*2.3. Calculation of Environmental Factors and Instantaneous Fresh Weight* 2.3.1. Calculation of Cumulative Radiant Heat Product

The effects of temperature and radiation on the fresh weight of lettuce can be measured by the cumulative radiant heat product. The specific calculation formula is as follows [26]:

$$R\_{TE} = \begin{cases} 0 & (T < T\_b) \\ \frac{T - T\_b}{T\_{ob} - T\_b} & (T\_b \le T < T\_{ob}) \\ 1 & (T\_{ob} \le T \le T\_{ou}) \\ \frac{T\_m - T}{T\_m - T\_{ou}} & (T\_{ou} < T \le T\_m) \\ 0 & (T > T\_m) \end{cases} \tag{1}$$

$$T\_{EP} = \sum R\_{TPP} \tag{2}$$

$$R\_{TPP} = \sum\_{i=1}^{24} \left( R\_{TEi} \times P\_{ARi} \times 3600 / 10^6 \right) \tag{3}$$

where *Tb* is the lower limit of growth temperature (◦C), *Tm* is the upper limit of growth temperature (◦C), *Tob* is the lower limit of optimum growth temperature (◦C), *Tou* is the upper limit of optimum growth temperature (◦C), *T* is the ambient temperature (◦C), *RTE* is the relative thermal effect, *RTEP* is the daily cumulative radiant heat product (MJ·m−2·d<sup>−</sup>1), *RTEi* is the relative thermal effect in the *i*-th hour, *PAri* is the average photosynthetically active radiation in the *<sup>i</sup>*-th hour (MJ·m−2·d−1), and *TEP* is the cumulative radiation heat product (MJ·m<sup>−</sup>2).

#### 2.3.2. Calculation of Crop Evapotranspiration

If reference evapotranspiration is used to replace crop evapotranspiration, there will be a large error. Therefore, in order to improve the accuracy of calculation of crop evapotranspiration, the crop coefficient was used to correct the reference evapotranspiration. The specific calculation formula is as follows [27]:

$$ET\_{\mathfrak{c}} = ET\_0 \cdot \mathbf{K}\_{\mathfrak{c}} \tag{4}$$

$$ET\_{0i} = \frac{0.408\Delta (R\eta - G) + \gamma \frac{1713}{T + 273} (e\_s - e\_a)}{\Delta + 1.64\gamma} \tag{5}$$

$$\Delta = \frac{2505 \cdot \exp\left(\frac{17.27T}{T + 237.3}\right)}{\left(T + 237.3\right)^2} \tag{6}$$

$$e\_s = \frac{e\_s(T\_{\text{max}}) + e\_s(T\_{\text{min}})}{2} \tag{7}$$

$$e\_s(T\_{\text{max/min}}) = 0.6108 \cdot \exp\left(\frac{17.27 T\_{\text{max/min}}}{T\_{\text{max/min}} + 237.3}\right) \tag{8}$$

$$e\_a = \frac{e\_s \left(T\_{\rm min}\right) \frac{RH\_{\rm max}}{100} + e\_s \left(T\_{\rm max}\right) \frac{RH\_{\rm min}}{100}}{2} \tag{9}$$

$$R = K \cdot P\_{AR} \tag{10}$$

$$R\_n = a \cdot R + b \tag{11}$$

where *ET0* is the reference evapotranspiration under full irrigation (cm·d<sup>−</sup>1), <sup>Δ</sup> is the slope of the saturated vapor pressure curve (kPa· ◦C<sup>−</sup>1), *Rn* is the net radiation of the crop canopy (MJ·m−2·d−1), *<sup>G</sup>* is the soil heat flux density (MJ·m−2·d−1), *<sup>γ</sup>* is the dry and wet table constant (kPa· ◦C<sup>−</sup>1), *T* is the daily average temperature at the height of 1.5 to 2.5 m above the surface (◦C), *Tmax/min* is the daily maximum or minimum air temperature at the height of 1.5 to 2.5 m above the surface (◦C), *es* is the average saturated vapor pressure at the height of 1.5 to 2.5 m above the surface (kPa), *ea* is the average actual vapor pressure at the height of 1.5 to 2.5 m above the surface (kPa), *RHmax/min* is the daily maximum or minimum relative humidity at the height of 1.5 to 2.5 m above the surface (%), *ETci* is the evapotranspiration of crops on the *i*-th day under full irrigation (cm/d), *Kc* is the crop coefficient, *<sup>R</sup>* is the total solar radiation (MJ·m−2·d<sup>−</sup>1), *PAR* is the photosynthetically active radiation (MJ·m−2·d<sup>−</sup>1), *<sup>K</sup>* is the conversion coefficient between photosynthetically active radiation and total solar radiation, and a and b are the conversion coefficients between net radiation and total radiation.

If *G* = 0, *γ* = 0.067, *Kc* = 0.7, 1.00 or 0.95 [28], *K* = 80/39 [29], a = 0.8277, and b = 0.2909 [30], then *ETc* can be calculated using Formulas (4)–(11) and the indoor temperature, humidity, and photosynthetically active radiation.

#### 2.3.3. Calculation of Instantaneous Fresh Weight and Fresh Weight Increment

Based on the previous research results of this research group [24], the online, nondestructive calculation of the fresh weight of substrate-cultivated lettuce grown in a solar greenhouse was realized by combining the data of phenotypic characteristics and environmental characteristics. Firstly, the collected lettuce canopy images were used to extract phenotypic characteristics such as shape, color, and texture. Then, using the online monitoring values of temperature and photosynthetically active radiation, cumulative radiant

heat product was calculated as an environmental factor. Finally, the above factors were introduced into the model for fresh weight estimation of substrate-cultivated lettuce grown in a solar greenhouse, and the instantaneous fresh weight of the lettuce was obtained (Figure 2). The fresh weight increment was obtained by subtracting the instantaneous fresh weight at one time point from another.

**Figure 2.** Schematic diagram of calculation of instantaneous fresh weight.
