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 degrees and 25 degrees C was ex amined by light and electron microscopy. Root and vascular cylinder di ameters were greater at 10 degrees C. At maturity vascular cylinder ce lls were longer at 10 degrees C than at 25 degrees C, but did not diff er in mean diameter(11-16 mu m). This effect was most pronounced in th e xylem sectors (626 vs. 429 mu m). Observation of cell ultrastructure showed cellular degeneration in some central xylem cells at both temp eratures in the region 20-24 mm from the root tip, but degeneration wa s more extensive at 25 degrees C. The ultrastructure of degenerating c ells was similar to that of senescing cells. At 25 degrees C, within t he region 25-29 mm from the tip, expanding metaxylem tracheary element s (MTEs) appeared to cause pressure on surrounding cells. This stress appeared to disrupt walls of degenerating cells, leading to the format ion of small lacunae in the region 30-34 mm from the root tip. The lac unae expanded by similar lysis of adjacent cells, usually causing a se ries of lens-shaped cavities to form in the center of the vascular cyl inder. The cavities further enlarged and connected to form a large, co ntinuous 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 ti p) cell divisions in xylem tissue was much less frequent in roots grow n at 10 degrees C, and these roots did not form cavities. We conclude that at 25 degrees C rapid development of MTEs, concurrent with accele rated central xylem cell autolysis, permits mechanical stress in the v ascular cylinder to break weakened cell waits and consequently induce cavity formation.