The effect of water stress on the temperature range for germination of Orobanche aegyptiaca seeds

Non-dormant seeds of Orobanche aegyptiaca were incubated at water potential s of 0 to -1.33 MPa and at constant temperatures from 5 to 29 degrees C. Ef fects of water potential and temperature on final germination were modelled . In general, germination increased with increased temperature from 5 to 20 degrees C and decreased above 26 degrees C. Maximum germination occurred a t 20-26 degrees C and 0 MPa. Germination was reduced as the water potential decreased. Water potential also affected the temperature range over which high germination was observed; at 0 MPa high germination occurred over 9 de grees (17-26 degrees C) compared with 3 degrees at -1.25 MPa (17-20 degrees C). The optimum germination temperature also tended to decrease with a dec rease in water potential. Final germination could be accounted for by seed- to-seed variation in the population assuming that each seed had a minimum t emperature for germination and a maximum temperature above which it would n ot germinate. Seed-to-seed variation in these characteristics was assumed t o be normally distributed, and it was further assumed that the two characte ristics were independent. Effects of water potential on these temperature r equirements were quantified, and the resulting empirical model accounted fo r final germination with reasonable accuracy (R-2 = 0.96).