NITRATE REDUCTION 



445 



result of supply and consumption, depending upon such factors as the 

 rate of photosynthesis and carbohydrate formation and the rate of migra- 

 tion of nutrients to the leaves, both partly regulated by the stomatal 

 movements, the temperature, etc., apart from the hypothetical direct 

 action of light on the nitrate reduction. Thus there are several ways in 

 which hght regulates the nitrate consumption indirectly, and three of 

 them deserve special attention. 



- &0 



90 



o 



8C a 

 o 



< 

 70 1 



X 



< 



S 



60 



50 ^ 



40 



- 30 H 

 < 

 S 

 o 



H20 '^ 



-J I 



8 A.M. 10 



12 



2 P M 



2 A.M. 4 



Sunset Sunrise 



Fig. 8-1. The diurnal variation in nitrate content of Helianthus annuus (July 10 and 

 11, 1945). I, nitrate content in stems: II, nitrate content in old leaves; III, nitrate 

 content in young leaves; IV, stomatal aperture, percentage of the maximal opening; 

 V, air temperature. Nitrate-content curves read on left ordinate ; stomata-aperture 

 and air-temperature curves, on their respective ordinates at left. {Experiments by H. 

 Rufelt, unpublished.) 



2. INDIRECT ACTIONS OF LIGHT ON THE NITRATE REDUCTION 



2-1. The Stomatal Factor. An important action of light is indicated in 

 Fig. 8-1 as the stomatal aperture, which shows a common kind of diurnal 

 periodicity with closure in the afternoon and opening after sunrise. If 

 the less important variations of temperature and humidity are neglected, 

 the transpiration should roughly follow the course of the stomatal move- 

 ments. It has further been shown that, with all other conditions con- 

 stant and the plants adapted to the particular external nutrient medium, 

 the absorption of ions and their transport to the leaves closely follow the 

 rate of transpiration (Hylmo, 1953). The supply of nitrate to the leaves 



