Effect of combined nitrogen and sulphur fertilization on yield and qualitative parameters of Camelina sativa [L.] Crtz. (false flax)

Abstract

Camelina (Camelina sativa (L.) Crtz., false flax) is described as a species requiring fewer inputs than other oilseed crops thus making it an interesting alternative in sustainable cropping systems. As information on the combined effects of nitrogen and sulphur on camelina yield and quality parameters is meagre, a pot fertilization experiment was carried out with nitrogen applied as NH₄NO₃ at three increasing rates equivalent to a range from 63 up to 127 kg ha⁻¹. These treatments were combined with sulphur additions applied as (NH₄)₂SO₄ to achieve a soil sulphate content of 25 or 45 mg kg⁻¹, respectively (equivalent to 75 and 135 kg S ha⁻¹). The medium and high nitrogen rates combined with the low sulphur fertilization level increased the number of branches per plant compared with the lowest nitrogen fertilization at the same sulphur rate. Camelina seed yield increased with increasing nitrogen doses at the same low sulphur level, whereas straw yield increased only at the highest rate of nitrogen. Thousand-seed mass increased at the highest nitrogen dose and with the low sulphur application rate. In addition, the increase in nitrogen fertilization reduced seed oil content from 39.8% to 37.1%. A highly negative correlation was observed between oil and protein content of seeds. Nitrogen fertilization increased total oil yield and total protein yield. Differences in crop parameters between the sulphur treatments were not significant statistically, although the higher sulphur treatment tended to increase seed yield as well as oil and protein content compared with the low sulphur treatment. Thus, the combined application of N and S as mineral fertilizers is only recommended when growing camelina on sulphur-deficient soils if the aim is to achieve both high oil and protein production.

Keywords:

• Oil content
• protein content
• seed
• straw
• TSM
• yield

Introduction

Camelina (false flax, gold-of-pleasure, Camelina sativa [L.] Crtz.) belongs to the Brassicaceae family and is a flexible oilseed crop that can be grown under different climatic and soil conditions (Zubr, 2003a). The interest in camelina as a crop is due to its low input requirements and resistance to drought (Putnam et al., 1993; Vollmann et al., 1996, 2005, 2007). Like olive and rapeseed oils, camelina oil in the diet can significantly reduce serum cholesterol concentrations in humans, among other effects (Karvonen et al., 2002). Camelina oil is a rich source of omega-3-fatty acids and increased consumption of camelina oil could improve the general health of the population (Zubr, 1997; Rokka et al., 2002; Zubr & Matthäus, 2002; Lu & Kang, 2008). The by-product from pressed camelina oil production (oil cake) is traditionally used as an animal feed ingredient (Zubr, 2003b). In non-food applications, camelina oil can be utilized in environmentally safe paints and coatings, in cosmetics and dermatological products and as a fuel for diesel engines (Bonjean & Le Goffic, 1999; Bernardo et al., 2003).

While its agronomic features have been considered acceptable, the oil content of camelina has been described as being low, in the range of 32–44% seed dry matter (Seehuber, 1984; Marquard & Kuhlmann, 1986; Vollmann et al., 1996; Zubr, 2003a). In comparison with camelina the content of oil in oilseed rape seeds ranges in principle from 39–45% (Lošák, 2003). Increasing the seed oil content would thus improve the competitiveness of camelina relative to other oilseed crops (Vollmann et al., 2005). The protein content in the seed is also an important parameter with regard to the nutritional value of the oil cake (Zubr, 2003a). The content of crude protein in camelina seed ranges from 25–45% (Nordestgaard, 1961; Plessers et al., 1962; Korsrud et al., 1978; Marquard & Kuhlmann, 1986).

Nitrogen is one of the most important nutrients involved in the production of oilseed crops (Nuttall & Malhi, 1991; Urbaniak et al., 2008a). A number of studies have evaluated the N requirements of C. sativa, with variable results. According to studies conducted in Europe (Zubr, 1997), C. sativa can be successfully grown with N levels of 100 kg N ha⁻¹. Camelina sativa grown in trials in the northern United States responded well to an N rate of 90 kg N ha⁻¹ (Budin et al., 1995). Nitrogen fertilization influences the overall amounts and relative proportions of seed and straw yield produced (Agegnehu & Honermeier, 1997), as well as seed oil and protein content and fatty acid composition (Honermeier & Agegnehu, 1996; StraŠil, 1997; Zadernowski et al., 1999; Szczebiot, 2002; Urbaniak et al., 2008a; Zheljazkov et al., 2008).

Oilseed plants are usually very sulphur-demanding in terms of yield and oil content of the seed. In the Czech Republic and elsewhere, the supply of S from the atmosphere has declined in recent decades and thus S deficiency is becoming common in crops (Lošák, 2003; Balík et al., 2009). Sulphur deficiency inhibits the plant use efficiency of N from fertilizers (Tandon, 1992; Schnug et al., 1993) and may therefore increase N losses. If the S supply is deficient, then increasing rates of N can intensify the deficiency symptoms and further reduce yields (Janzen & Bettany, 1984). Little attention has been devoted to the problem of fertilizing camelina with S and the results presented to date on the effects of S (Pearson et al., 1999) or of S and magnesium (Mg) application (Szczebiot, 2002) on camelina seed yields are contradictory. Nevertheless, fertilizers containing S are believed to be effective in camelina nutrition and application of 250 kg ha⁻¹ of gypsum prior to sowing is a standard fertilization measure (Urbaniak et al., 2008b).

The aim of the present study was therefore to explore the effect of combined N and S fertilization on the yield and qualitative parameters of false flax.

Materials and methods

A pot trial with 4 replications of every treatment was set up outdoors on 20 March 2005 at the experimental site of Mendel University. A total of 6-litre Mitscherlich pots were filled with 6 kg of fresh medium-heavy soil characterized as a fluvial.

Soil analysis

The soil was extracted using the Mehlich III method (CH₃COOH, NH₄NO₃, NH₄F, HNO₃ and EDTA). The content of available P (138 ppm) in the extract was determined colorimetrically and the content of available K (226 ppm), Mg (167 ppm) and Ca (2784 ppm) by means of AAS (Zbíral, 2002). All these nutrients were in high (P) or good supply. Before specifying the content of S- in the soil (25 mg kg⁻¹) extraction with demineralized water was conducted at a 1:5 ratio for 6 hours on a rotary shaker. Measurement proper was performed using capillary zonal electrophoresis (CZE) on the CES-1 apparatus (Dionex Corp., USA) with a silica capillary. The content of mineral nitrogen (Nmin) in the soil was low (3 mg kg⁻¹); N-NH₄ ⁺ was determined using colorimetry (dle Nesslera) and N-NO₃ ⁻ using the ion-selective electrode – ISE. The ion-selective electrode was used to determine the pH value and pH/CaCl₂ was 7.5 – slightly alkaline (Zbíral, 2002).

False flax cultivar and N and S applications

False flax cv. Calena was sown at a rate of 25 seeds per pot (=4 kg ha⁻¹) on 29 March 2005 and the plants began to emerge within 7 days. At the 4-leaf stage (27 April 2005), the plants were thinned to 7 plants per pot and each pot was fertilized with a starter dose of 0.2 g N (=21 kg N per ha) in the form of ammonium nitrate (34.5% N). Zubr (1997) recommends an N application at the 4–6-leaf stage rather than earlier to prevent any loss of N from the soil by leaching. At the leaf rosette (9–11 leaves) stage (9 May 2005), the plants were thinned to 6 plants per pot and side-dressed with combined N and S according to the plan presented in Table I. Sulphur was applied as ammonium sulphate (20.4% N, 24% S) and N was balanced by applying ammonium nitrate.

Table I. Experimental layout of the nitrogen/sulphur fertilization experiment.

Treatment no.	Description	N – dose (g pot^−1)	S- ^− content in soil (mg kg^−1)
1	N1S0	0.6	25
2	N1S1	0.6	45
3	N2S0	0.9	25
4	N2S1	0.9	45
5	N3S0	1.2	25
6	N3S1	1.2	45

Plant analysis

During the growing season, the aboveground biomass was repeatedly sampled to assess the dry matter weight of one plant per pot and the N and S content of the biomass. One plant per pot was sampled prior to N and S application at the leaf rosette stage (9 May 2005), then in the stage of stem elongation growth (17 May 2005) and in the stage of flowering (7 June 2005). The N content in plant biomass was determined by coulometry after wet combustion in H₂SO₄+H₂O₂ on an SL 02 apparatus (Zbíral, 1994). For S determination, the plant biomass samples were first wet-mineralized with a mixture of HNO₃ and H₂O₂. The S content in the samples was then determined using the ICP-OES method (Zbíral, 1994) on a JOBIN YVON 24 apparatus.

Irrigation and harvest

The pots were watered with demineralized water to a level of 60% of the maximum water-holding capacity and were kept free of weeds.

On 18 July 2005, four plants from each pot were harvested at the stage of full maturity and the number of branches per plant, seed and straw weight per plant, 1000-seed weight, oil content and protein content were determined.

Seed analyses

Sub-samples of about 15 g of dry seeds were analysed for oil and protein content by near-infrared reflectance spectroscopy (NIRS) using an InfraAnalyzer model 450 spectrometer and IDAS CALIBRATION software (Bran & Luebbe, Norderstedt, Germany). NIRS calibrations with validation r ² values of 0.94 and 0.90 for protein and oil content, respectively, had previously been developed with reference samples from eight different environments. All seed oil and protein content data were expressed as% of oven-dried seed dry matter.

Statistical analyses

The data were statistically analysed by means of the statistical program Statistica, using the variance analysis (ANOVA) with N rate and S rate as fixed effects. Tukey's test at α = 0.05 was used to determine statistically significant differences between the factors within methods of subsequent tests.

Results and discussion

Average content of plant nitrogen and sulphur, N/S ratio and dry matter of plants

Table II shows the contents of N and S in the aboveground biomass of camelina at the leaf rosette stage (including the N/S ratio) and the dry matter content of one plant before combined fertilization with N and S.

Table II. Dry matter of 1 plant and nitrogen and sulphur content in plant in successive dates of sampling.

			% in dry matter
Treatment no.*	Description	Dry matter of 1 plant (g)	N	S	N/S
Prior to N and S application at the leaf rosette stage (9 May 2005)
–	–	0.34	5.81	0.62	9.37
In the stage of stem elongation growth (17 May 2005)
1	N1S0	1.01	5.58	0.70	7.97
2	N1S1	1.13	5.31	0.65	8.16
3	N2S0	1.25	5.29	0.66	8.01
4	N2S1	1.20	5.40	0.61	8.85
5	N3S0	1.15	5.44	0.65	8.36
6	N3S1	1.09	5.31	0.62	8.56
In the stage of flowering (7 June 2005)
1	N1S0	9.47	2.14	0.53	4.03
2	N1S1	10.10	2.01	0.55	3.65
3	N2S0	11.03	2.43	0.51	4.76
4	N2S1	12.64	2.55	0.64	3.98
5	N3S0	11.00	2.84	0.48	5.91
6	N3S1	14.58	3.24	0.57	5.68
*For an explanation of treatment no. see Table I.

The results of chemical analyses at the stem elongation stage are shown in Table II. In these plants, which were sampled 8 days after combined N and S application, there were no marked differences in the dry matter content of the plant or in N and S content. The N content fluctuated between 5.29 and 5.58% and the S content between 0.61 and 0.70%. The N/S ratio ranged between 7.97 and 8.85 and was balanced. Haneklaus and Schnug (2001) reported that to obtain maximum yields of species in the family Brassicaceae, to which camelina belongs, the N/S ratio must be 4–8/1, and thus the values found in the present study are high but close to the optimum range. The dry matter content of plants from the individual treatments was balanced.

In the last sampling (at the flowering stage), the N and S content, particularly N, decreased compared with the previous sampling (Table II). This was most likely due to the ‘dilution effect’, where biomass production is not matched by a sufficient supply of nutrients (Mengel & Kirkby, 2001). The N and S contents in the biomass increased with increasing fertilizer dose (Table IV). This confirms findings by Urbaniak et al. (2008a), who reported that the total N content in whole plant (aboveground) biomass of the same cultivar of C. sativa increased from 1.04% to 1.46% with increasing N rate from 0 to 120 kg ha⁻¹. Similarly, Svecnjak and Rengal (2006) reported that increasing the N fertilizer rate resulted in a corresponding increase in the N content of Brassica napus biomass.

The N/S ratio in camelina plants in the present study ranged from 3.65 and 5.91 (Table II), and was considerably lower than at the previous sampling date (7.97–8.85; Table II). This positive trend can be attributed to the adequate S content in the tissues of camelina after S fertilization.

The dry matter weight of single plants ranged from 9.47 to 14.58 g (Table II). At each individual rate of N fertilization, the application of S further stimulated dry matter production by the plant.

Branches per plant

The number of branches per plant ranged between 10.44 and 15.92 and increased with increasing N fertilization but with no effect of sulphur (Table III). The N₂ and N₃ doses applied with a S₁ dose significantly increased the number of branches per plant compared with N₁S₁. The number of branches in treatments 4–6 was significantly higher than in treatment 2. The average number of branches was highest in treatment 6 and differed significantly from treatments 1–3. The number of branches also increased significantly (to 13.04 and 14.99) with the N₂ or N₃ treatments compared with N₁ (10.66). These findings are in agreement with Agegnehu and Honermeier (1997), who reported that increasing N fertilization rate from 0 to 120 kg N ha⁻¹ increased the number of branches per camelina plant from 4.6 to 7.3.

Table III. Seed and straw yield parameters as influenced by N–S treatments.

Treatment no.	Description	Branches (no. plant^−1) mean±SD	Seed yield (g plant^−1) mean±SD	Straw yield (g plant^−1) mean±SD	Straw/seed ratio mean±SD	1000-seed weight (g) mean±SD
1	N1S0	10.88 bd	3.83 b	15.23 b	3.98 c	1.07 cd
		±0.83	±0.19	±0.14	±0.23	±0.06
2	N1S1	10.44 b	3.75 b	15.06 b	4.02 ac	1.09 cd
		±0.55	±0.25	±0.38	±0.28	±0.02
3	N2S0	11.38 bcd	4.44 d	15.80 cdb	3.56 cd	1.04 bc
		±0.78	±0.18	±0.43	±0.16	±0.04
4	N2S1	14.70 ad	5.04 c	15.72 bc	3.13 bd	1.09 cd
		±1.36	±0.43	±0.93	±0.26	±0.08
5	N3S0	14.06 ad	5.11 ac	17.59 ad	3.45 bd	1.17 ad
		±1.60	±0.25	±0.96	±0.30	±0.08
6	N3S1	15.92 a	5.62 a	17.14 ac	3.05 bd	1.08 cd
		±2.95	±0.13	±1.43	±0.23	±0.03
Average	N1	10.66 b	3.79 c	15.15 b	4.00 a	1.08 a
		±0.69	±0.21	±0.28	±0.24	±0.04
	N2	13.04 a	4.74 b	15.76 b	3.34 b	1.06 a
		±2.05	±0.44	±0.67	±0.30	±0.06
	N3	14.99 a	5.36 a	17.36 a	3.25 b	1.12 a
		±2.41	±0.33	±1.15	±0.33	±0.07
	S0	12.10 a	4.46 a	16.21 a	3.66 a	1.09 a
		±1.79	±0.58	±1.18	±0.32	±0.08
	S1	13.69 a	4.80 a	15.97 a	3.40 a	1.08 a
		±3.00	±0.86	±1.28	±0.52	±0.05
a-d Means followed by the same letter in each column are not significantly different at α = 0.05 based on Tukey test.

Seed yield

Vollmann et al. (2007) reported then the camelina cultivar Calena gave higher seed yield (2248 kg seeds ha⁻¹) than the 29 other camelina genotypes they tested (1574–2095 kg seeds ha⁻¹). Seed yield per plant in the present study (Table III) ranged between 3.75 and 5.62 g (=2419–3625 kg seeds ha⁻¹), and increased significantly with increasing N rate from 3.79 g for N₁ (=2445 kg ha⁻¹) to 4.74 g for N₂ (=3057 kg ha⁻¹) and 5.36 g for N₃ (= 3457 kg ha⁻¹). At the levels of S₀–S₁ seed yields increased significantly with doses of nitrogen, i.e. N₁S₀ – N₂S₀ – N₃S₀, and N₁S₁ – N₂S₁ – N₃S₁, respectively (Table III). Likewise, Agegnehu and Honermeier (1997) reported that increasing the N dose (0 – 60 – 120 kg ha⁻¹) increased seed yield from 1.02 – 1.98 – 2.63 g per plant. However, in that study the differences were significant only between the highest rate of N fertilization and the non-fertilized treatment. In their experiments with camelina Marquard and Kuhlmann (1986) also discovered that yields increased with increasing doses of nitrogen, from 1.17 to 1.58 t ha⁻¹. Furthermore, Zubr (1997) reported higher yields in five camelina varieties with increasing nitrogen dose (70–130 kg N ha⁻¹) applied at the 4-leaf stage. In the present study, the positive effect of S addition was significant only between treatment N₂S₀ and treatment N₂S₁ (Table III).

Straw yield

Straw yield per plant ranged between 15.06 and 17.59 g (Table III). Straw yields increased significantly only with a dose of nitrogen, specifically between treatments N₃S₀ and N₁S₀, and N₃S₁ and N₁S₁. Straw yield increased significantly only with the highest rate of nitrogen (N₃; 17.36 g) compared with N₂ (15.76 g) and N₁ (15.15 g) (Table III). The effect of sulphur (S₀–S₁) was insignificant.

The ratio between straw and seed yield per plant ranged between 3.05 and 4.02 (Table III). With the highest rate of nitrogen the ratio decreased significantly compared to the lowest rate of nitrogen (N₃S₀ compared to N₁S₀) or (N₃S₁ compared to N₁S₁). The N₂ and N₃ doses significantly reduced this ratio (to 3.34 and 3.25, respectively) compared with N₁ (4.00). That meant that N fertilization increased the production of seeds at the expense of straw.

Thousand-seed mass (TSM)

Seed size of C. sativa is highly heritable (Marquard & Kuhlmann, 1986; Vollmann et al., 1996). Accessions with larger seeds have been developed, but with a corresponding decline in oil content and seed yield (Vollmann et al., 1996). The range of 1000-seed mass (TSM) found here was narrow (1.04–1.17 g) with a significant effect of only the highest nitrogen dose N₃S₀ compared with the N₂S₀ treatment (Table III). These results are in agreement with Marquard and Kuhlmann (1986), who found that the TSM ranged between 0.8 and 1.3 g in six camelina cultivars and was not affected by two doses of nitrogen. Similarly, Agegnehu and Honermeier (1997) found that rates of 0–120 kg N ha⁻¹ did not significantly affect TSM (1.45–1.48). Seehuber and Dambroth (1983) reported 1.15 g as the average TSM in hybrid varieties. Vollmann et al. (2005) reported that the average TSM in 130 camelina accessions ranged between 0.77 and 1.24 g, while Vollmann et al. (2007) found that the average TSM of cv. Calena was 1.09 g, which corresponds with our results. Zubr (1997) reported that TSM varied between 0.8 and 1.8 g depending on cultivar, growth conditions, nutrition, etc. Based on 3-year experiments, Angelini et al. (1997) reported the TSM range to be 0.70–1.04 g.

Oil content

The variation in seed quality of camelina can be attributed partly to the cultivar and mainly to the combined effects of the climate and soil conditions under which the crop is grown (Zubr, 2003a).

In the present study, the seed oil content ranged from 37.01–40.02% of seed dry matter (Table IV). This range corresponds to that reported in a number of previous studies (Seehuber, 1984; Marquard & Kuhlmann, 1986; Budin et al., 1995; Vollmann et al., 1996, 2005; StraŠil, 1997; Zubr, 1997, 2003a; Agegnehu & Honermeier, 1997; Gugel & Falk, 2006). In two experiments (2005–2006) at three locations in Canada, Urbaniak et al. (2008a) discovered that the oil content in a similar variety ranged between 37.28 and 39.65%.

Table IV. Oil and protein content as influenced by N–S treatments.

Treatment no.	Description	Oil content (%) mean±SD	Oil yield (g plant^−1) mean±SD	Protein content (%) mean±SD	Protein yield (g plant^−1) mean±SD
1	N1S0	40.02 a	1.53 bd	24.98 bd	0.96 b
		±0.71	±0.08	±0.76	±0.04
2	N1S1	39.64 ac	1.48 d	25.01 bd	0.94 b
		±1.27	±0.09	±0.68	±0.08
3	N2S0	39.06 acd	1.73 bc	25.72 bcd	1.14 b
		±0.52	±0.06	±0.60	±0.07
4	N2S1	37.86 bc	1.90 ac	26.86 ad	1.36 a
		±0.41	±0.15	±0.77	±0.14
5	N3S0	37.24 bd	1.90 ac	27.83 ac	1.42 a
		±0.89	±0.08	±0.98	±0.10
6	N3S1	37.01 b	2.10 a	27.90 a	1.57 a
		±0.98	±0.04	±1.63	±0.10
Average	N1	39.83 a	1.51 c	24.99 c	0.95 c
		±0.98	±0.08	±0.67	±0.06
	N2	38.46 b	1.80 b	26.29 b	1.25 b
		±0.77	±0.14	±0.89	±0.16
	N3	37.13 c	2.00 a	27.87 a	1.49 a
		±0.88	±0.12	±1.24	±0.12
	S0	38.77 a	1.72 a	26.17 a	1.17 a
		±1.37	±0.17	±1.45	±0.21
	S1	38.17 a	1.83 a	26.59 a	1.29 a
		±1.44	±0.28	±1.60	±0.29
a-d Means followed by the same letter in each column are not significantly different at α = 0.05 based on Tukey test.

In our experiment (Table IV) the oil content decreased significantly (39.83, 38.46 and 37.13%, respectively) with increasing rate of N fertilization (N₁, N₂ and N₃). The highest N dose significantly reduced the oil content of seeds in comparison to the lowest one applied with the same S rate (N₃S₀ compared with N₁S₀ or N₃S₁ compared with N₁S₁). This finding confirms the conclusions of Agegnehu and Honermeier (1997), who discovered that increasing the N dose from 0–120 kg N ha⁻¹ for a seed rate of 400 seeds m⁻² decreased the oil content of camelina from 41% to 39%. Bugnarug and Borcean (2000) and Urbaniak et al. (2008a) arrived at the same conclusion. It has been suggested by Rathke et al. (2005) that this may be a consequence of reduced availability of carbohydrates for oil synthesis with increasing N availability. In our experiment S fertilization did not affect the oil content of camelina, but a number of previous studies found that S had a positive effect and increased the oil content in some oilseed plants, particularly winter rape (Altaf et al., 2000; Chakraborty & Das, 2000; Lošák, 2003).

Oil yield

Oil yield in grams per plant ranged between 1.48 and 2.10 (Table IV). The effect of N dose on oil yield per plant was the opposite of the effect on oil content in seeds. Increasing N rate significantly increased the total oil yield in grams per plant from 1.51 (N₁) to 1.80 (N₂) and 2.00 (N₃) and was the result of a significant increase in seed yield with increasing nitrogen rate (3.79 g for N₁, 4.74 g for N₂ and 5.36 g for N₃), which compensated for the reduction in oil content after N fertilization (39.83% for N₁, 38.46% for N₂ and 37.13% for N₃). The oil yield increased significantly only when nitrogen was applied, between treatment N₃S₀ and N₁S₀ or N₃S₁ and N₂S₁ compared with N₁S₁ (Table IV). Vollmann et al. (2007) reported that the oil yield of cv. Calena was higher (983 kg ha⁻¹) than that of 29 other camelina genotypes (641–943 kg ha⁻¹). In sunflower, Zheljazkov et al. (2008) likewise discovered that increasing N rate reduced the seed oil concentration but increased seed yield and subsequently oil yield per unit area.

Protein content

The protein content in seeds ranged between 24.98 and 27.90% (Table IV), and increased significantly with increasing N rate (24.99% for N₁, 26.29% for N₂ and 27.87% for N₃). The content of protein increased significantly only with a dose of nitrogen; in concrete terms between treatments N₃S₀ and N₁S₀ or N₃S₁ and N₁S₁. Similarly, in two-year experiments with camelina in Canada, Urbaniak et al. (2008a) found that the protein content increased from 22.9% to 30.0% when the N rate was increased from 0 to 120 kg ha⁻¹. A high correlation was found in our experiment (Table V) between the N content in plant and protein content (r=0.7924) and oil content (r=–0.8021). Marquard and Kuhlmann (1986) found that the protein content of camelina ranged between 23.4% and 30.1% and the oil content between 37.1% and 40.9%, but with a negative correlation (r=–0.8711) between oil and protein content. In addition, Gugel and Falk (2006) reported that the highest oil contents of camelina were associated with the lowest protein contents. These findings are fully confirmed by our results (r=–0.9362) (Table V). The inverse relationship between oil content and protein content can be explained by their mutual competition for carbon skeletons during carbohydrate metabolism. Since carbohydrate levels are lower in proteins than in oils, an increase in N supply results in intensified protein synthesis at the expense of fatty acid synthesis (Rathke et al., 2005).

Table V. Correlation coefficients between some parameters.

	N content in plant (%)	S content in plant (%)	N/S ratio	Branches (no. plant^−1)	Seed yield (g plant^−1)	Straw yield (g plant^−1)	Straw/ seed ratio	1000-seed weight (g)	Oil content (%)	Protein content (%)	Oil yield (g plant^-1)	Protein yield (g plant^-1)
N content in plant (%)
S content in plant (%)	0.0000
	p=1.00
N/S ratio	0.9562	0.2928
	p=0.000	p=0.165
Branches (pcs plant^−1)	0.7105	0.3175	0.7724
	p=0.000	p=0.131	p=0.000
Seed yield (g plant^−1)	0.8925	0.2358	0.9224	0.8113
	p=0.000	p=0.267	p=0.000	p=0.000
Straw yield (g plant^−1)	0.7608	−0.0996	0.6983	0.6522	0.6479
	p=0.000	p=0.643	p=0.000	p=0.001	p=0.001
Straw/seed ratio	−0.7116	−0.3011	−0.7686	−0.6568	−0.8995	−0.2673
	p=0.000	p=0.153	p=0.000	p=0.000	p=0.000	p=0.207
1000-seed weight (g)	0.2867	−0.0627	0.2558	0.1240	0.1103	0.2641	0.0163
	p=0.174	p=0.771	p=0.228	p=0.564	p=0.608	p=0.212	p=0.940
Oil content (%)	−0.8021	−0.2176	−0.8306	−0.8347	−0.8254	−0.7315	0.6378	−0.3464
	p=0.000	p=0.307	p=0.000	p=0.000	p=0.000	p=0.000	p=0.001	p=0.097
Protein content (%)	0.7924	0.1414	0.7990	0.8928	0.8110	0.7973	−0.5750	0.3280	−0.9362
	p=0.000	p=0.510	p=0.000	p=0.000	p=0.000	p=0.000	p=0.003	p=0.118	p=0.000
Oil yield (g plant^−1)	0.8708	0.2340	0.9012	0.7768	0.9911	0.5996	−0.9160	0.0354	−0.7503	0.7443
	p=0.000	p=0.271	p=0.000	p=0.000	p =0.00	p=0.002	p=0.000	p=0.870	p=0.000	p=0.000
Protein yield (g plant^−1)	0.8950	0.2291	0.9229	0.8730	0.9841	0.7206	−0.8357	0.1774	−0.8886	0.9003	0.9583
	p=0.000	p=0.282	p=0.000	p=0.000	p=0.000	p=0.000	p=0.000	p=0.407	p=0.000	p=0.000	p=0.000

Protein yield

Protein yield in g per plant ranged between 0.94 and 1.57 and increased significantly more in treatments 4–6 than in treatments 1–3 (Table IV). The effect of S was positive with the N₂ rate, i.e. the protein yield of treatment N₂S₀ was 1.14 g, while in treatment N₂S₁ it increased significantly to 1.36 g. Increasing N rate also significantly increased protein yield (0.95–1.25–1.49 g per plant), the result of the positive action of N on seed yield and protein content. The yields of protein increased significantly with doses of nitrogen; in concrete terms in treatment N₃S₀ compared with N₂S₀ and with N₁S₀ or N₃S₁ and N₂S₁ compared with N₁S₁ (Table IV).

There were no significant differences between the S₀ and S₁ rates for any of the crop parameters studied. This was probably due to the relatively good supply of S in the original soil (25 mg kg⁻¹ S). However an increased S level of 45 mg kg⁻¹ stabilized or non-significantly increased the selected yield parameters (seed, oil and protein yield).

Camelina (false flax) is described as a crop requiring fewer inputs than other oilseed species. This study showed that if plant nutrition is balanced, the decisive macronutrient in terms of seed, oil and protein yield is nitrogen. Although the highest N dose significantly reduced the oil content of seeds in comparison to the lowest one applied with the same S rate, the oil yields per plant increased with a dose of nitrogen. Although S application did not significantly alter any of the crop parameters studied, it stabilized or increased the levels observed. Combined application of N and S as mineral fertilizer is recommended when growing camelina, particularly on sulphur-deficient soils.

Acknowledgements

This study was supported by Research Plan No. MSM6215648905 ‘Biological and technological aspects of sustainability of controlled ecosystems and their adaptability to climate change’, which is financed by the Ministry of Education, Youth and Sports of the Czech Republic.