Chapter12 Effect of Chemical Environment on Seed

Introduction

Soil is the natural physical and chemical environment of most seeds. Essentially, soil is a three-phase system consisting of solids, liquids and gases in varying propor-tions. In most soils, the solids are predomi-nantly mineral, derived from rock materials. Direct chemical effects of miner-als in soil on the germination of seeds are not known. Minerals inhibit germination non-specifically when they occur in high concentrations in soils. The effects of high salinity can be either osmotic or toxic. Soil may also contain organic matter. The amount of organic matter is determined partly by the rate at which fresh plant residues are added and partly by the rate at which they are decomposed by the microflora and fauna. Primary soil particles of different sizes are mixed in various pro-portions to give recognizable textural classes of soils, e.g. sand loams, sands, etc. (Currie, 1973). The matrix formed by such a mixture has a fundamental pore size, which reflects the proportions of the ingre-dients. Thus clay soils have many small pores; sands have fewer but larger pores. Soil structure depends on the primary composition of soil, on the interaction between different soil components and, in arable land, on the cultivation methods.Therefore, structure forms a heterogeneous pattern, which may show large open crevices in dry summers but may collapse under the impact of rain or pressure. Freshly ploughed fields contain clods, which, in turn, are broken down to become the crumbs of the seedbed during subse-quent cultivation. Soil structure is impor-tant to germinating seeds because it determines the distribution and availability of water, solutes and gases. The seed–soil contact and the process of water uptake have been described in detail by Hadas (1982).

The natural chemical environment of seeds is composed of the liquid and gas phases of soil. The dissolved substances in soil that may affect germination are either inorganic or organic. Most inorganic ions do not have any specific effect on seed ger-mination (Egley and Duke, 1985). Nitrate ions and, to a certain extent, ammonium ions are the notable exceptions.

Organic substances in soil that influ-ence germination partly originate from the material in the direct vicinity of the seeds, and are often neighbouring seeds, fruits or other maternal tissues. Mostly the sub-stances are added to the soil solution by leakage or secretion from living under-ground plant organs, by leaking from leaves or by decomposition of such plant

© CAB International2000. Seeds: The Ecology of Regeneration in Plant Communities, 2nd edition293 (ed. M. Fenner)