F. W. WENT 165 



the plant but a function of its actual size. That is to say, that the further 

 the growing points are removed from the leaves or supply organs, the 

 lower the nycto-temperature is because the more translocation becomes 

 limiting. 



Not only in tomato, but in many other plants, this Qio under one for 

 sugar translocation explains observations in the laboratory and in nature. 

 Sugar beets, for instance, attain a sugar concentration in their storage or- 

 gans which is inversely proj^ortional to the temperature, and therefore 

 sugar beets growing in cooler climates have invariably a higher sugar con- 

 tent than those grown in warmer climates. For this reason also, sugar beets 

 in the hot Imperial Valley can be grown during winter but have to be 

 harvested before the warm weather of summer. The explanation usually 

 given for this phenomenon, namely that at the higher temperatures the 

 sugar is removed by excessive respiration, is invalid because respiration 

 removes in such beets only a fraction (less than 30%) of the sugar formed 



40 

 30 h 



o . 



Fig. 2. Maximum growth rate 

 of Alaska pea (ordinate) kept ^^° 

 at different nyctotemperatures 

 From Went 1957. 



E 

 E 10 



Phototemp. 20° 

 Photoperiod 12 hrs. 



4° 7° 10° 14° 17° 20° 23° 26° 

 Nyctotemperature 



during the day in photosynthesis. Besides, it was found in tomatoes that the 

 sugar content of leaves is higher after a warm night than after a cool one, 

 due to the slow translocation at the high temperatures. 



The effect of temperature on the growth of peas is somewhat similar to 

 that described in tomatoes, but especially in young peas there is a con- 

 siderable range of temperatures where growth is not influenced by tem- 

 perature at all. Between 7° and 20° nyctotemperature (see fig. 2) the maxi- 

 mal growth rate of pea plants is exactly the same. This means that a 

 diffusion process is limiting growth over that region. At lower temperatures 

 the growth process is again limiting but at higher temperatures injurious 

 effects cause very rapid decline in growth rate with time. 



The effects of temperature on growth in general are mostly comparable 

 with those described in the previous paragraphs. Consequently, we can 

 draw another conclusion. Since apparently at the optimal temperatures 

 with a Qio near one we are dealing with processes limiting growth by dif- 

 fusion or a similar process, it is unlikely that it would be possible to speed 

 up growth beyond the maximum rate which can be attained at optimal 

 temperature. It was already shown that hormones are unable to increase 



