Records of Source—Sink Relations by Means of Respiration Measurements

A method for recording respiration rates of attached single soybean pods is described. By means of such records source-sink relations can be observed over an extended period of time if photosynthesis of leaves is recorded simultaneously. It is shown that there are direct influences on respiration, such as changes in temperature and indirect effects which influence primarily the assimilate influx into the pods, e.g. light and competition between pods. The shading of a single leaf next to the pod has only a small effect because the pod is supplied by more than one leaf. By girdling the plants, the export-import conditions can be studied on a single leaf as source and a single pod as sink. The results show that a trifoliate fully-grown leaf produces more assimilate than can be used by a single pod. If only one pod is growing within the girdled area then starch is accumulated and the photosynthetic rate of the leaf is greatly reduced. The respiration rate of the pod is only slightly increased, in spite of the abundant availability of assimilates. It appears therefore, that the filling rate of pods is determined mainly by the capacity of the pod to off-load the phloem, and cannot be increased beyond a certain level by additional sucrose influx. Our results show that, using this method, the source-sink relations can be recorded qualitatively over time periods of 3–4 weeks.