PHOTOPERIOD IN 3 XANTHIUM POPULATIONS FROM TROPIC OF CANCER IN MEXICO

Diverse photoperiodic responses were shown by three populations of Xanthium strumarium L. originating between 22° and 25°N on the western coast near Culiacán, Sinaloa; in the Chihuahuan Desert near Matehuala, San Luis Potośi; 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).