PHOTOSYNTHESIS OF GREEN PLANTS IN ULTRAVIOLET AND INFRARED 1 1 53 



yield drops rapidly at, or only a little beyond, the violet end of the visible 

 spectrum (400 m/x)- 



In vitro, the chlorophyll abso'rption extends throughout the near and 

 medium ultraviolet (c/. fig. 21.3). Whether other common components 

 of the plant cells, which absorb in this region, interfere with the light sup- 

 ply to chlorophyll (by acting as ''screens"), we do not know (this could 

 perhaps be elucidated by fluorescence measurements). Ursprung (1917, 

 1918) found evidence of photosynthesis in Phascolvs ivJgnris down to 330 

 him; Hoover (1937) and Burns (1942) ol)scr^•cd it in TrUiciun at 305 niju. 

 Gabrielsen (1940) calculated a yield of about 1 mg. CO2/5O cm.- hr. as the 

 possible contribution of the ultraviolet part of the solar spectrum to the 

 photosynthesis of leaves of Sinapis and Conjlus. 



Johnson and Levring (1946) found with six marine algae (green and red), 

 a decline in respiration by about 1 mg. O2 per hr. per g. dry weight (deter- 

 mined by Winkler's methods) in near-ultraviolet light (366 mn, intensity 

 5 X 10"^ cal./cm.- min.). This was interpreted as evidence of photo- 

 synthetic effectiveness of this light. Further in the ultraviolet (260-320 

 m^, intensity about equal to that of the erytheme-producing radiation in 

 sunlight), no such effect could be observ^ed. 



Light below 300 m/x is highly injurious to plants. According to Meier 

 (1932, 1934, 1936) an illumination of 1000 erg/cm.^ sec. kills ChloreUa 

 cells in 110 sec. at 260 m/i, and in 10,000 sec. at 302 m/x. Preferential in- 

 hibition of photosynthesis (with the cells still alive and respiring) by the 

 mercury resonance line 253.6 m^ was described by Arnold (1933) (c/. chap- 

 ter 13, Vol. I, page 344); it is not associated with a visible destruction of 

 chlorophyll. 



The presence of yellow pigments {e. g., flavones, anthocyanines or 

 other hydrophilic compounds, which may be present either in the cell sap 

 or in the cell walls) may cause a decline or complete cessation of photosyn- 

 thesis at comparatively long wave lengths. This is the probable reason why 

 Bums (1933, 1934) found that the photosynthesis of certain conifers — e.g., 

 spruce and white pine — ceases below 450-465 m/x {cf. page 1164). 



In the spectmm of extracted chlorophyll, the absorption declines rap- 

 idly above 680 m/x; but the absorption spectra of intact cells reveal, in 

 addition to a shift of the red absorption band from 660 to approximately 680 

 mn, an extension of absorption to much longer waves. This spreading of 

 the red band can be recognized, e. g., in figures 22.10-22.24, 22.44, 

 and 22.48; and it has also been observed in aqueous suspensions of 

 chloroplastic matter (Smith, fig. 21.28). In the latter case, the infrared 

 "tail" of the red band disappeared upon clarification of the suspension by 

 digitonin, and was ascribed by Smith to scattering ("false absorption"). 

 However, in other investigations, e.g., those of Noddack and Eichhoff 



