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Inhibition of Plant Growth by Irradiation

V. Radiation Effects on Initiation and Completion of Growth^{* ,}

(From the Departments of Physics and of Physiology, University of Illinois, and Carle Foundation, Urbana, Illinois)

1. The observations reported concern linear growth in Mung beans—growth of individual segments and of the entire hypocotyl and the first segment of the epicotyl in normal plants, after resection of various parts, and after total or partial irradiation with various doses.

2. The growth capacity is analyzed into two components: the capacity to initiate growth, which is located in the top 2 mm, is dependent on food stored in the cotyledons, and is determined by the age of the seedling; and the capacity to complete growth, which is spread along a zone of about a 15-mm. length, is locally autarkic, and is determined by the position of the segment observed. "Growth initiation and completion" are possibly, but not necessarily, identical with "cell division and stretching."

3. Irradiation with x-rays causes three distinct effects: temporary inhibition of growth completion, and permanent reduction of the capacities of both initiating and completing growth. Permanent stunting is an effect of local irradiation.

4. An analysis of dose-response relations shows that the apparatus for growth initiation is more vulnerable than the apparatus concerned with actual growth.

^* This investigation was supported in part by a grant from the National Cancer Institute, Public Health Service.

Presented at the Forty-first Annual Meeting of the American Association for Cancer Research, Atlantic City, N.J., April 16–18, 1950.

Received 4/29/50.