1. Introduction
Potato (
Solanum tuberosum L.) is an important food crop with a wide range of growing locations around the world [
1]. Globally, over 375 million tons of potatoes were produced in 2016 [
1]. Potatoes in the United States produce a raw product value of about USD 4 billion per year and are planted on around 400,000 hectares of land [
2]. In Wisconsin, which ranks as number three in the United States for potato production, potatoes are planted on 27,000 hectares of land, and potato production creates about USD 350 million in annual economic activity and over 2750 jobs statewide [
2,
3]. Approximately 50% of Wisconsin’s potatoes are sold for fresh market consumption and the other half are used for French fry or chip processing [
2] (Keene and Mitchell, 2010).
Compared to other crops, potatoes have a shallow rooting zone, high susceptibility to water stress, and a low water stress threshold required to close their stomata [
4,
5]. In addition, they are generally grown on soils that have low water-holding capacity, such as the sandy soils of the Central Sands in Wisconsin [
5,
6]. The Central Sands region is a sandy outwash plain marked by extremely coarse texture (90% sand) with a low soil organic matter content (<10 g kg
−1) and a relatively shallow groundwater table (0.9
–11 m) [
7]. In this region, irrigated agriculture has increased from 15,000 hectares in the 1960s to 74,000 hectares in 2000, while domestic water withdrawals have not significantly increased [
8]. The presence of irrigation in the Central Sands reduces the availability of groundwater as well as the volume of surface water bodies [
7]. Kraft et al. [
8] found that agricultural irrigation in the Central Sands has significant impact on the base flow of streams and water level in aquifers, lakes, and wetlands. In addition, heavily farmed areas with large amounts of irrigation in this region have been shown to strongly correlate with excessive amounts of nitrate contamination (greater than 10 mg/L of NO
3-N) in the groundwater [
9], which has been a pressing challenge in recent years. Phosphorous leaching can also occur in some irrigated cropping systems, although this is not a significant issue in the Central Sands region.
Regulated deficit irrigation is a strategy that reduces or removes irrigation during crop growth stages that are drought-tolerant in order to maximize water-use efficiency [
10] (Geerts and Raes, 2009). The overarching goal of regulated deficit irrigation is reducing evaporative water loss and avoiding excessive water use, while optimizing crop yield and quality and eliminating pests and diseases that would thrive in wet or humid environments [
10,
11,
12]. Through deficit irrigation, yield and plant health can remain stable while the water needed to maintain the crop is minimized. Regulated deficit irrigation may also reduce the possibilities of nitrogen leaching, leading to better nutrient uptake in crops as well as reduced water quality degradation [
13].
Potato crops can undergo yield reductions when soil moisture deviates by only 10% [
14]. Water stress can reduce leaf area through wilting and leaf elongation as well as reduce the amount of photosynthesis that takes place in a given leaf area through stomatal closure [
15]. However, the optimal amount of water needed can vary, depending on the characteristics of the plant, soil, and climate [
15]. Additionally, tuber production can suffer in terms of yield, size, and disease incidence from both under- and over-irrigation due to increased disease pressure, decreased soil aeration, and nutrient leaching [
11,
12,
16].
The tuber bulking stage is the growth period that determines a large part of tuber yield and quality [
15]. During this phase, tuber growth rates are relatively linear as sucrose moves from the leaves to the tubers and is converted to starch, and the tubers accumulate water, nutrients, and carbohydrates [
5]. Tubers can be remarkably susceptible to water stress that is related to misshaping or poor quality during the tuber bulking stage [
17]. Many studies have indicated that water stress during tuber initiation and early tuber bulking can decrease tuber yield and quality [
18,
19,
20,
21]. The study of Stark et al. [
22] conducted in Idaho concluded that providing full irrigation through mid-bulking followed by a slow reduction in irrigation amounts was the best scenario for reducing yield losses when water supplies were deficient. Aside from this yield analysis, little recent data are available about the impact of irrigation on the latter part of tuber bulking stage and the tuber maturation stage.
The objective of this study was to evaluate the yield, quality, irrigation efficiency, water-use efficiency, and post-harvest storability of common Wisconsin russet potato varieties that were subjected to different irrigation rates during the late tuber bulking and tuber maturation growth stages. The ultimate goal is to provide useful information that growers can use to make management decisions to achieve more sustainable irrigation management, to save operation costs associated with unneeded irrigation events, and to improve stewardship of the invaluable water resource for potato production.
2. Materials and Methods
2.1. Experimental Design and Agronomic Practices
This study took place over the course of the 2018 and 2019 field seasons and the 2018 storage season at the Hancock Agricultural Research Station (HARS) in Hancock, WI (latitude: 44°12.1413 N; longitude 89° 53.6840; elevation 328 m). The soil type of the field was a loamy fine sand (organic matter < 1%) from a depth of 0 to 45 cm and was a fine sand (organic matter < 0.5%) from a depth of 45 to 75 cm (
Supplemental Table S1). Soils in the area have an average permanent wilting point was 0.055 m
3 m
−3, an average field capacity of 0.162 m
3 m
−3, and an average available water content of 0.107 m
3 m
−3 [
23]. For the standard irrigation treatment, the maximum allowed soil water deficit was 40%. Irrigation was scheduled according to weather and crop conditions such that this maximum allowed deficit was not passed, with the goal of maintaining 60–80% soil moisture. Other pre-planting soil test results are shown in
Supplemental Table S2.
This experiment was a two-factor factorial design with four blocks, with irrigation rate and variety as the two factors. The irrigation rates are listed in
Table 1, and the varieties were Russet Burbank, Russet Norkotah, and Silverton. Each plot contained four 7.6 m rows of each variety with 0.3 m seed spacing and 0.9 m row spacing. For each variety, the outer two rows were guard rows to mitigate any variations resulting from edge effects, and the inner two rows were dug for at-harvest and post-harvest sampling. A weather station (Campbell Scientific, Logan, UT, USA) was located within 500 m of experimental plots to record precipitation and temperature data. Accumulated growing degree days (GDD) were calculated from 1 May to 1 July in 2011 to 2019 using the formula GDD = [min temperature + max temperature]/2 − 7 °C].
Seed potato pieces were planted on 1 and 2 May in 2018, and on 3 and 6 May in 2019. Emergence occurred on around May 20th of each year, and tuber initiation took place in the first week of June in each year. Irrigation management was performed with a linear overhead nozzle-controlled variable rate irrigation (VRI) system (Reinke Irrigation Company, Deshler, NE, USA) paired with a remote precision controller (Reinke Irrigation Company, Deshler, NE, USA) with touch technology and GPS. The system was 392 m wide and 764 m long. The sprinkler package was a Nelson Irrigation Model 03030 with nozzles rated at 28 L per minute. The nozzles are 30 to 35 psi inlet pressure regulated to 10 psi outlet side. The sprinklers had a height of 2.3 m and a spacing of 2.9 m. The overall system had a flow rate of 1934 L per minute.
Watering at 100% evapotranspiration (ET) is a standard practice at HARS as well as in the Wisconsin potato industry. In both years, all plots received irrigation at 100%ET before 25 July, which was the beginning of the irrigation treatment period. The date 25 July was chosen as it represented the average date across the three varieties to reach late tuber bulking, and 20 August was chosen as the date to change application rates in treatment 5 (
Table 1) because it was during the tuber maturation stage. All irrigation treatments ended at vine kill, which occurred on 10 September in 2018 and 9 September in 2019. The vine kill date was 14 days before harvest. Daily ET was estimated by the Wisconsin Irrigation Scheduling Program using the Priestley–Taylor equation [
24]:
where ET
pm,o is Penman–Monteith evapotranspiration in m day
−1; λ
v is volumetric latent heat of vaporization, 2453 MJ m
−3; Δ is slope of the saturation vapor pressure-temperature curve (kPa C
−1); R
n is net radiation (MJ m
−2 day
−1); G is soil heat flux density (MJ m
−2 day
−1); ρ
a is air density for a given air pressure (kg m
−3); c
p is specific heat of air (MJ kg
−1 C
−1); e
s is saturation vapor pressure (kPa); e
a is actual vapor pressure (kPa); r
a is air resistance (day m
−1); γ is the psychrometric constant (kPa C
−1); and r
s is surface resistance (day m
−1).
The irrigation rates used in this study were chosen to cover a wide spectrum of possible irrigation scenarios.
All other cultural practices including fertilization and pest management were based on the University of Wisconsin-Madison Extension’s recommendation [
25]. Total nitrogen applied was 340 kg N ha
−1, and application was split between planting, hilling, tuber initiation, early tuber bulking, and mid tuber bulking. The starter fertilizer was 6-30-22-4S. The hilling application was accomplished using side-dressing, where 21-0-0-24S ammonium sulfate was applied. All other applications were accomplished using an airflow fertilizer applicator, where dry 34-0-0 ammonium nitrate was applied (
Supplemental Table S3).
To monitor soil moisture content during the growing season, a water content reflectometer sensor (WCR) (model CS616, Campbell Scientific, Logan, UT, USA) was installed at two depths (30 and 60 cm) in one replication of each treatment. The sensors were installed in the first row of each subplot containing Russet Burbank. The WCR sensors recorded daily soil volumetric water content at 15-minute intervals.
2.2. In-Season Weather
Precipitation totals for the entire growing season and for the treatment period alone (25 July to vine kill) are shown in
Table 2.
Daily irrigation and precipitation data for the 2018 and 2019 growing seasons are shown in
Figure 1. In 2018, there were three periods of high rainfall (>2.5 cm per rain event) that largely disrupted irrigation treatments. These periods took place from 14 June to 22 June, 19 July to 23 July, and 21 August to 9 September (
Figure 1A). The intense rainfall period between 21 August and 9 September caused the second irrigation treatment of rate 5, which was expected to start on 20 August at a rate of 50%ET, to be disrupted and not to be effectively applied. In 2019, there were erratic rainfall events (<2.5 cm) throughout the treatment period (
Figure 1B).
2.3. Harvest Practices
All plots were harvested on 24 September 2018 and 23 September 2019 using a two-row digger (Gallenberg Technologies, Antigo, WI, USA). Tuber yield and quality were determined by assessing total final plot weight, total final weight of marketable tubers, specific gravity, and internal defects. The tubers were washed and graded for their individual weight using an AgRay Vision x-ray grading machine (AgRay Vision, Acampo, CA, USA). Tubers that were misshapen, rotten, or green due to sunlight exposure were culled before being graded. The year 2019 had an unusually high number of green tubers due to late season precipitation washing away topsoil. Cull weights were recorded in their own category and counted towards total yield from each plot.
Grading was categorized according to the following size profile based on individual tuber weight: 0–113 g, 113–170 g, 170–283 g, 283–367 g, and greater than 367 g. Marketable yield was then calculated as the yield of non-culled tubers that were larger than 170 g. In this study, no tubers bigger than 510 g were observed. These values were chosen to reflect regional industry standards.
Ten tubers from each plot weighing 170 g to 280 g each were randomly chosen to determine specific gravity as well as internal defects. Specific gravity was measured using a Weltech PW-2050 Dry Matter Assessment System. After taking the weight of a 2000–3000 g sample of tubers in the air and in water, the system used the following formula to calculate the specific gravity for the sample [
26]:
To assess internal defects, a separate sample of 10 tubers (170 g to 280 g) from each plot were cut in half from stem end to bud end to evaluate incidence of hollow heart.
Irrigation efficiency (IE) was defined as the ratio of the yield produced by the crop to amount of water supplied to the crop through irrigation. This efficiency was calculated as yield (in megagrams per hectare) divided by the total amount of irrigated water (measured in centimeters) applied over the course of the entire growing season. Water-use efficiency (WUE) was defined as the ratio of the yield produced by the crop to the amount of all water supplied to the crop and was calculated as yield divided by the total amount of precipitation and irrigation over the entire growing season. Both total yield and marketable yield were considered for these calculations. The major difference between these two efficiencies is that water-use efficiency considers all water that the crop receives, while irrigation efficiency disregards precipitation and only considers the yield achieved in relation to the amount of irrigation water applied.
2.4. Storage Management
After harvest, about 20 kg of healthy potatoes from each plot were placed into the University of Wisconsin Hancock Potato Storage Research Facility, which was maintained at 12.8 °C for the purpose of wound healing for thirty days. Temperatures were then decreased at a rate of 0.3 °C per day to a final holding temperature of 8.9 °C. The sprout inhibitor, Isopropyl N-(3-chlorophenyl)carbamate (CIPC, Decco Chemical, at 78.6% active ingredient), was applied to all tubers as a thermal aerosol at a rate of 22 mg kg−1 right before ramping temperature down to the holding temperature. Relative humidity was maintained at 95% ± 3% and CO2 level was kept below 3600 mg m−3 throughout the entire duration of storage.
Storage data were collected for the 2018 and 2019 storage seasons. Data were collected at the time of harvest as well as at 16 and 32 weeks after harvest. Only Russet Burbank was evaluated due to its use for fry processing purpose.
2.5. Fry Color Analysis
One fried plank (3.0 cm × 0.8 cm × length of tuber) from a 10-tuber sample from each variety/irrigation treatment/block was used for fry color determination within one week of harvest and then at 16 and 32 weeks in storage. This range of dates was recommended by regional industry potato storage experts. Planks were fried in canola oil at 191 °C for 3.5 min and blotted dry to remove extra oil. Fry color was determined within the first 3 min after frying using a model 577 Photovolt Reflectometer (model 577, Photovolt Instruments Inc., Minneapolis, MN, USA). As per the manufacturer’s recommendation, a green filter was used and calibrated, using a black-cavity standard as 0.0% reflectance and a white plaque as 99.9% reflectance. Measurements were taken on the bud and stem ends of each plank. A relationship between the USDA standard fry color and the photovolt reflectance was previously established. A USDA fry color rating 1 is equal to a 44.0 or greater reflectance reading, a USDA 2 rating is less than 44.0 to 35.0 reflectance reading, a USDA 3 rating is less than 35.0 to 26.0 reflectance reading, and a USDA 4 rating is less than 26.0 reflectance reading [
17]. Therefore, higher photovolt reflectance readings are associated with lighter fry color. In 2019, fry color evaluation at harvest and after 32 weeks of storage were measured based on photovolt readings, whereas fry color assessment at 16 weeks of storage used USDA ratings. Due to the different rating systems, an ANOVA was not run on the fry color data in 2019.
2.6. Sugar Analysis
Sucrose and glucose content were determined on the same 10-tuber sample used for the fry color determination by the method of Sowokinos et al. [
27] with modifications. Two hundred grams of tuber samples were sliced from both the stem end and bud end of Russet Burbank tubers. Samples were then juiced using an Acme Supreme Juicerator (Acme Equipment, Spring Hill, FL, USA) mixed with a 50 mM phosphate buffer. This phosphate buffer was a sodium phosphate monobasic and dibasic mixture. The samples were placed in a refrigerator for a minimum for 30 min, then a 1 mL sample from the top of the container was extracted. A YSI 2900 Select Biochemistry Analyzer (Sigma-Aldrich, St. Louis, MO, USA) was used to analyze sucrose and glucose concentration.
2.7. Data Analysis
Data analysis was performed following a two-factor factorial design, with irrigation treatment and variety as the two factors, and variability between the four blocks as the error term. Data from 2018 and 2019 seasons were analyzed separately due to variable weather conditions in those two years. Analysis of variance (ANOVA) was conducted in R [
28]. Means were separated using Tukey’s test at the α = 0.05 level.