396 Mineral Nutrition of Plants 



narrow photoperiod range; and (d) day-neutral plants indifferent to 

 photoperiod as regards flower bud initiation. In all these classes are 

 plants whose flowering is also influenced by temperature. With some 

 of these types in certain combinations of day and night temperatures, 

 photoperiod has little influence on the course of development, the 

 determination of the initiation of flower buds being controlled chiefly 

 by temperature. 



Numerous investigations have been conducted in an endeavor to cor- 

 relate photoperiodic reactions with mineral nutrient supply and metab- 

 olism. With but few reported exceptions, variations in the mineral 

 nutrient supply within the range capable of supporting growth have 

 had no determinative effect capable of altering the course of floral 

 initiation. The two environmental factors which appear to be deter- 

 minative are relative length of the light and dark periods and tempera- 

 ture. In spite of the negative results obtained, however, several inter- 

 esting correlations have been reported on the influence of mineral 

 nutrient levels on the rate of appearance of floral primordia and the 

 effect of photoperiod on the rate of appearance of nutrient deficiency 

 symptoms. 



Nitrogen relationships 



Particular attention has been paid to nitrogen relationships. While 

 the level of nitrogen appears to play no critical role in determining the 

 induction of flowering, low levels of nitrogen do significantly affect 

 the time of appearance of macroscopic flower buds. In short photo- 

 periods, decreasing or removing the nitrogen supply decreases the rate 

 of growth very markedly. In such cases, it has been found that the 

 macroscopic flower buds of short-day plants often appear later than 

 those of plants growing in a high nitrogen medium. In a long photo- 

 period such plants are vegetative at all nitrogen levels. This situa- 

 tion is exemplified in short-day plants, such as chrysanthemum (7), 

 Xanthium (jo, 5S), soybean (46, 58), Kalanchoe (7), Tinantia fitgax 

 (7), Setaria italica (7), Tithonia speciosa (5S), and Salvia splendens 

 (58). For short-day plants in general, abundant nitrogen supply appears 

 to hasten flowering. Without an adequate nitrogen supply, the number 

 of floral buds set is sparse and fruiting often fails to occur. 



