Guelph Millennium Research Articles

Transcriptomic analysis of Asparagus officinalis cultivars with varying levels of freezing tolerance over fall acclimation and spring deacclimation periods.

Asparagus (Asparagus officinalis L.) is an important vegetable crop in southern Ontario, Canada, where winter air and soil temperatures below 0°C are common. Consequently, cultivars growing in this area must possess winterhardiness and freezing tolerance for survival. Asparagus acquires freezing tolerance in the fall through cold acclimation and loses freezing tolerance in the spring through deacclimation. To understand the molecular bases of these processes, transcriptomic analysis (RNA-Seq) was conducted on two cultivars, one adapted, 'Guelph Millennium' (GM), and one unadapted, 'UC157' (UC), to the winter conditions of southern Ontario. RNA extracted from bud and rhizome tissues, sampled on three dates during early spring and late fall, was subjected to sequencing. In the fall, the numbers of differentially expressed (DE) genes at the second and third harvests increased, relative to the first harvest, in dormant buds and rhizomes as freezing tolerance of cultivars increased, and the majority of DE genes were downregulated. In spring, freezing tolerance decreased as plants deacclimated and most genes DE at second and third harvests were upregulated in both cultivars. GM had lower LT50 (lethal temperature at which 50% of plants die) values and hence higher freezing tolerance than UC on specific sampling dates during both spring and fall, and expression patterns of specific genes were correlated with LT50 differences. Functional analysis revealed that these genes were involved in carbohydrate metabolic process, plant hormone signal transduction (auxin and gibberellin), proline metabolism, biosynthesis of secondary metabolites, circadian rhythm, and late embryogenesis abundant proteins and could be associated with cold acclimation and deacclimation processes. These findings will help researchers understand the molecular mechanisms of freezing tolerance in asparagus, leading to breeding and genetic strategies to improve the trait.

Asparagus (Asparagus officinalis L.) Root Distribution: Cultivar Differences in Mature Plantings

Annual plant growth patterns and seasonal conditions have both been shown to influence asparagus (Asparagus officinalis L.) root development over time. Root biomass and distribution changes in mature asparagus cultivars are herein illustrated and described. Asparagus root length density and biomass were estimated from soil cores using a systematic field sampling approach each spring. Soil cores (0.9 m deep) were divided into 0.15 m lengths and fleshy roots collected from the soil. Root length density and dry weights were determined and root distribution maps generated from collected data. As asparagus plantings matured, the sampling year had a significant influence on root development. Fleshy roots grew deeper into the soil each year but the majority of roots of Atlas, Guelph Millennium, and Jersey Giant were found in the upper 60 cm of the soil profile. For the three cultivars evaluated, minor differences in root length and root weight occurred. By Year 6, Atlas showed a decrease in root length and weight when compared to Guelph Millennium and Jersey Giant. While spear yield differences between the varieties were not significant, Atlas tended to produce more very large and large spears compared to Guelph Millennium and Jersey Giant. These results increase our understanding of asparagus root development.

Asparagus (Asparagus officinalis L.) Root Distribution Varies with Cultivar during Early Establishment Years

Soil type, crop management practices, annual plant growth patterns, and seasonal changes have all been shown to influence asparagus (Asparagus officinalis) roots. This study describes the changes in root growth that occur over the three establishment seasons of three asparagus cultivars. Starting one year after planting, asparagus root length density and biomass were estimated from soil cores (55 mm diam. × 0.2 m long) collected to 0.9 m at three locations adjacent to the row (0.15, 0.3, and 0.6 m from row center). Samples were collected each spring during the spear harvest period (late April to early May). Soil cores were divided into 0.15 m lengths and fleshy roots were collected for the soil, root length density determined, and dry weights measured. The year of sampling had a significant effect on root development and, as time progressed, fleshy asparagus roots grew deeper into the soil. There was no difference in fleshy root length or fresh weight between the three cultivars evaluated. However, root distribution patterns varied between the cultivars. Fleshy roots for the cultivars Atlas and Jersey Giant extended further from the crown and deeper in the soil when compared to Guelph Millennium. Results can be used to improve crop management practices and increase our understanding of the dynamic changes of root development that occur over time in asparagus.

The role of drought stress on the acquisition of freezing tolerance in asparagus under controlled conditions

Cold acclimation induces freezing tolerance in asparagus ( Asparagus officinalis L.), which is important for survival and longevity in temperate climates. Previous studies indicated that cultivar “Guelph Millennium” (GM), adapted to Southern Ontario, acquired freezing tolerance earlier in the fall and maintained it later in the spring than “UC157” (UC), a cultivar commonly grown in desert climates. As the drought and cold acclimation metabolic pathways overlap, the objective of this research was to determine whether drought alone induced freezing tolerance or interacted synergistically with cold temperatures. Seedlings of GM and UC were subjected to control, drought, or cold acclimating conditions for 6 weeks. Additional treatments included 6 weeks of drought or control conditions followed by 6 weeks of cold acclimation, and plants grown for 12 weeks under control conditions. LT50, the temperature at which 50% of plants die, and metabolites or parameters associated with freezing tolerance were measured. GM had decreased LT50 levels (increased freezing tolerance) compared with UC under both drought and cold acclimating conditions, and no cultivar differences were observed under control conditions. Drought stress before cold acclimation resulted in an increased LT50 (reduced freezing tolerance) for UC compared with GM. A high root:shoot ratio and low crown water percentage were associated with increased freezing tolerance in GM, while a high crown sucrose concentration in UC was related to decreased freezing tolerance for the drought treatment. Overall, drought stress induced cultivar-specific adaptations causing differences in freezing tolerance, thus confirming the interrelationship between these two physiological pathways.

Effects of decreasing photoperiod on cold acclimation of asparagus seedlings

Cold acclimation is a vital process for surviving subzero temperatures in asparagus ( Asparagus officinalis L.). A winter hardy cultivar, “Guelph Millennium” (GM), cold acclimates, senesces, and develops freezing tolerance earlier in the fall compared to “UC157” (UC), a cultivar adapted to warm climates. Decreasing photoperiod can induce timely senescence and freezing tolerance in both woody and herbaceous perennials, although its effects on asparagus are unclear. The objectives of this research were to study the effects of daylength and cold, and their interaction, on the induction of freezing tolerance in asparagus. Seedlings of GM and UC were subjected to four treatments: constant (15.5 h) or decreasing (15.5–14 h) photoperiods with either high (23 °C) or low (11 °C) temperatures. Decreasing daylengths did not impact the acquisition of freezing tolerance at either temperature. For UC, low temperature accompanied by decreasing photoperiod diminished root:shoot ratio and increased crown water percentage, which are both indicators of reduced dormancy. A decreasing photoperiod and high temperature increased crown fructan (total and high-molecular weight) and decreased glucose and proline concentrations in GM compared to UC. These results suggest that decreasing daylengths, similar to those occurring in late summer in Southern Ontario, may be a signal for crown metabolite partitioning. This effect may be a contributing factor to the timely senescence and cold acclimation seen in GM compared to UC.

Managing drought risk in a changing climate: Irrigation and cultivar impacts on Michigan asparagus

Increasing temperatures and rainfall variability in the Midwestern U.S. have spurred interest in strategies to reduce risks of heat and drought stress in traditionally rainfed crops including asparagus. A long-term field experiment was conducted on sandy soils in Western Michigan from 2010-17 to evaluate the effects of three levels of irrigation (none, overhead or sub-surface drip) and two asparagus cultivars (Guelph Millennium [GM] and Jersey Supreme [JS]) for reducing these risks. Overhead irrigation during the fern growth period resulted in cumulative yield improvements of 10% for GM during the 2012-17 growing seasons, with the largest yield benefits (21%) occurring in 2012, following hot, dry conditions the previous summer. In contrast, cumulative yields of JS were unaffected by irrigation, and yield reductions of 13% due to irrigation were observed in 2017, following wet conditions the previous late summer and fall. Estimates of cultivar water-use by depth suggest that JS was better able to tolerate drought due to a deeper root system compared to GM. However, our results suggest that JS may also be more sensitive than GM to excessive soil moisture during fall senescence. Yield response did not vary with delivery system, but sub-surface drip used less water than overhead irrigation. These results demonstrate the important role of both genetics and management practices in mitigating drought risk.

Management of Stemphylium leaf spot (Stemphylium vesicarium) and rust (Puccinia asparagi) of asparagus (Asparagus officinalis) with cultivar selection and fungicides

Stemphylium leaf spot (SLS) and rust are foliar diseases of asparagus fern. If left unmanaged, both SLS and rust can contribute to early decline of an asparagus field and a reduction in yield in subsequent seasons. Currently, growers rely on protectant foliar fungicides and cultivar selection to control foliar disease in asparagus in Canada. However, it is unclear which new cultivars are less susceptible to foliar disease and if any new fungicides might provide adequate control. The objectives of this research were to: (1) assess cultivars and experimental breeding lines for sensitivity to foliar disease, and (2) evaluate fungicides for control of foliar disease in asparagus. A cultivar trial was established in 2011 to compare six cultivars and lines. Also in 2011, thirteen fungicides and combinations of fungicides were assessed for control of foliar disease in ‘Guelph Millennium’. In both studies, the fern was assessed for symptoms of SLS and rust. ‘Jersey Giant’ and ‘Guelph Equinox’ and lines UG010 and UG023 had lower rust severity (as measured as standardized area under the disease progress curve, sAUDPC) than ‘Guelph Millennium’ and ‘Tiessen’. ‘Jersey Giant’ also had lower SLS disease severity index (DSI) and sAUDPC than ‘Guelph Millennium’ and ‘Guelph Equinox’ and the line UG023. There was a negative correlation between SLS and rust for both sAUDPC and DSI assessments. In the fungicide screening trials, differences were observed in lesion counts and sAUDPC. In a 3-year-old field, azoxystrobin/cyproconazole, azoxystrobin/difenoconazole, benzovindiflupyr, cyproconazole, tebuconazole, and the tank-mixture of mineral oil + pigment + propiconazole had lower sAUDPC than the untreated control. In the 4-year-old field, only azoxystrobin/cyproconazole, azoxystrobin/difenoconazole, and cyproconazole had lower sAUDPC than the untreated control. Further research is warranted to investigate rust management programs with fungicides on slow-rusting asparagus cultivars.

Evaluation of the TOM-CAST Forecasting Model in Asparagus for Management of Stemphylium Leaf Spot in Ontario, Canada

In the last 5 years, asparagus acreage in Canada has increased by over 25%. Stemphylium leaf spot, caused by Stemphylium vesicarium, has emerged as the predominant foliar pathogen of asparagus. Typically, contact fungicides are applied every 14 days; however, regardless of the number of applications, growers are not achieving adequate control of the disease. The TOM-CAST forecasting model is used widely in Michigan asparagus fields, but it has never been assessed for suitability in Ontario or in the popular cultivar, Guelph Millennium. Six field trials were conducted in 2012 and 2013 to evaluate the TOM-CAST forecasting model in two asparagus cultivars. The fungicides chlorothalonil or azoxystrobin/difenoconazole were applied according to the forecasting model or on a 14-day interval. The effectiveness of the forecasting model differed between sites and cultivars. Even though TOM-CAST is used in all cultivars in Michigan, TOM-CAST was not effective on Guelph Millennium. In the cultivar Jersey Giant, however, TOM-CAST with a 20 disease severity value spray interval improved control of Stemphylium leaf spot without increasing the number of sprays, compared with a 14-day treatment. The results in Guelph Millennium differed between sites. At one site, TOM-CAST maintained similar levels of Stemphylium leaf spot, but increased the number of applications, compared with a 14-day application interval. Of more concern, none of the fungicide treatments differed greatly from the untreated control at the other site. Our results show that forecasting models need to be validated locally in asparagus cultivars relevant to production today.

Asparagus cultivars with varying adaptation to southern Ontario differ for induction of freezing tolerance in the fall

Asparagus (Asparagus officinalis L.) cultivars differ for adaptation in southern Ontario, and delayed or decreased acquisition of freezing tolerance in the fall could explain, in part, the diminished longevity observed in some germplasm. A field study was conducted to determine the relationship between LT50, the lethal temperature at which 50% of plants die, and physiological parameters related to freezing tolerance, in three cultivars with varying adaptation in southern Ontario: Guelph Millennium (GM) > Jersey Giant (JG) > UC 157 (UC). The experiment was replicated at two sites in one location, in each of two years. LT50 values for GM were lower (increased freezing tolerance) than those for UC in early October; levels for JG were intermediate. In late-October and early-November, the cultivars did not differ. Increased freezing tolerance was associated with high low-molecular-weight fructan (LF), protein and proline concentrations and low sucrose concentration in the rhizome, and high sucrose and proline concentrations and low LF concentration in the storage roots. Acclimation traits were generally consistent over years and deviations may have been related to differing temperature profiles. Results indicate that winter survival of asparagus is in part determined by timely fall acclimation allowing plants to survive exposure to early frosts.

Freezing Tolerance Attributes during Spring Deacclimation for Three Asparagus Cultivars with Varying Adaptation to Southern Ontario

Winterhardiness in asparagus (Asparagus officinalis) may be related to proper cold acclimation and induction of freezing tolerance in the fall, levels and maintenance of freezing tolerance in the winter, and the timing of deacclimation in the spring. Premature deacclimation and the inability to reacclimate could result in crown damage from spring freeze-thaw cycles. A field experiment was conducted, replicated over 2 years, to determine how three cultivars with varying adaptation to southern Ontario deacclimate in the spring by assessing LT50 (the temperature at which 50% of plants die) and biochemical and physiological parameters associated with freezing tolerance. ‘UC 157’ (UC), the least-adapted cultivar, deacclimated after soil temperatures rose above freezing; LT50 values increased linearly over time and were unaffected by fluctuations in soil temperature. ‘Jersey Giant’ (JG), a cultivar with moderate adaptation, rapidly deacclimated with increased soil temperature but appeared to partially reacclimate as temperatures decreased. For ‘Guelph Millennium’ (GM), the most-adapted cultivar, LT50 values did not change, maintaining the greatest levels of freezing tolerance during the spring sampling period. Although LT50 values did not differ among cultivars on the first spring sampling date, ranking for freezing tolerance at the final sampling in each year was GM>JG>UC, which is consistent with adaptation. Rhizome traits were most associated with freezing tolerance and included high concentrations of low-molecular-weight fructans (LFs), glucose, and proline and low percentage water and sucrose concentration. Overall, data suggest that the timing of deacclimation and loss of freezing tolerance in the spring may significantly affect winterhardiness; cultivars that lose freezing tolerance early and cannot reacclimate could suffer most from late spring freeze-thaw cycles.

Freezing tolerance assessment for seedlings of three asparagus cultivars grown under controlled conditions

Kim, J. and Wolyn, D. J. 2015. Freezing tolerance assessment for seedlings of three asparagus cultivars grown under controlled conditions. Can. J. Plant Sci. 95: 495–504. Asparagus (Asparagus officinalis L.) cultivars grown in southern Ontario must be winter-hardy. Development of a method to screen seedlings for freezing tolerance directly, or indirectly through metabolite analysis, could be useful in a breeding program. Ten-week-old seedlings of three cultivars with varying adaptation to southern Ontario, Guelph Millennium (GM), Jersey Giant (JG) and UC157 (UC), were acclimated under factorial combinations of two temperatures (7 or 23°C) and two photoperiods (8 and 16 h), with or without 5 additional days of sub-freezing acclimation at 3/−3°C (12/12 h) in darkness. Plants were then evaluated for metabolites and LT50, the temperature at which 50% of plants die. Photoperiod had no effect, but low temperature without sub-freezing acclimation decreased LT50 (increased freezing tolerance) of all three cultiva...

ANALYSIS OF COLD ACCLIMATIZATION IN TWO ASPARAGUS CULTIVARS WITH VARYING PATTERNS OF FALL SENESCENCE

Adequate winter hardiness is paramount for optimal performance of Asparagus officinalis L. cultivars in Southern Ontario, Canada, and may be influenced by fall senescence. Cultivar 'Guelph Millennium' (GM) turns yellow by mid-October prior to 'Jersey Giant' (JG), which will often 'freeze green'. Timely fall senescence may contribute to freezing tolerance and result in the enhanced stand longevity and yield observed for GM compared to JG. In order to understand the relationship between fall senescence and cold acclimation, a field experiment was conducted to assess phenological parameters in plant tissues through the late summer and fall. Once photoperiod and night temperature decreased below 12 h and 10°C, respectively, both cultivars showed increased storage root LT50 values as well as carbohydrate, nitrogen and proline concentrations, while fern chlorophyll and crown water concentrations decreased. Cultivar differences were found for some traits; GM initiated nitrogen and proline accumulation in the roots and chlorophyll catabolism in the fern two weeks prior to JG. Freezing tolerance of storage roots, however, was not significantly different between cultivars. LT50 values were correlated with carbohydrate concentrations and percent water, but not chlorophyll concentration or percent nitrogen. A greater percent of storage roots nitrogen in GM than JG through the fall could help explain the stand longevity of GM.

A method to assess cold acclimation and freezing tolerance in asparagus seedlings

Landry, E. J. and Wolyn, D. J. 2012. A method to assess cold acclimation and freezing tolerance in asparagus seedlings. Can. J. Plant Sci. 92: 271–277. Assessment of winter-hardiness using field-grown asparagus is complicated by variable, yearly climatic conditions and the large crown growing below the soil surface. The development of a seedling assay in controlled environments would be beneficial to study the physiology of winter-hardiness and to facilitate the selection of superior genotypes in breeding programs. Two cultivars, Guelph Millennium (GM) and Jersey Giant (JG), with differing patterns of autumn fern senescence in the field, where GM senesces earlier than JG, were compared. Seedlings were analyzed for physiological parameters after cold acclimation (10°C day/5°C night) or cold acclimation followed by sub-freezing (3°C to −3°C) in controlled environment chambers. Cold acclimation induced greater chlorophyll loss in GM than JG, consistent with previous field observations. LT50, the temperature at which 50% mortality occurs, decreased to approximately −8°C for both cultivars after the initial cold acclimation treatment. Subsequent subfreezing acclimation increased the LT50 for JG to −5°C, decreased freezing tolerance, while that for GM did not change. Early senescence and high proline concentration as well as stable protein and reducing sugar concentrations were associated with the freezing tolerance observed in GM. Further studies are required to establish if the cultivar differences for freezing tolerance identified here are correlated with experiments from field-grown plants.

A method to assess cold acclimation and freezing tolerance in asparagus seedlings

Landry, E. J. and Wolyn, D. J. 2012. A method to assess cold acclimation and freezing tolerance in asparagus seedlings. Can. J. Plant Sci. 92: 271-277. Assessment of winter-hardiness using field-grown asparagus is complicated by variable, yearly climatic conditions and the large crown growing below the soil surface. The development of a seedling assay in controlled environments would be beneficial to study the physiology of winter-hardiness and to facilitate the selection of superior genotypes in breeding programs. Two cultivars, Guelph Millennium (GM) and Jersey Giant (JG), with differing patterns of autumn fern senescence in the field, where GM senesces earlier than JG, were compared. Seedlings were analyzed for physiological parameters after cold acclimation (10°C day/5°C night) or cold acclimation followed by sub-freezing (3°C to -3°C) in controlled environment chambers. Cold acclimation induced greater chlorophyll loss in GM than JG, consistent with previous field observations. LT50, the temperature ...

Cold Acclimation Attributes of Two Asparagus Cultivars with Varying Patterns of Fern Senescence

Adequate winterhardiness is crucial for yield stability of asparagus (Asparagus officinalis) cultivars in southern Ontario, Canada, and could be influenced by pattern of the fall fern senescence. Fern of cultivar Guelph Millennium (GM) turns yellow or senesces by mid-October, before that of cultivar Jersey Giant (JG), which often remains green until a killing frost. Early fern senescence could be a signal for cold acclimation competency and consequently winterhardiness, explaining the superior stand longevity and yield observed for GM compared with JG. A field experiment was conducted from mid-August to November to measure physiological parameters related to cold acclimation in fern, rhizome, and storage roots. During fall, fern chlorophyll concentration, rhizome nitrogen concentration, percent water of the crown, and storage root LT50 (temperature at which 50% cell death occurs) decreased. Cultivars did not differ for storage root percent water; however, values were smaller (greater dehydration) for GM than JG in the rhizome. At the end of the sampling period, GM had higher and lower concentrations of rhizome low-molecular-weight, non-structural carbohydrates and sucrose, respectively, than JG, which could support a hypothesis of greater winterhardiness in GM. Storage root LT50 values of –19 °C and the lack of cultivar differences for this trait, in conjunction with differences between GM and JG for rhizome traits thought to be important for freezing tolerance, suggest characteristics of the rhizome in conjunction with timing of fern senescence may be important in cold acclimation of asparagus.

The effect of plant growth-promoting rhizobacteria on asparagus seedlings and germinating seeds subjected to water stress under greenhouse conditions

Plant growth-promoting rhizobacteria (PGPR) can have positive effects on vigour and productivity, especially under stress conditions. In asparagus (Asparagus officinalis L.) field culture, seeds are planted in high-density nurseries, and 1-year-old crowns are transplanted to production fields. Performance can be negatively affected by water stress, transplant shock, and disease pressure on wounded roots. PGPR inoculation has the potential to alleviate some of the stresses incurred in the production system. In this study, the effects of PGPR (Pseudomonas spp.) treatment were determined on 3-week-old greenhouse-grown seedlings and germinating seeds of 2 asparagus cultivars. The pots were irrigated to a predetermined level that resulted in optimum growth or the plants were subjected to drought or flooding stress for 8 weeks. The cultivars responded differently to PGPR: single inoculations of seedlings enhanced growth of 'Guelph Millennium' under optimum conditions and 'Jersey Giant' seedlings under drought stress. Seed inoculations with PGPR resulted in a positive response only for 'Guelph Millennium', for which both single or multiple inoculations enhanced plant growth under drought stress.

Activation of Defense Responses to Fusarium Infection in Asparagus densiflorus

Defense responses to Fusarium oxysporum f. sp. asparagi and F. proliferatum were compared after root inoculation of the asparagus fern, Asparagus densiflorus vars. Myersii and Sprengeri, and cultivated asparagus, A. officinalis cv. Guelph Millennium. Both varieties of A. densiflorus exhibited a hypersensitive response with rapid death of epidermal cells within 8–24 h and restricted the fungal growth. In A. officinalis roots, rapid cell death was not found, and necrotic lesions were observed 8–14 d after fungal inoculation. Peroxidase and phenylalanine ammonia-lyase activities increased significantly in inoculated A. densiflorus but not A. officinalis plants. Local and systemic induction of peroxidase activity was detected after pathogen inoculation in root and spear tissues, respectively, of A. densiflorus. POX activity decreased in roots of inoculated A. officinalis by 8 d post-inoculation. Germination and germ tube growth were inhibited when spores of F. oxysporum f. sp. asparagi were incubated in root exudates and on root segment surfaces of inoculated A. densiflorus plants exhibiting hypersensitive cell death. Spore germination of F. proliferatum and three fungi non-pathogenic to cultivated asparagus was inhibited as well. Rapid induction of hypersensitive cell death in A. densiflorus was associated with restriction of fungal growth, and activation of peroxidase and phenylalanine ammonia-lyase, two defense enzymes thought to be important for plant disease resistance.