Nitrogen and boron nutrition in grafted watermelon I: Impact on pomological attributes, yield and fruit quality

Watermelon is extensively consumed fruit across the globe. However, limited is known about interactive effect of nitrogen (N) and boron (B) nutrition on pomological, yield and fruit quality attributes of grafted watermelon. This two-year study tested the influence of different N and B doses on pomological, yield and fruit quality attributes of grafted watermelon under field conditions in Çukurova plains of Turkey. Four different N (0, 90, 180 and 270 kg ha-1) and two B doses (0 and 2 kg ha-1 B) were tested. The individual and interactive effects of N and B significantly altered pomological, yield and fruit quality attributes during both years. Overall, application of 270 kg ha-1 N and 2 kg ha-1 B improved yield, pomological and fruit quality attributes during both years. The highest values for yield, main stem length, stem diameter, fruit weight, fruit width, number nodes and branches per stem were recorded for 270 kg ha-1 N during both years. However, rind thickness was not altered by N application. Similarly, the highest values for quality attributes such as sucrose, glucose, fructose, citric acid, tartaric acid and ascorbic acid were noted for 270 kg ha-1 N during both years. Interestingly, no N application and 90 kg ha-1 N recorded the highest values of maleic acid during both years. The highest values of rind thickness, fruit length, fruit width and fruit weight were noted for 2 kg ha-1 B during both years, while B application had no effect on main stem length, main stem diameter, number of nodes and number of branches. Regarding N by B interactions, 180 and 270 kg ha-1 N with both B doses observed the highest values for yield, pomological and quality attributes during each year. These results indicate that N has significant contribution towards yield, pomological attributes and fruit quality of grafted watermelon. Therefore, N should be applied at the rate of 270 kg ha-1 for better yield, pomological attributes and fruit quality. Nonetheless, where necessary grafted watermelon should be fertilized with 2 kg ha-1 B for better fruit quality and pomological attributes.


Introduction
Nutrients are the most important component of growing environment for plants and influence their growth and yield. Yield and quality of field crops is positively influenced by major macro a1111111111 a1111111111 a1111111111 a1111111111 a1111111111 Application Center, Ç ukurova University, Faculty of Agriculture, Department of Soil Science and Plant Nutrition, Turkey during watermelon growing seasons of 2018 and 2019. Grafted watermelon cultivar 'Starburst' was used as experimental material. The experiment was laid according to split-plot design with N as main factor and B as sub-factor. All experimental treatments had four replications.
Seedlings were planted keeping 4 m distance between rows, 1.2 m between plants and 6 plants were grown in each replication. The soil was analyzed prior to the initiation of experiments and depending on the results of the soil analysis 25 kg of phosphorus (P 2 O 5 ) was applied per hectare at the time of planting.
Four different N doses, i.e., N 0 (0 kg N ha -1 ), N 1 (90 kg N ha -1 ), N 2 (180 kg N ha -1 ) and N 3 (270 kg N ha -1 ) and two different B doses, i.e., B 0 (0 kg ha -1 B) and B 2 (2 kg ha -1 B) were used in the study. Nitrogen was supplied by using urea and applied in three equal splits (i.e., at sowing, flowering and fruiting). Etidot67-B was used as B source and whole amount of B was applied at the time of sowing.
When plants were 45 days old, pomological characteristics such as main stem length (cm), number of branches (pieces), main stem diameter (mm) were measured with the help of meters and calipers in the field. Depending on the climatic conditions (the first week of July), the harvesting process was carried out by hand picking the fruits by drying the auricles and leeches on the fruit stalk. The yield was computed based on the weight of harvested fruits. The pomological features were realized by taking 3 fruits from each parcel.

Statistical analysis
The collected data for nutrient uptake were tested for normality by Shapiro-Wilk normality test [32]. The data were normally distributed; therefore, original data were used in statistical analysis. The differences among the years were analyzed by paired t test, which were significant. Therefore, data of both years were analyzed and presented, separately. Two-way analysis of variance (ANOVA) was used to test the significant differences among N and B doses, and their interaction [33]. Least significant difference at 5% probability was used to separate the means where ANOVA indicated significant differences.

Growth and yield attributes
The individual and interactive effect of N and B significantly altered various growth and yield attributes with some exceptions during both years (Table 1). Nitrogen doses significantly altered all growth attributes except number of branches during both years. However, the growth attributes were not altered by B doses during both years, except for the only significant effect on number of branches during 2 nd year. Regarding N by B interactions, all growth attributes were significantly affected except for the non-significant effect on number of branches during 1 st year ( Table 1).
The highest values of growth attributes such as main stem length, stem diameter and number of nodes per stem and yield were recorded for 270 kg ha -1 N during both years, whereas no N application observed the lowest values of these traits (Fig 1). The growth attributes were not influenced at all by B doses during both years (Table 2).
Regarding interactions, the highest values of all growth attributes were recorded 270 kg ha -1 N with both B doses during each year (Table 3).

Pomological attributes
Different N and B doses and their interaction significantly altered various pomological attributes with some exceptions (Table 1). Nitrogen doses significantly altered all pomological attributes except rind thickness during both years. Similarly, B doses also had significant effect on all pomological attributes during both years. Regarding N by B interactions, all pomological attributes were significantly affected during both years ( Table 1).
The highest values of pomological attributes such fruit length, width and weight were recorded for 270 kg ha -1 N during both years, whereas no N application observed the lowest  values of these traits (Fig 2). The application of 2 kg ha -1 B recorded the highest values of all pomological attributes both years (Table 2). Regarding interactions, the highest values of all pomological attributes were recorded for 270 kg ha -1 N with both B doses during each year (Table 3).

Fruit quality attributes
The individual and interactive effect of N and B significantly altered fruit quality attributes during both years (Table 4). Nitrogen doses significantly altered all fruit quality attributes, including sucrose, glucose, fructose, citric acid, tartaric acid, maleic acid and ascorbic acid during both years. Similarly, different B doses had significant effect on all fruit quality attributes during both years, with some exceptions. Regarding N by B interactions, all fruit quality attributes were significantly affected during both years ( Table 4).
The highest values of all fruit quality attributes except maleic acid were recorded for 270 kg ha -1 N during both years, whereas no N application observed the lowest values of these traits (Table 5). Similarly, 2 kg ha -1 B observed the highest values of fruit quality attributes during both years (Table 6).
Regarding interactions, the highest values of all fruit quality attributes were recorded for 270 kg ha -1 N with both B doses during each study year (Table 7).

PLOS ONE
Nitrogen and boron nutrition in grafted watermelon

Discussion
Individual and interactive effects of N and B altered growth, yield, pomological and fruit quality attributes. As hypothesized, increasing doses of N and B improved growth, pomological and fruit quality attributes. The highest values of growth, pomological and fruit quality attributes were recorded for the highest N and B doses used in the current study. These can be owed to the continuous availability of both nutrients throughout the growing season, which resulted in proper functioning, metabolism and nutrition. Collectively all metabolic processes improved growth, pomological and fruit quality attributes of grafted watermelon in the current study. Stem length or plant height is an important character related to plant productivity [34]. Main stem length was increased with increasing N doses. This could be attributed to the ability of N treatments to enhance plant growth via promoting cellular division and nutrients' uptake from soils [35]. Chlorophyll is an essential biomolecule for photon absorption, transmission, transportation and photosynthetic rate (Pn) in leaves [36]. Lawlor et al. [37] reported that increasing N fertilizer can restore the chlorophyll in plants. Thus, improved growth attributes are owed to higher chlorophyll synthesis with higher N dose in the current study.
Plants require N in large quantity for optimum growth and development. Nitrogen is necessary for various metabolic processes of crop plants. Chlorophyll synthesis and photosynthesis are directly influenced by N [29, [38][39][40][41]. Plant growth is poor under low N availability as it is part of amino acids, nucleic acid, proteins, chlorophyll and hormones [42]. Nonetheless, photosynthesis, flowering and fruit development are positively influenced by optimum N availability resulting in higher crop yields [29, 43,44]. Improved growth attributes are owed to these processes in the current study.  Boron is involved in numerous physiological processes of plants [20,45,46]. Principally, cell wall structural integration, and linkage of B with pectic polysaccharide rhamnogalacturonan regulate porosity and tensile strength of the cell wall [19]. However, limitation or excess of B adversely affect plant growth [21][22][23][24]. The obvious response of B-deficiency in several crops is inhibition of root growth because of reduced cell division [47]. Moreover, long-term deficient B condition provokes lipid peroxidation and reduces the activity of antioxidant enzymes [20,23]. Composition of cereal grains is significantly altered by N [48], whereas sugar concentrations and conversion of simple sugars and complex carbohydrates is also regulated by N availability [49]. Free amino acid concentrations of cereal grain is significantly affected by N supply [50][51][52]. Concentrations of sugars in tubers may be increased by 100% in potatoes under Ndeficiency [53]. Similarly, non-availability of N results in lower concentration of reducing sugars [54]. Increased N fertilizer has also been shown to cause a rise in free amino acid concentrations [55]. The improvement in fruit quality attributes, particularly of sugars is directly linked with N availability. Since both B doses resulted in similar fruit quality attributes with both B doses, it is concluded that N is the main determinant of fruit quality attributes rather than B.