Principal factors in the germination of Heracleum sphondylium L: importance of oxygenation

Abstract

Heracleum sphondylium is an umbellifer found on uncultivated soil which cannot tolerate intensive cultivation. This sensitivity may be explained not only by the biology of the hemicryptophyte, but also by its germination characteristics: the seedlings emerge nearly simultaneously at the beginning of spring, near the surface as well as from deeper levels, in such a way that no live seeds can persist if the soil has been disturbed. The regularity of germination at the end of winter is the consequence of dormancy breaking by cold and moisture, necessary conditions for the reduction of tegumentary inhibition, but more especially for the maturation of the embryo. Thus dormancy breaking as well as embryo swelling do not take place unless the temperature is below 10 °C, This thermal requirement may be explained simply by the need for a relatively high level of dissolved oxygen around the seed, which may be satisfied at a temperature at which the diffusion of this gas is facilitated, making it more soluble and limiting its fixation reactions. The seed envelopes (seed coats and endosperm) thus play a complex role in regulating the availability quantity of oxygen and nutritive elements. After a phase of hypoxia preventing germination in the off-season, there is an ideal flow rate for the development of the embryo by the end of winter, ie is when the surrounding atmosphere reaches a partial pressure of 10 to 15% oxygen. © 1992.