The main aims of the present investigations are (1) to determine the assimilation rates of
14CO
2 of the organs consisting the rice ear, i.e. hulled grain, hull, abortive grain and rachis-branch (including rachis and pedicel), and then (2) to determine the redistribution of
14C-assimilates among those organs after a given time. Plants from 10 to 14 days after ear emergence were used through the present experiments.
14CO
2 gas was applied to the materials in following two ways; one is to the ear, and another to the shoot. The materials were harvested immediately after one hour exposure to
14CO
2 for (1) experiment and at one week after exposure to
14CO
2 gas for (2) experiment. When
14CO
2 gas was applied to the ear, assimilation rates of
14CO
2, i.e.
14C/mg dry weight/hr and
14C/1OO spikelets/hr were high in the rachis-branch, while were very low and/or almost negligible in the hulled grain and the hull. The translocation rate of
14C-assimilates to the hulled grain was most rapid for one hour following one hour exposure to
14CO
2 gas, and became slow thereafter. The translocation of
14C-assimilates to the hulled grain, however, seems to continue for five to seven hours after exposure to
14CO
2 gas. For one week following exposure to
14CO
2 gas, the hulled grain functioned as a greedy "sink" organ of
14C-assimilates, while the rachis-branch and the abortive grain as a rich and a poor "source" orga'n of
14C-assimilates, respectively. On the other hand, the hull worked as a "source" organ in some cases, but in others. Each of "sink" and "source" capacities of the hull, however, was very small compared to the hulled grain and the rachis-branch. The assimilation rate of
14CO
2 of the rachis-branch,
14C/100 spikelets/hr tended to be higher in varieties which was heavier in dry weight of the rachis-branch per 100 spikelets. Tsuno, et al. (1975) showed that net ear photosynthesis correlated positively with chlorophyll content of the ear. Therefore, the basic benefit of heavy dry weight of the rachis-branch may be to associate with the dense thickness and the anatomy having relatively high proportion of photosynthetic components. It is interesting that the varieties showing high assimilation rate of
14C/ 100 spike1ets/hr tended to be also high in
14C/mg dry weight/hr. When
14CO
2 gas was applied to the shoot, the assimilation rates of
14CO
2 of leaf blade and sheath were the highest in flag leaf. The assimilation rate of
14CO
2 of the rachis-branch,
14C/mg dry weight/hr was similar to it of the flag leaf sheath and from one-half to one fourth of the flag leaf blade. The rachis-branch worked as only one "source" organ of
14C-assimilates in the ear, while others as "sink" organs. The proportion fixed by the ear out of the total amount of
14C-assimilates, however, was 4%, and it fixed by the rachis-branch 3%.
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