PLANT MORPHOGENESIS FOR SCIENTIFIC MANAGEMENT OF RANGE RESOURCES 



29 



studies which demonstrate that cold-resistant va- 

 rieties often are heat-resistant as well (5). I have 

 heat-hardened Bromus catharticus Vahl. and 

 then found it more tolerant of cold stress than 

 plants not heat-hardened. Likewise, cold-harden- 

 ed plants have been found more heat-tolerant 

 than plants not cold-hardened. This capacity may 

 prove to be a meritorious attribute for range 

 vegetation. 



The ability of a plant to curtail growth dur- 

 ing temperature stress and then to resume growth 

 after the stress terminates probably deserves more 

 attention than this behavior has received to date. 

 Plants, for a short time, can survive extremes of 

 temperature which are too severe to permit meas- 

 urable growth. Between the maximum tempera- 

 ture permitting growth and that temperature 

 producing mortality, the plant is in a state some- 

 times termed "rigor." Marinucci and Rivera (22) 

 exposed seedlings of Lupinus albus L. to 52° C. 

 for periods of 10 to 48 hours. They reported that 

 growth was suspended for intervals from 2 to 22 

 days depending on the duration of the heat stress. 

 Perhaps we should investigate the ability of 

 range vegetation to enter temperature rigor and 

 later to resume growth. 



A more commonly observed growth cessation 

 is that of dormancy in the perennial plant. A 

 complex of factors contribute to winter or sum- 

 mer dormancy including temperature, moisture, 

 and photoperiod. However, in different species 

 these factors change in relative importance. An 

 example of temperature playing a major role 

 with photoperiod is found in the summer dor- 

 mancy of Poa scdbrella (Thurb.) Benth. ex 

 Vasey (17). This species becomes dormant about 

 June 1 at Davis, Calif., even when soil moisture 

 is readily available. Investigation revealed that 

 high temperatures coupled with the long day- 

 lengths of that season triggered the onset of dor- 

 mancy. 



Disorders And Injuries From Heat And 

 Cold 



Factors of the physical environment are re- 

 sponsible for well over half of the disorders of 

 plants, according to Treshow (36). Still these 

 noninfectious factors can be most elusive. High 



or low temperatures are associated with a diverse 

 array of injuries. Treshow considered primarily 

 woody species and vegetables, and listed among 

 other conditions leaf chlorosis, leaf scorch, sun- 

 scald,- stem cracks and lesions, and frost at flow- 

 ering and fruiting. The literature treating grasses 

 (including cereals) and legumes provides ex- 

 amples of these and an array of additional symp- 

 toms. Though these studies have seldom used 

 range species, they are, in the opinion of this 

 writer, applicable to range vegetation and are 

 deserving of greater attention by range investi- 

 gators. 



Frischknecht (8) observed the breaking over of 

 seedlings upon emergence, numbers of seedlings 

 emerging as albinos, and frost heaving in studies 

 of grasses fall or spring planted at high eleva- 

 tions in Utah. Many of the broken seedlings re- 

 vealed etiolated frost banding near the soil sur- 

 face, and few of these survived. Seedlings which 

 emerged with little chlorophyll or as albinos 

 usually died. It would be well to note any effects 

 on later development in those seedlings which 

 did survive. Suneson and Stevens (34) observed 

 the later growth of frost-induced color-deficient 

 seedlings of barley. Approximately 48 hours after 

 seedlings at the two-leaf stage experienced a min- 

 imum night temperature of near —4° C, the 

 leaves lost chlorophyll. Some survived by pro- 

 ducing later leaves of normal color. However, sur- 

 vivors which had exhibited this early chlorosis 

 were identified for comparison with plants from 

 normal seedlings. At heading those from albino 

 seedlings had decidedly fewer matured tillers 

 and caryopses than plants which had not shown 

 this albinism. Competitive ability of the albinos 

 was greatly reduced. 



Heat banding of leaves has been reported in 

 the field in Russia by Bukharin (3) who noted 

 that seedling leaves of wheat exhibited pale yel- 

 low constrictions where they contacted hot soils. 

 Soil surface temperatures above 54.5° C. pro- 

 duced severe burns. The leaves bent or broke over 

 at the points of heat constriction. Heat banding 

 of barley leaves has been produced in a controlled 

 environment (18). When 5-day-old seedlings 

 were subjected to 54.4° C. for 4.75 hours a chlo- 

 rotic band appeared at the constricted point of 

 emegence of the first foliage leaf from the cole- 



