EXPERIMENTAL HYBRIDIZATION OF XANTHIUM STRUMARIUM (COMPOSITAE) FROM ASIA AND AMERICA. I. RESPONSES OF F HYBRIDS TO PHOTOPERIOD AND TEMPERATURE.

Responses to photoperiod and temperature were compared for hybrids between Asiatic plants in the indigenous strumarium morphological complex and plants in five American complexes: oviforme, italicum, chinense, cavanillesii, and pennsylvancium. The F hybrids between Hong Kong plants and various American plants showed intermediacy in photoperiodic response; however, the hybrid night requirement was more similar to that of the American parent. The Hong Kong plants are difficult to evaluate photoperiodically but showed a night length requirement of 9.25-9.50 hr. Day-neutral plants from India in the strumarium complex produced day-neutral F hybrids in crosses with most American plants having night requirements less than 10 hr. The F hybrids involving the day-neutral Indian plants and either Indian or Australian chinense plants showed a night requirement of 8.25-8.75 hr. The chinense parental plants had apparent critical nights of 10.25-10.50 hr. Crosses between the day-neutral Indian plants and Mexican plants with apparent critical nights of 10.75-11 hr produced F hybrids requiring nights of 8.75-9.00 hr. The various hybrids tended to show the broader temperature tolerances of the American parents. The ripeness-to-flower (maturity) responses of seedlings tended to show the genetic controls of the parent with the faster developmental rate. The hybridization evidence suggests that photoperiodic responses are quantitatively controlled and inherited independently of morphology and ripeness-to-flower responses. The populations of India are highly diverse and probably reflect recombinations of photoperiodic and temperature responses between indigenous day-neutral plants and photoperiodic chinense plants introduced from North America.