Nondormant Alfalfa Research Articles

Salinity and cultivar effects on alfalfa forage yield and nutritive value in a Mediterranean climate

AbstractBackgroundSoil and water salinity are increasing problems worldwide, causing significantly reduced crop yields. Alfalfa (Medicago sativa L.) is often listed as salt‐sensitive, but field testing of improved cultivars is limited. Forage systems and improved high‐quality alfalfa varieties are needed to enable crop production under high salinity (HS) conditions.MethodsThe objective of this study was to measure the yield and quality response of alfalfa to high saline conditions in the field and to document the relative saline tolerance of its varieties. HS irrigation water (electrical conductivity of water, or ECw 8.0–11.0 dS m−1) was applied to 33 nondormant alfalfa cultivars and were compared with low salinity (LS) treatments (ECw 0.5–1.2 dS m−1) over 4 years in a Mediterranean environment on a clay loam soil utilizing a split‐plot design. Crops were harvested seven to eight times per year, and the forage quality was measured on selected harvests utilizing near‐infrared spectroscopy.ResultsThe average yield loss due to HS treatment was 23.9% compared with LS treatment, but yields averaged 23.4 Mg ha−1 under HS over the 3 full years of production. This level of production is considered to be economically viable in this region. Differences in salinity tolerance between lines were identified in the field; individual cultivars lost 5%–35% of their LS yield when grown under HS conditions. Forage quality was significantly improved under HS versus LS conditions, but improvements were negatively correlated with biomass yield (R2 > 0.81), similar to responses observed in drought‐stressed alfalfa.ConclusionsThese yield results confirm greenhouse studies, indicating that alfalfa is highly salt tolerant once established in the field, with potential for further improvement with tolerant cultivars. Salinity tolerance should be chosen based on total biomass yield as well as on the salinity tolerance index (HS yield relative to LS yield). Agronomic practices to mitigate salinity and sodicity are critical, along with improved cultivars.

Release of ‘UF_AlfPers_2015’, a nondormant alfalfa cultivar for the southeastern United States

AbstractThe alfalfa (Medicago sativa L.) cultivar ‘UF_AlfPers_2015’ (Reg. no. CV‐208, PI 702791) was developed by the Agronomy Department, a unit of the Florida Agricultural Experiment Station, University of Florida Institute of Food and Agricultural Sciences (UF‐IFAS), and was released in 2021. The need for high‐yielding and persistent nondormant alfalfa cultivars adapted to the southeastern United States continues to be a factor limiting the adoption and use of alfalfa in the region. A group of 17 alfalfa plants selected from a pasture of ‘Florida 99’ were intercrossed in isolation to produce UF_AlfPers_2015. The experimental line was evaluated for herbage accumulation (HA), persistence, and nutritive value in Florida and other states. UF_AlfPers_2015 had superior HA (20% greater than Florida 99 and 30% than ‘Bulldog 805’ on average) and improved persistence and showed similar nutritive value compared with other cultivars in Florida. UF_AlfPers_2015 had greater HA than ‘Alfagraze 600 RR’ in Gene Autry, OK, and similar HA compared with Bulldog 505 and Bulldog 805 in Gene Autry, OK; Vashti, TX; and Brooksville, MS. UF_AlfPers_2015 is a nondormant (fall dormancy 9) alfalfa cultivar. A US Plant Variety Protection application has been filed, and breeder seed will be maintained by the Agronomy Department of UF‐IFAS.

Evaluation of Different Fall Dormancy-Rating Alfalfa Cultivars for Forage Yield in a Semiarid Environment

Alfalfa is one of the most important, nutritive, and high yielding forage legumes planted across the US. Fall dormancy in alfalfa influences forage yield characteristics and the plants persistence mostly under the cold and temperate climate. The objective of this study was to evaluate alfalfa cultivars with different fall dormancy-ratings for their forage yield at each cut and the annual forage yield. Two sets of 24 alfalfa cultivars were evaluated in a field experiment conducted at the Agricultural Science Center at Farmington, NM. The first set of 24 cultivars was planted late fall 2007 at seeding rate of 22.4 kg ha−1 and managed for the 2007–2011 period and the second set was planted late fall 2009 and managed during the 2009–2013 period. Average forage yield varied with years from 7.6 to 2.9 Mg ha−1, 6.8 to 4.3 Mg ha−1, 9.2 to 4.2 Mg ha−1, and 7.9 to 3.2 Mg ha−1 during the 1st, 2nd, 3rd, and 4th alfalfa cut, respectively. The results showed no statistical differences between the moderately dormant, dormant, and the non-dormant alfalfa cultivars while they showed higher forage yield than the very dormant and semi-dormant alfalfa cultivars. There was a decreasing trend in forage yield from the first cut to the fourth cut in each growing season. However, the very dormant cultivars showed the lowest forage yield. Alfalfa forage yield decreased from the cut 1 to the cut 4 which represented on average 33, 29, 22, and 16% of the annual yield. The semi-dormant cultivars obtained the lowest forage yield at the first and second cutting while there was no difference between the cultivars for the third and fourth harvests. Average forage yields per harvest were 5.7, 5.9, 6.0, 5.5, and 5.9 Mg ha−1 for the very dormant, dormant, moderately dormant, semi-dormant, and non-dormant alfalfa cultivars, respectively. Annual forage yield varied with alfalfa fall dormancy-ratings and ranged from 15.5 to 29.9 Mg ha−1 with the highest forage yield achieved during the third years of the production. The moderately dormant and the non-dormant cultivars showed the highest yield during the first harvest year while the very dormant cultivars and dormant cultivars had the lowest forage yield. Alfalfa cultivars with a fall dormancy range 4–5 may be considered for alfalfa production in northwest New Mexico however, the good agricultural practices (conservation tillage, fertilizer management based on soil residual available nutrient and crop requirement, recommended planting rate, weed and pest management, irrigation scheduling to match crop evapotranspiration) should be the most important to maximize alfalfa forage yield in the southwest US.

QTL mapping of flowering time and biomass yield in tetraploid alfalfa (Medicago sativa L.)

BackgroundThe genetic and genomic basis of flowering time and biomass yield in alfalfa (Medicago sativa L.) remains poorly understood mainly due to the autopolyploid nature of the species and the lack of adequate genomic resources. We constructed linkage maps using genotyping-by-sequencing (GBS) based single dose allele (SDA) SNP and mapped alfalfa timing of flowering (TOF), spring yield (SY), and cumulative summer biomass (CSB) in a pseudo-testcross F1 population derived from a fall dormant (3010) and a non-dormant (CW 1010) cultivars. We analyzed the quantitative trait loci (QTL) to identify conserved genomic regions and detected molecular markers and potential candidate genes associated with the traits to improve alfalfa and provide genomic resources for the future studies.ResultsThis study showed that both fall dormant and non-dormant alfalfa cultivars harbored QTL for early and late flowering, suggesting that flowering time in alfalfa is not an indicator of its fall dormancy (FD) levels. A weak phenotypic correlation between the flowering time and fall dormancy (FD) in F1 and checks also corroborated that alfalfa FD and TOF are not the predictors of one another. The relationship between flowering time and alfalfa biomass yield was not strong, but the non-dormant had relatively more SY than dormant. Therefore, selecting superior alfalfa cultivars that are non-dormant, winter-hardy, and early flowering would allow for an early spring harvest with enhanced biomass. In this study, we found 25 QTL for TOF, 17 for SY and six QTL for CSB. Three TOF related QTL were stable and four TOF QTL were detected in the corresponding genomic locations of the flowering QTL of M. truncatula, an indication of possible evolutionarily conserved regions. The potential candidate genes for the SNP sequences of QTL regions were identified for all three traits and these genes would be potential targets for further molecular studies.ConclusionsThis research showed that variation in alfalfa flowering time after spring green up has no association with dormancy levels. Here we reported QTL, markers, and potential candidate genes associated with spring flowering time and biomass yield of alfalfa, which constitute valuable genomic resources for improving these traits via marker-assisted selection (MAS).

How fall dormancy benefits alfalfa winter-survival? Physiologic and transcriptomic analyses of dormancy process

BackgroundFall dormancy and freezing tolerance characterized as two important phenotypic traits, have great effects on productivity and persistence of alfalfa (Medicago sativa L.). Despite the fact that one of the most limiting traits for alfalfa freezing tolerance in winter is fall dormancy, the interplay between fall dormancy and cold acclimation processes of alfalfa remains largely unknown. We compared the plant regrowth, winter survival, raffinose and amino acids accumulation, and genome-wide differentially expressed genes of fall-dormant cultivar with non-dormant cultivar under cold acclimation.ResultsAveraged over both years, the non-dormant alfalfa exhibited largely rapid regrowth compared with fall dormant alfalfa after last cutting in autumn, but the winter survival rate of fall dormant alfalfa was about 34-fold higher than that of non-dormant alfalfa. The accumulation of raffinose and amino acids were significantly increased in fall dormant alfalfa, whereas were decreased in non-dormant alfalfa under cold acclimation. Expressions of candidate genes encoding raffinose biosynthesis genes were highly up-regulated in fall dormant alfalfa, but down-regulated in non-dormant alfalfa under cold acclimation. In fall dormant alfalfa, there was a significantly down-regulated expression of candidate genes encoding the glutamine synthase, which is indirectly involved in the proline metabolism. A total of eight significantly differentially expressed transcription factors (TFs) related to CBF and ABRE-BFs were identified. The most up-regulated TFs in fall dormant alfalfa cultivar were ABF4 and DREB1C.ConclusionsFall dormant alfalfa drastically increased raffinose and amino acids accumulation under cold acclimation. Raffinose-associated and amino acid-associated genes involved in metabolic pathways were more highly expressed in fall dormant alfalfa than non-dormant alfalfa under cold acclimation. This global survey of transcriptome profiles provides new insights into the interplay between fall dormancy and cold acclimation in alfalfa.

Proteomics reveals key proteins participating in growth difference between fall dormant and non-dormant alfalfa in terminal buds

To explore the molecular mechanism of growth differences between fall dormant (FD) and non-FD alfalfa, we conducted iTRAQ-based quantitative proteomics on terminal buds of Maverick (FD) and Cuf101 (non-FD) cultivars, identified differential abundance protein species (DAPS) and verified expression profiling of certain corresponding mRNA by qRT-PCR. A total of 3872 protein species were annotated. Of the 90 DAPS, 56 and 34 were respectively up- and down-accumulated in Maverick, compared to Cuf101. They were grouped into 35 functional categories and enriched in seven pathways. Of which, auxin polar transport was up-regulated, while phenylpropanoid biosynthesis, pyruvate metabolism and transportation, vitamin B1 synthesis process and flavonoid biosynthesis were down-regulated in Maverick, comparing with Cuf101. In Maverick, mRNA abundances of l-asparaginase, chalcone and stilbene synthase family protein, cinnamyl alcohol dehydrogenase-like protein, thiazole biosynthetic enzyme, pyruvate dehydrogenase E1 beta subunit, and aldo/keto reductase family oxidoreductase were significantly lower at FD than at other stages, and lower than in Cuf101. We also observed opposite mRNA profiles of thiazole biosynthetic enzyme, chalcone and stilbene synthase family protein, pyruvate dehydrogenase E1 beta subunit in both cultivars from summer to autumn. Our results suggest that these DAPS could play important roles in growth difference between FD and non-FD alfalfa. Biological significanceUp to now, as far as we know, currently the proteins related with the growth differences between FD and non-FD alfalfa cultivars in autumn have not yet been identified in terminal buds. This study identified the protein species expressed in alfalfa terminal buds, selected differentially abundant protein species in terminal buds between Maverick (FD) and Cuf101 (non-FD) cultivars in autumn and identified the important protein species participated in the growth differences. This study lays a foundation for further investigation of the molecular mechanism of the growth differences between FD and non-FD alfalfa and the cultivation of advanced alfalfa cultivars.

De novo characterization of fall dormant and nondormant alfalfa (Medicago sativa L.) leaf transcriptome and identification of candidate genes related to fall dormancy.

Alfalfa (Medicago sativa L.) is one of the most widely cultivated perennial forage legumes worldwide. Fall dormancy is an adaptive character related to the biomass production and winter survival in alfalfa. The physiological, biochemical and molecular mechanisms causing fall dormancy and the related genes have not been well studied. In this study, we sequenced two standard varieties of alfalfa (dormant and non-dormant) at two time points and generated approximately 160 million high quality paired-end sequence reads using sequencing by synthesis (SBS) technology. The de novo transcriptome assembly generated a set of 192,875 transcripts with an average length of 856 bp representing about 165.1 Mb of the alfalfa leaf transcriptome. After assembly, 111,062 (57.6%) transcripts were annotated against the NCBI non-redundant database. A total of 30,165 (15.6%) transcripts were mapped to 323 Kyoto Encyclopedia of Genes and Genomes pathways. We also identified 41,973 simple sequence repeats, which can be used to generate markers for alfalfa, and 1,541 transcription factors were identified across 1,350 transcripts. Gene expression between dormant and non-dormant alfalfa at different time points were performed, and we identified several differentially expressed genes potentially related to fall dormancy. The Gene Ontology and pathways information were also identified. We sequenced and assembled the leaf transcriptome of alfalfa related to fall dormancy, and also identified some genes of interest involved in the fall dormancy mechanism. Thus, our research focused on studying fall dormancy in alfalfa through transcriptome sequencing. The sequencing and gene expression data generated in this study may be used further to elucidate the complete mechanisms governing fall dormancy in alfalfa.

Applicability of Predictive Equations for Alfalfa Quality to Southwestern United States and Northern Mexico

ABSTRACTModels to estimate alfalfa (Medicago sativa L.) neutral detergent fiber (NDF) and acid detergent fiber (ADF) concentrations using easily available variables were developed and tested in northern, humid regions of the United States but have not been validated in arid areas below 35° N latitude. The objective of this research was to test the performance of predictive equations for alfalfa quality (PEAQ) on nondormant alfalfa grown in irrigated desert regions in New Mexico and northern Mexico. Alfalfa with fall dormancy ratings of 8 and 9 were sampled over 1 or 2 yr at three irrigated locations. Five alfalfa samples were taken per field on each sample date between 0730 and 0830 h, leaving a stubble of approximately 4 cm. Observed NDF and ADF values were regressed on predicted values yielding r2 and root mean square error values in the range of those previously reported for alfalfa grown in northern states. The accuracy of predictions was improved by using field means (n = 5) rather than individual samples. In most cases regression equations were biased (slope ≠ 1 and/or intercept ≠ 0), but the magnitude of bias was relatively small. The index of agreement (d) was greater than 0.86 for all data, indicating an acceptable goodness of fit to predict NDF and ADF concentration using PEAQ. It is concluded that PEAQ can predict, with reasonable accuracy, fiber concentrations in alfalfa grown in desert regions of the southwestern United States and northern Mexico.

Deep sequencing of the microRNA expression in fall dormant and non-dormant alfalfa.

MicroRNAs (miRNAs) play a critical role in post-transcriptional gene regulation that down-regulates target genes by mRNA degradation or translational repression. Evidence is increasing for their crucial roles during plant development. Identification of miRNAs at the global genome-level by high-throughput sequencing is essential to functionally characterize miRNAs in plants. Alfalfa (Medicago sativa L.) is one of the most widely cultivated perennial forage legumes worldwide. Fall dormancy is an adaptive character related to the biomass production and winter survival in alfalfa. However, little is known about miRNA-mediated developmental regulation of fall dormancy in alfalfa. Here, we provide detailed experimental methods and analysis pipeline in our study to identify miRNAs that were responsive to fall dormancy (Fan W et al., Genome-wide identification of different dormant Medicago sativa L. microRNAs in response to fall dormancy, submitted for publication) for reproducible research. The data generated in our work provide meaningful information for understanding the roles of miRNAs in response to seasonal change and growth regulation in alfalfa.

Transcriptome profiling of gene expression in fall dormant and nondormant alfalfa.

Fall dormancy (FD) is an adaptive trait in alfalfa (Medicago sativa L.). It appeared in the regrowth process in late summer or early autumn after alfalfa was harvested. FD affects the biomass accumulation and winter survival in high latitude area. However, the molecular mechanism under FD is still not clear at present. Performing RNA-seq of fall dormant and nondormant alfalfa varieties at different time points, we obtained differentially expressed genes between different FD types or time points. These differentially expressed genes may relate to FD in alfalfa. Here, we provide detailed experimental methods and analysis pipeline in our study (Zhang S et al., De novo Characterization of Fall Dormant and Nondormant Alfalfa (Medicago sativa L.) Transcriptome and Identification of Candidate Genes Relate to Fall Dormancy, submitted for publication) for reproducible research. Data generated in our work provide a resource to help decipher the molecular mechanism of FD in alfalfa.

Development of an alfalfa SNP array and its use to evaluate patterns of population structure and linkage disequilibrium.

A large set of genome-wide markers and a high-throughput genotyping platform can facilitate the genetic dissection of complex traits and accelerate molecular breeding applications. Previously, we identified about 0.9 million SNP markers by sequencing transcriptomes of 27 diverse alfalfa genotypes. From this SNP set, we developed an Illumina Infinium array containing 9,277 SNPs. Using this array, we genotyped 280 diverse alfalfa genotypes and several genotypes from related species. About 81% (7,476) of the SNPs met the criteria for quality control and showed polymorphisms. The alfalfa SNP array also showed a high level of transferability for several closely related Medicago species. Principal component analysis and model-based clustering showed clear population structure corresponding to subspecies and ploidy levels. Within cultivated tetraploid alfalfa, genotypes from dormant and nondormant cultivars were largely assigned to different clusters; genotypes from semidormant cultivars were split between the groups. The extent of linkage disequilibrium (LD) across all genotypes rapidly decayed to 26 Kbp at r2 = 0.2, but the rate varied across ploidy levels and subspecies. A high level of consistency in LD was found between and within the two subpopulations of cultivated dormant and nondormant alfalfa suggesting that genome-wide association studies (GWAS) and genomic selection (GS) could be conducted using alfalfa genotypes from throughout the fall dormancy spectrum. However, the relatively low LD levels would require a large number of markers to fully saturate the genome.

Living Mulch Nutritive Value in a Corn‐Soybean‐Forage Rotation

Living mulches can function to supply forage in multifunctional cropping systems. Information quantifying nutritive value of forage plants in living mulch cropping systems is limited. The objective of this research was to quantify the nutritive value of forages from different plant functional groups managed as living mulches in 2 of 3 yr and as a forage crop in the third year of this 3‐yr corn (Zea mays L.)‐soybean [Glycine max (L.) Merr.]‐forage rotation. Alfalfa (Medicago sativa L.), Kura clover (Trifolium ambiguum M. Bieb.), and birdsfoot trefoil (Lotus corniculatus L.) were evaluated in sole seedings and binary mixtures, reed canarygrass (Phalaris arundinacea L.) and orchardgrass (Dactylis glomerata L.) were included in three‐way mixtures, and a nondormant alfalfa was seeded each spring for comparison as a control. Average crude protein (CP) and neutral detergent fiber (NDF) during 2005, 2006, and 2007 was 190 and 449 g kg−1 for Kura + alfalfa + reed canarygrass compared with 182 and 458 g kg−1 averaged across birdsfoot trefoil + alfalfa + orchardgrass and Kura + alfalfa + orchardgrass mixtures, 195 and 397 g kg−1 averaged across alfalfa, Kura + alfalfa, and alfalfa + birdsfoot trefoil, and 193 and 404 g kg−1 for the nondormant alfalfa treatment. Weed densities averaged 104, 178, and 116 weeds m−2 in 2005, 2006, and 2007 among living mulch treatments compared with 14, 84, and 67 weeds m−2 in the nondormant alfalfa. Despite high weed densities in the living mulch treatments, no specific treatment effect was detected for nutritive value. All of these living mulch treatments produced high nutritive value forage.

Living Mulch Forage Yield and Botanical Composition in a Corn‐Soybean‐Forage Rotation

Managing forages as living mulches during row crop production requires suppressing the forages to produce economical crop yields. The objective of this research was to identify forage plants with varied growth habit, persistence, and yield potential to provide desirable ecosystem functions in a multifunctional cropping system. Alfalfa (Medicago sativa L.), kura clover (Trifolium ambiguum Bieb.), and birdsfoot trefoil (Lotus corniculatus L.) were evaluated in sole seedings, binary mixtures, and reed canarygrass (Phalaris arundinacea L.) and orchardgrass (Dactylis glomerata L.) were included in three‐way mixtures in a corn (Zea mays L.)–soybean [Glycine max (L.) Merr.]–forage rotation. The forages functioned as cover crops in corn and soybean and were suppressed using a 25‐cm glyphosate band over the row. In the forage year, treatments containing alfalfa produced the highest yields (7824 kg ha−1 average dry matter 2005–2007). Three‐way mixtures provided greater weed suppression in the interrow than sole seedings or binary mixtures in 2 of 3 yr (80 vs. 127 weeds m−2, 2005–2006 average). Kura clover provided greater weed suppression than alfalfa in the former row (108 vs. 186 weeds m−2). Reed canarygrass exhibited better stability in sward composition (20% in 2005 and 22% in 2007) than orchardgrass (58% in 2005 and 16% in 2007). Consequently, the best combination of species to use as living mulches to provide high forage yields and lower weed densities includes alfalfa, kura clover, and reed canarygrass. Seeding a nondormant alfalfa in the spring of the forage year will supplement yield and suppress weeds in the former crop row.

Year‐round Irrigation and Fall Dormancy Affects Alfalfa Yield in a Semiarid, Subtropical Environment

Fall dormancy (FD) group selection and irrigation management may be valuable tools for increasing alfalfa (Medicago sativa L.) yield in semiarid, subtropical regions. Dry matter (DM) yields of moderately dormant, semidormant, moderately nondormant, and nondormant alfalfa as subplots (each represented by four varieties) were compared for four years under two irrigation regimes (furrow-irrigated once before each harvest or 11 times throughout the year) as whole plots in the Southern High Plains of the USA. Variety, year, and harvest effects also were tested. There were four randomized complete blocks. Year-round irrigation increased (P < 0.05) first harvest yield in 10 of 16 comparisons by > 0.39 Mg/ha, affecting each FD grouping in at least 2 of 4 years. Warmer spring temperatures magnified the difference. Typical dormancy effects on yield were observed in late summer and autumn, but they were inverted in spring and early summer. Differences existed between FD groups in average annual production over the four years of the study (17.65, 18.66, 19.62, and 18.89 Mg/ha for moderately dormant, semidormant, moderately nondormant, and nondormant, respectively; 5% LSD = 0.95). Year-round irrigation may have value during a warm winter when continued warm temperatures are forecast throughout spring.

Improving Winter Hardiness in Nondormant Alfalfa Germplasm

Alfalfa (Medicago sativa L.) adapted to the midwestern USA typically becomes dormant in autumn to survive winter stress. Nondormant alfalfa germplasm produces higher biomass yield during autumn but usually dies during the winter. The objective of this study was to test the hypothesis that recurrent selection could decrease winter injury in nondormant alfalfa populations. Between 1997 and 2000, three cycles of selection were conducted at Ames, IA, in four nondormant cultivars (5939, GT13R+, CUF101, and Magna 8) by recurrently selecting the most vigorous plants in the spring following establishment. In May 2001, the original cultivars and the three cycles of selection (C0, C1, C2, and C3) of each cultivar were transplanted with six check cultivars in the field at Ames, IA, and West Lafayette, IN. Biomass yield was measured in September and November 2001 at both locations. In November 2001, half of the plants in each plot were removed and roots were analyzed for total sugar, starch, and protein concentrations. The remaining plants were scored for winter injury in April 2002. Strong negative linear trends were observed across cycles for both winter injury and autumn plant height. In contrast, forage biomass yields showed little change. Root total sugar concentration in November increased with cycle of selection, but root starch, protein, and amino acid concentrations remained constant. Little change was noted among cycles for forage nutritive value. Selection based solely on winter injury can improve winter hardiness of nondormant populations. A concomitant decrease in autumn plant height may not affect autumn yield.

Understanding mechanisms of reduced annual weed emergence in alfalfa

Field experiments were carried out at the Facultad de Agronomía, Universidad de Buenos Aires, Argentina (34°25′S, 58°25′W), to evaluate the possibility of reducing weed seedling emergence through the use of alfalfa cultivars with low levels of winter dormancy and by increasing plant density from 200 to 400 plants m−2. It was hypothesized that these treatments would alter the temperature regime and the red (R)–far-red (FR) ratio of radiation to which seeds were exposed. Responses to management treatments were recorded for bull thistle, cotton thistle, plumeless thistle, tall rocket, mustard, curly dock, and pigweed. During the alfalfa establishment year, pigweed and curly dock emergence was reduced by the nondormant cultivar established at high density. This reduction disappeared when soil beneath the canopy was fitted with heaters that mimicked bare-soil temperatures. Crop canopy presence during the establishment year was not effective in reducing mustard, cotton thistle, bull thistle, plumeless thistle, and tall rocket emergence. During the second and third years after crop establishment, the canopy of the nondormant alfalfa cultivar was effective in reducing germination of weed seeds placed on the soil surface during fall and winter. In contrast, the winter-dormant cultivar allowed the establishment of weeds during the winter period. These reductions in weed emergence were associated with a modification in the R–FR ratio perceived by the seeds located at the soil surface and could largely be removed by using FR filters to increase the R–FR ratio. These results suggest that the selection of a nondormant cultivar combined with an increase in plant density could effectively reduce weed populations in alfalfa.

Comparative Chromosome Banding Studies of Nondormant Alfalfa Germplasm

A cytogenetic investigation was conducted on four historically putatively distinct nondormant alfalfa germplasm sources, African, Chilean, Peruvian, and Indian tetraploid alfalfa [Medicago sativa ssp. sativa (L.) L. &amp; L.; 2n = 4x = 32]. C‐banding, image analysis, and cluster analysis was used to test the hypothesis that chromosome structure differed among the four nondormant alfalfa populations. Cytogenetic analyses revealed polymorphisms for heterochromatic DNA in the number and location of constitutive heterochromatic DNA both within and among genotypes. However, this variability did not prevent recognition of homologous chromosomes. Karyotypes of Peruvian and Indian populations were developed. The reference African population was used to compare the karyotypes of Peruvian and Indian populations as well as the previously published Chilean population. In general, the number of heterochromatic DNA bands was similar for the African, Chilean, and Peruvian populations; however, the Indian population had significantly fewer heterochromatic bands than the other three. Cluster analysis based on all eight chromosomes yielded no clear separation of the nondormant alfalfa populations possibly because of the lack of chromosomal rearrangements, similar genetic backgrounds of the initial introductions, intercrossing of the different sources, genetic drift during maintenance, and/or common genetic backgrounds of the original parental germplasm sources.

Raffinose and Stachyose Accumulation, Galactinol Synthase Expression, and Winter Injury of Contrasting Alfalfa Germplasms

Large differences in winter hardiness exist among alfalfa (Medicago saliva L.) cultivars, but the physiological and molecular bases for these differences are not understood. Our objective was to determine how raffinose family oligosaccharide (RFO) accumulation and steady state mRNA levels for galactinol synthase (GaS) in roots relate to genetic variation in alfalfa winter survival. A GaS cDNA was isolated that possesses over 70% identity with GaS clones from other plant species. Induction of GaS transcripts in crowns of winter hardy alfalfa cultivars occurred within 8 h of exposure to 2°C, and was intensified by exposing plants to -2°C for 2 wk. Galactinol synthase transcripts increased in November in crown and root tissues of winter hardy alfalfa plants. This increase was accompanied by large increases in root RFO concentrations between October and December. A close positive association between RFO accumulation in roots in December and genetic differences in winter survival was observed in these alfalfa populations. Although roots and crowns of nondormant alfalfa cultivars accumulated both GaS transcripts and RFO, accumulation was delayed until December and these cultivars did not survive winter. Understanding the mechanisms regulating GaS gene expression and subsequent RFO accumulation in roots and crowns provides opportunity to genetically improve alfalfa winter hardiness.

Growth, sugar accumulation, and dark respiration of suspension cell cultures derived from contrasting alfalfa cultivars

Fall dormancy results in decumbent, slow shoot growth of alfalfa (Medicago sativa L.) in autumn and reduced shoot regrowth rates after herbage removal in summer. Although fall dormancy is used to predict alfalfa adaptation, we possess a poor understanding of the biological mechanisms underlying fall dormancy. Our objective was to examine growth and carbohydrate metabolism of suspension cell cultures derived from contrasting alfalfa cultivars that genetically differed in fall dormancy. Suspension cells were grown in B5h media containing 2% sucrose. Cells derived from fall non-dormant plants accumulated sugars more rapidly after transfer to fresh media and to higher concentrations than did cells derived from fall dormant alfalfa cultivars. Dark respiration rates of cells derived from non-dormant plants were similar to those derived from fall dormant plants when growth was limited at low cell sugar concentrations. However, both cell growth and dark respiration rates increased in cells derived from non-dormant cultivars in response to greater cell sugar concentrations. High growth rates of cells derived from rapid growing, fall non-dormant alfalfa cultivars were associated with rapid sugar uptake and higher cell respiration rates when compared to cells derived from dormant alfalfa cultivars.

Raffinose and Stachyose Accumulation, Galactinol Synthase Expression, and Winter Injury of Contrasting Alfalfa Germplasms

Large differences in winter hardiness exist among alfalfa (Medicago sativa L.) cultivars, but the physiological and molecular bases for these differences are not understood. Our objective was to determine how raffinose family oligosaccharide (RFO) accumulation and steady state mRNA levels for galactinol synthase (GaS) in roots relate to genetic variation in alfalfa winter survival. A GaS cDNA was isolated that possesses over 70% identity with GaS clones from other plant species. Induction of GaS transcripts in crowns of winter hardy alfalfa cultivars occurred within 8 h of exposure to 2°C, and was intensified by exposing plants to −2°C for 2 wk. Galactinol synthase transcripts increased in November in crown and root tissues of winter hardy alfalfa plants. This increase was accompanied by large increases in root RFO concentrations between October and December. A close positive association between RFO accumulation in roots in December and genetic differences in winter survival was observed in these alfalfa populations. Although roots and crowns of nondormant alfalfa cultivars accumulated both GaS transcripts and RFO, accumulation was delayed until December and these cultivars did not survive winter. Understanding the mechanisms regulating GaS gene expression and subsequent RFO accumulation in roots and crowns provides opportunity to genetically improve alfalfa winter hardiness.