PLANT MORPHOGENESIS FOR SCIENTIFIC MANAGEMENT OF RANGE RESOURCES 



27 



seedlings has been reported recently (Onwueme 

 et al., (25) ). Activity of this enzyme was assayed 

 during and following a heat stress of 43° C. for 

 24 hours under continuous light. This level of 

 stress evoked pronounced change in the activity 

 of the enzyme while producing no tissue necro- 

 sis in the seedlings. Nitrate reductase activity de- 

 clined to approximately 30 percent of its initial 

 value during 24 hours of heat stress. During the 

 next 24 hours the plants were held at 24° C. 

 and regained activity to near the prestress level. 

 Irreversible damage to the enzyme was not indi- 

 cated under the conditions of this experiment. 

 Extracts from heat-stressed and control leaves 

 were mixed in various proportions and gave ad- 

 ditive activity, suggesting the absence of inhib- 

 itors in the extracts. It was recognized that mois- 

 ture deficits can result in decreased enzyme ac- 

 tivity. These authors determined that the water 

 potential of stressed plants averaged 0.7 at- 

 mospheres lower than in the unstressed. However, 

 when a water potential 2 atmospheres below that 

 of freely watered plants was developed by with- 

 holding water, the nitrate reductase activity re- 

 mained near 87 percent that of plants freely 

 watered. Temperature, therefore, was viewed as 

 being the primary factor inducing the reduced 

 enzyme activity during the heat stress. These in- 

 vestigators further compared nitrate reductase 

 activity in heat-hardened and unhardened seed- 

 lings, by subjecting both to heat stress. The rate 

 of inactivation of the enzyme during stress did 

 not differ significantly between hardened and un- 

 hardened plants, suggesting that the hardening 

 treatments had not resulted in increased heat- 

 stability of the enzyme. It was further deter- 

 mined that nitrate reductase activity decreased 

 during darkness. Conceivably this might mean 

 that under field conditions the reduced activity 

 of the enzyme which starts during a hot after- 

 noon could continue during the night. Heat the 

 next day could impair further production of the 

 enzyme and also result in further decrease in 

 nitrate reductase activity. During a sequence of 

 hot days, therefore, the activity of nitrate re- 

 ductase may be depressed to low levels resulting 

 in some degree of protein depletion. This line 

 of reasoning may warrant further study. 



Enzyme response to low temperature has been 

 reviewed recently by Mayland and Cary (23) as 



part of a broad consideration of frost and chill- 

 ing injury. They view change in water structure 

 around the protein molecule as instrumental in 

 leading to protein denaturation. They noted that 

 researchers have established temperature levels 

 for some enzymes at which denaturation begins, 

 but also that other enzymes are not inactivated 

 by freezing and thawing. Obviously the degree 

 of cold lability differs among enzymes. We might 

 seek to determine which ones are most sensitive 

 to temperature rise or decline, thus gaining an 

 insight into what processes are first disrupted. 



Temperature On Growth And Development 



Some of the more visible responses associated 

 with temperature, though reflecting change 

 within cells, have been investigated from the 

 standpoint of their effects on growth and de- 

 velopment during or following the particular 

 temperature condition. These involve rate of ad- 

 vance through the life cycle, changes from the 

 vegetative to reproductive state, and growth in- 

 terruptions. 



Advance to maturity is often hastened by heat, 

 this being especially noticeable in the annual 

 grasses. Reduced vegetative growth coupled with 

 earlier heading and maturity is frequently asso- 

 ciated with fewer and smaller heads bearing 

 smaller numbers of seeds. This is often observed 

 in the field when both high temperatures and 

 moisture deficiency prevail. Bromus rubens L., 

 when in the two-leaf stage, was stressed for 5 

 hours at air-temperature of 54.4° C. (18). For a 

 month following this stress, the treated plants 

 were shorter and were slower to initiate new 

 leaves and tillers. During subsequent growth un- 

 der favorable environment, these effects slowly 

 disappeared, but later a significant delay in date 

 of heading was observed. Delay rather than ac- 

 celeration in heading in this case was attributed 

 to the severity of the stress used and to the par- 

 ticular ecotype studied. Some ecotypes responded 

 to this magnitude of stress by earlier heading 

 (6). 



Differences among five species of forage le- 

 gumes grown to flowering under controlled tem- 

 peratures ranging from 32° C. day, and 27° 

 night, to 15° C. day and 10° night were reported 

 by Smith (31). Generally, flowering was delayed 

 at the cooler temperatures. However, birdsfoot 



