Plant and Soil 123, 239-241 (1990). © Kluwer Academic Publishers. Printed in the Netherlands. PLSO PN 164 Genetic studies on the acidification capacity of sunflower roots induced under iron stress E. ALCANTARA, M. FERNANDEZ and M.D. DE LA GUARDIA Departamento de Agronomia, Escuela TOcnica Superior de Ingenieros Agrrnomos, Universidad de Crrdoba, E-14080 Crrdoba, Spain Key words: Helianthus annuus, inheritance, iron stress, proton efllux, sunflower Abstract Under stress of iron deficiency roots of sunflower (Helianthus annuus L.) increase proton efflux which acidifies the root medium, increase the ferric reducing capacity and the exudation of phenolic compounds. Differences have been found previously among sunflower inbred lines in the capacity of their roots to lower pH and it was also found that this character is under genetic control. This work presents the results of an inheritance study made by crossing two genotypes, one (CMS HA 89) without acidification capacity and another (RHA 271) with it. Plants were grown individually in 75 mL vessels with an aerated solution low in iron. After 4 days, solutions were changed to one without iron and the pH of the medium was measured during the following days. Results from FI, F2, and backcross generations can be explained with two pairs of alleles controlling the character, being the relation between alleles of complete dominance at both gene pairs, but either gene, when dominant is epistatic to the other. Introduction Sunflowers, as many other dicotyledonous species, develop under iron deficiency cytological, morphological and physiological changes in their roots which increase iron uptake and translocation. These responses in sunflower involve differentia- tion of peripheral cells into transfer cells and development of abundant root hairs in a swollen zone near the root apex, increase in the ferric reduc- ing capacity, proton efflux and release of com- pounds with capacity to chelate and reduce ferric ions (Kramer et al., 1980; R6mheld and Marschner, 1979). R6mheld and Marschner (1986) discuss the importance of these responses in dicotyledonous species and compare them with the responses of graminae. Genetic studies on iron efficiency have been made in several species: soybean (Cianzio and Fehr, 1982; Weiss, 1943), tomato (Wann and Hills, 1973) and bean (Coyne et al., 1982). In previous work we have found differences 239 among sunflower inbred lines in the acidification capacity and that this character was under genetic control (Alcfintara and De la Guardia, 1986; 1987). The purpose of this work was to study the inheri- tance of this character in several generations derived from the cross between two inbred lines of sunflower with different acidification capacity under iron deficiency. Materials and methods The original parental plant materials used in this study were the American lines CMS HA 89 (P1) and RHA 271 (P2). Line CMS HA 89 is cyto- plasmic male sterile and line RHA 271 is a fertility restorer, both lines are used to produce commercial hybrids. Under iron deficiency stress, line CMS HA 89 showed a uniform response without acidification capacity of the roots medium. Line RHA 271, although it had acidification capacity, had an uneven response, it was then selected for several