Effect of Planting Density and Row Spacing on the Yielding and Morphological Features of Pea (Pisum sativum L.)

Abstract

The aim of the study was to evaluate the effect of planting density and row spacing in pea on the plant growth, yield, structural seed yield components, and some plant indices in the central-and-northern part of Poland. Field experiments were performed in 2016–2019. The study involved a Polish pea cultivar, Batuta (semi-leafless). Three planting densities were applied (70, 90, and 110 seeds per 1 m²), with two row spacings (16 and 32 cm), in four replications on 20 m² plots There was considerable pea sensitivity to precipitation deficit; in higher precipitation years, the seed yield was almost half higher than in the dry years. There was no recorded effect of planting density or the row spacing (except in 2019) on pea yield, and their variation in successive research years was nonsignificant. Similar conclusions also applied to the protein yield; however, its significantly highest value was recorded in the hottest vegetation period with the lowest total precipitation. The deteriorating precipitation conditions across the years were accompanied by a lower number of pods per plant, the number of seeds and their weight per pod, and 1000 seed weight. Moreover, those results were not significantly correlated with the factors under study except for the planting density in HI.

1. Introduction

Legumes should be used as a source of N, as unrenewable sources of energy are being exhausted [1]. They play a key role as a source of protein; however, the yields undergo greater fluctuations than in other crops, the reasons for which are not yet sufficiently known, and this can have a negative impact on the way they are perceived [2]. Pea is grown in Europe for both human consumption and animal feed [3]. It is one of the most important legume species [4,5,6], with a high sensitivity to water stress, especially when the plants increase the share of dry weight in the vegetative organs, including the roots, decrease the allocation of dry weight to pods and seeds, and increase flower shedding [1,7], which limits the number of pods, the number of seeds per pod, and 1000 seed weight considerably [8]. According to Martin et al. [9] and Grabowska and Banaszkiewicz [10], water stress is the key cause of a decrease in pea yield in a temperate climate.

Under Mediterranean conditions [8], the planting density of 40 pea plants per 1 m² is most appropriate, and a further increase in the sowing density does not lead to a significant increase in the seed yield. In South-East Poland, Gugała and Zarzecka [11] obtained the highest pea yield from a sowing density of 125 pea plants per 1 m² and the lowest from a density of 75 plants per 1 m². In Croatia, on the other hand, Krizmanić et al. [5] found that pea sowing at a varied density considerably modified the seed yield, plant height, and the number of pods per plant, and that it affected neither the number of seeds per pod nor the 1000 seed weight.

The planting density in pea modifies the dynamics of mass accumulation in the respective organs, and the decrease in row spacing from 35 to 25 cm in Poland resulted in an increase in the pea biomass yield [12]. The pea plants grown at a wider row spacing were higher, and their leaf area was lower. In Pakistan, Shaukat et al. [13] reported a higher number of branches, pods per plant, and seeds per pod in pea at a wider row spacing, and the row spacing did not have a significant effect on the protein content in seeds.

The aim of the present research has been to evaluate the effect of row spacing and sowing density on pea yield and structural components in Poland.

2. Materials and Methods

2.1. Experimental Design and Conditions

The research oon the effect of the row spacing (first factor) and sowing density (second factor) on Batuta pea (semi-leafless) yielding was performed in Poland in 2016–2019, 53°13′ N and 17°51′ E (north-central Poland), at 89.8 m above sea level. Batuta plants reach up to 95 cm in height, the stems are rigid at flowering (white flowers), and they maintain a good rigidity at harvest. Batuta is a general-purpose cultivar with a high protein content and with plants also resistant to fusarium wilt, powdery mildew, and anthracnose. Pods include numerous large and very large yellow seeds with a high 1000 seed weight. The experiments were located in Haplic Luvisol, the profile of which included horizons Ap, Eet, Bt, and C. The grain size composition of the soil was loamy sand in the surface horizon and sandy loam in horizon C, of very good rye complex. The soil type classified according to the WRB, as Haplic Luvisols (Cutanic), was a typical lessive soil formed with light loamy sand, deposited in a shallow layer on light loam [14]. The content of phosphorus was very high, potassium was high, and magnesium was low. The content of available forms of potassium and phosphorus was assayed with the Egner-Rhiem DL method, and magnesium with the Schachtschabel method. The content of nitrate and ammonium ions was determined with colorimetric tests with the Behelot and Griess-Ilosvay reactions. The soil pH was potentiometrically measured in 1 mol·L⁻¹ KCl. In all the research years, the soil pH was adequate for pea cultivation (

Table 1. Chemical properties of soil prior to pea sowing in 2016–2019.

Year	P	K	Mg	$\mathbf{N}{\mathbf{O}}_3^{-}-\mathbf{N}$	$\mathbf{N}{\mathbf{H}}_4^{+} -\mathbf{N}$	pH
	(mg·kg−1 of Soil in 0–60 cm)			(mg·kg−1 in Dry Matter in 0–60 cm)
2016	109.0	127	30.0	9.53	5.73	6.3
2017	92.0	149	31.0	5.90	7.96	6.3
2018	82.0	112	27.0	6.12	6.97	6.8
2019	77.0	149	27.0	8.69	2.27	6.5
Mean	90.0	134	28.7	7.56	5.73	6.5

).

In the present study, a high and well-distributed total precipitation was recorded in June and July 2016 (98.8 mm and 133.8 mm) and in 2017 (118.9 mm and 126.1 mm) (

Table 2. Weather conditions at the experiment site.

Month	2016	2017	2018	2019	1981–2010	2016	2017	2018	2019
Total precipitation (mm)						Deviations from the total precipitation of 1981–2010 (%)
April	28.7	40.8	40.4	1.5	27.0	6.3	51.1	49.6	−94.4
May	51.4	56.3	14.2	89.2	49.3	4.3	14.2	−71.2	80.9
June	98.1	54.3	26.4	17.7	52.8	85.8	2.8	−50.0	−66.5
July	133.8	118.9	86	22.4	69.8	91.7	70.3	23.2	−67.9
April–July	312.0	270.3	167.0	130.8	198.9	56.9	35.9	−16.0	−34.2
Average air temperature (°C)						Deviations from the average air temperature of 1981–2010 (°C)
April	8.1	6.8	12.2	9.3	7.9	0.2	−1.1	4.3	1.4
May	14.4	13.3	16.9	12.0	13.3	1.1	0.0	3.6	−1.3
June	17.7	16.8	18.4	21.9	17.0	0.7	−0.2	1.4	4.9
July	18.2	17.6	20.4	18.5	15.5	2.7	2.1	4.9	3
April–July	14.6	13.6	17.0	15.4	14.0	0.6	−0.4	3.0	1.4

), which facilitated a very good growth of pea plants, their development, and yield. However, in the successive research years, soil drought occurred; in 2018, there was a single rainfall of 86 mm in July and 23.7 mm in August, and in 2019, only 22.4 mm and 37.7 mm, respectively. Each year, the peas were planted on 23 March, 28 March, 6 April, and 28 March, while the following harvest occurred at 122, 128, 103, and 111 days after sowing.

2.2. Crop Management

The experiment forecrop in all the years was winter barley. Pre-sowing, 60 kg P, and 80 kg K kg·ha⁻¹, as well as 30 kg N·ha⁻¹ were applied. The plots for sowing were 21.24 m² in size, and the plots for harvest were 20.0 m², with a row spacing of 16 and 32 cm, and a sowing rate of 70, 90, and 110 of germinating seeds per m². The seeds were sown at the depth of 3–5 cm. Right before sowing, the sowing material was inoculated with Nitragina. Stomp Aqua-pendimetalina (di-nitroanilina_ at 45 g·L⁻¹ (39%) at 2.5–3.5 L·ha⁻¹ was applied after sowing (BBCH 00-01), while Corum 502.4 SL-bentazon (group of diazin) at 480 g·L⁻¹ (43.0%) and immazamoks (a compound from the group of immidazolinones) at 22.4 g·L⁻¹ (2%) were applied after plan emergence. The yield of pea seed was given at 15% of the moisture content.

2.3. Measurements and Observations Made during Pea Growth

Pea plant density after full emergence (BBCH 13) and before harvest (BBCH 89) was calculated to 1 m² in four replications at each field. At the full flowering stage (BBCH 65), the SunScan Canopy Analysis System (∆T Devices Ltd., Burwell, UK) was used to determine the LAI (Leaf Area Index), while N-tester determined the content of chlorophyll in leaves. Right before harvest, the number of pods per plant, the number of seeds per pod, the weight of seeds per pod, and the 1000 seed weight were evaluated. Moreover, the yield of the dry weight of straw from each plot was assessed to calculate the HI value (Harvest Index), and SPAD was used to measure the crop nitrogen status.

2.4. Data Analysis Methods

The data were collected from a two-factor experiment design in randomized complete blocks with four replications. For the statistical evaluation, the two-way analysis of variance (ANOVA) was used; while the significance of the results was verified with the HSD (honestly significant difference) Tukey’s test at the significance level of α = 0.01 and 0.05. For the purpose of analysis, long-term observations involved repeated ANOVA measures. The Pearson correlation coefficients were used for the dependence between precipitation and seed yield. The data were processed using STATISTICA version 13.3 software (StatSoft, Tulsa, OK, USA). The means in the tables and the charts provided with the same letters did not differ significantly.

3. Results and Discussion

3.1. Pea Seed Yield

Pea production provides several agronomic advantages in the Mediterranean region supporting more sustainable cropping systems and reduced nitrogen fertilizer use [3]. The recent studies by Reckling et al. [15] point to the legume yields being the same as other spring crops in Europe, and the yielding instability, according to the latest research, is only slightly lower than, e.g., in cereal crops and, therefore, their share in the crop structure should keep on growing. According to Karkanis et al. [3], the shorter vegetation season in Europe puts pea in an unfavorable situation in comparison to cereals.

One of the reasons for legume yield instability is varied-across-years precipitation and precipitation distribution. Krizmanić et al. [5] and Martin et al. [9] claimed that water stress was the key reason for decreased pea yield; hence, use of the appropriate variety depends on the agroclimatic conditions of the growing region [6]. For the strongly varied total precipitation across the years of the present research, the mean 4-year Batuta pea seed yield was high (3.98 t·ha⁻¹), mostly due to high and well-distributed precipitation in 2016 and 2017 (

Table 3. Effect of row spacing and planting density on the pea seed yield (t·ha⁻¹) in 2016–2019.

Factor		Year				Mean
		2016	2017	2018	2019
Row spacing (R)	16	4.84 a	5.55 a	2.56 a	3.21 a	4.04 a
	32	4.76 a	5.61 a	2.39 a	2.96 b	3.93 a
Planting density (D)	70	4.81 a	5.90 a	2.45 a	3.05 a	4.05 a
	90	4.85 a	5.52 a	2.49 a	3.10 a	3.99 a
	110	4.74 a	5.32 a	2.48	3.09 a	3.90 a
	Mean	4.80 B	5.58 A	2.47 D	3.08 C	3.98
	R	ns	ns	ns	*	ns
	D	ns	ns	ns	ns	ns
	R × D	ns	ns	ns	ns	ns

Values of a parameter followed by the same small letter in columns and big ones in rows did not differ significantly (ANOVA followed by Tukey’s HDS test, p < 0.01 and 0.05). ANOVA results: * p < 0.05; ns, not significant.

). We confirmed a significant effect of the weather pattern on pea yield; in 2016 and 2017, with high favorably-distributed precipitation, the seed yield was significantly higher (4.80 and 5.58 t·ha⁻¹, respectively), while in the unfavorable conditions, in 2018 and 2019, it was significantly lower (2.47 and 3.08 t·ha⁻¹, respectively). It is also worth noting that the higher pea yields were accompanied by a lower average air temperature during the growing season. However, in our own earlier studies, despite a significant humidity differentiation during the vegetation period in following years, especially during generative development, pea crop did not vary significantly [16]. Interestingly, only in 2019, a significantly higher yield of pea seeds grown in 16 cm than in 32 cm rows was found.

The flowering rate and beginning of the filling of pea seeds in pods according to Dumoulin et al. [17] do not differ significantly and do not depend significantly on environmental conditions, which was also observed in our own studies. What is interesting here is the effect of the mean monthly temperature and the total precipitation on pea yield. In the years with a lower mean monthly temperature in June and July, the pea seed yield was significantly higher than in the years with a higher temperature (except for 2018–2019 in June). According to Silim et al. [18], drought stress during flowering (BBCH 60–69) in June shows a stronger effect on decreasing the seed yield than in July (BBCH 71–79) at pod filling.

Importantly, in the present research the mean temperature in June and July in the research years was quite even at 18.7 °C (from 17.2 °C in 2017 to 20.2 °C in 2019), whereas the total precipitation decreased linearly from 116 mm in 2016 to only 20 mm in 2019, which decreased the pea yield considerably, similar to that reported by Martin and Jamieson [19]. Grabowska and Banaszkiewicz [10] stated that the air temperature and, more importantly, the sum of rainfall determined, on average, 84.5% of the level of pea yield. Therefore, favorable precipitation conditions affected the pea seed yield more than the other yield components [18,20]. According to Kuznetsov et al. [21] there was a correlation between the length of the growing season and the pea yield, which was also found in the presented research. The many-year total precipitation in May–June and May–July was highly but non-significantly correlated with the pea seed yield, and only in the in the last third of July was there a high significant correlation of the factors under study (

Table 4. Pearson correlation coefficients (r) for the total precipitation in months, the entire pea vegetative period, and seed yield.

Month	Decade Ten Days of Month
	I	II	III
April	−0.360 ns	0.727 ns	0.404 ns
May	0.878 ns	−0.358 ns	0.311 ns
June	0.589 ns	0.459 ns	0.603 ns
July	0.632 ns	0.105 ns	0.973 **
May–July	0.861 ns
April–July	0.853 ns

** p < 0.01; ^ns, not significant.

). In the successive parts of the vegetation period, in most cases, we observed a quite high but nonsignificant effect of the total precipitation on pea yield.

In the present research, we did not find a significant effect of the row spacing (with one exception in 2019) or the planting density on pea yield, which was also confirmed by the results reported by some authors [4,22]. Podleśny [12], said that row spacing modifies the dynamics of mass accumulation by respective organs, and a decrease in the width between rows from 35 to 25 cm only results in an increase in the pea biomass yield.

According to Spies et al. [23], the recommended pea sowing density is 59–88 plants per m², and a further increase in the planting density results only in lower plant branching. Martin et al. [9], on the other hand, reported that a higher planting density increased the pea plant weight yield, and its effect on the seed yield was inconsiderable; this was similar to the present research. Kuczkowski et al. [24] recorded a higher pea yield when increasing the field planting density from 75 to 125 seeds per m²; however, in the dry year, for all the cultivars studied, the sowing rate of 75 seeds per m² was enough. Moreover, Türk et al. [25] and Krizmanić et al. [5] observed a significant increase in the pea seed yield when increasing the planting density from 75 to 150 plants per m²; however, they did not find a permanent relationship between the pea seed yield and the planting density, similar to Salter and Williams [22].

3.2. Pea Protein Yield

The mean pea protein yield was 892 kg·ha⁻¹; only in 2019, it was significantly higher when pea was grown in narrow rows; however, the planting density did not differentiate that feature significantly (

Table 5. Effect of row spacing and planting density on the protein yield (kg·ha⁻¹) in 2016–2019.

Factor		Year				Mean
		2016	2017	2018	2019
Row spacing (R)	16	1037 a	1237 a	592 a	784 a	912 a
	32	1007 a	1197 a	561 a	722 b	872 a
Planting density (D)	70	1009 a	1259 a	564 a	734 a	892 a
	90	1033 a	1196 a	579 a	733 a	887 a
	110	1026 a	1198 a	585 a	792 a	899 a
	Mean	1022 B	1217 A	576 D	753 C	892
	R	ns	ns	ns	*	ns
	D	ns	ns	ns	ns	ns
	R × D	ns	ns	ns	ns	ns

Values of a parameter followed by the same small letter in columns and big ones in rows did not differ significantly (ANOVA followed by Tukey’s HDS test, p < 0.01 and 0.05). ANOVA results: * p < 0.05; ns, not significant.

). In 2017, the protein yield was significantly higher than in 2016, and it was significantly higher in 2019 than in 2018. With that in mind, one must note that temperature, the total precipitation, and precipitation distribution were the key factors lowering not only the pea seed yield but also the protein yield, similar to other Fabaceae (2, 15, 20, 21).

Water stress has a strong negative impact on the dry weight of seeds and N accumulation in seeds, although it does not have a special effect on the N allocation between aboveground and underground parts (1); the content of protein in pea seeds varies a lot due to the sowing date, and it is not affected by row spacing (13). The present study identified no significant effect of row spacing, planting density, or their interaction (row spacing × planting density) on the protein content in pea seeds, which in 2016–2019 was, on average, 22.2% (Figure 1). The decrease in the total precipitation (from 116 mm in 2016 to 20 mm in 2019) and increasing temperature (from 18 °C in 2016 to 20.2 °C in 2019) in June and July resulted in a significant increase in the protein content in pea seeds, which is almost opposite to the seed yield.

3.3. Pea Yield Components

Unlike the seed yield, the values of the structural pea yield components depended significantly not only on the weather pattern in the research years but also on the row spacing and on the planting density (Table 5). The mean number of pods per plant was 5.26, and in the years favorable in terms of the total precipitation (2016 and 2017), it was higher than in the less favorable years (2018 and 2019), similar to the pea seed yield. The other structural pea yield components differed only slightly less considerably due to the total precipitation. According to Górynowicz at al. [20], favorable precipitation conditions affect the seed yield and the number of pods per plant, as well as the number of seeds per pod and 1000 seed weight, less considerably. The number of pods, seeds per pod, and seed weight per pod were significantly higher when pea was sown in narrow (16 cm) rather than wide (32 cm) rows. Shaukat et al. [13], on the other hand, recorded a higher number of plants and seeds per pod when pea was grown with a wider row spacing; however, Kuczkowski et al. [24] reported the increased row spacing from 15 to 25 cm resulted in a significant increase in the seed weight per plant and 1000 seed weight, which, in the present research, was not confirmed for the seed weight per plant. Krizmanić et al. [5] claimed that a higher planting density had no significant effect on the number of seeds per pod, the number of pods, and 1000 seed weight. According to Górynowicz et al. [20], favorable precipitation conditions affected the number of seeds per pod and 1000 seed weight, less considerably; although, according to Martin and Jamieson [19], water stress, in general, does not affect the number of seeds per pod.

The planting density had a significant effect on most of the pea yield structural components, except for 1000 seed weight (

Table 6. Pea yield components depending on row spacing and plant density across the years.

Factor		No. of Pods per Plant	No. of Seeds per Pod	Seed Weight per Pod (g)	Weight of 1000 Seeds (g)
Year (Y)	2016	4.77 c	4.49 a	1.212 b	269 b
	2017	6.73 a	4.39 a	1.311 a	299 a
	2018	3.78 d	3.76 b	0.878 c	234 c
	2019	5.73 b	3.68 b	0.774 d	211 d
Row spacing (R)	16	5.59 a	4.17 a	1.074 a	255 a
	32	4.95 b	3.99 b	1.014 b	251 a
Planting density (D)	70	5.76 a	4.26 a	1.092 a	254 a
	90	5.08 b	4.08 b	1.055 a	256 a
	110	4.93 b	3.90 b	0.984 b	250 a
	Mean	5.26	4.08	1.044	253
	Y	**	**	**	**
	R	**	*	**	ns
	D	**	**	**	ns
	Y × R	ns	ns	ns	ns
	Y × D	*	ns	ns	ns
	R × D	ns	ns	ns	ns
	Y × R × D	ns	ns	ns	ns

Values of a parameter followed by the same letter did not differ significantly between the year, row spacing, and planting density (ANOVA followed by Tukey’s HDS test, p < 0.05) ANOVA results: ** p < 0.01; * p < 0.05; ns, not significant.

). With the lowest planting density (70 plants per m²), there were a significantly higher number of pods and number of seeds per pod than for the planting density of 90 plants, and the seed weight per pod was higher than for 110 plants. According to Dumoulin et al. [17] the yield of pea seeds is generally strongly correlated with the number of seeds in a pod, which was observed in the authors’ own research for a smaller row spacing and a lower stocking density. The mean 1000 pea seed weight was 253 g, and neither the row spacing applied at sowing nor the varied planting density made it significantly different. Türk et al. [25] found that increasing the planting density from 75 to 150 per m² decreased the number of seeds per pod, which was not observed in the present research; whereas, according to Krizmanić et al. [5], the planting density per m² did not have a significant impact on the number of pods or the number of seeds per pod; neither did it have a significant effect, as in the present research, on 1000 pea seed weight.

Neither were there identified any significant interactions between the seed yield components under study in the research years and the row spacing or the planting density applied. Yucel [8], on the other hand, reported the planting density of 40–50 in pea had a significant positive effect on the number of pods per plant and the number of seeds, which, in the present study, was only confirmed with the assumed lowest planting density. According to Salter and Williams [22], the seed weight per plant in pea decreased regularly with an increase in planting density, which, in the present study, was observed for the decreasing seed weight per pod, similar to that reported by Türk et al. [25], with a growing planting density 75–100–125–150 per m² resulting in a significant decrease in the number of seeds per pod and the protein yield.

3.4. Plant Indexes

The mean value of the Harvest Index (HI) for pea in 2016–2019 accounted for 50.8%, and it was decreased linearly in successive research years; yet, the row spacing did not differentiate it significantly. However, for the planting density of 70 and 90 plants, its value was significantly higher than for 110 plants per m² (

Table 7. Harvest Index (HI), Leaf Area Index (LAI), and Soil Plant Analysis System (SPAD) for pea depending on the year, row spacing, and plant density across the years.

Factor		HI	LAI	SPAD
Year (Y)	2016	59.4 a	4.75 a	41.1 b
	2017	53.4 b	4.35 a	42.7 b
	2018	48.3 c	2.47 b	52.1 a
	2019	42.0 d	2.28 b	43.8 b
Row spacing (R)	16	50.5 a	2.97 a	45.3 a
	32	51.0 a	3.09 a	44.6 a
Planting density (D)	70	52.04 a	2.88	45.3
	90	51.12 a	3.06	45.2
	110	49.19 b	3.15	44.1
	Mean	50.8	3.03	44.9
	Y	**	**	**
	R	ns	ns	ns
	D	**	ns	ns
	Y × R	ns	ns	ns
	Y × D	ns	ns	ns
	R × D	ns	ns	ns
	Y × R × D	ns	ns	ns

Values of a parameter followed by the same letter did not differ significantly between the year, row spacing, and density (ANOVA followed by Tukey’s HDS test, p < 0.05). ANOVA results: ** p < 0.01; ns, not significant.

). Similarly, Martin et al. [9] found a decreasing HI value with an increase in the pea planting density.

At the full flowering stage each year, the LAI value per m² of soil was measured with the Sun Scan Canopy Analysis System. A significantly higher LAI value was found only for the pea plants cultivated in the years with a high total precipitation (2016 and 2017); however, there was no significant impact of the row spacing and planting density on its value. Richard et al. [26] demonstrated that a higher planting density in a humid period resulted in an increase in the LAI value, which was also confirmed in the present study (in 2016 and 2017) with a high total precipitation the LAI value was almost two-fold higher than in the dry years (2018 and 2019). According to Podleśny [12], in the pea plants grown with a wider row spacing, the LAI value was lower, which was not found in the present research. Only the dry year, 2018, recorded a significantly higher SPAD value; however, neither the row spacing nor the sowing density differentiated its value significantly. In the dry period, with a water deficit, SPAD values can be considered a drought stress marker to evaluate the capacity for water use and the productivity of pea cultivars rather than the extent of stomata resistance [27].

4. Conclusions

The length of the growing season of pea in Middle Europe may limit its yield potential in relation to crops obtained at lower latitude and a longer growing season. In our own research, row spacing and planting density, in the vast majority, had no significant effect on pea yield or protein yield, and the most important factor determining their values was the sum of the rainfall distribution over the plan vegetation period. Most of the results did not depend on the interaction between the two factors significantly, in partial agreement with the many results cited, which were recorded in Europe or in Asia.