a sensitive:‘mechaniam, of;correlative regulation. .Auxing exist ih a free, readily extractable.'stateinall living cells of the plant. -Thd chief.-: ° Bite et.sox | Synthesis:Sis: 5the:erowing.terminal. bud. of.the ‘high r. ple 2ve we : wee meeSideF= SouSlaeen,J2UcTaetinghagegla esesrpgRees, The.‘response:ofthexplant..to. directly er indirectly. causdd va tions in hormone‘level. has-been intensively studied im the past] aS decades. Since .a mmberofthe observed effects of high energyJradiations are similar in’effect. to those caused by changes in auxinjlevel,. . the effect of such: radiation on auxin and auxin economy becomes] perti-. he nent to research upon the: mechanisms of radiation effects. The auxin native to. the plant is chiefly, if not. wholly,} the heterocyclic ‘compound indoleacetic acid. Investigators et Argdnne National Laboratory have been studying the effect of ionizing wediation (1) on auxin in in vitroand in vivo systems, (2) on the biosyythesis of auxin in the living plant tissues, and (3) on Physiological processes controlled ‘by ‘auxin. vo X-radiation inactivates auxin in aqueous solutions with[apparent first order kinetics. Accompanying organic compounds areablefto pro- ° tect auxin against inactivation by ionizing radiation. Such protective action was shown by ethanol, glucose, ascorbate, cysteine, glugkathione, and citrate--all natural components of a plant. What, then, the effect of ionizing radiation on auxin in the living plant? Here we must distinguish between auxin and auxin formation, or biosynthesi Using kidney bean, cocklebur, and cabbage plants, free feuxin levels were determined immediately after various single doses radiation. Radiation doses as low as 25 to 100 roentgens cau: diate drop in free auxin levels. As the radiation dose is in the absolute amounts of free auxins in the tissue progressively decrease. Depending on the plant material, it takes from 50 to tions produced in the tissues to inactivate one molecule of X-radiation is initiated. This is in contrast to the initial] tion efficiency of one molecule per ionization when a pure a tion of the hormone is irradiated. We have seen that indole is readily protected in vitro by other oxidizable organic mo by periodic sampling after irradiation. 1$000 ionizainactiva-" Auxin formation is in the kidney bean by as little as 25 roentgens, with progrepsively stronger inhibition by higher doses. Recovery to control ldvels of syn- thesis after irradiation is. attained in from several days tq 2 weeks, depending on the dose. Doses of 10,000 roentgens result infincreasingly depressed biosynthesis, with mo recovery. The effectiveness of low doses of ionizing radiationfcan be UNCLASSIFIED == TOT oe er rr rr as |