The mentor pollen phenomenon in black cottonwood

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Abstract
Black cottonwood (Populus trichocarpa T. & G. ex Hook.) is in the Tacamahaca section of poplars and cannot normally be crossed with members of the Leuce section. Earlier studies had shown that such crosses succeed if black cottonwood is pollinated with a mixture of pollen, combining foreign with irradiated pollen of the maternal species. This ‘mentor’ pollen phenomenon was further studied during 1970–1975. Black cottonwood pollen, irradiated with a 60Co source at 0.4, 1, 10, and 100 kR, was indistinguishable from normal pollen in germination and tube growth in situ and promoted fruit development on cut branches but failed to set seed at the highest dosage. Pollen from P. alba var. bolleana and P. tremuloides (both Leuce) showed normal germination and stigma penetration but delayed or slowed tube growth and led to premature catkin abscission. Mixes of 20-, 40-, and 80-kR-irradiated mentor pollen mixed with foreign pollen resulted in good fruit development but little seed set, probably because the mentor pollen successfully competed for available ovules without producing embryos. Hybrid seedlings were weak and susceptible to early mortality. Incompatibility in these crosses is manifest inside the stigmatic and stylar tissue and resembles gametophytic self-incompatibility. The irradiated mentor pollen in black cottonwood seems to facilitate hybridization indirectly through the promotion of fruit development rather than as a provider of recognition substances to the associated foreign pollen, as described for other poplars.