Development of Vacuolar Volume in the Root Tips of Pea

Cell and vacuole areas were measured by light microscopy in longitudinal and transverse sections cut at 0.4-mm intervals along the apical 7.2 mm of the primary root of pea. The vacuolar area (or volume) fraction — that is, vacuole area (or volume) divided by cell area (or volume) — increased from about 15 % in cells 0.4 mm from the distal boundary of the apical meristem (the cap /root junction), to about 85% in cells situated 6.8–7.2 mm from that boundary. At each distance, vacuole development tended to be greater in the cortex than in the stele. Vacuoles occupied about 22% of the tissue volume in the first 1 mm length of root (measured from the cap/root junction), about 31 % of the tissue volume in the first 2 mm, and about 45% when summed over the apical 5-mm length of root. Phosphorus supply or deprivation produced only minor and non-significant changes in vacuole development. The results have implications affecting previous estimates of cytoplasmic and vacuolar phosphate concentrations in pea root tips.