Timing, magnitude, and location of initial soluble aluminum injuries to Mungbean roots

Abstract

Despite a century's knowledge that soluble aluminum (Al) is associated with acid soils and poor plant growth, it is still uncertain how Al exerts its deleterious effects. Hypotheses include reactions of Al with components of the cell wall, plasmalemma, or cytoplasm of cells close to the root tip, thereby reducing cell expansion and root growth. Digital micros copy was used to determine the initial injuries of soluble Al to mungbean (Vigna radiata L) roots Roots of young seedlings were marked with activated carbon particles and grown in 1 mM CaCl2 solution at pH 6 for ca. 100 min (control period), and AICl3 solution was added to ensure a final concentration of 50 µM Al (pH 4). Further studies were conducted on the effects of pH 4 with and without 50 iM Al Four distinct, but possibly related, initial detrimental effects of soluble Al were noted First, there was a 56–75% reduction in the root elongation rate, first evident 18–52 min after the addition of Al, root elongation continuing at a decreased rate for ca. 20 h. Decreasing solution pH from 6 to 4 increased the root elongation rate 4-fold after 5 min, which decreased to close to the original rate after 130 min. The addition of Al during the period of rapid growth at pH 4 reduced the root elongation rate by 71% 14 min after the addition of Al The activated carbon marks on the roots showed that, during the control period, the zone of maximum root growth occurred at 2,200–5,100 im from the root tip (i.e the cell elongation zone) It was there that Al first exerted its detrimental effect and low pH increased root elongation Second, soluble Al pre vented the progress of cells from the transition to the elongation phase, resulting in a considerable reduction of root growth over the longer term. The third type of soluble Al injury occurred after exposure for ca. 4 h to 50 µM Al when a kink developed at 2,370 im from the root tip. Fourth, ruptures of the root epidermal and cortical cells at 1,900–2,300 im from the tip occurred ≥4.3 h after exposure to soluble Al The timing and location of Al injuries support the contention that Al initially reduces cell elongation, thus decreasing root growth and causing damage to epidermal and cortical cells.