## 3 Results and Discussion  
### SPAD Index  
The measured values of SPAD readouts ranged from 538 to 695. They were similar to the ranges of values listed in reference publications for maize grown in Poland [25]. [PERSON] et al. [26] conducted a study on maize at BBCH 83-85 and reported higher values ranging from 719 to 846 (in control variants) to 742.6 to 859.4 (in fertilized variants). [PERSON] et al. [27] observed lower values of SPAD readouts, which depended on the phase of maize development during the measurements. In our study, differences in SPAD readings were also observed between consecutive measurement dates. In the first time point, they ranged from 538 to 661. On the consecutive measurement dates, the SPAD index values increased, in the second time point, they ranged from 555 to 689, whereas they were the highest in the third time point and ranged from 539 to 695 (Figure 1). The growth dynamics of the leaf greenness index from the day of plant emergence was illustrated by equations for the average values from both study years and all mineral fertilization variants. The dynamics of changes in the SPAD index of the variety Delitop were described (R\({}^{2}\) = 0.94) with a simple linear regression equation, y = 1.115x + 533.74. Changes in the variety Magitop (R\({}^{2}\) = 0.99) were described with the following equation: y = 1.37x + 548.14, where: y is the SPAD index value, and x is the number of days since the sowing of maize seeds. [PERSON] et al. [28] conducted their research in eastern Canada and made similar observations. [PERSON] and [PERSON] [29] found that as the time passed and the plants developed, the SPAD index in maize leaves tended to increase regardless of the nitrogen fertilization level. The latter authors observed the highest SPAD index values 84-89 days after plant emergence, i.e., at the beginning of the panicle ejection phase. The date of measurement had a greater influence on the SPAD index than nitrogen \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline \multirow{2}{*}{**Month**} & \multicolumn{3}{c}{**Average Air Temperature (°C)**} & \multicolumn{3}{c}{**Total Rainfall (mm)**} \\ \cline{2-7}  & **The First** & **The Second** & **Long-Term** & **The First Year** & **The Second** & **Long-Term** \\  & **Year** & **Year** & **Average** & **Year** & **Average** \\ \hline March & 4.2 & 4.3 & 2.8 & 75.7 & 65.1 & 37.5 \\ April & 8.7 & 11.7 & 7.7 & 8.7 & 13.3 & 37.2 \\ May & 15.2 & 13.4 & 13.1 & 19.5 & 85.3 & 54.7 \\ June & 19.1 & 15.7 & 16.3 & 8.6 & 79.3 & 64.6 \\ July & 20.0 & 19.7 & 17.9 & 80.1 & 68.1 & 77.5 \\ August & 18.8 & 19.7 & 17.4 & 171.5 & 31.4 & 62.7 \\ September & 13.9 & 15.6 & 13.0 & 29.8 & 50.0 & 47.0 \\ October & 10.0 & 7.9 & 8.5 & 74.9 & 73.3 & 40.7 \\ November & 5.7 & 6.7 & 3.4 & 34.3 & 45.4 & 44.1 \\ Mean/Sum & 12.8 & 12.7 & 11.1 & 503.1 & 511.2 & 466.0 \\ March–November & & & & & & \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Weather conditions during the vegetation period in RGD Brody in the years of study. fertilization, plantation location, and maize variety. [PERSON] and [PERSON] [30] made similar observations in their study. They found that the date of measurement had the greatest influence on the total variability of SPAD index readouts and determined them from 57% to over 90%, depending on the year of the study. In addition, [PERSON] et al. [25] noted the highest SPAD index at BBCH 67 (full flowering), regardless of the nitrogen fertilization level. They recorded significantly lower values at BBCH 75, i.e., the phase of full milk maturity. Similarly, [PERSON] et al. [31] observed that the SPAD index value increased systematically until the beginning of cob formation (phase R1), but then it decreased. The maximum chlorophyll level in the leaves is referred to as 'photosynthetic maturity' in reference publications. In our study, the mean values of the leaf greenness index (SPAD) differed significantly depending on the level of mineral nitrogen fertilization.