BGN 2: Radiation-induced growth depression and gibberellic acid content of barley seedlings BARLEY GENETICS NEWSLETTER, VOL. 2, II. RESEARCH NOTES
Nawar et al., pp. 60-62

II.21. Radiation-induced growth depression and gibberellic acid content of barley seedlings.

M. M. Nawar, E. G. Sideris and R. A. Nilan. Program in Genetics and Department of Agronomy and Soils, Washington State University, Pullman, Washington 99163, U.S.A.

Ionizing radiation depresses seedling growth (a widely used criterion of biological damage) while application of exogenous Gibberellic acid (GA) to seeds enhances the growth rate (1). A preliminary study was conducted with the following objectives: a) to observe the correlation, if any, between radiation-induced seedling growth depression and the GA activity in barley seedlings obtained from irradiated seeds, and b) to see if the exogenous application of GA to irradiated barley seeds reverses, partially or totally, the radiation-induced seedling growth depression.

Air dried barley (Hordeum vulgare var. Himalaya, CI 620) seeds (1969 crop) were used. Irradiation was performed at room temperature in a 60Co facility delivering gamma rays at a dose rate of 14,300-14,600 R/min. Control and irradiated seeds were planted on blotters and supported vertically in polyethylene racks under continuous illumination at 23+2°C. After one day the seeds were transferred to a medium containing either .001 M sodium acetate (pH 4.8) + .10 M calcium chloride (buffer) or to a solution of 10-4 M GA made in the same buffer.

In the extraction of GA, the method of Jones (1968) was employed (2). GA activity was estimated by measuring the activity of alpha amylase released from barley seed halves (3). The release of alpha amylase under these conditions is proportional to the common logarithm of GA concentration (4). Other details have been published elsewhere (4,5).

Three-day-old barley seedlings were used in the present study because they exhibit a relatively high GA activity (activity per gm fresh weight comparable to the activity shown by a 10-7 M solution of GA) and are easy to handle. When GA activity was computed on the fresh weight basis, the decrease in number of seedlings per gm fresh weight observed with increase in age of seedling apparently led to a decrease in GA activity. When GA activity was computed on a per seedling basis, however, a gradual increase in activity with increase in age of seedling was observed (Table 1). Consequently, throughout the present study GA activity was computed per gm fresh weight and per seedling.

Table 1. Variation in activity of gibberellic acid with age of barley seedlinga,

Results of the present investigation indicate a positive correlation between the radiation-induced growth depression and the decrease in GA activity per seedling (Table 2), The observed reduction in GA activity may be due to a radiation-induced curtailment in the biosynthesis of GA. The extent of reduction in GA activity increased with radiation dose particularly when GA activity was computed on seedling basis. The reduction in GA biosynthesis may contribute, in part, to the radiation induced growth depression observed as a decrease in seedling length. Application of exogenous GA to irradiated seeds, however, did not overcome the relative growth depression resulting from irradiation (Table 2).

Table 2. Radiation-induced growth depression and gibberellic acid activity in three-day-old barley seedlings.

References:

l. S. Kumar, Indian J of Genet. and Plant Breeding 27, 154 (1967).

2. R. Jones, Planta 81, 95 (1968)

3. R. Jones and J Varner, Planta 72, 155 (1967).

4. E. G. Sideris, A. Kleinhofs, R. A Nialan, Radiation Botany 9, 349 (1969).

5. E. G. Sideris, M. M. Nawar, R. A. Nilan, Submitted to Radiation Botany.

Supported in part by the U. S. Atomic Energy Commission Contract AT(45-1)-2221. AEC RLO-2221-T2-6.

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