Impact of Cutting the Growing Apex and Nitrogen Fertilizer on the Growth and Yield of Brussels Sprout (Brassica olelecea var. gemmifera)

Brussels sprout (Brassica olelecea. var gemmifera) is one of the cruciferous family plants. It is an annual plant considered a nutritional and economic value vegetable. The part that is eaten is the sprout, which grows on the main stem under the leaves. Likewise, the rosette leaves can be eaten after cutting (Combs and Ernst, 2019). It contains vitamin A and C, iron, calcium, thiamine, niacin, riboflavin, high centralizations or concentrations of carotenoids and natural sulfur phytochemicals that increase their cancer prevention agent limit. It is rich in chlorophyll, glycosinolates, sulfuric substances and other compounds that give it a sharp taste (Natasa, 2006). In Iraq, Brussels sprouts are not grown, except for scientific research purposes, while it is widely cultivated in Europe, especially England and France, as well as in America, Canada, Australia and Japan (Turan et al., 2009).
       
Plant growth is affected by several environmental factors, such as temperature, humidity, nutrition, etc. The nutrition factor, especially the nitrogen element, has a major role in plant growth, as the plant needs it in large quantities during the growth stage, bud formation and yield increase (Williams et al., 1996; Al-Silmawy and Abdul-Ratha, 2023) and the amount added depends on weather conditions, soil type and fertility, in addition, it’s content of mineral (Omar, 2017; Abo Al Maged, 2019; Booij, 2000). Saud et al., (2016) found that application of nitrogen with 20,40,60,80,100 Kg/N/Ha increased all growth and yield attributes of mustard. Al-Leela  et al. (2019) revealed that the addition of urea fertilizer significantly improved head length, number of leaves per plant, stem length, the total weight of the head and total yield of the head in lettuce. Also, Al-Dabbagh et al., (2020) illustrated that spraying cabbage plants with mineral fertilizer (Agroleaf power), which contains 31% nitrogen, gave a significant superiority over the comparison treatment in the yield characteristics. Al-Dabbagh and Kahlel (2024) found that spraying cabbage plants with nano NPK 20:20:20 increased vegetative growth and yield parameters compared to control.
       
A common technological practice in Brussels sprouts production is the removal of the apical plant apex before harvest by a few weeks to stimulate the development of lateral buds (sprouts) and increased yield. This practice essentially redirects the plant’s energy from vertical growth to producing larger and more uniform sprouts. It is well-known that this measure positively influences the size of the sprouts (Jakopic et al., 2016). In addition, the number of buds can be increased by cutting the growing apex process. Removing the growing top stimulates the formation of lateral buds. The results of previous studies indicated that it is possible to increase the productivity of some vegetable plants when removing the growing tops of the plants. It was found an increase in plant yield was found when removing the growing top and this was mentioned by (Sajjan et al., 2002; Sailaja and Panchbhai, 2014 and Sideman et al., 2023) .
       
Since no such study has been carried out on Brussels sprout in Iraq (Mosul) so far, it was undertaken to investigate the effect of urea and cutting the growing apex of the plant under the environmental conditions of Mosul city.

The experiment was conducted in Horticulture and Landscape Department / College of Agriculture and Forestry / University of Mosul/Iraq to study the impact of cutting the growing apex vis a vis no cutting and adding nitrogen fertilizer (urea 46% N), with four levels (0, 50, 75 and 100) kg N per hectare in growth and yield of Brussels sprout.
       
The seeds of Brussels Sprout, Oliver variety, produced by Bejozaden Company, were planted on 1st September  2020, in germination trays filled with peat moss. When the seedlings reached the stage of 3-4 true leaves, they were transplanted into large plastic pots (35 cm in diameter and 30 cm in height), which were filled with soil mixed with organic fertilizer residues (remains from mushroom cultivation) at a ratio of 1:3. The cutting of the growing apex was at 15^th Jan. 2021.
       
Nitrogen fertilizer was added in 2 batches, the first on 26 Dec. 2020 and the second on 26 Jan. 2021 (after one month between one batch and the next), with 1.43 gr. fertilizer for each pot for 50 kg N. ha^-1. Level, 2.5 gr. N for 75 kg N ha^-1 level and 2.90 gr. N  for 100 kg N ha^-1level.                                                                                                                   
       
The study was conducted in a factorial experiment in RCBD design with 8 treatments (2 x 4) and three replicates, with five plants for each experimental unit.
       
The following characteristic measurements were recorded:
       
1-Plant height (cm), 2-Number of leaves/plants, 3-Diameter of main stem mm/plant, 4-Total Chlorophyll in leaves (SPAD),  5-Leaf area of plant (cm² ),  6-total weight of heads/plant (gm.), 7- Number of heads/plants, 8- Diameter of head (mm), 9- Length of head (cm), 10- The percentage of dry matter in the leaves, 11-The percentage of elements (N, P, K) in heads, 12- Protein in green heads.
The calculated protein was according to the equation:
 

 Protein percentage = Percentage of nitrogen x 6.25.

 
The data were statistically analyzed and the means were tested according to Duncan’s at the 0.05 level (Al-Rawi and Khalaf Allah, 2000).

Data from Table 1 revealed that the treatment of cutting the growing apex caused a significant difference in the characteristics of plant height, number of leaves and leaf area. The highest mean value of plant height was 57.392 cm, from treatment without cutting the growing apex, while the lowest value was 27.125 cm, from cutting the growing apex. Similarly, the highest number of leaves per plant was 58.225, from the treatment without cutting the growing apex, compared to the lowest value, 26.750, from cutting the growing apex. Again, the highest value of leaf area of the plant was (4578.8 cm²) from treatment without cutting the growing apex, compared to the values when cutting the growing apex, which recorded 1505.4 cm². The treatment of cutting the growing apex had no significant effect on stem diameter (cm) and total chlorophyll in leaves (SPAD).


    
The addition of nitrogen fertilizer (urea), increased significantly all the studied traits compared to the control treatment (without adding nitrogen fertilizer).The highest values were 49.250 for plant height and 46.383 for leaves number,  21.117, for head diameter, 87.917 for chlorophyll and 3624.3 for leaf area from an additional 100 kg N/ha., while the lowest value was recorded from the control treatment which is 24.533 cm for plant height, 37.117 for leaves number per plant, 17.967 cm. for steam diameter, 29.217 SPAD for total chlorophyll and 2147.7 cm² for leaf area of plant.
       
The interaction treatments between cutting the growing apex and nitrogen fertilizer had significant differences in all the characteristics mentioned in Table (1). The interaction treatment of without cutting the growing apex and addition of 75 kg N/ha of urea fertilizer was superior in giving the highest plant height (68.50 cm) and highest stem diameter (22,100 mm). While the highest number of leaves (67,133), the highest percentage of chlorophyll (88,600 SPAD) and the highest leaf area (5733.9 cm²) were from the interaction treatment without cutting the growing apex and addition of 100 kg N/ha of urea fertilizer.
       
On the other hand, the lowest value of plant height was 18.767 cm., leaves number per plant 21.300, steam diameter 19.200 cm, total chlorophyll in leaves 28.267 SPAD and leaf area per plant 1211.0 cm² was from interaction treatment between cutting the growing apex and 0 kg N/ha of urea fertilizer.
       
This result is in harmony with (Abou El- Magd, 2019; Abou El- Magd et al., 2019) in Brussels sprout and Patil et al., (2003) in Kohlrabi, who revealed that increasing nitrogen fertilizer increased the vegetative growth parameters of plants. It is clear that the application of nitrogen plays an important role in cell division and enlargement and consequently in plant growth. Also, the nitrogen element may work to increase and activate the biochemical processes that increase plant growth, such as the synthesis of amino acids, which are the building blocks of proteins and enzymes and thus control all the important biological reactions that occur within the plant (Abou El- Magd et al., 2019; Sadik et al., 2016).
       
Nitrogen also enters into the composition of nucleic acids such as DNA and RNA, which are necessary for cell division (Williams et al.,1996) and is involved in the construction of some growth regulators such as auxins and gibberellins, which encourage cell division, cell elongation and increased plant height (Omar,  2017, Abou El- Magd,  2019).
       
The data presented in Table (2) revealed that the treatment of cutting the growing apex of the plant caused a significant difference in head weight and the length of heads, the values were (431.45  g and 4.022 cm.)   compared to the not cutting the growing apex which gave 268.76 gr. and 2.903 respectively, while no significant effect showed in number of head per plant and head diameter.


       
The significant effect of cutting growing apex treatments in the values of some characteristics studied may be because any cutting of plant apex stimulates the growth of lateral buds, as the number of lateral buds increases and the nutritional status of the plant increases and the nutritional compounds go to the terminal bud directly (Sideman et al., 2023 ) and this encourages the growth of these buds and since the nature of the growth of  Brussels sprouts plants stems have short internodes and the lateral buds are close together, we found that these buds begin to form a group of leaves wrapped together instead of taking on elongation growth and begin to form small heads, which are a group of leaves wrapped together. (Jakopic et al., 2016).
       
The nitrogen fertilizer treatments led to differences in all the characteristics mentioned in Table 2. The addition of nitrogen (50,75,100 kg N.) was significantly superior to the control (0 N) and the treatment of adding 100 kg N/ha was significantly superior in head weight per plant (482.37 g) and number of heads (195.33) compared to the rest fertilizer treatments. The same treatment was also significantly superior in head diameter (20.737 mm.) and length of the head (4.003 cm) compared to the control treatment only.
       
However, interaction treatments between cutting the growing apex and nitrogen fertilizer caused significant differences in all studied traits.
       
This result corresponds with (Abou El Magd,  2019), who found that a high level of nitrogen increased (head weight, head number, total sprout yield, head diameter and height). Kaur and Sharma, 2018. Rai et al., 2003 found that broccoli head yields were increased by increasing N levels; they added that the effect of N level on the yield of terminal curd was highly significant. Nitrogen plays a great role in photosynthesis and increases its products through plant tissues. in addition, chlorophyll A and B concentrations increased in leaf tissues by nitrogen increments.Chlorophyll A and B are the major light-absorbing pigments of the plant leaves and play an important role in the metabolic activities in plant tissues, obtaining higher vegetative growth and dry matter accumulation. (20,21 Ridha et al., 2020, AL-Zaidyand AL-Ubaidy.  2017, Dhatt et al., 2023).
       
The interaction treatments of cutting growing apex with the addition of 100 kg. N/ha was superior in terms of head weight per plant (613.65 g.), the highest values of number of heads (200.35) and head diameter (22.437) was from interaction treatment between without cutting the growing apex and nitrogen fertilizer 75 kg N/ha, while the highest value of head length (4.700) was from interaction treatment of cutting the growing apex and nitrogen fertilizer 50 kg N/ha. The lowest values of head weight (48.03 g.), number of heads per plant (60.67) and head length (1.877 cm.) were recorded from interaction treatment without cutting the growing apex and 0 nitrogen.
       
Table 3 revealed the impact of cutting the growing apex and nitrogen fertilizer on some chemical properties of Brussels sprout. It was found that the non-cutting growing apex treatment led to a significant increase in the percentage of nitrogen (3.38%), percentage of potassium (3.975%), as well as the protean percentage (21.125%), but the phosphorus and dry matter percentage increased when cutting the growing apex and recorded (0.690%, 17.00%) respectively.


       
All nitrogen fertilizer treatments led to significant differences in the percentages of the studied traits except the dry matter percentage compared to the control treatment. The highest percentage of nitrogen and protein was from adding 100 kg N/ha and recorded (3.65%, 22.812%), with superior to other treatments. The treatment of adding 50 kg N/ha had a significant effect in increasing the percentage of phosphorus (03765%) and potassium (4.110%) compared to the rest fertilizer treatments.
       
In addition, the interaction treatments between the two factors created significant differences in the percentage of the studied traits and the highest value for nitrogen (4.30%) and protein (26.875 %) was from the non-cutting growing apex and the addition of 75 kg N/ha. The highest percentage of phosphorus (0.830%) was from cutting the growing apex with the addition of 50 kg N/ha. The non-cutting growing apex treatment with the addition of 50 kg N/ha led to the highest percentage of potassium (4,300%) and the highest percentage of dry matter (17.65%) was from cutting the growing apex with 0 addition of nitrogen.                                                                                                                                           
               
While the lowest value of nitrogen (1.69%), phosphorus (0.530%), potassium (2.290%) and protein (10.562%) were from interaction treatment between cutting the growing apex and 0 nitrogen, the lowest percentage of dry matter (12.78%) was from interaction treatment between without cutting the growing apex and 75 nitrogen’s.

From the results, it can be concluded that the treatment of cutting the growing apex increased the total weight of heads per plant, head length, dry matter percentage and protein percentage. While the treatment without cutting the growing apex increased plant length, leaf number and leaf area of plant. Also, the nitrogen fertilizer gave a significant increase for all parameters compared to the control (0 nitrogen) and the highest values were from adding 100 Kg. N. Ha^-1.

We extend our thanks and appreciation to the presidency of the Horticulture and Landscape Design Department, Deanship of the College of Agriculture and Forestry/ University of Mosul, for providing research requirements and continuous support for us that helped us to complete this research.

All authors declare that they have no conflict of interest.