Root Elongation of Soybean Genotypes in Response to Acidity Constraints in a Subsurface Solution Compartment

rhizotoxicity by reducing Al activity through increased solution ionic strength and increased competition beAluminum-tolerant germplasm is needed to overcome subsurface tween Ca and Al at binding sites external to the plasacidity constraints to root growth and plant access to water and nutrients. Root elongation of four soybean [Glycine max (L.) Merr.] genomalema (Kinraide and Parker, 1987). Other roles for types exposed to varying concentrations of Al, H, and Ca were comCa include increasing membrane hydrophobicity; prepared in two experiments using a vertically split root system. Roots venting cell leakiness; strengthened cell walls; and, in extending from a limed surface soil compartment grew for 12 d into the cytoplasm, acting as a “second messenger” that a subsurface compartment with nutrient solution treatments. In Exp. mediates changes in enzyme activity (Hanson, 1984; 1 root growth for cv. Ransom and Plant Introduction 416937 (PI) Marschner, 1995). were compared in solutions with factorial combinations of pH (4.2, Different classes of roots for the same genotype can 5.2) and Al (0, 7.5, 15 mM ) with Ca maintained at 10 mM. In Exp. exhibit differential elongation responses to rhizotoxic 2 soybean line N93-S-179 (N93), PI, and cultivars Ransom and Young Al. Bushamuka and Zobel (1998), in a comparison of were compared in solutions with factorial combinations of Ca (2 and elongation of tap, basal, and lateral roots between limed 10 mM ) and Al (7.5 and 15 mM ) maintained at pH 4.6. Ransom and PI had similar responses in tap and lateral root elongation to solution and unlimed subsurface layers of Al-toxic soil among pH and Al treatments in Exp. 1, but mean tap root length of Ransom in several maize (Zea mays L.) and soybean cultivars, the subsurface compartment exceeded that of PI by 22%. Aluminum found that tolerance to Al in the unlimed subsurface inhibited the length of lateral roots more than tap roots in both layer by all three root classes was only observed in one experiments. Molar activity ratios between Ca and Al31 {Ca/Al31} maize cultivar. However, among the other maize and accounted for most of the differences in root elongation response soybean cultivars, either one, two, or none of these root among solution treatments in Exp. 2. A 50% reduction in relative classes were tolerant to the soil Al stress. Sanzonowicz length of tap roots for all genotypes occurred with a {Ca/Al31} value et al. (1998a, 1998b) found that subsurface solution H of 891. Values of {Ca/Al31} for 50% reductions in relative length of and Al inhibited the length of lateral roots more than lateral roots differed among genotypes and were 1.6 to 3.5 times tap roots for Ransom soybean. When exposed to similar greater than for tap roots. On the basis of the {Ca/Al31} indices for lateral root length, line N93 and Ransom exhibited greater tolerance levels of toxic H or Al, a higher solution Ca concentrato subsurface solution Al than PI and Young. tion was needed to increase length of lateral roots relative to tap roots. The objective of this investigation was to compare root elongation response to varying solution concentraD of soybean roots in acid soils is retions of H and Al and the ameliorative effects of Ca stricted by low pH, low contents of Ca and Mg, among different classes of roots for selected soybean and high Al (Goldman et al., 1989; Carter and Rufty, genotypes exposed to rhizotoxic treatments in the sub1993; Spehar, 1994). Liming alleviates constraints of soil surface compartment of a vertically split root system. acidity in surface layers (Dierolf et al., 1997), but subsurface layers may remain acid and can have low Ca conMATERIALS AND METHODS centrations (Ritchey et al., 1980). Subsoil acidity reduces crop yields by restricting root growth and thereby reducA vertically split root system (Sanzonowicz et al., 1998a) ing nutrient acquisition and plant access to soil water was used to evaluate elongation of soybean roots extending reserves during periods of drought stress. Potential alfrom a limed surface soil into a subsurface zone containing ternatives to the direct amelioration of subsoil acidity solutions with various combinations of Al, pH, and Ca. Plastic tubes, 52 cm long with an internal diameter of 10 cm, were include the use of Al-tolerant germplasm (Foy, 1988). divided into two sections separated by a root-permeable memAluminum rhizotoxicity for several plant species can brane. The membrane was formed by dipping cheesecloth be alleviated by increasing the Ca concentration in nutriattached to the surface compartment into a mixture of paraffin ent solutions (Alva et al., 1986a, 1986b; Horst, 1987; and petrolatum (1:2) at 808C for 10 s. Both tube sections were Runge and Rode, 1991). Calcium additions alleviate Al disinfected with 20.6 M ethanol. The 12-cm-long surface compartment was filled with 1.1 kg Armando Ferrufino, Programa de Apoyo a la Estrategia de Desarollo of sterilized (1218C, 0.1 MPa for 1 h) loamy sand from the Ap Alternativo (PRAEDAC), P.O. Box 2395, Cochabamba, Bolivia; T. horizon of a Wagram soil (loamy, kaolinitic, thermic Arenic Jot Smyth and Daniel W. Israel, USDA-ARS Soil Science Dep., North Kandiudult). Selected chemical properties of this soil are deCarolina State University, Raleigh, NC 27695-7619; Thomas E. Carter, Jr., USDA-ARS Crop Science Dep., North Carolina State University, Abbreviations: Ala, fraction of Al in solution that reacts immediately Raleigh, NC 27695-7620. Research supported by the Soil Management with the ferron reagent; Alb, fraction of Al in solution that reacts with Collaborative Research Support Program and funded in part by Grant ferron according to first order kinetics; AlT, all Al in solution that no. DAN-1311-G-00-1049-00 from the USAID. Received 5 Feb. 1999. reacts with ferron; AlFN30, Al in solution reacting with ferron by 30 *Corresponding author ([email protected]). s; AlMono, Al31 1 Al(OH)21 1 Al(OH)1 2 1 Al(OH)3 1 Al(OH)2 4 ; {Ca/Al31}, molar activity ratio between Ca and trivalent Al; N93, Published in Crop Sci. 40:413–421 (2000). breeding line N93-S-179; PI, Plant Introduction 416937.