CONTROLLED ENVIRONMENTAL CONDITIONS 311 



The Carbohydrate Nitrogen Ratio in Plants 



By wide variations in growth conditions ^ the ratio of easily hydrolyzable 

 carbohydrates (fiber not included) to total nitrogen content of plants can 

 be varied widely in some plants. In other plants it can be varied only 

 moderately. The carbohydrate per cent of course increases wdth rise in 

 conditions that favor photosynthesis — increase in light intensity, day 

 length, and CO2 concentration of the air up to the optimum for each. The 

 nitrogen content rises with the per cent of available nitrogen in the soil 

 or nutrient solution. In some plants the increase in the nitrogen fraction 

 occurs over a wide range of nitrogen supply, while with others the plants 

 themselves limit nitrogen absorption under a high available supply. 



Kraus and Kraybill -^ had concluded that the tomato, which is indif- 

 ferent to day length as to fruitmg, fruited only when there was a proper 

 carbohydrate/nitrogen (C/N) ratio in the tissue. Arthur and associates 

 grew the tomato under a wide range of conditions which gave also a wide 

 range in the C/N ratio. The tomato plant did not fruit in the 5-hour 

 day, but did fruit in any length of day from 7 to 19 hours. It did not 

 fruit in a 24-hour day where extreme foliar injury occurred. In the range 

 of 7- to 19-hour day the C/N ratio rose as _the day length increased. 

 The gantry crane greenhouse with 6 hours of artificial light at night and 

 0.3 per cent CO2 also gave fruiting plants ^vith very high C/N ratio; these 

 showed a variation in C/N ratio from 7.5 to 39. Arthur and associates 

 suggest that the failure of the tomato to fruit is not caused by an improper 

 C/N ratio but by a deficiency in carbohydrates or nitrogen compounds, 

 just as the deficiency in other nutrient factors such as phosphorus or 

 potassium might lead to failure to set fruit. Several other plants were also 

 studied as to possible significance of the C/N ratio as a fruiting factor. 



Radish plants showed a great variation in C/N ratio with variation in 

 growth conditions. The total carbohydrate, dry weight basis, varied from 

 7.47 to 34.73 per cent among the plants that were flowering or showed 

 flowering response. There was a range of 8.95 to 21.23 per cent among those 

 that did not flower. Total nitrogen varied from 1.51 to 5.92 per cent in the 

 first group and from 2.77 to 7.27 per cent in the second. The radish flowered 

 on a long day and failed to flower on a short day, regardless of the C/N 

 ratio in either case. Radish plants growing in the greenhouse during the 

 wdnter flowered when illuminated 6 hours at night with 170 foot-candles 

 of artificial light, although this extra light gave no appreciable increase in 

 the carbohydrate content; they also flowered with 6 hours of 700 foot- 

 candles of artificial light at night which increased the carbohydrate content 

 considerably. Radish plants grown alternately in 24 hours of light and 

 24 hours of darkness flowered, whereas those gro^vIl in 12 hours of daylight 

 and 12 hours of darkness did not flower, although the C/N ratio in both 

 was similar. The variety of lettuce studied was a long-day plant like the 

 radish. In this also somewhat less extensive chemical analyses indicated 



