P}$ variants) were considered. Particular attention was paid to the following aspect: Is a modification of the assimilate distribution, conditioned by phosphate withdrawal, only associated with the reduction in the shoot/root ratio or do changes in distribution already occur before morphological parameters are influenced? Further it is known from literature that after a contemporary interruption of the phosphate supply plants display an increased phosphate uptake rate when the supply is restored in comparison to plants nourished normally. In this respect the following aspects were of interest: Does an increased uptake rate, for which sufficient energy must be made available, have a direct effect on thetransport of assimilates to the root? Is the carbohydrate content of the roots possibly a parameter with which the difference in efficiency of the two varieties may be described? $^{14}$CO$_{2}$ was supplied to a single leaf (1$^{st}$ trifoliate) in the first series of experiments. This enabled the net photosynthesis rate of this leaf to be determined, as well as the export rate of radioactively labelled assimilates into other parts of the shoot and into the root. In a second experimental approach, $^{14}$CO$_{2}$ was supplied to the whole shoot. In this case, apart from uniform nutrition for the entire root system, individual root strands of theplant were supplied with or denied phosphate (split-root system). In both experimental approaches $^{14}$CO$_{2}$ was supplied for a period of 100 minutes. The central results of the experiments are: In comparison to $\textbf{+ P}$ variants, the transport of $^{14}$C labelled assimilates into the roots of $\textbf{- P}$ variants was significantly increased. For example in $\textbf{+ P}$ variants approx. 11-12% of the assimilated $^{14}$C was found in the root and 15-16% for - P variants after single-leaf application. The increased transport of assimilates to the root can be attributed to an increased export rate of photosynthesis products from the 1$^{st}$ trifoliate. The overall export rate in experiments with single-leaf application was thus between 20-24% for $\textbf{+ P}$ variants and between 27-30% for $\textbf{- P}$ variants of the radioactivity taken up by the single leaf. Reduced radioactivity contents in the 1$^{st}$ trifoliate pointing to a stimulated export rate were also established during the application of $^{14}$CO$_{2}$ to the entire shoot. The results of the $^{14}$C assimilate distribution were supported by an analysis of the carbohydrate in the plant tissue. An increased export rate from the 1$^{st}$ trifoliate was thus accompanied by a decreased sucrose content in this leaf. The stimulated transport of radioactively labelled assimilates to the root was accompanied by a significantly increased hexose content (glucose and fructose). The Century variety, which was characterized as more efficient with respect to phosphate uptake, displayed higher carbohydrate contents in the roots in comparison to the Woodworth variety in all variants, which may possibly explain the difference in efficiency. If in the split-root system phosphate was only withheld from individual root strands in a plant, their carbohydrate status was altered in the same way as in whole root systems. This showed that the sink effect of each individual root strand in a plant is controlled separately as a function of the phosphate nutrition. It may be concluded that the moderate stress situation caused a modification of the assimilate distribution in favour of the root. This modification was already established before the plants morphological parameters were affected. The increased carbohydrate content in the root tissue was used to supply energy for the increased phosphate uptake rate, to reduce the pH of the nutrient medium by active proton deposition, to create a high membrane potential and possibly to build up functional membrane systems. These are factors which may be interpreted as the plant's reaction to phosphate stress.