Chapter X 

 WATER LOSS AND WATER RETENTION 



Introduction : — The maintenance of a favorable balance between water 

 loss and water absorption is essential in the economy of a plant, for growth 

 and development depend upon an adequate water supply. Favorable, as 

 used here, does not imply a positive hydrostatic pressure within the xylem 

 conductors, nor does it necessarily mean a positive supply in the soil. It 

 has been shown that a plant may absorb water from the soil against a dififu- 

 sion pressure deficit variously estimated to be 4 to 16 atmospheres, and it is 

 obvious that tensions within the xylem may attain much higher values in 

 plants that are developing normally. The inference is that there are definite 

 limits of DPD for water in the soil, and water in the plant, above which the 

 plant cannot operate normally. And because of these limits, water loss 

 must be eventually counterbalanced by an equal uptake if the plant is to 

 grow and develop and maintain life throughout a normal span. Because 

 the larger vessels of the xylem, and many parenchyma cells of xylem, 

 phloem, and cortex may act temporarily as reservoirs, the plant may survive 

 diurnal periods when loss exceeds uptake. However, such deficits must be 

 replenished, at least during periods of active growth, if the plant is to thrive. 

 And because of the interrelation between the various processes of absorp- 

 tion, movement, and loss, an effect on one will cause a correlated effect on 

 the others. The term water balance is commonly used by plant physiologists 

 to describe these processes as they relate to the water economy of plants. 



Transpiration, that is, loss of water from plants in the vapor form, has 

 been termed both beneficial (Clements, 1934&) and a necessary evil 

 (Barnes, 1902; Curtis, 1926). Probably both viewpoints have some 

 justification. Certainly the maintenance of moist wall surfaces within the 

 stomatal chambers of leaves, for absorption of CO2 during rapid photo- 

 synthesis, results inevitably in loss of water by evaporation, and much 

 evidence is at hand to prove that such evaporation is usually far in excess 

 of any requirement, because plant growth in tropical regions where the air 

 approaches water saturation is very luxuriant. 



Although the work of Hasselbring (1914), Kiesselbach (1916), 

 MuENSCHER (1922), and Mendiola (1922) indicates that different rates 

 of transpiration may occur without materially affecting mineral salt absorp- 

 tion, experiments on translocation of minerals within the xylem by Clem- 

 ents and Engard (1938), Wright (1939), and Stout and Hoagland 

 (1939) prove that transpiration facilitates the distribution of minerals 

 within the plant. More recent studies by Hoagland ( 1944) show that the 

 previous salt status of the plant is extremely important in such studies. It 

 seems therefore that some loss of water by transpiration is necessary; on 

 the other hand, transpiration in most terrestrial environments is far in 

 excess of requirements for purposes of translocation. Hence, conserva- 

 tion of water by the plant is important both economically, where the supply 

 may fail and so limit growth, and essentially for xerophytes, where the 

 water supply may determine survival. 



Most studies on transpiration emphasize water loss and the relation of 

 the various physical mechanisms and physiological processes to it. From 



