EFFECT OF REDUCTION OF SINK STRENGTH IN DEVELOPING SEEDS ON VEIN LOADING PATTERNS OF PHOTOASSIMILATE IN SOURCE LEAVES OF PISUM-SATIVUM L

The effect of sink strength reduction in developing seeds on vein loading of photoassimilate has been studied in Pisum sativum L. The sink strength was manipulated by means of the 'open seed coat technique'. Sink strength of the operated ovules was controlled by the osmolality of a substitute medium replacing the embryo. A high osmolality of the medium (400 mM mannitol) or a low osmolality of the medium (without mannitol) was used to maintain a high or low sink strength, respectively. To study the effect of sink strength reduction on vein loading, macro-autoradiography was used. After applying (CO2)-C-14 to the source leaf for 10 to 20 min autoradiographs of plants with different sink strength showed differences in distribution of C-14-photoassimilate over the mesophyll and the veins. Under low sink strength conditions vein loading of C-14-photoassimilate was reduced. After longer time spans (40 to 60 min) no clear differences in vein loading were visible in the autoradiographs. However, measurement of C-14 in plant parts along the path of transport from source to sink showed a decrease of the rate of export of C-14 from the source leaf under low sink strength conditions. Apparently, accumulation of C-14-photoassimilate into the phloem and export towards sink regions can be reduced by lowering the sink strength. A signal must have been transferred from sink all the way to the source regions. The mechanism of such a signal is discussed.