Experimental hybridization in Xanthium strumarium of American complexes with diverse photoperiodic adaptation

Plants with different critical periods of darkness for flowering were hybridized for a comparison of photoperiodic adaptation. The reciprocal F1 progenies showed a photoperiodic requirement which was intermediate between those of the parental types. The leaf and bur morphologies and the sesquiterpene lactones of the hybrids also showed intermediacy. The American morphological complexes involved in the artificial crosses were italicum, chinense, cavanillesii, and pennsylvanicum and included indigenous populations as well as those introduced to Australia. The plants that were hybridized were from lower latitudes and had apparent critical nights of 9.25–9.5, 9.75, 10, 10.25, and 10.5 h.