1034 BIOLOGICAL EFFECTS OF RADIATION 



nutrition medium of Sinapis alba increases the photosynthetic rates 

 at all light intensities. 



Gregory and Richards (35) observed that barley plants deficient 

 in nitrogen and phosphorus showed almost normal photosynthesis at 

 low light intensities, but subnormal photosynthesis at high intensities 

 of light. Deficiency in the potassium ion caused subnormal photo- 

 synthesis at all light intensities. Gassner and Goeze (33), on the other 

 hand, found the highest assimilation value with potassium deficiency. 



Chemical Inhibitors. — Warburg (133a) has investigated the inhibitory 

 influence of cyanides on photosynthesis at different light intensities. 

 He has found that the inhibition by cyanides is relatively more pro- 

 nounced at the higher light intensities. Van der Paauw (126) has 

 shown that 0.0001 molar HCN stimulates photosynthesis in Hormidium 

 at all light intensities, while 0.001 molar concentration produces a 

 retardation at all intensities. The retardation is greater at high intensi- 

 ties; this is so great that the photosynthetic rate passes through a maxi- 

 mum. Other experiments with HCN show that the photosynthesis 

 actually goes below the compensation point. Phenylurethane at 

 2.4 X 10~* moles per liter produces a lowering of the photosynthetic 

 rate at all the light intensities investigated by van der Paauw. 



Temperature. — The results on the simultaneous change of temperature 

 and light intensity are so complicated that it is possible to give only the 

 barest outlines of the effects produced. 



Investigating the influence of temperature on the photosynthetic 

 rate of Chlorella, at constant light intensity, Emerson (23) found a 

 sigmoid relationship. In an attempt to calculate the heat of activation 

 of the photosynthetic process by the Arrhenius equation he found that 

 it varied with the temperature, thus indicating a complex reaction 

 mechanism. Mliller (81) observed that the rate of photosynthesis at 

 low light intensities was greater when the temperature was low than 

 when it was high. At higher light intensities, however, a greater photo- 

 synthetic rate was observed at higher temperatures. Neydel (83) 

 has demonstrated graphically the difference in plants relative to their 

 temperature-photosynthesis relations at different light intensities. 

 For Trichomanes the rate of change of photosynthesis with temperature 

 is constant at three different light intensities. The sun alga Cladophora 

 passing through the same temperature range shows a maximum. Fonti- 

 nalis and Spirogyra have also been found to have temperature optima 

 (Matsubara, 75). Photosynthesis at lower temperatures (5°C.) is 

 much greater for Fontinalis than for Spirogyra. 



LundegS.rdh (73) found that photosynthesis in the potato leaf passes 

 through several temperature optima at constant light intensity. This 

 observation has thrown a new aspect on the problem. This type of 

 behavior has been confirmed by Yoshii (152) for the bean leaf, and by 



