Cc. Baskin et al., TEMPERATURE REQUIREMENTS FOR DORMANCY BREAK AND GERMINATION IN ACHENES OF THE WINTER ANNUAL LINDHEIMERA-TEXANA (ASTERACEAE), The Southwestern naturalist, 40(3), 1995, pp. 268-272
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.