*3.3. LAImax*

LAImax values varied from 2.8 to 6.8 for common wheat and from 3.6 to 8.6 for spelt wheat (see Appendix A). Multi-year and multi-variety LAImax of spelt were 26.8, 22.8, 4.4 and 9.9% higher than that of common wheat across N fertilizer levels. These significant differences (corresponding *p* values were less than 0.027) clearly indicate the spelt is more vigorous in tillering, especially at low N levels. LAImax values grew significantly with the increasing N fertilisation level (Figure 2). Common wheat showed considerably more fertilisation-related LAImax growth. LAImax of the N120 treatment was 52% higher than that of the N0 treatment for common wheat while this difference was only 31% for spelt wheat. The observed LAI maximums of spelt wheat were considerably greater (even two times greater) than those reported in other studies [40], while the common wheat LAImax values were in good agreemen<sup>t</sup> with other studies [25,41]. Inter-annual variability could be a simple reason for this, as crop production could leave the so-called average range in certain years. Furthermore, results obtained at certain sites could be valid to other sites having different environmental conditions to a limited extent only. Thus, it is better to say that our results complement and do not contradict the previous findings on the maxima of spelt wheat leaf area index.

### *3.4. Chlorophyll Content of the Flag Leaf*

The measured SPAD values (see Table A1 in the Appendix A) and the corresponding leaf chlorophyll contents overlapped with the equivalent values of forty winter wheat varieties investigated in an independent experiment at two nitrogen levels (N0 and N120) in three consecutive cropping seasons (2012/2013, 2013/2014 and 2014/2015) at Martonvásár, where the chlorophyll content ranged between 45 and 468 mg m<sup>−</sup><sup>2</sup> [42]. In our experiment the chlorophyll content of spelt and common wheat ranged between 227 and 338 and 195 and 451 mg m<sup>−</sup>2, respectively (Figure 2). Similarly to the yield, N fertilisation significantly increased the chlorophyll content of the flag leaf of both crops for every N dose increment with only one exception: the N40 – N80 increment caused a non-significant increase in the spelt wheat chlorophyll content (*p* = 0.48). The chlorophyll content of common wheat showed a considerably stronger reaction to the increasing doses of N fertilisation. Spelt wheat chlorophyll contents were significantly higher than the common wheat chlorophyll contents in all three year at the N0 and N40 fertilisation levels. This again emphasizes the fact that spelt wheat has the capacity to use the resources of the soil more vigorously in limited environmental conditions.

### *3.5. Normalized Di*ff*erence Vegetation Index (NDVI)*

NDVI values (Table A1 and Figure A1 in the Appendix A), varied from 0.54 to 0.80 for common wheat and from 0.51 to 0.82 for spelt wheat, which were in good agreemen<sup>t</sup> with the measurements of Piekarczyk and Sulewska [22] who observed 0.72 and 0.71 average NDVI values for spelt and common wheat, respectively around flowering. Though there is a constant demand for deriving LAI data from NDVI, which is a standard component of remotely sensed datasets, the reality is that LAIs around and above 3 m<sup>2</sup> m<sup>−</sup><sup>2</sup> are not distinguishable with NDVI data [43]. To make the issue even more complicated, according to our results, considerable interannual variable can be observed in the NDVI-LAImax correlation (Figure 3). The interannual di fference is much more pronounced for spelt wheat, but the years 2016 and 2019 were considerably di fferent for common wheat, as well. In general, there is certainly a positive correlation between the LAImax and the NDVI measured at flowering, but in certain years considerable deviations could be observed. If the 2016 and 2018 spelt wheat data are compared, similar LAImax values (5.2 and 4.8 m<sup>2</sup> m<sup>−</sup>2; non-significant di fference *p* = 0.35) correspond to significantly di fferent NDVI values (0.54 and 0,72; *p* ≈ 0). The di fference between the two wheat species requires further investigation and highlights the fact that the NDVI-LAI interrelation is highly dependent on the plant species and probably on other environmental conditions as well.

**Figure 3.** Correlation of leaf area index maximum (LAImax) and NDVI values for spelt (*T.spelta*, left) and common wheat (*T. aestivum*, right) at Martonvásár (Hungary) in 2016 (dots), 2018 (diamonds) and 2019 (triangles).

### *3.6. Multivariable Linear Yield Estimation Equation*

The linear regression (calibration) resulted in the following estimation equations for the two crops:


The equations were capable of estimating the yield with 0.64 and 0.37 t ha−<sup>1</sup> mean absolute error, that correspond to 11.2% and 7.1% relative errors for common and spelt wheat, respectively. When the equations were applied to the validation datasets the results were more moderate:


These kind of equations could be useful extensions to yield forecasting applications such as AgrometShell [44], since they add crop-specific parameters to the estimation beyond the already incorporated soil and weather-specific parameters.
