Paper No. 5 



ENVIRONMENTAL CONTROL OF THE GROWTH AND DEVELOPMENT 

 OF PERENNIAL HERBAGE SPECIES 



By R. J. Bula 2 



Abstract 



Under temperate, humid conditions, radiant 

 energy levels and temperature play a dominant 

 role in regulating growth and development of 

 perennial herbage species. Light intensity rather 

 than photoperiod appears to be an important en- 

 vironmental variable during seedling develop- 

 ment. High levels of radiant energy during this 

 stage of development insure rapid seedling 

 growth and development of an adequate root sys- 

 tem, resulting in a strongly competitive plant. 



Development of regenerative tissue (tillers) is 

 favored by short photoperiods and cool tempera- 

 tures. Leaf expansion is favored by long photo- 

 periods and warm temperatures. Such responses 

 coincide with the characteristic development of 

 perennial, long-day plants. Development of tillers 

 occurs during the fall and spring. During late 

 spring and early summer, when environmental 

 conditions provide long daylengths and warm 

 temperatures, most of the photosynthetic energy 

 is utilized for tiller elongation, ultimately leading 

 to flowering. This growth pattern may principally 

 account for the seasonal variations in yield ob- 

 served for many perennial herbage species. 



Additional key words: Photoperiod response, 

 floral development, seedling growth, tillering, 

 light intensity, temperature response. 



Introduction 



Net productivity of a herbage ecosystem is 

 dependent largely on the number of harvests per 

 growing season. Invariably, herbage yields de- 



1 Joint contribution from the Agricultural Research 

 Service, U.S. Department of Agriculture, and Department 

 of Agronomy, Purdue University, Lafayette, Ind. 47907. 



"Agronomist, Agricultural Research Service, U.S. De- 

 partment of Agriculture, Lafayette, Ind. 



cline at each successive harvest from spring to 

 fall. For example, alfalfa {Medicago sativa L.) 

 yields in the southwestern U.S. are reported to 

 decrease to a seasonal low during late summer 

 followed by a slight increase during the autumn 

 period (16). A similar midsummer decline in 

 growth and productivity of herbage grass spe- 

 cies has been observed (14)- Although several en- 

 vironmental factors have been implicated in these 

 yield responses, it is obvious that dry matter ac- 

 cumulation or harvestable yield is dependent on 

 a complex of interacting physiological and en- 

 vironmental factors. 



Adding to this complexity is the fact that 

 many herbage species are perennials whose de- 

 velopmental physiology is considerably different 

 from that of annual species. The diversion of 

 photosynthate to storage tissues and activation of 

 regenerative tissue are important characteristics 

 of perennial species that have a bearing on the 

 net productivity of a herbage community. Thus, 

 the effects of specific environmental factors on 

 plant response are different, depending on the 

 stage of development of a perennial plant. Eluci- 

 dation of the relationships of an environmental 

 factor or combination of factors with the physio- 

 logical processes involved in dry mattter accu- 

 mulation would be a major contribution toward 

 realizing maximum net productivity of a herb- 

 age ecosystem. 



Although radiant energy levels establish the 

 ultimate net productivity of crop systems, under 

 natural conditions the environmental control of 

 plant growth and development involves other fac- 

 tors including temperature, water, and nutrients. 

 The importance of each factor in limiting plant 

 growth varies with the climatic conditions of the 

 geographic region. Under temperate, humid con- 

 ditions, light and temperature play the most 

 dominant role. Here I will discuss some of the 



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