Assimilation capacity by non-destructive in situ measurements in longterm experiment of maize (Zea mays L.) under different plant density and nitrigen supply in Chernozem

© Slovak University of Agriculture in Nitra Faculty of Agrobiology and Food Resources


Introduction
Maize has a significant role in both world (2 nd largest production area) and Hungarian (largest production area of arable land) field crop production. It has very high genetic potential (it is a C4 photosynthetic type plant), but only 25-35% of which is currently utilised in Hungary (country average yield have been between 6.5-8.5 t ha -1 in last decade). In addition to its high yield potential, maize is largely affected by changes in ecological and agrotechnical conditions. From a crop formation perspective, changes in the dynamics of the assimilation capacity (leaf area, relative chlorophyll content) of the crop stand has a significant role in the vegetation period (Carter, 1994;Martinez and Guiamei, 2004;Hawkins et al., 2009). It is crucial to measure LAI and SPAD values in maize populations, as it makes it possible to gather data on photosynthetic activity via in situ nondestructive methods. LAI and SPAD values are affected by the year, the hybrid, fertilization and plant density as well. Fertilization makes huge changes in the SPAD (Yu et al., 2010;Széles et al., 2011) and LAI values of hybrids (Novoa and Loomis, 1981;Oikeh et al., 1998, Micskei et al., 2012. Some other references show that increasing plant density decreased the SPAD values (Su et al., 2012;Tajul et al., 2013) and increased the LAI values (Ahmad et al., 2010;Valadabadi and Farahani, 2010). Many researchers examined the relationship between maize SPAD and LAI values and yield. The relative chlorophyll concentration of maize (SPAD) had a positive correlation with nitrogen supply and maize yield (Széles, 2008;Bencze and Futó, 2017). Research results showed a strong positive correlation between LAI values at maize flowering and yield (Oikeh et al., 1998;Bavec and Bavec, 2002;Ma et al., 2005). However, research data by Esechie (1982), Remison and Lucas (1992) showed no correlation between leaf area index (LAI) and maize yield.
Several research shows that maize yield can be increased significantly by fertilization (Berzsenyi, 2010;Széll et al., 2010;Vári and Pepó, 2011). However, the effects of fertilizers were largely affected by the water supply of the year (Azeez, 2009;Ványiné et al., 2012) as well as the hybrid (Körshens, 2006;D'Haene et al., 2007). At the same time, there is a strong correlation between optimum plant density and yield in maize production. Optimal plant density was affected by the water supply of the vegetation period (Sárvári and Pepó, 2014;Nagy, 2010;Berzsenyi et al., 2011) as well the water reservoirs of soil (Fulton, 1970;Dóka, 2015). In addition to water supply and year, the plant density response of the hybrids also plays a significant role in yield formation. Up-to-date hybrids can utilise more yield potential at higher crop density (Carlone and Russel, 1987;Russel, 1991;Haegele et al., 2014). The experimental results of Murányi (2014, 2015) show that the yield of crop hybrids is also affected by cropping area (row spacing).
In our long-term chernozem soil experiment we studied the responses of different maize hybrids to fertilization and plant density, as well as measure photosynthesis capacity values (LAI, SPAD) in the vegetation season of maize. We examined the relations between LAI and SPAD values and maize yield.

Material and methods
In 1983, a long-term trial on calcareous chernozem soils (code CH accordind to WRB classification) was set up in the Hajdúság (Eastern Hungary), 15 km from Debrecen (latitude 47 o 33' N., and longitude 21 o 27' E.). The chernozem soil of the long-term experiment contains 2.7-2.8% humus, and total depth of the humus enriched horizon was about 0.8 m (Table 1). When the trial was set up, the soil contained 130 mg kg -1 AL-soluble P 2 O 5 and 240 mg kg -1 AL-soluble K 2 O. The calcareous chernozem soil is characterized by a specific plasticity index (KA) of 40 and nearly neutral pH (pH KCl = 6.46). The soil has favourable water management characteristics.
During the long-term trial we applied treatment with 6 nutrient doses. In addition to the control treatment, a basic dose of N =30 kg ha -1 + P 2 O 5 = 22.5 kg ha -1 + K 2 O = 26.5 kg ha -1 was applied in double, triple, quadruple and quintuple quantities. In the trial, among these doses, the following treatments were examined: Nitrogen fertilizer was applied to the plots 50% in the autumn and 50% in the spring, before sowing. The full amount (100%) of the phosphorous and the potassium was applied in the autumn before ploughing.
Two plant densities (65 thousand ha -1 and 85 thousand ha -1 ) were set up in our long-term experiment with two different genotype (Sushi (FAO 340), Fornad (FAO 420). The trial was arranged in a split-split-plot design. The gross and net plot areas were 9.12 m 2 and 7.60 m 2 , respectively. The trial involved four repetitions. The forecrop was winter wheat. The optimal agricultural elements (tilling, sowing, crop protection, harvesting) were used, which matched to modern maize production.
Important weather information for trial years are shown in Tables 2 and 3.
The meteorical data of experimental years proved that the rainfall of pre-vegetation periods (from October to Mach) was slightly (+20.9 mm) and highly (+131.7 mm) higher, compared with the 30-years mean in 2017 and 2018, respectively. The amount and the distribution of rainfall in the vegetation period (April-September) were much favourable in 2017 year, compared with 2018 year. The monthly average temperatures of vegetation periods were over the mean average in both years which modified the absolute values and dynamics of SPAD and LAI readings. In our long-term experiment the leaf area index (LAI) and relative chlorophyll content (SPAD) were measured 5 times in the vegetation period of maize. The LAI values were determined using a SunScan Canopy Analysis System (SSI) portable leaf area measuring instruments in four repetitions with five measurements per repetition. SPAD and LAI values were measured during the morning period between 9-11 a.m. After SPAD max the readings of SPAD had a moderate decreasement in 2017 (28.3-54.9 at the time of the 01/09 measurement) and a significant drop in 2018 (9.3-14.5 as measured on 07/09), which was due to the temperature differences of July and August between the two years. Fertilization had significant increasements in both hybrids at most measurement times, plant density made no differences on them. There was no significant difference in the relative chlorophyll content of the two genotypes either. The temporal changes in leaf area index (LAI) showed similar dynamics comparing with relative chlorophyll values (SPAD) ( Table 4, 5). The hybrids gave their LAI max values in early July in 2017 (3.0-5.3 m 2 m -2 on 04/07) and early August in 2018 (3.5-4.4 m 2 m -2 on 06/08). As opposed to SPAD values, the decreasement of LAI values followed a similar trend in both years (1.5-3.0 m 2 m -2 , on 01/09/2017 and 1.1-2.3 m 2 m -2 on 07/09/2018). The LAI max values were higher in 2018 than in 2018. Increasing fertilization doses and plant density resulted the higher LAI values of hybrids at all times of measurements. As a result of fertilization, there were significant differences in the N 150 + PK treatment in 2017, whereas differences were not significant in most cases in 2018 and LAI max values were reached in the N 90 + PK treatment.
The effects of excellent chernozem soil, favourable water supply and near-optimal agrotechnology could moderate the negative temperature conditions of both years, so we obtained high yields in our long-term    (Table 6). In both years the fertilization had significant effects on the yield of maize hybrids. The yield surpluses of hybrids due to fertilization were 1.8-3.7 t ha -1 at 65 thousand ha -1 plant density, 2.1-5.9 t ha -1 at 85 thousand plant density, and 0.2-2.6 t ha -1 vs 1.1-2.3 t ha -1 in 2018, respectively. Increasing plant density lead to an increase in yield in both years, which was triggered by the significant amount of soil water supplies in spite of unfavourable temperature conditions in the vegetation season. The plant density of hybrids were different, i.e. in the Sushi hybrid increased plant density gave no significant yield increasement in the two years of our experiment (-0.2-+0.6 t ha -1 in 2017, vs 1.1-1.6 t ha -1 in 2018). In the Fornad hybrid, increased plant density realized a significant increase in yield (0.6-4.6 t ha -1 in 2017, 2.9-3.9 t ha -1 in 2018, respectively). The excellent nutrient and water supplies of the soil in the long-term experiment were proven by the high yield in the control treatment (10.2-11.1 t ha -1 in 2017, 9.3-13.6 t ha -1 in 2018, respectively). Sushi hybrid showed maximum yield in the N 150 + PK (2017) and the N 90 + PK (2018) treatment, whereas Fornad showed maximum yield in the N 90-150 + PK (2017) and N 90 + PK (2018) treatment.
We applied the Pearson's correlation analysis to assess the correlations between the SPAD (Table 7) and LAI (Table 8) values with the hybrids, agrotechnological factors (fertilization, plant density) and yield at different measurement times. SPAD values showed relatively high correlations with nutrient supply in both years. In 2017, SPAD 3, SPAD 4 and SPAD 5 showed correlation coefficients of r = 0.630**, 0.773** and 0.795** with fertilization, respectively. In 2018, SPAD 2, SPAD 3 and SPAD 4 showed correlation coefficients of r = 0.799**, 0.530** and 0.593**, respectively. We found no correlations between the hybrids, plant density and SPAD values during the years of our experiments. According to our research data there were a very weak correlation between SPAD and yield (r = -0.018-0.424**), except at the time of the SPAD 4 (r = 0.619**) and SPAD 5 (r = 0.590**) measurements in 2017. Similarly, the Pearson's correlation analyses proved no significant correlations between LAI values measured at different times, hybrids and plant density. Fertilization   Pearson's correlation analysis proved very weak correlations (r = -0.450-374**) between yields and hybrids in 2017 and 2018 (Table 9), but we had a relatively high correlation (r = 0.672**) between yields and fertilization in 2017. Similar correlation value (r = 0.517**) was between the plant density and the maize yield in 2018.

Conclusions
According to our 2017 and 2018 research results, the SPAD and LAI values of maize hybrids showed special dynamics in the vegetation season. SPAD and LAI values were growing from late May until early July (2017), and until early August (2018). Fertilization had a positive effect on SPAD values (Bencze és Futó, 2017, Yu et al., 2010 and the leaf area index (LAI) (Novoa and Loomis, 1981, Oikeh et al.;1998;Micskey et al., 2012). As opposed to other researchers (Ahmad et al., 2010, Valadabadi andFarahani, 2010), in our experiments maize LAI values were not significantly increased by higher plant density.
Soil with excellent nutrient and water husbandry could significantly reduce the negative effects of the unfavourable weather conditions (high temperature values) of the vegetation season. The chernozem soil in our long-term experiment had excellent natural nutrient supplying capacity, which so we obtained high yields (10.2-11.1 t ha -1 in 2017, 9.3-13.6 t ha -1 in 2018, respectively) in the control plots. The maximum yield in 2017 was 15.4 t ha -1 in Sushi hybrid vs 16.7 t ha -1 in Fornad hybrid, whereas the maximum yield values in 2018 were 13.0 t ha -1 and 15.9 t ha -1 , respectively. Thus, our research results show that maize yield was affected by water supply (Azeez, 2009;Dóka, 2015;Ványiné et al., 2012Ványiné et al., ), hybrid (2009Körshens, 2006) and plant density (Sárvári and Pepó, 2014). Similarly to other research findings (Berzsenyi, 2010;Berzsenyi et al., 2011, Széll et al., 2010, Vári and Pepó, 2011, fertilization had the most significant effect on maize yield in our long-term experiment. As a result of fertilization, the yield surpluses compared to the control treatment were was 1.8-5.9 t ha -1 in 2017, and 0.2-2.6 t ha -1 in 2018, depending on hybrids and plant density. We have got a special interactive effect between fertilization and plant density in 2017. In case of no nutrient supply (control treatment), higher plant densities had minimal yield increasement (-0.2-+06 t ha -1 ) as compared to the yield surpluses in the N 150 + PK treatment (+0.6-+4.6 t ha -1 ).  Using Pearson's correlation analysis we could state that fertilization was the main factor which significantly effects on SPAD and LAI values in the different maize phenophases. Correlations between fertilization and SPAD (r = 0.6**-0.8**) as well as LAI were relatively high (r = 0.5**-0.8**). According to our results we stated weak correlation (r = 0.1-0.3) among plant density, hybrid and SPAD and LAI values in both years. As opposed to research findings of Széles (2008), we found relatively weak correlations between SPAD and yield (r = -0.018-424**). We could prove moderate correlations (r = 0.4**-0.8**) between LAI values and yield at certain measurement times, as opposed to the research findings of Oikeh et al. (1998), Bavec and Bavec (2002) and Ma et al. (2005). The Pearson correlation analysis showed that hybrid (-0.450-0.374**) and plant density (0.284-0.517**) had very weak correlations with yield, whereas we could prove a moderate correlation between fertilization and yield (0.247-0.672**).