PHOTAUTOXIDATION IN STARVED PLANTS 



529 



As found by Noack, some carbon dioxide liberated by respiration (and photoxida- 

 tion) was reduced by the leaves before it reached the alkali. Franck and French 

 determined this residual photosynthesis by experiments in nitrogen containing only 

 1.5% oxygen, which is sufficient to saturate the respiratory apparatus but not enough 

 to cause measurable photoxidation (c/. Figs. 58 and 59). The net rate of oxygen 

 consumption, determined under the higher partial pressures of oxygen, was then cor- 

 rected by adding the rate of residual photosynthesis and substracting the rate of dark 

 respiration. (This presupposes that photosynthesis was not affected by photoxidation, 

 although the latter may have produced more carbon dioxide than did respiration.) 



Photautoxidation proceeded for a while at a constant rate, until it 

 slowed down for lack of substrates ; before stopping altogether it attacked 



thread 

 paraffin 



'KOH (olution 

 I 2 cm. 



»««'•= I III l| M m|m I I I .. .. 



10 



15 20mm. 



Fig. 57. — Apparatus for the study of photoxidation in carbon dioxide-deprived 

 leaves (after Franck and French 1941). The leaf is mounted on a rotor; its lower 

 surface, which contains the stomata, is just above a potassium hydroxide solution. 

 The vessel is joined to a manometer for the measurement of o.xygen consumption. 



the pigments and other vital constituents of the cell. This "light injury " 

 can be postponed by providing an external supply of oxidation substrates: 

 in leaves whose stems dipped into a glucose solution, the rate of phot- 

 autoxidation was both higher and steadier than without glucose. Similar 

 results were obtained by Mevius (1936), who left C02-starved leaves on 

 the stem and allowed other leaves to photosynthesize in the normal way 

 and supply the photoxidizing leaves with sugars by translocation. 



