Drought and Growth Hormone Effects on Yucca Whipplei and Yucca Brevifolia Root Contraction
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Yucca seedlings can longitudinally contract and radially expand their roots, pulling the seedlings down into the soil for protection from evapotranspiration and heat. This contraction occurs through a rearrangement of microtubules in the perivascular cortical tissue. Plant microtubule arrays are correlated with cellulose microfibrils in the cell wall, and therefore plant cell growth and polarity. Within the cortical array of the basal region of the root, microtubules can reorient and in turn cause the cell to grow laterally. Drought and the growth hormone gibberellic acid were both studied as variables in the rate of root contraction. Anatomical changes in root tissue were studied in Yucca brevifolia and Yucca whipplei . Physical measurements of seedling roots in soil were used to quantify the amount of contraction over a period of time. Root contraction was observed in all of the species studied. All seedling roots measured shortened over three weeks. Ridges also formed in the contracted basal region of the root epidermis, providing external evidence of contraction. Drought negatively affected the rate of contraction in Y. whipplei but not Y. brevifolia seedlings. Within the perivascular cortical tissue, lateral elongation of cells was verified using microscopy. Microtubule reorientation was observed using immunofluorescence microscopy within contracting root cells within the cortical tissue. Gibberellin did not cause any change in the rate of contraction, but immunofluorescence microscopy showed a thickening of microfilament bundles within cells dosed with it.