Crosses (Fig. 45) using tetraploid var idahoense as the seed parent and var montanum (diploid) as the staminate parent were successful 25-100% of the time with low pollen stainability of 14-64%. The reverse of this cross was successful 0-100% of the time, with pollen stainability low (0-51%). Crosses using tetraploid var idahoense as the staminate parent and tetraploid var montanum as the seed parent were successful 100% of the time and pollen stainability was high (82-98%).
Using diploid var idahoense as the seed parent and var montanum (diploid) as the staminate parent, crosses (Fig. 46) were successful 50-100% of the time with relatively high pollen stainability (53-100%). The reverse of this cross was successful 0-100% of the time and all had relatively high pollen stainabihty (46-100%). The cross of tetraploid var montanum with diploid var idahoense was successful 14-33% of the time.
A cross between var idahoense (tetraploid) and var idahoense (diploid) was successful 56-100% of the time, but pollen stainability was low (35-61%).
Hybrids from the above crosses (with the exception of some from tetraploid-diploid crosses) were vegetatively vigorous. Basal leaves (Fig. 47) appeared to be the main feature noticeably intermediate in the F1's. Thlaspi montanum var idahoense
and var montanum undoubtedly interbreed where their ranges overlap in Idaho. As expected from greenhouse studies, herbarium specimens indicate the intermediate expression of the basal leaves at the outer limits of the range of var idahoense.