*3.1. Fresh Weight Growth Curve of Lettuce*

It can be seen from Figure 5 that on both sunny and cloudy days, the changes in fresh weight at nighttime are not obvious, while the changes in fresh weight during the daytime are relatively obvious. The fresh weight tends to decrease in the morning when the sun suddenly becomes stronger. The fresh weight then rises slowly and gradually recovers. When the sun is shining brightly at noon, the fresh weight tends to decrease again. The fresh weight recovers slowly in the afternoon, and it tends to remain stable. The main reason is that the transpiration during the daytime is obviously higher than that at nighttime [34], and the lettuce water content changes faster under the high temperatures, strong light, and low humidity of the daytime. Transpiration is an important indicator for measuring plant water content [35], and its strength is closely related to the degree of water loss in plants [36]. Moreover, water absorption through roots is the main way that water content is maintained in plants [37]. When the water lost by transpiration is higher than that absorbed by roots, the fresh weight of lettuce shows a downward trend. With the decrease of water content in a lettuce plant, a larger pull force is created, forcing the root to absorb more water to maintain normal metabolism and to supplement the water lost through transpiration. When the rate of water absorption by the roots increases slowly, approaching and exceeding the rate of water loss by transpiration, the fresh weight decreases slowly, stops gradually, and begins to increase. Finally, the fresh weight approaches the previous fresh weight range. During the processes of losing water through transpiration and absorption of water through the roots, and with the increase of photosynthesis of the lettuce leaves, the content of organic matter produced by photosynthesis gradually increases in the plant, making the lettuce larger in volume and allowing more water to be stored in the plant. The fresh weight of lettuce will then increase.

**Figure 5.** Fresh weight growth curves in different weather conditions. (**a**) On a cloudy day, (**b**) on a sunny day.

By comparing the change of fresh weight on a sunny day with that on a cloudy day, it was found that the variation in fresh weight growth on the sunny day was higher than that on the cloudy day. This was mainly due to the higher temperatures, stronger illumination, and lower humidity on the sunny day than on the cloudy day, meaning that the volumes of water lost through transpiration and absorbed by the roots were greater and the variations of fresh weight were stronger. There is no sunlight at nighttime and there is little change in temperature and humidity. The water lost by transpiration and the water absorbed by roots

is relatively stable. At the same time, compared with a cloudy day, lettuce has a higher level of photosynthesis and accumulates more organic matter on a sunny day, which enables lettuce to absorb more water, increasing its fresh weight.

In order to accurately construct the relationship between the environmental factors and fresh weight growth, the calculation time of cumulative environmental factors and instantaneous fresh weight of lettuce was set at 8:00 AM.

#### *3.2. Optimum Response Time*

It can be seen from Figure 6 that the response relationship between cumulative environmental factors and fresh weight growth over different cumulative days was different during the growth process among different samples in the same batch. With the increase in the number of cumulative days, the predicted determination coefficient showed a trend of increase at first. There was an individual decline in this process, but it did not affect the trend of increase. When the cumulative time reached 12 days, the determination coefficients for samples 1, 2, and 3 reached maximum values of 97.02%, 95.64%, and 97.06%, followed by a trend of decrease. In this process, there was an individual increase, but it did not affect the decreasing trend. The optimum response time of the most significant correlation between cumulative environmental factors and fresh weight growth among the different samples in the same batch was 12 days.

**Figure 6.** The response relationship between cumulative environmental factors and fresh weight growth among the different samples in the same batch. (**a**) Sample 1, (**b**) sample 2, (**c**) sample 3.

It can be seen from Figure 7 that the response relationship between cumulative environmental factors and fresh weight growth in different cumulative days was different during the growth process among the different samples in different batches. With the increase of cumulative days, the determination coefficient showed a trend of gradual increase at first. In this process, there was a decline in some cases, but it did not affect the trend of increase. When the determination coefficient reached the maximum value, it began to decrease. In this process, there was an increase in some cases, but it did not affect the decreasing trend. In the samples from the first batch, the coefficient of determination reached a maximum value of 97.57% for 13 cumulative days. The determination coefficient for 12 cumulative days was 97.29%, which was very close to the maximum value of the determination coefficient, and only 0.28% lower. In the samples of the second batch, the determination coefficient reached a maximum value of 94.14% for 13 cumulative days. The determination coefficient for 12 cumulative days was 93.47%, which was very close to the maximum value of the determination coefficient, and only 0.67% lower. In the samples of the third batch, the determination coefficient reached a maximum value of 97.72% for 10 cumulative days. The determination coefficient for 11 cumulative days was 97.39%, which was very close to the maximum value, and only 0.33% lower. The determination coefficient for 12 cumulative days was 97.09%, which was very close to the maximum value, and only 0.63% lower.

**Figure 7.** The response relationship between cumulative environmental factors and fresh weight growth among the samples in different batches. (**a**) Samples from the first batch, (**b**) samples from the second batch, (**c**) samples from the third batch.

It can be seen in Table 1 that with the increasing number of cumulative days, the average value of the determination coefficient in the three batches of samples showed a trend of first increasing and then decreasing. When the cumulative time reached 12 days, the average of the determination coefficient reached its maximum value of 95.95%, indicating that the optimum response time of the most significant correlation between cumulative environmental factors and fresh weight growth among different samples in the different batches was 12 days.

**Table 1.** Numerical distribution table of adjacent regions with maximum values of the coefficient of determination in different batches.

