Selection for Aluminum and Acid-Soil Resistance in White Clover

based studies are conducted, the effects of Al are usually confounded with other factors such as levels of P, Ca, Despite good resistance to acid-soil stresses, white clover (TriMo, Mn, Zn, organic acids, and other soil components folium repens L.) is not found on some acid soils. Our objectives (Edmeades et al., 1995; Villagarcia et al., 2001). An exwere to develop a large-leafed white clover with acid-soil resistance, to relate seedling Al to mature plant acid-soil resistance, and to valiception is studies of the growth of very young seedlings. date our soil-on-agar procedure. We used a two-stage selection proThis is possible because (i) the effect of Al on root cedure. In Stage 1 we used the soil-on-agar technique to select for growth is rapid, occurring within hours, and can be draseedling Al-resistance and Al-susceptibility in Brown Loam Synthetic matic (Kochian, 1995), (ii) Al and proton activity at the No. 2 and ‘Grasslands Huia’ white clover. In Stage 2 we used convenroot surface are the major factors limiting white clover tional pot studies with two soil pH treatments, 4.2 and 5.2, and stemprimary root growth, with Al being more toxic than protip cuttings of Al-resistant selections from Brown Loam to select for tons (Brauer, 1998), and (iii) initial root growth is depenacid-soil resistance. The same Stage 1 and 2 techniques were used to dent primarily on nutrient reserves in the seed rather evaluate 12 experimental populations and both parents for seedling than nutrients from the soil (Edmeades et al., 1995). Al resistance and mature plant acid-soil resistance. Across two cycles The soil-on-agar technique (Voigt et al., 1997) takes of selection, both Brown Loam and Huia Al-resistant and susceptible populations diverged. For Brown Loam, progress was made toward advantage of the above factors to provide a biological both increased Al resistance and susceptibility. For Huia, progress apassessment of soils at the critical establishment stage peared more toward Al susceptibility than toward Al resistance. Poputhat is closely related to the toxic form of Al found in lations developed from two-stage selection were more acid-soil resisthe soil solution (Voigt et al., 1998). The procedure can tant than their parent. However, populations selected only for seedling be used to relate seedling Al resistance to acid-soil resisAl resistance or Al susceptibility were usually no more acid-soil resistance of small-seeded forage legumes (Voigt et al., 1997; tant than their parent. We were able to increase the acid-soil resistance Voigt and Mosjidis, 2002). For this paper, results from of Brown Loam white clover. But, the soil-on-agar procedure was not the soil-on-agar procedure will be discussed as Al resisan effective technique for developing acid-soil-resistant white clover tance, results from pot studies as acid-soil resistance. germplasm. As a species, white clover occurs on a wide array of soils that range from calcareous to acid (Snaydon, 1962). Plants from populations growing on acid soils were betF a physiological perspective, a plant’s response ter adapted to these soils than plants from calcareous to stress, for example, temperature extremes or soils (Snaydon and Bradshaw, 1962). Also, plants from drought, can be characterized as either avoidance (the the acid-soil populations were less well adapted to other factor is excluded from the plant tissue) or tolerance soils or to high levels of lime application. Extensive (the factor penetrates the tissue but the tissue survives) variation in Al resistance among 50 white clover culti(Levitt, 1964). Kochian (1995) extended this mechanisvars was reported by Wheeler and Dodd (1995). Solutic terminology to stress from Al using the terms exclution Al activity, at which root yields were reduced by sion, rather than avoidance, and tolerance. Physiologi50%, ranged from 0.7 for ‘Menna’ to 3.0 for ‘Pronitro’. cally, the term resistance is mechanism neutral; that is, Similar variability was observed for herbage yields. Howit implies neither tolerance nor exclusion. In this paper, ever, in soil-based studies where Al(SO4)3 was added we will use the term resistance to characterize the reto a pH 5.0 soil to induce stress, differences were not sponse of white clover to Al and acid-soil stress. detected in acid-soil resistance among white clover cultiWhite clover is relatively resistant to acid-soil stresses, vars (Caradus et al., 1987; Mackay et al., 1990). The but it is not found on some highly acid soils even though experiment did suggest, however, that variation existed it can be abundant on adjacent sites where pH is higher. within white clover cultivars and that selection for acidOn such low-pH soils, initial seedling establishment is soil resistance within otherwise adapted germplasm might especially critical, although vegetative persistence is also be a useful approach to white clover improvement. important, in determining white clover presence or abGenotypes with increased acid-soil resistance and sussence (Voigt, 1997, unpublished data). Toxic levels of ceptibility were identified from Huia white clover (CaraAl are believed to be a major component of much of this dus et al., 1991). Results from a 6 6 diallel cross of acid-soil stress. three resistant and three susceptible genotypes sugThe classical procedure used to determine affects of gested that acid-soil resistance was recessive. DistribuAl on plants is growth in solution culture. When soiltions of acid-soil resistance among the hybrids illustrate the problem of breeding for this character (Caradus and USDA-ARS, Appalachian Farming Systems Research Center, 1224 Mackay, 1995). Regardless of parental classification as Airport Road, Beaver, WV 25813-9423. Received 5 Aug. 2002. *Correresistant or susceptible, most hybrids were susceptible. sponding author ([email protected]). However, crosses between acid-soil-resistant genotypes Published in Crop Sci. 44:38–48 (2004).  Crop Science Society of America Abbreviations: CoVE, coefficient of velocity of root emergence; T50E, time to 50% root emergence. 677 S. Segoe Rd., Madison, WI 53711 USA