**1. Introduction**

Constructed green wetlands with horizontal surface for wastewater treatment are getting social recognition. They usually have plants as an essential component of the design. Many coun‐ tries have published innovative experiences with this technology: the USA, Canada, the UK,

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Germany, France, the Netherlands, Switzerland, Norway, Poland, Slovenia, Lithuania, Italy, Spain, Portugal, Australia, Japan, China, India, Taiwan, South Africa, Turkey, Kenya, Uganda and México [1, 2]. A great variety of wastewaters from industries have been treated: chemical, petrochemical, textile, pulp and paper, tannery, abattoir, food processing, distillery and winery factories. Effluents from pig farms, fish farms, shrimp culture, cobalt recovery, mining or coke plants have also been managed this way. Run-offs from airports, highways, hospitals, agricul‐ tural activities or storm water have been tested as well. Finally, landfill leachate or polluted rivers have been experimentally decontaminated using constructed wetlands with vascular plants as the principal component [1, 3]. Uptake of rare earth elements (REEs) from obsolete equipment of modern technologies has also been reported [4].

Different species have been used to date, all over the world. They belong to different genera and families. Frequently they are helophytes, which means plants growing in marsh partly submerged in water, so that they regrow from buds below the water surface. The helophytic plants used to build wastewater green filters in constructed wetlands have in common with these significant characteristics: (a) a rich below-ground organ, root or rhizome, as to pro‐ vide substrate for attached bacteria and oxygenation (as much as possible) of areas adjacent to the radicular apparatus; (b) a high-tolerance nutrient and organic loadings; and (c) the production of a high amount of above-ground biomass for winter insulation in cold and temperate regions, as well as nutrient removal via harvesting [1].

From a botanical point of view, a great list of species could be used in each country, because plant biodiversity is high. Ad hoc species for each biogeographical region could be found, attending to the above-mentioned characteristics. However, in practical terms, a limited number of species have been tested by the industry. Vymazal [1] summarised the most important, and we have taken its publication as a framework. Later references [4–12] have also been consulted, and **Table 1** shows the helophytes that we have considered most relevant.

For the production and multiplication of plants in nurseries to the global market, it is necessary to standardise the operations aimed at getting living plants that can be installed in the constructed sewages. Plant reproduction can be made by vegetative way (cuttings) or by seeds (sexual way). Nowadays international trade is essential, so those systems better adapted to international transport and management are more competitive. For this reason seed multiplication systems are very interesting. Seeds are more resistant and better prepared than cuttings, to adverse environmental conditions (light, humidity, temperature), that can occur during international transport. Thus, seed technology development provides interesting tools to produce in nurseries good quality plants. It is a sustainable new business.

Various authors have highlighted that studies on seed germination of aquatic and lacustrine plants are very few [13]. This topic is a much less studied subject that other aspects of seed biology, physiology or ecology. However it has a great importance from an applied point of view. That is why we have made the following bibliographic review. It will let us summa‐ rise and analyse the state of the art of this interesting topic.
