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MISCELLANEOUS PUBLICATION 1271, U.S. DEPARTMENT OF AGRICULTURE 



periods do not appear to cause a progressive re- 

 duction in the capacity of the shoot to respond to 

 rewatering. 



Renewed root growth also requires the initia- 

 tion of new primordia and renewed cell division 

 and enlargement in the root meristem. By com- 

 parison with the shoot, however, renewed root 

 growth often appears to be delayed, perhaps 

 because of the need for a supply of fresh as- 

 similates from the leaves. In consequence, if 

 periods of stress recur before full rates of root 

 elongation have resumed, a progressive decline 

 in rates of root growth during each cycle may 

 occur. 



This phenomenon has been demonstrated by 

 Kaufmann (16), in studies of root and shoot 

 elongation in potted loblolly (P. taeda) and white 

 (P. strobus) pine seedlings. Kaufmann imposed 

 two watering regimes (5- and 7-day cycles), the 

 most severe of which induced water potentials of 

 the order of * = —16 bars before rewatering. 

 The control plants were watered daily. The rela- 

 tive responses of the shoots and roots to these 

 treatments are shown (fig. 3). It is apparent 

 that, although shoot growth is depressed in the 

 water stress treatments, the amount of shoot 

 growth in each cycle is maintained relatively 

 constant. By comparison, a progressive decline 

 occurred in the amount of root growth as the 

 second and third stress cycles were imposed. 



Although these experiments were conducted 

 with woody perennial evergreen species, it seems 

 likely that similar responses occur in annual and 

 herbaceous species. 



In Kaufmann's study, however, the entire root 

 system was under stress. Under natural condi- 

 tions, deeper roots may be in moist soil and New- 

 man's work, cited earlier, suggests that such roots 

 will continue to elongate even though roots in 

 shallower, drier, soil may reflect the pattern just 

 described. 



Effect Of Water Stress On Reproductive 

 Development 



In focussing the general effects of water stress 

 on to the specific area of reproductive develop- 

 ment, there are two main stages to be considered. 

 The first of these is the stage of floral initiation 

 and inflorescence development when the potential 

 fruit number is determined. The second is the 



stage of anathesis and fertilization when this 

 potential is realized, and fixed. This is followed 

 by a third stage of fruit maturation during which 

 fruit or seed weight progressively increases. This 

 stage, of great importance to economic yield in 

 many agricultural crops, is of reduced signifi- 

 cance in browse species and will not be considered 

 here. 



It is extremely difficult to generalize as to the 

 effects of water stress on floral development in 

 different species because of the markedly differ- 

 ent patterns which exist. Even so, it is desirable 

 to bring out as many general features as possible. 



The first stage to be considered is that of floral 

 induction. The kej r question is whether the transi- 

 tion from the production of vegetative to floral 

 primordia is affected by water stress and. if so. 

 in which direction? 



A change in environmental conditions is some- 

 times thought to induce initiation of floral pri- 

 mordia (26) and water stress has been regarded 

 as one such stimulus (27). However, in many de- 

 terminate annual herbs and grasses, the number 

 of vegetative primordia formed before the ap- 

 pearance of the first floral primordium does not 

 appear to be greatly affected by water stress. In 

 tobacco, for example, Hopkinson (13) showed 

 that water sti-ess could lead to the laying down 

 of additional vegetative primordia before floral 

 initiation occurred, but the maximum number 

 observed did not exceed five. In barley, Nicholls 

 and May (22) found no effect of stress on the 

 number of vegetative primordia laid down. 



On the other hand, in sorghum, Whiteman and 

 Wilson (35) found that, as onset of stress ap- 

 proached the normal time of floral initiation, 

 mean leaf number could be reduced up to three. 

 These workers considered that the leaf number 

 at which floral initiation commenced depended on 

 the interaction between the amount of vegetative 

 development necessary to allow the formation of 

 a floral stimulus, and the extent to which stress 

 may suspend development. With stress imposed 

 well ahead of the normal time of floral initiation, 

 there was insufficient vegetative development so 

 that, on rewatering, leaf formation was resumed, 

 and the number of vegetative primordia formed 

 was unaffected. 



By comparison, when stress was imposed closer 

 to the normal time of floral initiation, there was 



