I have been using zb4 to test linkage with other loci (hcf3, hcf6) on chromosome arm 1S. The penetrance of the trait is variable. I find that it depends on night temperatures. In progeny that should be segregating 1/4 zb4 zb4, the highest frequency I have observed is 0.15. Often a second planting of an ear that expressed zb4 did not display the zb4 phenotype, a yellow green/green banding pattern. Expression tended to be strongest in greenhouse plantings during early fall, when heat had not been turned on in the greenhouse and nights were cool. To test the temperature dependence, I grew flats of material segregating zb4 zb4 in 85F chambers and on germination, transferred some to a 55F refrigerator overnight (14 hours) with return to lights and 85F during the day. Material which remained in growth chamber (70F during 8hr dark) did not express zb4, whereas the night-refrigerated seedlings developed fine, yellow-green horizontal bands on leaves that were expanding at the time of cold treatment. The maximum number of bands per leaf equalled the number of nights (3) spent in the refrigerator. I have since been able to obtain expression of zb4 using 16 hours light at temperatures of 84-86F and 8 hours darkness at 64F. Generally expression was not observed in seedlings until the fourth leaf had begun to emerge.
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