Acid-Soil Resistance of Forage Legumes as Assessed by a Soil-on-Agar Method

plants, some white clover plants survived for 2 to 3 yr at a pH as low as 4.2 (Voigt, unpublished data, 1996). Establishment of small-seeded legumes on strongly acid soils can Thus, acid-soil resistance at seedlings stages may be even be even more difficult than maintaining those stands once established. more critical than acid-soil resistance at older stages of Our objectives were to use a soil-based procedure for characterization of acid-soil resistance of small-seeded forage-legumes and to detergrowth. mine the validity of primary root growth and specifically the soil-onEvaluation and selection of germplasm for resistance agar technique, using germplasm that varied widely in seed size and to acid-soil stress require simple and reliable methods. acid-soil resistance. We ran four experiments to evaluate 28 cultivars A soil-on-agar procedure, intended for use with very of 15 species. In general, species differences observed were in good young seedlings of small-seeded species, was proposed agreement with previous knowledge. Large differences in acid-soil for these purposes (Voigt et al., 1997). This simple, resistance, for example among crimson (Trifolium incarnatum L.), nondestructive procedure evaluates root emergence of white (T. repens L.), and berseem clover (T. alexandrinum L.) were germinated seedlings from a thin layer of soil into watervisually obvious. Smaller differences were not as clear but could still agar during a ≈10-d period. Results for white clover, be detected statistically; for example, the greater acid-soil resistance based on eight soils varying widely in Al saturation, of striate [Kummerowia striata (Thunb.) Schindl.] compared with Korean lespedeza [K. stipulacea (Maxim.) Makino]. Relative acidindicated a strong relationship between root emergence soil resistance of rose clover (T. hirtum All.) is reported for the first of white clover and the species of Al in the soil solution time. Within species, the known greater acid-soil resistance of ‘AU that are toxic to the roots of dicotyledonous plants Dewey’ birdsfoot trefoil (Lotus corniculatus L.), compared with other (Voigt et al., 1998). The usefulness for the characterizacultivars of that species, was detected. One major difference between tion of germplasm of this procedure, which is based on our results and solution culture studies, kura clover (T. ambiguum primary root growth, remains to be demonstrated. M. Bieb.) was more acid-soil resistant than white clover. ‘Cossack’, The abilities of clovers to grow on acid and calcareous a kura clover bred from germplasm developed on a high pH soil, soils vary widely (Hoveland and Evers, 1995). Clovers was more sensitive to acid-soil stress than several other cultivars. that are among the more resistant to acid-soil stress, A procedure characterizing primary root growth, the soil-on-agar such as crimson and subterranean clover (T. subtertechnique, can do an effective job of evaluating acid-soil resistance of small-seeded legumes. raneum L.), can be among the more susceptible to iron chlorosis when grown on calcareous soils (Rogers, 1947; Gildersleeve and Ocumpaugh, 1988). In contrast, berseem clover remains green and grows well on calcareous P that are resistant to Al toxicity, or by extensoils (Gildersleeve and Ocumpaugh, 1988) but is not sion acid-soil, are those, “. . . that exhibit superior recommended for acid soils (Hoveland and Evers, root growth which ultimately result in enhanced plant 1995). Even within species, where wide genetic variation vigor on acidic, Al-toxic soils or solutions” (Kochian, is known, germplasm that is more resistant to acid-soil 1995). Aluminum tolerance and Al exclusion imply spestresses can be more susceptible to iron chlorosis. Oscific and different mechanisms responsible for that reborne et al. (1981) studied the response of seven sub sistance. clover cultivars to Al in acid solution cultures. Those Establishment of small-seeded forage species is fresame cultivars were included in studies of iron chlorosis quently more difficult than that of larger-seeded crops. by Gildersleeve and Ocumpaugh (1989). Rank correlaOur observations at a field site in southern West Virtion coefficients calculated between Gildersleeve and ginia indicated that as pH dropped below 5.0, and Al Ocumpaugh’s chlorosis scores at 6 wk and the relative availability increased, successful establishment was reroot growth characteristics (4 mg kg 1/0 mg kg 1 Al duced. At a pH of 4.9, reduced numbers of ‘Grasslands 100) of Osborne et al. were r 0.95 and 0.90 (n Huia’ white clover seedlings were established and the 7, P 0.01) for the Paritta soil (clayey, mixed, active, resulting seedlings were less vigorous and more chlohyperthermic, shallow Petrocalcic Paleustolls) and root rotic than seedlings at pH 5.0 and above (Staley, perweight and the Denhawken soil (fine, smectitic, hypersonal communication, 2001). Observations along a pH thermic Vertic Haplustepts) and root length, respecgradient at the edge of the same field showed a dramatic tively. Thus, in evaluating results from the soil-on-agar drop in occurrence of white or red clover (T. pratense procedure with those reported in the literature, not only L.) when the soil pH was below 4.9 (Voigt, unpublished acid-soil and Al resistance should be considered, but data, 1996). In contrast, when established from transalso iron chlorosis resistance. Our hypothesis was that the soil-on-agar procedure, P.W. Voigt, USDA-ARS, Appalachian Farming Systems Research which uses primary root growth to characterize, could Center, 1224 Airport Road, Beaver, WV 25813-9423; J.A. Mosjidis, be used to determine the acid-soil resistance of smallDep. of Agronomy and Soils, Alabama Agric. Exp. Stn. and Auburn Univ., Auburn, AL 36849-5412. Received 4 May 2001. *Corresponding author ([email protected]). Abbreviations: CoVE, coefficient of velocity of root emergence; RI, resistance index. Published in Crop Sci. 42:1631–1639 (2002).