Published in Agron. J. 95:1201-1208 (2003).
© American Society of Agronomy
677 S. Segoe Rd., Madison, WI 53711 USA
WHEAT
Vernalization Studies with Pacific Northwest Wheat
D. M. Balocha,
R. S. Karow*,b,
E. Marxc,
J. G. Klingb and
M. D. Wittd
a 768 Robson St., Vancouver, BC V67 1A1, Canada
b Dep. of Crop and Soil Sci., 107 Crop Science Bldg., Oregon State Univ., Corvallis, OR 97331-3002
c Larimer County Ext., 1525 Blue Spruce Rd., P.O. Box 543, Fort Collins, CO 80522-0543
d Southwest Area Ext. Office, 4500 E. Mary St., Garden City, KS 67846
* Corresponding author (Russell.S.Karow{at}oregonstate.edu).
Received for publication October 14, 2002.
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ABSTRACT
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Lack of rain or excess soil moisture may create problems for fall sowing of winter cereals in the Pacific Northwest. Growers unable to plant in the fall question the potential for late winter sowings, which is determined to a large extent by cultivar vernalization requirement. This study was conducted to gain a better understanding of the vernalization requirement of winter wheat (Triticum aestivum L.) cultivars grown in the region and to determine if a tool could be developed for predicting latest winter sowing dates. A growth chamber–greenhouse procedure was developed to determine the relative vernalization requirement of new wheat cultivars. Twenty cultivars were sown in the field at intervals from early October through late March during the 1991–1993 cropping seasons. Accumulated heat units from sowing were used to determine vernalization days (VD). Heading date and yield were measured. Field results indicated that cultivars could be categorized in three groups—those with low, intermediate, and high vernalization requirements. Growth chamber–greenhouse results verified these groupings. Field data suggest that if 50% relative yield (late-sown yield is 50% of that for the same cultivar when sown in fall or early winter) is acceptable, then cultivars with a low vernalization requirement can be sown with 60 VD expected after sowing while cultivars with a high vernalization requirement require 70 expected VD. If 70% relative yield is desired, all cultivars tested showed similar performance.
Abbreviations: GDD, growing degree day(s) • VD, vernalization day(s)
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INTRODUCTION
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CLIMATIC CONDITIONS in the Pacific Northwest, dry summers and wet winters, have always been a critical problem for the successful sowing of winter wheat cultivars in the region. Growers sometimes cannot plant all of their winter wheat in fall and question the potential for success of late-sowing winter cultivars. The vernalization requirement of cultivars plays a large role in the success or failure of such sowings. Vernalization is the exposure of plants to low temperatures and increases a plant's readiness to flower, with longer vernalization generally leading to reduced time to flowering (Ritchie, 1991). Vernalization plays a fundamental role in the adaptation of winter cereals to their growing environment by allowing plants to escape cold injury during reproductive development (Flood and Halloran, 1986). Wheat cultivars differ in vernalization requirement. Winter grains show low to high requirements (Flood and Halloran, 1986; Rawson et al., 1998; Sutton and Bacon, 1988) while spring grain response is more controversial. Spring grains have been shown to respond to vernalization, exhibiting this response as a shortening of time to heading (Halse and Weir, 1970; Levy and Peterson, 1972; Jedel et al., 1986). On the other hand, Halloran (1967) found no response among spring cultivars. Gardner et al. (1993) reported that spring-type wheat cultivars require no vernalization and can be grown over a wide range of sowing dates in southern USA while winter-type cultivars require vernalization and should be sown in early winter at lower latitudes. Similar results were obtained by King and Bacon (1992) when they evaluated 25 winter and 5 spring oat (Avena sativa L.) cultivars by vernalizing germinated seeds at 5°C for 0, 12, 24, and 48 d. They found that all winter oat genotypes responded to cold temperature with a decrease in days to heading, whereas none of the spring genotypes responded to vernalization.
Wheat cultivars can be classified into groups, according to their vernalization response. Gardner and Barnett (1990) studied five soft red winter wheat cultivars, two spring wheat cultivars, and one triticale (Triticosecale spp.) with various duration of natural winter and refrigerator cold exposure. They found three types of vernalization response: (i) cold obligate, winter-hardy types (qualitative) requiring 6 to 8 wk of cold for heading; (ii) cold stimulated, mild-winter types (quantitative) heading earlier then the control with 2 to 4 wk of cold exposure; and (iii) cold neutral, spring types not stimulated by cold exposure.
Many researchers have conducted studies to determine the optimal temperature range for vernalization. It is generally accepted that temperatures between 2 and 10°C are effective for vernalization in wheat (Purvis, 1948; McKinney and Sando, 1935; Vavilov, 1951). A comprehensive study to determine the effective temperature limits of vernalization for winter wheat was conducted by Trione and Metzger (1970). They found that the effectiveness of cold treatment in wheat seedlings was maximum at 7°C and significantly less when the temperature was raised to 9°C or lowered to 3°C. Rawson et al. (1998) found that a vernalization temperature of 3°C was most likely to produce the minimum leaf number in wheat, but earliest heading was obtained with temperatures ranging from 6 to 19°C, depending on the cultivar. A synthesis of results from 11 studies worldwide showed that optimum vernalization temperatures lie between 3.8 and 6.0°C (Porter and Gawith, 1999). Base temperatures for vernalization were found to average -1.3°C, and maximum temperatures were 15.7°C.
Reversal of vernalization (devernalization) is observed in vernalized plants when they are exposed to high temperatures (Lang, 1965). Purvis and Gregory (1937) first observed devernalization in winter rye (Secale cereal L.) when plants were exposed to 25, 30, and 40°C immediately after cold treatment of 1°C for 45 d. Similar results were obtained by Chujo (1970) for wheat. He vernalized a winter wheat cultivar at 1, 4, 8, 11, and 15°C for 40 d. Immediately after cold treatment, he exposed the plants to temperatures of 12, 18, and 24°C for 10 d. He reported devernalization by exposure to the 18 and 24°C temperatures.
Genetic studies have shown that there are three groups of genes controlling flowering time in wheat: Ppd genes regulate photoperiod response; Vrn genes control vernalization response; and Eps genes control earliness per se (Snape et al., 2001). Isogenic lines of wheat that possessed the allele for spring growth habit (Vrn-A1) and winter habit (vrn-A1) were compared at staggered sowing dates from fall through spring in the United Kingdom. The line with the vernalization-insensitive allele (spring habit) produced both vegetative and reproductive primordia at a faster rate than the winter line. Similar experiments with isogenic lines for alleles controlling photoperiod response did not show a major effect of these genes on rate of primordia development. Rather, the allele for photoperiod insensitivity brought forward the time of terminal spikelet, and thus reduced the number of spikelets produced.
Some studies have shown that photoperiodism and vernalization are physiologically interactive although they are controlled by different genes (Flood and Halloran, 1986; González et al., 2002). Optimum daylength for a number of long-day crops, including wheat, was found to be 17.7 h and appeared not to vary among species or cultivars (Major and Kiniry, 1991). Wheat phenology models based on the effects of temperature, vernalization, and photoperiod have been developed. They predict developmental stages with reasonable accuracy and can be applied over a wide geographic range (Cao and Moss, 1997; Wang and Engel, 1998).
The objectives of this research were to (i) determine the vernalization requirement of winter wheat cultivars commonly grown in the Pacific Northwest, (ii) develop a simple growth chamber–greenhouse procedure for identifying the vernalization response of new Pacific Northwest cultivars, and (iii) determine if an easy-to-use tool to predict latest winter sowing date could be developed.
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MATERIAL AND METHODS
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Field Experiments
Trials were conducted on the Hyslop Field Experimental Station of the Department of Crop and Soil Science, Oregon State University, Corvallis, OR, during the 1991 through 1993 crop seasons. A set of 20 cultivars was included in each trial—14 soft white winter wheat, two winter club wheat, one hard red winter wheat, one soft white spring wheat, and two winter triticales. All cultivars are adapted to the Pacific Northwest. Winter types included cultivars known to have a low or high vernalization requirement, based on previous field observation, along with those of unknown requirement. Cultivars are described in Table 1.
In 1990–1991, an attempt was made to seed every other week in October through mid-January, then weekly until late March. As fall seedings showed similar performance in 1990–1991, monthly seedings were used in October, November, and December 1991–1992 and 1992–1993, with weekly seeding from mid-January to late March. Sowing dates are shown in Table 2. Average monthly weather data are shown in Table 3. Each cultivar was sown in a single 2.4-m row on each sowing date. Plots were not replicated due to the difficulty of hand sowing and managing trials under field conditions. Spacing between rows was 60 cm. A seeding depth of 2.5 to 3.8 cm and seeding rate of 66 seeds m-1 row were used.
Plots were sown on ground that had been fallowed the previous year. Soil type at the site is a Woodburn silt loam (fine-silty, mixed, mesic, Aqualtic Argixerolls). This soil is deep and moderately well drained with high N mineralization capacity (USDA Soil Conserv. Serv., 1987). Neither fall fertilizer nor herbicides were applied. Plots were hand-weeded. Cultivars in each sowing date were differentially topdressed by hand with 56 kg N ha-1 as urea as they reached the late-tillering stage. Propiconazole {1-[[2-(2,4-dichlorophenyl)-4-propyl-1,3-diox-olan-2-yl]methyl]-1H-1,2,4-triazole]} applications were begun at the late-tillering stage and continued at 21-d intervals to control leaf diseases [yellow stripe rust (Puccinia striiformis L.), leaf rust (P. triticina L.), and Septoria leaf blotch (Septoria tritici L.)]. No insecticide was used. Aphids and other insects were not a problem.
An array of agronomic characteristics (heading date, plant height, spikes per 60-cm of row, yield components, and grain yield) were measured, and data are available on request. Heading and yield data are reported in this paper.
- Heading—date when 50% of spikes in 1 m of row were fully emerged from the boot.
- Grain yield—calculated as grams m-1 row and reported in this paper as relative yield. In 1990–1991, 30 cm of row was harvested from each treatment and threshed in a stationary thresher to determine yields. In subsequent years, row lengths were measured and entire rows combined with a plot combine. Grain in all years was cleaned using a rub-bar cleaner before being weighed. Relative yield is reported as a percentage and was calculated by dividing yield for an individual cultivar at a specific sowing date by the highest yield for that same cultivar in that same year. In most cases, highest yields were observed in late-November to early-December sowings.
Calculation of Growing Degree Days
Daily weather data for all 3 yr were obtained from a weather station located on the Hyslop Experiment Station and were used to calculate heat units expressed as growing degree days (GDD) accumulated from sowing through heading for each treatment. A GDD is equivalent to thermal time as defined by Ritchie and NeSmith (1991).
Daily GDD were calculated by using the formula:
where Tmax is maximum air temperature (°C) for a day, Tmin is minimum air temperature, and Tx is a base temperature. For wheat and triticale, 0°C was used as base temperature. When the mean daily temperature was less than zero, a heat unit value of zero was assigned. Accumulated heat units were calculated by summing daily heat units from the date of sowing to 50% heading.
Vernalization Day Calculations
Vernalization days were used as the unit against which cultivar performance was measured. Use of VD allowed standardization of sowing date data over years. Vernalization days are an estimate of accumulated cold. To calculate VD, daily GDD value was multiplied by an effectiveness factor. In this study, CERES wheat model functions (Ritchie et al., unpublished, 1987) were used to calculate VD values for each sowing date. In the CERES model, vernalization temperatures in the 3 to 6°C range are assigned greatest vernalization effectiveness. Temperature effectiveness curve segment equations are
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where X equals daily GDD and Y equals VD.
A typical VD accumulation for Corvallis, OR, using 30-yr average daily maximum and minimum temperature data for the calculation of GDD, is shown in Fig. 1
. In this figure, VD are accumulated from 1 September. For a given sowing date in these trials, accumulated VD were calculated from the day of sowing to 31 May of the harvest year. Based on the 30-yr weather record, no vernalization is expected to occur at this location after 31 May, and devernalization potential is expected to be minimal as mean high temperature for the month is 18°C.
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Fig. 1. Relationship between daily growing degree days (GDD) and accumulated vernalization days (VD) based on 30-yr average maximum and minimum daily temperatures for Corvallis, OR.
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Statistical Analyses
Due to the difficulty of sowing in midwinter, and the many sowing dates and large number of cultivars tested, single-row, nonreplicated plots were used in this study. Data were normalized on the basis of accumulated VD across years.
Cate–Nelson analysis (Cate and Nelson, 1971) is routinely used to determine critical levels of plant nutrients. Nelson and Anderson (1977) provide further detail on use of this analysis procedure. We reasoned that a vernalization threshold was like a nutrient threshold and that a Cate–Nelson procedure could be used. In a typical Cate–Nelson analysis, a scatter plot of data points is separated into four quadrants by a vertical and horizontal line. These lines are adjusted to maximize the number of data points in two opposing quadrants and to minimize data point numbers in the others. These lines can be determined by visual analysis or by using a spreadsheet to determine mean square errors in each quadrant as line positions are changed. The point where the vertical line crosses the x-axis is defined as the critical level. In our heading analyses, the y-axis intercept was set at a GDD level of 1100, the GDD required by a fall- or early-winter–planted cultivar to reach 50% heading. In our yield analyses, relative yield was set at 50 or 70%, and then the critical value for VD was determined. These relative yield points were selected for analysis as long-term comparisons of winter vs. spring wheat yields in cultivar trials in Oregon (data not shown) show that spring cultivar yields range from 50 to 70% those of winter cultivars. From a risk management standpoint, late-sown winter cultivars must yield at or above these levels to make late sowing a logical decision.
Growth Chamber–Greenhouse Experiments
As field determination of vernalization response is both difficult and time consuming, an attempt was made to develop a growth chamber–greenhouse procedure with low labor and resource requirements that could be used to classify cultivars as having a low, intermediate, or high vernalization requirement. The same set of cultivars as used in the field studies was used in growth chamber–greenhouse tests.
The experiment was performed in two phases. In the first phase, five cold treatment periods were used—7, 14, 21, 28, and 35 d. In this initial study, all cultivars were fully vernalized (i.e., they subsequently headed) after only 28 d of cold treatment, most after 21. In the second phase of testing, cold treatments were narrowed to 5, 10, 15, and 20 d. In both phases, 50 to 60 seeds of each cultivar were soaked in distilled water at room temperature for 24 h before sowing so that seedling emergence would be as uniform as possible for all treatments. Imbibed seed was sown in vermiculite in 45- by 35-cm pressed peat trays. For each cultivar, 20 healthy looking seeds of uniform size were sown in 35-cm-long rows, nine rows per tray in a completely random design with two replications. Sown trays were placed in a walk-in growth chamber (2 by 5 by 3 m) set at 6 to 7°C constant temperature for vernalization treatment. Approximately 0.96 VD would be accumulated each day in this environment using the CERES model for VD calculation. Lights in the growth chamber (mixed fluorescent and incandescent) were set at 8 h light/16 h dark, with average photosynthetically active radiation of 800 µmol photons m-2 s-1.
After treatment, but before transferring to the greenhouse, trays were placed in a 15°C chamber for 1 wk to reduce the risk of devernalization (Chouard, 1960). Greenhouse temperature was set at 21°C but was not closely controlled.
A commercial water-soluble fertilizer solution (Peters Professional 20–20–20, Scotts Testing Lab., Allentown, PA) was prepared according to label directions and applied at least weekly during all phases of the study. Trays were watered when the surface appeared dry. Differential watering was done as fan and light position affected drying rates of individual trays. Trays were rotated in both environments on a daily basis.
In the first study, after 8 wk, 10 plants from each treatment were dissected to determine if a head had formed. Plants remaining in the trays were allowed to mature. Vernalized plants exhibited 50% heading in 10 to 14 wk after the 15°C stabilization treatment. In the second study, a 50% headed or not headed determination was made after 14 wk.
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RESULTS AND DISCUSSION
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Heading Date
Heading data for a group of low (‘Hoff’, ‘Oveson’, and ‘Whitman’) and high (‘Eltan’, ‘Kmor’, and ‘Yamhill’) vernalization requirement cultivars are shown in Fig. 2
. Data for each cultivar in each trial year are shown and were used in the Cate–Nelson analysis. These cultivars represent the extremes in the cultivars tested. The other 14 cultivars evaluated in these trials showed heading patterns between the boundaries set by these extreme groups (data not shown but available on request). All cultivars exhibited uniform heading when they obtained at least 120 VD. Both sets of cultivars required a minimum of 1100 GDD to reach 50% heading with 120 VD or more. With later sowing and fewer VD accumulated, increased GDD were required for heading. Some high vernalization requirement cultivars did not head when fewer than 70 VD were accumulated. Failure to head is shown as a 4000 GDD value in Fig. 2. Cultivars with a high vernalization requirement showed a more rapid increase in GDD required for heading per reduced VD (26.2 GDD VD-1) compared with those with a low requirement (19.6 GDD VD-1). High vernalization requirement cultivars appeared to require more heat units to compensate for decreased vernalization. Low vernalization requirement cultivars headed at low VD accumulation though GDD required was nearly three times that of a fall-sown crop.
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Fig. 2. Regression of GDD required for 50% heading on accumulated VD for three low and three high vernalization requiring cultivars.
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Heading data from the growth chamber–greenhouse study are shown in Table 4. All data from the first trial and 5- and 10-d vernalization treatments from the second trial are shown. There were no differences in individual cultivar performance between replications in a trial or between similar treatments in the two trials (14 vs. 15 d; 20 vs. 21 d)—50% headed or not headed ratings were identical. Whitman and Oveson, a winter triticale and soft white winter wheat, respectively, headed with only 5 d of cold treatment. ‘Treasure’, a soft white spring wheat cultivar, did not head with 5 d of cold treatment, suggesting that it has a vernalization requirement, as observed by researchers in other spring cultivars (Levy and Peterson, 1972; Jedel et al., 1986). Greenhouse observations paralleled field observations on heading performance. Based on the growth chamber–greenhouse test, cultivars can be readily grouped into low, intermediate, and high vernalization requirement categories. Qualitative ratings of this type are useful in making recommendations to growers on cultivars that can and should not be late-season sown.
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Table 4. Effect of vernalization on heading of 20 wheat and triticale cultivars exposed to different periods of cold treatment under growth chamber conditions.
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Yield Data
Relative yield data vs. accumulated VD are shown in Fig. 3 and 4
for three low vernalization requirement cultivars (Hoff, Oveson, and Whitman) and three high vernalization requirement cultivars (Eltan, Kmor, and Yamhill). These cultivars represent the extremes in the cultivars tested. The other 14 cultivars evaluated in these trials showed yield reduction patterns between the boundaries set by these extreme groups (data not shown but available on request).
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Fig. 3. Relationship between 50% relative yield and VD for three low and three high vernalization requiring cultivars.
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Fig. 4. Relationship between 70% relative yield and VD for three low and three high vernalization requiring cultivars.
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Figure 3 shows a 50% yield level assessment. The low vernalization requirement cultivars needed 60 VD to reach a 50% yield level, but all headed and produced grain with as few as 20 VD. Cultivars with a high vernalization requirement needed 70 VD to reach 50% relative yield, and all showed a dramatic reduction in yield with fewer VD. None of these high-VD–requiring cultivars headed or produced grain at fewer than 35 VD. If 50% relative yield is acceptable from an economic point of view, then cultivars with low vernalization requirements can be sown with 60 VD expected while high-vernalizing cultivars require 70 expected VD.
Figure 4 shows a 70% yield level assessment. Both low and high vernalization requirement cultivars showed a 70 VD requirement to reach 70% relative yield. The difference between the two classes is that high-requirement cultivars show dramatic yield reduction with fewer VD in comparison to low-requirement cultivars. Other cultivars tested showed a similar pattern. As conventional wisdom suggests, there is greater risk in late-sowing a cultivar with a high-vernalization requirement. If weather is warmer than expected and 70 VD are not accumulated, the magnitude of potential yield losses is significantly greater with the high-VD–requiring cultivars.
Given the consistency of our data across an array of cultivars with very different genetic backgrounds, we were interested in assessing the utility of the vernalization cutoff concept for an independent set of data. Witt (1996) conducted a sowing date trial with the hard red winter wheat ‘TAM 107’ in Kansas between 1985 and 1990. Detailed yield and weather data for these trials were obtained from Dr. Witt, and relative yields were calculated and plotted against accumulated VD for each year. These data are shown in Fig. 5
. Cate–Nelson analysis with relative yield fixed at a 70% level shows that between 60 and 80 VD are required to reach this yield level. There appears to be general agreement in VD requirement between the two studies. This finding is not unexpected as Porter and Gawith (1999) noted, after an extensive review of literature on the effects of temperature on the growth and development of wheat, that cardinal mean temperatures for many developmental processes are consistent between studies and well defined in wheat.
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Fig. 5. Relationship between 70% relative yield for the cultivar TAM 107 and accumulated VD for six years of delayed sowing date trials at Garden City, KS.
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CONCLUSIONS
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Using a 70-VD requirement and historic temperature data for wheat-producing areas in Oregon, we have predicted latest sowing dates by backward accumulation of VD from 31 May. In some regions, a date later than 31 May may need to be used if VD are still accumulated beyond this date and devernalization temperatures (temperatures >20°C) do not occur. Other scientists will need to determine if the proposed 70-VD cutoff value is meaningful for cultivars in their region.
While this procedure may be useful in assessing the potential success of late-sown wheat, users should compare late-sown winter wheat yields to those of spring wheat. In Oregon, we found that in many instances, spring wheat yields were typically 50 to 70% those of winter wheat. As there would be less risk in sowing spring wheat that has no or a low vernalization requirement, this may be the better management decision unless winter seed must be used for some other reason.
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NOTES
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Contribution no. 03-318J from the Kansas Agric. Exp. Stn.
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REFERENCES
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TOP
ABSTRACT
INTRODUCTION
