Abstract
In the present study, Acacia saligna (Labill.) H.L.Wendl. wood blocks with dimensions of 0.5 × 1 × 2 cm were inoculated with five molds (Aspergillus niger, A. flavus, Alternaria tenuissima, Fusarium culmorum, and Trichoderma harzianum) and the changes in the organic acids (oxalic, citric, tartaric, succinic, glutaric, acetic, propionic, and butyric) of powdered wood were analyzed by HPLC. The effects of the five inoculated fungi on the alterations to the wood cell wall ultrastructures were examined by TEM. The wood became more acidic as it was inoculated with the studied fungi. From the HPLC analysis, the oxalic acid (293.34 µg/g o.d.) in the A. saligna, A. tenuissima (167.33 µg/g o.d.), and T. harzianum (245.01 µg/g o.d.) wood decreased, but it increased in the A. flavus (362.08 µg/g o.d.), A. niger (1202.53 µg/g o.d.), and F. culmorum (431.85 µg/g o.d.) inoculated wood. Citric acid was observed in the wood inoculated with A. flavus (110 µg/g o.d) and A. niger (2499.63 µg/g o.d). Tartaric (1150.98 µg/g o.d), acetic (2.04 µg/g o.d), and propionic (1.79 µg/g o.d) acids were found in the wood inoculated with A. niger. Butyric acid was found in small amounts. A loss of wood substances appeared as the electron-lucent increased in the middle lamella and the layers of the secondary wall. Within the secondary cell wall regions, checks and splits were also noted, which resulted from the effects of the acids on the carbohydrates, according to the fungus type and the acids. In conclusion, increasing the amount of organic acids in the wood samples through inoculation with fungi results in more degradations in the wood, especially in the wood inoculated with A. niger.
Highlights
Wood, as a natural organic material, consists of hemicellulose, cellulose, lignin, and minor amounts of organic extractives, as well as ash as an inorganic mineral [1]
The pH decreased from 5.5 to 4.0 during the fermentation process of wheat straw, which indicates the production of organic acids by the white rot fungus Nematoloma frowardii, where malate, fumarate, and oxalate were present at a high level [31]
The fungal isolate T. harzianum was identified by the specific pair of primers THITS-F2 and THITS-R3 and the degenerated primers published by Miyazaki et al [39], while A. tenuissima isolate was identified upon sequence of the rDNA ITS region amplified by ITS1 and ITS4 primers and H3-1a, H3-1b primers for histone 3 gene amplification [40]
Summary
As a natural organic material, consists of hemicellulose, cellulose, lignin, and minor amounts of organic extractives, as well as ash as an inorganic mineral [1]. An increase in organic acids leads to accelerating the degradation of wood Many fungi, including both the brown and white rot fungi, produce calcium oxalate crystals that adhere to the hyphae [16,17]. Oxalic acid production by brown-rot decay fungi has eluded many researchers for decades [19,20,21] This is followed by the secretion of hydrolytic enzymes that decompose the polysaccharides’ plant cell wall [22,23]. The pH decreased from 5.5 to 4.0 during the fermentation process of wheat straw, which indicates the production of organic acids by the white rot fungus Nematoloma frowardii, where malate, fumarate, and oxalate were present at a high level [31]. The present work aims to illustrate the effects of five fungi on the contents of organic acids as well as the type of degradation in the ultrastructure compositions of wood from Acacia saligna
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