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Among the cultures used for the production of biofuels, the sunflower is one of the most important. Although some information exists, the water and nutritional needs of sunflower in the north east of Brazil are not well known. To fill knowledge gaps, an experiment was carried out to evaluate the effect of nitrogen (N), phosphorus (P), potassium (K) fertilization and available soil water (ASW) on sunflower yields. The sunflower cultivar Embrapa 122-V2000 was subjected to 44 treatments on a completely randomized design generated by the Baconian Matrix with four rates of N (0, 60, 80 and 100 kg ·ha <sup>-1</sup>), four rates of P2O5 (0, 80, 100 and 120 kg ·ha <sup>-1</sup>), four rates of K2O (0, 80, 100 and 120 kg ·ha <sup>-1</sup>), and four available soil water (ASW) levels (55%, 70%, 85% and 100%) replicated three times. Urea was used as a source of N, triple super phosphate as P and potassium chloride as K. In all the experimental units was applied 2 kg ·B ·ha <sup>-1</sup> as boric acid. The components of production evaluated were dry matter of the head, total number of achenes, total achenes’ weight and 1000 achenes’ weight. The results of this research showed that nitrogen had a significant effect on the dry matter of the head, total number of achenes and total achenes’ weight. Phosphorus affected all production components and potassium affected the total number and the weight of achenes. With the exception of the 1000 achenes’ weight, all the production components of the sunflower increased with the increased ASW level influenced significantly at 0.01 level of probably the total number of achenes. The highest rates of N, P and K (100, 120 and 120 kg ·ha <sup>-1</sup>, respectively) and 100% of available soil water produced the highest production.

Sunflower (Helianthus annuus L.) occupies a prominent place among oilseed crops, as it contributes approximately 12% to global edible oil production. Water and nutrients play an important role in improving seed yield and oil quality of sunflowers [

The number of achenes per head is a reflection of action of N in critical early stages of flowering in sunflower development. The potential number of flowers is determined very early, and subsequently affects the number of achenes and head diameter [^{−}^{1}). However, this increase in head diameter does not continue with further increases of N. Sachs et al. [^{−}^{1}, 41 kg ha^{−}^{1}of K_{2}O and 46 kg∙ha^{−1} of P_{2}O_{5}. The achenes’ oil content increased with the application of K_{2}O and P_{2}O_{5} and the protein content decreased with increasing K_{2}O application.

Shortage of water, which is the most important component of life, limits plant growth and crop productivity, particularly in arid regions. Sunflower is commonly regarded as a plant that is tolerant to drought and it uses water efficiently. Nevertheless, the crop consumes a large amount of total water due to the fact that it produces high yields and a large vegetative bulk. It also has a long growing period coinciding with the warm months of spring and summer. Water stress on sunflower reduces plant height, root length, stomata number and causes early flowering, early maturity and seed yield reduction. Drought adversely influenced leaf area leaf area, days to maturity, leaf diameter, 1000-achene weight and achene yield per plant [

The present research aimed to investigate the yield of sunflower affected by the interaction between NPK fertilization treatments and irrigation regimes under semiarid Brazilian conditions.

The experiment was carried out from March to July 2011 under greenhouse conditions at the Agricultural Engineering Department of the Federal University of Campina Grande, Paraiba State, Brazil.

The sunflower cultivar used was the Embrapa 122-V2000. A total of 44 treatments on a completely randomized design generated by the Baconian Matrix (^{−1}), four of P_{2}O_{5} (0, 80, 100 and 120 kg∙ha^{−1}), four of K_{2}O (0, 80, 100 and 120 kg∙ha^{−1}), four available soil water (ASW) levels (55%, 70%, 85% and 100%) and three replicates resulting in 132 experimental units. Urea was used as a source of N; triple super phosphate as P and potassium chloride as K. In the soil in all the experimental units was applied, as pure solution, 2 kg∙B∙ha^{−1} as boric acid.

Each experimental unit consisted of a plastic pot filled with 32 kg of an Alfisol with the following attributes using the procedures recommended by [^{−1}; silt = 117.30 g∙kg^{−1}; clay = 329.30 g∙kg^{−1}; pH (H_{2}O) = 6.6; Ca^{2+} = 1.45 cmol_{c}∙kg^{−1}; Mg^{2+} = 1.65 cmol_{c}∙kg^{−1}; Na = 0.17 cmol_{c}∙kg^{−1}; K^{+} = 0.21 cmol_{c}∙kg^{−1}; H^{+} + Al^{3+} = 0.79 cmol_{c}∙kg^{−1}; organic matter = 22.2 g∙kg^{−1}; Available P (Mehlich) = 8.1 mg∙kg^{−1}.

Soil water content was monitored daily at three depth intervals: 0 - 10, 10 - 20 and 20 - 30 cm, using a Frequency Domain Reflectometry (FDR) segmented probe, inserted into the soil through an access tube installed in the pots. The volume of water required to maintain ASW for each treatment was calculated based the difference between field capacity and the permanent wilting point, and the FDR measurements. Irrigation was performed daily.

Five sunflower seeds were sown directly in the pots at a 2 cm depth. Twenty days after sowing, seedlings were thinned to one plant per pot.

When the experiment was finalized, plants were harvested and measured for dry matter (grams) of the head (DMH), total number of achenes (TNA), total weight (grams) achenes (TWA) and 1000 achenes weight (W1000A).

The results were analyzed statistically through the analyses of variance (ANOVA) described by [

Treatments. | N | P_{2}O_{5} | K_{2}O | Water | Treatments | N | P_{2}O_{5} | K_{2}O | Water |
---|---|---|---|---|---|---|---|---|---|

------kg∙ha^{−1}----- | % | ------ kg∙ha^{−1}----- | % | ||||||

1 | 0 | 0 | 0 | 55 | 23 | 0 | 0 | 0 | 85 |

2 | 0 | 80 | 80 | 55 | 24 | 0 | 80 | 80 | 85 |

3 | 80 | 80 | 80 | 55 | 25 | 80 | 80 | 80 | 85 |

4 | 100 | 80 | 80 | 55 | 26 | 100 | 80 | 80 | 85 |

5 | 60 | 0 | 80 | 55 | 27 | 60 | 0 | 80 | 85 |

6 | 60 | 100 | 80 | 55 | 28 | 60 | 100 | 80 | 85 |

7 | 60 | 120 | 80 | 55 | 29 | 60 | 120 | 80 | 85 |

8 | 60 | 80 | 0 | 55 | 30 | 60 | 80 | 0 | 85 |

9^{*} | 60 | 80 | 80 | 55 | 31^{*} | 60 | 80 | 80 | 85 |

10 | 60 | 80 | 100 | 55 | 32 | 60 | 80 | 100 | 85 |

11 | 60 | 80 | 120 | 55 | 33 | 60 | 80 | 120 | 85 |

12 | 0 | 0 | 0 | 70 | 34 | 0 | 0 | 0 | 100 |

13 | 0 | 80 | 80 | 70 | 35 | 0 | 80 | 80 | 100 |

14 | 80 | 80 | 80 | 70 | 36 | 80 | 80 | 80 | 100 |

15 | 100 | 80 | 80 | 70 | 37 | 100 | 80 | 80 | 100 |

16 | 60 | 0 | 80 | 70 | 38 | 60 | 0 | 80 | 100 |

17 | 60 | 100 | 80 | 70 | 39 | 60 | 100 | 80 | 100 |

18 | 60 | 120 | 80 | 70 | 40 | 60 | 120 | 80 | 100 |

19 | 60 | 80 | 0 | 70 | 41 | 60 | 80 | 0 | 100 |

20^{*} | 60 | 80 | 80 | 70 | 42* | 60 | 80 | 80 | 100 |

21 | 60 | 80 | 100 | 70 | 43 | 60 | 80 | 100 | 100 |

22 | 60 | 80 | 120 | 70 | 44 | 60 | 80 | 120 | 100 |

^{*}Reference level used by the sunflower growers of the region.

The ANOVA results are presented in

For plants treated with N the DMH increased with increasing N rate whose data were fitted to a quadratic regression model (^{−1}) 25% higher than the DMH found for the control treatment. The DMH was also influenced significantly by increasing P doses, and these results were fitted to a quadratic regression model (^{−1}) showing a superiority of 556% when compared with the reference level. The results corroborate [

Potassium rates did not affect the DMH values (

The DMH increased linearly with increasing ASW from 18 to 24 g, for the lowest (55%) to the greatest ASW doses (100%), respectively (

Source | DF | Mean square | Pr > F | Mean square | Pr > F | Mean square | Pr > F | Mean square | Pr > F |
---|---|---|---|---|---|---|---|---|---|

DMH | TAN | TWA | W1000A | ||||||

N | 3 | 141.76 | <0.0001 | 77234.76 | <0.0001 | 51.95 | 0.0013 | 618.67 | 0.0625 |

P | 3 | 1682.16 | <0.0001 | 423580.92 | <0.0001 | 669.36 | <0.0001 | 5869.42 | <0.0001 |

K | 3 | 5.40 | 0.7513 | 5677.88 | 0.0009 | 27.59 | 0.0338 | 270.98 | 0.3511 |

ASW | 3 | 61.78 | 0.0047 | 50590.70 | 0.0020 | 46.66 | 0.0026 | 54.61 | 0.8804 |

N^{*} ASW | 9 | 19.08 | 0.1894 | 32852.11 | 0.0010 | 15.84 | 0.0924 | 249.73 | 0.4311 |

P^{*} ASW | 9 | 25.66 | 0.0593 | 14096.31 | 0.1659 | 24.62 | 0.0079 | 252.92 | 0.4211 |

K^{*} ASW | 9 | 18.56 | 0.2063 | 7348.53 | 0.6415 | 5.33 | 0.8070 | 250.49 | 0.4287 |

Contrast | DF | ||||||||

N linear | 1 | 109.05 | 0.0054 | 207053.43 | <0.0001 | 142.18 | 0.0002 | 43.10 | 0.6760 |

N quadratic | 1 | 177.65 | 0.0004 | 1227.04 | 0.7202 | 0.75 | 0.7745 | 229.97 | 0.3353 |

P linear | 1 | 2417.57 | <0.0001 | 556461.89 | <0.0001 | 1031.08 | <0.0001 | 8932.15 | <0.0001 |

P quadratic | 1 | 1139.87 | <0.0001 | 318864.65 | <0.0001 | 644.27 | <0.0001 | 2925.41 | 0.0008 |

K linear | 1 | 2.22 | 0.6850 | 207053.43 | <0.0001 | 142.18 | 0.0002 | 43.10 | 0.6760 |

K quadratic | 1 | 11.04 | 0.3664 | 1227.04 | 0.7202 | 0.75 | 0.7745 | 229.97 | 0.3353 |

ASW linear | 1 | 124.20 | 0.0030 | 62694.06 | 0.0118 | 101.99 | 0.0012 | 130.48 | 0.4675 |

ASW quadratic | 1 | 5.39 | 0.5275 | 87700.72 | 0.0031 | 17.67 | 0.1677 | 26.92 | 0.7411 |

N × ASW | 1 | 51.84 | 0.0522 | 53772.80 | 0.0195 | 21.65 | 0.1272 | 555.46 | 0.1357 |

P × ASW | 1 | 0.0007 | 0.9941 | 6512.35 | 0.4100 | 9.58 | 0.3085 | 359.46 | 0.2290 |

K × ASW | 1 | 19.93 | 0.2258 | 3238.75 | 0.5608 | 6.20 | 0.4122 | 591.56 | 0.1238 |

ASW was increased from 55% to 100%. Similarly [

The total number of achenes (TAN) was significantly affected, at 1% level of probability, by the increase of N, P and K doses and available soil water. The interaction among N and ASW at 1% level of probability was also significant (

The increase of the number of achenes with the N doses was fitted to a linear model as observed in

The effect of P on the TAN was fitted to a quadratic model (^{−}^{1}. This kind of adjustment was also reported by [^{−}^{1}.

The increase in available soil water content increased, in a quadratic manner, the achene number from 301 to 448 when water increased from 55% to 100% ASW, an increase of 52.30% (

The effect of ASW on the sunflower is similar to other studies [

The interaction among N and the available soil water content on TAN was significant to the 1% level of probability (

with the N application and with the ASW. Thus the highest total number of achenes (844) was obtained with the highest N application (100 kg∙ha^{−1}) and the highest ASW (100%).

The total achenes’ weight was significantly influenced by increasing N, P, K rates and the ASW (^{−1}). ^{−1}). These highest weights were 136% and 728% superior to the control treatment, respectively.

The TWA increased linearly with increasing K (^{−}^{1}. The ASW treatments increased the total achenes weight linearly obtaining 8.94 and 15.22 g for the lowest and highest treatment, respectively. There was an increase of 70.25% between the lowest and highest water treatments.

The 1000 achenes’ weight of sunflower was only affected by the P application, at the 1% level of probability. The regression was adjusted to a quadratic model (^{−}^{1} (60 g).

The dry matter of the head, the total achenes’ number and the total achenes’ weight increased significantly with the nitrogen doses applied.

All the sunflower variables studied increased significantly with the phosphorus doses applied.

The total achenes’ number and the total achenes’ weight increased significantly with the potassium doses applied.

With the exception of the 1000 achenes’ weight, whose effect was not significant, all the production components of the sunflower increased with the available water in the soil.

The interaction between N and the available soil water content was significant only for the TAN. It increased with the N application and with ASW.

The highest rates of N, P, and K (100, 120 and 120 kg∙ha^{−1}, respectively) and the 100% available soil water produced the highest production.

Thanks to the Coordination of Improvement of Higher Education (CAPES) for the scholarship award to the second author at the Graduate School.