Influence of nitrogen level and plant density on growth parameters and economic efficiency of sunflower cultured in Thua Thien Hue province

. Sunflower ( Helianthus annuus ) cultivation is gaining popularity among Vietnamese farmers. Two separate experiments were conducted from January to April 2018 in a randomized complete block design with three replicates. Four levels of each factor were included in the experiments: 0, 20, 40, and 60 kg/ha for nitrogen and 11, 13, 16, and 20 plants/m 2 for the density. The nitrogen fertilizer (up to 60 kg/ha) substantially affects the leaf area, diameters of stem plant, receptacle and flower head, and cut flower vase life. Low plant density works well on plant height, leaf number, width and length at the flowering stage, with the largest head diameter (17.2 cm) at the 13 plants/m 2 density. Based on the results of both growth ability and economic efficiency, we suggested applying N at 60 kg/ha and planting at 16 plants/m 2 density for growing F1 cut sunflowers in Thua Thien Hue province.


Introduction
Sunflower (Helianthus annuus L.) is grown for various purposes, such as seed and oil, pot and cut flowers, husbandry feeding materials, or green manure [1][2][3]. In Vietnam, sunflower has widely been cultivated in Nghe An and other provinces, such as Lam Dong, Ha Noi, Bac Ninh, and the central areas, primarily for feeding materials, achene harvest, and floral photography.
Sunflower is also favoured as cut or pot flowers because of their big inflorescent diameter and golden petals. Recently, sunflower has been introduced for trial cultivation in Thua Thien Hue to study the potential of local farming conditions.
Cultural methods, including fertilizer application and cultivated density, should be investigated to improve culture practices.
Applying nutrition is one of the most crucial factors influencing plant growth and yield.
Among all plant nutrients, nitrogen (N) is an essential macronutrient and plays a vital role in plant cell formation and division [4]. Nitrogen increases the plant height, head diameter, husk percentage, and seed and oil yields [2,3,5].
However, information regarding the impact of this important nutrient on sunflower cultivation is less available in Vietnam.
An optimum plant density is another important factor that affects the sunflower yield [6]. Plant density significantly affects various cut flowers, such as gerbera [7], chrysanthemum [8], and single-stemmed rose [9]. Most previous studies targeted the optimum plant density for maximum achene, oil content, and protein yield.
There is less information on the cut flower yield of sunflowers in the literature [5,10].

Data collection and analysis
Ten plants were randomly selected in each

Weather conditions
Weather data were obtained from the

Effect of nitrogen level on growth parameters and economic efficiency
Most of the seeds (>90%) germinated within five to six days after sowing (DAS) ( Table 2). The time from sowing to budding, flowering, and harvesting was the shortest at the 60 kg/ha N dose and the longest in the plot without N application.
Although the changes in these values were small, the sunflower speed growth tended to increase when the N levels increased.  Note: y Different letters in each column indicate significant differences among treatments in each growth parameter at p ≤ 0.05. Although the 60 kg/ha N treatment provided the best flower quality, it caused the appearance of diseases and insects ( Table 4). The stalk rot was the most widespread among the diseases, and the insects were primarily green worms, grey worms, and pentatomidae. The highest brown eyespot rate was observed with the 40 kg/ha N dose (22%), whereas the lowest was in the control treatment (13%). Table 5

Effect of plant density on growth parameters and economic efficiency
The germination time was 6-7 days after sowing, and the germination rate was higher than 94% in all treatments ( Table 6)  The plant height fluctuated significantly among treatments. The difference was from 1 to 9 cm within each period in the density treatments (Table 7). On the 15th DAS, the plant height was Regarding leaf growth, the density did not affect the leaf area [1]. Thus, there are the least significantly differences among of leaf number, leaf width, and leaf length between treatments in each period.

Conclusion
In this study, the effects of nitrogen application and plant density on the growth and flowering of F1 sunflowers were investigated. Based on the results, we concluded that the increase in nitrogen application could promote the growth and flowering of sunflowers while plant density did not affect these parameters. We suggest that nitrogen at the 60 kg/ha dose and the 16 plants/m 2 density be applied to cultivation in other seasons with different cultivars to evaluate their impacts.