Translocation of 14C-assimilate among Main Stem and Tillers in Rice Plants

The following experiment was conducted to evaluate the translocation of 14C-assimilate among main stem and tillers at tillering stage in rice plants. Rice plants, cv. Nihonbare, were grown outdoors in 1/5000 a Wagner pots. When the plants reached a leaf age of about 10, leaf blades of the upper two to four leaves on the main stem, tiller II, III, IV, V or tiller VI (the tiller on 2nd, 3rd, 4th, 5th or 6th node of main stem, respectively) were exposed to 14CO2 for 30 min. each, in a growth chamber at 30°C and 30 klx of light. The plants were harvested at 24 hours after exposure and radioactivity in the main stem and each tiller was determined. For a better understanding of the compensatory translocation of assimilate which may occur when leaves of tiller and/or main stem are lost, some plants were defoliated in various ways on main stem and tillers 1 day prior to 14CO2 exposure. The results are summarized as follows: 1. The 14C-assimilate from the exposed leaves was distributed in the largest quantity to the other parts of the exposed tiller (or main stem) itself. And the contribution of the 14C-assimilate to the exposed tiller itself showed larger in the upper node tiller than in the lower node tiller. 2. The main stem supplied much more assimilate to the primary tillers than to the secondary tillers. Among the primary tillers, the upper node tiller received more assimilate from the main stem than did the lower node tiller. On the contrary, in the secondary tillers which were on the same primary tillers, the lower node secondary tiller received more assimilate from the main stem than did the upper node secondary tiller. And this suggested that more 14C-assimilate was translocated to the tillers nearer the source leaves of the main stem. 3. The translocated 14C-assimilate from the exposed tiller (mother stem) was distributed mostly in the secondary tillers (daughter tillers) on the exposed tiller itself. The distribution of 14C-assimilate from the mother stem decreased as the daughter tillers became more developed, and those tillers which had 3 or more expanded leaves received only a small part of the 14C from the mother stem. 4. Translocation of the 14C-assimilate from the primary tillers to the main stem was observed. It was found that the nutritional contribution to the main stem was larger from the lower node tiller than from the upper node tiller. In addition, the translocation of 14C-assimilate among primary tillers was also observed. As determined by dpm/mg dry weight, a higher concentration of 14C was transported to the upper node tiller, especially to the 2nd tiller above the exposed tiller, than to the lower node tiller. 5. Translocation of 14C-assimilate from the exposed tillers toward the youngest primary tillers increased when the main stem and all the tillers below the exposed tiller were defoliated. However, at the same time, translocation toward the defoliated tillers decreased.