DEW: FACTS AND FALLACIES 



51 



The free energy of atmospheric water vapour Ha is related to relative 

 humidity and absolute temperature by the equation 



H^ = 47oT(-lnr) 



= -i-38.io*hir atr=293°K 



(19) 



and Fig. 3 shows equivalent scales of — Ha and r. In leaf cells (assumed 

 completely vacuolated), the free energy deficit { — Hl) is the difference 

 between the osmotic potential energy of the vacuolar fluid and the elastic 

 potential energy of the ceU wall, a quantity known to physiologists as 



region oF rapid 

 absorption fronn 

 saturated air 



<^'^y so,i 



sandy 



Fig. 3. Variation of water potential with soil moisture (per cent dry weight) for two 

 idealised soils. The horizontal axis shows the relation between water potential and 

 equilibrium relative humidity. The water potential of the clay soil at S% moisture 

 content is about — 100 m and absorption of atmospheric water vapour would occur 

 only when relative humidity exceeded 99%- 



diffusion pressure deficit (DPD). The water deficit of plant leaves after a 

 period of transpiration is normally met by the transfer of water from other 

 parts of the plant and ultimately from the roots; but the direct uptake of 

 atmospheric water vapour through the cuticle is possible when Hl<Ha- 

 Leaves at the conventional '15 atmosphere' wilting point {Hl= — i55 i"^) 

 will absorb vapour from an atmosphere with relative humidity exceeding 

 98-8%, and for a plant surviving at Hl= - i 120111 (Slaty er, 1958) absorption 

 is possible when the relative humidity is 92%. 



The free energy of soil moisture Hs is determined by its salt concentra- 

 tion and by the radius of the largest water-occupied pores. At the soil 

 surface, and in the neighbourhood of plant roots, Hs decreases during the 

 day as moisture content decreases. The condition for uptake of atmospheric 

 water vapour during the night is Hs<Ha. 



