PLANT MORPHOGENESIS FOR SCIENTIFIC MANAGEMENT OF RANGE RESOURCES 



15 



and how 7r eoi i influences plant growth, other than 

 through its effect on lowering * B , will be included 

 in the discussion to follow. 



Since Slatyer has reviewed the effects of water 

 stress, primarily water stress resulting from in- 

 creased T S oii, I would like to confine my discussion 

 to the effects of 7r so n on plants, particularly as 

 it affects leaf morphogenesis. To do this, I will 

 discuss the response of plants to increased 7r so u in 

 terms of osmotic adjustment of the plant cells, 

 changes in resistance to water movement, how 

 these factors can lead to development of water 

 stress in the leaves and its resultant effects on 

 leaf growth, and how osmotic adjustment of the 

 cells directly influences leaf initiation and growth 

 in the absence of water stress. I also will discuss 

 the reversibility of these changes. Unless specifi- 

 cally stated otherwise at points in the discussion, 

 the plant to which I will be referring is the red 

 kidney bean (Phaseohis vulgaris L.). Also, unless 

 stated otherwise, I will be referring to osmotically 

 adjusted plants that have been subjected to grad- 

 ually increasing levels of NaCl in the root en- 

 vironment (2, lf.lt). Seeds were germinated in 

 paper towels, transferred to nutrient solution 

 when 7 days old, and NaCl additions started on 

 the ninth day. Salt additions were completed by 

 the 15th day (U). 



Osmotic Adjustment 



For many years the effect of increased 7r 30l i was 

 considered to be the same as increased t B0 u. This 

 viewpoint was implicit in development and ac- 

 ceptance of the concept of physiological drought. 

 This view was popular in spite of the demon- 

 stration many years ago that, as 7r soil increased, so 

 did 7Tp lant (36). This is illustrated by the data in 

 table 1. Even though this demonstration of inter- 

 nal osmotic adjustment was confirmed several 

 times in the following years, not until after the 

 papers of Bernstein (2) and Slatyer (53) ap- 

 peared, did the realization of plant osmotic ad- 

 justment become widespread. Slatyer (54) has re- 

 viewed the literature on this subject. 



When the 7r P i a nt increases in response to increase 

 in 7r soil , there is not always a direct correspondence 

 between the two. Sometimes the osmotic adjust- 

 ment is less than complete (26, 27, IfO) ; sometimes 

 it is complete (53) , and 7r p i ant has even been found 



Table 1. — Osmotic adjustment of corn roots in 



response to increased salinity of the soil solution 1 



[w = osmotic pressure] 



jt of growth meduim 



ir of root cells 



(bars) 



1.21 



1.99 



3.38 



4.96 



7.22 



(bars) 

 4.59 

 5.48 

 6.61 

 7.51 

 8.19 



'Adapted from McColl and Millar (36). 



to increase more than the increase in ■* of the 

 root medium (6). This osmotic adjustment occurs 

 whether the increase in v of the root medium is 

 slow or fast (39, 52), within limits. Furthermore, 

 the adjustment occurs whether the solutes respon- 

 sible for the increase in n of the root medium are 

 permeating (11, 12) or nonpermeating (26, 52), 

 and it occurs in both glycophytes (IfO, 53) and 

 halophytes (5). In fact, 7r p i ant even has been found 

 to increase proportionally with decrease in soil 

 moisture, under certain conditions (50). Thus, 

 osmotic adjustment in plants is truly a response 

 to decreased water potential in the rhizqsphere 

 and not simply a response to increased concentra- 

 tion of permeating solutes. 



Some of the variation in degree of adjustment 

 found by various investigators undoubtedly is 

 due to the normal fluctuations in it exhibited by 

 plants. That is, a normal or non-salinized plant 

 will exhibit fluctuations in internal osmotic pres- 

 sure, dependent on age and environmental con- 

 ditions. Thus, a more reasonable indication of 

 how much a plant osmotically adjusts, in response 

 to alterations in 7r so n, may be given by comparing 

 the difference in it of salinized and control plants 

 with the difference in v of the ambient solution 

 around the roots of the respective plants. Such a 

 comparison is illustrated in table 2. We increased 

 the it of the nutrient solution, in which bean 

 plants were growing, by 1 bar every 2 days. 

 Every 2 days, plants growing in both salinized 

 and nonsalinized solutions were sampled. The 

 7r of control plants growing in nonsalinized nu- 

 trient solution fluctuated between 5.8 and 7.7 

 bars over the 2-week period. The plants growing 



