Hormonal Mechanism of Growth Inhibition by Radiation 551 



ated plants is reduced as a result of a decrease in cell wall plasticity. 

 This must mean that a decrease in endogenous gibberellin causes a 

 reduction in plasticity. Gibberellin, then, is necessary for the main- 

 tenance of normal (maximum) cell wall plasticity. Experimental 

 measurements have demonstrated that treatment of irradiated plants 

 with gibberellic acid will result in high cell wall plasticity, equal to 

 that of dark-grown plants. 



Relation of Gibberellin to the Photochemical Reaction 



The same red far-red photochemical process affects various mor- 

 phological responses of plants in addition to stem length. In the 

 same plants, i.e., Pisnm seedlings, irradiation affects stem length, leaf 

 development, epicotyl hook opening, and rate of node formation. 

 Gibberellin treatments will completely reverse the effect of radiation 

 on stem growth, but it has no effect on these other photomorphogenic 

 responses. Therefore, applied gibberellin does not act directly on 

 the initial photochemical act, or even on subsequent thermochemical 

 processes which are common to all these reactions. Rather, gibberellin 

 must act on the terminal reactions controlling stem growth. It clearly 

 does not influence any reaction common to all these photomorpho- 

 genic processes (16). 



Gibberellin Reversal of Stem-Growth Inhibition in Various Species 



Interactions between gibberellic acid and radiation-inhibition of 

 stem growth have been examined in various other species in addition 

 to Pisnm (18). Responses of plant species so far examined appear to 

 fall into four groups, as follows: 



(1) Gibberellin results in complete reversal of radiation inhibi- 

 tion. Examples: Pisnm sativum 'Alaska' (tall) and 'Morse's Progress 

 No. 9' (dwarf), Helianthns annuus, Hordeum vulgare. 



(2) Gibberellin results in partial reversal of radiation-inhibition. 

 Examples: Cucurbita pepo, Cucumis sativus. 



(3) Gibberellin elicits no growth response in either irradiated or 

 dark-grown plants. Example: Sinapis alba. 



(4) Gibberellin gives no growth promotion in darkness, but pro- 

 motes growth markedly when plants are irradiated. Examples: Phase- 

 olus vulgaris, various dwarf and normal clones. 



It is not yet known, then, just how widespread is the interaction 

 between gibberellic acid and visible radiation or in how many species 

 radiation acts through the endogenous gibberellin system. Studies 

 on the mechanism of interaction have been pursued with Pisum, 

 since the interaction is complete in this species, and since it is a de- 

 sirable experimental plant in other respects. Just how far the results 

 obtained with Pisum may be extrapolated to other species remains 

 to be seen. 



