28 



MISCELLANEOUS PUBLICATION 1271, U.S. DEPARTMENT OF AGRICULTURE 



trefoil flowered later at the warmest temperature 

 than at intermediate levels. This delay in flower- 

 ing was accompanied by greater dry weight ac- 

 cumulation. Birdsfoot trefoil, accordingly, did 

 not follow the trend shown by the other species; 

 namely, that of greater dry matter production at 

 the lower temperatures. 



Vernalization temperatures play a significant 

 role in time of heading. Many species from tem- 

 perate climates require a period of cool condi- 

 tioning before floral induction. Following this 

 "winter" requirement, which must be associated 

 with short-day lengths in some species, the plant 

 advances through floral initiation and develop- 

 ment terminating in flowering. McKinney (2 4) 

 reviewed the subject, noting studies on cool season 

 crops for which vernalization temperatures from 

 freezing to 10° C. are found effective in hastening 

 flowering, and on warm season crops for which 

 temperatures from 20° to 30° C. have been used, 

 but often with less certainty of success. Most re- 

 search has involved small grains, with less at- 

 tention being directed to forage crops. Some of 

 the morphological responses noted with rye and 

 wheat when earliness is hastened are fewer leaves 

 per culm, and fewer tillers, each bearing smaller 

 numbers of seeds. Less rapid advance to maturity 

 results in the opposite trend in these characters. 

 In forage production, slower advance to maturity 

 often would seem preferable to extend the period 

 of vegetative growth. This may be accomplished 

 either by warm temperatures nullifying the low- 

 temperature effect or by inadequate vernalization. 

 Lindsey and Peterson (21) demonstrated the re- 

 versal of thermal induction in Merion Kentucky 

 bluegrass (Poa pratensis L.), a plant which re- 

 quires both cold and short days followed by 

 warm and long days to flower copiously. They 

 found that a single day reaching 35° C. for a 

 few hours reduced the percent of tillers heading 

 after vernalization, and a period of 16 days reach- 

 ing 35° C. prevented flowering in previously ver- 

 nalized plants. 



To gain the most vegetative growth from 

 grasses, Peterson, Cooper, and Vose (26) sug- 

 gested the use of nonflowering strains of herbage 

 grasses. This they point out may be achieved in 

 those species which have an obligatory require- 

 ment for cold and possibly for short day for 

 floral induction, with flowering occurring later 



under long, warm days. By growing these plants 

 in regions where either the cold is inadequate or 

 the subsequent daylength is too short, .flowering 

 is greatly depressed or even prevented. Seed pro- 

 duction in such culture, of course, must be devel- 

 oped in regions other than those of forage pro- 

 duction. Of the two limitations to flowering, in- 

 adequate cold or photoperiod, it is easier to em- 

 ploy photoperiod, since this varies seasonally de- 

 pending on latitude while temperatures at any 

 site are much more erratic. Still, inadequate low- 

 temperature vernalization may be characteristic 

 of some areas and may limit flowering. With 

 range species, such manipulation of the vegeta- 

 tive-reproductive balance has not been attempted 

 to the extejit that it has been with cultivated pas- 

 ture plants. Still, the same principles apply to 

 the plants of both categories. 



Morphological characters which might indicate 

 the hardiness of a plant have long been sought 

 with little success. Levitt (20) reviewed work 

 reporting morphological features associated with 

 frost hardiness. No single structural character- 

 istic was consistently identified. Small cell size 

 appeared to be the most reliable indicator, but 

 this he considered only a secondan^ factor to 

 which numerous exceptions were found. 



During the development of hardiness to heat 

 or cold, growth retardation consistently occurs as 

 temperatures gradually rise or lower. Weiser 

 (37) emphasized that in woody plants growth 

 cessation is a prerequisite to the attainment of 

 cold hardiness. Smith (30) reviewed low tem- 

 perature responses of forage plants and noted 

 that resistance to cold is reduced by conditions 

 which stimulate growth. He cited work in Flor- 

 ida and Georgia showing less over-wintering 

 ability in certain species following nitrogen ap- 

 plications. Fall applications of nitrogen, however, 

 may be used to stimulate growth in the cool 

 weather of autumn and early spring. Such is the 

 case with annual grasses on the low foothills in 

 California (19), where winterkilling is infre- 

 quent. In these regions, having day temperatures 

 warm enough to promote some growth, vegetation 

 fertilized with nitrogen appears less injured by 

 frosty nights than does unfertilized vegetation. 



Levitt (20) noted similarities in resistance to 

 heat and to cold. This has been borne out in 



