548 ]. A. Lockharl 



The stem apex is the site of gibberellin production in Pisuin, a 

 closely related species (17). Thus irradiation of the elongating region 

 probably inactivates gibberellin in situ. Recovery would occur when a 

 new supply of gibberellin is received from the nonirradiated stem 

 apex. Irradiation of the apex would inactivate gibberellin in a region 

 of relatively high gibberellin accumulation and would result in a 

 greater and longer lasting effect. Still unexplained is the rapidity of 

 transmission of inhibiting stimulus from tip to elongating region com- 

 pared to the slow recovery on direct irradiation of the growing region. 



THE ROLE OF GROWTH HORMONES IN THE EFFECTS 

 OF RADIATION ON GROWTH 

 Gibberellins 



An interaction between gibberellin and visible radiation was sug- 

 gested in some of the earliest work on the effects of gibberellin on 

 higher plants. Treatment of plants with large doses of gibberellin 

 results in elongated internodes and long, narrow leaves in monocots 

 and extensive stem elongation in dicotyledonous plants. Decreases in 

 leaf area have also been reported (42), although this is less common 

 (3,41,43). 



Direct evidence that application of gibberellin reverses growth 

 inhibition caused by visible radiation has been reported for etiolated 

 plants by Lockhart (16) and for light-grown plants as well by Lona 

 and Bocchi (23). 



Inhibition of light-grown Perilla was accomplished by following 

 the daily light period with exposure to red radiation, while relatively 

 noninhibited plants were produced by following each light period 

 with far-red radiation. Gibberellic acid treatments completely pre- 

 vented the inhibition of growth due to red radiation. It resulted in 

 equal growth in red or far-red irradiated plants. 



With a tall variety of Pisum sativum, 'Alaska,' normal stem growth 

 in complete darkness is unaffected by gibberellin treatment. However, 

 when gibberellin is given immediately prior to exposing plants to con- 

 tinuous visible radiation, growth inhibition is completely prevented. 

 Dwarf varieties of Pisum respond to gibberellin treatment even when 

 grown in complete darkness. At saturating doses of gibberellin, how- 

 ever, the growth rate of treated plants is identical, whether irradi- 

 ated or dark-grown (Table 1). 



In Pisum, application of gibberellic acid will prevent inhibition 

 of stem growth at continuous irradiances as high as 2,000 ergs- cm.-^ 

 •sec.-i, both red and blue (18,38). These arc the highest intensities 

 so far tested and are of the same order of magnitude as the high- 

 energy blue far-red inhibition of stem growth reported by Mohr (26). 



On the other hand, inhibition of stem growth by ultraviolet radia- 



