Seedling Tolerance to Aluminum Toxicity in Hard Red Winter Wheat Germplasm Brett F. Carver,* William P. Inskeep, Nancy P. Wilson, and RobertL. Westerman ABSTRACT Critically acid soils (pH < 5.0) now exist in the major wheat (Trlticum aestlvum L.) production areas of the Southern Great Plain s. This condition has encouraged theidentification and development of hard red winter (HRW) wheat genotypes that can tolerate higher amounts of soluble soil AI. The objectives of this research were to identify Al-tolerant HRW genotypes better adapted to soils high in soluble A! and to determine alternative gene sources. Seedlings were grown in nutrient solutions containing 0.18,0.36,and 0.72 mM AI. As a measure of tolerance, the degree of staining on root tipsof 5- d-old seedlings was visually scored after exposure to hematoxylin. A total of 156 HRW pureline cultivars, ancestors, and parental lines used in HRW breeding programs was classified into four tolerance levels, i.e., very susceptible (54%), moderately susceptible (15%), intermediate (14%), and tolerant (17%). Several ancestors from other wheat classes showed intermediate or tolerant responses including ’Kenya 58’, ’Mediterranean’, ’Purplestraw’, and ’Red Fife’; yet, the predominant ancestor of HRW wheat, ’Turkey’, was very suscep- tible. Eight of the 16tolerant USA cultivars shared similar pedi- grees, and thus probably shared thesame source(s) of Aitolerance (i.e., ’Sono~-a 64’ and ’Trapper’). Confirmed sources of tolerance in remaining cultivars included ’Centurk’ and ’Prelude’, although sev- eral were derived from parents classified as very susce~ptible. Nine of the 15intermediate USA cultivars were separated intotwo groups based on their distantly related Al-tolerant parents (Kenya 58 and Prelude vs. Mediterranean). In summary, AI tolerance in HRW wheat could not betraced in a single lineage to any single ancestor orto a set of closely related lines. These results provide an information base to test for different genes among sources and to help develop recommendations of more tolerant cultivars for acid soils high in soluble AI. Additional Index Words: Tritlcum aestivum L., Aluminum toler- ance, Soilacidity, Soluble aluminum, Genetic diversity. A GRADUAL DECLINE in the pH of surface soils in some areas of the southern Great Plains has reached the point that it now poses a serious threat to hard red winter (HRW) wheat production. Results a comprehensive soil-test survey revealed acid soil conditions of pH < 5.0 for 15%of the total wheat acreage in Oklahoma in 1985 (14). Critical soil acidity also occurs in major wheat production areas of Kansas (31). Thepotential exists for more extensive acid soil developmentin other areas of the HRW wheat region if applications of acid-forming fertilizers continue without corrective lime applications. Soluble A1 in medium to heavy-textured soils, such as the PondCreek (fine-silty, mixed, thermic Pachic Argiustolls) and Grant (fine-silty, mixed, thermic Udic B.F. Carver, N.P. Wilson, and R.L. Westerman, Dep. of Agronomy, Oklahoma State Univ., Stillwater, OK 74078; and W.P. Inskeep, Dep. of Plant and Soil Science, Montana State Univ., Bozeman, MT 95717. Journal manuscript J-5189 of the Oklahoma Agric. Exp. Stn., work supported in part by the Oklahoma Wheat Res. Foun- dation and the Oklahoma Agric. Exp. Stn., Projects H-1867 and H- 1963. Received 11 May 1987. *Correspondingauthor. Published in CropSci. 28:463-467 (1988). Argiustolls) series in Oklahoma and Kansas, is con- sidered one of the primary toxic factors in acid soils. For every unit decrease in soil pH, the concentration of soluble A1 theoretically increases 1000-fold (18), re- sulting in different levels of phytotoxicity depending on the soil type and plant genotype (11). Aluminum- stressed wheatroots are typically short, thick, and de- ficient in secondary root development, resulting in less efficient nutrient and water supply to vegetative ma- terial (11,15,21,28), particularly when moisture is lim- ited in dry year s. Restricted shoot development of Al- stressed wheatis also associated with decreased chlo- rophyll concentration and photosynthesis (22). Acid tolerance of wheat has been related to its capacity to withstand high concentrations of soluble AI (5,17,30), specifically A13+ (4,23). Wheat cultivars having excep- tional A1 tolerance (Colonias, Frondosa,Frontana, and Fronteira) were developed in Brazil where strongly acid soils prevail (12,19). Other cultivars having notable tolerance levels were developed in traditionally acid- soil regions of the southeastern USA and in Ohio (12,13). Neutralization of soil acidity via liming effectively reduces A1 toxicity (23,29). However, lime application has not been a general practice in the Southern Great Plains, due in part to the high cost of transporting lime to agricultural areas with greatest need. Thus, genetic improvement of A1 tolerance in HRW wheat is re- ceiving more emphasis among wheat breeders in the region. Identification of Al-tolerant wheat genotypes in the USA has traditionally centered upon soft red winter (SRW) wheat, because this class is grownin a geo- graphical region generally having high surface and subsoil acidity (1). Only recently was attention solely given to differential A1 tolerance of HRW cultivars (30). Themajority of cultivars screened in that study wereextremely sensitive. If Al-tolerant genotypes could be identified within HRW germplasm, further devel- opment and release of Al-tolerant cultivars would be expedited without severely disrupting favorable gene complexes previously selected. Sufficient levels of tol- erance may already exist in the HRW class, either as a result of genes present in HRW ancestors and in- advertently retained during selection for current cul- tivars, or as a result of selection for linked genescon- tributed by extraneous germplasm developed in acid- soil regions (20). To increase the potential contribu- tion from other germplasm, breeders have more re- cently increased the genetic diversity of HRW wheat by hybridizing lines from other classes with adapted material (10). The purposes of this research were to identify A1- tolerant genotypes within the HRW class better adapted to acid soils high in soluble AI and to deter- mine alternative gene sources among those genotypes. Older cultivars and ancestral lines were examined to determine possible genetic sources of the AI tolerance 463 Published May, 1988