50 



MISCELLANEOUS PUBLICATION 1271, U.S. DEPARTMENT OF AGRICULTURE 



effects these two important environmental vari- 

 ables have on growth and development of peren- 

 nial herbage species. 



Seedling Development 



If soil moisture is adequate, seed germination 

 and seedling emergence are primarily affected by 

 temperature. Generally, increases in temperature, 

 within limits, increase the rate of germination 

 and emergence. The optimum temperature ap- 

 pears to be species or even cultivar related (10). 

 On the other hand, seedling growth has been 

 shown to be closely related to light intensity and 

 temperature. Bula et al. (If) noted that total dry 

 weight of 14- or 28-day old alfalfa seedlings was 

 essentially linearly related to light intensity over 

 the range of 8,000 to 32,000 lux (fig. 1). Combina- 

 tions of light intensity levels during the 15-hr. 

 illumination period resulted in greater dry 

 weight accumulation after 14 days of growth 



than an exposure to 16,000 lux during the entire 

 illumination period even though all these light 

 treatments provided equal amounts of light en- 

 ergy. 



Dry weight accumulation in red clover (Tri- 

 folium pratense L.) seedlings at the end of a 28- 

 day growing period increased markedly when 

 grown at 16,000 lux compared to 8,000 lux (2). 

 Only a slight increase in seedling growth Avas 

 noted when the red clover were grown at 32,000 

 compared with 16,000 lux (fig. 2). Length of the 

 illumination period also had an effect on seedling 

 growth and development. An 8-hr. illumination 

 period resulted in relatively low growth rates at 

 all three intensities. A 20-hr. illumination period 

 resulted in growth rates approximately six times 

 higher than those noted for the 8-hr. period. 



The effects of light intensity and illumination 

 period on red clover seedling development are 

 illustrated in table 1. Less than 30 percent of the 

 plants of 'Pennscott' red clover produced flower- 



et: 



Ll! 

 Q. 



r- 

 X 



o 



UJ 



or 



Q 



75- 



50 



25 



l-8-3hr, l6-3hr, 32-3hr, l6-3hr, 8-3hr 

 2-8-5hr, 32-5hr, 8-5hr 

 3-8-IOhr, 32-5hr 

 4-32-5hr, 8-IOhr 



2 Weeks 



750h 



500 



250 







4 Weeks 



/ 



8 16 32 w 8 



LIGHT INTENSITY, K LUX 



16 



32 



Figure 1. — Dry weight accumulation in alfalfa seedlings after 2- and 4-week growing periods under various light 



regimes. 



