The aim of this study was to evaluate the effect of constant temperatures on the seed germination of Tridax procumbens, a weed of increasing importance in Brazil. Germination was carried out with 4 replicates of 100 seeds (achenes) from 3 different samples. Seeds were placed between two layers of blotting-paper, soaked in distilled water inside closed transparent plastic boxes incubated for 12 hours of light per day, at temperatures of 15, 20, 25, 30, 35 and 40 ºC. In general, total germination was higher at 25, 30 and 35 ºC, with highest germination velocity index at 30 ºC, and the best distribution of germination over time at 35 ºC. The results from the seed sample with the best germination performance were 94.5% at 25 ºC, 93.4% at 30 ºC and 92% at 35 ºC, with germination velocity index of 14.28, 17.43 and 12, respectively. The overall germination at other temperatures tested was very low, i.e., 1.3% at 15 ºC, 5.4% at 20 ºC and 3.4% at 40 ºC, as well as in an additional treatment, under alternating temperature at room temperature (temperatures varying from 20 to 28 ºC). Seeds incubated at 15 and 20 ºC achieved full germination after being transferred to 30 ºC, while those incubated at 40 ºC lost 50 to 80% of their viability. Although in small proportions, some seeds germinated up to 80 days of imbibition. Primary root protrusion initiated after 2 to 4 days of imbibition at 25, 30 and 35 ºC, even when seeds had been recently shed from the mother plant.
weed; medicinal plant; germination ecology
