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MISCELLANEOUS PUBLICATION 1271, U.S. DEPARTMENT OF AGRICULTURE 



49) studies of wheat. Williams showed that, 

 rather than by a single exponential curve, growth 

 could be considered as a sequence of exponential 

 curves ; the change from one exponential curve to 

 the next representing successively, initiation of 

 secondary roots, cessation of leaf growth, cessa- 

 tion of stem growth, and the development of 

 floral structures. 



In another series of curves Williams (48) 

 showed changes in dry matter production of 

 leaves, stems, and inflorescences (fig. 2). 



Figure 2 shows that a decrease in growth of 

 leaves coincides with an increase in growth of 

 stems and a decrease in growth of stems by an 

 increase in growth of inflorescences. In his text, 

 Williams states that root growth stopped at the 

 same time as leaf growth; stem growth declined 



4 



3 ■ 



! 



Stems 



/ 



Inflor. 



/ /"^V 



/ / ^~~- v Leaves 



/ / - 



in 



i r . 



20 



2S 



Weeks 



Figure 2. — Growth of developing organs of the wheat 

 plant. (After Williams, f t 8.) 



as spikelets developed and, in turn, growth of 

 these declined as the floral parts began to de- 

 velop rapidly. In effect energy reserves appear to 

 be channelled successively to newly developing 

 organs. This led Williams to suggest that the 

 annual habit may be a result of self-starvation 

 from transference of energy substrates and other 

 nutrients to the developing caryopses, the form 

 of the plant in the dry season. 



Defoliation In Relation To Shoot Apex 

 Development 



In annuals, growth of tillers is determinate; 

 each tiller produces a single flowerhead and dies 

 after flowering and seed production (13). 



At or shortly after floral initiation, elongation 

 of stem internodes raises the shoot apex from 

 near ground level to a height where it eventually 

 becomes accessible to removal by cutting or graz- 

 ing. Removing the shoot apex of an elongating 

 tiller results in the death of that tiller but may 

 stimulate the growth of lateral buds, and, in 

 most annuals, removal of shoot apices prior to 

 flowering is followed by a resurgence of new 

 tillers. 



In some species, continued removal of shoot 

 apices by grazing or cutting may lead to contin- 

 ued production of fertile tillers throughout the 

 season. These tillers become reproductive with 

 fewer leaves than the primary tillers, and they 

 produce flower heads more rapidly. If decapita- 

 tion takes place after flowering, removal of nu- 

 trients in the inflorescence may mean that energy 

 levels could be too low for axillary tiller bud 

 development. As Milthorpe and Davidson (24) 

 showed, axillary tiller growth is slow if the sup- 

 porting stems carried ripening seeds when they 

 were cut. 



Aspinall (6) postulates that development of 

 axillary tillers following decapitation of inflo- 

 rescences is due to both a growth substance and 

 nutrients and their interaction on apical domi- 

 nance. 



According to Mc William (23), death of an- 

 nual pasture grasses is due largely to senescence 

 following water stress at the end of the growing 

 season. 



Effects of defoliation at different stages of de- 

 velopment are now considered for the following 



