Effects of plant density on the phenology and vegetative traits of onion cultivars
Days to physiological maturity
The number of days required to reach physiological maturity was significantly (P < 0.05) influenced by the main effects of onion cultivar, and density. The two-way interaction of cultivar density did not influence the physiological maturity of onion (Table 4).
Plant height (cm)
The combined analysis across locations revealed that onion plant height was influenced (p < 0.001) by cultivar and density. However, the interactions between cultivar and density did not influence onion plant height (Table 4).
The highest plant height was recorded for the Bombay Red cultivar, which was not significantly different from that of Red Coach and Jambar, while the lowest plant height was recorded for Russet cultivar. Onion seedlings transplanted at the lowest density of 222,222 and 333,333 plants ha− 1 showed the longest plants with a height of 52.56 cm and 51.8 cm, respectively, while high density plants had the shortest plant height (Fig. 3).
Effects of cultivar and plant density on onion plant height combined across the three locations.
Scholars have also observed significant differences in plant height between onion cultivars14,22,44. Plant height is directly influenced by the availability of growth factors and their capacity to compete for these factors within a plant population. In densely populated plants, there is strong competition for water, nutrients, and light that leads to the production of short plants. However, in less populated plants, plants can fulfill their requirements for growth and development, thus leading to the production of longer plants. These findings are in line with the results of Alemu Diribsa et al.18 and Wubetie et al.22, who observed tallest onion plants at the lowest plant densities.
Leaf number per plant
The number of leaves per onion plant was highly significantly (p < 0.001) influenced by cultivar and plant density. However, the two-way interaction of cultivar and density did not influence the number of onion leaf per plant (Table 4). Consequently, the highest mean leaf number was obtained for the Bombay Red cultivar which was significantly statistically similar to that of Jambar. The densest onion plants produced the lowest number of leaves per plant (8.27), while the less populated plants produced the highest number of plants, as indicated in Table 5.
These results are in agreement with the findings of other scholars who reported significant differences in leaf number between onion cultivars45,46,47. In this regard, Steer48 reported that Bombay Red bulbs produced more splitter bulbs/shoots from multiple growing points, which facilitated the production of more leaves per plant as indicated in the present study.
As indicated in the present study, the planting density and the number of leaves are inversely related. As the population of the plant increased, the number of leaves per onion plant decreased. The lowest number of leaves observed at the highest plant density was probably attributed to the increased competition between plants for nutrients and moisture. Brewster 1, suggested that onion plants with approximately 6 to 8 fully expanded leaves are capable of sustaining optimal photosynthetic activity. Researchers also reported similar results in which more leaves per plant were recorded at the lowest plant density, while fewer leaves were produced on highly populated onion plants18,49,50. Akoun51 also observed that the plant population significantly influenced different onion parameters including the number of leaves produced per plant, leaf area, bulb diameter, and yield.
Leaf diameter (cm)
The results across the three locations indicated that the Bombay Red cultivar took the longest time to reach maturity compared to other cultivars. On the other hand, the Russet cultivar displayed earlier maturity dates across the three testing sites (Table 3). These findings are in line with the results of Jilani et al.31, who also reported differences in days to maturity of different onion cultivars.
The Russet onion cultivar reached maturity earlier than Bomby Red cultivar, with differences of 13 days, as indicated in Table 3. Previous studies have shown differences in maturity among onion cultivars32,33. Variations in physiological maturity also reflect genotypic differences in onion cultivars in the length of growth period and maturity, which affects the transfer of photosynthetic materials from leaves to bulbs and the improvement of the growth rate, thus early bulb initiation and maturity34. The physiological maturity of the hybrid cultivars is generally earlier than that of the open-pollinated cultivars due to heterosis and genetic uniformity. These plants also exhibit increased photosynthetic efficiency and the breeding is targeted for early maturity, ensuring timely harvests and multiple cropping cycles35.
Similar results were also reported where significant variations in days to maturity among onion cultivars were observed18,36. Furthermore, the Russet onion cultivar showed early physiological maturity due to its ideal leaf number of leaves per plant (9). These findings are in line with the findings of Kebede30, Cramer37, and Birhanu and Tilahun38, who claimed that hybrid onion cultivars matured earlier than open-pollinated ones.
With respect to plant density, a significant delay in maturity was observed as the plant population decreased. Early maturity was recorded at the highest plant density, while onion plants planted at low plant population required longer days to mature across the three locations (Table 3). The early maturity of a high density plant population may be caused by an inadequate supply of water and nutrients to prolong growth and metabolic processes. Plant density also influences physiological maturity by intensifying competition for abiotic factors such as light, water, and nutrients, which ultimately accelerate maturity as plants allocate more resources to reproduction39,40. These results are consistent with the findings of Hidayatullah and Jillani41, who reported that plants grown at greater density had the shortest time to mature. Similar findings have been reported where plants grown at the lowest density took longer time to reach maturity while those at the highest density matured earlier42,43.
The combined analysis of variance showed that the main effects were highly significantly (p < 0.001) influenced the diameter of the onion leaf. On the other hand, the interactions between cultivar and density did not influence onion leaf diameter (p > 0.05) (Table 4).
The greatest mean leaf diameter was recorded for the Russet and Jambar cultivars. Similarly, plants grown at the lowest population density had the greatest leaf diameter, while the lowest leaf diameter was recorded from plants grown at the highest plant density (Table 5). Generally, the leaves of hybrid onion cultivars (Russet and Jambar) were wider than those of the open pollinated Bombay Red cultivar. Cramer37 and Chen and Chen52 reported that hybrid onion cultivars tend to produce wider leaves than open-pollinated cultivars. Breeders intentionally select wider leaf traits from parental lines to develop hybrids with enhanced qualities, yield, disease resistance, and adaptability to diverse environmental conditions.
The wider leaf diameter recorded at the lowest plant density might be due to the presence of sufficient light in the wider-spaced plants, which is essential for photosynthesis and nutrient uptake compared to those of the closely spaced plants. These results are in agreement with the findings of many authors who also reported significant variation in leaf diameter due to differences in plant density53,54,55,56.
Effects of plant population on bulb yield and yield-related traits
Bulb neck diameter (cm)
The results of analysis of the variance indicate that the diameter of the onion bulb neck was significantly (P < 0.01) influenced by the main effects of density and cultivar. However, the two-way interaction of cultivar and density did not significantly influence onion bulb neck diameter (p > 0.05) (Table 4).
The Bombay Red cultivar had the widest neck diameter which was statistically similar to that of the Red Coach, while the lowest bulb neck diameter was obtained from Russet, which was statistically similar to that of the Jambar cultivar. On the other hand, the highest bulb neck diameter was recorded from the lowest (222,222 ha–1 plants) plant density while the highest plant density (666,666 plant ha–1) recorded the lowest bulb neck diameter (Fig. 4).
Effects of cultivar and density on the diameter of the onion bulb neck at the three locations.
The genetic composition and inherent traits of onion cultivars contribute to the differences in bulb neck diameters as reported by Fery and Eissa57, which could also be the case in the present study. The findings by Gautam et al.58 also indicated varying bulb neck diameter in different onion genotypes.
The wider bulb neck diameter observed in plants grown with a wider plant spacing (lowest plant density) is associated with less competition from plants for growth factors. Under these circumstances, the vegetative growth of the plants is enhanced which in turn leads to wider bulb neck diameter, as indicated in the present study. On the other hand, high plant populations may have exerted pressure on the scarcity of growth factors such as light, space, moisture, and nutrients, leading to reduced vegetative growth and narrow bulb neck diameter59. According to Wubetie22, planting onions at higher densities can lead to smaller bulbs and reduced neck diameters, as plants compete for growth resources, while lower planting densities can promote better bulb growth and potentially result in larger neck diameters.
Average bulb diameter (cm) and bulb weight (g)
The results of analysis of variance indicated a highly significant difference (P < 0.01) among the cultivars and plant densities for the diameter of the onion bulb. However, the effects of the cultivar-density interaction did not influence the diameter of the onion bulb (Table 4). The average bulb weight of onions was influenced (p < 0.001) by the main effects of cultivar and density. The interactions between cultivar and density did not have a significant impact on onion bulb weight (p > 0.05) as shown in Table 4.
Hybrid onion cultivars produced bulbs with the largest diameter and weight, which were statistically comparable to each other. In contrast, the Bombay Red cultivar exhibited the smallest diameter and weight (Table 6). The higher plant density resulted in smaller bulb diameters and lighter weights than lower plant density did (Table 6).
The diameter and weight of the bulb signify the potential for onion yield. These hybrid cultivars excel compared to the local open pollinated Bombay Red cultivar due to inherent differences. This finding aligns with the findings of Kokobe and Hirut47, who noted significant variations in onion cultivar for length, diameter, and weight.
Intense competition for resources negatively impacts overall onion plant performance and reduces bulb diameter and weight. Densely populated onion plants experience restricted nutrient availability, limited sunlight interception, decreased photosynthetic activity, and water stress, which hinder bulb expansion. Previous researches conducted by Brewster1, Alemu Direbsa et al.18, and Yldrm and Ciftci60, support the findings of the present study. Furthermore, studies by Jilani et al.31, Sikder et al.61; Muhammad et al.62 confirmed that decreasing plant density results in increased weight and size of individual onion bulbs.
Total bulb yield (t ha− 1)
The combined analysis of variance showed that cultivar and plant density significantly influenced the total bulb yield (p < 0.0001). The two-way interaction of cultivarand density did not affect total bulb yield (Table 4).
Russet had the highest bulb yield, which was statistically similar to that of Jambar, while Bombay Red had the lowest yield. Increasing the plant density to 666,666 plants ha− 1 (Table 7) increased the total bulb yield. The different plant densities contributed to 42.43% of the yield variation. This underscores the importance of cultivar and planting density in optimizing the onion yield.
The yield of onions depends on cultivar, environmental conditions, and agronomic practices and is influenced by the genetic makeup and the interaction of the genotype and environment44,46,58,63,64,65,66. Hybrid onion cultivars consistently outperformed open-pollinated Bombay Red cultivar, which also align with the findings in tomato cultivation67,68. An increase in plant density also correlates with the increase in garlic bulb yield as supported by Hemla and Hosmani69, and Fikreyohannes Gedamu70. This highlights the impact of genetic factors and planting density on improving onion and garlic productivity.
The plant population, the number of plants per unit area, directly affects the yield of onion bulbs. Similar to the results of the present study Geremew24, Kantona et al.71, AVRDC72, Sara Belay73, and Yohannes Gebremichael et al.74 reported that increasing the plant population from 222,222 to 666,666 plants ha− 1 led to a 42.4% increase in total bulb yield. Kantona et al.71 also observed an increase in onion yield from 17.4 to 39.5 t ha− 1 with an increase in plant population from 50 to 150 plants per square meter. However, the higher population density resulted reduction in the diameter and weight of individual onion bulbs (Table 6), which is consistent with the findings of other researchers25,61. According to Zubelidia and Gases25, optimizing plant populations prevents intense competition between plants for growth factors and helps efficient utilization of available cropland. He et al.75 and AL-Naggar et al.76 reported that high plant density resulted in the highest grain yield for hybrid maize cultivars, which reflects that hybrids exhibit greater density tolerance, contributing to their superior performance in terms of grain yield.
Marketable bulb yield (t ha− 1)
Analysis of variance revealed that cultivar and plant density influenced (p < 0.001) the marketable bulb yield of onion, while the two-way interactions did not influence the marketable bulb yield of onion (Table 4).
The highest marketable bulb yield across the three locations was obtained from the Russet and Jambar cultivars, where the yields were statistically similar. However, the lowest marketable bulb yield was recorded for the cultivar Bombay Red. Onion plants grown at the highest plant density produced the highest marketable bulb yield followed by those grown at the medium plant density. The plants grown in the lowest plant population produced the lowest marketable bulb yield as shown in Table 7.
The marketable yield of onion generally showed an increasing trend and reached a plateau at higher densities, which was found to be optimal for onion production across all locations. However, a further increase in the plant population reduced the marketable bulb yield, as shown in Fig. 5. The marketable yield increased with increasing plant density due to the accommodation of more plants per unit area, which in turn increased the bulb yield per unit area. However, this increase in bulb yield depends on the availability of enough nutrients and other growth factors. In this regard, Khan et al.59. and Yemane Kahsay et al.64 indicated that as the plant density decreased from 666,666 plants ha− 1 to 333,333 plants ha− 1, the marketable bulb yield decreased from 34.49 t ha− 1 to 28.10 t ha− 1. Similarly, Kantona et al.71 also reported an increase in the number of marketable bulb yields with the increased onion plant density. Therefore, plant density is the determining factor of marketable yield at varying locations77.
Trends in marketable onion bulb yield are influenced by plant density across locations.
As indicated in Fig. 6a and 6b, marketable yield of onion has a positive relationship with diameter and weight of onion bulbs. On the other hand, bulb weight has a negative relationship with days to physiological maturity (Fig. 6c).
Trends of marketable bulb yield as influenced by bulb diameter (a) bulb weight (b) bulb weight as related to days to physiological maturity (c) across plant density and location.
The highest marketable yields observed by Russet and Jambar in the present study were attributed to the increase in the diameter and weight of onion bulbs as indicated in Table 6. The difference in yield potential is associated with the genetic variation of the cultivars as reported by various researchers where marketable bulb yield variations were observed between onion cultivars;14,18,24. In recent meta- analyses conducted by Garcia and Nguyen78, onion cultivar had a significant impact on marketable bulb yield. The analysis revealed that specific cultivars consistently outperformed others, regardless of the planting density or specific environmental conditions.
Unmarketable bulb yield (%)
The analysis of variance indicated that the cultivar influenced (P < 0.0001 the unmarketable bulb yield of onion, while the density and the two-way interactions did not influence the unmarketable bulb yield (Table 4).
The highest percentage of unmarketable bulb yield was obtained from the Bombay Red and Red Coach cultivars, while the lowest percentage of unmarketable bulb yield was recorded from the Russet and Jambar cultivars (Table 7). The difference in unmarketable yield could be attributed to the genetic variation of the onion cultivars. Fikre and Mensa36 reported similar results, in which variations in unmarketable bulb yield were observed between different onion cultivars. Wubetie et al.22 indicated that the unmarketable bulb yield of onion cultivars was not significantly affected by density and the interaction of cultivar and density.
Harvest index
Analysis of variance revealed that cultivar influenced (P < 0.001) the onion harvest index, while plant population and two-way interactions did not influence the onion harvest index (Table 5).
The highest mean harvest index was recorded for the Russet cultivar, which was statistically similar to that of the Jambar cultivar, while the lowest harvest index was recorded for the Bombay Red cultivar as indicated in Table 6. The harvest index is a measure of the success of the partitioning of photosynthetic assimilates. The harvest index is variable and calculated to reflect the crop performance79. In this sense, the greater harvest index of the Russet and Jambar onion cultivars observed in the present study could be attributed to the higher average bulb weight of the cultivars, as indicated in Table 6. These results are in agreement with the findings of Tegbew Walle et al.66 who reported significant variations in the harvest index between onion varieties.
Marketable bulb yield distribution
The combined analysis of variance revealed that cultivar and plant population/density influenced (p < 0.001) the percentage of small, medium and large bulbs of onion. The two-way density–cultivar interaction significantly (p < 0.001) influenced the percentages of medium and large bulbs (Table 4).
Increasing the density of the plant significantly increased the percentage of small bulb yield. The highest small bulb percentages were recorded for the Bombay Red cultivar and the highest planting density, respectively (Table 8).
In the interaction effect the highest percentage of medium-sized bulbs was recorded by the treatment combination of Russet x high population density, Jamber x high population density and Bombay red x medium population density, which were statistically similar when compared each other. Similary, Russet and Jamber combined with high plant density recorded the highest precentage of large-sized bulbs (Table 9).
Increasing the density of the plant significantly increased the percentage of small bulbs, but decreased the percentage of large bulbs of onion cultivars as indicated in Fig. 7. This could be due to resource constraints at high densities that encourage the growth of smaller bulbs to ensure survival and reproduction even in the face of scarce supplies. According to Uddin et al.80, high plant population causes a larger proportion of small bulbs in the total output.
Trends in the distribution of bulb size as affected by plant density across the three locations.
Several scholars have reported similar results in which increasing the plant density decreased onion bulb sizes31,64. According to Yemane et al.64, decreasing the plant population from 666,666 plants ha− 1 to 333,333 plants ha− 1 increased the percentage of large bulbs from 9.3 to 20.3%. On the other hand, planting onion plants at higher densities results in the formation of smaller bulbs81.
All onion cultivars grown at a medium planting density produced the highest percentage of medium-sized bulb yield (Table 9). Onion bulbs of Russet and Jambar grown at the highest plant density produced significantly greater percentage of medium-sized bulbs than bulbs grown at a lower plant density. This might be due to proper utilization of resources for all onions compared to lower densities80. These results are in agreement with the findings of Rumpel et al.82, who reported that the yield of medium bulbs increased with increased density while the yield of large bulbs decreased. Similarly, Nasir et al.49 and Rumpel et al.82 reported that maximum weights of medium-sized bulbs were obtained at higher planting densities.
Quality parameters of onion
Total soluble solid (oBrix)
The combined analysis of variance showed that cultivar, plant density, and their two-way interactions influenced (p < 0.01) the total soluble solid (TSS) of the onion plants (Table 4). Bombay Red, which was grown at the lowest density followed by Jambar had the highest total soluble solids, while the Red Coach cultivar recorded the lowest TSS. Onion bulbs harvested from plants grown in the lowest plant population recorded the highest TSS, while those harvested from highly populated plants recorded the lowest TSS (Table 10).
Total soluble solids are the amounts of soluble solids in liquid. The TSS affects the taste of a given product and is dominated by the total sugar content and a small portion of soluble proteins, amino acids, and other organic materials. Therefore, the level of sweetness of a product is proportional to its TSS content as indicated by Kusumiyati et al.83, which could be affected by the cultivar, growing environment, and agronomic practices. The reduced TSS of bulbs produced from densely populated onion plants is apparently due to the strong competition of plants for growth factors that lead to the production of low-soluble solids and thus lowers TSS in the bulbs.
Firmness of the bulb (Newton)
The firmness of the bulb was influenced (p < 0.01) by the main effects, as well as the two-way interaction (Table 4).
Bulbs of the Russet cultivar had the highest firmness, which is at par with that of the Jamber cultivar, while the bulbs of the Bombay Red cultivar had the lowest firmness. Firmer bulbs were also obtained from plants grown at the lowest plant density while bulbs from the highest and medium plant densities were less firm (Table 10). This could be attributed to the fact that fewer planting density allows individual onion plants to get more water and nutrients from the soil than they would in congested areas. Firmer bulbs may resulted from higher cellulose deposition and thicker cell wall growth, which are facilitated by better resource intake Wubetie et al.22. These findings agree with those of Lancaster et al.84, Chope et al.85, and Larsen et al.86, who reported significant variation among onion cultivars concerning bulb firmness.
