Plum Trees’ Leaf Area Response to Fertilization and Irrigation in the Nursery

Abstract

This study addressed a significant and relevant issue, specifically the production of high-quality fruit planting material linked to an economically viable nursery operation. The process considered both the pedo-climatic conditions of the region where the fruit planting material was cultivated and the technological elements utilized. The objective of this research was to gather information regarding the necessity and effectiveness of implementing localized irrigation for plum trees in the nursery in the context of various fertilization treatments. It also aimed to investigate the variations in leaf area among Cacanska Lepotica and Stanley plum cultivars subjected to various irrigation (non-irrigated control, 10 mm, 20 mm, and 30 mm) and fertilization (unfertilized control, N₈P₈K₈, N₁₆P₁₆K₁₆, and N₂₄P₂₄K₂₄) methods. The study was conducted within a private nursery situated in the northwest region of Romania using a 4 × 2 × 4 split-split-plot design with five replications. This research took place in the summer of 2024, in the second field of the nursery during the growth stage of grafted trees. The implementation of various NPK fertilization methods (8%, 16%, and 24%) led to enhancements in leaf surface developments (increased by 6.53–16.14% compared to the control). The application of fertilization ranging from 8 to 16% and subsequently from 16 to 24% was effectively absorbed by the plum trees, resulting in a substantial growth of 180–226 cm². Irrigation with 30 mm generated significant increases in the leaf area of 4.42–14.27% compared to the control. To obtain optimal yields of grafted trees, it is advisable to utilize a combination of irrigation and NPK fertilization. To promote the appropriate growth and development of the trees, it is essential to monitor the soil moisture levels and to implement irrigation during times of water shortage when the trees exhibit heightened water usage. The research findings indicated that both cultivars experienced similar advantages from 24% NPK fertilization and 30 mm of irrigation; therefore, the implementation of the aforementioned technological elements is strongly recommended.

1. Introduction

The development of fruit trees in nurseries, like all cultivated species, is greatly impacted by the existing climatic and soil conditions. Water, in conjunction with heat, light, air, and minerals, is one of the key factors that greatly contributes to this development. According to Smith [1], water constitutes approximately 75–85% of the total weight of various tree organs, and in some cases, this proportion may exceed that range. Water not only facilitates the transport of nutrients from the soil to the plant but also serves as a fundamental component in the synthesis of all organic substances that form the tissues of both rootstocks and trees. Therefore, it is essential for plants to have a consistent and adequate supply of water to ensure that growth processes occur at the optimal intensity [2].

The management of water and nutrients applied through fertilizers constitutes two primary factors that influence growth and productivity within tree nurseries. The emphasis on water and sustainable fertilizers in these nurseries has become increasingly important, leading to the implementation of management strategies that ensure satisfactory yields and thereby enhancing the efficiency of both fertilizer and water usage. In recent years, there has been a significant market demand for fruit trees, with the harvested fruit serving as a vital source of vitamins. Additionally, the planting materials produced in the nursery have substantial water requirements, necessitating irrigation throughout the growing seasons. Nurseries have faced challenging production periods due to weather-related factors in recent years. Consequently, it is deemed essential to gather information regarding the effective combination of irrigation practices and irrigation volumes. This research topic was selected due to the limited availability of data regarding the impacts of localized irrigation when combined with additional technological elements on fruit tree seedlings in nurseries. The current body of literature and prior studies have mainly focused on irrigation and fertilization practices in fruit tree orchards, rather than in nursery settings. This research holds considerable scientific importance as it may provide essential insights for fruit tree specialists engaged in seedling production. Practically, it aims to implement various irrigation and fertilization standards for maiden plum trees in nurseries, thereby enhancing the horticultural sector and elevating productivity levels. The anticipated outcomes resulting from the application of differentiated irrigation and fertilization practices include achieving greater economic efficiency compared to nurseries that do not utilize the aforementioned methods. Prior findings indicate that, when fertilization is applied, the following factors must be considered: soil conditions, species, cultivar, rootstock, planting density, and expected yield.

With regard to the need for water in the nursery, the present research started from the premise that it is necessary for the trees to have sufficient water available at all times so that the growth processes can take place with the greatest possible intensity. While a substantial quantity of water from the soil is absorbed by the roots for the synthesis of organic compounds, only a small fraction is utilized by the rootstock, with the remainder being lost through transpiration. Rootstocks cultivated in saturated, nutrient-deficient soils characterized by lower concentrations of fertilizing elements in the soil solution generally exhibit higher transpiration rates compared to the rootstocks planted in more fertile soils. In Romania, their cultivation is primarily concentrated in regions with abundant rainfall.

The addition of fertilizers and the provision of adequate amounts of water in the soil decrease the transpiration coefficient, thereby enhancing the efficiency of water usage by the rootstock; the transpiration coefficient represents the amount of water eliminated in the process of perspiration in the period in which one gram of dry substance is biosynthesized [3].

In horticulture, the administration of irrigation systems involves multiple components concerning water management and distribution, plant care, and the maintenance of irrigation infrastructure. The effectiveness of irrigation is significantly dependent on the correct implementation of such strategies. Research indicates that neglecting the aforementioned aspects may decrease the effectiveness of investments allocated to irrigation initiatives. Consequently, administering water to plants without considering their specific needs can adversely affect not only the current year’s yield but also the overall quality of the produce [4].

Experiences in horticulture so far have demonstrated that providing a controlled supply of supplementary water to soil and plants beyond what is naturally available supports consistent agricultural output at high levels [5]. This approach ultimately aims to foster conditions for efficient and sustainable agricultural practices while safeguarding environmental elements. Among these factors, soil plays a crucial role, necessitating the maintenance and enhancement of its fertility. It is advisable to integrate irrigation, a vital method for combating drought, with other technical and agricultural practices, such as drainage (when necessary), fertilization, and the selection of superior plant cultivars [6,7,8].

Sullivan et al. [9] associated irrigation with a crucial technological intervention that has the potential to greatly improve agricultural productivity, irrespective of natural precipitation levels. Irrigation can effectively mitigate water shortages by providing additional water in line with the requirements of plants, which in turn enhances the efficient utilization of fertilizers due to the increased solubility of nutrients. Furthermore, it has the potential to improve product quality, prolong the growing season, and enable more efficient production planning.

In the climatic conditions in Romania, irrigation serves to supplement the naturally available water from rainfall during times when it is insufficient compared to crop requirements. In fact, it is used to effectively mitigate the variability in harvests from year to year, which is caused by natural factors. The aim of implementing irrigation is to achieve stable yields that align closely with the productive potential of crops under specific soil and climate conditions. Research conducted in Romania has revealed that there are years when inadequate rainfall leads to significantly diminished harvests, sometimes resulting in total crop failure [10].

Furthermore, studies in horticulture indicate that irrigating only along tree rows does not adversely affect root system development. Under such conditions, roots tend to cluster in a more confined area, without significantly shortening their overall length, thus maintaining their capacity for water and nutrient absorption [11].

In Romania, the circumstances are unique and influenced by its geographical location, which results in areas defined by a temperate continental climate with average annual rainfall ranging between 400 and 550 in irrigated agricultural regions. Thus, precipitation levels within the aforementioned limits prevent the agglomeration of the root system, especially along the rows of the plants. The roots grow both horizontally and vertically, similarly to the patterns observed with irrigation techniques such as sprinkling and surface runoff, without significant differences [12]. In fruit nurseries, the water content in the soil should be maintained within the limits of the active moisture range at a level higher than the minimum threshold. Thus, the soil water content should be kept above the wilting coefficient, near which plants undergo a series of changes during the growing process. Soil moisture in the range between the wilting coefficient (Co) and the field soil water capacity (C) represents the water that plants can utilize. The greatest quantity of water available to plants is characterized by the accessible water capacity (Caa) or the useful water capacity (Cau) of the soil. The moisture interval between these two constants is referred to as the active moisture range (AWR).

In modern fruit growing, fertilization, along with irrigation, stand out as some of the most crucial technological components. As fruit plants tend to occupy the same plot of land for extended periods, they develop extensive root systems and, due to their high yields, draw significant amounts of nutrients from the soil during the harvest time. In such circumstances, as pointed out by Jäger [13], yearly fertilization interventions in several rounds might ensure, on the one hand, the achievement of a certain level of production and, on the other hand, a certain level and ratio of nutrients. In horticulture, the fertilizer application system is primarily structured to address issues related to the effective replenishment of easily accessible nutrients in the soil and to restore these nutrients in a balanced manner, according to the requirements of rootstocks and seedlings. This is a crucial factor, as every harvest depletes the soil of specific mineral and organic compounds. Additionally, it is important to provide organic matter to the microflora of the soil; otherwise, these microorganisms may compete with higher plants for essential mineral nutrients [14]. Most microorganisms in the arable layer are heterotrophic, which means that, to obtain the energy needed for nutrition, they rely on organic matter previously synthesized by other organisms. Fertilization plays crucial roles in regulating the ionic composition of the soil and in adjusting the pH levels to meet the biological needs of rootstocks and seedlings.

Shibiao et al. [15] demonstrated that the application of fertilizers in fruit nurseries is intended to enhance nutritional conditions and boost organic matter synthesis while also facilitating substantial and cost-effective production with high-quality standards. This approach ensures that plant resistance to diseases and pests remains intact and minimizes environmental pollution. The effectiveness of fertilizers in increasing production is maximized only when they are integrated into a comprehensive system of technological practices, with the application rates tailored to the specific plant, soil and climatic conditions, and cultivation techniques.

Furthermore, replenishing soil nutrients depleted during harvest and providing organic matter to support the microflora is most effectively achieved through the application of both chemical and organic fertilizers.

The suitable dosages for each situation in the field should be determined based on a thorough analysis of various soil characteristics, alongside a good understanding of crops’ requirements and the need to maintain specific qualitative and quantitative production standards. Given the wide range of conditions and circumstances, fertilization practices in orchards should offer tailored solutions for each unique case. Research on fertilizers, whether conducted in field settings or in greenhouses, seeks to clarify the mechanisms governing the mineral nutrition of trees and assesses the nutrient balance within both the plant and the soil. This research might also explore ways to improve fertilization in an efficient way [16].

Concerning the specific needs of plum trees in the nursery, fertilizers are applied during the active season, typically 2–3 times throughout the summer or more frequently, albeit in smaller amounts. Fertilizers with a high nitrogen content are utilized to promote the vegetative growth of plum trees. Nitrogen plays a crucial role in encouraging the vegetative growth of maiden plum trees, aiding in the development of leaves and green branches. A sufficient supply of nitrogen is vital for healthy tree growth and enhances the production of leaves, which are essential for photosynthesis and energy generation. Phosphorus is critical for the establishment of the root system in trees, as it aids in the formation of robust roots and facilitates the effective uptake of water and other nutrients from the soil. Potassium improves the trees’ resilience to environmental stress and bolsters their defence mechanisms against diseases. However, the application of excessive fertilizers to maiden plum trees in the nursery may lead to rampant vegetative growth, reduced drought resistance, or complications in the absorption of other nutrients.

In the climatic context of recent years, Romania is encountering the issue of complex agricultural drought, a climatic hazard that results in the most severe repercussions for agriculture. In particular, within the nursery sector, irrigation provides the opportunity to produce vigorous, viable, high-quality, healthy, and damage-free planting material that is resistant to diseases and pests. Besides water, another crucial element in the production of quality fruit planting material is the fulfilment of fertilizer requirements, which underpins the rationale for this research. The objective of this research has been to establish the influences of irrigation, fertilization, and the plant cultivar on a physiological character in the plum species in the nursery. Based on this research, it can be concluded that the efficient and optimal utilization of fertilizers within a rational fertilization system is possible only by adhering to all the necessary requirements dictated by the cultivation technology. Effective agricultural techniques, even when employing reduced amounts of fertilizers, can result in higher crop yields as a consequence of the improved fertilizer utilization coefficient. The efficiency of the fertilization system is higher when the physical–chemical properties of the fertilizer are considered and a differentiated application is made, corresponding to the soil properties and the biological particularities of the species. This research holds practical significance through the implementation of various irrigation and fertilization techniques on maiden plum trees within the nursery aimed at advancing the horticultural sector and enhancing productivity. The anticipated outcomes from the application of these diverse irrigation and fertilization methods include the production of robust and healthy fruit planting material, an increase in nursery output, and the attainment of improved economic efficiency compared to nurseries that do not utilize irrigation or fertilization.

2. Materials and Methods

2.1. Characterization of the Climatic Conditions of the Experimental Field

This research was carried out in a private nursery in field II. It is important to note that although the rootstocks are located in field I of the nursery, they transition to field II after grafting in the subsequent spring. In this field, the scion is thus present as a bud that has been grafted onto the rootstock. Regardless of the establishment method used, the efforts undertaken in field II of the coppice are focused on maximizing the number of grafted shoots that are established in vegetation. In this particular instance, the rootstocks were planted in the spring of 2023, grafted during the summer of that same year, and by the spring of 2024, they had transitioned to field II. The nursery is located in the northwestern part of Romania on a plain unit, being the result of a long process of accumulation and erosion through the diversion of the hydrographic network, which descends from the mountainous and hilly areas.

Table 1. The monthly average temperatures and monthly average rainfall in the research field in 2024.

Month	I	II	III	IV	V	VI	VII	VIII	IX	X	XI	XII
monthly average temperatures (°C)	−1.5	4.4	6.9	11.0	14.5	20.2	21.6	23.2	18.9	12.7	5.1	4.9
monthly average rainfall (L/mp)	13.5	45.2	43.8	15.4	60.0	101.5	48	45	48	48	35	70

I—January, II—February, III—March, IV—April, V—May, VI—June, VII—July, VIII—August, IX—September, X—October, XI—November, XII—December.

displays the average monthly temperatures and average monthly precipitation for that specific year. The warmest month was August with 23.2 °C, and the coldest month was January.

In 2024, recognized as the driest year, three periods could be identified based on the fluctuations in monthly precipitation relative to the multi-year average. The initial period, spanning from January to May, exhibited negative deviations ranging from 2 to 16 mm, followed by the months of June and July, with positive deviations of 39–85 mm and the July–December period, characterized by negative variations of 16–25 mm. The distribution of monthly precipitation throughout these three periods in 2024 also reflected negative variations of 2–48 mm in the period between January and May and of 9–21 mm in the last part of the year. Rainfall exceeding the multiannual average by 39–85 mm during June and July significantly contributed to the overall precipitation for the year, positioning it as intermediary between the other two years in terms of rainfall amounts (Table 1). Information regarding climate conditions was sourced from the meteorological station, which monitors the temperature and precipitation patterns for each area. The rainfall patterns were assessed using a rain gauge placed in the experimental field.

2.2. Soil Characteristics and the Organization of the Experimental Field

To identify, delimit, and characterize the soil units within the land where the study was conducted, a pedological profile was developed. From it, samples were gathered on 4 horizons (

Table 2. Physical and hydrophysical indices of alluvial gley soil from the nursery.

Soil Horizon	Sampling Depth (cm)	AD (t/m3)	HC (%)	WC (%)	FC (%)	AHR (%)	Tmin (%)
Ap	0–30	1.26	9.04	13.56	26.85	13.29	20.21
A0	30–50	1.27	8.96	13.44	26.51	13.07	19.98
AC	50–80	1.33	8.87	13.31	26.39	13.09	19.85
Cg	80–120	1.34	8.83	13.25	25.81	12.57	19.53

AD—apparent density; HC—hygroscopicity coefficient; WC—wilting coefficient; FC—field capacity; AHR—active humidity range; Tmin—minimum threshold of soil humidity.

). The assessments conducted and the methodologies employed were as follows: potentiometric pH measurement using a glass electrode in a 1:2 aqueous soil/water suspension; humus percentage determination via the Walkley–Black method; a mobile phosphorus assessment using the Eqner–Riehm–Domingo method; and a mobile potassium analysis also following the Eqner–Riehm–Domingo method. The concentration of potassium in the analysed soil extract was determined using a potassium interference filter and a photoelectric cell, and the mechanical analysis was conducted via the Kacinschi method after pre-treatment with hexametaphosphate for dispersion. Following the analyses, the identified soil taxonomic was arable, weakly glaciated, loamy clay on fluvial deposits. The soil texture was fine throughout the depth of the studied profile. The soil reaction was slightly alkaline at 0–30 cm and 50–80 cm, and neutral at 30–50 cm and 80–120 cm.

As a rule, fruit trees thrive in soil that has a neutral pH, as well as in slightly acidic or slightly alkaline conditions. The base saturation level for our experiment was notably high, ranging from 84% to 90%. The humus content was moderate, while the levels of phosphorus and potassium were exceptionally elevated.

The observed density within the examined soil profile ranged from 1.26 and 1.34 t/m³, which can be classified as average. The hygroscopicity coefficient ranged from 8.83% to 9.04%, which correlated with elevated wilting coefficient values of 13.25% to 13.56% and field capacity values of 25.81% to 26.85%. The minimum threshold recorded values ranged between 19.53% and 20.21%, corresponding to relative values of 75% to 76% of the field capacity.

The study was conducted utilizing a 4 × 2 × 4 split-split-plot design with five replications, featuring plots of four two-year-old trees planted at a spacing of 0.7 × 0.25 m². The primary factor was irrigation, which included four levels (non-irrigated control, 10 mm, 20 mm, and 30 mm), while the second factor was the cultivar (Stanley and Cacanska Lepotica), and the third factor was fertilization at four levels (unfertilized control, N₈P₈K₈, N₁₆P₁₆K₁₆, and N₂₄P₂₄K₂₄). To achieve the three levels of fertilization (N₈P₈K₈, N₁₆P₁₆K₁₆, and N₂₄P₂₄K₂₄), a complex fertilizer with a ratio of 16:16:16 was applied in three increments (50, 100, and 150 kg/ha). The irrigation schedule was based on variations in soil moisture, with applications occurring on 17 August, 22 August, and 27 August. Fertilizers were incorporated into the soil at a sampling depth of 15 cm on 19 August, 25 August, and 29 August. The rootstocks were planted in the spring of 2023 and grafted during the summer of the same year. Leaf area measurements were conducted in August 2024, following leaf drop after the first frost, which typically occurs in October. Considering that the trees are removed from the second field of the nursery and prepared for sale, they are not affected by frost, since when frost occurs, the trees are already planted in the orchard. The leaf area was measured on one tree in each plot. One typical tree was chosen in each plot, and was normally developed, healthy, and without defects. Different leaves of an average size located on the average annual growth area were chosen. Measurements were made on 10 leaves from each tree, and the arithmetic mean of the results obtained was calculated. A leaf area meter, AM 350, produced by ADC BioScientific Ltd. (Hoddesdon, UK), bought from a Romanian distributor from Bucharest, was employed for these measurements, featuring a scanning speed of up to 20 mm/s, a maximum width of 103 mm, a maximum length of 2 m, linear precision/repeatability of 1%, area accuracy of ±2%, perimeter accuracy of ±5%, and a resolution of 0.065 mm².

2.3. Calculations

To assess the direct water usage, the soil water balance was evaluated by measuring the soil water reserves at the beginning and the end of each month from April to September. The soil moisture content was determined using a gravimetric method, where soil samples were collected from the field at the beginning and midpoint of each month, and weighed before and after being dried in an oven. The calculation of the soil moisture content was based on the difference between the two weight measurements using the following formula [17]:

W = 100 × (B − A)/A;

(1)

where W represents soil moisture (%); A is the mass of the wet soil sample (g); and B is the mass of dry soil sample.

When the soil moisture fell below 10%, it was deemed low, potentially signalling a water deficit, which could necessitate the application of irrigation. The irrigation rate was calculated by applying the following formula [17]:

M = 110 × H × Gv (C − P min.)

(2)

where m is the watering rate (m³/ha); H is the thickness of the soil layer intended to be supplied with water in meters; Gv is the volumetric weight of the soil (t/m³); C is the field capacity for water, in weight percentages relative to dry soil; and P is the minimum threshold or momentary supply, as a percentage of dry soil weight.

The calculation of water consumption from the soil, which includes the volume of water absorbed by the trees in the nursery during transpiration along with the water lost through evaporation from the land surface, was performed using the following formula [17]:

ET = ∑ (t + e)

(3)

where ET = evapotranspiration (m³/ha), t = the amount of water lost from the soil through plant transpiration (m³/ha); and e = water evaporated on the soil surface (m³/ha).

The application of the initial irrigation, defined as the period between plant emergence and the first watering, was determined by the initial water reserves in the soil at the start of the growing season, the specific water requirements of the particular crop, and the rainfall that occurred during that timeframe [17]:

$$\mathrm{T} = {\displaystyle \frac{\mathrm{R}\mathrm{i}-\mathrm{p} \mathrm{m}\mathrm{i}\mathrm{n}}{\left(\mathrm{e}+\mathrm{t}\right)-\mathrm{P}}}$$

(4)

and the interval between two waterings was calculated according to the following formula [17]:

$$\mathrm{T} = {\displaystyle \frac{\mathrm{m}}{\left(\mathrm{e}+\mathrm{t}\right)-\mathrm{P}}}$$

(5)

In this context, T represents the interval measured in days; e + t denotes the total daily water consumption for the specific crop, expressed in m³/ha/day; Ri refers to the initial water reserve present in the soil at the depth of saturation at the onset of the growing season, which can be determined using a formula akin to that used for calculating the final reserve, but substituting the withering coefficient with the field water capacity of the soil (m³/ha); and P indicates the immediate moisture content (water level at the time of irrigation) of the soil layer H, represented as a percentage of the dry soil’s weight.

Beneficial precipitation is determined by the disparity between total precipitation and the losses incurred through evaporation and excessive infiltration [17]:

P_e = (P₀ − E) − (I + R)

(6)

where P₀ is the total precipitation, E is evaporation, I is infiltration, and R is surface runoff.

The irrigation standard for the growing season, in the context of a closed-loop balance (a hydrological irrigation regime that does not involve groundwater recharge), was determined as follows [17]:

∑m = ∑ (E + T) + Rf + Ri − Pv (m³/ha)

(7)

-

∑ (E + T) the total consumption of water from the soil during the vegetation period;

-

The soil water reserve Rf, at the conclusion of the growing season is established through a relationship involving the volumetric weight of the soil at the specified depth, the thickness of the soil layer designated for irrigation, and the wilting coefficient at that same depth (m³/ha);

-

The initial soil water reserve at the wetting depth at the onset of the growing season is similarly calculated using a relationship akin to that of the final reserve, with the soil water capacity in the soil field (m³/ha) substituted for the wilting coefficient;

-

The sum of summer precipitation is defined as being equal to or exceeding 5 mm.

The data regarding the leaf area were statistically processed using ANOVA associated with the 4 × 2 × 4 split-split-plot design, and an LSD test at p < 0.05 [18]. The results were expressed as means ± standard errors.

2.4. Analysis of the Water Balance and Consumption Under Different Irrigation Conditions

In April 2024, the recorded soil water reserve ranged from 2405 to 2463 m³/ha, representing about 86.5% of the field water capacity, indicating a favourable soil supply. However, due to a negative water balance in April, the soil water reserve at the start of May fell to between 2210 and 2228 m³/ha, which was marginally above the minimum threshold (

Table 3. The elements of the soil water balance for maiden plum trees in the research field.

Irrigation Norm		Month
	Elements	April	May	June	July	August	September
Non-irrigated	Ri (m3/ha)	2405	2210	2223	2291	2296	1864
0 mm	Pe (m3/ha)	150	542	850	1048	277	295
	ET (m3/ha)	345	529	782	1044	708	464
	Rf1 (m3/ha)	2210	2223	2291	2296	1864	1695
	M (m3/ha)	-	-	-	-	-	-
	Rf2 (m3/ha)	-	-	-	-	-	-
Irrigated with	Ri (m3/ha)	2431	2214	2251	2305	2294	2106
10 mm	Pe (m3/ha)	150	542	850	1048	277	295
	ET (m3/ha)	367	505	796	1059	765	528
	Rf1 (m3/ha)	2214	2251	2305	2294	1806	1873
	M (m3/ha)	-	-	-	-	300	-
	Rf2 (m3/ha)	-	-	-	-	2106	-
Irrigated with	Ri (m3/ha)	2463	2228	2222	2313	2247	2275
20 mm	Pe (m3/ha)	150	542	850	1048	277	295
	ET (m3/ha)	385	548	759	1114	849	562
	Rf1 (m3/ha)	2228	2222	2313	2247	1675	2008
	M (m3/ha)	-	-	-	-	600	-
	Rf2 (m3/ha)	-	-	-	-	2275	-
Irrigated	Ri (m3/ha)	2419	2210	2218	2294	2258	2519
30 mm	Pe (m3/ha)	150	542	850	1048	277	295
	ET (m3/ha)	359	534	774	1084	916	584
	Rf1 (m3/ha)	2210	2218	2294	2258	1619	2230
	M (m3/ha)	-	-	-	-	900	-
	Rf2 (m3/ha)	-	-	-	-	2519	-

Ri—initial reserve; Pe—useful precipitation; ET—evapotranspiration; Rf₁—final reserve without irrigation; M—irrigation rate; Rf₂—final reserve with irrigation.

).

In June and July, the abundant rainfall, associated with a relatively stable soil water balance, facilitated the maintenance of soil moisture above the minimum threshold. In August, the low level of rainfall and high temperatures generated a significant loss of soil water that reached values between 1619 and 1864 m³/ha, well below the minimum threshold. To compensate for the water deficit, irrigation was applied in accordance with the norms of 300, 600, and 900 m³/ha so that the soil water reserve exceeded the minimum threshold, reaching levels of 2106–2519 m³/ha. In September, due to a low amount of rainfall, the water stress experienced in the previous month persisted, resulting in the soil water reserves dropping below the minimum threshold for both the non-irrigated variant (1695 m³/ha) and the irrigated variant with the norm of 10 mm (1873 m³/ha). Throughout the 2024 vegetation period, the average daily consumption exhibited a steady rise until July (refer to Figure 1). Thus, against the background of values ranging from 11.5 to 12.83 m³/ha/day observed in April, the climatic conditions in May generated an increase in water consumption with rates ranging between 33.2% and 48.35%.

The peak water consumption level, recorded at 33.67–35.94 m³/ha in July, was primarily influenced by the substantial precipitation levels in conjunction with the irrigation applied, rather than the temperature increase of 1.4 °C compared to June. Water consumption declined during the April to June period compared to the previous year, whereas an increase was noted in July and August, and fairly close values in September. At the beginning of April 2024, the soil moisture content in the non-irrigated variant had a value of 22.8%, close to that recorded in the previous year. During the May–June period, the soil moisture maintained its values above the minimum threshold (Figure 2).

Against the background of high temperatures in association with the lower level of precipitation in August, the soil moisture level decreased to 17.52%, a value that was significantly lower than the minimum threshold. In September, the climatic conditions led to a heightened water deficit, resulting in a reduction in the soil moisture content to 15.93%. For the variant involving a 10 mm irrigation standard, the water deficit experienced in August was mitigated by the application of an irrigation volume of 300 m³/ha, which facilitated an increase in the soil moisture content of approximately 2.8%, bringing it close to the minimum threshold, of 16.97% (Figure 3).

The drought experienced in August necessitated, based on the variant associated with the 20 mm standard, the implementation of three irrigation sessions, amounting to an irrigation norm of 600 m³/ha, which maintained a soil moisture level of 21.38%, approximately 1.5% above the minimum threshold, of 15.74% (Figure 4).

The water deficit in September resulted in a reduction in the soil moisture levels by as much as 18.87%, which was 2.9% higher compared to the non-irrigated variant. For the irrigated variant with the norm of 30 mm, three irrigations totalling 900 m³/ha were administered in response to the water deficit caused by the drought in August. This resulted in a soil moisture level of 23.87% by the end of the month, exceeding the minimum threshold, of 15.22% (Figure 5).

The water shortage in September led to a decrease in the soil moisture content of up to 20.96%. However, that level remained above the minimum threshold and was notably around 5% higher than that of the non-irrigated variant.

3. Results

3.1. Analysis of the Variance Components

The findings from the variance analysis presented in

Table 4. Analysis of variance regarding the effects of irrigation, the cultivar, and fertilization on the leaf area.

Source of Variation	SS	DF	MS	F Test
Total	43,868,124	159
Replication	216,761	4	54,190	0.59
Irrigation (A)	8,493,021	3	2,831,007	30.75 **
Error A	1,104,856	12	92,071
Cultivar (B)	9986	1	9986	0.12
A × B	970,711	3	323,570	3.84 *
Error B	1,349,748	16	84,359
Fertilization (C)	9,982,917	3	3,327,639	22.07 **
A × C	3,044,252	9	338,250	2.24 *
B × C	1,398,818	3	466,273	3.09 *
A × B × C	2,819,710	9	313,301	2.08 *
Error C	14,477,344	96	150,806

* Significant at p < 0.05; ** significant at p < 0.01.

indicate that only irrigation and fertilization had significant impacts on the leaf area, with irrigation demonstrating a more pronounced effect compared to the cultivar, which had a negligible influence. Furthermore, while the combined effects of these factors were significant as regards the leaf area, they were notably less impactful than their individual effects. The interaction between the cultivar and irrigation was particularly emphasized due to its substantial effect, followed by the interaction between fertilization and the cultivar.

3.2. Effects of Irrigation and Cultivar on the Leaf Area

An analysis of the interaction between irrigation and the cultivar on the leaf area of plum trees revealed that for the Cacanska Lepotica cultivar, the three irrigation levels resulted in significant differences when compared to the control, amidst variations among the irrigation levels. Conversely, the Stanley cultivar demonstrated effective utilization of only the 20 and 30 mm irrigation levels, leading to significant enhancements in this characteristic relative to the control, whereas the 10 mm level had a non-significant effect and was inferior to that observed in the Cacanska Lepotica cultivar (

Table 5. The effects of irrigation and the cultivar on the leaf area.

Cultivar	Irrigation				Cultivar Mean
	Control	10 mm	20 mm	30 mm
Stanley	4316 ± 59 a z	4498 ± 71 a yz	4651 ± 65 a xy	4821 ± 86 a x	4572 ± 41 A
Cacanska Lepotica	4234± 65 a z	4429 ± 60 a z	4738 ± 88 a y	4948 ± 90 a x	4587 ± 49 A

Cultivar LSD_5% = 97 cm²; cultivar–irrigation LSD_5% = 193 cm²; irrigation–cultivar LSD_5% = 195 cm²; the same letter (a) in the column indicates non-significant differences (p > 0.05) between cultivars; different superscript letters (x, y, z) in the row indicate significant differences (p < 0.05) between watering norms. The same letter for the cultivar means (A) indicates a non-significant difference (p > 0.05).

).

Given the interaction between irrigation and the leaf area in Stanley cultivar trees, it was observed that only irrigation with 20–30 mm allowed a significant variation of 7.76–11.7%, while the effect of the 10 mm watering norm was smaller and non-significant. Under the effect of different watering norms, trees of the Cacanska Lepotica cultivar recorded a leaf area ranging between 4234 cm² in the case of the control and 4821 cm² for the 30 mm watering norm, against a background of a 9.5% variability between watering norms. Compared to the control, in the trees of this cultivar, the three watering norms generated significant increases of 4.61–16.86%. The gradual adjustment of the watering standard by 10 mm resulted in changes in this character, ranging from 210 to 309 cm².

3.3. Effects of Irrigation and Fertilization on the Leaf Area

The leaf surface exhibited a variation of 682 cm² due to different fertilization treatments, with measurements ranging from 4226 cm² in the unfertilized agricultural setting to 4908 cm² when a dose of 24% NPK was applied, reflecting a variability of 6.3% across the treatments (

Table 6. The effects of irrigation and fertilization on the leaf area.

Irrigation	Fertilization				Irrigation Mean
	Control	N8P8K8	N16P16K16	N24P24K24
Control	4006 ± 63 b y	4196 ± 45 c y	4343 ± 76 c xy	4557 ± 57 c x	4275 ± 43 D
10 mm	4216 ± 91 ab y	4364 ± 64 bc y	4520 ± 64 bc xy	4753 ± 62 bc x	4464 ± 46 C
20 mm	4267 ± 58 ab z	4608 ± 55 ab yz	4856 ± 43 ab xy	5048 ± 71 ab x	4695 ± 54 B
30 mm	4415 ± 47 a z	4838 ± 98 a y	5008 ± 86 a xy	5277 ± 77 a x	4885 ± 62 A

Irrigation LSD_5% = 148 cm²; irrigation–fertilization LSD_5% = 327 cm²; fertilization–irrigation LSD_5% = 345 cm²; different letters (a, b, c) in the column indicate significant differences (p < 0.05) between watering norms; different superscript letters (x, y, z) in the row indicate significant differences (p < 0.05) between NPK levels. Capital letters were used for irrigation mean (A, B, C, D) comparisons.

).

The application of NPK fertilization variants determined the recording of significant increases in the growth of this character by 6.53–16.14% compared to the non-fertilized agricultural background. The addition of fertilization from 8 to 16% and from 16 to 24% was effectively utilized by the seedlings that achieved a significant growth of 180–226 cm².

By analysing the impact of fertilization on the growth of the leaf structure at a specific watering rate, it is obvious that the greatest variation was noted with the 30 mm norm, whereas the 10 mm norm exhibited a smaller range of variation between the NPK dosage levels.

Without fertilization, the examined irrigation standards resulted in leaf surface area measurements ranging from 4006 cm² under non-irrigated conditions to 4415 cm² at the 30 mm irrigation level, indicating a variability of 5.79% among the different watering standards.

Compared to the non-irrigated version, the three irrigation standards facilitated an increase of 5.24 to 10.21% in the leaf apparatus in these conditions; however, only the effect of the 30 mm standard was significant. The modification of watering norms did not generate substantial increases against the background of variations in the leaf surface area of 51–199 cm². Under fertilization conditions involving 8% NPK, a variation in this character was evident, ranging from 4196 cm² in the non-irrigated case to 4838 cm² when applying a watering norm of 30 mm. On that agricultural land, only the watering norms of 20 and 30 mm showed significant effects associated with increases of 11.81–15.31% compared to the non-irrigated version. Furthermore, adjusting the watering rate by 10 mm had a negligible impact; thus, only the increase in the watering rate from 10 mm to 30 mm resulted in a significant variation of 10.86% in the leaf surface area. Under fertilization conditions involving 16% NPK, the leaf surface ranged between 4343 cm² and 5008 cm². As such, in that case, irrigation with 20–30 mm generated a significant increase of 11.81–15.31% of that character compared to the non-irrigated variant. The adjustment in the irrigation rate from 10 mm to 20 mm resulted in a change of 7.43% in the leaf surface area, whereas a further increase in irrigation from 20 mm to 30 mm produced a lesser and statistically insignificant effect of 4.54%.

In the context of fertilization with 24% NPK, the application of irrigation with 20–30 mm favoured a significant increase in that character by 491–720 cm². In the case of this agricultural background, only the increase in the watering rate from 10 to 30 mm showed a high efficiency, which materialized as the significant development of the leaf apparatus with 524 cm². According to the data presented in Figure 4, the non-irrigated variant demonstrated that the exponential regression revealed an average foliar apparatus development rate of 22.96 cm² for every kilogram of NPK applied. That rate was 18.3 cm² at application levels of 8 to 16% and increased to 26.75 cm²/kg NPK for doses between 16 and 24%. The estimates provided are highly precise, with a confidence level of 99.62%, based on a leaf area of 4009 cm² in the absence of fertilization.

For the norm of 10 mm, the effect of fertilization on that character was expressed by a regression that was based on a precision of 99.08% and indicated an average growth with a relatively constant rate of 18–19.5 cm²/kg NPK up to the dose of 16% and a higher rate of 26.75 cm²/kg NPK between the last levels. Under irrigation conditions of 20 mm, the leaf surface exhibited a variation corresponding to an average rate of 32.52 cm²/kg NPK, associated with a coefficient of determination of 97.67%, with the initial value of that characteristic being 4310 cm² in the non-irrigated agricultural context. In the context of irrigation at 30 mm, the average growth rate of the leaf structure was 35.92 cm²/kg NPK applied, with different values from one dose to another (21.25–52.87 cm²/kg NPK) under conditions that allowed for an estimation of the accuracy of that character at 96% (Figure 6).

Compared to the non-fertilized agricultural background, the treatments with 8–16% resulted in a positive impact, reflected by minor increases of 4.74–8.41% in that character. Conversely, the treatment with 24% NPK had a significant effect of 13.75%. Notably, the change in the NPK dosage from 8 to 24% was associated with a significant effect of an 8.6% increase in the leaf area.

In the context of irrigation set at a standard of 10 mm, the fertilization methods employed resulted in values ranging from 4216 cm² for the unfertilized agricultural setting to 4753 cm² for the application of 24% NPK, with an amplitude of variation of 537 cm² and a variability of 6.52% between treatments. Under such soil moisture conditions, only the treatment with 24% NPK generated a significant increase of 12.74% compared to the unfertilized version. Furthermore, the application of 24% NPK demonstrated a markedly greater impact on that characteristic when compared to the 8% NPK dosage. Under the effect of irrigation with 20 mm, fertilization with 16–24% NPK showed a significant influence on the increase in the leaf surface, associated with increases of 589–781 cm². Only the addition of fertilization from 8 to 24% generated a significant variation of 9.55% in that character. An examination of the impact of fertilization on the leaf surface area of seedlings at varying watering rates reveals that in the absence of irrigation, the seedlings recorded values ranging from 4006 cm² in the case of the unfertilized variant up to 4557 cm² in the 24% NPK variant, with a variability between treatments of 6.46%.

In the context of applying a watering norm of 30 mm, the seedlings exhibited leaf surface area values ranging from 4415 cm² in the unfertilized variant to 5277 cm² in the variant supplemented with 24% NPK, with a variability of 8.06% observed between the treatments. Compared to the non-fertilized agricultural background, the NPK treatments had a significantly higher efficiency, which materialized as increases of 9.58–19.52%. Changing the dose of NPK from 8 to 24% was associated with a significant increase in the development of the leaf apparatus of 9.07%.

3.4. Effects of the Cultivar and Fertilization on the Leaf Area

Upon examining the interaction between the cultivar and fertilization on leaf surface area characteristics (

Table 7. The effects of the cultivar and fertilization on the leaf area.

Cultivar	Fertilization
	Control	N8P8K8	N16P16K16	N24P24K24
Stanley	4293 ± 63 a z	4500 ± 82 a yz	4657 ± 88 a xy	4837 ± 79 a x
Cacanska Lepotica	4160 ± 65 a z	4503 ± 85 a y	4707 ± 82 a xy	4980 ± 94 a x
Fertilization mean	4226 ± 69 W	4502 ± 71 Z	4682 ± 76 Y	4908 ± 85 X

Fertilization LSD_5% = 172 cm²; cultivar–fertilization LSD_5% = 15 cm²; fertilization–cultivar LSD_5% = 7 cm²; the same letter (a) in the column indicates non-significant differences (p > 0.05) between cultivars; different superscript letters (x, y, z) in the row indicate significant differences (p < 0.05) between NPK levels. Capital letters were used for fertilization mean (X, Y, Z, W) comparisons.

), it was observed that in the Stanley cultivar, only the treatments utilizing 16–24% NPK produced noteworthy positive effects, amidst relatively minor differences across the treatments. Conversely, for the seedlings of the Cacanska Lepotica cultivar, fertilization had a significant impact on this type.

Concerning the impact of fertilization on the leaf surface area of each cultivar (

Table 8. The combined impacts of irrigation and fertilization on the leaf area of the two cultivars.

Specification	0 mm
	Control
	Fertilization
Cultivar	Control	N8P8K8	N16P16K16	N24P24K24
Stanley	4124 ± 80 a x	4261 ± 73 a x	4363 ± 150 a x	4518 ± 100 a x
Cacanska Lepotica	3888 ± 68 a y	4131 ± 41 a xy	4323 a ± 55 x	4595 a ± 63 x
	10 mm
	Fertilization
Cultivar	Control	N8P8K8	N16P16K16	N24P24K24
Stanley	4308 ± 167 a y	4406 ± 94 a xy	4483 ± 106 a xy	4794 ± 123 a x
Cacanska Lepotica	4125 ± 74 a y	4322 ± 94 a xy	4558 ± 32 a xy	4713 ± 38 a x
	20 mm
	Fertilization
Cultivar	Control	N8P8K8	N16P16K16	N24P24K24
Stanley	4322 ± 72 a y	4519 ± 87 a xy	4873 ± 68 a xy	4891 ± 70 a x
Cacanska Lepotica	4212 ± 91 a y	4697 ± 47 a xy	4839 ± 59 a x	5204 ± 77 a x
	30 mm
	Fertilization
Cultivar	Control	N8P8K8	N16P16K16	N24P24K24
Stanley	4417 ± 44 a z	4814 ± 197 a yz	4910 ± 143 a xy	5143 ± 98 a x
Cacanska Lepotica	4414 ± 37 a z	4862 ± 66 a yz	5107 ± 91 a xy	5410 ± 70 a x

Cultivar LSD_5% = 459 cm²; fertilization LSD_5% = 487 cm²; the same letter (a) in the column indicates non-significant differences (p > 0.05) between cultivars; different superscript letters (x, y, z) in the row indicate significant differences (p < 0.05) between NPK levels.

), it could be noticed that for the Stanley cultivar, the values ranged between 4293 cm² for the unfertilized variant and 4837 cm² in the case of applying the dose of 24% NPK. Compared to the unfertilized version, only the treatments with 16–24 kg had significant effects and were associated with increases of 364–544 kg. It was also found that only the addition of fertilization from 8 to 24% NPK generated a significant increase in that character of 7.5%.

As regards the leaf surface area of the seedlings from the Cacanska Lepotica cultivar, the variation in the effects of fertilization was more pronounced, being characterized by an amplitude of 820 cm² ranging from 4160 cm² on untreated soil to 4980 cm² at a 24% NPK application rate. The three treatments resulted in an improvement, with an increase of 8.25% to 19.71% in the leaf apparatus. The addition of fertilization by increasing the NPK levels from 8 to 16 and 24% determined a significant increase in this character of 273–477 cm².

Figure 5 provides data based on the exponential regression suggesting that for the Cacanska Lepotica cultivar, the leaf surface area varied by an average of 34.17 cm² for each kg of NPK fertilizer applied, with fluctuations between 25.5 cm/kg NPK for levels ranging from 8 to 16% and reaching 42.87 cm²/kg NPK at the 8% level. These estimates were made with a high degree of accuracy, specifically 98.61%, based on a leaf surface area of 4194 cm² when no fertilization was used.

In the case of the Stanley cultivar, the impact of fertilization on the growth of the leaf structure was reduced, showing an average growth rate equivalent to 22.67 cm²/kg NPK against the background of variations from 19.62 cm²/kg NPK between treatments with 8–16% NPK and 25.97 cm²/kg NPK between the levels of 8% and the control.

The predictability of the logarithmic regression between the fertilization dose and the variation of that character for the Stanley cultivar was 99.55%, based on an initial value of 4306 cm² on the unfertilized agricultural background (Figure 7).

3.5. Combined Effects of Irrigation, Fertilization and the Cultivar on the Leaf Area

Considering the combined effects of the three factors (Table 8), fertilization did not significantly influence the leaf surface area of the Stanley cultivar on the non-irrigated agricultural background under the circumstances of a 394 cm² amplitude. For the Cacanska Lepotica cultivar, an enhancement of 435–707 cm² in the leaf surface area was observed solely with fertilization using 16–24% NPK.

On the arable land irrigated with 10 mm, only the fertilization with 24% NPK determined a significant increase in the leaf surface area of both cultivars of 486–688 cm² under the conditions of small and insignificant variations between the three doses applied.

When irrigated with 20 mm, an increase in the leaf area was recorded, with the Stanley cultivar seedlings showing an increase of 569 cm² and the Cacanska Lepotica cultivar seedlings exhibiting an increase of 992 cm², both fertilized with 24% NPK, compared to the untreated variant; however, alterations in the NPK dosage resulted in negligible variations in this character.

The application of irrigation with 30 mm revealed that fertilization had a more pronounced effect on the leaf surface area in both cultivars that was associated with differences of 725–996 cm² compared to the non-fertilized variant and insignificant variations between treatments. Within agricultural contexts that represented combinations between fertilization and irrigation, there were no significant distinctions between the two cultivars for that character.

4. Discussion

Utilizing irrigation and fertilization in nurseries results in the production of robust and healthy fruit planting material, enhancing overall yield and providing superior economic returns compared to nurseries lacking irrigation and fertilization. Although nurseries are typically situated on fertile land, the application of fertilizers is essential to create optimal conditions for the growth of rootstocks and trees, as well as to support the activity of soil microorganisms. While climatic conditions generally ensure the normal growth of trees without irrigation, in excessively dry years, irrigation becomes a necessity. Prolonged periods of drought hinder the growth of plants within nurseries. Additionally, it is crucial for trees in a nursery to achieve rapid growth within a limited timeframe, particularly when the root systems have been significantly reduced through shaping, necessitating swift restoration to enable them to access a larger volume of soil. Therefore, it is essential to apply fertilizers and take measures to enhance soil fertility in order to achieve high yields of fruit trees in nurseries. Nursery fertilization should encompass a series of actions that contribute to the long-term enhancement of physical and chemical properties while ensuring the availability of necessary nutrients in forms that can be assimilated during each phase of vegetation, according to the requirements of the trees [19]. This research highlights that the climate and soil conditions are favourable to tree cultivation and do not pose any limiting factors.

Additionally, irrigation must maintain optimal water levels, ensuring a supply of 65–75% of the total soil retention capacity, avoiding excessive moisture levels beyond 80%.

In terms of fertilizer application within the nursery, it is essential to note that nitrogen (N), phosphorus (P), and potassium (K) are the key nutrients that should be utilized to ensure optimal plant growth in the field, as shown by Rosen and Swanson [20]. Fertilization influences crop development, nutrient uptake, and overall productivity [21]. Regarding the application of fertilizers, the current understanding indicates that factors such as soil conditions, species, cultivar, rootstock, density, forecasted production, etc., must be considered. Establishing the optimal doses for each situation in the field must be performed after analysing a series of soil properties, knowing the requirements imposed by the crop and those related to ensuring a certain quantitative and qualitative level of production. Although at first glance they seem to be expensive elements of technology, they are actually only rational methods of using water and fertilizers to obtain planting material. The improper use of fertilizers in nurseries, particularly when applied in excessive, unilateral, and unbalanced quantities, as well as the use of fertilizer types that do not align with the biological needs of specific species and soils, can lead to detrimental outcomes. Instead of achieving the anticipated increases in tree production, a decline may be observed. Furthermore, the economic viability and yield in irrigated agriculture are largely influenced by the irrigation practices employed for each crop. A significant consequence of the overuse of chemical fertilizers is the leaching of nutrients from the soil due to irrigation or rainfall, which can infiltrate groundwater and exacerbate the eutrophication of watercourses.

The investigations carried out so far on the use of irrigation and fertilization in the nursery refer to fruit shrubs or woody species, not to fruit tree planting materials. Consequently, it is not possible to compare the various results obtained. But as a general idea, other recent research shows that both irrigation and fertilization have been proven to improve plant growth, accelerate photosynthesis and growth metabolism activities, and eventually achieve higher crop yield and quality [22,23,24]. Besides obtaining a high yield, another objective of nursery owners is to obtain a profit. According to Fulcher and Fernandez [25], irrigation can shorten the production period for field nursery crops and increase quality, which has a positive impact on nursery profitability. Drip irrigation and fertilization involves some additional costs in terms of purchasing and installing equipment, but it is compensated by a high and good quality of fruit tree production.

In the same study, the effects of irrigation, fertilization, and the cultivar on different morphological traits of plum, including shoot growth, shoot weight, crown diameter, branching growth, and tree yield, were also determined. Concerning the physiological traits of plum, the leaf area and photosynthesis rate were examined under the influences of irrigation and fertilization.

5. Conclusions

The implementation of various NPK fertilization methods resulted in statistically assured enhancements in leaf surface growth, ranging from 6.53% to 16.14% when compared to the control group that did not receive fertilization. Especially when added in amounts between 8 to 16% and 16 to 24%, fertilizers were effectively absorbed by the grafted trees, leading to substantial growth improvements of 180–226 cm². In the Stanley cultivar, only the treatments with 16–24% NPK showed significantly positive influences on the leaf surface area against the background of small variations between the treatments. In the case of the Cacanska Lepotica cultivar, fertilization had a substantially higher influence on the leaf area.

Irrigation led to increases in the leaf area ranging from 4.42% to 14.27% when compared to the control group. For the Cacanska Lepotica trees, the applied watering norms resulted in significant differences in the leaf area relative to the control, highlighting the substantial variations among the watering norms. Conversely, the Stanley cultivar trees effectively utilized only the 20 mm and 30 mm watering norms, which resulted in significant enhancements of this characteristic compared to the control. As future research directions, it is appropriate to analyse the economic efficiency of producing grafted trees under different conditions of irrigation and fertilization.

It is essential for nursery owners and horticulture enthusiasts to consider various factors, such as soil conditions, species, cultivar, rootstock, density, and anticipated production, when implementing irrigation and fertilization practices. Additionally, understanding the rainfall patterns, the low water permeability of the soil, and groundwater levels can be helpful for establishing appropriate irrigation standards. The optimal level of NPK in the field must be set after analysing a series of soil properties, knowing the requirements imposed by the crop and those related to ensuring a certain quantitative and qualitative level of production. It is also recommended to carry out chemical laboratory analyses on the qualitative and quantitative presence of various nutrients in the soil.