EFFECTS OF LIGHT I X TENSITY 751 



and absorbed more nitrogen. Wiesmann (119) found that plants grown 

 in full sunlight absorbed more mineral nutrients than those grown in lower 

 light intensities. 



Cooper (23) found a correlation between the tolerance of plants to 

 shade and to strong ions. Plants which make best growth on fertile 

 soils and require strong ions were intolerant of shade and had a high 

 carbohydrate reserve, whereas plants which grow on poor soils and endure 

 large quantities of weak ions were more tolerant of shade but had a lower 

 carbohydrate reserve. 



Tyson (116) made chemical analyses of sugar beets grown in the open 

 and under 1, 2, 3, and 4 layers of cheesecloth, and under 2 layers of cheese- 

 cloth plus 1 layer of black calico. Ash content of roots and leaves, based 

 on percentage of dry weight, increased somewhat with decreasing light. 

 This was reflected in increased magnesium, phosphorus, and calcium. 

 The sugar percentage of the beets on a dry-weight basis was about the 

 same for all light intensities. Total yield of beets and tops increased 

 directly with increasing light intensity. Catalase activity was favored 

 by increasing light intensity and oxidase activity by decreasing light 

 intensity. 



The effect of light intensities on the rate of reproduction of Lemna 

 major was studied by Clark (21). The plants were grown in nutrient 

 solution and subjected to illuminations of 400 and 900 foot-candles. The 

 rate of reproduction, which is a measure of photosynthetic activity, 

 increased in both cases with increased length of day up to continuous 

 illumination. The plants with 900 foot-candles reproduced more rapidly 

 than those with 400 foot-candles, regardless of the length of day, and the 

 difference between the two treatments became greater as the daily period 

 of illumination was increased. With continuous illumination the plants 

 tended to become chlorotic. Changing the solutions every 12 hr. avoided 

 this difficulty, the iron in the solution being no longer available after a 

 period of continuous illumination. 



Loehwing (60), studying the influence of light on the hydrogen ion 

 concentration of the cell sap of wheat, found that plants grown on alkaline 

 soil at high light intensities became chlorotic. Under such conditions 

 the iron did not move from the roots up the stems. Full sunlight caused 

 a decrease in the acidity of the cell sap. In the alkaline cell sap, iron 

 was insoluble and hence could not be transported. Shaded plants were 

 able to remain green under the same soil conditions. 



The work of these investigators, together with observations of other 

 workers who grew plants in nutrient solutions, indicates that under certain 

 conditions in which the soil or nutrient solution is near the neutral point, 

 or slightly alkaline, strong light intensity may interfere with the iron 

 nutrition of the plants. This question undoubtedly merits further 

 investigation and should prove a fruitful field for further study. 



