How is Nicotiana exhibit self-incompatibility? Explain its mechanism.

Explanation

Self - sterility means that the pollen from a plant is unable to germinate on its own stigma and will not be able to bring about fertilization in the ovules of the same plant. East (1925) . Observed multiple alleles in Nicotiana which are responsible for self - incompatibility or self - sterility The gene for self - incompatibility can be designated as S, which has allelic series `S_1,S_2,S_3,S_4 and S_5`.
The cross - fertilizing tobacco plants were not always homozygous as `S_1S1" or "S_2S_2`, but all plants were heterozygous as `S_1S_2,S_3S_4 and S_5S_6` . when crosses were made between different `S_1S_2` plants the pollen tube did not develop normally. But effective pollen tube development was observed when crossing was made with other than `S_S_2` form example `S_3S_4.`
When crosses were made between seed parent with `S_1S_2` and polllen parents with `S_2S_3` , two kinds of pollen tubes were distinguished. Pollen tubes were distinguished pollen grain carrying `S_2` were not effective, but the the pollen grains carrying `S_3` were capable of fertilization Thus, from the cross `S_1S_2 X S_3S_4`, all the pollens were effective and four kinds of progeny resulted : `S_1S_3,S_1S_4,S_2S_3 and S_2S_4`.