NO. 14 EFFECTS OF INFRA-RED ON PLANTS — JOHNSTON II 



The experiments described in the present paper set forth the gen- 

 eral growth characteristics of tomato plants grown under two ranges 

 of wave lengths for two different intensities. One group of plants 

 was exposed to light, a large proportion of whose energy was in the 

 red and near infra-red region, the other exposed to light limited 

 to the visible wave length region. Thus, an attempt is here made to 

 separate any near infra-red effects from those brought about by 

 other wave lengths. 



The growth of these tomato plants as measured by their total dry 

 weights clearly shows that the plants receiving infra-red radiation 

 were considerably heavier, with less difference occurring in the 

 stronger light. It is conceivable that if the illumination were further 

 increased the plants receiving no infra-red would surpass in weight 

 those receiving these longer wave lengths. Such an increase in 

 illumination would undoubtedly have been beneficial since Arthur (2) 

 found that where light of the same composition as sunlight was re- 

 duced to 35 per cent of full sunlight tomato plants thrived best. 

 Assuming 10,000 foot-candles as a value for full sunlight, then an 

 illumination of 3,500 foot-candles would be the optimum illumination 

 for tomatoes. In these experiments the higher estimated intensity was 

 1,966 foot-candles, approximately half their optimum intensity. 



The general form of the plants grown under the near infra-red 

 radiation was somewhat characteristic of plants grown under shade 

 conditions in that the internodes were long. However, the leaves 

 were not small as might be expected for shade conditions. The 

 water requirement was also lower than that of the plants receiving 

 only visible radiation. This point is rather interesting because shading 

 is usually considered an environmental factor increasing the water 

 requirement of plants. Thus, some general growth habits of the plants 

 exposed to the near infra-red radiation are common to plants grown 

 under shade conditions and others to those grown under normal 

 light conditions. 



Although the dry weight production was greater for plants ex- 

 posed to the infra-red radiation, these plants were distinctly less 

 green than those receiving only the visible wave lengths. In the 

 higher intensity of the infra-red the lower leaves were rather yellow. 

 This evident destruction of chlorophyll in the near infra-red region 

 is extremely interesting. Although considerable work has been done 

 on growing plants in different colored lights, many of the early re- 

 sults are questionable because of inadequate light filters and a lack 

 of suitable measuring devices for evaluating the intensity factors. 

 In recent years many of these difficulties have been overcome. 



