Physiological mechanisms by which gypsum increases the growth and yield of Lentinula edodes

Lentinula edodes is one of the most important commercially cultivated edible mushrooms. It is well known that gypsum (CaSO₄·2H₂O) supplementation in sawdust medium increases the yield of L. edodes, while the physiological mechanisms remain unclear. Our previous study showed that the acidification of the medium to pH 3.5-4.0 was essential for the growth of L. edodes. In this study, it was found that the oxalic acid excreted by L. edodes was responsible for the acidification of the medium. The biosynthesis of oxalic acid was regulated by the ambient pH and buffer capacity of the medium. To acidify the sawdust medium, the concentrations of total and soluble oxalate were 51.1 mmol/kg and 10.8 mmol/kg, respectively. However, when the concentration of soluble oxalate was 8.0 mmol/kg, the mycelial growth rate decreased by 29% compared with the control. Soluble oxalate was toxic to L. edodes, while soluble sulfate was nontoxic. CaSO₄ reacted with soluble oxalate to form nontoxic insoluble CaC₂O₄ and the strong acid H₂SO₄. When the CaSO₄ supplemented in sawdust medium was more than 25 mmol/kg, the soluble oxalate decreased to less than 1 mmol/kg, and the mycelial growth rate increased by 32% compared with the control. In conclusion, gypsum improved the growth and yield by relieving the toxicity of oxalate and facilitating the acidification of sawdust medium. KEY POINTS: • L. edodes excretes oxalic acid to acidify the ambient environment for growth. • Soluble oxalate is toxic to L. edodes. • Gypsum increases growth by reacting with oxalate to relieve its toxicity.