The function of casing is to induce fruiting, support mushroom growth, and provide a source of water to the mushroom that compensates for its water lost through evaporation and transpiration. Sometimes, there has been an overemphasis on the influence of the moisture layer in the casing on mushroom yield and quality. A large variation in the compost substrate moisture may have more influence on mushroom yield and fresh mushroom quality. It is not to say that casing moisture does not influence yield or quality, but it may take a larger disparity in moisture to markedly influence the end product.

The mushroom relies on water potential gradients to move water through the mycelium. Water flows from regions of higher water potential to lower water potential, which helps maintain turgor pressure necessary for mycelial tip extension and growth. This same gradient also affects nutrient transport: nutrients, ions, and metabolites often move along with water via bulk flow or are actively transported across membranes, but their distribution is shaped by the underlying water potential differences within the mycelial network and between the fungus and the compost.

In a mycelial network, water potential gradients help the translocation of nutrients—moving them from the compost, the resource-rich region, to actively growing hyphal tips or fruiting bodies. Under dry conditions, the external water potential becomes too low, water movement into the hyphae slows, reducing nutrient uptake and impairing growth. Conversely, favorable water potential promotes efficient uptake and redistribution of resources throughout the mycelium.

Think of thicker rhizomorphs in the casing as the big pipes that move water and nutrients from the compost to the developing mushrooms. To keep mushrooms healthy and productive, this pipe system needs to stay in good working order all through the crop. Just like it’s easier to move water through a fire hose than a garden hose, well-developed rhizomorphs make water delivery more efficient. Keeping the casing wet or moist is what allows these larger “pipes” to form and continue feeding the mushrooms. If the casing dries out during production, most strains will give lower yields and poorer quality mushrooms. In the end, this whole water network relies on the compost as the main source of water for the crop.

Water is constantly moving during the cropping cycle. Mushrooms take up water into their cells, while water is also lost through evaporation and transpiration. Growers replace this loss mainly by watering the casing layer. However, we still know little about exactly how mushroom mycelium absorbs and transports water, or how water moves through rhizomorphs into the mushroom itself.

Some research suggests that water uptake depends on differences in water potential between the mycelium and the compost solution (Kalberer, 1987). As cropping progresses, mushrooms use water from both the compost and casing, while evaporation, transpiration, and respiration continually reduce the water content of the crop. This loss lowers the water potential inside mushroom cells. Because water moves from higher to lower potential, this gradient may allow mushrooms to absorb water with little energy cost. Another idea is that nutrient absorption helps drive water uptake (Holtz, 1971; 1979; Schroeder & Schisler, 1981; Kalberer, 1987). When nutrients are actively absorbed by the mycelium, the osmotic potential inside the cells decreases. Water then follows passively, moving in response to the concentration of nutrients. The developing mushrooms produce the sugar mannitol and therefore have a much higher concentration than the vegetative mycelium (Holtz, 1976). It was suggested that this different concentration of mannitol creates the osmotic and water potential gradient responsible for “pumping” nutrients and water from the compost mycelium through the casing rhizomorphs and into the developing mushrooms.

In summary, mushroom fruit body development depends on a balance of water movement between the compost, casing, and the developing fruit mushrooms. While casing moisture is important for yield and quality, compost moisture may play an even greater role. Water and nutrients move through the mycelium by water potential gradients, flowing from wetter regions in the compost toward drier hyphal tips and mushrooms. Rhizomorphs act like large pipes, efficiently transporting water and nutrients when casing moisture is maintained; if the casing dries, yields and quality decline. Water uptake is largely passive, driven by gradients in water and osmotic potential, which are influenced by nutrient absorption and the accumulation of compounds like mannitol in mushrooms. Together, these processes form a dynamic water transport system that sustains mushroom development throughout cropping.


REFERENCES
Holtz, R.B. 1971. Qualitative and quantitative analysis of free neutral carbohydrates in mushroom tissue by gas-liquid chromatography and mass spectrometry. J. Agr. Food Chem. 19 (6):1272-1273.
Holtz, R.B. and Smith, D.E. 1979. Lipid metabolism of mushroom mycelia. Mushroom Sci. 10 (Part 1):437-444.
Kalberer, P.P., 1987. Water potentials of casing and substrate and osmotic potentials of fruit bodies of Agaricus bisporus. Sci. Hort. 32:175-182.
Schroeder, G.M. and Schisler, L.C. 1981. Influence of compost and casing moisture on size, yield, and dry weight of mushrooms. Mushroom Sci. 11:495-509.