Abstract text: Xyloglucan is the most abundant hemicellulose in eudicots and is found in the primary walls of root hairs. It is important for root hair growth, since xyloglucan deficiency results in root hairs with abnormal morphologies[1], but the underlying cellular and mechanical reasons for these developmental defects are poorly understood. Here, we studied the growth, cellular properties, and mechanical characteristics of root hairs in two mutants, xxt1 xxt2 and cslc-∆5, that lack any detectable xyloglucan. We found that both mutants not only had shorter but also fewer and wider root hairs with thicker cell walls, and that these root hairs grew slower than wild-type. Estimation of turgor pressure using incipient plasmolysis revealed that xyloglucan-deficient root hairs had lower turgor than wild-type. Quantification of wall mechanical properties, such as stiffness and viscosity, using Brillouin microscopy revealed that cell walls in root hair bulges of xyloglucan-deficient mutants were softer and less viscous, whereas mature root hairs in mutants had mechanical properties similar to those of wild-type. Taken together, these data help explain the causes of root hair defects in xyloglucan-deficient mutants. Currently, we are constructing and testing Finite Element models to understand root hair growth in these mutants under different mechanical constraints.