12 



MISCELLANEOUS PUBLICATION 1271, U.S. DEPARTMENT OF AGRICULTURE 



and a general stunting of the tops of the plants. 

 Root growth is also likely to be suppressed more 

 or less in proportion to shoot growth. Flowering 

 and fruit set will probably occur at much the 

 same ontogenetic stage as well watered species, 

 but are likely to be delayed in time. Both seed 

 number and seed size are likely to be reduced, and 

 seed viability may also be reduced. If stress is 

 more severe, continued growth and development 

 is likely to be stopped and the plant will finally 

 die unless rewatered. 



In perennial grasses, a similar general effect 

 on shoot development — of reduced leaf size and 

 internode length — can be expected to occur. Root 

 growth, however, will be controlled more by local 

 levels of soil-water potential than by mean plant- 

 water potential and can be expected to continue 

 in moist soil. If stress is severe, shoot growth 

 will stop but will be resumed upon rewatering. 

 Reproductive development is likely to be delayed 

 and floral initiation may not occur unless stress 

 is mild or absent. If the stress is sufficiently severe 

 to induce shoot dieback, new tillers will develop 

 from basal buds when water is reapplied. The 

 residual tussocks are extremely tolerant of water 

 stress and will probably persist through lengthy 

 drought periods if they are not damaged by 

 grazing. 



Woody evergreens tend to reflect the same pat- 

 tern of stress response as perennial grasses in that 

 shoot growth may cease while root development 

 continues in moist soil. However, the pattern 

 of shoot development can be influenced over 

 lengthy periods in cases where growth is largely 

 seasonal and is based in part on development of 

 overwintering buds. In such cases water stress 

 at the times of bud development has an impor- 

 tant bearing on subsequent shoot development, 

 both vegetative and reproductive. In general, the 

 number of leaves and flowers will be controlled 

 by water stress during bud development, but leaf 

 size and shoot length may be more influenced by 

 water stress during the postwinter period, as will 

 the amount of fruit set and final seed size. In 

 species without such strong seasonal control, con- 

 temporary water stress operates in much the same 

 way as in herbaceous species. 



Woody perennials also tend to be extremely 

 drought tolerant. There may be some drought 

 induced leaf shedding, but unless the stress is 



almost lethal, leaf shedding tends to be greatest 

 after water is reapplied and when new growth 

 is commencing. 



Literature Cited 



(1) Alvim, P. DeT. 



1960. MOISTUBE STRESS AS A REQUIREMENT FOR FLOW- 

 ERING of coffee. Science 132 : 354. 



(2) Aspinall, D., and I. Husain. 



1970. THE inhibition of flowering by water stress. 

 Austral. J. Biol. Sci. 23: 925-936. 



(3) Boyer, J. S. 



1968. relationship of water potential to growth 

 of leaves. PI. Physiol. (Lancaster) 43: 1056- 

 1062. 



(4) Brown, D. S. 



1952. relation of irrigation practice to the dif- 

 ferentiation AND DEVELOPMENT OF APRICOi 

 flower buds. Bot. Gaz. 114: 95-102. 



(5) Childers, N. F. 



1961. modern fruit science. Horticultural Publi- 

 cations, U.S.A. 893 pp. 



(6) Clements, J. R. 



1970. shoot responses of young red pine to water- 

 ing applied over two seasons. Canad. J. 

 Botany 48: 75-80. 



(7) Evans, L. T., and I. F. Wardlaw. 



1966. independent translocation of "c-labelled 

 assimilates and the floral stimulus in 

 lolium temulentum. Planta 68 : 310-326. 



(8) Gardner, W. R., and R. H. Nieman. 



1964. lower limit of water availability to plants. 

 Science 143: 1460-1462. 



(9) Gates, C. T. 



1955a. the response of the young tomato plant to 



A BRIEF PERIOD OF WATER SHORTAGE. I. THE 



whole plant and its principal parts. Aus- 

 tral. J. Biol. Sci. 8: 196-214. 



(10) 



1955b. THE RESPONSE OF THE YOUNG TOMATO PLANT TO 

 A BRIEF PERIOD OF WATER SHORTAGE. II. THE 



individual leaves. Austral. J. Biol. Sci. 8: 

 215-230. 



(11) 



1957. THE RESPONSE OF THE YOUNG TOMATO PLANT 

 TO A BRIEF PERIOD OF WATER SHORTAGE, m. 

 DRIFTS IN NITROGEN AND PHOSPHORUS. Aus- 

 tral. J. Biol. Sci. 10: 125-146. 



(12) 



1968. WATER DEFICITS AND GROWTH OF HERBACEOUS 



plants. In Water Deficits and Plant 

 Growth, pp. 135-190. Vol. II, T. T. Kozlowski, 

 ed. Academic Press Inc., New York. 



(13) Hopkinson, J. M. 



1968. EFFECTS OF EARLY DROUGHT AND TRANSPLANT- 

 ING ON THE SUBSEQUENT DEVELOPMENT OF THE 



tobacco plant. Austral. J. Agric. Res. 19 : 

 47-57. 



