Threshold temperatures for seed germination in nine species of Verbascum (Scrophulariaceae)

Light and fluctuating temperatures are two important factorstriggeringseed germination.The aim of thiswork was to: (1) elucidate the effect of temperature, light regime and storage time on seed germination in nine taxaofVerbascum spp.,collectedfromdifferenthabitats in the Mediterranean area; and (2) estimate threshold temperatures for seed germination. For all taxa, germination assays were performed at constant and fluctuating temperatures, both in continuous darkness (D) and in alternating light/dark (L/D; 12h photoperiod). Final germinated proportions (FGPs), base (Tb), optimal (To) and cut-off (Tc) temperatures were derived. At constant temperatures, seed germination was strongly suppressed under the D regime. In L/D, the effect of storage time was very small and the highest FGPs were observedfrom15to308C(40–100%),dependingonthe species. Tb ranged from below 7 to above 108C and it appeared to be constant within each seed lot. To and Tc showed some within-lot variability and were higher for fast-germinating seeds in each lot. Considering fluctuatingtemperatures,germinationappearedtobequicker and more complete than at constant temperatures. The germination of Verbascum spp. is favoured in L/D and fluctuating temperatures, which explains their pioneer character when positioned near the soil surface and under a low vegetation canopy. V. arcturus and V. pinnatifidum were shown to be less favoured by L and fluctuating temperatures, which might explain their ability to germinate in rocky areas or sandy dunes, even when they are not directly exposed to the light.