Paper No. 2 



DEVELOPMENT AND REVERSAL OF PLANT RESPONSES TO SALINITY 



AND WATER STRESS T 



By James W. O'Leary 2 



Abstract 



As the osmotic pressure of the solution around 

 plant roots increases, plants increase their in- 

 ternal osmotic pressure. This osmotic adjustment 

 of plant cells is reversible. The water permea- 

 bility of roots in osmotically adjusted plants is 

 irreversibly decreased, and this allows water 

 stress to develop in these plants. Fresh weight, 

 dry weight, and leaf area are reduced by the 

 increased osmotic pressure, as is leaf initia- 

 tion. Upon removal of the osmotic stress, growth 

 rates of the stressed plants return to normal, 

 except that leaf expansion is still slightly sup- 

 pressed. This is considered to be due to decreased 

 extensibility of cell walls resulting from the in- 

 creased osmotic pressure of the cells. Data sup- 

 port the conclusion that one of the major effects 

 of increased osmotic pressure in the cells is an 

 acceleration of cell aging. 



Additional key words: Salinity, osmotic stress, 

 water relations, water stress 



Introduction 



The equilibrium water status of a plant is 

 dependent on the plant's environment. For a ter- 

 restrial vascular plant, that means the equilib- 

 rium plant-water potential will be equal to or 

 less than the soil-water potential under most 

 conditions (If5). That is, the soil-water potential 

 (* s ) determines the maximum plant- water poten- 

 tial (* p ) that can be developed. It is for this rea- 

 son that considerable emphasis is placed on main- 

 taining soil-water potential as high as is reason- 

 ably practical. The value of the soil-water poten- 



1 This research was supported by funds from the Rocke- 

 feller Foundation. 



2 Professor of Biological Sciences and Plant Physiolo- 

 gist, Environmental Research Laboratory, University of 

 Arizona, Tucson 85721. 



14 



tial depends almost exclusively on matric (t) and 

 osmotic pressures (r.). This relationship can be 

 indicated as : 



*„ 



(1) 



As the water content of a soil decreases, both 

 matric and osmotic pressures increase, and * s is 

 lowered accordingly. Under nonsaline soil-water 

 conditions, w is so small relative to t that it 

 usually is neglected, and equation 1 becomes: 



* B 



(2) 



As the salinity of the soil-water increases, -k be- 

 comes more important. This is a matter of great 

 concern because it makes possible the existence of 

 low soil-water potentials, even at high soil-water 

 content. For example, since there is such a close 

 relationship between t and soil-water content, 

 measurements of water content or t are the soil- 

 water measurements typically employed. How- 

 ever, if the salt content of the soil-water was such 

 that 7r soi i was 3 bars, then neither of those meas- 

 urements would reveal the fact, and more im- 

 portant, the * s would be _ 3 bars even at field 

 capacity. Under most conditions, the salinity of 

 the soil-water gradually increases over time and 

 may not be apparent until it has become quite 

 high. Thus, it is important to include measure- 

 ment of 7T soi i along with the other soil-water 

 measurements, even if direct psychometric meas- 

 urement of * s is made. 



If 7T SOi i affected plant growth only insofar as it 

 reduces * s , that is. if it only acted in the same 

 fashion as r soi i. then there would be no need for 

 concern, but if plant growth is reduced in some 

 other way, then it is important. For many years, 

 it was thought that 7r sol i and T eo n acted in the 

 same way in affecting plant growth, that is. they 

 simply lowered * s . but that idea has been ques- 

 tioned (31). Why this idea has been questioned 



