Enhancement of biostimulant activity of spent Pleurotus substrate for seedling production

Spent mushroom substrate is considered as one of the ingredients of growing media which could be used partially as peat substitution in the near future. The aim of this research was to enhance the plant biostimulant activity of a spent Pleurotus substrate (SPS) coming from mushroom cultivation on wheat straw, and containing 39.5±1.3% cellulose, 14.7±0.5% hemicellulose, 19.6±1.1% lignin, 7.8±0,3% chitin and 18.4±0.6% water extractible. The dried SPS was treated with a suspension of spores of Brevibacillus parabrevis B50 (NCAIM B 001413), reaching 10⁷ cfu spores g^-1 dried substrate. The B. parabrevis bio-enhanced SPS was pelletized into a pellet mill. Rate of survival of embedded plant beneficial bacterial spores where higher than 80%. The release of soluble silicon and polyamines from the bio-enhanced and pelletized SPS was tested, using pelletized SPS (without bacterial spores) as control. Treatment with bacterial spores significantly increased the release of soluble silicon and polyamines, compounds with a plant biostimulant activity. Also, bio-enhanced and pelletized spent mushroom substrate was tested on germination and seedlings growth of two vegetables, tomato (Solanum lycopersicum L. 'Menhir F₁') and cucumber (Cucumis sativus L. 'Karolina F₁'), in greenhouse conditions. The experiments were done in completely randomized design. Both vegetable seedlings were grown in 6 different media, obtained from bio-enhanced and pelletized SPS, mixed in various ratios (2:1; 1:1; 1:2; 1:3; 1:4, v:v) with peat. There were two controls: M1, peat, and M2, obtained from pelletized SPS (not bio-enhanced), mixed with peat (1:2, v:v). Controls showed similar growth and development pattern for both vegetables. Compared to controls, significant increase in plant height, stem diameter, fresh and dry weights were found in treatments with maximum 25% content of bio-enhanced pelletized SPS, in both vegetables.