The experiment was conducted at the Agronomic Research Area of, University of Sargodha (32˚05''N, 72˚67''E), Pakistan during the spring seasons of 2013. The soil is sandy clay loamy somewhat poorly drained with pH ranging from 7.9 - 7.33. The nitrogen level was 0.066 to 0.052 are shown in Table 1.

The experiment was set in a Split plot arrangement under RCBD having 3 replications. The net plot size was 4.2 m × 6 m having row to row spacing 70 cm and plant to plant distance 20 cm. Sunflower hybrids (Hysun-33, S-278) were kept in main plots and N levels (0, 45, 90,135 and 180 kg/ha) in sub plots. The crop was sown by dibbler method using seed rate of 5 kg/ha. Phosphorus and potash were applied at the rate of 80 - 40 kg/ha in all plots. Nitrogen, Phosphorus and potassium were used in the form of urea, DAP and Potassium (K₂SO₄). The nitrogen

was used in three splits, 1/3 dose of nitrogen and all of the phosphorous and potash fertilizer were applied at the time of sowing. Remaining 2/3 of nitrogen was used in two splits, at first irrigation and flowering stage. All other agronomic practices such as hoeing, weeding, irrigation and plant protection measure were kept normal for the crop.

A sample of 5 plants were selected randomly from each plots for measuring growth parameters such as leaf area index, leaf area duration. At final harvest 10 plants fortnightly were selected form each plots for measurement of days to maturity, number of achene’s per head, 1000-achene weight(g), achene’s yields (kg/ha) and total dry matter (kg). First growth sampling was conducted after 15 days of sowing, then each sampling every 10 days interval. The leaf area was measured from 10 g fresh leaves from harvested material from each fifteen days interval. An area meter (CI 202. Portable Laser Leaf Area Meter) was used for the measurement of leaf area.

Data collected on growth and yield components was analyzed statistically by employing the Fisher’s analysis of variance technique and significant of treatment means was tested using least significance difference (LSD) test at 5% probability level (Steel et al. [16] ) Calculating Root Mean Square error Value (RMSE) i.e. Residual variation among observed and stimulated data tested accuracy of model.

Leaf area index is the ratio of leaf area per unit land area. It is unit less quantity due to ratio of same units. Greater the leaf area index, increase the capability of plants to harvest the solar energy and converted into chemical energy. The two sunflower hybrids were statistically different from each other, maximum leaf area index (3.88) was observed in hybrid Hysun-33 as compared to S-78 (3.62) as shown in Table 1. Same results are reported by (Kho [17] ; Aleman et al. [18] ). Leaf area index was significantly increased ranging 3.05 to 4.52 with higher supply of nitrogen fertilizer from 0 to 180 kg∙ha⁻¹ respectively. The application of nitrogen 180 kg∙ha⁻¹ attained the highest leaf area index (4.52), followed by N₄ (135 kg N ha⁻¹) treatment 4.26. The leaf area index (3.05) was achieved without use of nitrogen fertilizer and it was statistically at par with the plot where nitrogen was applied at the rate 45 kg∙ha⁻¹ (3.23). These findings were in close conformity with result of Nasim et al. [19] ) who reported that increase leaf area index with enhancing the nitrogen levels. The statistically significant results were also observed during the interaction between hybrids and different nitrogen levels as shown in Table 2, nitrogen application 180 kg∙ha⁻¹ in hybrid Hysun-33 attained maximum leaf area index (4.76) statistically at par with same application level of S-78 (4.27). The lowest leaf area index (2.95) was recorded where no application of nitrogen fertilizer.

Figure 1 represents that leaf area index significantly affected by different nitrogen levels with hybrid Hysun-33. In all nitrogen treated plots LAI was enhanced with increasing the nitrogen levels. Maximum leaf area index (4.7) was attained by hybrid Hysun-33 with nitrogen level of 180 kg∙ha⁻¹ on 70 days and lowest (3.2) was recorded in control treatment. The leaf area index was increased in all fertilized nitrogen treated plots by applying of various levels with S-78 hybrid was shown in Figure 2. Show that highest leaf area index (4.4) was achieved by hybrid S-78 with nitrogen level of 180 kg∙ha⁻¹ on 70 days and lowest (2.95) was noted in treatment where no nitrogen was applied.

Data reported in Table 2 demonstrated that Leaf area duration was statistically significant for different hybrids. The maximum leaf area duration (172.47) was

recorded in Hysun-33 as compared to S-78 hybrid (154). Applying different levels of nitrogen fertilizer, the maximum leaf area duration (198.33) was observed in N₅ treatment (180 kg N ha⁻¹), followed (187.40) by the Treatment N₄ (135 kg∙ha⁻¹). However, the minimum leaf area duration (127.75) was recorded in control plots. These results are in close agreement with the finding of and also was same results (Albrizio et al. [20] ) who concluded that leaf area duration was increase by enhancing the nitrogen application level.

The interaction between hybrids and nitrogen application affecting the leaf area duration was significant as shown in Table 3. Maximum leaf area duration (212.83) was noted in H₁N₅ combination which was followed by same hybrid at nitrogen level of 135 kg∙ha⁻¹ having LAD of 198.15. Hybrid S-78 attained maximum leaf area duration (183.83) at rate of nitrogen 180 kg∙ha⁻¹ which was statistically at par with nitrogen level 135 kg∙ha⁻¹ of same hybrid achieved LAD 176.65.

Mean having different letters differ significantly from each other by Tukey HSD (P = 0.05).

The penology of crop plants is an important yield contributing parameter. The different sunflower hybrids have taken varying number of days to anthesis (R.5.3) stage. Hybrid hysun-33 took more days to R.5.3 stage (70 days) as compared to S-78 hybrid (59 days) as shown in Table 2. Difference for days to anthesis among hybrids was also observed by (Akhtar [21] ; Nasim [22] ; Ali et al. [23] ). In all the treated plots with nitrogen, days to anthesis were significantly different over control. The 180 kg N ha⁻¹ application significantly increases days for anthesis than 135, 90, 45, 0 kg N ha⁻¹. Nitrogen levels at the rate of 135, 90, 45 kg∙ha⁻¹ took 65, 64.16, 63.33 days for completion of 80 % anthesis which were statistically at par with each other. These results are supported by the findings of (Sadras [24] ; Cechin et al. [25] ). The interaction between hybrids and nitrogen levels for R.5.3 stage was non-significant.

The maturity of plant plays important role for obtaining the highest yield of crop. Data (Table 2) indicate that plant maturity was significantly affected by different hybrids and nitrogen levels. The two sunflower hybrids were statistically different from each other. Hysun-33 taken more days to maturity (120 Days) as compared to S-78 (102 days). The similar result was observed from (Bakht et al. [15] ; Bakht et al. [26] ). The plant maturity was enhanced by different levels of nitrogen fertilizer application. The 180 and 135 kg N ha⁻¹ showed at par effect on days to plant maturity. The lowest number of days for R. 9 was recorded in control plot. These results are close agreement with (Bakht et al. [15] ) also observed that plant maturity was enhance with increasing the nitrogen rates. The statistically significant results were observed during the interaction between hybrids and varying nitrogen rates. The data in Table 3 showed that more no of days (123) was required to plant maturity in hybrid Hysun-33 with treatment level of 180 kg Nha⁻¹ at par with 135 kg N ha⁻¹ (121). The S-78 hybrid was observed in treatment H₂N₅ and H₂N₄ with nitrogen level of 180 and 135 kg∙ha⁻¹, required days 105 and 103 respectively. These are statistically at par with each other.

Head diameter is an important character indicating the plant yield. Data in Table 2 presents that head diameter was significantly affected by different sunflower hybrids as well as different levels of nitrogen fertilizer. The hybrid S-78 produced significantly greater size of head diameter (21 cm) as compared to Hysun-33 (16 cm).Head diameter was enlarged with increasing the rates of fertilizer from 0 to 180 kg N ha⁻¹. The maximum size of head diameter (22 cm) was recorded in treatment N₅ (180 kg N ha⁻¹) and it is statistically at par with nitrogen rate of 135 kg∙ha⁻¹The lowest head diameter (16 cm) was noted in treatment N₁ the control plots. The statistically significant results were also observed during the interaction between hybrids and different nitrogen levels. The data in Table 3 exhibited that highest head diameter 25 cm and 20 cm of S-78 hybrid was observed in treatment H₂N₅ and H₂N₄ with nitrogen level of 180 and 135 kg∙ha⁻¹, respectively. The Hysun-33 hybrid with same level of Nitrogen 180 and 135 kg∙ha⁻¹ produced smaller head diameter of 18 cm and 17cm respectively, which was statistically at par with the control treatment of S-78. These results are supported by the findings of Khaliq and Cheema [27] .

The data regarding number of achene’s per head is given in Table 2 exhibited that hybrid S-78 produced more numbers of achene per head (1054) as compared to Hysun-33 (917) which were statistically different from each other. Number of achenes per head was significantly increased with higher nitrogen fertilizer rates. The application of 180 kg N ha⁻¹ produced highest number of achenes per head (1199) and it is statistically at par with the plot where nitrogen was applied at the rate 135 kg∙ha⁻¹. The lesser number of achenes per head (767) was recorded in control treatment. The interaction between hybrids Hysun-33 and S-78 with varying levels of nitrogen was significant. Results presented in Table 3 revealed that highest number of achenes per head (1320) was noted in treatments N₅ (180 kg N ha⁻¹) and N₄ (135 kg N ha⁻¹), respectively, in S-78 hybrid. The same treatment N₄ (135 kg N ha⁻¹) for Hysun-33 produced 960 number of achenes per head which was statistically at par with the application of nitrogen fertilizer 45 Kg∙ha⁻¹ of hybrid S-78. These results are supported by the findings of (Nazir et al. [28] ; Akhtar and Malik. [29] ) who concluded that interaction between hybrids and nitrogen application was significant.

The most essential parameter is achene yield in sunflower crop and number of components are included for obtaining the highest yield are, head diameter, number of achenes per head and 1000 achene weight. The statistically significant results were observed between hybrids as shown in Table 2. The hybrid S-78 produced higher achene yield (3714 kg∙ha⁻¹) than Hysun-33 (3464 kg∙ha⁻¹). The data (Table 2) indicated that increased nitrogen levels promote the achene yield significantly. Among the nitrogen levels N₅ (180 kg∙ha⁻¹) produced significantly higher achene yield (4015 kg∙ha⁻¹), which was statistically at par with N₄ treatment where the yield obtained was 3970 kg∙ha⁻¹. However, the lowest achene yield (3067 kg∙ha⁻¹) was recorded in control treatment.

The interaction of hybrids and nitrogen application was found to be significant in Table 3. Maximum achene yield (4095 kg∙ha⁻¹) was noted in H₂N₅ combination which was statistically at par with H₁N₄ (3884 kg∙ha⁻¹) and H₂N₄ (4055 kg∙ha⁻¹). The lowest achene yield was noted in hybrid Hysun-33 (2923) without application of nitrogen fertilizer. These results are in close agreement with the finding of (Khaliq and Cheema. [28] ; Akhtar and Malik. [29] ; Hussain et al. [30] who concluded that interaction was significant between hybrids and nitrogen application.

The data (Table 2) depict that total dry matter was significantly affected by different hybrids and applying of various nitrogen fertilizer levels. The total dry matter production of Hysun-33 hybrid was more (9747 kg∙ha⁻¹) as compared with S-78 (8593 kg∙ha⁻¹). As regards of different nitrogen fertilizer application levels, total dry matter production in all the fertilized plots was significantly higher than treatment receiving no fertilizer. The highest rate of nitrogen fertilizer (180 kg N ha⁻¹) showed the greatest total dry matter 10655 kg∙ha⁻¹ however this rate of nitrogen fertilizer was statistically in par with 150 kg N ha⁻¹. The plots was treated with nitrogen level 0 and 45 kg∙ha⁻¹ produced total dry matter (6745 and 8103 kg∙ha⁻¹) respectively, which was statistically at par with each other.

The statistically significant interaction was observed between hybrids Hysun-33 and S-78 with varying levels of nitrogen affecting the total dry matter. The data in Table 3 concluded that Hysun-33 at 90 kg N ha⁻¹ produced total dry matter (9726 kg∙ha⁻¹), which was statistically in par with hybrid S-78 (9723 kg∙ha⁻¹) at application of nitrogen level 180 kg∙ha⁻¹. The highest total dry matter (11586 kg∙ha⁻¹) noted in hybrid Hysun-33 with nitrogen application of 180 kg∙ha⁻¹ as well as lowest total dry matter was obtained from S-78 with least application of nitrogen fertilizer (control). The results are close with finding of (Nasim et al. [20] ) who reported a significant interaction was between hybrids and nitrogen application. There was positive and strong liner relationship between total dry matter and achene yield with value of (R² = 0.41) shown in Figure 3.