*3.4. Interactive Effects of GA3 and Nitrogen Fertilization on Growth and Yield of Sugar Beet*

The interaction of nitrogen fertilizer levels and concentrations of GA3 foliar spraying significantly affected root fresh weight/plant (Figure 1) and root yield/ha (Figure 2) in the first season. Root fresh weight/plant and root yield/ha were significantly increased with increasing nitrogen fertilizer levels and GA3 foliar spraying concentrations, where the highest values of root fresh weight (971.5 g/plant) and root yield (80.4 t/ha) were obtained from 275 kg N/ha and foliar spraying with GA3 at 240 mg/L. On the other hand, the lowest values of the above-mentioned traits were recorded with the application of 165 kg N/ha and foliar spraying with tap water. No significant differences were detected for root fresh weight/plant (Figure 3) and root yield/ha (Figure 4) in the second season. Sugar beet importance is not confined only to the root yield but also to its byproducts, where its foliage is considered to be a good source of livestock feed. The results reveal that the interaction effect of nitrogen fertilizer levels and GA3 foliar concentrations on root fresh weight/plant and root yield/ha in the first season were similar to those effects seen on foliage fresh weight/plant (Figures 5 and 6) and foliage yield/ha (Figures 7 and 8) in both seasons. It should be noted that fertilizing beets with 275 kg N/ha and foliar spraying with GA3 at 240 mg/L resulted in the highest means of foliage fresh weight (610 and 572.5 g/plant) and foliage yield (50.5 and 46.7 t/ha) over the two seasons, respectively. Such results are mainly due to the role of nitrogen in increasing cell division, protein content, and potassium and phosphorous utilization, in addition to the role of GA3 in increasing the enzymatic and non-enzymatic antioxidants activities/levels and stimulating the production of mRNA molecules in the cells.

**Figure 1.** Root fresh weight (g/plant) as affected by the interaction between nitrogen fertilizer levels and GA3 spraying concentrations in the first season (2014/2015).

**Figure 2.** Root yield (t/ha) as affected by the interaction between nitrogen fertilizer levels and GA3 spraying concentrations in the first season (2014/2015).

**Figure 3.** Root fresh weight (g/plant) as affected by the interaction between nitrogen fertilizer levels and GA3 spraying concentrations in the second season (2015/2016).

**Figure 4.** Root yield (t/ha) as affected by the interaction between nitrogen fertilizer levels and GA3 spraying concentrations in the second season (2015/2016).

**Figure 5.** Foliage fresh weight (g/plant) as affected by the interaction between nitrogen fertilizer levels and GA3 spraying concentrations in the first season (2014/2015).

**Figure 6.** Foliage fresh weight (g/plant) as affected by the interaction between nitrogen fertilizer levels and GA3 spraying concentrations in the second season (2015/2016).

**Figure 7.** Foliage yield (t/ha) as affected by the interaction between nitrogen fertilizer levels and GA3 spraying concentrations in the first season (2014/2015).

**Figure 8.** Foliage yield (t/ha) as affected by the interaction between nitrogen fertilizer levels and GA3 spraying concentrations in the second season (2015/2016).

#### **4. Discussion**

Nitrogen is an essential element for plants. It is considered to be a major constituent of many biomolecules, including protein and chlorophyll, and it also has an important role in many physiological processes [39]. Our results show that the highest values of root dimensions (length and diameter), root and foliage fresh weights/plant, root and foliage yields/ha, and sugar yield/ha resulted from the increase of nitrogen fertilizer up to the highest level (275 kg N/ha). The increase in the above mentioned traits with the increase of applied levels of nitrogen fertilizer may be attributed to the role of nitrogen in enhancing rapid early growth, encouraging the uptake and utilization of other nutrients including potassium and phosphorous, increasing protein content through synthesize amino acids, and controlling the overall growth of the plant [40,41]. Similar results have been reported by Abdelaal and Tawfik [16], Mekdad [17], Afshar et al. [21], and Zarski et al. [22]. A moderate supply of nitrogen fertilizer is an essential limiting factor for optimum yield, but the excess in nitrogen fertilizer amounts may result in an increase in root yield with lower sucrose content and juice purity [4–7]. Over fertilizing sugar beet with more nitrogen than needed for maximum sucrose production led to decreased sucrose yield [8,9]. With increasing nitrogen supply, sugar concentration decreased, while root yield, sugar yield, and white sugar yield increased and reached maximum values when sugar beet was fertilized at 159, 136, and 129 kg N/ha, respectively [42]. Increasing nitrogen fertilizer levels led to a significant decrease in TSS%, though there was a partitioning of more photosynthetic metabolites to sugar beet tops than to the roots [43]. Additionally, Prvulovic et al. [14] concluded that, when the nitrogen supply increased, the α-amino-N concentration increased considerably and sucrose decreased. Dastorani and Armin [44] reported that an increase in nitrogen levels reduced the impure sugar content, while it increased both root and sugar yields as well as the content of a-amino and sodium. Moreover, it has been stated by Mekdad and Shaaban [45] that, with an increase in the nitrogen fertilizer level from 190 to 290 kg/ha,

the sucrose, extractable sugar, and purity percentages decreased. The resulting increase in TSS and sucrose percentages by using the lowest nitrogen rate (165 kg/ha) in our study may be attributed to the fact that it gave the lowest root size and moisture content, therefore the concentration of TSS% and sucrose% increased. Regarding this, Abd El-Lateef et al. [6], Abdelaal and Tawfik [16], and Mekdad and Rady [18] came to the same conclusion.

Gibberellic acid is responsible for stimulating the production of mRNA molecules in the cells, and mRNA, produced in this form, codes for the hydrolytic enzymes, which in turn improves the chances of fast growth [46]. The observed increase in both root length and diameter, root and foliage fresh weights/plant, root and foliage yields/ha, and sugar yield/ha with the gradual increase of spraying with GA3 concentrations (Tables 2 and 3) might explain why GA3 is one of the most favorable substances for improving plant growth through encouraging the canopy to grow more, thus increasing utilization of solar radiation in a good photosynthesis that produces more carbohydrates that are transported to roots. This is in addition to its role in increasing the activities/levels of enzymatic and non-enzymatic antioxidants and vita organic osmolytes, which improves sugar content, chlorophyll content, and leaf area index [47]. Such results are in agreement with those stated by Qotob et al. [48], who reported that spraying sugar beets with GA3 led to an increase in N use efficiency, which resulted in enhanced plant growth and productivity. Given the effect of GA3 concentrations on quality traits in sugar beet roots, it can be concluded that lower sucrose percentage resulting from using higher concentrations of GA3 may be attributed to the fact that higher concentrations of GA3 may reduce dry matter percentage and thus increase the water content of the root [49,50]. Moreover, the negative effects of GA3 on sucrose, TSS, and purity percentages, as well as its positive effects on total sugar yield, was mentioned by Abdou [27].

The superior effect of GA3 spraying at 60 days after planting compared with spraying at 120 days after planting for all studied traits (Tables 2 and 3) can be attributed to the fact that plants were in their first half of life, thus the absorption efficiency was high, which enabled plants to absorb the full dose, which consequently promoted root and vegetative growth [27]. Early spraying of GA3 leads to rapid leaf growth during the vegetative growth phase; therefore, photosynthesis production in the leaves achieves more than the basic needs of the plant, which leads to sugar beet plants storing photosynthesis products, thereby increasing sucrose. This occurs naturally when the foliage growth reaches its maximum size under appropriate climatic conditions. Nelson and Wood [33] came to the same conclusion when they reported that applying gibberellic acid at 100 mg/L, 3 to 6 weeks before harvest time (late stage), decreased the sucrose percentage. Additionally, the superior effect of GA3 on growth and yield was also mentioned by Rahman et al. [51] when they reported that spraying GA3 on Soybean 30 days after sowing significantly increased all growth and yield parameters.

#### **5. Conclusions**

Generally, for raising sugar yield/ha, it can be concluded that fertilizing sugar beet plants with 275 kg N/ha or the foliar application of GA3 with a concentration of 160 mg/L 60 days after planting is the recommended treatment. Meanwhile, fertilizing sugar beet plants with the same dose (275 kg N/ha) or foliar application of GA3 with a concentration of 240 mg/L 60 days after planting is the recommended treatment for raising foliage and root yields/ha under the ecological circumstances of this research.

**Author Contributions:** Conceptualization, data, formal analysis, investigation, writing—original draft, A.A.A.L.; writing—review and editing, N.K. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research received no external funding.

**Data Availability Statement:** All the data generated or analyzed during the current study are included in the published article.

**Conflicts of Interest:** The authors declare no conflict of interest.
