Cellular Changes Precede Cavity Formation in the Vascular Cylinders of Pea Roots (Pisum sativum L. cv Alaska)

The anatomy of the vascular cylinders of 150-mm-long pea primary roots (Pisum sativum cv Alaska) grown at 10° and 25°C was examined by light and electron microscopy. Root and vascular cylinder diameters were greater at 10°C. At maturity vascular cylinder cells were longer at 10°C than at 25°C, but did not differ in mean diameter (11-16 μm). This effect was most pronounced in the xylem sectors (626 vs. 429 μm). Observation of cell ultrastructure showed cellular degeneration in some central xylem cells at both temperatures in the region 20-24 mm from the root tip, but degeneration was more extensive at 25°C. The ultrastructure of degenerating cells was similar to that of senescing cells. At 25°C, within the region 25-29 mm from the tip, expanding metaxylem tracheary elements (MTEs) appeared to cause pressure on surrounding cells. This stress appeared to disrupt walls of degenerating cells, leading to the formation of small lacunae in the region 30-34 mm from the root tip. The lacunae expanded by similar lysis of adjacent cells, usually causing a series of lens-shaped cavities to form in the center of the vascular cylinder. The cavities further enlarged and connected to form a large, continuous cavity along the center of the root axis. Many cell divisions occurred among xylem cells between the onset of cell degeneration and cavity initiation. Within the comparable region (25-29 mm from the tip) cell divisions in xylem tissue was much less frequent in roots grown at 10°C, and these roots did not form cavities. We conclude that at 25°C rapid development of MTEs, concurrent with accelerated central xylem cell autolysis, permits mechanical stress in the vascular cylinder to break weakened cell walls and consequently induce cavity formation.