The years of the experiment had different hydrothermal regimes; therefore, experimental data for individual years are presented.
3.1. Variation of SPAD and Fv/Fm of Pea Varieties
According to a three-way ANOVA, chlorophyll index (SPAD) was significantly influenced by pea growth stage (GS) (factor A) (
p ≤ 0.01), variety (factor B) (
p ≤ 0.01), and their interaction (A × B) (
p ≤ 0.05,
p ≤ 0.01) during all four years of the experiment (
Table 1). The influence of fertilization (factor C) on SPAD was significant only in 2014 and 2015 (
p ≤ 0.05,
p ≤ 0.01). GS and variety were the main factors determined at 2.9–60.2% and 8.8–63.9% of the total variability of SPAD in different experimental years. Fertilization explained the least part of SPAD (1.4–2.9%).
In all years of the experiment, at later GS, SPAD was significantly higher than at the first assessment at BBCH 19, by 3.8–23.1% and 5.4–24.2% at BBCH 65 and BBCH 69, respectively. At an even later stage of development, when peas form pods and grow seeds in them, SPAD decreases, as assimilates are transferred from the leaves to the seeds. This is demonstrated by the study of Szpunar-Krok [
26] with foliar fertilizers and biostimulants applied to peas, in which SPAD at the end of pod development (BBCH 79) was significantly lower (by 21.6–28.5%) than at BBCH 65. The significant influence of the pea development stage on SPAD was also found in tillage experiments [
28], where the highest SPAD values were recorded at the full flowering stage (65 BBCH), but with the onset of pea leaf senescence at BBCH 75, the SPAD index decreased significantly. In a study involving three pea varieties, comparing NPK foliar application vs. NPK soil application, it was found that both fertilizer application methods significantly increased the total chlorophyll content [
30]. The data ibid showed that the varieties responded differently to fertilization; the content of green pigment chlorophyll was significantly higher in variety Climax at vegetative GS with soil application, while Aleena had higher chlorophyll content with the foliar spray at maturity GS.
The variety Respect demonstrated the highest SPAD values and, compared to the trial mean, significantly surpassed it by 5.0–14.1%. The SPAD of variety Simona was 0.3–6.2% lower, and the variety Ieva DS was 4.5–8.3% lower than the trial mean. Our findings are consistent with those of Szpunar-Krok [
26], when in a study involving eight pea varieties, the reduction in SPAD along the vegetation season was observed in each fertilization treatment and all tested varieties. The author also indicated significant differences in SPAD between the tested pea varieties. A study involving six green pea varieties with different leaf types and maturity showed similar trends; here, the SPAD values differed because different varieties reacted differently to the moisture regime [
34].
In the current experiment, significant differences in SPAD values were obtained only in the first two years of the experiment. The influence of fertilization was negative, compared to unfertilized pea (control, NPK 0:0:0). In 2014, SPAD decreased significantly in all fertilization treatments, and in 2015, which was characterized as dry, SPAD was significantly reduced only by the highest fertilization rates—NPK 60:40:80 and NPK 60:80:160, compared to control.
Knowledge of the physiological functions of plants and understanding the factors influencing them are among the most important issues in plant breeding, agronomy, and ecology today. Such research is inseparable from the direction of increasing yields [
35,
36]. The efficiency of solar energy conversion is particularly low, with less than one percent of the energy used in crops being utilized. C3 plants convert 2.4%, while C4 plants convert 3.7% of solar energy conversion [
37,
38]. Photosynthesis remains an unexploited area for crop productivity improvement. New tools and technologies offer the opportunity to improve photosynthesis, while increasing crop productivity [
36].
One way to improve the efficiency of photosynthesis is to optimally supply plants with nutrients. Nutrient availability is an influential factor in the photosynthesis process [
39]. Nitrogen fertilization increases leaf area, and nitrogen is also considered a key component of chlorophyll, increasing photosynthetic capacity and crop growth and yield [
40]. The effect of nutrient supply on plant physiological processes has been well documented in many studies [
24,
27,
29,
39,
41]. The photosynthetic duration is closely concerned with leaf senescence; it is very important to maintain the green and functional leaf area for as long as possible [
42,
43].
Our data in 2014 and 2015, when SPAD values decreased under the influence of fertilizer, are contrary to the study of Ejaz et al. [
44], where it was found that different NP fertilization regimes significantly increased chlorophyll in pea leaves compared to the control. The foliar application of biostimulants also had a beneficial effect on the amount of green pigment, significantly increasing it at BBCH 65 and BBCH 79 GS [
26]. Nutrients are mobilized from the older leaves to the younger leaves and finally to the flag leaf, which contributes most of the nutrients and photosynthesis assimilates used for the grain filling [
42].
A three-way ANOVA showed that maximum quantum efficiency (Fv/Fm) was significantly (
p ≤ 0.01) influenced by GS (factor A), variety (factor B), and their interaction (A × B) (
Table 2). GS was the main factor responsible for 4.3–34.0%, variety—for 3.6–11.6%, and their interaction—for 8.1–20.6% of Fv/Fm data variation.
Compared to BBCH 19, Fv/Fm values in most cases were higher at BBCH 65 and BBCH 69, respectively, by 1.9–9.9% and 3.7–18.7%. There was one exception in 2015 when the Fv/Fm values in subsequent GS were close to the values established by BBCH 19. Fv/Fm allows you to understand the physiological state of a plant and assess its stress level. In studies with cereals, it was found that values of Fv/Fm consistently decreased after the heading stage, which denoted physiological leaf senescence and transport of assimilates from leaves to reproductive organs [
42,
45].
The effect of variety differed in separate years of the experiment. Compared to the trial mean, it was found that in the 2015 and 2016 years, the variety Respect was distinguished for its low Fv/Fm values (by −1.3 and −2.7%, respectively), while the variety Ieva DS significantly surpassed the trial mean (by 2.2 and 1.7%, respectively), but in the 2014 year, Fv/Fm values of the variety Ieva DS were significantly lower (−2.8%) than the trial mean. The influence of fertilization on Fv/Fm was weak, with one exception when it was established that applying NPK 60:40:80 significantly decreased Fv/Fm by 1.5–3.8%.
Chlorophyll fluorescence is indicated as an applicable method for assessing the effects of biotic and abiotic stress and the efficiency of the photosynthetic apparatus [
45,
46]. Stress can be caused by many factors, such as poor nutritional conditions, drought, and high temperature [
25,
40,
47]. According to the study of Lima-Moro et al. [
24], the application of micronutrients in soybeans had a negative effect on Fv/Fm. The response of pea foliage to foliar nutrition is reported by Škarpa et al. [
14], in which the foliar application of phosphorus had the opposite effect on pea fluorescence indices, increasing them both 14 and 28 days after the foliar application.
3.2. Variation of Leaf Gas Exchange Parameters
Photosynthetic rate (A) was significantly influenced by GS (
p ≤ 0.01), variety (
p ≤ 0.05,
p ≤ 0.01), fertilization (
p ≤ 0.05,
p ≤ 0.01), and A × B interaction (
p ≤ 0.01) (
Table 3). GS (factor A) explained 10.6–27.7%, variety (factor B)—2.5–12.6%, fertilization (factor C)—6.3–9.5%, and A × B interaction governed 5.8–28.4% of the A variations.
Compared to BBCH 19, A was in most cases significantly higher by 8.9–85.3% in the middle of the pea flowering (BBCH 65). The A values decreased at the end of flowering (BBCH 69), in comparison to the middle of flowering (BBCH 65). In the dry year 2015, during the last evaluation at BBCH 69, the A values were 34.3% lower than at BBCH 19. Meanwhile, in the years of optimal moisture, such as 2014, 2016, and 2017, the A values were higher by 30.7%, 1.7%, and 35.7% than at BBCH19, respectively.
The intensity of photosynthesis in the varieties was different in individual years. Ieva DS distinguished with the highest A in 2014 and 2017, when, compared to the trial mean, the indicator values were 17.4% and 14.3% higher, respectively. In 2015 and 2016, the highest intensity of photosynthesis was determined in the Simona variety, which exceeded the trial mean by 30.7% and 14.1%, respectively. The variety Respect showed the lowest A in all the years of the experiment, and the difference with the trial mean was in the range of 0.9–13.6%.
Fertilizers in most cases had a positive effect on the intensity of photosynthesis, but the effect of fertilizers was not the same in different years and depended on the moisture regime. The lowest values of the A were determined in 2014 and dry 2015 when the application of NPK 0:40:80 and NPK 30:40:80 did not affect the A values. According to average data, compared to unfertilized pea, the application of NPK increased the A values by 13.4, 15.5, 39.4 (p ≤ 0.05), and 43.2% (p ≤ 0.05), respectively, in the treatments NPK 0:40:80, NPK 30:40:80, NPK 60:40:80, and NPK 60:80:160. Compared to NPK 0:40:80, applying balanced fertilization with N, nitrogen increased the A on average by 3.3%, 22.1% (p ≤ 0.05), and 29.8% (p ≤ 0.05) in NPK 30:40:80, NPK 60:40:80, and NPK 60:80:160 treatments, respectively. The largest and most significant differences in A values were found in the most abundantly fertilized variant NPK– 60:80:160.
The intensity of net photosynthesis was dependent on the developmental stage of the pea plant [
39,
40]. Our studies are consistent with this statement, as we found that the highest values of physiological indicators were at BBCH65, and then they decreased. The relationship between nitrogen and photosynthetic processes has been widely described for various plant species [
1,
24,
46]. Nitrogen is necessary for plants due to its indispensable role in various metabolic and regulatory processes in plant cells. Adams et al. [
48] established that nitrogen always had only a positive influence on both A and gs values. Similar data were obtained by Ju et al. [
49], who studied the application of five nitrogen fertilizer rates in oats and found that the N90 rate mainly contributed to the improved leaf photosynthesis traits A, E, gs, SPAD, and Ci. There is evidence to the contrary, that higher N application is not always an indicator of higher photosynthetic activeness [
50]. In the present study, the influence of fertilization on the A values was confirmed, which in most cases (except 2016) significantly increased when applying the highest NPK rates.
The results of three-way ANOVA revealed that all three factors and A × B interaction had a significant (
p ≤ 0.05,
p ≤ 0.01) influence on the transpiration rate (E). GS (factor A) was the main factor determining from 3.7 to 43.7%, and variety (factor B) explained from 2.4 to 32.5% of the total variability of E (
Table 4). Fertilization (factor C) was responsible only for 1.8–4.8% of the E data variation.
In comparison to BBCH 19, at BBCH 65, the E values were significantly lower by 11.1–33.3%, with one exception in dry 2015, when the E was higher by 82.6%. The highest E was obtained at BBCH 69, and compared to BBCH 19, the E values were higher by 16.7–103.9%, and the differences were in most cases significant.
The lowest transpiration was observed in the Respect variety, in comparison to the trial mean, the E values were by 9.0–40.0% lower. In 2015, not only Respect but also the Ieva SD variety reacted sensitively to the dry conditions of the period, when the E was determined to be 22.9% lower than the trial mean. The Simona variety exceeded the trial mean by 6.4–60.0%, but the differences were uneven in individual research years.
In 2015 and 2016, fertilizers reduced the E values by 12.2–26.8% and 16.9–35.1%, respectively. Meanwhile, in 2017, applying fertilizers, especially NPK, regardless of the fertilizer rate, increased the E by 24.2–34.7%.
A three-way ANOVA showed that GS (factor A) had a significant (
p ≤ 0.01) effect on water use efficiency (WUE) and was responsible for 9.2–20.8% of the differences between treatments (
Table 5). The effect of fertilization (factor C) was significant (
p ≤ 0.01) only in 2014, and the variety did not influence WUE values.
Under normal humidity conditions, WUE at BBCH 65 was the highest and was 76.4–159.3% higher than that measured at BBCH 19. In the dry year of 2015, WUE, on the contrary, was the highest measured at BBCH 19 during the first assessment and decreased consistently and significantly in subsequent GSs.
According to the average data, Simona was characterized by low and Ieva DS by higher WUE values.
The influence of fertilizers on WUE values was not the same during the research years. According to the average data, fertilizers increased WUE. According to Shangguan et al. [
51], the WUE of the plants under high N application was decreased by a larger value than that under low N application due to a larger decrease in photosynthetic rate than in the transpiration rate. In contrast, in studies with triticale, it was found that nitrogen application did not affect WUE [
52]. In the present study, the lowest WUE was found at the last assessment at BBCH 65 GS. This decrease was influenced by the decrease in A and E.
According to three-way ANOVA data, the effect of GS (factor A) was significant (
p < 0.01), and it was found to be the main factor governing most of the variation in the data of stomatal conductance (gs) (14.6–67.9%) (
Table 6). The influence of variety (factor B) on gs was significant (
p ≤ 0.05,
p ≤ 0.01) and determined 1.5–13.6% of the total variability of gs. The influence of fertilization (factor C) on gs was significant only in 2015 and 2017 (
p ≤ 0.05,
p ≤ 0.01) and determined only 1.6–2.8% of gs differences.
The gs values at BBCH 19 were low—from 0.04 to 0.09. At BBCH 65, gs was found to be 28.6% to 3.7 times higher than at BBCH 19. At the end of pea flowering (BBCH 69), compared to BBCH 19, gs values decreased by −28.6–66.7% during optimal moisture seasons; whereas, gs increased significantly in the dry year 2015.
The variety Ieva DS exhibited significantly higher gs (+3.8–25.0%), the variety Respect exhibited significantly lower gs (−3.8–28.6%) in comparison to the trial mean, and gs values of the variety Simona were close to the trial mean.
Among fertilization treatments, significant gs differences were determined only in 2015, when applying NPK 30:40: and NPK 60:80:160 gs increased by 28.6 and 47.6%, respectively.
There are conflicting studies on the influence of N on gs; under high N application, gs increased, in comparison to low N treatment [
51], and conversely, N fertilization did not influence gs changes [
52]. We can state that meteorological conditions play a significant role in the influence of nitrogen on gs values.
The intercellular CO
2 concentration (Ci) was significantly influenced by GS (factor A) (
p ≤ 0.01), variety (factor B) (
p ≤ 0.01), and fertilization (factor C) (
p ≤ 0.05,
p ≤ 0.01) (
Table 7). GS explained the largest part (10.3–41.3%) of the total variability of Ci. Variety was responsible for 4.1–13.8% of the differences between the treatments and applying fertilizers—only for 4.5–6.4 of the differences.
In most cases, compared to the values at BBCH 19, Ci decreased with an increase in the pea development stage. The exception was the dry year of 2015, when Ci values at BBCH 65 and BBCH 69 increased by 4.3% and 28.7%, respectively, compared to BBCH 19.
The influence of varieties on Ci was different during the experimental years. Under dry weather conditions in 2015, the variety Respect exceeded the trial mean by 6.0%, while the varieties Ieva DS and Simona had lower Ci values than the trial mean. Under optimal moisture conditions in 2016 and 2017, Ci values of the variety Respect were 10.1 and 8.3% lower than the trial mean, respectively. According to the average Ci data, the varieties were ranked in descending order: Ieva DS > Simona > Respect.
Fertilization had an inconsequential influence on the Ci in most cases, with one exception in 2015, when applying NPK 30:40:80 led to a significant Ci reduction. Similar data were obtained in a study with three levels of N fertilization, and it was found that due to the increased total chlorophyll content, A, E, and gs values were enhanced, under applying low (N1) and medium (N2) rates. However, Ci values, on the contrary, decreased with increasing nitrogen fertilization rates [
53]. Studies with elevated carbon dioxide levels have also shown that the fertilization N100 significantly reduced the Ci in wheat, faba bean, and the intercropping of both species [
54]. Ju et al. [
49] states that N90, one of the N rates studied, had the most effective positive effect on Ci.
GS (factor A) significantly (
p ≤ 0.01) influenced apparent carboxylation efficiency (ACE), governing 10.8–27.8% of the total variability of ACE (
Table 8). The effect of variety and fertilization was significant (
p ≤ 0.05,
p ≤ 0.01); however, it resulted in a smaller part of the data variability. These two factors were responsible for 2.6–9.7% and 5.7–7.7% of ACE data variation, respectively.
Under optimal humidity conditions (2014, 2016, 2017), compared to BBCH 19, ACE noticeably increased in the later development stages, as follows: at BBCH 65—by 67.5–92.7%, at BBCH 69—by 25.0–245.0%. During the dry season in 2015, ACE decreased by 8.7 and 55.4% at BBCH 65 and BBCH 69, respectively, in comparison to BBCH 19.
The differences between varieties were small and the character of the effect varied in different experimental years. The ACE of variety Simona was significantly higher than the trial mean in 2015 and 2016 (by 29.2 and 20.1%), but in 2017, the ACE was 21.0% lower than the trial mean. On average data, ACE was similar among the studied varieties. On the contrary, Nerva et al. [
55] proposed that the effects of tested products on ACE varied between varieties.
Fertilization increased ACE in most cases, but the differences were not always significant. The highest NPK rates had the greatest impact on ACE, with NPK 60:40:80 and NPK 60:80:160, increasing ACE by 19.3–104.0% and 21.1–80.0%, respectively, compared to unfertilized peas.
3.3. Correlation Analysis Between Physiological Characteristics, Seed Yield, and Meteorological Indices
We assessed the correlation between physiological characteristics, seed yield, and meteorological indices at different growth stages (
Table 9). Data averaged across variety showed that seed yield significantly (
p ≤ 0.05,
p ≤ 0.01) and positively correlated with A at BBCH 19 and BBCH 69; with E and gs at BBCH 19; with ACE at BBCH 69. The negative relationship was assessed between seed yield and gs and Ci at BBCH 69. The negative correlation seed yield with gs can be explained by the disruption of photosynthetic processes and is primarily due to impaired chlorophyll function and reduced CO
2 availability from stomatal closure during the later stages [
56]. At BBCH 65 and BBCH 69, the relationship between seed yield and Fv/Fm was negative and significant. No correlation was found between SPAD and seed yield.
The correlation between A and precipitation was significant and positive at BBCH 65 and BBCH 69. The accumulated growing degree days (AGDD > 5 °C and AGDD > 10 °C) in most cases negatively correlated (p ≤ 0.05, p ≤ 0.01) with E, Ci, and SPAD, regardless of GS. At BBCH 19 and BBCH 65, the relationship between AGDD and A, WUE, and ACE was significant (p ≤ 0.05, p ≤ 0.01) and positive in most cases; however, at the end of the pea flowering (BBCH 69), the direction of the relationship changed and became negative. Sunshine duration negatively correlated with E, gs, and Ci at BBCH 19 and with A and ACE at BBCH 69.
Meteorological conditions are one of the important problems related to pea production, the variability of physiological characteristics, and productivity characteristics. In experiments for a dryland and an irrigated location, it was found that the length of reproductive growth and pea seed yield increased with seasonal precipitation, and pea was sensitive to heat. Strong relationships were observed between pea development and daily temperature during reproductive growth [
57]. In an experiment lasting 9 years with the 15 most promising varieties, a close and significant relationship was established between the duration of pea development stages and temperature conditions from germination to BBCH 65 and from BBCH 65 to BBCH 75 (r = −0.472 and r = −0.788, respectively) [
58]. In our study, at prior GS (BBCH 19 and BBCH 65), the relationship between AGDD 5 and AGDD 10 with physiological parameters was significant in most cases. However, at BBCH 69, the relationship with gas exchange parameters was found to be weaker and negative in most cases. We found a positive significant relationship between precipitation and A, E, and Ci at BBCH 19 and BBCH 65 GS, but a negative relationship between gs, Ci, and WUE at BBCH 69. Adverse weather conditions negatively affect photosynthetic processes in plants [
47,
59,
60]. Our findings confirm the previous result [
58], where a significant relationship was also found between the duration of the vegetation period, the efficiency of physiological processes, and the amount of precipitation (r = 0.937) and the HTC (r = 0.927).
Temperature stress is one of the most common abiotic stresses in nature. It strongly affects plant development and metabolism, causing changes in photochemistry and the breakdown of thylakoid membranes [
60]. High temperatures are known to cause leaf wilting, accompanied by severe damage to photosynthetic pigments and a decrease in Chl content [
61]. In a study with six green pea varieties with different maturity and leaf types, under three irrigation conditions (irrigated, water deficit, non-irrigated), it was found that during the vegetation period, when drought conditions prevailed, reduced SPAD values can be considered a drought stress marker [
34].
The relationship between seed yield and A was described by linear equations (
Table 10). The correlation was significant at BBCH 69 for all varieties. Regression equations show that when increasing the A values, seed yield also increased. This estimation showed that, at BBCH 69, about 20%, 22%, and 22% of the yield variations could be explained by variations in A values, respectively, for the Ieva DS, Simona, and Respect varieties.
The multiple linear regression model (y = a + bx1 + cx2 + dx3 + ex4 + fx5) revealed that weather conditions influenced the seed yield (SY) and physiological indices of peas at all evaluated growth stages (
Table 11). Averaged across varieties, it was established that the interaction of meteorological factors influenced SY by 45%, 38%, and 28% at BBCH 19, BBCH 65, and BBCH 69, respectively. The relationship between tested indices was moderate to strong in most cases. At BBCH 19, the strongest correlation of meteorological factors was with E, Ci, and SPAD (respectively, R = 0.647, R = 0.752, R = 0.696) at BBCH 65 with Fv/FM, gs, A, and WUE (respectively, R = 0.900, R = 0.732, R = 0.681, R = 0.667) and at BBCH 69 with gs, Fv/FM, E, and A (respectively, R = 0.922, R = 0.800, R = 0.753, R = 0.651) (
p ≤ 0.01).