544 J. A. Lockliart 



served at very low energy levels as well. The most obvious effect of 

 low intensity or brief high intensity irradiation is usually morpho- 

 logical. Internodes which are elongating at the time of irradiation 

 are markedly inhibited but growth of subsequent nodes may be pro- 

 moted proportionally. In many cases, then, low radiation energies 

 may reduce the length of the first internodes but have little or no 

 effect on total growth rate. This appears to be essentially a problem 

 in rate of node formation, since irradiated plants have more nodes, 

 but the same total stem length as dark-grown plants. Essentially 

 nothing is known as yet about the mechanism which controls node 

 formation, but considerable information is available on the mechan- 

 ism of control of total stem elongation. 



THE NATURE OF THE VISIBLE RADIATION AFFECTING 



STEM GROWTH 



Several distinct effects of visible radiation on stem growth have 

 been observed. Radiation energy level and apparently species as well 

 determine the response of plants to radiation. Early workers utilized 

 filters to isolate broad spectral regions of high-intensity solar radia- 

 tion (cf. 8). They found, generally, that blue radiation produced 

 short, thick stems while green and red radiation yielded relatively tall, 

 slender plants. These responses have been confirmed with higli- 

 intensity fluorescent lamps (25, 37, 39). At low energy levels, on the 

 other hand, red radiation is the most inhibitory to stem growth. As 

 radiation energies are increased, a cross-over point is reached at which 

 red is no longer more effective than blue. At irradiances above the 

 cross-over point, blue radiation is most effective. Thus, red radiation 

 is more inhibitory at low intensities, but maximum inhibition is at- 

 tained with high-energy blue radiation. AMiether or not different 

 mechanisms are involved is not known. The red far-red pigment is the 

 photoreceptor for the low-energy red response (2, 5, 29). Its action 

 may also be manifest in a somewhat diffeient manner. If plants are 

 grown in white light for 8 hrs. a day, their groAvth takes place largely 

 during the 16-hr. dark period (33). Wassink and Stolwijk (39) found 

 that the extent of growth during the dark period was controlled by 

 the spectral distribution of radiation received immediately preceding 

 the dark period. Ihis radiation may be of high or low intensity; 

 growth depends only on the ratio of red to far-red radiation (6). 

 Thus the red far-red pigment system acts to modify growth even when 

 plants are grown in high-intensity artificial or solar radiation. The 

 only results available (33) suggest that the growth inhibition by high- 

 intensity blue radiation is not reversed by far-red irradiation at the 

 beginning of the dark period. 



