Photoperiod in three xanthium populations from the tropic of cancer in Mexico.

Diverse photoperiodic responses were shown by three populations of Xanthium strumarium L. originating between 22 degrees and 25 degrees N on the western coast near Culiacán, Sinaloa; in the Chihuahuan Desert near Matehuala, San Luis Potoś; and on the Gulf Coast near Ciudad Mante, Tamaulipas, respectively. A combination of differences in critical night length and in ripeness-to-flower response (maturity) appears to be the basis for reproductive adaptation of these populations to different climatic regimes that prevail at the same latitude (and photoperiodic regime).