identifying base temperature for alfalfa germination ... · the temperature at which alfalfa seeds...

crop science, vol. 56, september–october 2016 www.crops.org 2833

RESEARCH

Alfalfa is the most common forage legume produced in the United States with >5.5 million ha planted in 2014

(National Agriculture Statistics Service, 2015). In the Upper Midwest, alfalfa is often seeded in the spring using herbicides for weed control (Undersander et al., 2011). For organic production, where herbicides are not permitted, alfalfa is seeded in spring with companion crops (Sheaffer et al., 2014). For both conven-tional and organic systems, spring seeding in late April to early May is encouraged to increase establishment success as a result of accessibility to early season moisture and reduced competition with weeds (Undersander et al., 2011). Frost seeding is an alter-native approach in which alfalfa and other small seeded legumes are broadcast onto frozen ground in effort to introduce N-fixing legumes into pastures and winter small grains (Blaser et al., 2006). Successful frost seeding relies on soil freeze–thaw cycles in late winter and early spring to improve soil–seed contact. However, if winter or early spring air temperatures stimulate germination and seedling emergence and are followed by temperatures lethal to emerged seedlings, a frost-seeded stand could fail. Understanding the temperature at which alfalfa seeds germinate will help iden-tify when and where frost seeding alfalfa can be most effective.

Temperature is a critical component of most developmental pro-cesses in plants including germination. A large body of research has reported the relationship between temperature and growth in alfalfa

Identifying Base Temperature for Alfalfa Germination: Implications for Frost Seeding

Jacob M. Jungers,* Mary Brakke, Aaron Rendahl, and Craig C. Sheaffer

ABSTRACTFrost seeding alfalfa (Medicago sativa L.) can be convenient and economical for establishing or renovating forage stands; however, prema-ture seedling emergence triggered by unusu-ally warm temperatures followed by fatally cold temperatures can lead to seedling mor-tality and stand failure. Delaying germination could improve establishment success in frost-seeded stands. Our objective was to measure the effect of temperature and water potential (Y) on germination across a range of alfalfa variet-ies. Germination rate (1 divided by days to 50% germination) was estimated for 11 varieties at nine constant temperatures (−1.1 to 10°C) and three Y (0, −0.2, and −0.6 MPa). Linear regres-sion between temperature and germination rate was tested for all variety–Y combinations. Base temperature (Tb; minimum temperature for 50% germination) and thermal constant (DD; time to 50% germination in growing degree-days) were determined by calculating the intercept on the temperature axis and the inverse of the slope parameter from each regression, respectively. The Tb ranged from −0.55 to 0.49°C across varieties and was greater in low Y conditions. The DD was negatively correlated with Tb, which supports the hypothesis that seeds with lower Tb require more degree-days to germinate. Selecting alfalfa varieties with higher Tb could delay germination and reduce the risk of frost mortality; however, these varieties are also likely to have higher DD, which would expedite seed-ling emergence and potentially offset delays from high Tb. Based on small variations in ger-mination parameters compared with field tem-peratures, no variety was identified as superior for frost-seeding success.

J.M. Jungers, M. Brakke, and C.C. Sheaffer, Dep. of Agronomy and Plant Genetics, University of Minnesota, 411 Borlaug Hall, 1991 Upper Buford Circle, Saint Paul, MN, 55108 USA; A. Rendahl, School of Statistics, University of Minnesota, 224 Church Street SE, Minneapolis, MN, 55455 USA. Received 19 Feb. 2016. Accepted 27 Apr. 2016. *Corresponding author ( [email protected]).

Abbreviations: DD, thermal constant; FD, fall dormancy; Tb, base temperature; Y, water potential.

Published in Crop Sci. 56:2833–2840 (2016). doi: 10.2135/cropsci2016.02.0109 © Crop Science Society of America | 5585 Guilford Rd., Madison, WI 53711 USA All rights reserved.

Published August 30, 2016

2834 www.crops.org crop science, vol. 56, september–october 2016

(Christian, 1977) including studies on biomass accumula-tion (Pearson and Hunt, 1972), flowering time (Sharratt et al., 1989), and chemical composition (Vough and Marten, 1971). A number of studies have investigated the effects of temperature on germination rate in forage legumes (Brar et al., 1991; McElgunn, 1973; Stone et al., 1979), and studies show that alfalfa can germinate at temperatures near freez-ing (Arakeri and Schmid, 1949). In a study with Kansas common alfalfa, Coffman (1923) reported that 24% of the alfalfa seed germinated while encased in ice at a tempera-ture near 0°C. However, previous studies have not tested temperature ranges appropriate for detecting the minimum temperature required for germination (i.e., base tempera-ture) with modern alfalfa varieties. Base temperature is a necessary piece of information for determining thermal time requirements for growth (Trudgill et al., 2005); an applied example is estimating forage yield and quality using tools such as growing degree-day models (Sharratt et al., 1989).

Water is essential for plant metabolic functions includ-ing germination (Hegarty, 1978; Bradford, 1990). Alfalfa germination rates fell to zero when Y was between −1.0 and −1.5 MPa (Redmann, 1974). Reduced germination rates translated to low establishment in water-limited field conditions (Triplett and Tesar, 1960). Moreover, the inter-active effects of water and temperature on alfalfa germi-nation have not been measured and are necessary for pre-dicting frost-seeding establishment success.

Fall dormancy is a traditional metric used to rank a variety’s capacity to acclimate to cold temperature and shortened photoperiod (Chen and Chen, 1988; Schwab et al., 1996; Brummer and Klos, 2000). Strong associations between fall dormancy and winter hardiness were observed in early germplasms (Larson and Smith, 1963; Schwab et al., 1996); however, breeders have successfully dissoci-ated fall dormancy and winter hardiness so that varieties can be optimized for both characteristics (Brummer and Klos, 2000; Weishaar et al., 2005). Identifying correlations between reported growth characteristics, such as fall dor-mancy and germination parameters, would provide addi-tional information for producers who need to choose a vari-ety for frost seeding. Although previous work has searched for correlations between fall dormancy and germination parameters (Larson and Smith, 1963), genetic associations that controlled such correlations may have been altered during the last few decades as a result of alfalfa breeding.

Our objectives were to determine Tb and germina-tion rates across a range of alfalfa varieties under vari-ous Y levels. This information could assist in identifying growing conditions that are best for avoiding premature germination and cold mortality in frost-seeded fields. We tested for relationships between germination parameters and traditional growth trait information, such as fall dor-mancy, to predict germination responses to frost-seeding conditions across varieties.

MATERIALS AND METHODSA diverse set of alfalfa varieties was selected for use in this study. Varieties were selected that represented a range of yield poten-tial, persistence, fall dormancy, and disease resistance. Alfalfa seeds from multiple lots within each variety were acquired when possible. To determine the potential variation in seed germina-tion across lots within varieties, we mechanically scarified seeds that were not treated with fungicides or rhizobia and then ger-minated them at 10°C and −0.2 MPa using the equipment and procedures described below. We compared germination rates across three lots within each of five varieties and two lots within seven varieties. Multiple lots were not available for two variet-ies. Germination was measured from three replicates of each lot after 2 and 3 d of incubation. One-way analysis of variance was used to determine if variation in germination among lots within varieties was statistically significant a = 0.05. Two vari-eties included lots with significantly different germination rates; therefore, those lots were retained and treated independently for the subsequent experiment. For varieties where germina-tion was not significantly different across lots, the lot with the germination rate most similar to the average germination (across all lots within a variety) was selected for the subsequent experi-ment. Based on this initial study, 14 seed lots, representing 11 varieties were selected for the experiment.

The experiment was a completely randomized factorial of 14 seed lots, nine temperatures (−1.1, 0.0, 1.1, 2.2, 3.3, 4.4, 5.5, 7.8, and 10.0°C), and three osmotic potential treatments (0, −0.2, and −0.6 MPa). Osmotic solutions were created with pure, deionized water or polyethylene glycol 8000 MW dissolved in deionized water according to Michel (1983). Six separate incu-bators were used for temperature control during the course of the study. Each temperature treatment was replicated three times. Temperature levels were randomized over time using a balanced incomplete block design, and temperature levels were assigned to incubators randomly over the treatment period. Water potential and alfalfa varieties were completely random-ized within temperature levels. Initially, counts of germinated seeds were made at intervals of 24, 48, 96, 192, and 240 h after placing in incubators. As the study progressed, counts were made daily for 14 d. Counts were made over a period of 31 d for the temperature of 1.1°C. The cumulative germination was based on total seeds excluding hard seeds (those which imbibi-tion did not occur).

Fifty seeds of each variety were sampled, weighed, and average mass was determined. Sterile, disposable Petri dishes were prepared with two layers of sterile filter paper with medium porosity. Ten milliliters of the appropriate osmotic solution was added to the dishes and seeds were sprinkled onto the moistened filter paper. Seeds were not covered by liquid. Petri dishes were placed in plastic bags to reduce evaporative loss of solution and then placed in darkened incubators. Data loggers (Spectrum Technologies) recorded air temperature within the incubator at 30-min intervals. Seeds were consid-ered germinated if the radicle was 2 mm or greater in length. Germinated seeds were removed as counted and discarded.

Data AnalysisParametric survival curves were fit to cumulative proportions of seed germination for each treatment combination, that is, for


where m is the slope of the regression between 1/D and envi-ronmental temperature (Te). The equation can be rewritten as

e b1

DD

T T

D

-=

where

b

bT

m= -

and

DD =1

mA mixed-effects analysis of covariance model was used to

identify a relationship between Tb and DD as well as Tb and fall dormancy (FD). Both models included Y and its inter-action between DD and FD as main effects and seed ID as random effects. The marginal R2 value was used to report the variance explained by the fixed factors using R functions in the piecewiseSEM package (Lefcheck, 2016; Nakagawa and Schielzeth, 2013).

RESULTSThe minimum growth chamber temperature at which we observed alfalfa seed germination was 0.0°C. No seeds germinated at −1.1°C, indicating that the temperature range used in this study captured the base temperature required for germination. Figure 1 shows the observed

each replicate of every variety–temperature–Y combination, using the following logistic curve:

( ); , , 1b tap t c Z D c e-æ ö÷ç ÷ç= - ÷ç ÷÷çè ø

where D is the duration in days for 50% germination, b = D/1 + Z, a = Z b/log2, and p is the probability that a seed germi-nated on or before time t (Sakanoue, 2010). The parameters c, Z, and D were determined by optimizing the maximum log likelihood estimates. The profile likelihood of D was computed for each set of seeds because some seed sets, especially those with low germination rates, did not meet certain assumptions for normal approximation methods. Using these likelihoods, 95% confidence intervals for D were computed for each set. A random-effects model was used to estimate the variability of D between reps. Assuming that the variability was the same on the log scale, one variance and separate means were fit for each variety–temperature–Y combination.

Base temperature and DD were determined by regressing germination rate (1/D) against temperature for each variety–Y combination (Garcia-Huidobro et al., 1982) using nonlinear least squares and maximum likelihood estimates of D. Using the following equation

e

1mT b

D= +

Fig. 1. Cumulative germination percentage of three alfalfa varieties through time at three different temperatures and water potentials. The dashed line indicates 50% germination, the percentage used to calculate germination rate as the response variable for subsequent analyses.


germination percentages at various days following incu-bation initiation for one replicate of three alfalfa variet-ies. The three alfalfa varieties shown here represent the range of fall dormancy ratings within the sample (Table 1), and the three temperatures represent the range where germination occurred for all varieties. The day at which seed germination reached 50% often occurred between sampling dates, which was the impetus for fitting a logistic curve to estimate germination rate (D = days to 50% ger-mination). A regression of the inverse of the estimated D based on the logistic function and incubation temperature for each variety–Y combination was used to estimate Tb and DD. Figure 2 shows the 1/D estimates at all incuba-tion temperatures for the three varieties presented in Fig. 1. A positive linear relationship between 1/D and tem-perature was significant for all variety–Y combinations, therefore meeting the assumptions necessary to calculate Tb and DD (Trudgill et al., 2005).

Base temperature varied by alfalfa variety (F = 5.27; P = 0.002) and Y (F = 46.24; P < 0.001), but there was no variety × Y interaction (F = 0.59; P = 0.885). Post hoc pairwise comparisons revealed that most varieties had similar Tb and that the variety V-3 had a Tb 0.50, 0.51, and 0.52°C greater than P-2, P-1, and P-9, respectively (Table 1). On average, Tb was similar under no and mod-erate moisture limited conditions, but increased by 0.43°C when Y was −0.6 MPa. The thermal constant varied by alfalfa variety (F = 3.56; P = 0.002; Table 1) but not Y(F = 1.18; P = 0.320). A pairwise comparison showed that DD was 4.14°C days lower for P-12 than P-2.

Base temperature was negatively associated with DD (F = 19.41, P < 0.001, marginal R2 = 0.63; Fig. 3), and the relationship was similar across Y (interaction term: F

= 2.15, P = 0.140). On average, DD decreased by 14.5°C days per 1°C increase in Tb (Fig. 3). Fall dormancy was not associated with DD (F = 4.54, P = 0.055), but FD was negatively associated with Tb (F = 8.19, P = 0.014, mar-ginal R2 = 0.68; Fig. 4). There was a significant interaction between FD and Y (F = 5.12, P = 0.014). Base temperature declined less in varieties with higher FD without moisture limitation compared with conditions with low Y.

DISCUSSIONThis study is the first to measure Tb for germination across a range of Y and varieties of alfalfa. Previous studies found that germination percentage varied across alfalfa variet-ies as a function of temperature and osmotic potential (Stone et al., 1979; Redmann, 1974), so it is not surprising that our results show statistically significant variation in Tb across alfalfa varieties. However, this variation could be biologically negligible in terms of frost-seeding suc-cess. Averaged across Y values, Tb ranged from −0.39 to 0.12°C, which is small compared with typical winter and spring daily temperature fluctuations in the Upper Mid-west. At these Tb values, germination date would likely be the same for all varieties. However, DD ranged from 24.0 to 28.2°C days and, when combined with Tb in a thermal time model, could lead to agronomically sig-nificant variation in germination and emergence time across varieties. To show this, we used daily maximum and minimum temperatures from 2011 to 2013 at three research stations in Minnesota to predict the date at which 50% germination would occur if frost seeded on January 1 (Table 2). For some site-years, the time span for which various alfalfa varieties are expected to reach 50% ger-mination is within a few days (e.g., Lamberton, MN, in 2011 and Saint Paul, MN, in 2013). However, at Waseca, MN, in 2012, the variety P-1 would have reached 50% germination on 21 February, while others such as V-3 would not have reached 50% germination until 7 March (Table 2). Between these dates, minimum temperatures at Waseca fell below −4.4°C (temperatures lethal to alfalfa seedlings; Undersander et al., 2011) during five different days. Therefore, the probability of frost mortality would be higher for the P-1 variety than V-3.

We conducted a second analysis that used 30 April (the typical alfalfa seeding date for the Upper Midwest) as the seeding date. The results predicted that all variet-ies would reach 50% germination on the same day within all site-years. This exercise was conducted to illustrate the potential influence of the variation of our Tb and DD measurements relative to variation in field conditions. Daily air temperature was the most reliable data avail-able for this exercise. A more precise prediction of actual emergence date would have to incorporate soil temper-ature and moisture data, rather than air temperature, as demonstrated by Izquierdo et al. (2013).

Table 1. Variety and fall dormancy (FD) score for 14 alfalfa samples used to estimate base temperature (Tb) and thermal constant (DD) for 50% germination. Estimates are averaged over water potential for 14 seed lots from 11 varieties.

Label Variety name FD Tb DD

°C °C days

P-9 Pioneer 55V48 5 −0.39a† 24.77ab

P-1 Pioneer 56S82 6 −0.39a 26.37ab

P-2 Pioneer 56S82 6 −0.37a 28.17a

A-2 Ameristand 403T 4 −0.27ab 27.32ab

G-2 Genoa 2184–375 4 −0.21ab 25.44ab

P-8 Pioneer 53V52 4 −0.17ab 26.89ab

LG-2 Legendairy YPQ 3 −0.15ab 24.62ab

DL-6 DairyLand DS761 2 −0.07ab 24.49ab

P-10 Pioneer 55V48 5 −0.07ab 25.95ab

P-6 Pioneer 54H91 4 −0.01ab 26.82ab

DL-4 DairyLand HybriForce 4 0.02ab 24.68ab

DL-3 Magnum V 4 0.09ab 24.58ab

P-12 Pioneer 54H91 4 0.10ab 24.04b

V-3 Vernal 2 0.12b 25.11ab

† Values within a column followed by the same letter are not significantly different based on Bonferroni-corrected multiple comparisons.


Fig. 2. Regressions between germination rate and incubation temperature for three alfalfa varieties spanning fall dormancy ratings and incubated at three water potentials. The x-intercept represents the base temperature and the inverse of the slope represents the thermal constant for each variety–water potential combination.

Fig. 3. Scatterplot and linear regression lines of thermal constant and base temperature for alfalfa varieties germinated at three water potentials.

Fig. 4. Scatterplot and linear regression lines of fall dormancy rating and base temperature at three water potentials.


Variation in Base Temperature and Moisture EffectsHigh, but not moderate, moisture limitation increased Tb (Table 1). Although the Tb at Y of −0.6 MPa was statistically higher than Tb of seeds without moisture limitation, the difference may not affect alfalfa establishment in the field. The effect of lower Y increased Tb by <0.5°C, which is well within the range of typical daily temperature fluctuations during winter–spring conditions in the Upper Midwest. Germination studies on other crops and the common weed lambsquarters (Chenopodium album L. CHEAL) showed relatively larger increases in Tb under similar negative Y compared with our findings (Fyfield and Gregory, 1989; Roman et al., 1999), and that the relationship between Tb and Y can be linear (Kebreab and Murdoch, 1999). Water potential did not affect DD (Table 1), and since changes in DD were more influential to predicted emergence date, it is unlikely that variations in Y within the range we tested will lead to differences in emergence date.

The base Y for germination (Yb) has been determined for a number of crop and weed species (Guillemin et al., 2013; Masin et al., 2010), which can then be incorporated with Tb to predict seedling emergence using a hydrother-mal time model (Gummerson, 1986; Bradford, 2002). Incorporating Y into seed germination models improves the predictive capability compared with models without Y (Masin et al., 2010). However, the simple empirical hydro-thermal time model assumes that Tb is independent of Y. In corroboration with other studies (Dahal and Bradford, 1994; Kebreab and Murdoch, 1999; Bradford, 2002), here we show that Tb is not independent of Y, and that these variables should be used with mechanistic models to pre-dict seedling emergence (Forcella et al., 2000).

Base Temperature–Thermal Time Tradeoff and ImplicationsOur results support the hypothesis that there is a trad-eoff between Tb and DD (Fig. 3). Trudgill et al. (2005) examined several studies and found that temperate spe-cies with low Tb had higher DD, whereas tropical species with higher Tb had lower DD. It has been hypothesized that such a tradeoff describes two different evolutionary strategies to permit the coexistence of competing species within a community (Trudgill and Perry, 1994). The rela-tionship holds for many field crops (Angus et al., 1980), and here we show that it holds across varieties of alfalfa.

Since the Tb–DD tradeoff exists in alfalfa, it is possible that these germination parameters are not helpful for dif-ferentiating frost-seeding success across varieties. Variet-ies with lower Tb could begin germinating earlier than varieties with higher Tb, which could make those varieties more susceptible to frost damage. However, varieties with lower Tb could also require more time to reach 50% ger-mination than varieties with higher Tb, which might delay their emergence to a point where temperatures that would permit frost damage are unlikely. Although breeding for traits such as yield and persistence often result in physi-ological changes that might be important for other aspects of crop profitability (e.g., establishment success after frost seeding), our results show that indirect effects of alfalfa breeding have not resulted in populations that deviate from the Tb–DD tradeoff. Although we found variation in Tb and DD among varieties, a more robust field germi-nation–emergence experiment is needed to identify alfalfa varieties superior for frost seeding.

Alfalfa varieties are ranked for traits including winter survival, FD, and pest resistance, all of which could influence their potential to successfully establish when frost seeded. Since there is very little information on

Table 2. Estimated emergence dates of 14 seed lots from 11 alfalfa varieties frost seeded on 1 January at three locations for 3 yr based on base temperature and thermal constant measured from growth chamber experiments.

Seed lot

Waseca, MN Lamberton, MN Saint Paul, MN

2011 2012 2013 2011 2012 2013 2011 2012 2013

P-9 21 March 23 February 5 April 20 March 16 January 31 March 20 March 7 March 7 April

P-1 22 March 21 February 6 April 20 March 31 January 31 March 21 March 7 March 8 April

P-2 22 March 2 March 7 April 21 March 31 January 31 March 21 March 7 March 8 April

A-2 22 March 7 March 7 April 21 March 31 January 31 March 21 March 8 March 8 April

G-2 22 March 2 March 6 April 21 March 31 January 31 March 21 March 7 March 8 April

P-8 23 March 7 March 7 April 21 March 31 January 4 April 22 March 8 March 8 April

LG-2 23 March 2 March 6 April 21 March 31 January 31 March 21 March 7 March 8 April

DL-6 22 March 7 March 6 April 21 March 31 January 31 March 21 March 7 March 8 April


P-6 31 March 7 March 7 April 21 March 31 January 4 April 22 March 11 March 9 April




V-3 31 March 7 March 7 April 21 March 31 January 4 April 21 March 11 March 9 April


frost-seeding success across varieties and environments, identifying correlations between trait ratings and factors that might influence frost-seeding success could be useful for selecting varieties for frost seeding. Fall dormancy rat-ings are a relative measure of plant growth in fall and are used to measure a variety’s acclimation response to winter. Over a wide range of conditions, fall dormancy is cor-related with other trait ratings such as winter hardiness, which is useful for predicting winter survival for new varieties that have not been exposed to harsh winters in field trials (Schwab et al., 1996; Barnes et al., 1978). Since FD values were available for all the varieties in this study, we tested for correlations between FD, Tb, and DD.

Fall dormancy was not related to DD, the compo-nent of seed germination that varied most across variet-ies, but it was related to Tb (Fig. 4). A significant interac-tion between FD and Y showed that only when moisture was limited, varieties with less fall growth (low FD) had higher Tb. This relationship is surprising given that variet-ies with low FD were bred with high proportions of parent material from northern latitudes, which would have lower Tb based on the hypothesis and analysis by Trudgill et al. (2005). Despite having a sample with good representation of the variation across FD ratings—ranging from 2 to 6 on a scale from 1 to 11—our sample was somewhat lim-ited by the number of observations within each FD group. This relationship between FD and Tb should be confirmed with additional information from both field and labora-tory experiments that include more observations from varieties at the low and high end of the FD range. Field research that quantifies establishment rates and emergence timing in frost seeded alfalfa stands with different varieties is needed before using FD as a proxy for predicting frost-seeding establishment success.

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identifying base temperature for alfalfa germination ... · the temperature at which alfalfa seeds...

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