546 



]. A. Lockharl 



trol growth has been investigated from time to time (e.g., 7, 10, 11, 32). 

 However, the results obtained represented only correlations between 

 these various factors antl the radiation regime. 



In more recent investigations, however, all factors kno^vn to in- 

 fluence growth rate have been examined in relation to both radiation 

 regime and growth rate (19). Of the various factors which might affect 

 growth in pea seedlings, only one, cell wall plasticity, shows changes 

 which parallel changes in growth rate in response to irradiation. 

 Within two hrs. after the onset of irradiation, when growth rate has 

 been reduced 50 to 70 per cent, cell wall plasticity has decreased by 

 about 75 per cent as compared to dark-grown plants. 



Plasticity is here defined as the attribute of a tissue which is 

 measured by irreversible bending or stretching. It was measured as 

 the amount of residual bending (that left after removal of the load) 

 after the tissues had been subjected to a standard transverse load for 

 a standard time (Figure 1). Plasmolysis of these curved sections did 



en 20 



UJ 

 LU 



ca 



iS> 



UJ 

 Q 



CD 



UJ 

 CO 



40 



60 La 



"T" 



"T" 



1 1 1 



Gibberellic Acid Treated 



I 



1 r 



-1 r 



Untreated 



o Irradiated 

 • Dark grown 



16 



16 



MINUTES 



Fig. 1. Plasticity of the elongating region of stems of 'Alaska' peas, dark-grown or 

 after 3 hrs. red irradiation. Plants treated with gil)berellic acid (1 ng. per plant) 

 3 hrs. prior to time of irradiation. Plasticity is measured either as the slope of 

 linear bending or as the amount of residual bending after the load has been re- 

 moved. Illustrated are the standard deviations of the means at representative points. 

 Seven plants per treatment. 



