In last year's News Letter (MGCNL 55:2-3, 1981) I presented a rather lengthy account of the striping phenomenon observed in Mu lines on some occasions. This phenomenon was found only rarely in plants grown in Iowa, Hawaii and Davis, California but was observed frequently in Mike Freeling's nursery in Berkeley, California. In last year's report, I suggested that the Berkeley climatic conditions might account for the striping. On returning to Iowa State University last spring, I looked up the average minimum temperature and average maximum temperature for Berkeley during the growing period. These turned out to be about 55 F and 70 F respectively.
In an attempt to duplicate these conditions, I grew both mutator and standard lines in a growth chamber on a 12-hour dark 12-hour light cycle with a 55 F dark temperature and a 70 F light temperature. These conditions, of course, fall considerably short of duplicating Berkeley's growing conditions. For one thing the light-70 F period should have been considerably longer and the dark-55 F period shorter. In Berkeley, of course, the temperature does not drop down to a constant 55 F as soon as the sun sets or go up to a constant 70 F as soon as the sun rises. Also, it is not possible to program in the periods of time when the temperatures in Berkeley fail to reach or exceed both the average maximum and minimum temperatures. All things considered, I would guess the growing conditions in the growth chamber are more adverse than those experienced in Berkeley. Plants grow very slowly under these conditions and frequently are observed to have pale yellow leaf tips.
We did indeed get striping in mutator plants under these conditions. However, the same striping pattern is found in standard lines as well. Several different plant virologists, shown these plants (both Mu and standard), have indicated that the striping is similar to that found in plants infected with viruses.
The cool nighttime temperatures seem to be necessary for the striping response. Plants grown at 70 F night and day do not show this striping.
Donald S. Robertson
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