Investigating the Effects of Optimized Mineral Fertilization on Plant Growth, Physiological Traits, Tuber Yield, and Biochemical Contents of Potato Crop

Abstract

Plants of two potato (Solanum tuberosum L.) varieties ‘Spunta’ (mid-early maturity) and ‘Kensa’ (mid-late maturity) were subjected to three nitrogen (N), phosphorus (P), and potassium (K) fertilization regimes, with T1 optimal rates (as recommended by the Tunisian Potato Technical Centre), T2 high rates (+25% of T1), and T3 low rates (−25% of T1). Plant growth, yield components, chlorophyll fluorescence (Fo, Fm, Fv/Fm), photosynthetic active radiation (PAR), real evapotranspiration (RET), and biochemical parameters in mature leaves and tubers (total soluble sugars (TSSs) and starch in both leaves and tubers) were evaluated. Our results showed a significant effect of fertilizer rates on plant growth, physiological, yield, and quality traits, as well as on biochemical contents of leaves and tubers, as well as on the variety. The application of high rates (T2) resulted in increased chlorophyll fluorescence (Fo) and high ratios of Fv/Fm, and it reduced Fm and photosynthetic active radiation (PAR). The highest yield per plant (615.4 g of tubers) and average number of tubers/plant (6.44) were observed in cv. ‘Spunta’ subjected to optimal fertilizer rate (T1), while more than 50% of tubers of this variety were classified as size C1 (>50 mm). On the other hand, high rates (T2) increased the yield per plant (436.74 g; approximately 9.3% compared to T1) and the number of tubers per plant (5.70) in cv. ‘Kensa’, with approximately 56% of tubers being classified in the C1 category. High rates also increased sucrose and starch content in tubers, regardless of the variety, without being significantly different from the other fertilization regimes. In conclusion, our results provide important information regarding the effect of fertilization practice on potato growth and yield parameters and the biochemical composition of leaves and tubers. Therefore, it could be suggested that the application of reduced NPK rates (−25% of optimal rates) in mid-early varieties (namely cv. ‘Spunta’) could reduce the production cost without compromising yield and quality components.

1. Introduction

Potato (Solanum tuberosum L.) is the fourth largest food crop in the world besides wheat, rice, and maize and plays a vital role in addressing human food security [1]. Potato tubers contain high levels of K and provide approximately half the daily adult requirement of vitamin C as well as of vitamins A, B, and E [2,3]. The total harvested area of potatoes worldwide was 17,788,408 hectares in 2022, slightly less than the previous year, with approximately 375 million tons of potatoes being produced globally. The most important producers on a global scale were China (95.5 million tons) and India (56 million tons) [4]. Moreover, potato is one of the most consumed vegetables, after tomatoes in North African countries such as Tunisia and Algeria, while it is a basic ingredient in many local dishes.

In Tunisia, potato is cultivated throughout the year with three cropping seasons, namely the late season (arrierre saison; planted from mid-August to mid-October and harvested from mid-January to mid-February), main season (saison; planted from mid-January to mid-March and harvested from mid-June to the end of July), and early season (primeur; planted from November to mid-December and harvested from early March to mid-May) [5]. The crop is cultivated throughout the country with a total harvested area of 22,552 ha and a total production of 400,000 tons for 2022 [4], while the main cropping areas are Cap Bon, Sahel, and in central Tunisia [6]. In Algeria, potato is cultivated in the Saharan El Oued region (south of the country), with a total harvested area of 36,200 ha and production of 1,136,000 tons in 2019 [7].

According to the literature, the selection of cultivar maturity and the date of planting of tuber seeds are essential for crop yield, since they facilitate the acclimatization of plants at the environmental conditions that prevail in each region [8]. Therefore, the selection of middle-to-late-maturity cultivars are more productive than ones that mature earlier, as long as temperatures meet crop temperature requirements for vegetative growth and tuberization [9]. On the other hand, early-maturity cultivars could have higher yields compared to late maturity when plants grow under water limitations or drought stress, especially under rain-fed conditions [8,10].

There are several factors that determine the growth and yield of potato plants, including seed quality [11,12], genotype [13], and fertilization management [14,15,16,17]. Potato plants have specific nutrient requirements throughout their growth cycle, which should be fulfilled through fertilizer application [18,19]. Macrominerals such as N, P, and K are essential nutrients that influence potato growth and production [20,21]. Among the macrominerals, N is essential for plant growth at vegetative stage. It accounts for 1–4% of plant’s dry matter, and it is taken up from the soil in nitrate (NO₃⁻) or ammonium (NH₄⁺) form [22,23,24]. Nitrogen is a main component of protoplasm, and it is involved in chlorophyll synthesis [25], it can increase the dry matter and protein content of tubers and total tuber yield [26]. Nitrogen deficiency results in reduced transpiration rates and stomatal conductance, as well as in reduced levels of carotenoids, chlorophyll, and soluble sugar content; it can also reduce the activity of the photosynthetic enzymes of PSII [25,27], thus resulting in the production of very small tubers, low dry matter content, overmature tubers, and increased susceptibility to fungi infections [28,29]. On the other hand, excessive N rates may induce plant growth instead of tuberization [30,31], thus leading to the formation of small and immature tubers with high sugar levels and moderate dry matter content, as well as increased susceptibility to diseases and bruising [32,33].

Phosphorus and potassium are also important macronutrients which regulate plant growth through endogenous metabolism and photosynthetic activity. Phosphorus (P) is essential for increasing tuber yield and quality, since it affects cell division, starch synthesis, and tuber storage life, while it may increase the concentrations of ascorbic acid, N, and protein in the tubers, thus having an effect on the size and dry matter content of tubers [34,35]. Potassium also affects the production and quality of tubers and increases the tolerance of plants to drought and frost stress [36,37]. It is a mobile element widely distributed in plant tissues, which plays an important role in photosynthesis through the metabolism of carbohydrates, the regulation of osmotic pressure, the translocation of assimilates, and the uptake of N by facilitating CO₂ diffusion through the leaf mesophyll [14,38]. In addition, it increases the tolerance of plants against various biotic and abiotic stressors, such as pathogens, drought, and extreme temperatures [23].

Moreover, environmental factors, particularly temperature and the photoperiod, may have a major influence on the growth and development of potato plants, since they regulate several physiological processes [39,40,41,42,43]. On the other hand, several studies have reported that plant growth and yield attributes are determined by genetic and environmental factor interactions, which show a complex relationship [44,45,46]. This interaction can have a positive effect on the growth and development of potato plants, but the crop productivity and quality features of tubers could also be affected by the cropping system (organic vs. conventional farming), as well as by the implementation of fertilization regimes and the cultivated varieties [46,47,48]. Another important aspect of nutrient management through the application of fertilizers is related not only to the amount and type of fertilizer but also to the application time, with special consideration of the earliness of the cultivar (namely, cultivars with early, mid-early, or late maturity) [14,49].

The application of NPK fertilizers has a marked effect on potato yield, with the highest total tuber yield and tuber dry matter content being observed at maximum rates [50]. Contrarily, when fertilizers are applied at excessive or deficient rates, the tuber yield is usually negatively affected. In a recent study where the effect of four NPK fertilizer rates (0:0:0, 50:50:50, 100:100:60, and 150:150:90 kg NPK/ha) on the growth, yield, and economic profitability of three potato varieties was assessed, it was reported that all growth traits and yield components were consistently higher in the studied varieties when plants received 150:150:90 kg/ha of NPK compared to the lower rates implemented. Accordingly, Parganiha et al. [51] stated that the efficiency of N, P, and K fertilization is significantly influenced by the application rates. Nevertheless, the highest doses of NPK resulted in higher soil nutrient levels, higher availability of N, P₂O₅, and K₂O, and, consequently, in higher nutrient uptake, which also varies greatly between potato varieties due to genetic differences [50,52].

A great deal of research has been devoted to the optimization of nutrient rates and application time throughout the life cycle of potato plants, aiming to improve crop performance and achieve higher yields with optimum tuber size [11,53]. Despite that, the amounts of N, P, and K nutrients used for potato production in many African countries are often considered excessive, leading to an imbalance of essential elements in the soil [52], antagonistic effects among nutrients [54], reduced nutrient use efficiency, and, consequently, the reduced yield and quality of tubers [54] and soil salinity [55,56]. Therefore, this study was carried out to determine the effects of high and low rates of NPK compared to the optimal doses recommended by the Tunisian Technical Centre for Potatoes on plant growth, yield, and yield components of two potato cultivars (namely ‘Spunta’ and ‘Kensa’), as well as to identify the effects of fertilization regimes on physiological traits such as chlorophyll fluorescence parameters, real evapotranspiration rate, and photosynthetic active radiation. Finally, the chemical composition of leaves and tubers was also evaluated in terms of total soluble sugar and starch content in relation to the studied fertilizer rates.

2. Materials and Methods

2.1. Plant Material and Growth Conditions

The experiment was conducted at the High Agronomic Institute of Chott Mariem-Sousse-Tunisia (latitude 35°54′22.21′′ N and longitude 10°32′47.81′′ E) from 5th March until the end of June 2023. Seed potatoes (tubers) of two varieties, namely ‘Spunta’ and ‘Kensa’, with mid-early and mid-late maturity, respectively, were obtained from a certified seed potato production of the Technical Centre of Potato, Tunisia (TCP). The required number of seeds (namely 90 seeds per variety and treatment in 3 replications) were selected for size homogeneity with approximately 45–55 mm transverse diameter and around 70–80 g of weight. The selected seeds were left to sprout for 20 days under room temperature (23 ± 1 °C) and planted in 10 L plastic pots with a diameter of 22.5 cm, containing a growth medium composed of a commercial peat (EC: 25 mS/m ± 25%; pH: 5.5–6.5; NPK: 140–100–180 g/m³ and organic matter: 0.11 mg/kg) and perlite (2:1; v/v). One seed per pot was used, and then pots were placed in a growth room. The average photosynthetic photon flux density was approximately 450 μmol/m²/s during the first stage of growth, and supplementary lighting was provided by two additional lamps (NATRALOX high-pressure sodium lamps evenly distributed throughout the growth chamber) per m² with a corresponding luminous flux (100 h). The photoperiod regime was set at 16/8 h light/darkness during the growth stage and sprouting, and then reduced to 10/14 h day/night during the tuberization phase. A daytime temperature of 18 °C and a night-time temperature of 16 °C were also applied.

2.2. Fertilization Treatments

Three NPK rates were applied throughout the growth cycle of plants. In particular, the following fertilization regimes were implemented:

-

Optimum rates (T1; according to TCP) which were used as the control treatment, with an average of 117.5 kg/ha (N), 145 kg/ha (K₂O), and 19 L (P₂O₅);

-

High rates (T2; +25% of the optimum rate);

-

Low rates (T3; −25% of the optimum rate).

The details of these rates and potato stages are given in

Table 1. NPK rates applied in this study in relation to the stages of the plant, based on a density of 32,000 plants/ha for field conditions.

Treatments and Fertilzer Application Rates		Percentage of NPK Applied in Relation to Plant Life Cycle Stages
		Vegetative Growth and Sprouting	Tuberization	Tuber Bulking
T1: Optimum rates	N (3.68 g/plant)	60%	20%	20%
	P2O5 (0.57 mL/plant)	40%	30%	30%
	K2O (4.52 g/plant)	20%	30%	50%
T2: High rates (+25% of T1)	N (4.60 g/plant)	60%	20%	20%
	P2O5 (0.71 mL/plant)	40%	30%	30%
	K2O (5.65 g/plant)	20%	30%	50%
T3: Low rates (−25% of T1)	N (2.76 g/plant)	60%	20%	20%
	P2O5 (0.43 mL/plant)	40%	30%	30%
	K2O (3.39 g/plant)	20%	30%	50%

. The commercial fertilizers used in this experiment were ammonium nitrate (33.5%) for N, potassium sulphate (51%) for K₂O, and phosphoric acid (53%, 1.6 density) for P₂O₅. Moreover, during the life cycle, MgO (magnesium sulphate 16%) was added at a dose of 0.2 g per plant. Fertilization started two weeks after planting and stopped two weeks before the end of the cycle (i.e., the duration of fertilizer application was 10 weeks). Plants were irrigated at regular intervals depending on water requirements at an average of 2 times per week, while the third consecutive irrigation was combined with the fertigation of plants using the rates described in Table 1.

2.3. Evaluation Criteria

The effects of the different fertilizer application rates were assessed using characteristics related to plant growth (see Section 2.3.1), physiological traits, yield, and yield components, as well as those related to the chemical composition of leaves and tubers.

2.3.1. Plant Growth Assessment

To evaluate the effect of different fertilizer rates on potato plant growth, the following growth parameters were assessed both at the end of the vegetative growth and sprouting stage: stem number per plant (SN) and average stem diameter (SD) measured at the middle of the main stem using a calliper (cm); plant length (PL) in relation to the main stem (cm); the total number of leaves per plant (TLN); and the number of photosynthetic leaves (Phot.L) referring to those leaves whose area was ≥50% of the final dimension of an adult leaf [57].

2.3.2. Evaluation of the Chlorophyll Fluorescence Parameters

Chlorophyll fluorescence was measured on the 5th fully expanded healthy leaf with a fluorometer (Fluorescence Monitoring System, Plant kit stress, Opti-Sciences model, Hudson, NH, USA), which automatically estimated the minimum and maximum fluorescence (Fo and Fm) and the fluorescence yield ratio, Fv/Fm.

2.3.3. Evaluation of Photosynthetic Active Radiation and Real Evapotranspiration

Photosynthetic active radiation (PAR) (μmol/m²/s) and real evapotranspiration (RET) (mm H₂O/day) were measured according to the method applied by Saadaoui et al. [58] on the 5th fully expanded leaf from the apex on a clear day (at 11 a.m.), 50 days after the first fertilizer application using the fluorometer described above (Plant kit stress, Opti-Sciences model, Hudson, NH, USA).

2.3.4. Yield and Yield Component Criteria

At the end of the experiment (120 days after planting), tubers were harvested manually, separately for each plant and treatment. Then, they were counted and weighed using an electronic balance and graded into three classes (C1–C3; C1 > 50 mm, 40 < C2 < 50 mm, and C3 < 40 mm) based on the medium diameter. Thus, the yield and yield component traits were evaluated on the basis of yield per plant (g), the total number of tubers per plant, tuber diameter at the centre of the tuber (mm), and tuber size (length of tuber expressed in mm).

2.3.5. Sugar and Starch Content Evaluation

Total Soluble Sugar Determination

Samples of mature leaves (100–150 g fresh weight) and tubers (200 g fresh weight), collected at harvest time, were immediately frozen in liquid N and stored at sunset to ensure that the soluble sugar content in the source leaves was the highest [59]. Briefly, samples were extracted with 1 mL of 80% ethanol at 80 °C for 2 h. The supernatant was used for the determination of total soluble sugars (glucose, fructose, and sucrose). After 2 h, the pellet obtained at the bottom of the extraction tube was washed three times with 80% ethanol and then incubated at 70 °C until drying. Leaf and tuber samples were homogenized in 800 μL of 0.2 MKOH and then incubated at 95 °C for 1 h. The pH was adjusted to neutral or slightly acidic by the addition of 140 μL of 1 M acetic acid, and then centrifuged for 5 min (10,000× g). Approximately 50 μL of the supernatant was mixed with 100 μL of amyloglucosidase (2 mg/mL 50 mM sodium acetate buffer, pH 5.0) and incubated overnight at 55 °C. Sucrose standards were used to quantify sugars, expressed as mg/g fresh weight.

Starch Content Evaluation

The starch determination was performed as described by Berry and Bjorkman [60]. From the same samples used for total soluble sugars, the residue from the extraction of TSS was dried at 70 °C. Then, starch was hydrolyzed in 52% perchloric acid for 30 min and centrifuged for 10 min (2000× g). The procedure was repeated three times, and the supernatants were combined and analyzed by the phenol-sulphur method [61]. Starch content was expressed as mg/g fresh weight.

2.4. Statistical Analysis

The experiment was conducted according to the fully randomized design layout with two factors, namely ‘Variety’ (V) and ‘Fertilization regime’ (F). All plant measurements and yield parameters were recorded in four replications, while sugar and starch contents were investigated in three replications. Each replication consisted of 5–6 plants, and the results were expressed as mean values ± standard deviation. Data were subjected to a two-way analysis of variance (ANOVA). Means were compared based on the Duncan multiple range test (DMRT) at p < 0.05. Statistical analyses were performed using SAS software V9.2 (SAS Institute, Cary, CA, USA).

3. Results

3.1. Effect of Fertilization Regime and Variety on Plant Growth and Physiological Traits

The results of the ANOVA on the effects of the fertilization regime and variety on plant growth and physiological characteristics of two potato varieties are presented in

Table 2. The results of the analysis of variance (means squares) for traits related to plant growth and the physiological parameters of two potato varieties under different fertilization regimes (T1: optimum rates NPK; T2: high rates (+25% of T1); and T3: low rates (−25% of T1)).

S.O.V.	DF	SN	SD	PL	TLN	Phot.L	Fo	Fm	Fv/Fm	RET	PAR
Variety (V)	1	7.13 *	33.74 *	13.63 ns	406.12 *	3838.82 **	38,388.69 **	10,522.88 *	0.06 **	4272.35 *	1561.12 **
Fertilization regime (F)	2	1.58 ns	5.56 *	50.39 *	1168.64 **	1152.00 **	11,520.37 **	21,268.79 **	0.02 **	3025.90 **	3577.02 **
V × F	2	1.80 ns	10.30 *	25.58 ns	17.37 *	1176.91 *	11,769.13 **	22,056.07 *	0.01 *	5494.41 *	5209.57 *
Error	152	1.66	2.20	1.78	32.02	36.62	1022.56	1589.90	0.002	2056.30	2043.80
CV (%)	-	22.19	20.57	33.29	24.10	39.23	17.64	19.45	5.83	19.48	15.20

S.O.V.: source of variation; CV: coefficient of variation; DF: degrees of freedom; SN: stem number/plant; SD: stem diameter; PL: plant length; TLN: total leaf number; Phot.L.: photosynthetic leaf number; Fv/Fm: maximum quantum yield of PSII; RET: real evapotranspiration; PAR: photosynthetic active radiation. ^ns: non-significant; *: significant at p < 0.05; **: significant at p < 0.01.

. Regarding the growth traits, the obtained data showed a significant interaction among the tested factors for most of the recorded traits, except for stem number (SN) and plant length (PL). On the other hand, the fertilization regime (F) significantly affected all the recorded traits, apart from stem number (SN), while the effect of variety (V) was also significant for all the traits except for PL. Interestingly, physiological parameters, such as the maximum and the minimum fluorescence yield (Fm and Fo, respectively), maximum quantum yield of PSII (Fv/Fm ratio), real evapotranspiration (RET), and photosynthetic active radiation (PAR), varied significantly (p < 0.001) among the tested fertilizer rates (Table 2), indicating the positive effect of the fertilization regime on physiological processes. Accordingly, the varied response of the tested varieties to physiological and morphological traits gave evidence of significant genotypic variability. The number of stems per plant varied between the tested varieties, mainly due to the physiological age of the mother seeds, while no significant effect was observed for fertilizer rates for the same trait.

3.2. Effect of Fertilization Regime Across Varieties in Relation to Plant Growth

Regarding the growth parameters studied, only plant length (PL) was significantly affected by the fertilization regime, regardless of the variety, since for the rest of the parameters, a significant interaction between the studied factors was recorded. In particular, both T2 and T3 treatments increased plant length (29.83 and 28.95 cm, respectively) over the control treatment (26.54 cm) (Figure 1A). On the other hand, the number of stems was not significantly affected by the fertilization regime, regardless of the variety (Figure 1B).

3.3. Effect of Varieties Across Fertilization Regimes in Relation to Plant Growth

Our results showed a significant effect of varieties only on the number of stems, regardless of the fertilization regime, since for the rest of the parameters, a significant interaction of the tested factors was recorded. In general, the ‘Spunta’ variety formed a higher number of stems than ‘Kensa’ (Figure 2A). On the other hand, plant length was not significantly affected by variety, regardless of the fertilization regime (Figure 2B).

3.4. Interaction Effects of Fertilization Regime × Variety on Plant Growth and Physiological Traits

Effects on Plant Growth and Physiological Traits

Table 3. The response of two potato varieties to different fertilization regimes in terms of plant growth and physiological traits.

Variety	Treatment	TLN	Phot.L.	SD	Fo	Fm	Fv/Fm Ratio	RET (mmH2O/day)	PAR (μmol/m2/s)
‘Spunta’	T1: Optimal rates (control)	22.04 * ± 4.45 b	12.62 ± 4.12 c	7.06 ± 1.44 b	553.26 ± 43.1 b	1753.40 ± 488.18 b	0.68 ± 0.04 c	170.60 ± 19.75 a	396.29 ± 14.95 b
	T2: High rates (+25% T1)	37.74 ± 11.27 a	15.44 ± 6.51 b	8.51 ± 2.26 a	598.89 ± 74.97 ab	1718.59 ± 367.85 b	0.74 ± 0.03 b	87.03 ± 16.33 b	457.26 ± 20.84 a
	T3: Low rates (−25% T1)	35.42 ± 8.29 a	15.66 ± 5.54 b	7.45 ± 1.35 ab	420.22 ± 63.10 c	2356.26 ± 204.65 a	0.70 ± 0.03 c	66.54 ± 20.80 bc	469.70 ± 29.11 a
‘Kensa’	T1: Optimal rates (control)	35.14 ± 9.34 a	16.37 ± 6.14 b	5.93 ± 1.08 b	624.52 ± 51.16 a	1530.48 ± 89.89 c	0.75 ± 0.04 b	64.67 ± 19.82 bc	197.76 ± 14.95 d
	T2: High rates (+25% T1)	39.96 ± 9.58 a	21.62 ± 5.01 a	7.66 ± 1.09 a	621.22 ± 66.70 a	1601.15 ± 87.02 c	0.79 ± 0.05 a	86.31 ± 26.32 b	260.48 ± 19.68 c
	T3: Low rates (−25% T1)	23.59 ± 5.14 b	12.81 ± 3.15 c	5.68 ± 1.42 c	618.70 ± 58.11 a	1732.33 ± 90.65 b	0.73 ± 0.03 bc	75.74 ± 23.71 b	276.01 ± 15.80 c

* Data are shown as means ± SD. TLN: total leaf number per plant; Phot.L.: photosynthetic leaf number per plant; SD: stem diameter; Fv/Fm: maximum quantum yield of PSII; RET: real evapotranspiration; PAR: photosynthetic active radiation. Values with the same letter in each column are not significantly different according to the Duncan multiple range test (DMRT) at p ≤ 0.05.

presents the effect of the interaction between the studied factors on plant growth and physiological processes.

The two-way ANOVA showed that the two varieties responded differently depending on the fertilizer rates applied (Table 3). Both varieties recorded the highest total leaf number for the T2 treatment, without significant differences from the optimal and low rates in ‘Kensa’ and ‘Spunta’ varieties, respectively. Similarly, the highest stem diameter was recorded for the high fertilizer rates in both varieties, while the lowest value was observed in the ‘Kensa’ variety (5.68 mm). Regarding the photosynthetic leaf number and the Fv/Fm ratio, the highest values were recorded for the ‘Kensa’ variety and the T2 treatment. On the other hand, no significant differences were recorded for Fo values among the studied fertilization regimes in the case of the ‘Kensa’ variety, although T1 treatment resulted in the highest overall value, whereas low rates resulted in the highest overall values for Fm in the ‘Spunta’ variety. Regarding real evapotranspiration, the highest value was observed in the ‘Spunta’ variety and T1 treatment, while PAR values were the highest in T2 and T3 treatments for the same variety. Interestingly, the optimal rates resulted in lower PAR values compared to either high or low rates for both varieties.

3.5. Effect of Fertilization Regime and Variety on Yield and Yield Components and Chemical Composition of Tubers and Leaves

The analysis of variance (ANOVA) for yield and yield components showed that both the tested factors and their interaction significantly affected tuber yield and the tuber number per plant as well as tuber size, whereas tuber diameter was only affected by the fertilization regime (

Table 4. The results of the analysis of variance (mean squares) for yield, yield components, and carbohydrate content of two potato varieties under different fertilization regimes.

					Tuber Size			Carbohydrate Contents
S.O.V.	DF	Yield/Plant	Tubers/ Plant	Tubers Diameter	C1 > 50 mm	40 mm < C2 < 50 mm	C3 < 40 mm	Gluc.T	Fruct.T	Sucr.T	Starch T	Gluc.L	Fruct.L	Sucr.L	Starch L
Variety (V)	1	1095.20 **	66.76 **	0.18 ns	15.43 **	9.38 **	2.00 *	0.81 ns	0.02 ns	0.84 *	2.75 *	3.55 **	1.07 **	0.64 *	12.79 **
Fertilization regime (F)	2	2188.38 **	20.32 **	44.96 **	9.37 **	3.57 *	1.85 **	0.43 ns	0.29 *	0.58 *	46.96 *	0.03 ns	0.67 **	0.39 *	3.49 *
V × F	2	954.68 *	4.84 *	13.14 ns	0.93 *	1.46 *	0.02 *	0.62 ns	0.17 ns	0.15 *	4.93 *	3.79 *	0.43 *	2.10 *	5.06 **
Error	152	234.80	1.86	5.27	0.90	1.02	0.30	0.31	0.10	0.16	0.84	0.26	0.14	0.18	1.70
CV(%)	-	13.67	24.85	10.07	27.86	26.39	17.89	29.70	24.70	14.84	25.66	35.45	45.67	33.46	35.98

S.O.V.: source of variation; CV: coefficient of variation; DF: degrees of freedom; Gluc.T: glucose content in tuber; Fruct.T: fructose content in tuber; Sucr.T: sucrose content in tuber; Starch T: starch content in tuber; Gluc.L: glucose content in leaf; Fruct.L: fructose content in leaf; Sucr.L: sucrose content in leaf; ^ns: non significant; *: significant at p < 0.05; **: significant at p < 0.01.

). Similarly, carbohydrate content in tubers and leaves was also affected by fertilizer rates, variety, and their interaction, apart from glucose, which was not affected by any factor, the fructose of tubers, which was only affected by fertilizer rates, and glucose in leaves, which was not affected by the fertilization regime (Table 4).

3.5.1. Effect of Fertilization Regimes Across Varieties in Relation to Yield and Yield Components and Chemical Composition of Tubers and Leaves

Regarding the diameter of tubers, low rates resulted in tubers with significantly larger diameters (23.80 mm) compared to both optimal and high rates (22.03 and 22.55 mm, respectively) (

Table 5. The effect of fertilization regimes and variety on yield components and carbohydrate content in tubers.

		Carbohydrate Content (mg/g Fresh Weight)
Fertilization Regime	Tuber Diameter (mm)	Gluc.T	Fruct.T
T1: Optimal rates (control)	22.03 b *	1.42 a	1.42 b
T2: High rates (+25% T1)	22.55 b	1.44 a	1.64 a
T3: Low rates (−25% T1)	23.80 a	1.46 a	1.46 b
Variety
Kensa	22.83 a	0.81 a	0.40 a
Spunta	22.76 a	0.76 a	0.37 a

* Data are shown as means. Gluc.T: glucose content in tuber; Fruct.T: fructose content in tuber. Values with the same letter in each column, for fertilizer treatments and varieties separately, do not differ significantly according to Duncan’s multiple range test (DMRT) at p < 0.05.

). In addition, the fertilization regime affected the fructose content of tubers, which was the highest in plants treated with T2 (1.64 mg/g fresh weight), whereas no significant differences were noted for T1 and T3 treatments. In contrast, the fertilization regime did not affect glucose content in tubers.

3.5.2. Interaction Effects of Fertilization Regime × Variety on Yield and Yield Components and Chemical Composition of Tubers and Leaves

The data presented in

Table 6. Response of the studied potato varieties to different fertilization regimes in relation to yield, yield components, and carbohydrate content in leaves and tubers (date are shown only for traits where a significant interaction of the tested factors was recorded).

				Tuber Size			Carbohydrate Content (mg·g−1 FW)
Variety	Treatment	Yield/Plant (g)	Tubers/Plant	C1 > 50	40 < C2 < 50	C3 < 40	Sucr.T	Starch T	Gluc.L	Fruct.L	Sucr.L	Starch L
Spunta	T1	615.40 ± 17.4 a	6.44 ± 2.11 a	3.66 ± 0.87 ab	2.14 ± 0.27 a	0.63 ± 0.09 a	0.37 ± 0.08 c	6.36 ± 0.33 c	1.52 ± 0.06 b	0.73 ± 0.04 c	0.75 ± 0.04 a	1.37 ± 0.04 c
	T2	472.52 ± 18.64 c	5.44 ± 1.08 b	3.22 ± 0.69 b	1.96 ± 0.65 ab	0.49 ± 0.07 b	0.48 ± 0.07 ab	7.73 ± 0.41 b	1.33 ± 0.05 c	0.68 ± 0.03 c	0.62 ± 0.04 b	1.64 ± 0.06 b
	T3	566.05 ± 10.72 b	6.15 ± 1.14 a	4.30± 1.02 a	2.00 ± 0.55 ab	0.26 ± 0.07 c	0.45 ± 0.05 b	7.53 ± 0.97 b	1.03 ± 0.03 d	0.39 ± 0.02 d	0.22 ± 0.02 c	2.04 ± 0.08 a
Kensa	T1	399.40 ± 15.21 d	5.26 ± 0.30 c	2.78 ± 0.06 c	1.37 ± 0.14 b	0.40 ±0.05 b	0.13 ± 0.09 d	7.16 ± 0.83 b	0.57 ± 0.02 e	0.69 ± 0.03 c	0.61 ± 0.04 b	0.91 ± 0.03 d
	T2	436.74 ± 15.73 c	5.70 ± 0.13 ab	3.18 ± 0.17 b	2.03 ± 0.15 ab	0.31 ± 0.05 c	0.52 ± 0.03 a	8.41 ± 0.16 a	1.55 ± 0.05 b	0.94 ± 0.05 b	0.56 ± 0.03 b	1.62 ± 0.08 b
	T3	368.66 ± 11.61 e	5.59 ± 0.69 b	3.37 ± 0.65 ab	1.26 ± 0.07 c	0.26 ± 0.07 c	0.45 ± 0.05 b	7.56 ± 0.83 b	1.89 ± 0.06 a	1.06 ± 0.07 a	0.79 ± 0.05 a	0.81 ± 0.02 d

Data are shown as means ± SD. T1: optimal rates (control); T2: high rates (+25%T1); T3: low rates (−25%T1); C1, C2, and C3: tuber size classification; Sucr.T: sucrose content in tuber; Starch T: starch content in tuber; Gluc.L: glucose content in leaf; Fruct.L: fructose content in leaf; Sucr.L: sucrose content in leaf; Starch L: starch content in leaf. Values with the same letter in each column are similar according to Duncan’s multiple range test (DMRT) at p < 0.05.

show that the ‘Spunta’ variety produced the highest tuber yield under the optimal rates (615.40 g), followed by low and high rates (566.05 and 472.52 g, respectively), whereas high rates were the most beneficial for the ‘Kensa’ variety (436.74 g). Moreover, ‘Spunta’ recorded a higher yield per plant than ‘Kensa’ for all the fertilizer treatments tested. Similar trends were recorded for the number of tubers per plant, where T1 and T2 were the most effective fertilizer rates for ‘Spunta’ and ‘Kensa’ varieties, respectively (Table 6); T3 was more beneficial for ‘Spunta’ than for ‘Kensa’. In terms of the size classification of tubers, low rates increased the number of tubers classified in the C1 category for both varieties. On the other hand, the number of tubers in the C2 category was not affected by fertilizer rates in the case of the ‘Spunta’ variety, while high rates increased the number of tubers in this category in ‘Kensa’ plants. Finally, optimal rates resulted in a higher number of tubers belonging to the C3 category in both varieties.

The chemical composition of leaves and tubers varied depending on the fertilizer rate and the variety (Table 6). In particular, high fertilizer rates increased sucrose content in tubers, while the opposite trend was recorded for optimal rates. Moreover, the high and optimal rates resulted in the highest and lowest starch content in tubers for both cultivars, respectively. The optimal and low rates resulted in the highest and lowest content of glucose in the leaves of ‘Spunta’ plants, whereas the opposite trend was recorded for the plants of the ‘Kensa’ variety. Similar trends were recorded for fructose and sucrose. Finally, starch content in leaves increased in low and high rates for ‘Spunta’ and ‘Kensa’ varieties, respectively.

4. Discussion

The present study was conducted to identify the fertilizer rates that may increase the growth and yield parameters of two potato varieties that varied in the duration of their growth cycle, namely ‘Spunta’ and ‘Kensa’, with mid-early and mid-late maturity, respectively. There were significant differences regarding plant length, the total and photosynthetic leaf number, plant stem diameter, physiological traits, and real evapotranspiration rate. Significant differences were detected among the studied fertilizer rates (optimal, high, and low) for both varieties in terms of plant growth traits, except for the number of stems per plant, which was affected only by the genotype. Similar findings were reported by Zou et al. [62], who also found significant differences in the number of main stems per tuber among different varieties, as well as in tubers of the same variety that were stored at different temperatures. High and low NPK rates significantly increased plant length compared to the control, especially at high fertilizer rates, whereas no significant differences were recorded between the high and low fertilizer rates. Moreover, high rates resulted in thinner stems compared to the rest of the treatments, which partly explains the higher values of plant length for this particular treatment. According to the literature, high doses of N result in thin stems and long internodes, while increased rates of N and K may increase the number of stems per plant [17,63,64], findings which are in agreement with the results of our study. This effect could be associated with the pivotal role of macronutrients in promoting cell division, growth, and stem elongation, which results in increased plant length.

The high and low rates (T2 and T3, respectively) increased the total and photosynthetic leaf number in ‘Spunta’, whereas in Kensa, only high rates had a beneficial effect on leaf number, thus indicating differences in nutrient requirements between the tested varieties. However, apart from nutrient availability, the number of leaves is an inherent genotypic trait associated with the number and length of internodes, which also showed significant differences between the tested varieties. These findings reflect the direct effect of fertilizer application rates on leaf development, which is consistent with previous studies, where N fertilizer rates were correlated with the leaf area index and the aboveground biomass of potato crop [65,66]. Moreover, Zaleem et al. [67] and Abdel Naby et al. [68] reported that NPK fertilizers may affect the aboveground biomass yield of potato plants. According to Xing et al. [69], potato is a N-intensive crop with low uptake efficiency, although differences among varieties have been recorded. It is well known that depending on the nature of the soil and the environmental conditions, macronutrients can act synergistically or antagonistically. Potassium, in particular, regulates the N metabolism and osmotic adjustment, which are crucial for the development and growth of the leaves [70]. However, in contrast to the reports of Sai and Paswan [50] and Oliveira et al. [71], no significant effects were observed in our study for the low rates in the case of the ‘Spunta’ variety. This contradicting response highlights the varietal effects, as previously observed by Petropoulos et al. [16] and Mozumder et al. [72]. Additionally, the differences in duration of the growth cycle between the studied varieties could explain the varied response to fertilization, since in varieties with mid-late maturity such as ‘Kensa’, high nutrient availability for a longer period is essential for crop performance in terms of both above- and belowground biomass development. Moreover, Allison et al. [73] suggested that the increased rates of K fertilizers were more effective in early varieties, where the leaf area index is increasing early in the growing season, thus resulting in increased photosynthetic area and in increased yield.

The effect of variety was highly significant for all growth traits. ‘Spunta’ performed better than ‘Kensa’ for all plant growth parameters, except plant length, indicative of the intrinsic genetic effect and the variable adaptability of the tested varieties. In ‘Spunta’, which is known for its short vegetative cycle, low doses were equally effective to achieve optimal rates for growth parameters such as stem diameter, total leaf number, and photosynthetic leaf number. Contrarily, in the case of ‘Kensa’, which is characterized by a longer cycle, the low rates could not cover plant nutrient requirements throughout the growth cycle and especially at the phase, thus resulting in a decrease in the respective growth parameters compared to the ‘Spunta’ variety.

Physiological traits were also affected by fertilizer rates, while a varied response was recorded between the tested varieties, where ‘Spunta’ showed a better performance compared to ‘Kensa’ variety. Low fertilizer rates resulted in low Fo and high Fm values, and a low Fv/Fm ratio was also associated with high PAR values, thus indicating a better photosynthetic performance of both varieties at these particular rates. The assessment of photosystem (PSII) chlorophyll fluorescence represents a significant parameter for the evaluation of potato plant performance under different fertilizer rates and is a powerful tool in environmental and nutritional studies [74]. The application of a saturating pulse (light) to a dark-adapted leaf induces maximum fluorescence (Fm) by closing the reaction centres. At this point, in a healthy, unstressed plant, there is no NPQ (non-photochemical quenching) because the tissue is fully dark-adapted, and the maximum possible fluorescence Fm is recorded. The difference between Fo and Fm is the variable fluorescence (Fv). It is well established, both theoretically and empirically, that Fv/Fm is a robust indicator of the maximum quantum yield of PSII chemistry [75]. Furthermore, in unstressed leaves, the value of Fv/fm is highly consistent at around 0.83 and correlates with the maximum quantum yield of photosynthesis. Any type of stress that induces PSII inactivation damage or prolonged quenching leads to a decrease in Fv/Fm [76]. Interestingly, high values of Fm and low values of Fv/Fm, indicating photosynthetic efficiency [77], varied significantly between varieties and treatments (Table 3). For both varieties, low fertilizer rates increased Fm values and reduced the Fv/Fm ratio, especially in ‘Spunta’. However, high rates of NPK did not show any advantages compared to the optimum dose, whereas they are associated with additional production costs and environmental burdens. This response was consistent with the plant metabolites’ profile in terms of soluble sugars and starch, which generally showed a high amount of soluble sugars in the leaves at high and/or low rates. Both these fertilizer rates were associated with the highest amount of starch in tubers, which represent the main sink organs of potato plants during tuberization and tuber bulking phases. In this respect, we observed significant differences between soluble sugar and starch content in tubers and leaves when the interaction of the tested factors was considered. Interestingly, high and low rates induced similar sucrose and starch in leaves, indicating their efficiency in photosynthetic activity. On the other hand, optimal rates were more effective in increasing soluble sugar content in leaves in ‘Spunta’ plants, whereas the same trend was recorded for low rates in the case of the ‘Kensa’ variety.

Moreover, K supplementation may improve photosynthetic activity through the enhancement of RuBisCO activity, CO₂ fixation, and ion transportation across thylakoid stromal membranes [78], while it is also associated with improved N uptake and translocation [56]. Moreover, K and N must be readily available to plants in specific amounts to promote protein synthesis [50], since K and N are involved in N microbial transformation and the K–protein pathway [30]. The interrelationship between the main macronutrients (NPK) and their importance in plant physiological processes and consequently in crop performance has been reported by several authors [21,48,50], who suggested that higher levels of N and P may facilitate a greater efficiency of K fertilization and better crop performance.

The variation in growth traits also had an effect on yield components depending on fertilizer rates and variety. ‘Spunta’ recorded a higher yield than ‘Kensa’ for all the studied fertilizer rates, especially at T1 treatment, where the highest overall yield was recorded, while ‘Kensa’ had a better performance when plants were treated with high fertilizer rates (T2). These findings indicate that apart from nutrient availability, the genotype also plays a pivotal role in crop performance. Moreover, nutrient uptake depends not only on the availability of nutrients but also on the variety and their varied nutrient requirements throughout their growth cycle depending on the maturation process. Therefore, in our study, high rates of NPK may cause an imbalance in nutrient uptake through an antagonistic effect in the case of ‘Spunta’ due to the earliness in maturity of this particular variety. El-Hadidi et al. [79] and Job et al. [80] pointed out that the K excess could reduce the uptake of some necessary nutrients such as NH₄⁺, Ca²⁺, and Mg²⁺ and thus affect the nutritional balance between the other nutrients. Furthermore, the application of high K levels may negatively affect the uptake of Mg, an essential nutrient for photosynthesis, and consequently affect crop performance [23,80]. In the study by Setu and Mitiku [64], increasing N rates (up to 200 kg/ha) resulted in higher tuber yield due to a higher number of tubers per hill, while the mean tuber size remained unaffected. On the other hand, Maha et al. [17] suggested similar trends for K fertilization, where increased rates were associated with increased yields due to a higher number of tubers per plant. Moreover, Petropoulos et al. [16] recorded differences in the yield of two varieties, namely ‘Spunta’ and ‘Kennebec’, while both of them showed a similar response to the tested fertilization regimes (namely standard fertilizers, slow-release fertilizers, and soil amendments such as manure and zeolite). Moreover, the same authors suggested that the size of potato seed may affect yield, with larger tubers resulting in higher yield, while slow-release fertilizers were also more effective in terms of crop performance, since they facilitate better nutrient availability throughout the growth cycle and especially during tuberization and tuber bulking [16]. It also has to be noted that yield increase through the fertilizer application rates was associated mostly with tuber size rather than the number of tubers per plant, which is controlled by the genotype [81,82]. According to Bélanger et al. [26], increased nitrogen fertilization is associated with increased tuber fresh weight, although varietal differences were also reported. The higher yield and proportion of larger tubers (C1 category) in the ‘Spunta’ variety than ‘Kensa’ could also be associated with the higher vigour of plants (namely the stem number and total and photosynthetic leaf number) and the differences in maturity [82], as well as with the varied response of the tested varieties to fertilizer rates in terms of the stem and leaf number. For instance, ‘Spunta’ responded better to high fertilizer rates compared to ‘Kensa’, while low rates had a lesser effect, since ‘Spunta’ matures earlier than ‘Kensa’, meaning that plants of the former variety did not experience any lack of nutrient availability. Finally, the fertilizer application time and the form of applied nutrients may also affect crop performance, since these agronomic practises regulate nutrient availability, the partitioning of plant biomass, and the allocation of assimilates in tubers [83,84]. On the other hand, excessive fertilizer rates may have negative effects on the environment through the leaching of nutrients or on tuber quality (low specific gravity and high nitrate content) [26,85], without being associated with increased biomass production and tuber yield [20].

5. Conclusions

This study evaluated the effect of different fertilizer rates on plant growth, photosynthetic efficiency, and metabolite accumulation and productivity in two potato varieties, one mid-early (‘Spunta’) and one mid-late variety (‘Kensa’). In the case of ‘Spunta’, similar growth characteristics were observed at both high and low NPK rates, whereas for Kensa, the low rates reduced plant growth. Interestingly, the low rates improved photosynthetic activity, with low Fo and high Fm values being recorded in both varieties. On the other hand, high rates of NPK were more efficient in accumulating sucrose and starch in the sink organs. Nevertheless, these rates did not improve the yield per plant, thus indicating the possible negative effects on the nutrient balance between major and minor nutrients. In addition, our results indicate that low fertilizer rates were insufficient to meet the nutrient needs of the ‘Kensa’ variety, with a significant reduction in yield per plant being observed compared to the optimum and high rates. In conclusion, NPK fertilization is often considered excessive in many African countries, causing an imbalance in nutrients and resulting in high production costs and the salinization of agricultural soils. Therefore, based on our findings, a reduction in fertilizer rates could be recommended, as a sustainable and eco-friendly agronomic practice, without compromising the yield of potato crops, although the differential response among the tested varieties indicates that this practice could be applied on a genotype-dependent basis. However, further field studies are needed to validate our results before extrapolating them to commercial farms.