39$ Mineral Nutrition of Plants 



during the winter months when the daylight intensity was low and the 

 photoperiod was short. Supplemental incandescent lamp irradiation 

 was used to lengthen the photoperiod. Thus, the light intensity was 

 relatively low as compared with summer conditions and the growth 

 rate was correspondingly lower. These authors conclude that the lack 

 of severity of boron deficiency symptoms under a short photoperiod 

 is correlated with cessation of cambial activity and the occurrence of 

 floral induction. 



Manganese and iron 



Steckel (50), working with soybean, has observed the appearance 

 of deficiency symptoms for iron and manganese under conditions of 

 different light intensities and day lengths, using varying ratios of iron 

 and manganese. He found that the time of appearance of the symp- 

 toms could be correlated with light conditions. The symptoms ap- 

 peared earlier at high light intensities than at low and earlier in long 

 photoperiods than in short. Weight and top-root ratio data indicated 

 that one of the chief factors probably responsible for the delay in ap- 

 pearance of the deficiency symptoms in short photoperiods and low 

 intensities as compared with long photoperiods and high light inten- 

 sities was a slower rate of growth. 



CHLOROPHYLL SYNTHESIS 



Chlorosis is one of the most conspicuous symptoms of both macro- 

 nutrient and micronutrient deficiencies in higher plants and yet rela- 

 tively little is known as to the biochemistry of chlorophyll synthesis 

 and the mechanisms by which the lack of each element produces its 

 own characteristic pattern of chlorophyll insufficiency. Nitrogen and 

 magnesium are essential in the structure of the protochlorophyll and 

 chlorophyll molecules. Galston (/j) has reported that the chloroplasts 

 contain from 30 to 40 per cent of the total nitrogen in both chlorotic 

 and green leaves of oat. Granick (18) found similar results for tomato 

 and tobacco, with 10 per cent of the chloroplast nitrogen present in the 

 chlorophyll molecule. About 10 per cent of the magnesium in the plant 

 is found in chlorophyll, with magnesium comprising about 2.7 per cent 

 of the chlorophyll molecule. Several of the micronutrients, such as iron, 



