Interactions of temperature and dormancy-relieving compounds on the germination of weed seeds

Dormant weed seeds were germinated at constant and alternating temperatures using five chemicals. Germination of Chenopodium album when expressed in normal equivalent deviates, increased linearly with the logarithm of chemical dose up to an optimal concentration. Ethephon (2-chloroethyl phosphonic acid) and sodium azide slightly stimulated germination of C. album seeds at 20 degrees C while potassium nitrate, thiourea and azide each interacted positively with alternating temperatures (5 degrees/25 degrees C). Relief of Avena fatua dormancy by azide exhibited a positive interaction with cool constant temperatures (3-10 degrees C), and chemicals had smaller or no effects at higher and alternating temperatures. No chemical mixture, including that of nitrate, thiourea, ethephon, azide and hydrogen peroxide, promoted more germination than azide on its own in A. fatua or nitrate together with ethephon in C.album. In C. album, this result arose largely because nitrate and azide antagonized each other in the presence of ethephon. Peroxide and thiourea together relieved this antagonism so that the fourth order interaction was positive. The negative interaction of nitrate, azide and ethephon in C. album was avoided in a sequential treatment with nitrate plus ethephon for seven days followed by sodium azide. Relief of dormancy by nitrate and ethephon was optimized at 67mM nitrate with 1.4 mM ethephon for C. album at 20 degrees C and 20 mM nitrate with 1.4mM ethephon for A. fatua at 15 degrees C. None of the chemicals used in this investigation stimulated the germination of Rumex crispus seeds. The results are discussed with respect to chemical control of soil seed banks.