358 



FACTORS AFFECTING PHOTOSYNTHESIS 



J00I7 



pronounced than its direct effects. In other words deficiency of water as 

 a raw material is not commonly a limiting factor in photosynthesis. Never- 

 theless, a reduction in the water content of leaves generally results in a de- 

 crease in the rate of photosynthesis (Fig. 92). There are probably two prin- 

 cipal ways in which this effect can be exerted : ( i ) reduction in the diffusive 

 capacity of the stomates resulting from the decreased leaf water content, and 



(2) reduction in the hydration of the 

 chloroplasts and other parts of the 

 protoplasm. 



There is no question that decrease in 

 the water content of leaves causes a re- 

 duction in the diffusive capacity of the 

 stomates. The results of Mitchell 

 (1936) indicate, however, that reduc- 

 tion in the diffusive capacity of the 

 stomates does not have as marked an 

 effect upon photosj^nthesis as is some- 

 times supposed. Leaves of tomato and 

 Pelargonium were found to utilize car- 

 bon dioxide approximately as rapidly 

 under high humidities as under low 

 ones, although under the latter condi- 

 tion the stomates appeared to be almost 

 entirely closed. The probable explana- 

 tion of this seemingly anomalous find- 

 ing is that stomates which appear closed 

 to microscopic observation are not 

 entirely closed to the passage of gases. 

 It should also be recalled that the dif- 

 fusive capacity of only slightly open stomates is much greater than the area 

 of the apertures would suggest (Chap. XIII). 



In general it appears that the rate of photosynthesis is less affected by 

 reduction in leaf water content than the rate of transpiration. This is indi- 

 cated by the results of Heinicke and Childers (1936) who determined the 

 average rates of photosynthesis and transpiration over a week's period in two 

 apple trees, one of which was well watered while the other was growing in 

 soil which was gradually drying out. The rate of photosynthesis of the plant 

 in the dry soil was about half as great as in the plant which was watered. 

 The rate of transpiration, however, was only about one-fourth as great in the 

 former plant as in the latter. 



.0001 



1.04 1.06 1.08 I.IO 1.12 1.14 



WATER CONTENT GRAMS PER lOOCM? LEAF AREA 



Fig. 92. Relation between rate of 

 photosynthesis in attached leaves of 

 Abutilon dariuin'ti and their water 

 content. Data of Dastur (1925). 



