Paper No. 1 

 EFFECTS OF WATER STRESS ON PLANT MORPHOGENESIS 



By R. O. Slatyer J 



Abstract 



Water stress has a profound effect on metab- 

 olism and morphogenesis, affecting many aspects 

 of growth and development. At the cellular level, 

 both cell division and cell enlargement are sensi- 

 tive to stress. Both appear to be progressively in- 

 hibited as water deficits develop, but cell en- 

 largement generally ceases before cell division, 

 frequently at water potentials of the order of 

 * = — 5 bars. Initiation of vegetative and floral 

 primordia appears to be completely suppressed 

 by quite small water deficits. Stem and root elon- 

 gation and leaf and inflorescence development 

 appear to parallel the behavior of individual cells. 

 Stress may affect root and shoot development dif- 

 ferently, root development being less influenced 

 by internal levels of water potential and more in- 

 fluenced by local levels of soil water potential. 



The transition from formation of vegetative to 

 floral primordia in woody plants is sometimes 

 hastened by water stress. In herbaceous plants 

 there is no consistent pattern but, even when the 

 ontogenetic stage at which flowering occurs is 

 advanced, the date of flowering is generally de- 

 layed by the stress-induced retardation of most 

 metabolic and morphogenic processes. 



Additional hey words : Water potential, growth, 

 flowering 



Introduction 



Water stress affects the growth and develop- 

 ment of plants in many ways and the mechanisms 

 involved have been studied extensively, particul- 

 arly by workers in subhumid and arid environ- 

 ments. However, despite this activity and the 

 large amount of information which has accumu- 



1 Professor of Biology and Head, Department of En- 

 vironmental Biology, Australian National University, P.O. 

 Box 475 City, Canberra, A.C.T., 2601 Australia. 



lated, it is still not possible to give a clear gen- 

 eral account of the sequential effects of water 

 stress on metabolism and morphogenesis. 



In part, this is because plant water status is a 

 highly dynamic parameter, strongly influenced by 

 conditions in the soil and atmospheric microen- 

 vironment and also regulated to different degrees, 

 in different situations and with different species, 

 by physiological factors. It therefore constitutes 

 a difficult parameter to examine experimentally. 



Of greater importance, though, is the fact that 

 both metabolism and morphogenesis are intim- 

 ately related through feed back control mechan- 

 isms. Thus, a direct effect of dehydration on cell 

 enlargement may indirectly influence protein syn- 

 thesis by affecting the demand of the plant for 

 the metabolites involved in cell division and en- 

 largement. This leads to an end-product inhibition 

 of protein synthesis. Since dehydration may also 

 affect protein synthesis directly, it sometimes be- 

 comes extremely difficult to separate cause and 

 effect relationships and to identify the sequence 

 of metabolic and morphogenic events associated 

 with stress effects. Furthermore, most factors 

 which affect growth have repercussions through- 

 out the whole plant. Thus, water stress not only 

 directly affects such processes as nutrient uptake 

 and photosynthesis, but the demand for mineral 

 nutrients and assimilates is itself affected by 

 growth rate. 



Effect Of Water Deficits On Cellular 

 Processes 



Growth and development of a plant depend, 

 in the simplest terms, on continuing cell division, 

 and on the differentiation and expansion of in- 

 dividual cells until the characteristic form of the 

 plant is realized. Associated with this (basically 

 genetically determined) phenomenon is an inter- 

 connected web of metabolic events which involves 

 the uptake of nutrients, the synthesis of metab- 



3 



