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MISCELLANEOUS PUBLICATION 1271, U.S. DEPARTMENT OF AGRICULTURE 



cies and the influence that environment has on 

 grass morphology. It would reveal morphological 

 aspects of adaptation. 



Morphogenetic responses to defoliation are 

 known for many herbage grasses, but little is 

 known of range species. For instance, what is 

 the effect of defoliation on AstrebloJ. Does it 

 cause a cessation of root growth or a dieback 

 of roots? Are all reserves mobilized to restore 

 the lost leaf area? Is tillering affected, and if 

 so, in what way? Is flowering promoted? These 

 questions are easy to ask, but the answers are 

 needed in order to determine the best manage- 

 ment practices for promoting plant welfare. 



Finally, in regard to carbohydrate reserves and 

 regrowth, Whalley and Davidson (13) showed 

 that in some summer-growing perennial grasses 

 of semiarid Australia, available carbohydrates 

 in the base of plants declined with the initial 

 growth after rain and increased as the soil dried 

 out so that the plant entered the dry period 

 with carbohydrate reserves comparable to that 

 of winter-dormant herbage grasses at the start 

 of winter. Hay don (6) showed that both starch 

 and sugars remain at a low level (2.0 percent and 

 1.1 percent, respectively) until flowering in As- 

 trebla lappacea and then, in a 30-day period 

 prior to drying off, these carbohydrates increase 

 to 9.0 percent for starch and 2.0 percent for 

 sugars. There appears to be no information aA r ail- 

 able on the pattern of carbohydrate reserves in 

 range grasses which have been grazed. In herbage 

 grasses of temperate regions, reserves appear to 

 be mobilized for the rapid production of new leaf 



U)- 



The importance of morphogenetic information 

 in determining management techniques appears 

 to have fairly wide acceptance (3). The questions 

 about plant welfare emerge clearly. Should the 

 management system of vegetative plants allow 

 for the development of a high leaf area during 

 favorable growth periods in order to maximize 

 productivity which can be utilized by the grazing 

 animal? If this is desirable, then more intensive 

 stock management with rotational grazing will 

 be necessary. What effects would such a manage- 

 ment procedure have on transpirational water 

 loss? Does this management allow for subsequent 

 recovery of the plants grazed during the dry 

 season ? None of these questions take into account 



the added expense of more intensive manage- 

 ment. 



Perennation 



Probably the main justification for dealing 

 with perennial range grasses separately from the 

 annuals is the capacity of perennials to survive 

 extended periods of drought as established plants 

 and to resume growth when favorable condi- 

 tions return. Despite the considerable physiologi- 

 cal and morphogenetic interest in these growth 

 characteristics of perennial range grasses, we 

 have little direct evidence on them in Australia. 



Wareing (12) draws the distinction between 

 imposed dormancy or quiescence and innate dor- 

 mancy. In imposed dormancy, plant growth is 

 arrested as a result of unfavorable environmental 

 conditions — such as drought or excessively high 

 temperatures — and resumes immediately on the 

 return of favorable conditions. In innate dor- 

 mancy, growth fails to occur even though external 

 conditions appear to be favorable; growth re- 

 sumes only after a certain amount of time passes 

 or after a particular environmental or genetic 

 trigger is released. 



Jozwik 6 and Jozwik and others (8) indicated 

 the possibility that Astrebla is capable of imposed 

 dormancy, although specific studies are needed to 

 confirm this. In terms of plant adaptation, im- 

 posed dormancy is a reaction which fits plants 

 to the arid rangeland environment since it allows 

 them to exploit favorable conditions -whenever 

 they occur. However, this system of dormancy 

 control carries the risk of growth, stimulated by 

 rainfall, inadequate to allow the completion of 

 either seed formation or resting bud formation. 



The physiology of drought and high-tempera- 

 ture tolerance is worthy of investigation in this 

 genus and in others because the perennating or- 

 gans are frequently exposed to considerable water 

 tension as well as very high temperatures. 



With regard to the flowering of Astrebla, it 

 would be interesting to know at what stage differ- 

 entiation of the inflorescence takes place. Since 

 inflorescences emerge 28 to 32 days after the 

 first rains, 7 initiation may occur at the end 

 of the previous growth period or even during the 



See footnote 4, p. 84. 

 7 See footnote 5, i>. 84. 



