394 Mineral Nutrition of Plants 



Recently, Lepeschkin (j/) has reported that sunlight and ultraviolet 

 radiation from a mercury arc accelerates the exosmosis of salts from the 

 leaves of Sambucus and Parthenocissus and from potato tuber tissue. 

 These results were obtained by measuring the changes in electrical con- 

 ductivity of distilled water in contact with the excised leaves and potato 

 tuber discs. 



Hoagland, Hibbard, and Davis (21) and Lundegardh (■?■?) have 

 shown that Nitella cells and excised roots, respectively, can carry out 

 ion accumulation only in the presence of an adequate supply of oxi- 

 dizable respiration substrates as carbohydrates. Thus, ion accumulation 

 by green tissues is indirectly accelerated by light through photosynthesis. 

 This factor seriously complicates the interpretation of experimental data 

 obtained with green leaves. The fact that the shorter wave lengths of 

 the visible and the near ultraviolet are the most effective spectral re- 

 gions and that the effects are observable in nonchlorophyllous tissue, 

 such as potato, would indicate that light appreciably alters the capacity 

 of cells to exchange solutes other than by providing a source of carbo- 

 hydrates for accumulation processes. 



Stalfelt (49) and Virgin (52), using centrifugation methods, meas- 

 ured the change in viscosity in the leaves of Helodea densa in response 

 to incandescent lamp irradiation. Virgin observed cyclic short- and long- 

 term fluctuations, the pattern of which varied with light intensity; at 

 intensities of 200 to 2000 foot-candles, a short transient increase in vis- 

 cosity was observed followed by a rapid fall, while at the very low in- 

 tensity of 0.05 foot-candle the viscosity decreased and remained low for 

 several hours of continuous irradiation. The significance of these results 

 in terms of the movement of solutes is not immediately apparent, but 

 experiments of this nature should be borne in mind in evaluating the 

 effect of light on solute movement and metabolism. 



It is possible that light-induced permeability and viscosity changes 

 may be the result of sensitized photochemical oxidations similar to 

 those recently observed by Galston (14, 75) on the photooxidation of 

 auxin and other cellular constituents with riboflavin, a fluorescing pig- 

 ment of the respiratory enzyme systems, as the photosensitizer. Irradi- 

 ation within the range of the principal absorption band of riboflavin, 

 about 4600 A., results in the transfer of the energy to neighboring mole- 



