112 • Chemical Control of Flowering 



The suggestion that nitrogen nutrition plays an important role 

 in the control of flowering and fruiting in a manner related to the 

 considerations above was strongly supported, although not origi- 

 nated, by Kraus and Kraybill in 1918 (see Kraus, 1925). They 

 concluded that fruitfulness in the tomato plant depended on the 

 ratio of carbohydrate to nitrogen— the C/N ratio. Under a given 

 light intensity (to supply the carbohydrates) and at a given tem- 

 perature (which would govern the rate at which they are metab- 

 olized), the C/N ratio can obviously be controlled by controlling 

 the nitrogen supply. In Kraus and Kraybill's experiments, a 

 moderate ratio was favorable to flowering and fruiting, whereas 

 a low ratio (high nitrogen) favored luxuriant vegetative growth 

 but little reproductive development. This conclusion in generalized 

 form was for a while inflated out of all proportion to the data 

 supporting it, which appear to have been valid largely for the 

 particular conditions used. However, one should note in fairness 

 that Kraus and Kraybill were chiefly interested in later flower 

 development and fruiting, not in flower initiation. 



A more recent study by Wittwer and Teubner (1957), also on 

 tomato, does not support the notion that high nitrogen favors 

 vegetative growth at the expense of flowering. On the contrary, in 

 solution culture the highest nitrogen level used gave the best 

 flowering even under optimal temperature conditions. With respect 

 to photoperiodic plants, El Hinnawy (1956) found that high nitro- 

 gen promoted earlier flowering in Perilla and Kalancho'e (both 

 SDP) under inductive conditions, slowed it in mustard (Brassica) 

 and dill, and had no effect on spinach (all three LDP) under induc- 

 tive conditions. It had no effect on the photoperiodic response as 

 such, and he concluded that the effects of nitrogen and other major 

 element changes were highly indirect. 



Eguchi et al. (1958) have studied the responses of some photo- 

 periodic, vernalizable, or daylength-indifferent plants to levels of 

 nitrogen and phosphate nutrition. They concluded that in the first 

 two types the time of flowering, both chronologically and develop- 

 mentally, was almost unaffected. In the daylength-indifferent plants, 

 however, which included tomato, pepper (Capsicum), and eggplant 

 (Solanum), there was a much greater effect. In a tomato variety, 

 for example, flowering was earliest at the highest levels of nitrogen 

 and phosphate used, with the first flower at node 8 or 9. Reducing 



