Role of temperature in regulating the timing of germination in Portulaca oleracea

The peak of germination of autumn-sown seeds of Portulaca oleracea was between 21 May and 21 June, when mean daily maximum and minimum temperatures were 30.7 and 17.4^oC, respectively. Fresh seeds collected in October germinated to 13 and 94% at 30:15 and 35:20^oC thermoperiods, respectively, in light, and to 0% in darkness. Seeds were buried in October 1975 and exhumed in December 1975 through September 1976. In light, exhumed seeds germinated to 69-100% at 30:15 and 35:20^oC, to 1-80% at 20:10^oC, and to 0-52% at 15:6^oC; in darkness they germinated to 5-55% at 30:15 and 35:20^oC and to 0% at 20:10 and 15:6^oC. Germination at 20:10^oC did not exceed 50% until mid-April, and it did not exceed 50% at 15:6^oC until June. The minimum temperature at which ≥50% of fresh seeds germinated in light decreased with an increase in afterripening temperature. The high temperature requirement for complete afterripening and for germination of partially afterripened seeds prevents germination of this summer annual in temperate regions until late spring and early summer. -from Authors