Xylem, cell wall alterations

Alterations in either lignin e.g., re/S ) or cell wall carbohydrate polymers can lead to changes in the physical properties of cell walls, which often result in similar phenotypic consequences. Such was the case with the irregular xylem (irx) mutants that exhibit collapsed tracheary elements. These mutants were isolated by microscopic inspection of stem hand sections. The irxl, irx2, and irx3 mutants exhibited reductions in cellulose synthesis and/or deposition. The irx3 mutant was later characterized to encode a cellulose synthase. ... 

Osmotic pressures in the wall space are believed to explain the observation that nontranspiring plants have negative water potentials. Cosgrove and Cleland conclude that the internal gradient in water potential from the xylem to the epidermis, which sustains cell enlargement, is small. Auxin does not alter the hydraulic conductance of stem tissue either at the cellular or whole tissue level. How extracellular nitrogenous compounds contribute to or modify this response remains unknown. 

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