THE EFFECT OF SOIL COMPACTION ON DIFFERENTIATION OF LATE METAXYLEM IN COMMON BEAN (PHASEOLUS-VULGARIS L)

Recent studies among several plant species have shown that maturation of the largest vessels in primary xylem of roots occurs much later than is commonly assumed. These results have important implications for studies of water and nutrient uptake since the condition of the vessels, termed late metaxylem (LMX), might have a large effect on the potential conductivity of the xylem. To determine whether this phenomenon occurred in common bean (Phaseolus vulgaris L.), patterns of root xylem differentiation were studied in young bean plants. Soil bulk density was varied in one trial to determine whether differentiation of LMX was sensitive to the growing medium. Vessels of LMX lost cell contents and-became functional conduits between 100 and 150 mm from the root apex. Increasing soil bulk density caused the zone of maturation of LMX to shift toward the root apex, but this zone was never closer than 67 mm. In the region where the primary root increased in diameter as it merged with the hypocotyl, a zone was found where vessels increased in number, had a reduced diameter, and were arranged in a ring, the normal tetrarch arrangement of the xylem being loSt. Potential conductivity in this zone was considerably less than in zones with conventional large LMX vessels, so the zone appears to present an important restriction to water transport from the root to the shoot. Thus, while the phenomenon of late maturation of LMX occurs in common bean, its significance in transport of water from roots to shoots is unclear © 1992 Annals of Botany Company.