52 



MISCELLANEOUS PUBLICATION 1271, U.S. DEPARTMENT OF AGRICULTURE 



8 16 32 



LIGHT INTENSITY, K LUX 



Figube 3. — Ratios of aerial/root tissue dry weights in 

 alfalfa seedlings after a 2-week growing period at 

 three light levels (4). 



comparison of dry matter accumulation in seed- 

 lings of red clover, alfalfa (Medicago sativa L.), 

 ladino clover {Trifolium repens L.), and birds- 

 foot trefoil (Lottos corniculatvs L.) grown at 

 temperatures from 15° to 35° C. is shown in fig. 4. 

 Plants were grown at 43,000 lux and 18-hr. 

 photoperiod. Dry weights were determined at the 

 end of a 3-week growing period following seed- 

 ling emergence. A constant temperature of 15°, 

 even at these relatively high light intensities, 

 severely limited seedling growth and develop- 

 ment. Over the range of 15° to 30°, dry matter 

 accumulation increased proportionately to tem- 

 perature. The optimum temperature for these 

 legume species was near 30°. At 35°, red clover 

 plants exhibited severe high temperature stress 

 and would not have matured. Growth and seed- 

 ling development of the other legume species was 

 reasonably satisfactory at this high temperature. 

 Dry matter accumulations in bromegrass (Bro- 

 mus inermus Leyss.), orchardgrass {Dactylis 

 glomerata L.), and timothy (Phlevm pratense 



L.) grown at temperatures from 15° to 35° are 

 shown in figure 5. The grass plants were grown 

 under environmental conditions analogous to 

 those described for the legume species. The op- 

 timum temperature for the grass species was 

 near 25°. Seedling growth of all three species 

 was greatly reduced at 35° and in this respect re- 

 sembled that reported for red clover. 



Another interesting aspect of the temperature 

 response of grasses and legumes was the compara- 

 tive root growth. Ratios of aerial to root tissue 

 weights are given in table 2. Plants grown at low- 

 er temperatures had the lowest ratios. Those spe- 

 cies that were extremely sensitive to the high 

 (35° C.) temperature had high stem to root ra- 

 tios. Under the conditions of these experiments, 

 cool temperatures favored the diversion of photo- 

 synthate to root growth. Whei. the seedlings de- 

 veloped under temperatures that approached 

 le'hai levels, root growth wns more severely re- 

 t aider! than top growth. 



20 25 30 



TEMPERATURES, C 



Figure 4. — Dry matter accumulation in legume seedlings 

 grown for 2 weeks at temperatures ranging from 15° 

 to 35° C. 



