Robert D. Berghage

TREE-FUNGUS INTERACTIONS IN ECTOMYCORRHIZAL SYMBIOSIS

The mycorrhiza is probably the most common symbiosis in which higher plants engage. More than 80% of higher terrestrial plants may be mycorrhizal.1 This may make the mycorrhiza the most common symbiosis in all of nature! Mycorrhizal fungi, those fungi that participate in the mycorrhizal symbiosis, are important and sometimes essential to their hosts, primarily because they can provide an avenue for nutrient (mostly N and P) acquisition in addition to the roots.

Plastics in Greenhouse Production

The use of plastics has revolutionized the greenhouse industry. Plastic is used for everything from the exterior greenhouse coverings, or claddings, to structural components, to nearly all aspects of the interior production systems, including pots, trays, irrigation systems and almost everything else. It is safe to say that without the use of plastics the greenhouse industry would be unrecognizable. Prior to the introduction of plastic covered greenhouses after WWII, greenhouses were covered with relatively small glass panes. These were heavy and required a lot of supporting sash bars which were generally wood, and that support required a lot of maintenance in the form of painting and reduced light transmission into the greenhouse. These glasshouses were expensive and time consuming to build. Plastic coverings, particularly polyethylene films changed all that. Suddenly you could construct an inexpensive frame to support large sheets of inexpensive plastic covering. This led to a rapid expansion of the use of greenhouses around the world to produce all sorts of crops (Wittwer, 1993). These greenhouses could have larger open spaces inside with fewer or in some cases no supporting columns facilitating better space utilization and greatly improved productivity. The use of plastic sheeting or film as a greenhouse covering probably had more impact on the greenhouse industry as a whole than any other single application of plastics (Jacobson, 2011). But this was not the only revolutionary change in greenhouse production that can be attributed to the use of plastic. Another game changing application was the use of plastics in pots. By the 1970s and 1980s plastic pots had almost completely replaced the clay pots that were used previously. Clay pots were heavy and fragile which greatly limited growers ability to ship and market flowering potted plants, potted nursery plants, and bedding plants. Plastic pots were relatively inexpensive, rugged, and perhaps most importantly light weight. The use of plastic containers caused another huge change in greenhouse production in the 1980s. The development of plastic plug trays with small individual cells combined with mechanical seeders capable of placing one or more seeds with precision in each of those individual cells revolutionized propagation practices in the greenhouse. Crop time was greatly reduced by eliminating transplant shock, and the whole process became much more efficient, with large-scale specialty propagators, or plug producers growing the plug seedlings while other growers focused on finishing the plants resulting in great economies of scale and labor requirements (Styer and Koranski, 1997). Another area where plastics completely changed a segment of the greenhouse industry is in their use in hydroponics. Modern hydroponic production systems depend on the use of plastics. Nutrient Film Technique, a common hydroponic production system for leafy greens and herbs, would not be possible without extruded plastic troughs and trays (Resh, 2013).