528 RESPIRATION 



respiration and in assimilation in the non-tuberous tissues is relatively small 

 the yield of potatoes will be curtailed. Hence profitable crops of potatoes 

 cannot be grown in relatively warm climates, since the proportion of the food 

 manufactured which can be utilized in tuberization is less than in cooler cli- 

 mates. This statement is not controverted by the fact that potatoes are an im- 

 portant crop in Florida, Texas, and other warm climate regions, since this 

 crop is raised in such areas only during the winter and spring months when 

 comparatively cool weather prevails. 



The same principle also applies to other plants although the temperature 

 which will result in the maximum P/R ratio varies considerably with the 

 species. Accumulation of foods in any plant is reduced at relatively high 

 temperatures. In most staple crop plants this means a reduction in yield, 

 since the commercially valuable parts of most species are the food accumulating 

 organs such as seeds, fruits, tubers, or roots. 



Analysis of the effect of temperature on the P/R ratio is complicated 

 by the fact that plants in their out-of-door habitats are not exposed to a 

 constant temperature, but to a daily cycle of temperature variations, which 

 varies greatly in its pattern, depending upon climatic conditions and even 

 in individual habitats within any climatic center. A daily alternation be- 

 tween relatively cool night temperatures and moderately high daytime tempera- 

 tures will result in a greater P/R ratio than if both day and night tempera- 

 tures are relatively high, because under the former condition night rates of 

 respiration are lower. 



A relatively low P/R ratio will not only check accumulation but will also 

 retard or may entirely inhibit assimilation. Hence the growth rate of a plant 

 may be greatly slowed down by a low rate of photosynthesis relative to the 

 rate of respiration. In extreme cases death of the plant may result. The 

 effect of variations in the P/R ratio is illustrated by the results of Nightingale 

 (1933) who grew tomato plants under continuous temperatures of 55° F. 

 (13° C.) 70° F. (21° C), and 95° F. (35° C). The plants survived, 

 developed, and stored carbohydrates at both the lower temperatures, indicating 

 that in these two groups of plants the P/R ratio was considerably in excess 

 of one. At 35° C, however, the carbohydrate content of the plants decreased 

 rapidly and many of them died in a relatively short time. 



Discussion Questions 



1. Would the rate of respiration in the leaves of a maple tree be greater on a 



cloudy day or a clear day if air temperatures were the same? 



2. How can respiration occur at night in leaves in which the stomates are 



closed ? 



