Interspecific Hybridization in <i>Trifolium</i> L., Sect. <i>Trifolium</i> Zoh. I. Diploid Hybrids among T. <i>alpestre</i> L., T. <i>rubens</i> L., T. <i>heldreichianum</i> Hausskn., and T. <i>noricum</i> Wulf.

Interspecific hybrids of Trifolium alpestre L. ✕ T. heldreichianum Hausskn. and reciprocal, T. alpestre ✕ T. rubens L. and T. rubens ✕ T. noricum Wulf. were produced by hand pollination in a greenhouse. Only the T. alpestre ✕ T. heldreichianum and T. alpestre ✕ T. rubens hybrids flowered. Hybridization barriers included apparent embryo abortion in the T. rubens ✕ T. alpestre and T. rubens ✕ T. heldreichianum cross, total F 1 seedling inviability in the T. heldreichianum ✕ T. alpestre and T. rubens ✕ T. noricum cross and poor F 2 seed germination and survival in the T. alpestre ✕ T. heldreichianum and T. alpestre ✕ T. rubens hybrids. Hybrids of T. alpestre ✕ T. heldreichianum and T. alpestre ✕ T. rubens had 16 somatic chromosomes and a mean of 7.95 and 7.77 bivalents per Pollen Mother Cell (PMC) respectively. No cells with multivalents were observed and frequency of laggards at anaphase I and micronuclei at the quartet stage was less than 0.20 per PMC. Pollen stainability of T. alpestre ✕ T. heldreichianum and T. alpestre ✕ T. rubens was 53 and 51%, respectively. Self compatibility was dominant to self incompatibility, and expressions of rhizomes and leaf mark were dominant to absences of rhizomes and leaf mark in the T. alpestre ✕ T. heldreichianum hybrid. Presence of rhizomes was also dominant in the T. alpestre ✕ T. rubens hybrid. Second‐generation hybrids were produced by selfing; intercrossing and backcrossing to their parents. No hybrids were produced by crossing either of the interspecific hybrids with red clover, T. pratense L.