Deficit Irrigation and Nitrogen Application Rate Influence Growth and Yield of Four Potato Cultivars (Solanum tuberosum L.)

Abstract

Potatoes have high nitrogen (N) and irrigation requirements. Increasing water scarcity and environmental concerns highlight the need for efficient resource management. This study evaluated the effects of deficit irrigation and reduced N on yield and growth parameters in four potato cultivars (Canela Russet, Mesa Russet, Russet Norkotah3, and Yukon Gold) at Colorado State University’s San Luis Valley Research Center over two growing seasons. Three irrigation levels (~70%, ~80%, and 100% ET replacement) and two N rates (165 and 131 kg/ha) were evaluated. Measurements included total and marketable yield, tuber size distribution, tuber bulking (TB), leaf area index (LAI), and stem and tuber numbers. Yield losses were absent with ≤18% irrigation reduction in Canela Russet, Mesa Russet, or Yukon Gold but occurred with larger deficits. Russet Norkotah3 experienced yield decline with 16–23% reductions in irrigation. A twenty percent reduction in N application had no effect on Mesa Russet or Russet Norkotah3 yields, while the other varieties experienced a yield decline in one out of two years. Early-season LAI and late-season TB were positively correlated with yield, particularly for Canela Russet and Russet Norkotah3. These findings suggest irrigation and N inputs can be reduced without compromising productivity, but reductions must be determined on a cultivar-by-cultivar basis.

1. Introduction

Potato (Solanum tuberosum L.) is an important food crop in many countries and is the fourth most important staple crop in the world after rice, wheat, and maize due to its short growing cycle and large production per unit area [1,2]. The growing population’s demand for food puts pressure on land and irrigation water supplies, and this contributes to increased pressure on water resource availability in the future. Therefore, it is crucial to improve water and nutrient use efficiency in the field by using efficient and effective irrigation and nutrient management [3]. Water and nitrogen (N) supplies are key challenges in the long-term production of potatoes [4], since increased or decreased irrigation and N application during the growing season can have adverse effects on the quantity and/or quality of potatoes [5].

Adequate N is essential for growth, development, productivity, and tuber quality [6]. Nitrogen is the most important limiting nutrient, influencing the productivity and quality of the crop; however, excessive N can result in nitrate leaching to the groundwater [7]. Likewise, excessive N fertilizer use may reduce tuber yield and quality [8]. Moreover, high N application may increase the degree of branching and the number of leaves per plant, delay maturity, and reduce tuber quality and dry matter concentration [7,9]. Management of N fertilization rate, timing, and method of application and water scheduling are important to improve N uptake and maintain high yield and quality while reducing N leaching [10]. Therefore, N use efficiency (NUE) must be improved to increase yields, reduce production costs, and decrease environmental pollution.

The availability of agricultural irrigation water has decreased due to an increased demand for water from non-agricultural sectors. Arid and semi-arid regions, including the San Luis Valley of southern Colorado (USA), are the areas most negatively impacted by irrigation water shortages [4]. For crop production in arid and semi-arid areas where rainfall is low and evapotranspiration is very high, declining irrigation water availability has become the main concern for maintaining crop productivity [3]. Therefore, the increasing demand for water and decreasing availability of agricultural irrigation water resources require the optimization of irrigation management practices to increase crop productivity and improve irrigation water use efficiency (WUE) [4,11,12].

Excessive irrigation can promote vegetative growth and reduce potato crop yield while increasing groundwater contamination [13]. On the other hand, deficit irrigation can increase productivity while saving irrigation water, especially in arid and semi-arid regions [4]. Scientists have shown that it is possible to increase WUE by 15 to 40% through improved nutrient and soil management practices, which can positively affect crop yield while conserving water [14,15].

The most sensitive potato growth stages to deficit irrigation are tuber bulking and tuber ripening [16]. Many studies report that potatoes are very sensitive to water stress due to their sparse and shallow root system [12,16]. Deficit irrigation has achieved success with several crops; however, it is difficult to manage potatoes because irrigation is essential for higher production of potatoes [12]. Sufficient irrigation supply before and during tuber initiation increases the number of tubers per plant, while after tuber initiation, tuber size is promoted by irrigation [16]. In addition, leaf area index (LAI) and biomass under deficit irrigation treatment are significantly less than under full irrigation, limiting photosynthate production [12].

Understanding the interactions between water and N over time is very important for improving the sustainability of agriculture in arid and semi-arid areas [17]. Interactions between water and N regulate the geochemical cycling of N and influence crop yield, grain size and protein concentration, photosynthesis, senescence and root architecture, root-to-shoot translocation, and soil microbial enzyme activity [17].

Soil water content directly affects the availability of N for crop uptake, and N affects plant canopy size and crop water use, which controls evapotranspiration [17]. The ability of plants to take up N is influenced by the irrigation regime and amount of water in the soil [18]. Therefore, increasing the availability of N and water to crops are management practices that often result in increased yield and profit [17]. The issues of water and N supply are interconnected and should not be considered separately [7]. Water use between 80% and 100% of ET was optimal to achieve maximum N uptake, high yield, biomass, and quality of an irrigated potato crop in Italy [18]. Although their study, conducted under different environmental and soil conditions in Italy, offers a useful reference, it does not fully account for the unique characteristics of the San Luis Valley, such as its high elevation, cool climate, alkaline soils, and potato varieties. Therefore, while informative, existing research provides limited insight into how these practices affect potato cultivars grown in the San Luis Valley.

Since irrigation water and N management have been high-priority issues for global agricultural production and the global economy, there is an urgent need to use irrigation water and N fertilizer efficiently in potato production systems while sustaining productivity. The objective of this study was to evaluate the effect of different levels of irrigation and N application rates on tuber yield and growth parameters of four potato cultivars grown in the San Luis Valley of Colorado (USA). This research will provide guidance for farmers striving to conserve water and N while maintaining productivity.

2. Materials and Methods

2.1. Experimental Site

Field experiments were laid out at Colorado State University’s San Luis Valley Research Center (latitude 37°43′ N, longitude 106°9′ W, and 2310 m altitude) during the 2016 and 2017 potato cropping seasons. The soil is classified as a Norte gravelly sandy loam [loamy-skeletal, mixed (calcareous), frigid Aquic Ustorthents]. Typically, the area receives annual rainfall of 10 to 18 cm. Prior to the potatoes, the preceding crop was dry beans. Soil pH was 8.0–8.1, and residual soil NO₃-N was 1.86 mg kg⁻¹ and 1.98 mg kg⁻¹ in 2016 and 2017, respectively.

Four potato cultivars (certified, disease-free seed) were evaluated in the experiment: Canela Russet (medium maturity with high specific gravity), Mesa Russet (medium maturity with medium specific gravity), Russet Norkotah3 (early maturity, fresh market, with medium to high specific gravity), and Yukon Gold (early to medium maturity, with yellow flesh, and high specific gravity).

2.2. Experimental Design and Field Layout

Experiments were laid out with six treatments (3 irrigation levels × 2 fertilizer treatments) and four replications. Each plot consisted of four rows, 8 m long, and 0.9 m between rows. Cut or whole seeds were planted 0.3 m apart within rows. Cut seed (~80 to 85 g) of each cultivar was planted to a depth of 10 to 13 cm using a potato planter on 19 May 2016 and 3 June 2017. The four cultivars were planted in separate blocks as independent experiments.

2.3. Irrigation Water Treatments

Daily reference evapotranspiration (ET) was estimated using the Hargreaves method [19], with data obtained from a weather station located at the San Luis Valley Research Center (https://coagmet.colostate.edu/ (accessed on 2 April 2025)). There were three irrigation treatments: the control, which provided 100% ET replacement for the crop, and two deficit irrigation levels targeting 80% and 70% of ET replacement (

Table 1. Irrigation treatments for four potato cultivars grown in 2016 and 2017 at the Colorado State University San Luis Valley Research Center. Irrigation amounts are given as a percentage of evapotranspiration replacement and in parentheses as a depth measurement in cm. Deficit irrigation treatments began about 60 days after planting. IRRG1 = lowest irrigation treatment, IRRG2 = moderate irrigation treatment, and IRRG3 = full irrigation treatment.

Cultivar and Year	Irrigation Treatment
	IRRG1	IRRG2	IRRG3
Canela Russet
2016	67% (36 cm)	84% (46 cm)	100% (54 cm)
2017	55% (31 cm)	82% (46 cm)	100% (56 cm)
Mesa Russet
2016	75% (37 cm)	94% (46 cm)	100% (49 cm)
2017	66% (33 cm)	90% (45 cm)	100% (50 cm)
Russet Norkotah3
2016	67% (36 cm)	84% (45 cm)	100% (54 cm)
2017	61% (34 cm)	77% (43 cm)	100% (56 cm)
Yukon Gold
2016	69% (36 cm)	85% (44 cm)	100% (52 cm)
2017	61% (28 cm)	83% (38 cm)	100% (46 cm)

). All experimental plots were provided with 100% ET replacement until initiation of tuber bulking and were then started on different levels of deficit irrigation about 60 days after planting (DAP). A single-line source irrigation technique was used to achieve deficit irrigation levels (Figure 1); therefore, irrigation treatments could not be randomized. However, soil properties were uniform within the study area, thus limiting potential bias due to the experimental design.

Unfortunately, due to variability in the irrigation system, Mesa Russet received a higher ET percentage for the deficit irrigation treatments than the other varieties; this was particularly true for the moderate irrigation treatment, where Mesa Russet received 90–94% ET compared to the other varieties, which received 77–85% ET. Although the actual amount of water applied under the IRRG2 treatment was similar across cultivars in both years, the percentage of ET replacement for Mesa Russet appears higher because the full irrigation (IRRG3) treatment for this cultivar received less total water (49 and 50 cm) for 2016 and 2017, respectively, compared to most of the other cultivars (52–56 cm). As a result, the same volume of water under IRRG2 represents a greater proportion of full ET for Mesa Russet, artificially inflating its percentage value relative to the other cultivars.

2.4. Fertilizer Application Treatments

Before planting potatoes, 68 kg N ha⁻¹ [urea ammonium nitrate (32-0-0)] was applied to all plots, banded 5 cm below and 5 cm to the side of each seed piece. In addition, 67 kg ha⁻¹ of ammonium phosphate (10-34-0) and 45 kg ha⁻¹ of potash [KCl (0-0-60)] were applied to all plots as pre-plant fertilizers based on Colorado State University (CSU) fertilizer recommendations for potatoes [20].

Two N application rates were applied to separate plots within each irrigation treatment. After tuberization, in-season N fertilizer was applied in three split applications at weekly intervals, with the first in-season N application at 64 and 50 days after planting (DAP) in 2016 and 2017, respectively, to achieve total N rates of either 131 or 165 kg N ha⁻¹ (

Table 2. Nitrogen fertilizer application treatments used in 2016–2017 potato experiments at the Colorado State University San Luis Valley Research Center.

Treatment	Low Nitrogen Application (N1)	Recommended Nitrogen Application (N2)
	-------------------------kg N ha−1-------------------------
Pre-plant	75	75
In-season	56 (split application 22/22/12)	90 (split application 34/34/22)
Total nitrogen applied	131	165

). In-season N fertilizer was applied using a boom sprayer and was immediately watered in after spraying.

2.5. Data Collection

2.5.1. Irrigation Water Measurement

Rain gauges were installed in the middle of each line of plots above the crop canopy to catch and measure the rainfall and the amount of water applied with each irrigation (Table 1). Reading was performed immediately after water application to prevent evaporation from the rain gauges.

2.5.2. Plant Sampling

Two weeks after deficit irrigation and N application rate treatments were initiated, two plants were sampled from each plot for a total of eight sampled plants per treatment. One of the two middle rows was maintained for sampling, and the other was for harvesting. Samples were collected 82, 103, and 113 DAP in 2016 and 104, 111, and 118 DAP in 2017 to measure the number of stems, number of tubers, leaf area index (LAI), and tuber bulking. Sampled plants were separated into aboveground vegetative growth (stems and leaves) and tubers. Tubers were washed and weighed to quantify tuber bulking at each sampling date. A leaf area meter (LI-COR, Inc., LI-3100 Area Meter, Lincoln, NE, USA) was used to measure total leaf area. The leaf area index was calculated as green leaf area/unit area of land. The number of stems and tubers per plant was counted in each plot.

2.5.3. Tuber Yield and Tuber Size Distribution

Potatoes were harvested at 115 DAP in 2016 (27 September) and 120 DAP (10 October) in 2017. Vines were killed by mechanical flailing, and a one-row tractor-mounted potato digger was used to dig one of two middle rows that had been reserved for end-of-season tuber yield and tuber size distribution. The harvested tubers from each plot were weighed for total tuber yield. Tubers were separated into various size distribution groups based on weight (>114 g, 170 g, 285 g, 114–285 g, and 170–454 g). Premium potatoes are those greater than 170 g, and marketable yield is the yield of potatoes >114 g.

2.5.4. Statistical Analysis

Statistical analysis was conducted on all data collected, using analysis of variance (ANOVA). Data analysis was conducted using the PROC MIXED procedure in SAS (Statistical Analysis System, version 9.4, Cary, NC, USA) with water and nitrogen treatments as main effects. Cultivars were analyzed separately. Fisher’s protected least significant difference (LSD) test was used for mean separation with a p-value of 0.05. If significant interaction (p < 0.05) between irrigation and N treatments was detected, pairwise comparisons were performed on the interaction using the SLICE statement to compare water levels within each N level using the Tukey adjustment. The Pearson Correlation Coefficient Analysis was used to evaluate the relationship between total yield vs. leaf area index (LAI) and tuber bulking (TB) for each of the cultivar by year combinations.

3. Results

3.1. Climate

The San Luis Valley Research Center received precipitation during the 2016 and 2017 growing seasons, as shown in Figure 2 (https://coagmet.colostate.edu/ accessed on 2 April 2025). In 2016, precipitation was fairly evenly distributed, except for a major peak in August, aligning with the tuber bulking stage, and total precipitation was 86 mm. In 2017, precipitation was highly variable, with dry early-season conditions and peaks occurring in July and September, with total precipitation of 108 mm. Although total rainfall was higher in 2017 compared to 2016, the poor distribution and timing of precipitation may have limited its benefit for potato growth and yield. Research indicates that not only the amount but also the distribution and timing of precipitation significantly influences potato yields [21].

Growing Degree Days (GDDs) are beneficial for tracking growth parameters and can be used as an indicator of yield [22]. The results from two years of GDD indicate that cumulative GDD was slightly higher in 2017 than in 2016, particularly during the early and mid-growth stages (Figure 3) (https://coagmet.colostate.edu/ accessed on 2 April 2025). However, the difference between the two years was minimal and diminished by the end of the growing season.

3.2. Total Yield and Tuber Size Distribution

Each potato variety was grown in a separate experimental block; therefore, the varieties cannot be directly compared and will be discussed individually.

3.2.1. Canela Russet

In 2016 and 2017, reducing irrigation by 16% and 18% (IRRG2), respectively, resulted in total yields of Canela Russet that were not significantly different from full irrigation (IRRG3); however, IRRG2 did reduce the yield of some size fractions in 2017 (

Table 3. Effect of deficit irrigation and nitrogen fertilizer application rate on potato yield and tuber size distribution of Canela Russet in 2016 and 2017.

Treatments	2016
	Total Y		>114 g	>170 g	>285 g	114–285 g	114–454 g	170–454 g
Water	Yield (Mg ha−1)
IRRG3	45 a		39 a	31 a	10 b	29 a	37 a	29 a
IRRG2	47 a		41 a	33 a	13 a	28 a	39 a	31 a
IRRG1	36 b		28 b	20 b	6 c	23 b	28 b	19 b
Nitrogen
N1	42 a		36 a	27 a	10 a	27 a	35 a	26 a
N2	42 a		36 a	29 a	10 a	26 a	35 a	27 a
Source of Variation	DF		p-Value
Water	2	0.0004	<0.0001	<0.0001	0.0006	0.0036	<0.0001	0.0001
Nitrogen	1	0.9468	1.0000	0.3563	0.9173	0.5906	0.6904	0.5394
W × N	2	0.9600	0.9941	0.8846	0.1833	0.5170	0.5166	0.7572
	2017
Water
IRRG3	40 a		35 a	27 a	9 a	26 a	34 a	26 a
IRRG2	38 a		30 b	20 b	6 ab	24 a	29 b	19 b
IRRG1	27 b		19 c	10 c	2 b	17 b	19 c	10 c
Nitrogen
N1	32 b		24 b	15 a	3 b	20 b	24 b	15 b
N2	38 a		32 a	23 b	7 a	25 a	31 a	22 a
Source of Variation	DF		p-Value
Water	2	<0.0001	<0.0001	<0.0001	0.0069	0.0010	<0.0001	<0.0001
Nitrogen	1	0.0003	<0.0001	0.0001	0.0212	0.0214	<0.0001	<0.0001
W × N	2	0.1269	0.3913	0.6799	0.1285	0.7071	0.2117	0.7169

Means within the same columns followed by the same letters are not significantly different (ns) at the 0.05 level of probability. IRRG1 = lowest irrigation treatment, IRRG2 = moderate irrigation treatment, and IRRG3 = full irrigation treatment. N1: Nitrogen reduced level (131 kg N ha⁻¹). N2: Nitrogen recommended level (165 kg N ha⁻¹). DF = degrees of freedom. W × N = The interaction between water (W) and nitrogen (N) treatments.

). In addition, a reduction of 33% and 45% for 2016 and 2017, respectively (IRRG1), significantly decreased total yield and marketable yield (114–285 g or 114–454 g). In 2016, nitrogen treatment had no effect on yield. However, in 2017, the recommended nitrogen level (N2) significantly increased total yield and marketable tubers (114–285 g and 114–454 g) compared to the lower nitrogen level (N1). No significant interactions were observed in either year.

3.2.2. Mesa Russet

In 2016 and 2017, reducing irrigation by 6% or 10% (IRRG2) had no significant effect on total yield or marketable tubers of Mesa Russet (114–285 g and 114–454 g) compared to full irrigation (IRRG3) (

Table 4. Effect of deficit irrigation and nitrogen fertilizer application rate on potato yield and tuber size distribution of Mesa Russet in 2016 and 2017.

2016
Treatments	Total Y		>114 g	>170 g	>285 g	114–285 g	114–454 g	170–454 g
Water	Yield (Mg ha−1)
IRRG3	43 a		28 a	17 a	3 a	26 a	28 a	17 a
IRRG2	48 a		34 a	19 a	4 a	29 a	34 a	19 a
IRRG1	31 b		14 b	5 b	0 b	14 b	14 b	5 b
Nitrogen
N1	41 a		27 a	14 a	2 a	24 a	27 a	14 a
N2	40 a		24 a	13 a	3 a	22 a	24 a	13 a
Source of Variation	DF		p-Value
Water	2	0.0001	<0.0001	0.0001	0.0018	0.0001	<0.0001	0.0001
Nitrogen	1	0.7835	0.4327	0.8909	0.4006	0.2299	0.4327	0.8909
W × N	2	0.2372	0.5445	0.5000	0.2428	0.6346	0.5445	0.5000
2017
Water
IRRG3	38 a		28 a	16 a	6 a	22 b	28 a	16 a
IRRG2	36 a		30 a	15 a	4 a	27 a	30 a	15 a
IRRG1	26 b		14 b	4 b	0 b	14 c	14 b	4 b
Nitrogen
N1	34 a		25 a	12 a	3 a	22 a	25 a	12 a
N2	33 a		23 a	12 a	3 a	20 a	23 a	12 a
Source of Variation	DF		p-Value
Water	2	<0.0001	<0.0001	<0.0001	0.0008	<0.0001	<0.0001	<0.0001
Nitrogen	1	0.5664	0.3012	0.9165	0.7855	0.1822	0.2980	0.9165
W × N	2	0.7432	0.9026	0.7312	0.9546	0.7778	0.9051	0.7312

Means within the same columns followed by the same letters are not significantly different (ns) at the 0.05 level of probability. IRRG1 = lowest irrigation treatment, IRRG2 = moderate irrigation treatment, and IRRG3 = full irrigation treatment. N1: Nitrogen reduced level (131 kg N ha⁻¹). N2: Nitrogen recommended level (165 kg N ha⁻¹). DF = degrees of freedom. W × N = The interaction between water (W) and nitrogen (N) treatments.

). However, a reduction of 25% or 34% (IRRG1) resulted in significantly lower total yield and marketable tubers in both years. No significant differences were observed between nitrogen levels in either year, and no significant interactions were observed across all tuber size categories.

3.2.3. Russet Norkotah3

A significant difference was observed in Russet Norkotah3 yields among irrigation treatments in both 2016 and 2017 (

Table 5. Effect of deficit irrigation and nitrogen fertilizer application rate on potato yield and tuber size distribution of Russet Norkotah3 in 2016 and 2017.

2016
Treatments	Total Y	>114 g	>170 g	>285 g	114–285 g	114–454 g		170–454 g
Water		Yield (Mg ha−1)
IRRG3		49 a	41 a	31 a	10 a	32 a	41 a	31 a
IRRG2		44 b	36 b	25 b	8 ab	28 ab	36 b	25 b
IRRG1		40 b	30 c	18 c	5 b	25 b	29 c	18 c
Nitrogen
N1		46 a	36 a	25 a	7 a	29 a	36 a	25 a
N2		43 a	35 a	24 a	8 a	27 a	35 a	24 a
Interaction
Nitrogen	Water
N1	IRRG3	52 a	42 a	32 a	7 a	35 a	42 a	32 s
	IRRG2	45 a	35 a	24 a	8 a	28 a	35 a	24 s
	IRRG1	41 a	31 a	19 a	7 a	24 a	30 a	19 s
N2	IRRG3	46 a	40 a	30 a	12 a	28 a	40 s	30 s
	IRRG2	44 a	36 a	26 a	8 ab	28 a	36 s	26 a
	IRRG1	39 a	28 a	17 a	3 b	25 a	28 s	17 a
Source of Variation	DF	p-Value
Water	2	0.0024	0.0002	<0.0001	0.0210	0.0157	<0.0001	<0.0001
Nitrogen	1	0.1868	0.3368	0.4875	0.6701	0.2077	0.3442	0.5861
W × N	2	0.5043	0.7196	0.5592	0.0484	0.1032	0.6447	0.6210
2017
Treatments		Total Y	>114 g	>170 g	>285 g	114–285 g	114–454 g	170–454 g
Water		Yield (Mg ha−1)
IRRG3		43 a	31 a	24 a	13 a	18 a	26 a	19 a
IRRG2		27 b	15 b	9 b	3 b	11 b	14 b	9 b
IRRG1		24 b	12 b	8 b	2 b	10 b	12 b	8 b
Nitrogen
N1		31 a	19 a	14 a	7 a	12 a	18 a	12 a
N2		32 a	19 a	14 a	6 a	14 a	17 a	11 a
Source of Variation	DF	p-Value
Water	2	<0.0001	<0.0001	<0.0001	<0.0001	0.0056	<0.0001	0.0007
Nitrogen	1	0.4777	0.8019	0.9279	0.4651	0.3782	0.7336	0.5909
W × N	2	0.6271	0.7275	0.4077	0.4388	0.9980	0.6074	0.5108

Means within the same columns followed by the same letters are not significantly different (ns) at 0.05 level of probability. IRRG1 = lowest irrigation treatment, IRRG2 = moderate irrigation treatment, and IRRG3 = full irrigation treatment. N1: Nitrogen reduced level (131 kg N ha⁻¹). N2: Nitrogen recommended level (165 kg N ha⁻¹). DF = degrees of freedom. W × N = The interaction between water (W) and nitrogen (N) treatments.

). In both years, full irrigation (IRRG3) produced the highest total yield and marketable tuber yield (114–285 g and 114–454 g). Reducing irrigation by 16% (IRRG2) reduced yield in most tuber size categories compared to full irrigation, and reducing irrigation by 33% or 39% (IRRG1) for 2016 and 2017, respectively, led to significant yield losses in every category. Nitrogen treatments had no effect on total yield or size fractions in either year. No significant interactions were found except for the category >285 g (large tuber), which showed a significant interaction of water within the N level in 2016. These findings indicate that reducing irrigation by more than 16% ET negatively affected Russet Norkotah3 tuber yields.

3.2.4. Yukon Gold

In 2016 and 2017, reducing irrigation by 15% or 17% (IRRG2) had no significant effect on total yield or marketable tubers (114–285 g and 114–454 g) of Yukon Gold compared to full irrigation (IRRG3) (

Table 6. Effect of deficit irrigation and nitrogen fertilizer application rate on potato yield and tuber size distribution of Yukon Gold in 2016 and 2017.

2016
Treatments		Total Y	>114 g	>170 g	>285 g	114–285 g	114–454 g	170–454 g
Water		Yield (Mg ha−1)
IRRG3		41 a	33 a	23 a	7 a	28 a	34 a	23 a
IRRG2		40 a	31 a	18 b	3 b	28 a	31 a	18 b
IRRG1		34 b	21 b	11 c	1 b	20 b	21 b	11 c
Nitrogen
N1		40 a	30 a	19 a	4 a	26 a	30 a	19 a
N2		37 b	26 b	16 b	2 a	25 a	27 a	16 b
Interaction
Nitrogen	Water
N1	IRRG3	45 a	37 a	27 a	9 a	27 a	37 a	27 a
	IRRG2	38 b	30 b	18 a	3 a	27 a	30 a	18 a
	IRRG1	38 b	23 c	12 a	0 a	23 a	23 a	12 a
N2	IRRG3	38 a	29 a	19 a	4 a	28 a	32	19 a
	IRRG2	41 a	31 a	18 a	2 a	29 a	31 a	18 a
	IRRG1	31 b	19 b	10 a	1 a	18 a	19 a	10 a
Source of Variation	DF	p-Value
Water	2	0.0031	<0.0001	<0.0001	0.0002	0.0012	<0.0001	<0.0001
Nitrogen	1	0.0245	0.0116	0.0253	0.0741	0.5888	0.1247	0.0370
W × N	2	0.0262	0.0316	0.0712	0.0537	0.1716	0.2545	0.1004
2017
Treatments		Total Y	>114 g	>170 g	>285 g	114–285 g	114–454 g	170–454 g
Water		Yield (Mg ha−1)
IRRG3		37 a	28 a	18 a	6 a	22 ab	28 a	18 a
IRRG2		36 a	27 a	15 b	2 b	25 a	27 a	15 b
IRRG1		29 b	20 b	10 c	1 b	19 b	20 b	10 c
Nitrogen
N1		32 b	23 b	13 b	3 a	20 b	23 b	13 b
N2		36 a	27 a	16 a	3 a	24 a	27 a	16 a
Source of Variation	DF	p-Value
Water	2	0.0021	0.0009	0.0003	<0.0001	0.0329	0.0009	0.0003
Nitrogen	1	0.0142	0.0143	0.0207	0.5377	0.0361	0.0143	0.0207
W × N	2	0.6485	0.1862	0.2013	0.1641	0.1934	0.1862	0.2013

Means within the same columns followed by the same letters are not significantly different (ns) at the 0.05 level of probability. IRRG1 = lowest irrigation treatment, IRRG2 = moderate irrigation treatment, and IRRG3 = full irrigation treatment. N1: Nitrogen reduced level (131 kg N ha⁻¹). N2: Nitrogen recommended level (165 kg N ha⁻¹). DF = degrees of freedom. W × N = The interaction between water (W) and nitrogen (N) treatments.

). However, a reduction of 31% or 39% (IRRG1) significantly decreased total yield and marketable tuber yield in both years. The effects of N varied; lower N levels produced higher total yields but were not significant for marketable yield (114–285 g and 114–454 g) in 2016, while higher N levels increased total and marketable yields in 2017. No significant interactions were observed, except for total and marketable yields in 2016, when full irrigation (IRRG3) resulted in the highest yield under low N (N1), while the recommended nitrogen level (N2) increased tolerance of moderate water stress (IRRG2). However, severe water deficit (IRRG1) reduced yield regardless of the N rate.

3.3. Tuber Bulking (TB)

3.3.1. Canela Russet

Water treatment significantly affected TB of Canela Russet at all measured stages (p < 0.0001) in 2016 and 2017 (

Table 7. Effect of deficit irrigation and nitrogen fertilizer application rate on potato tuber bulking of Canela Russet in 2016 and 2017.

	Years and Days After Planting (DAP)
	2016				2017
Treatments		82 DAP	103 DAP	113 DAP	104 DAP	111 DAP	118 DAP
Water
IRRG3		765.69 a	1120.31 a	1211.48 a	979.41 a	1145.62 a	1144.53 a
IRRG2		726.94 b	909.84 b	1140.61 b	960.72 a	892.17 b	928.29 b
IRRG1		460.53 c	596.02 c	678.53 c	565.40 b	718.50 c	664.66 c
Nitrogen
N1		660.82 a	884.19 a	1054.08 a	808.79 b	886.43 b	821.32 b
N2		641.28 a	866.60 a	966.34 b	861.56 a	951.09 a	1003.67 a
Interaction
Nitrogen	Water
N1	IRRG3	695.71 b	1236.06 a	1158.03 b	1017.41 a	1023.76 a	1038.91 a
	IRRG2	787.31 a	824.14 b	1263.39 a	917.41 b	947.50 b	754.55 b
	IRRG1	499.44 c	539.59 c	740.81 c	491.55 c	688.04 c	670.49 c
N2	IRRG3	835.66 a	1004.56 a	1123.20 a	941.40 a	1267.48 a	1250.15 a
	IRRG2	666.58 b	995.55 a	1159.56 a	1004.03 a	836.84 b	1102.04 b
	IRRG1	421.61 c	652.45 b	616.25 b	639.25 b	748.96 c	658.83 c
Source of Variation	DF	p-Value
Water	2	<0.0001	<0.0001	<0.0001	<0.0001	<0.0001	<0.0001
Nitrogen	1	0.1758	0.1086	<0.0001	0.0021	<0.0001	<0.0001
W × N	2	<0.0001	<0.0001	0.0051	<0.0001	<0.0001	<0.0001

Means within the same columns followed by the same letters are not significantly different (ns) at the 0.05 level of probability. IRRG1 = lowest irrigation treatment, IRRG2 = moderate irrigation treatment, and IRRG3 = full irrigation treatment. N1: Nitrogen reduced level (131 kg N ha⁻¹). N2: Nitrogen recommended level (165 kg N ha⁻¹). DF = degrees of freedom. W × N = The interaction between water (W) and nitrogen (N) treatments.

). IRRG3 (full irrigation) had the highest TB, followed by IRRG2 (moderate deficit irrigation), while IRRG1 (lowest deficit irrigation) had the lowest values in both years. In 2016, N showed a significant effect on TB at the end of the season at 113 DAP (p < 0.0001), with N1 (low N) producing a higher TB than N2 (recommended N). However, in 2017, N treatment showed a significant effect on TB at all three sampling dates; the recommended level of N (N2) had greater tuber bulking than the lower N rate (N1). The interaction between the water and N treatments was significant at all stages in both years, and full irrigation consistently produced the highest tuber bulking across both N rates, except at the low N (N1) rate in 2016.

3.3.2. Mesa Russet

Water levels significantly affected TB at all sampling dates in both years (

Table 8. Effect of deficit irrigation and nitrogen fertilizer application rate on potato tuber bulking of Mesa Russet in 2016 and 2017.

	Years and Days After Planting (DAP)
	2016				2017
Treatments		82 DAP	103 DAP	113 DAP	104 DAP	111 DAP	118 DAP
Water
IRRG3		947.42 a	908.46 c	1094.40 b	801.14 a	845.79 a	969.53 a
IRRG2		871.80 b	1104.71 a	1050.23 c	670.01 b	800.14 b	847.13 b
IRRG1		798.83 c	1054.44 b	1198.81 a	643.51 b	761.83 c	776.47 c
Nitrogen
N1		943.50 a	1236.05 a	1187.33 a	740.73 a	827.55 a	867.60 a
N2		801.86 b	809.03 b	1041.63 b	669.04 b	777.62 b	861.15 a
Interaction
Nitrogen	Water
N1	IRRG3	985.50 a	1187.99 b	1164.03 b	880.99 a	959.15 a	995.31 a
	IRRG2	910.81 b	1289.80 a	1046.50 c	764.04 b	775.68 b	790.69 b
	IRRG1	934.19 ab	1230.36 ab	1351.45 a	577.15 c	747.83 b	816.79 b
N2	IRRG3	909.34 a	628.94 b	1024.78 a	721.29 a	732.43 b	943.74 a
	IRRG2	832.79 b	919.61 a	1053.95 a	575.98 b	824.61 a	903.56 a
	IRRG1	663.46 c	878.53 a	1046.18 a	709.86 ab	775.83 ab	736.15 b
Source of Variation	DF	p-Value
Water	2	<0.0001	<0.0001	<0.0001	<0.0001	0.0007	<0.0001
Nitrogen	1	<0.0001	<0.0001	<0.0001	0.0017	0.0028	0.6074
W × N	2	<0.0001	<0.0001	<0.0001	<0.0001	<0.0001	<0.0001

Means within the same columns followed by the same letters are not significantly different (ns) at the 0.05 level of probability. IRRG1 = lowest irrigation treatment, IRRG2 = moderate irrigation treatment, and IRRG3 = full irrigation treatment. N1: Nitrogen reduced level (131 kg N ha⁻¹). N2: Nitrogen recommended level (165 kg N ha⁻¹). DF = degrees of freedom. W × N = The interaction between water (W) and nitrogen (N) treatments.

). In 2017, IRRG3 (full irrigation) produced the highest TB on every sampling date, but in 2016, results were more variable. Nitrogen had a strong effect on TB at most stages in 2016 and 2017, where N1 (low nitrogen) led to higher TB than N2 in both years. Water × Nitrogen interactions were significant on every sampling date.

3.3.3. Russet Norkotah3

Water treatments significantly affected TB of Russet Norkotah3 at all sampling dates in both years (

Table 9. Effect of deficit irrigation and nitrogen fertilizer application rate on potato tuber bulking of Russet Norkotah3 in 2016 and 2017.

	Years and Days After Planting (DAP)
	2016				2017
Treatments		82 DAP	103 DAP	113 DAP	104 DAP	111 DAP	118 DAP
Water
IRRG3		937.59 a	1251.69 a	1433.91 a	956.39 a	903.32 b	1020.69 b
IRRG2		928.16 a	1140.33 b	1223.40 b	735.94 b	997.32 a	1094.78 a
IRRG1		585.36 b	796.36 c	865.77 c	631.38 c	742.75 c	706.66 c
Nitrogen
N1		736.40 b	1091.75 a	1179.41 a	787.25 a	831.48 b	955.67 a
N2		897.68 a	1033.84 b	1169.30 a	761.89 a	930.78 a	925.75 a
Interaction
Nitrogen	Water
N1	IRRG3	822.58 a	1180.60 a	1299.46 b	1018.80 a	977.31 a	899.34 b
	IRRG2	844.10 a	1203.44 a	1355.71 a	710.18 b	838.93 b	1098.68 a
	IRRG1	542.51 b	717.48 b	883.06 c	632.79 c	678.21 c	869.00 b
N2	IRRG3	1052.61 a	132,278 a	1568.35 a	893.98 a	829.33 b	1142.05 a
	IRRG2	1012.21 a	1091.09 b	1091.09 b	761.71 b	1155.71 a	1090.89 a
	IRRG1	628.21 b	848.48 c	848.48 c	629.98 c	807.29 b	544.31 b
Source of Variation	DF	p-Value
Water	2	<0.0001	<0.0001	<0.0001	<0.0001	<0.0001	<0.0001
Nitrogen	1	<0.0001	0.0032	0.4210	0.0546	<0.0001	0.0496
W × N	2	0.0014	<0.0001	<0.0001	<0.0001	<0.0001	<0.0001

Means within the same columns followed by the same letters are not significantly different (ns) at the 0.05 level of probability. IRRG1 = lowest irrigation treatment, IRRG2 = moderate irrigation treatment, and IRRG3 = full irrigation treatment. N1: Nitrogen reduced level (131 kg N ha⁻¹). N2: Nitrogen recommended level (165 kg N ha⁻¹). DF = degrees of freedom. W × N = The interaction between water (W) and nitrogen (N) treatments.

). IRRG3 (full irrigation) had the highest TB in 2016, but in 2017, IRRG2 had higher TB in two out of three sampling times. IRRG1 had the lowest TB in all years and sampling times. Nitrogen treatment sometimes had a significant effect on TB, but the effect was inconsistent. The interaction between water and N treatments was significant on all dates; full irrigation (IRRG3) usually produced the highest TB at both nitrogen rates.

3.3.4. Yukon Gold

Water treatments significantly affected the TB of Yukon Gold at all stages in both years (

Table 10. Effect of deficit irrigation and nitrogen fertilizer application rate on potato tuber bulking of Yukon Gold in 2016 and 2017.

	Years and Days After Planting (DAP)
	2016				2017
Treatments		82 DAP	103 DAP	113 DAP	104 DAP	111 DAP	118 DAP
Water
IRRG3		990.53 a	1215.50 a	1274.23 b	770.75 b	820.34 a	913.71 b
IRRG2		946.89 b	1143.19 b	1431.72 a	815.88 a	808.19 a	853.22 c
IRRG1		617.36 c	904.73 c	821.44 c	805.83 ab	732.84 b	982.03 a
Nitrogen
N1		892.00 a	1077.92 a	1262.24 a	750.03 b	776.71 a	994.53 a
N2		811.19 b	1097.69 a	1089.35 b	844.94 a	797.54 a	838.10 b
Interaction
Nitrogen	Water
N1	IRRG3	1105.59 a	1176.53 a	1538.83 a	715.23 b	736.74 b	855.21 a
	IRRG2	928.78 b	1144.68 a	1410.06 b	865.68 a	899.35 a	770.19 a
	IRRG1	641.63 c	912.55 b	837.83 c	669.20 b	694.05 b	888.91 a
N2	IRRG3	875.48 b	1254.48 a	1009.63 b	826.28 b	903.94 a	972.20 a
	IRRG2	965.00 a	1141.70 a	1453.38 a	766.08 b	717.04 c	936.25 a
	IRRG1	593.09 c	896.90 b	805.05 c	942.46 a	771.64 b	1075.15 a
Source of Variation	DF	p-Value
Water	2	<0.0001	<0.0001	<0.0001	0.0473	<0.0001	<0.0001
Nitrogen	1	<0.0001	0.1921	<0.0001	<0.0001	0.0939	<0.0001
W × N	2	<0.0001	0.0356	<0.0001	<0.0001	<0.0001	0.2324

Means within the same columns followed by the same letters are not significantly different (ns) at the 0.05 level of probability. IRRG1 = lowest irrigation treatment, IRRG2 = moderate irrigation treatment, and IRRG3 = full irrigation treatment. N1: Nitrogen reduced level (131 kg N ha⁻¹). N2: Nitrogen recommended level (165 kg N ha⁻¹). DF = degrees of freedom. W × N = The interaction between water (W) and nitrogen (N) treatments.

). In 2016, IRRG3 had the highest TB except at 113 DAP, when the moderate irrigation IRRG2 was highest, and the low irrigation water treatment (IRRG1) had the lowest TB on every sampling date. In 2017, TB was highest at 104 DAP with moderate irrigation and at 111 DAP with full or moderate irrigation levels. Nitrogen significantly influenced tuber bulking at 82 and 113 DAP in 2016 and 104 and 118 DAP in 2017; N1 (low nitrogen) produced higher TB than N2 in most cases. The interaction was significant in most sampling times in both years, and full or moderate irrigation generally produced the highest TB.

3.4. Leaf Area Index (LAI)

3.4.1. Canela Russet

Water treatment significantly affected the LAI of Canela Russet at 82 and 103 DAP in 2016 and 104 DAP in 2017 (

Table 11. Effect of deficit irrigation and nitrogen fertilizer application rate on leaf area index (LAI) of Canela Russet in 2016 and 2017.

	Years and Days After Planting (DAP)
		2016			2017
Treatments		82 DAP	103 DAP	113 DAP	104 DAP	111 DAP	118 DAP
Water
IRRG3		2.9625 a	2.550 a	0.375 a	1.4875 a	1.100 a	-
IRRG2		2.7375 a	2.1875 a	0.225 a	1.0625 b	0.775 a	-
IRRG1		1.4375 b	1.0625 b	0.2125 a	0.500 c	0.5875 a	-
Nitrogen
N1		2.425 a	1.9334 a	0.29167 a	1.0667 a	1.6560 a	-
N2		2.333 a	1.9334 a	0.2500 a	0.9667 a	0.5917 b	-
Interaction
Nitrogen	Water
N1	IRRG3	2.600 a	2.6250 a	0.350 a	1.825 a	0.875 a	-
	IRRG2	3.000 a	2.175 a	0.275 a	0.75 b	0.500 a	-
	IRRG1	1.675 a	1.00 a	0.250 a	0.325 b	0.400 a	-
N2	IRRG3	3.325 a	2.475 a	0.400 a	1.150 a	1.325 a	-
	IRRG2	2.475 a	2.200 a	0.175 a	1.375 a	1.05 a	-
	IRRG1	1.200 a	1.125 a	0.175 a	0.675 a	0.775 a	-
Source of Variation	DF	p-Value
Water	2	0.0002	<0.0001	0.2236	0.0007	0.1291	-
Nitrogen	1	0.7230	1.000	0.6166	0.5679	0.00322	-
W × N	2	0.1039	0.8130	0.7289	0.0155	0.9365	-

Means within the same columns followed by the same letters are not significantly different (ns) at the 0.05 level of probability. IRRG1 = lowest irrigation treatment, IRRG2 = moderate irrigation treatment, and IRRG3 = full irrigation treatment. N1: Nitrogen reduced level (131 kg N ha⁻¹). N2: Nitrogen recommended level (165 kg N ha⁻¹). DF = degrees of freedom. W × N = The interaction between water (W) and nitrogen (N) treatments.

). The highest LAI was consistently observed under IRRG3 (full irrigation) in both years, while IRRG1 had the lowest LAI. Nitrogen levels generally had no significant effect on LAI at any stage in 2016 or 2017. In general, there was no significant interaction effect on LAI in 2016, but in 2017 at 104 DAP, there was a significant interaction of levels of irrigation within N treatment.

3.4.2. Mesa Russet

Water had a significant effect on the LAI of Mesa Russet at 82 and 113 DAP in 2016 and 104 and 111 DAP in 2017 (

Table 12. Effect of deficit irrigation and nitrogen fertilizer application rate on leaf area index (LAI) of Mesa Russet in 2016 and 2017.

	Years and Days After Planting (DAP)
		2016			2017
Treatments		82 DAP	103 DAP	113 DAP	104 DAP	111 DAP	118 DAP
Water
IRRG3		3.5375 a	3.000 a	1.4375 b	1.3250 a	1.1375 a	-
IRRG2		2.9750 ab	2.4625 a	0.9625 c	0.7625 b	0.6875 b	-
IRRG1		2.400 b	2.350 a	1.700 a	0.600 b	0.5875 b	-
Nitrogen
N1		2.9417 a	2.7250 a	1.2750 a	0.9833 a	0.8833 a	-
N2		3.000 a	2.4833 a	1.4583 a	0.8083 a	0.7250 a	-
Interaction
Nitrogen	Water
N1	IRRG3	3.325 a	3.200 a	1.275 b	1.575 a	0.925 a	-
	IRRG2	2.900 a	2.175 a	0.75 c	0.800 a	0.625 a	-
	IRRG1	2.600 a	2.800 a	1.800 a	0.575 a	0.625 a	-
N2	IRRG3	3.750 a	2.800 a	1.600 a	1.075 a	1.350 a	-
	IRRG2	3.050 a	2.525 a	1.175 a	0.725 a	0.750 a	-
	IRRG1	2.200 a	2.125 a	1.600 a	0.625 a	0.550 a	-
Source of Variation	DF	p-Value
Water	2	0.0336	0.3600	<0.001	0.0004	<0.0001	-
Nitrogen	1	0.859	0.5388	0.0761	0.1748	0.0601	-
W × N	2	0.5799	0.5435	0.0376	0.1929	0.0563	-

Means within the same columns followed by the same letters are not significantly different (ns) at 0.05 level of probability. IRRG1 = lowest irrigation treatment, IRRG2 = moderate irrigation treatment, and IRRG3 = full irrigation treatment. N1: Nitrogen reduced level (131 kg N ha⁻¹). N2: Nitrogen recommended level (165 kg N ha⁻¹). DF = degrees of freedom. W × N = The interaction between water (W) and nitrogen (N) treatments.

). In most cases, IRRG3 had the highest LAI, while IRRG1 had the lowest. Nitrogen had no significant effect on LAI in either year. There was no significant interaction observed in 2017, but in 2016, the interaction between water and N levels was significant at 113 DAP.

3.4.3. Russet Norkotah3

Water treatment significantly affected the LAI of Russet Norkotah3 on most sampling dates (p < 0.05) in both years; IRRG3 had the highest LAI, and IRRG1 was the lowest (LAI decreased with increased water deficit) (

Table 13. Effect of deficit irrigation and nitrogen fertilizer application rate on leaf area index (LAI) of Russet Norkotah3 in 2016 and 2017.

	Years and Days After Planting (DAP)
		2016			2017
Treatments		82 DAP	103 DAP	113 DAP	104 DAP	111 DAP	118 DAP
Water
IRRG3		2.9875 a	2.5625 a	0.6250 a	1.9875 a	1.6375 a	-
IRRG2		2.4875 a	1.9250 ab	0.4250 ab	1.3875 ab	1.1125 b	-
IRRG1		1.4250 b	1.2000 b	0.1500 b	0.9250 b	0.5000 c	-
Nitrogen
N1		2.5500 a	2.0583 a	0.4833 a	1.5083 a	1.3500 a	-
N2		2.0500 a	1.7333 a	0.3167 a	1.3583 a	0.8167 b	-
Source of Variation	DF	p-Value
Water	2	0.0035	0.0261	0.0636	0.0132	0.0002	-
Nitrogen	1	0.1450	0.3929	0.2915	0.5724	0.0072	-
W × N	2	0.9981	0.6490	0.8588	0.0825	0.1770	-

Means within the same columns followed by the same letters are not significantly different (ns) at the 0.05 level of probability. IRRG1 = lowest irrigation treatment, IRRG2 = moderate irrigation treatment, and IRRG3 = full irrigation treatment. N1: Nitrogen reduced level (131 kg N ha⁻¹). N2: Nitrogen recommended level (165 kg N ha⁻¹). DF = degrees of freedom. W × N = The interaction between water (W) and nitrogen (N) treatments.

). The N treatment was only significant on 111 DAP in 2017, and the interaction never had a significant effect on LAI.

3.4.4. Yukon Gold

Water treatment significantly affected the LAI of Yukon Gold at 82 DAP in 2016 and 104 and 111 DAP in 2017 (

Table 14. Effect of deficit irrigation and nitrogen fertilizer application rate on leaf area index (LAI) of Yukon Gold in 2016 and 2017.

	Years and Days After Planting (DAP)
		2016			2017
Treatments		82 DAP	103 DAP	113 DAP	104 DAP	111 DAP	118 DAP
Water
IRRG3		1.5863 a	0.3375 a	0.02500 a	0.6125 a	0.400 a	0.0000 a
IRRG2		1.6863 a	0.2188 a	0.01250 a	0.4375 ab	0.11250 b	0.1250 a
IRRG1		1.0625 b	0.1825 a	0.00000 a	0.2250 b	0.06250 b	0.0000 a
Nitrogen
N1		1.5000 a	0.2992 a	0.01667 a	0.4667 a	0.21667 a	0.08333 a
N2		1.3900 a	0.1933 a	0.00833 a	0.3833 a	0.16667 a	0.0000 a
Source of Variation	DF	p-Value
Water	2	0.0226	0.4492	0.3874	0.0171	0.0036	0.3874
Nitrogen	1	0.5448	0.3147	0.5709	0.4102	0.5143	0.3306
W × N	2	0.6797	0.4081	0.7209	0.9516	0.1512	0.3874

Means within the same columns followed by the same letters are not significantly different (ns) at the 0.05 level of probability. IRRG1 = lowest irrigation treatment, IRRG2 = moderate irrigation treatment, and IRRG3 = full irrigation treatment. N1: Nitrogen reduced level (131 kg N ha⁻¹). N2: Nitrogen recommended level (165 kg N ha⁻¹). DF = degrees of freedom. W × N = The interaction between water (W) and nitrogen (N) treatments.

). IRRG3 generally had the highest LAI, while IRRG1 had the lowest. Nitrogen had no significant effect at any stage, and the interaction was not significant on any sampling date.

3.5. Stem and Tuber Number

The numbers of stems and tubers per plant were rarely significant (p > 0.05) across water, N treatments, or their interaction for all four potato cultivars (Canela Russet, Mesa Russet, Russet Norkotah3 and Yukon Gold) in either 2016 (

Table 15. Effect of deficit irrigation and nitrogen fertilizer application rate on number of stems and tubers in four potato cultivars (Canela Russet, Mesa Russet, Russet Norkotah3, and Yukon Gold) 103 days after planting (DAP) in 2016.

Cultivars		Canela Russet		Mesa Russet		Russet Norkotah3		Yukon Gold
Treatments		No. of Stems Plant−1	No. of Tubers Plant−1	No. of Stems Plant−1	No. of Tubers Plant−1	No. of Stems Plant−1	No. of Tubers Plant−1	No. of Stems Plant−1	No. of Tubers Plant−1
Water
IRRG3		3.12 ab	7.50 a	3.62 a	8.62 a	4.25 a	9.12 a	3.00 a	8.75 a
IRRG2		2.81 b	6.62 a	3.94 a	10.56 a	4.56 a	9.62 a	3.18 a	9.12 a
IRRG3		3.75 a	6.56 a	4.06 a	11.12 a	4.56 a	7.75 a	3.25 a	9.12 a
Nitrogen
N1		3.17 a	7.25 a	4.08 a	11.46 a	3.96 a	9.08 a	3.21 a	9.33 a
N2		3.29 a	6.54 a	3.67 a	8.75 a	4.96 a	8.58 a	3.08 a	8.67 a
Interaction
N1	IRRG3	3.12 a	7.62 a	4.00 a	10.25 a	4.25 a	9.38 a	3.12 a	9.12 a
	IRRG2	3.38 a	6.38 a	4.25 a	11.88 a	3.75 a	10.88 a	3.38 a	9.38 a
	IRRG1	3.00 a	5.62 a	4.00 a	12.25 a	3.88 a	7.00 a	3.12 a	9.50 a
N2	IRRG3	3.12 b	7.38 a	3.25 a	7.00 a	4.25 a	8.88 a	2.88 a	8.38 a
	IRRG2	2.25 b	6.88 a	3.62 a	9.25 a	5.38 a	8.38 a	3.00 a	8.88 a
	IRRG1	4.50 a	7.50 a	4.12 a	10.00 a	5.25 a	8.50 a	3.38 a	8.75 a
Source of Variation	DF	p-Value
Water	2	0.0443	0.4680	0.7823	0.3029	0.8440	0.0980	0.8856	0.9257
Nitrogen	1	0.6669	0.3113	0.4347	0.0584	0.0624	0.4773	0.7744	0.4672
W × N	2	0.0054	0.4491	0.7631	0.9538	0.3850	0.0867	0.8225	0.9914

Means within the same columns followed by the same letters are not significantly different (ns) at the 0.05 level of probability according to the LSD test. IRRG1 = lowest irrigation treatment, IRRG2 = moderate irrigation treatment, and IRRG3 = full irrigation treatment. N1: Nitrogen reduced level (131 kg N ha⁻¹). N2: Nitrogen recommended level (165 kg N ha⁻¹). DF = degrees of freedom. W × N = The interaction between water (W) and nitrogen (N) treatments.

) or 2017 (

Table 16. Effect of deficit irrigation and nitrogen fertilizer application rate on number of stems and tubers in four potato cultivars (Canela Russet, Mesa Russet, Russet Norkotah3, and Yukon Gold) 110 days after planting (DAP) in 2017.

Cultivars		Canela Russet		Mesa Russet		Russet Norkotah3		Yukon Gold
Treatments		No. of Stems Plant−1	No. of Tubers Plant−1	No. of Stems Plant−1	No. of Tubers Plant−1	No. of Stems Plant−1	No. of Tubers Plant−1	No. of Stems Plant−1	No. of Tubers Plant−1
Water
IRRG3		2.69 a	6.06 a	3.38 a	6.81 a	4.62 a	6.25 a	3.56 a	6.31 a
IRRG2		2.94 a	6.50 a	4.44 a	7.12 a	4.75 a	6.00 a	3.94 a	6.75 a
IRRG3		2.87 a	6.44 a	3.69 a	7.19 a	4.25 a	5.94 a	3.25 a	6.12 a
Nitrogen
N1		2.62 a	6.58 a	4.42 a	7.29 a	4.62 a	5.83 a	3.71 a	6.38 a
N2		3.04 a	6.08 a	3.25 b	6.79 a	4.46 a	6.29 a	3.46 a	6.42 a
Interaction
N1	IRRG3	2.00 a	5.00 b	4.00 a	6.75 a	5.00 a	7.00 a	3.75 a	6.38 a
	IRRG2	3.12 a	8.00 a	5.38 a	7.88 a	4.25 a	5.00 a	4.38 a	7.00 a
	IRRG1	2.75 a	6.75 a	3.88 a	7.25 a	4.62 a	5.50 a	3.00 a	5.75 a
N2	IRRG3	3.38 a	7.12 a	2.75 a	6.88 a	4.25 a	5.50 a	3.38 a	6.25 a
	IRRG2	2.75 a	5.00 a	3.50 a	6.38 a	5.25 a	7.00 a	3.50 a	6.50 a
	IRRG1	3.00 a	6.12 a	3.50 a	7.12 a	3.88 a	6.38 a	3.50 a	6.50 a
Source of Variation	DF	p-Value
Water	2	0.8014	0.8744	0.2092	0.8850	0.7313	0.8961	0.2134	0.7875
Nitrogen	1	0.2057	0.5095	0.0264	0.4595	0.7579	0.4357	0.4249	0.9565
W × N	2	0.1016	0.0373	0.4590	0.5682	0.3242	0.0637	0.2068	0.7875

Means within the same columns followed by the same letters are not significantly different (ns) at the 0.05 level of probability according to the LSD test. IRRG1 = lowest irrigation treatment, IRRG2 = moderate irrigation treatment, and IRRG3 = full irrigation treatment. N1: Nitrogen reduced level (131 kg N ha⁻¹). N2: Nitrogen recommended level (165 kg N ha⁻¹). DF = degrees of freedom. W × N = The interaction between water (W) and nitrogen (N) treatments.

).

3.6. Correlation of Tuber Bulking and Leaf Area Index with Yield

In general, LAI was significantly positively correlated with total yield, especially for Russet Norkotah3 and Canela Russet at the early sampling dates (

Table 17. Pearson correlation coefficients for total yield vs. leaf area index (LAI) and tuber bulking (TB) for four potato cultivars for 2016 and 2017.

		LAI			TB
Year	Cultivar	82 DAP	103 DAP	113 DAP	82 DAP	103 DAP	113 DAP
2016	Canela Russet	0.640 **	0.560 **	0.231 ns	0.704 ***	0.589 **	0.752 ***
	Mesa Russet	0.435 *	–0.070 ns	–0.612 **	0.253 ns	0.097 ns	–0.383 ns
	Russet Norkotah3	0.427 *	0.591 **	0.503 *	0.201 ns	0.583 **	0.496 *
	Yukon Gold	0.339 ns	0.360 ns	0.107 ns	0.728 ***	0.472 *	0.660 ***
2017		104 DAP	111 DAP	118 DAP	104 DAP	111 DAP	118 DAP
	Canela Russet	0.515 *	0.500 *	—	0.777 ***	0.773 ***	0.858 ***
	Mesa Russet	0.225 ns	0.276 ns	—	0.399 ns	0.265 ns	0.420 *
	Russet Norkotah3	0.555 **	0.536 **	—	0.799 ***	0.124 ns	0.369 ns
	Yukon Gold	0.406 *	0.298 ns	−0.088 ns	0.222 ns	0.366	0.03 ns

ns = not significant. * Significant at p < 0.05. ** Significant at p < 0.01. *** Significant at p < 0.001.

). These correlations may be due to increased leaf area enhancing the plant’s photosynthetic capacity, which in turn, supports greater tuber development (tuber bulking). In general, TB presented a high positive correlation with total yield, particularly for Canela Russet, which was significant for every sampling date in both years; TB was generally a better yield predictor than LAI for Canela Russet, as shown by their higher correlation coefficients at every sampling date in both years. Yukon Gold and Russet Norkotah3 also showed significant correlation between TB and yield, but only about half the time (Table 17). In addition, the results showed that early-stage LAI (82 DAP) and late-stage TB (113 DAP) were generally most highly correlated with yield.

4. Discussion

4.1. Total Yield and Tuber Size Distribution

Canela Russet maintained high total and marketable yields with a 16–18% reduction in irrigation (Table 3). However, in 2017, this cultivar exhibited sensitivity to reduced irrigation and N inputs, as the large-size marketable yield was significantly lower under deficit irrigation, and most tuber sizes were smaller under reduced N application. The total yield of 38–47 Mg ha⁻¹ was achieved under full and moderate deficit irrigation, and this aligns with findings by Colorado State University (CSU), where yield under full irrigation ranged from 24–52 Mg ha⁻¹ [23]. Our results demonstrate Canela Russet’s ability to maintain productivity under moderate resource constraints.

Mesa Russet also showed results (Table 4) that confirmed that moderate reduction in water and N had no effect on total or marketable yield. Research by Colorado State University (CSU) showed that the average yield for Mesa Russet is between 39 and 50 Mg ha⁻¹ [24]; our results showed the average yields under full and moderate deficit irrigation to be 38–48 Mg/ha. These results align with previous research showing that moderate irrigation deficits did not compromise yield, due to reduced leaching and improved nutrient availability [25,26]. It is important to note that Mesa Russet received higher irrigation rates under deficit on a % ET basis than those of the other cultivars in both years, although the actual irrigation amounts were similar, thus confounding cultivar comparisons (Table 1).

On the other hand, Russet Norkotah3 exhibited significant yield reductions under moderate deficit irrigation (Table 5), confirming its high sensitivity to water stress. Russet Norkotah3 is the earliest maturing variety evaluated in this study and is, therefore, less drought tolerant. Similar yield declines have previously been reported under drought stress [27]. Due to its earliness, it needs a large quantity of water to build its vegetative structure (photosynthetic apparatus) early in the season to support early TB and early tuber maturity. In addition, Russet Norkotah3 has earlier canopy senescence and a shallower root system, leaving it more vulnerable to water stress during late-season bulking [28]. The yield potential for Russet Norkotah has been reported to be 45 Mg ha⁻¹ [29]. Our plots yielded 43–46 Mg ha⁻¹ under full irrigation; however, any reduction in irrigation water resulted in a yield decrease, indicating sensitivity to even slight reductions in irrigation. These findings are supported by others who demonstrated that limited soil water reduces potato productivity, restricting growth, tuber size, and overall yield [12,16,30]. However, Russet Norkotah3 maintained yields with reduced N application in both years.

The moderate deficit irrigation treatment had no significant effect on total or marketable yield of Yukon Gold in either year (Table 6). This supports previous research that found that deficit irrigation strategies, when applied either during specific growth periods or throughout the season, do not necessarily reduce yields [31]. The most severe negative effects of deficit irrigation occur during mid-to-late-season TB [32]. Average total yield of Yukon Gold has been reported to range from 26 to 57 Mg ha⁻¹ under full irrigation [33]. In contrast, our results with full or moderate deficit irrigation indicate yields of 37–41 Mg ha⁻¹, within the average range.

Our findings indicate that cultivars responded differently to reductions in irrigation and N application rates. While Canela Russet, Yukon Gold, and Mesa Russet tolerated moderate irrigation deficits without yield loss, Russet Norkotah3 required full irrigation for optimal productivity. A twenty percent reduction in N had no effect on Mesa Russet or Russet Norkotah3 yields but reduced the yields of the other varieties in one out of two years. These results highlight the need for each cultivar to be managed with specific water and nutrient management strategies to optimize yield, improve resource efficiency, and ensure sustainable potato production. Overall, a moderate reduction in both irrigation and N can be implemented without compromising the productivity of most potato cultivars, and moderate irrigation reduction also reduces the potential for nitrate leaching [34].

The San Luis Valley produces about 20,234 ha of potatoes annually. If Canela Russet or Yukon Gold was grown on all of this land, and irrigation was reduced by 8.5 cm annually without affecting yield, this would result in a savings of 17.2 billion L (17.2 million m³) of water annually in the San Luis Valley alone. This water savings is equivalent to 850 m³ ha⁻¹, within the range reported by others for a 20% reduction in irrigation [35]. Alternatively, if Russet Norkotah3 or Mesa Russet were grown, then the N application rate could be reduced by 34 kg N ha⁻¹ or 688 Mg N yr⁻¹.

4.2. Tuber Bulking (TB)

Tuber bulking demonstrated a strong and consistent positive correlation with total yield for Canela Russet (Table 17). Therefore, factors such as water and N availability, which influence physiological processes during TB, can alter the duration and effectiveness of this critical period, ultimately affecting total tuber yield. Our findings are in agreement with previously reported results showing that a reduction in irrigation water by 10% to 21% did not influence TB of Rio Grande Russet potatoes [13]. The TB rate and duration are important because they directly influence yield [36]. A faster TB rate is associated with higher yield and depends upon the amount of total available N and soil moisture. Insufficient moisture levels throughout the various stages of potato growth, particularly during tuber initiation and bulking, significantly affect the yield of potato crops [37], and excessive N may delay TB [38]. Reducing water gradually during TB provides a more favorable opportunity for the crop to adapt to stressful water conditions compared to exposing the crop to continuous water stress throughout the whole growing season [39]. The timing and rate of N fertilizer application also have a significant impact on tuber bulking; application of N at planting, followed by additional applications during TB, resulted in increased tuber size and yield [40]. Potatoes are sensitive to water stress during TB [41]. Specifically, deficit irrigation during the initial TB stage has led to reductions in tuber grade from US Number One tubers to US Number Two tubers [42]. The ability of plants to obtain N is influenced not only by the presence of N in the soil but also by the availability of water stored in the soil [43]. There is an optimal range of water and N requirements for each stage during the potato growing season, and this varies by cultivar. Providing inadequate water and N can have negative effects on TB as well as yield. Likewise, it is important to maintain a balanced and appropriate supply of water and N throughout the growing season to maximize potato production.

4.3. Leaf Area Index (LAI)

LAI was significantly positively correlated with total yield for Canela Russet and Russet Norkotah3; however, this pattern was not consistent for Mesa Russet or Yukon Gold (Table 17). Mesa Russet and Yukon Gold have broader leaves compared to the other two cultivars. Together with the geometric orientation of the leaves of Mesa Russet and Yukon Gold, there is a possibility of mutual shading among the leaves, thus preventing many of the middle and lower leaves from photosynthesizing optimally. These middle and lower leaves draw photosynthates from the plant at the expense of tuber growth, without contributing much photosynthate production [44]. Therefore, early and mid-season LAI measurements could be used as a yield predictor for Canela Russet and Russet Norkotah3, but not for Mesa Russet or Yukon Gold. In addition, most of the cultivars showed either a significant positive correlation between LAI and yield at 82 DAP, which corresponds to peak leaf growth. After 100 DAP, leaf senescence begins, and the correlation generally weakens as tubers mature [45].

Previous research has reported that LAI is an indicator of a potato plant’s ability to maximize light interception and photosynthetic capacity, which are positively associated with growth, development, and yield [46,47]. In addition, others have found that the leaf area of the potato plant at the time of tuber initiation and its capacity to capture solar radiation during the TB stage have a major impact on final tuber yield [48]. This relationship aligns with previous findings that show that prolonged maintenance of the green canopy extends the TB period, thereby increasing the crop yield [49]. In addition, yield has been reported to be correlated with the net photosynthetic rate during the TB stage [25].

There were significant effects of reduced water and N levels on LAI. The IRRG1 treatment significantly reduced LAI of Russet Norkotah3 on all sampling dates in both years (Table 13). In addition, N rate had no effect on LAI of Mesa Russet or Yukon Gold in either year and rarely affected LAI of the other two varieties. Our findings are supported by several studies showing that LAI increased significantly with increased irrigation water level [50,51]. Our findings indicate that higher irrigation levels initially promote greater LAI, but the effect diminishes over time. Water stress has been shown to decrease the photosynthetic rate and leaf area [52], and net photosynthetic rate, total leaf area, and aboveground biomass are optimized when plants receive sufficient N [32]. In addition, different amounts of N affect LAI differently depending on the phenological stage of potato [53]. LAI is significantly affected by water and N supply, resulting in lower LAI under less irrigation or lower N rates [34]. In previous research in the San Luis Valley, reducing irrigation by more than 15–20% significantly reduced LAI [13]. Moreover, it is possible to reduce the amount of water and N applied without impacting the LAI, and this relationship varies with cultivar.

5. Conclusions

Reducing irrigation by 15–18% did not significantly affect total yield, LAI, stem number, or tuber number in Canela Russet or Yukon Gold under most conditions. However, both varieties exhibited a significant reduction in yield under 20% reduced N application in one out of two years. Mesa Russet also exhibited no yield reduction in IRRG2 or reduced N treatments; however, due to the variability of the irrigation system, IRRG2 only represented a 6–10% reduction in water application. In contrast, Russet Norkotah3 showed significant sensitivity to reductions in irrigation, resulting in yield decline; however, the IRRG2 treatment represented a 16–23% reduction in water application, more than the other varieties. A twenty percent N reduction had no effect on Russet Norkotah3 or Mesa Russet yield.

There was significant positive correlation between yield, LAI, and tuber bulking for Canela Russet and Russet Norkotah3. Although the cultivars responded differently to N application under various irrigation levels, N rate alone had little to no significant effect across treatments. Clearly, the optimal balance of irrigation and N input is cultivar dependent. However, further investigation should explore genotype-by-environment interactions and physiological traits such as root architecture, stomatal conductance, and chlorophyll content to better understand cultivar-specific drought tolerance. Additionally, studies should examine N concentrations in the soil and groundwater throughout the entire growing season to assess the environmental benefits of reduced N application.