Root Physiology of Less Fall Dormant, Winter Hardy Alfalfa Selections

in nondormant, nonhardy cultivars when compared with fall dormant plants (Volenec, 1985; Boyce and Volenec, The physiological mechanisms causing fall dormancy (FD)-induced 1992; Castonguay et al., 1995; Cunningham and Volenec, differences in alfalfa (Medicago sativa L.) shoot growth in autumn and winter hardiness are not understood. The objective of this research 1998). Therefore, accumulation of sugars, and especially was to examine root physiology of experimental germplasms selected RFOs, rather than root total nonstructural carbohydrate for decreased FD that also were selected simultaneously for high (TNC) levels in general, may influence alfalfa winter winter hardiness. Dormant and semi-dormant cultivars and germhardiness. plasms had high root sugar concentrations that were positively associThe impact of root N on growth and stress tolerance ated with winter hardiness. Root amino N and protein levels in Decemof alfalfa has come under study recently (Volenec et ber were greater for germplasms selected for decreased FD and al., 1996). Changes in amino and non-amino N concenincreased winter hardiness than for cultivars with comparable levels trations have been observed during alfalfa cold acclimaof winter hardiness. Among the five most fall dormant cultivars Vernal tion (Bula et al., 1956; Wilding et al., 1960; Hendershot incurred the greatest amount of winter injury and it had lower root and Volenec, 1993). Unlike non-hardy cultivars that amino-N concentrations when compared with three of the other four dormant cultivars and germplasms. Germplasm 98-132 with an interexhibited little change in amino and non-amino N conmediate FD, incurred relatively low winter injury, similar to that of fall centrations between August and December, winter dormant Vernal, when compared with other intermediate dormancy hardy cultivars increased root amino and non-amino N cultivars and germplasms. This germplasm had root sugar concentralevels by 20 and 31%, respectively (Wilding et al., 1960). tions that were similar to plants with FD ratings of 1 to 3. Creation Bula et al. (1956) found a positive correlation between of even less FD germplasms that possess high winter hardiness would increased levels of soluble protein in autumn and enfacilitate our understanding of the physiological and molecular mechahanced winter hardiness of dormant alfalfa cultivars. nisms controlling these two very important agronomic traits of alfalfa. Hendershot and Volenec (1993) reported that soluble amino-N and buffer-soluble protein increased in autumn and early winter, and declined in spring as herbage T extensive genetic and phenotypic variation growth resumed, with nondormant genotypes having found in alfalfa permits its cultivation in diverse decreased amino N and protein concentrations when climates. Fall dormancy has an especially large impact compared with dormant genotypes. Three vegetative on alfalfa adaptation to particular environments bestorage proteins (VSPs) were identified whose concencause of its association with winter survival (McKenzie tration in roots increased markedly during autumn cold et al., 1988). Fall dormant cultivars produce short, prosacclimation. These proteins were extensively utilized as trate shoots in autumn, exhibit slow rates of shoot elonan N source for initial shoot growth in spring and during gation after harvest in summer, and possess high winter shoot regrowth after defoliation in summer (Henderhardiness. In contrast, nondormant alfalfa plants grow shot and Volenec, 1993; Avice et al., 1996; Barber et extensively in autumn producing tall, erect shoots, real., 1996; Cunningham and Volenec, 1998; Noquet et sume rapid shoot elongation after defoliation in spring al., 2001). Roots of fall dormant, winter hardy cultivars and summer, but nondormant cultivars are not winter and populations accumulate more protein in their roots hardy (Zaleski, 1954; Smith, 1961; Busbice and Wilsie, in December when compared with roots of nondormant 1965; Stout and Hall, 1989; Sheaffer et al., 1992). plants that do not survive winter (Li et al., 1996; CunThe specific physiological, biochemical, and molecuningham et al., 1998; 2001). In addition, several coldlar mechanisms causing FD-induced differences in shoot induced polypeptides appear in roots and crown buds growth in autumn and their association with poor winter of winter hardy cultivars during cold acclimation that hardiness is not completely understood. The improved are not present in roots and buds of nondormant plants winter survival exhibited by fall dormant cultivars is that winter kill (Castonguay et al., 1993; Cunningham closely associated with sugar accumulation in roots and et al., 1998). crown buds (Graber et al., 1927; Bula and Smith, 1954; Investigators have identified several alfalfa cold harVolenec et al., 1991; Castonguay et al., 1995; Cunningdiness genes (Mohapatra et al., 1987; Mohapatra et al., ham and Volenec, 1998; Cunningham et al., 2001). In 1989; Wolfraim et al., 1993). Northern blot analyses addition, Castonguay et al. (1995) reported a close assorevealed a positive association between transcript levels ciation of raffinose family oligosaccharide (RFO) accufor these genes and alfalfa FD and winter survival. Almulation with improved alfalfa winter survival. In conthough these differentially regulated transcripts have trast, root and bud starch concentrations were higher been identified in roots of cultivars possessing increased Dep. of Agronomy, Purdue Univ., West Lafayette, IN 47907-1150. Contribution from the Purdue Univ. Agric. Exp. Stn., Journal Series Abbreviations: car, cold acclimation responsive; FD, fall dormancy; No. 16828. Received 22 July 2002. *Corresponding author (jvolenec@ RFO, raffinose family oligosaccharide; SDS-PAGE, sodium dodecylpurdue.edu). sulfate polyacrylamide gel electrophoresis; VSP, vegetative storage protein. Published in Crop Sci. 43:1441–1447 (2003).