PLANT MORPHOGENESIS FOR SCIENTIFIC MANAGEMENT OF RANGE RESOURCES 



21 



during the stress period (13). However, even dry 

 matter production has been found to increase 

 when stress was relieved (48), and there is a sug- 

 gestion that grass plants may even be able to 

 maintain a higher photosynthetic rate at a lower 

 leaf turgor, after relief of water stress (59). It 

 has been pointed out, however, that plants sub- 

 jected to salinity stress do not show such behavior 

 upon removal of the stress and, in fact, exhibit 

 growth rates still lower than in nonstressed plants 

 (31, 54). The data of Greenway (19) are used to 

 support this conclusion. Meiri and others (40) 

 also found that growth rates do not equal control 

 plant growth rates when stress is relieved. When 

 plants were alternated daily between saline and 

 nonsaline conditions, however, they showed a 

 burst of growth upon being placed in the non- 

 saline conditions, and this growth rate exceeded 

 the growth rate in the control plants (60). Also, 

 Strogonov (57) found that cotton plants sub- 

 jected to salinity stress for a month and then re- 

 turned to nonsaline conditions exhibited acceler- 

 ated growth rates even 6 weeks later. 



We subjected plants to four bars of added 

 NaCl and then returned them to nonsaline con- 

 ditions. Growth was measured 5 days and 10 days 

 later. Fresh weight of plants returned to nonsa- 

 line conditions increased faster than fresh weight 

 of the plants remaining in saline solution or of 

 control plants (fig. 6). This was due to accelerat- 

 ed rates of fresh weight increase in both leaf 

 blades and roots. Even though the rate of dry 

 weight increase was accelerated considerably upon 

 removal of the saline medium, it did not surpass 

 the rate of control plants (fig. 7). It did, how- 

 ever, equal the growth rate of control plants. 

 These increases did not occur until the second 

 5-day period. This is in good agreement with 

 Greenway 's (19) results, since his measurements 

 of growth were taken 6 days after stress was re- 

 lieved. 



Conclusions 



Much of the data discussed in this paper and 

 elsewhere (46) support the conclusion that one 

 of the primary effects of water stress and osmotic 

 adjustment is in acceleration of the aging process 

 in cells. The effects of osmotic and water stresses 

 on hormone metabolism and the resultant effects 



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TOTAL 

 PLANT 



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 PETIOLES 



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TIME (days) 



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Figure 6. — Fresh weight of total plant and different 

 plant parts as affected by growth in solutions with 4 

 bars added NaCl. Some of the plants were returned 

 to nonsalinized solution after 2 weeks growth in the 

 salinized solutions (Day 0). The solid circles repres'ent 

 control plants, the open circles represent plants grown 

 continuously in solution with 4 bars added NaCl, and 

 the half-filled circles represent plants removed from 

 the salinized solution on Day 0. Each point is the 

 average of 5 plants. 



on senescence have been discussed by Itai and 

 Vaadia (% 25) and O'Leary (46). During the 

 past decade, much evidence has been obtained 

 showing the effects of stress on cellular metabo- 

 lism. The most frequently observed disruptions 

 in cell metabolism involve nucleic acid and pro- 

 tein metabolism. This, in turn, is reflected by 

 chloroplast breakdown and other events that are 

 characteristic of senescence. Much of the data is 

 conflicting, and a discussion of the subject of 

 stress and cell metabolism that is less than com- 

 prehensive in scope would be misleading and 

 subject to misinterpretation. Thus, I offer no more 

 than my subjective analysis of the enormous 

 amount of data relating cell physiology and stress 

 which is summed up in my general conclusion 

 that water stress, salinity stress, and probably 

 other forms of stress as well, act primarily to 

 accelerate senescence in plants. 



Of course, when environmental conditions are 

 such that they cause a reduction in cell turgidity, 



