FACTORS AFFECTING SALT ABSORPTION 



49 



- 



o. 



Light 



2 3 4 5 6 



Time, days 



Dark 



Light 



20 40 60 60 



Time, hr 



too 



120 



Fig. 15. Light and salt absorption 

 a. Absorption of phosphate by maize plants during successive periods of light 

 and darkness (redrawn from Alberda, 1948); b. Influence of light and 



darkness on the potassium content of Ulva lactuca. plants in continuous 



light; — plants transferred from darkness to light after 80 hr 



(redrawn from Scott and Hayward, 1953). 



Structure of leaves and the condition of stomata, which influence 

 transpiration, and thus indirectly affects salt absorption (see p. 112f). 

 It is known that photosynthetic organisms kept in the dark 

 gradually cease to absorb salts and finally release them as respiratory 

 substrates become depleted. Alberda (1948) found that the rate of 

 absorption of phosphate by maize plants fell to zero about 4 days 

 after they were transferred from light to darkness, and began to 

 rise again as soon as they were reilluminated (Fig. 15a). Ketchum 



