除草剤の選択殺草作用機構に関する生理生化学的研究

抄録

(1) Modes of the selective action of several herbicides on plant species, especially gramineous plants, were studied from the biochemical and physiological viewpoints. Not only for agricultural practice but also for scientific interest is the selective action of hercicides one of the most important aspects of their function because various characteristic to plant species can thus be revealed. In the study, the mechanisms underlying the selectivity of herbicides were traced through investigations of their absorption by either roots or shoots of plants, translocation in plant tissues or organs, chemical transformation by plant metabolism and effects on plant metabolic systems. Contribution of each process to the selectivity was evaluated by comparing these processes among plant species. How each process limits the concentration of the active chemicals in plants as well as their affinity to action site (s) was also considered. Use of herbicides labeled with radioactive isotopes was found to extremely informative for this type of study.
(2) Selectivity of propanil between rice and barnyardgrass was studied at first. By using carbonyl-¹⁴C-labeled propanil with organic phosphorus insecticides such as dipterex it was shown that the absorption was not a rate-limiting process in the selectivity. It was also shown that rice leaves hydrolysed considerably the compound to 3, 4-dichloroaniline and acid unlike barnyardgrass. Out of series on N-(3, 4-dichlorophenyl)-alkylcarboxamides and N-(chloro-substituted-phenyl)-ethylcarboxamides, only a few were found to be hydrolysed by barnyardgrass. It was concluded that barnyardgrass had arylacylamidase activity with a substrate specificity strictly limited in anilides. Solan, cypromid or CMPT were not hydrolysed by rice plants.
Thiadiazolylurea derivatives such as DBTU were synthesized as potent herbicides and N-demethylation was found to be the mechanism of detoxification. Cotton seedlings whidh are very tolerant to diuron were susceptible to DBTU and lacked the N-demethylation detoxification mechanism of the latter. Substrate specificity of the demethylating enzyme from different plant species was investigated.
(3) When propanil was applied at very low concentrations to roots of rice and barnyardgrass for several days, selectivity between the two species was achieved. Photosynthetic activity of barnyardgrass leaves decreased, unlike that of rice leaves. Concentration of propanil in leaves and recovery rates from the inhibition were investigated and a mechanism other than the hydrolytic degradation was proposed to account for the tolerance of rice to propanil.
(4) Several examples of herbicide selectivity where either absorption or translocation was the main limiting factor were identified. Orbencarb was shown to be absorbed more by crabgrass than by wheat, suggesting that the former plant may be susceptible to the herbicide due to its enhanced absorption. Barban showed a selectivity between wheat and oat. When it was applied to intact oat leaves, protein and RNA synthesis was markedly inhibited unlike in wheat. However when it was applied to leaves that had been cut in section protein and RNA synthesis in both plant species was similarly inhibited. It was observed that thiochlormethyl was actively translocated from the roots to the shoots in susceptible plants such as tomato compared with tolerant plants such as rice or cucumber.
(5) In an attempt to gain a better understanding of the specificity to herbicides of a large number of plant species, comparison between cultivated varieties was made. It was found that some of the japonica type varieties of rice were much more tolerant to simetryn than some of the indica and japonica×indica hybrid-types. The former metabolized almost 90% of the simetryn applied within 24 hours while the latter only less than 50%.
(6) Effect of ambient temperature on the selectivity of simetryn was studied.