Responses of Sunflower Genotypes to Aluminum in Acid Soil and Nutrient Solution

The overall objective of this research is to develop plant genotypes for better adaptation to acid soil stress factors, such as Al toxicity in subsoils, which cannot be economically corrected by conventional means. The specific objective of the work reported here was to determine the range of A1 tolerance available within the sunflower (Helianthus annuus L.) species. Thirteen sunflower genotypes were grown for 23 days in greenhouse pots (1.82 kg) of fertilized, acid, Al-toxic Bladen soil treated either with no lime or with 3,000 ppm CaCO₃. With vegetative growth as an index, the genotypes differed widely in tolerance to the acid soil. For example, on the unlimed soil (pH 4.1) the top yield of ‘CM-2,’ ‘Manchurian,’ ‘Greystripe,’ ‘Peredovik,’ and ‘Romania HS-52’ were 42, 38, 27, 23, and 11%, respectively, of those on the limed soil (pH 5.5). Corresponding relative root yields (no lime/lime %) were 69, 55, 50, 29, and 7%. The differential Bladen soil tolerance of the Manchurian and Romania HS-52 genotypes was confirmed in three additional experiments. The Manchurian genotype was also significantly more tolerant to excess Al in nutrient solutions than Romania HS-52. For example, with 2, 4, and 6 ppm Al added at initial pH 4.6, the relative top yields (Al1/no Al %) were 64, 28, and 28% for Manchurian and 64, 18, and 15% for Romania HS-52. Corresponding relative root yields were 65, 28, and 21% for Manchurian and 59, 16, and 8% for Romania HS-52. The greater acid soil sensitivity of the Romania HS-52 genotype was associated with a chlorosis of young leaves which did not occur in Al-toxic nutrient solutions. The stems of Bladen sensitive Romania HS-52 plants accumulated significantly higher concentrations of Zn than did those of the more tolerant Manchurian genotypes when both were grown on the unlimited Bladen soil. Genotypic differences in Bladen soil sensitivity or susceptibility to the symptoms of leaf chlorosis were not related to any significant differences in the concentrations of Al, Ca, P, Fe, Mn, Cu, or Ni in the plant tops. Fertilization of the unlimed Bladen soil with Mo reduced the chlorosis in Bladen-sensitive Romania HS-52 but did not increase growth. Results of these studies suggest the possibility of breeding genotypes with greater tolerance to acid subsurface soils and the subsequent expansion of sunflower acreage into soil regions where the crop previously has not been successful.