Effect of nitrogen fertilization and genotype on the yield and yield components of winter wheat

The effect of nitrogen fertilization on the yield and yield components of various wheat cultivars was studied in a small-plot long-term experiment with two factors arranged in a split-plot design in four replications. The experiment was carried out in the years 2006/2007, 2007/2008, and 2008/2009 at the Agricultural Institute of the Centre for Agricultural Research in Martonvásár. Seeds were sown on 19 October 2006, 17 October 2007, and 16 October 2008. In the long-term crop rotation experiment, the crop sequence was pea, winter wheat, maize, and spring barley. The dose of N fertilizer formed the main plot and the wheat cultivar the subplots. The doses of N fertilizer (calcium ammonium nitrate) were 0, 80, 160, and 240 kg N · ha^–1 (designated as N₀, N₈₀, N₁₆₀, and N₂₄₀, respectively) and were applied in two splits: one-third before sowing and the other two-third in early spring at tillering. All the plots were given the same doses of phosphorus and potassium (120 kg · ha^–1 of each). The three Martonvásár wheat genotypes sown in the subplots were Mv Toborzó (extra early), Mv Palotás (early), and Mv Verbunkos (mid-early). The soil properties in the 0- to 20-cm layer were determined at the end of the experiment (Table 1). The ploughed layer of the chernozem soil, a humus-containing loam, was slightly acidic, with moderate supplies of phosphorus and good supplies of potassium. The NO₂+NO₃ nitrogen content of the soil increased in response to N fertilization and was significantly higher in the N₂₄₀ treatment than in N₀. By the ammonium acetate-lactate (AL) method, the P₂O₅ content of the soil was 236–282 mg · kg^–1 and the AL-K₂O content was 205–233 mg · kg^–1; no significant differences were observed between the N treatments.

The rainfall quantity and the temperature during the growing season (September–July) for each year are shown in Figure 1. In the dry year of 2007, the total rainfall during the growing season was only at one-third (200 mm) compared to 2008 and 2009 (638 and 617 mm, respectively). The rainfall distribution was also unfavorable in 2007, while in 2008 and 2009, both the quantity and distribution of the rainfall were satisfactory (with the exception of the lack of rain in April 2009). The mean temperature during the growing season was higher in 2007 (12°C) than in the other two years (10°C), which could be attributed partly to the very mild winter.

Figure 1

The leaf chlorophyll index of the flag leaves (SPAD) was measured during flowering with the aid of an SPAD 502 chlorophyll meter (Minolta Corp., Tokyo, Japan), which records optical density measurements at two wavelengths and converts them into digital signals and then into an SPAD index (Minolta, 1989).

Harvest was performed on 2 July 2007, 30–31 July 2008, and 10^–13 July 2009. The number of spikes per square meter, the grain number per spike, the TKW, and the HI were determined on plant samples taken from a 1-m row (45–55 plants) before harvest. TKW was determined by counting and weighing 200-kernel samples. HI was calculated as the ratio of the grain yield to the total aboveground biomass and expressed as a percentage.

The statistical analysis of the data was performed using the GenStat 16 program. Correlation analysis was used to determine the relationship between grain yield, grain number, TKW, SPAD, and HI. The correlations between the yield, as dependent variable, and the two yield components (grain number per square meter and TKW), as independent variables, were determined for each year, cultivar, and N treatment using linear regression analysis for two independent variables. Linear regression analysis was also used to reveal the relationship between yield and HI and between yield and SPAD index.

The effect of N fertilization and the cultivar on the yield was significant in all the years. The results of analysis of variance are presented in Table 2. The interaction between N and cultivar was not significant in any of the years. The grain yields for the N treatments and cultivars in each year are illustrated in Figure 2. In all the years, the in 2007 (6.25 and 6.21 t · ha^–1, respectively) and for Mv yield was lowest in treatment N₀ (averaging 5.45 t · ha^–1), Verbunkos in 2008 and 2009 (7.63 and 7.51 t · ha^–1, re-with a significant increase in the N₈₀ treatment in 2007 spectively). Averaged over the treatments, the grain yield and 2008 (6.45 and 7.99 t · ha^–1, respectively) and in the was significantly higher in 2008 and 2009 (7.28 and 7.11 N₁₆₀ treatment in 2009 (7.44 t · ha^–1). Higher N doses had no further significant yield-increasing effect. The effect of the cultivar on the yield was significant in all three years, with the highest yields for Mv Palotás and Mv Verbunkos in 2007 (6.25 and 6.21 t · ha^–1, respectively) and for Mv Verbunkos in 2008 and 2009 (7.63 and 7.51 t · ha^–1, respectively). Averaged over the treatments, the grain yield was significantly higher in 2008 and 2009 (7.28 and 7.11 t · ha^–1, respectively) than in 2007 (6.11 t · ha^–1).

Figure 2

The effect of N fertilization and genotype on the yield components (spike number per square meter, grain number per spike, and TKW) for the three years is presented in Table 3. Nitrogen fertilization had a significant effect on the spike number per square meter, the grain number per spike (except in 2007), and the TKW. The spike number per square meter and the grain number per spike were highest in treatments N₁₆₀ and N₂₄₀, while TKW dropped significantly in the N₁₆₀ and N₂₄₀ treatments. The cultivar effect was significant in 2007 and 2008 for the spike number per square meter and in all the years for the grain number per spike and the TKW. The spike number of Mv Toborzó (639) was greater than that of Mv Palotás (607) or Mv Verbunkos (603). Among the cultivars, Mv Verbunkos had the highest grain number per spike (31.6), followed by Mv Palotás (28.2) and Mv Toborzó (21.7). The highest TKW was recorded for Mv Toborzó (52.5 g). The spike number per square meter was considerably higher in the dry year of 2007 (693) than in 2008 and 2009 (596 and 560, respectively). At the same time, the grain number per spike was almost three times higher in 2008 and 2009 (35.5 and 33.4 grains per spike, respectively) than in 2007 (12.6 grains per spike). TKW was higher in 2007 (47.8 g) than in 2008 (44.0 g) or 2009 (44.7 g).

The HI was significantly influenced by the N treatments in all three years and by the cultivar in 2008 and 2009 (Table 4). In 2009, the N treatment × cultivar interaction was also significant. The value of HI rose in response to N treatment and was highest in the N₁₆₀ treatment (43.8%). Among the cultivars, Mv Palotás achieved a HI of 43.3%, with values of 42.8% for Mv Verbunkos and 40.6% for Mv Toborzó. HI was highest in 2009 (43.9%), while the values in 2007 and 2008 were similar to each other (41.8 and 41.2%, respectively).

The SPAD index exhibited significant differences as the result of N treatment in all three years (Table 5), being lowest in the N₀ treatment (45.1). The value was significantly higher in the N₈₀ treatment (54.2) in 2007 and in the N₈₀ and the N₁₆₀ treatment in 2008 and 2009 (53.8). The cultivar effect was also significant in all three years. Averaged over N treatments and years the highest chlorophyll content was recorded for Mv Verbunkos, with a SPAD index of 52.8, followed by Mv Palotás (51.5) and Mv Toborzó (50.7). The mean SPAD index was 52.9 in 2007, 51.0 in 2008, and 51.1 in 2009.

The correlations between the yield and the grain number, TKW, HI, and SPAD index are illustrated for each N treatment in Figure 3, which shows that there was a highly significant (P<0.001) positive correlation between the yield and the grain number per square meter (r=0.85) and between the yield and the SPAD index (r=0.59). A negative correlation was detected between the yield and the TKW (r=–0.44), and a positive correlation between the yield and the HI (r=0.51), significant at the P<0.01 level.

Figure 3

Regression analysis was performed to find regression between two yield components on the yield for each N treatment. Significant relationships were found in the case of N fertilization but not for year or cultivar. Averaged over the three years, the effect of N fertilization was not significant in the control (N₀) treatment but was significant in all the other N treatments (Table 6).

In the N₀ treatment, neither of the yield components had a significant effect. The grain number per square meter had a significant effect in plots with N fertilizer application, but the effect of TKW was not significant in any of the N treatments. Of the two yield components, the grain number per square meter had a decisive effect on the yield, based on the standardized regression coefficient (β) (data not shown). The adjusted R² values indicated that 54.9% of the variance in yield could be attributed to the two yield components in the N₈₀ treatment, 88.6% in the N₁₆₀ treatment, and 76.9% in the N₂₄₀ treatment (Table 6).

Linear regression analysis showed that the SPAD index was responsible for 92.4% of the yield variance in the dry year of 2007 (at the P<0.001 level) but only 60.5% in 2008 and 61.5% 2009 (at the P<0.01 level) when conditions were more favorable. Averaged over the three years, the HI explained 23.4% of the yield variability at the P<0.05 level.