CHAPTER 15 



Temperature 



Temperature is obviously of much importance for the physiological ac- 

 tivities of a plant since the rate of metabolic processes is markedly 

 affected by it. Though its chief significance is in physiology, it also 

 influences development in various ways. These have been summarized 

 by Went (1953). 



Both light and temperature apparently produce their morphogenetic 

 effects by speeding up or slowing down particular physiological processes. 

 What the effect in a given case will be evidently depends on the sensi- 

 tivity of the plant to these stimuli in a particular part of its body or on a 

 particular phase of its development. The effect of temperature is especially 

 important on rate of growth. The optimum temperature for this may be 

 different in different regions of the plant, at different stages of develop- 

 ment, or even at different times of day. This may result in a change in 

 proportions of various parts and thus of form and structure. Biinning 

 (1935), for example, observed that the later in the season the seeds of 

 beans mature, the shorter is the epicotyl of seed and seedling and the 

 quicker do the primary leaves reach maturity. This he found to be a 

 temperature effect, for high temperature during the 5 weeks preceding 

 seed maturity produces longer epicotyls and a slower development of 

 primary leaves. 



In Ipomoea, Njoku ( 1957 ) found that the higher the night tempera- 

 ture (with a good level of mineral nutrition) the less deeply lobed were 

 the leaves (Fig. 15-1). Here what temperature seems to affect directly 

 is the rate of production of leaves at the growing point, and this, in turn, 

 is correlated with depth of lobing. Many other cases have been found 

 where temperature thus exerts an indirect effect on form and structure 

 because of the fact that different parts are differently susceptible to 

 its influence. 



Thermoperiodism. There is often a daily rhythm in reaction to tem- 

 perature as there is to light. At any temperature that is constant through- 

 out the 24 hours many plants will grow less rapidly than if their environ- 

 ment is relatively cool at night and warm during the day. The optimum 



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