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 potentials of 0 to -1.33 MPa and at constant temperatures from 5 to 29 degrees C. Effects 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 at 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 degrees (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 decrease in water potential. Final germination could be accounted for by seed-to-seed variation in the population assuming that each seed had a minimum temperature for germination and a maximum temperature above which it would not germinate. Seed-to-seed variation in these characteristics was assumed to be normally distributed, and it was further assumed that the two characteristics were independent. Effects of water potential on these temperature requirements were quantified, and the resulting empirical model accounted for final germination with reasonable accuracy (R(2) = 0.96).