The cellulose microfibril (CMF) orientation in the secondary cell walls of tension wood, compression wood, and primary cell walls of onion abaxial epidermis was studied with sum frequency generation (SFG) vibration spectroscopy. The SFG spectral features of tension wood and the epidermis of the outermost onion scale changed depending on the angle between the polarization of the incident IR beam and the preferential alignment axis of CMFs. In onions, the epidermal cells are elongated along the onion bulb axis. SFG showed that the outer most scales have an average CMF orientation over the entire thickness of the cell wall, oriented perpendicular to the bulb axis and more dispersed in the inner scales of the bulb. The surface orientation of CMFs imaged by AFM also showed a similar trend.  Tension wood contains highly-crystalline and highly-aligned CMFs. The change in alkyl/hydroxyl intensity ratio in tension wood, with respect to the azimuthal angle between incident polarization of IR beam and preferential alignment axis of CMFs, meant that CMFs are highly aligned along the wood grain axis. Compression wood did not show any dependence on the orientation of the incident IR polarization, implying that the overall orientation of cellulose microfibrils in compression wood is not highly aligned. However, the change in SFG alkyl/hydroxyl intensity ratio in compression wood was due to larger separation between CMFs and lower cellulose content. The SFG analysis of CMFs in onion epidermis and reaction wood showed that SFG spectral features are sensitive to average CMF orientation and inter-fibrillar separations. This capability of SFG analysis is useful to understand the correlation of CMF orientation and assembly with cell wall biomechanical properties, growth mechanism, and cell wall response to external stress.

Vibrational sum-frequency-generation (SFG) spectroscopy study of cellulose microfibril orientation and assembly in onion epidermis and reaction woods

249th National Meeting (2015)

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