Hormonal Mcchanisni of Groiuth Inhibition by Radiation 545 



A specific high-energy radiation effect on stem growth has been 

 reported by Mohr (26). Stem growth of Sinapis alba was found to be 

 unaffected by the low-energy red far-red pigment system; thus, the ac- 

 tion spectrum for a higli-intensity growth inliibition could be de- 

 termined unambiguously for this species. Two spectral regions of 

 maximum effectiveness were found. The most effective region in- 

 cludes the red far-red region, from about 660 to 740 m^u,. The second 

 region, in the blue, shows a maximum at about 455 m^u. This effect 

 is presumably comparable to the high energy effects observed in other 

 plants. If so, these results would represent a "composite" action spec- 

 trum near the cross-over point. The cross-over point would be at un- 

 usually low^ intensities in this species, because of its relative insensi- 

 tivitv to red. 



Recently, Mohr (27) has redetermined the blue far-red action 

 spectrum. One peak is found in the blue (ca. 440 m/x) and a much 

 higher peak in the far-red (ca. 730 ni/x), with a shoulder extending 

 through the red. 



Ultraviolet radiation (254 ni/x) may also inhibit stem growth, 

 but it does so only at relatively high irradiances. The mechanism of 

 growth inhibition is different from that of visible radiation (see 

 below). 



INTRACELLULAR MECHANISM OF RADIATION 



INHIBITION 



Visible radiation inhibits stem growth primarily through a de- 

 crease in cell elongation. This conclusion is supported by cell counts 

 (33, 35) and by findings that growth inhibition occurs in the morpho- 

 logical region of cell elongation (14; Lockhart, unpubl.). The only 

 known exception to this generalization is in Gramineae (i.e., Avena) 

 where radiation-inhibition of the first internode is due to a reduction 

 of cell division as well as of cell elongation (1). 



Cell elongation and, thus, to a substantial extent, stem elongation 

 are controlled by the rate and extent of plasticization of the primary 

 cell walls. The cell walls become more plastic while the internal os- 

 motic pressure remains constant. Thus, more water moves into the 

 cell, resulting in an increase in cell volume. In an elongating stem 

 this increase is mostly an increase in cell length. The osmotic pres- 

 sure in the cell is subsequently restored to its original volume by up- 

 take of solutes (or hydrolysis of existing substrates). A further in- 

 crease in plasticity results in a second increment of growth, etc. 

 (28, 34). 



What phase of this growth process is affected by irradiation? The 

 relation of radiation to various individual factors which might con- 



