Botanical fractions of rice straw colonized by white-rot fungi: changes in chemical composition and structure

Three species of white-rot fungi (Cyathus stercoreus (Cs) ATCC-36910, Phanerochaete chrysosporium (Pc) BKM, and Pleurotus sajor-caju (Ps) 537) were grown on leaf blade (leaf) or stem plus leaf sheath (stem) of rice straw for 30 d by solid state fermentation (SSF). Physical and chemical methods were employed to evaluate substrate specificity, substrate composition and histology. Changes in histology of decayed material were evaluated before and after ruminal digestion by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Control leaf and stem were similar in IVDMD (38%), although leaf was higher in crude protein and lower in cell wall compared to stem (3.7 vs. 2.8%; 73.9 vs. 80.7%, respectively). The changes were due mostly to a higher concentration of silica in leaf compared to stem (17.0 vs. 13.1%). After 30 d of SSF, Cs and Ps increased the IVDMD of leaf from 38.1 to 49 and 46.3%, respectively, by selective degradation of hemicellulose as opposed to cellulose. In contrast, Pc degraded cellulose and hemicellulose indiscriminately in leaf and lowered the IVDMD of leaf to 30.1%. Partially degraded lignin, silica and hemicellulose of leaf were negatively correlated (r) with IVDMD in contrast to cellulose (r = −0.49, −0.54, −0.16 and 0.85, respectively). Prediction of IVDMD of fungal-decayed leaf was primarily a function of hemicellulose and cellulose with a coefficient of IVDMD = −0.155 + 2.14 (cellulose) −0.87 (hemicellulose); R2 = 0.98. Stem decayed by Pc and Cs became less digestible compared to the control (18.5 and 20.3% vs. 39.7%, respectively), although hemicellulose and cellulose of stem were poorly degraded after SSF. Only Ps improved the IVDMD of stem compared to the control (44.1 vs. 39.7%). SEM sections of leaf decayed by Pc showed complete degradation of mesophyll but the more recalcitrant vascular and epidermal tissues resisted rumen degradation and resulted in lower IVDMD. Leaf tissues colonized by Cs and Ps showed presence of all tissues but after 72 h rumen microorganisms completely degraded mesophyll tissue which resulted in a higher IVDMD. Observation of TEM sections showed that fungal treatment facilitated rumen microbial penetration of lignified tissues. Improvement of digestibility of decayed straw depends upon the fungal species, the plant substrates and the botanical fractions.