Factors Controlling Germination of Some Dune Annuals

Abstract

Most dune annuals are characterized by autumn germination and spring flowering and consequently are regarded as 'winter annuals' (Salisbury 1952). In a previous paper (Pemadasa & Lovell 1974a) the factors controlling the timing of flowering of a group of annuals in the dune system at Aberffraw, Anglesey, were discussed. This paper is concerned with the factors determining the precise date of germination of these species. The literature suggests that both innate and enforced dormancy mechanisms (Harper 1957) are involved in controlling the timing of germination of winter annuals, and four plausible explanations can be suggested for their germination in autumn but not in spring or summer. (a) The germination capacity of some species is very, or relatively, low owing to innate dormancy. However, by autumn owing to after-ripening (Baskin & Baskin 1971a, b, 1972; Newman 1963; Ratcliffe 1961) and/or softening of the hard seed coat (Williams & Elliott 1960; Quinlivan 1961), which overcome the innate dormancy, rapid germination is possible. (b) Germination of fresh seeds of some species is favoured by temperatures below those prevailing in the field in spring and summer (Hulbert 1955; Newman 1963), but as the seed ages the temperature range for germination is widened thus enabling germination in autumn. (c) The temperatures in spring and summer are above the maximum or optimum for germination; thus seed is subjected to enforced dormancy. The lower temperatures in autumn allow germination (Juhren, Went & Phillips 1956; Went 1948, 1949; Went & Westagaard 1949), although there is no 'change in temperature response' (Newman 1963) with seed ageing. (d) The soil moisture in summer is below the minimum for germination; thus seeds are forced to remain dormant. The increased precipitation in autumn increases the soil moisture regime to a favourable level allowing germination. Although the generalizations made above may be applicable to a wide variety of winter annuals, it must be emphasized that the germination requirements differ widely between species. This paper concentrates mainly on dormancy, temperature regime and soit moisture regime. Other factors, including light and darkness, are given only brief mention. The species examined were Aira caryophylleat, A. praecox, Cerastium atrovirens, Erophila verna, Mibora minima, Saxifraga tridactylites and Vulpia membranacea. A detailed account of the distribution of these species in the dune system at Aberffraw has * Present address: Department of Botany, University of Sri Lanka, Peradeniya, Sri Lanka. t Nomenclature follows Clapham, Tutin & Warburg (1962).