PLANT MORPHOGENESIS FOR SCIENTIFIC MANAGEMENT OF RANGE RESOURCES 



ment upon relief of stress, is a phenomenon that 

 has been frequently cited (9, 10, 12, 30). 



Renewed development upon rewatering may 

 proceed at a more rapid rate than in the controls, 

 sometimes enabling stressed plants to "catch up." 

 This phenomenon, evident in figure 1, has also 

 been demonstrated in the field. Morton and Wat- 

 son (19), for example, showed that a crop of 

 sugar beets, exposed to repeated cycles of water 

 stress interspersed by irrigations, developed over- 

 all at the same rate as the well irrigated controls. 



When stress is more severe or is prolonged, the 

 total number of primordia formed is substantially 

 reduced. In barley, for example, Husain and As- 

 pinall (15) showed that total primordial number 

 is significantly reduced by withholding water for 

 several weeks (during which time the plant water 

 potential dropped to levels corresponding ap- 

 proximately to permanent wilting) . Even though 

 initiation of primordia was resumed upon rewat- 

 ering, their rate of appearance was insufficient to 

 permit "catching up" to the well watered controls 

 (fig. 2A). Similarly, in white pine (Pinus strobus 

 L.) Zahner (37) reported that the number of 

 needle primordia laid down in the terminal buds 

 of late summer could be reduced by 40 percent 



during summers in which mild droughts occurred 

 at bud formation stage. 



Cell division in organs that have been already 

 initiated does not appear to be as sensitive as in 

 the apical meristems, and may continue during 

 stress, although at a progressively reduced rate, 

 until quite severe conditions (* = — 10 to — 20 

 bars) exist. 



In the study on barley by Husain and Aspinall 

 (15) for example, slow but persistent differentia- 

 tion of pre-formed primordia continued during 

 the stress period (fig. 2B). It has also been noted 

 that the total number of cells in stressed leaves 

 may be the same as in well- watered controls (29). 

 Other corroborative evidence was obtained by 

 Gardner and Nieman (8) from studies of DNA 

 synthesis in stressed radish cotyledons. Although 

 a marked reduction in the rate of increase of 

 DNA occurred at small stress levels, some in- 

 creases occurred even when severe stress (* = 

 — 15 bars) was imposed. This phenomenon per- 

 mits relatively rapid resumption of growth and 

 development when stress is removed (fig. 2B). 



The relative growth rates of plants measured 

 following relief of water stress may be more 

 rapid than those of control plants (9, 10, 36). 



6 12 18 24 30 36 42 48 6 12 18 24 30 36 42 48 



A R 



Days from planting 



Fiqute 2. — Total number of primordia on main axis (A) and stage of development of main apex (B) of barley sub- 

 jected to prolonged water stress. Plants watered throughout (0) : not watered after day (•) ; not watered 

 between days and 30 (A) (after Husain and Aspinall (15)). 



