TEMPERATURE REQUIREMENTS FOR DORMANCY BREAK AND GERMINATION IN ACHENES OF THE WINTER ANNUAL LINDHEIMERA-TEXANA (ASTERACEAE)

Achenes of the winter annual Lindheimera texana (Texas star) were dorm ant at maturity in mid-May, and they came out of dormancy when subject ed to alternate wet-dry or to continuously dry conditions at simulated habitat temperatures during summer. As achenes first began to come ou t of dormancy in July, they germinated to 1% to 5% in light only at 12 /12 h daily thermoperiods of 15/6 degrees C, 20/10 degrees C and 25/15 degrees C. In October, achenes germinated to 19 to 81% at 15/6 degree s C, 20/10 degrees C and 25/15 degrees C and to 1 to 8% at 30/15 degre es C and 35/20 degrees C. Achenes given wee (1 day)-dry (14 days) trea tments twice each month during summer germinated to 86% at simulated h abitat temperatures (28/15 degrees C) in October, while those kept con tinuously dry all summer germinated to only 15%. Since achenes came ou t of dormancy during summer, and optimum temperatures for germination were 20/10 degrees C, 25/15 degrees C and 28/15 degrees C, achenes can germinate in the field as soon as soil moisture becomes nonlimiting i n October and November. After 16 weeks of burial in moist soil at 15/6 degrees C and 20/10 degrees C, achenes were still dormant; however, 7 5% to 94% of those buried at 5 degrees C, 25/15 degrees C and 30/15 de grees C were nondormant. Thus, achenes have the ability to come out of dormancy during summer or winter, and the species appears to have the potential to behave as a nonstrict winter annual. However, summer dro ught seems to restrict germination to late autumn and early winter, th us L. texana behaves as a strict winter annual.