Effects of Drip Irrigation with Plastic on Photosynthetic Characteristics and Biomass Distribution of Muskmelon

: An experiment was conducted in China to develop guidelines for the mulching drip irrigation of commercial muskmelon crops. Three sets of factors were laid out in rows to give a three × three factorial design. First, plastic covers were placed over the entire growing area (rows and inter-rows, or full), over the rows (half), or no plastic applied (none). Second, there was one irrigation pipe per row (T 1 ), three pipes for four rows (T 3 / 4 ), or one pipe for two rows (T 1 / 2 ). Finally, the plants were irrigated when the soil water content fell to 60%, 70%, or 80% of ﬁeld water capacity (FC). Information was collected on net CO 2 assimilation ( Pn ), plant growth, and yield. Overall, maximum Pn occurred with half plastic covering, one irrigation pipe for two rows, and irrigation at 80% FC. Plant fresh weight was higher with half plastic covering, one irrigation pipe per row, and irrigation at 70% or 80% FC. Yield was higher with half plastic covering, and irrigation at 70% or 80% FC. There were only small di ﬀ erences in the yield across numbers of irrigation pipes. These results suggest that overall productivity was better with plastic covers over the rows and irrigation at 70% or 80% FC. Di ﬀ erences in productivity with di ﬀ erent numbers of irrigation lines per row were small.


Introduction
Net CO 2 assimilation (Pn) is a determinant for crop growth and biomass accumulation. It is directly affected by temperature, light, and moisture [1], as well as by agronomic measures, such as irrigation [2,3]. Various agronomic measures are taken to regulate crop net CO 2 assimilation and photosynthetic product distribution. For example, crop intercropping makes full use of light energy and increases yield [4]. The improvement of fertilization technology can increase roots' nutrient absorption and leaves' net CO 2 assimilation [5]. Microelement and CO 2 fertilizer can regulate mesophyll cell activity and photosynthate accumulation [6][7][8][9]. In arid and semi-arid areas, water shortages are the limiting factor for crop growth. Drip irrigation with a plastic covering (DIP) is widely used due to its high-efficiency water savings. DIP involves factors such as irrigation volume, the plastic covering mode, and drip irrigation pipe density. Different combinations of these factors will cause differences in soil moisture, temperature, and enzymes [10]. These combinations may affect a crop's photosynthetic characteristics, biomass accumulation, and yield. Previous studies have found that crop yield is mainly affected by irrigation volume [11]. The water deficit stress caused by inadequate drip irrigation reduces the crop leaves' net CO 2 assimilation rate (Pn), transpiration rate (Tr), and stomatal conductance (Gs) [12], but it can optimize photosynthate distribution and increase yield [13]. Other studies indicate that different plastic covering methods can regulate soil moisture and temperature environment and improve crop Pn, Gs, and yield [14,15]. In addition, drip irrigation pipe density can significantly affect the spatial heterogeneity of soil moisture and temperature and the activity of soil enzymes in the crop root-zone; these changes indirectly affect the distribution of crop photosynthetic products [16]. Some studies have reported that plastic-mulching and irrigation management changed soil urease activity [17] and that soil urease regulated nitrogen cycling and increased crop yields [18]. Therefore, different combinations of irrigation volume, plastic covering mode, and drip irrigation pipe density could regulate crop photosynthetic characteristics, both theoretically and practically, thereby affecting photosynthetic product distribution and yield. Some researchers have increased crop Pn and yield through different combinations of irrigation volume and plastic covering modes [3,19]. However, it is not clear how different combinations of irrigation volume, plastic covering modes, and drip irrigation pipe densities can regulate a crop's photosynthetic characteristics and yield. Is it possible to further improve the photosynthetic efficiency and yield of crops by optimizing the combined parameters of their different irrigation volumes, plastic covering modes, and drip irrigation pipe densities? This is a very interesting question and worthy of further study to enhance the productivity potential of DIF measures.
Muskmelon is a healthy fruit rich in nutrients with and a unique taste. It has a large market demand and high economic benefits. China has become the largest producer of melon in the world [20]. However, there are problems with the cultivation of this fruit, such as the necessary large water and fertilizer input, soil nitrogen accumulation [21], and low photosynthetic efficiency [22]. DIF is a common method for muskmelon cultivation. The effects of mulching methods and irrigation rates on muskmelon growth have been studied by many researchers [23,24]. However, the effect of drip irrigation pipe density has received little attention. In addition, the regulation mechanism of DIF on muskmelon's photosynthetic characteristics and biomass distribution is not clear. This study uses an orthogonal design to evaluate the effects of different combinations of irrigation volume, plastic covering modes, and drip irrigation pipe densities on muskmelon leaves' photosynthetic parameters, biomass accumulation, and distribution in an attempt to further exploit the potential of DIF and regulate muskmelon photosynthetic product distribution and yield.

Experimental Design
The experiment was carried out in a greenhouse (108 m long and 8 m wide) in Yangling Agricultural High-Tech Zone (34 • 16 N, 108 • 08 E), Shaanxi Province, China, from October 2014 to May 2015. The soil's bulk density was 1.35 g·cm −3 , and the soil field's water capacity was 31.54%. The climate in this area is semi-arid, the average annual sunshine is about 2164 h, and the frost-free period is about 210 d. The experiment included three factors: the plastic covering method (P), the drip irrigation pipe density (T), and the lower limit of irrigation (L). Each factor had three levels. There were nine treatments and three repetitions for each treatment, with a total of 27 plots. The experiment was based on an orthogonal design table L9 (3 4 ), as in Table S1. The specific experimental design is shown in Table 1. 3 pipes for 4 rows (T 3/4 ) Three pipes were laid in three spaces between four rows of a group. 1 pipe for 2 rows (T 1/2 ) One pipe was laid between two prows.
Lower limit of irrigation 60% field water capacity (F) (L 60 ) The upper limit was 70% F. Irrigation water was derived from local agricultural water. 70% field water capacity (L 70 ) The upper limit was 80% F. 80% field water capacity (L 80 ) The upper limit was 90% F. The muskmelon cultivar "Shantian No. 1" was sown in seedling plugs, and its seedlings of 20 d were transplanted in double rows with a row spacing of 0.6 m. The planting plot was 6.0 m long and 1.0 m wide with double ridges. The cross-section of each planting plot was an inverted trapezoid, with an upper bottom width of 0.8 m, a lower bottom width of 0.4 m, and a depth of 0.2 m. The base fertilizer, consisting of 90 kg·ha −1 ternary compound fertilizer (N/P 2 O 5 /K 2 O = 15:15:15) and 160 kg·ha −1 organic fertilizer, was applied once before planting.
When muskmelon seedlings were transplanted, the irrigation volume was the same in each plot. The water management (L 60 , L 70 , and L 80 ) began after 13 days of transplanting. The information on the actual irrigation amount is provided in Table S2. During the experiment, three 1.0 m probes (Field TDR 200; Spectrum, USA) were installed in each planting plot to monitor the soil moisture at intervals of 10 cm, and the monitoring value was corrected by the drilling and drying gravimetric method. Water replenishment was calculated according to the following formula [16]: where M is the irrigation amount, m 3 ; s is the wet area in the soil, m 2 ; ρ b is the bulk density of the soil, 1.35 g·cm −3 ; p is the wetting ratio; h is the wetting depth of the soil, 40 cm; θ f is the maximum field water capacity, 31.54%; q 1 and q 2 are the upper limits of irrigation and the measured soil moisture, %F; and η is the water use coefficient, 0.95. The muskmelon cultivar "Shantian No. 1" was sown in seedling plugs, and its seedlings of 20 d were transplanted in double rows with a row spacing of 0.6 m. The planting plot was 6.0 m long and 1.0 m wide with double ridges. The cross-section of each planting plot was an inverted trapezoid, with an upper bottom width of 0.8 m, a lower bottom width of 0.4 m, and a depth of 0.2 m. The base fertilizer, consisting of 90 kg·ha -1 ternary compound fertilizer (N/P2O5/K2O = 15:15:15) and 160 kg·ha -1 organic fertilizer, was applied once before planting.
When muskmelon seedlings were transplanted, the irrigation volume was the same in each plot. The water management (L 60 , L 70, and L 80 ) began after 13 days of transplanting. The information on the actual irrigation amount is provided in Table S2. During the experiment, three 1.0 m probes (Field TDR 200; Spectrum, USA) were installed in each planting plot to monitor the soil moisture at intervals of 10 cm, and the monitoring value was corrected by the drilling and drying gravimetric method. Water replenishment was calculated according to the following formula [16]: where M is the irrigation amount, m 3 ; s is the wet area in the soil, m 2 ; b ρ is the bulk density of the soil, 1.35 g·cm -3 ; p is the wetting ratio; h is the wetting depth of the soil, 40 cm; f θ is the maximum field water capacity, 31.54%; q1 and q2 are the upper limits of irrigation and the measured soil moisture, %F; and η is the water use coefficient, 0.95.

Leaf Photosynthetic Characteristics
The photosynthetic characteristics were measured using an LI-6400 portable photosynthetic system (LI-COR, Lincoln, Nebraska, NE, USA) during the flowering period (FP), fruit swelling period (FSP), and mature period (MP). In each measurement activity, three plants of uniform growth were selected for each treatment, and three leaves were selected for each plant with sufficient light exposure and a relatively consistent leaf position. The determination time was selected as the morning of a clear day with few clouds for each growth period from 9:00 to 11:00 a.m. Specific measurement indicators included leaves Pn, Tr, Gs, and Ci. Each leaf was measured three times, and the mean value was calculated. The system contained a built-in light source and an open-air path, and the illumination was set to 800 μmol·m −2 ·s −1 .

Biomass, Yield, and Total Nitrogen Contents
Before the muskmelon fruits were harvested, three evenly grown plants were selected per treatment. The fruits were harvested separately for each treatment, and the yield (t·hm −2 ) was calculated. The above-ground plants were reaped and collected. The underground roots were collected using the excavation method, with a 60 × 50 × 50 cm rectangular body formed by the centerlines of the adjacent plants. The root samples were separated from the soil and packed into mesh bags and then brought back to the laboratory for cleaning and measuring. The rhizosphere soil samples were placed in sterile plastic tubes and taken to the laboratory for measuring. The plant and root samples were weighed by a 0.01-g electronic balance.
The nitrogen analysis was performed after the fruits were received. The plant and fruit samples were dried in a forced-air oven (fixed at 105 °C for 30 min and dried to a constant weight at 75 °C). The total nitrogen contents of the plant, root, and fruit were determined using the semi-micro Kjeldahl method.

Soil Urease Activity
The soil samples in FP, FSP, and MP were collected using the method, as shown in Section 2.2.2. Soil urease activity (mg NH3-N·g −1 ·d −1 ) was measured using the phenol-sodium hypochlorite colorimetric method [17].

Leaf Photosynthetic Characteristics
The photosynthetic characteristics were measured using an LI-6400 portable photosynthetic system (LI-COR, Lincoln, Nebraska, NE, USA) during the flowering period (FP), fruit swelling period (FSP), and mature period (MP). In each measurement activity, three plants of uniform growth were selected for each treatment, and three leaves were selected for each plant with sufficient light exposure and a relatively consistent leaf position. The determination time was selected as the morning of a clear day with few clouds for each growth period from 9:00 to 11:00 a.m. Specific measurement indicators included leaves Pn, Tr, Gs, and Ci. Each leaf was measured three times, and the mean value was calculated. The system contained a built-in light source and an open-air path, and the illumination was set to 800 µmol·m −2 ·s −1 .

Biomass, Yield, and Total Nitrogen Contents
Before the muskmelon fruits were harvested, three evenly grown plants were selected per treatment. The fruits were harvested separately for each treatment, and the yield (t·hm −2 ) was calculated. The above-ground plants were reaped and collected. The underground roots were collected using the excavation method, with a 60 × 50 × 50 cm rectangular body formed by the centerlines of the adjacent plants. The root samples were separated from the soil and packed into mesh bags and then brought back to the laboratory for cleaning and measuring. The rhizosphere soil samples were placed in sterile plastic tubes and taken to the laboratory for measuring. The plant and root samples were weighed by a 0.01-g electronic balance.
The nitrogen analysis was performed after the fruits were received. The plant and fruit samples were dried in a forced-air oven (fixed at 105 • C for 30 min and dried to a constant weight at 75 • C). The total nitrogen contents of the plant, root, and fruit were determined using the semi-micro Kjeldahl method.

Soil Urease Activity
The soil samples in FP, FSP, and MP were collected using the method, as shown in Section 2.2.2. Soil urease activity (mg NH 3 -N·g −1 ·d −1 ) was measured using the phenol-sodium hypochlorite colorimetric method [17].

Data Processing
The mean errors, one-way variance (one-way), and two-factor interactive analysis of the measured indicators were performed using SPSS 22.0 (IBM, Armonk, NY, USA). Tables and figures were developed using Excel 2010. The structural equation model analysis was performed using AMOS 25.0 (IBM, Armonk, NY, USA).

Results
3.1. Net CO 2 Assimilation Rate (P n ) P n was significantly affected by the plastic covering method and drip irrigation pipe density (p < 0.05; Table 2). The half plastic covering (H) increased P n by 29.54%, 9.52%, and 11.80%, respectively, in FP, FSP, and MP compared to the full plastic covering (F). Similarly, H increased Pn by 25.32% and 10.26%, respectively, in FP and FSP compared to the no plastic covering (N) group. Using one pipe for two rows (T 1/2 ) increased the P n in FSP by 20.24% and 27.58% compared to using one pipe for two rows (T 1 ) and three pipes for four rows (T 3/4 ), respectively. However, the P n of T 1/2 in MP showed an increase of 11.85% compared to T 3/4 . T 3/4 increased P n in FSP by 12.62% compared to T 1 . However, the lower limit of irrigation only had a significant effect on P n in FSP. An 80% field capacity (L 80 ) increased the P n in FSP by 43.62% and 11.36% compared to the results for 60% field capacity (L 60 ) and 70% field capacity (L 70 ), respectively. Therefore, the results of the single factor analysis showed that the optimal factors were H and T 1/2 . Overall, the optimal combination of H, T 1/2 , and L 80 can improve P n . Table 2. Effect of plastic covering method, drip irrigation pipe density, and lower limit of irrigation on net CO 2 assimilation rate (P n ) (µmol CO 2 ·m −2 ·s −1 ).

Growth Periods Single Experimental Factors
Flowering period (FP) P Note: P is plastic covering method (full, half, and no plastic plastic covering, i.e., F, H, and N); T is drip irrigation pipe density (one pipe per one row (T 1 ), three pipes for four rows (T 3/4 ), and one pipe for two rows (T 1/2 )); L is and lower limit of irrigation (60%, 70%, and 80% field capacity, i.e., L 60 , L 70 , and L 80 ). *and ns show significant (p < 0.05) and not significant difference of experimental factors, respectively. The different lowercase letters show significant difference (p < 0.05) in different level of the same factor. The following is the same.

Stomatal Conductance (G s )
The G s in FSP was significantly affected by the applied plastic covering method (p < 0.05; Table 3). H increased G s in FSP by 14.81% and 6.89% compared to F and N, respectively, whereas N promoted G s in FSP by 7.40% compared to F. The G s of the three periods was significantly affected by different levels of drip irrigation pipe density and the lower limit of irrigation. Specifically, the magnitudes of these effects were expressed as T 1/2 > T 3/4 > T 1 and L 60 = L 80 > L 70 in FP, T 1 > T 1/2 > T 3/4 and L 70 > L 80 > L 60 in FSP, and T 3/4 > T 1/2 > T 1 and L 80 > L 60 = L 70 in MP. After a comprehensive analysis, H, T 1/2 , and L 80 were shown to form the best combination for increasing G s , while the second-place combination was H, T 3/4 , and L 80 . Table 3. Effect of plastic covering method, drip irrigation pipe density, and lower limit of irrigation on stomatal conductance (G s ) (mmol·m −2 ·s −1 ).

Growth Periods Single Experimental Factors
Flowering period (FP)

Muskmelon Biomass
Compared with F and N, H increased fruit fresh biomass by 7.98% and 29.24%, plant fresh biomass by 10.83% and 22.77%, and total fresh biomass by 5.65% and 24.15%. T 1/2 and T 3/4, respectively, increased fruit fresh biomass by 8.59% and 5.05% compared to that of T 1 . L 70 and L 80, respectively, increased fruit fresh biomass by 18.42% and 16.46% compared to that of L 60 and, respectively, increased the total fresh biomass by 14.79% and 15.97% compared to that of L 60 .
The fresh biomass distribution was significantly affected by experimental factors (Figure 2). Compared with F and N, H reduced the proportion of plant fresh biomass to the total biomass by 5.64% and 9.44%, respectively, but increased the fruit fresh biomass proportion by 2.20% and 4.10%, respectively. T 1/2 and T 3/4 decreased the plant fresh biomass proportion by 36.71% and 28.83%, respectively, compared to T 1 ; however, they increased the fruit fresh biomass proportion by 12.16% and 10.48%, respectively. L 70 decreased the plant fresh biomass proportion by 9.88% and 9.05% compared to that of L 60 and L 80 , respectively. In addition, L 70 increased the fruit fresh biomass proportion by 4.44% and 3.83% compared to that of L 60 and L 80 , respectively. Therefore, H, T 1/2 , and L 70 significantly increased the fruit fresh biomass proportion.
Agriculture 2020, 10, x FOR PEER REVIEW 7 of 15 Agriculture 2020, 10, x; doi: FOR PEER REVIEW www.mdpi.com/journal/agriculture T1/2, and L80 were shown to form the best combination for increasing Gs, while the second-place combination was H, T3/4, and L80. Table 3. Effect of plastic covering method, drip irrigation pipe density, and lower limit of irrigation on stomatal conductance (Gs) (mmol·m −2 ·s −1 ).

Growth periods Single experimental factors
Flowering period (FP)

Muskmelon Biomass
Compared with F and N, H increased fruit fresh biomass by 7.98% and 29.24%, plant fresh biomass by 10.83% and 22.77%, and total fresh biomass by 5.65% and 24.15%. T1/2 and T3/4, respectively, increased fruit fresh biomass by 8.59% and 5.05% compared to that of T1. L70 and L80, respectively, increased fruit fresh biomass by 18.42% and 16.46% compared to that of L60 and, respectively, increased the total fresh biomass by 14.79% and 15.97% compared to that of L60.
The fresh biomass distribution was significantly affected by experimental factors (Figure 2). Compared with F and N, H reduced the proportion of plant fresh biomass to the total biomass by 5.64% and 9.44%, respectively, but increased the fruit fresh biomass proportion by 2.20% and 4.10%, respectively. T1/2 and T3/4 decreased the plant fresh biomass proportion by 36.71% and 28.83%, respectively, compared to T1; however, they increased the fruit fresh biomass proportion by 12.16% and 10.48%, respectively. L70 decreased the plant fresh biomass proportion by 9.88% and 9.05% compared to that of L60 and L80, respectively. In addition, L70 increased the fruit fresh biomass proportion by 4.44% and 3.83% compared to that of L60 and L80, respectively. Therefore, H, T1/2, and L70 significantly increased the fruit fresh biomass proportion.   Fresh biomass accumulation and distribution by the plastic covering method, drip irrigation pipe density, and lower limit of irrigation. Note: F, H, and N are full, half, and no plastic covering, respectively. T1, T3/4, and T1/2 are one pipe per one row, three pipes for four rows, and one pipe for two rows, respectively. L60, L70, and L80 are 60%, 70%, and 80% field water capacity, respectively. a (b,c), a' (b',c'), A' (B',C') and A (B, C) mean the significant difference (p < 0.05) of root, plant, fruit fresh and total biomass of different treatments, respectively.

Simple Correlation Analysis and Structural Equation Model
The correlations of the photosynthetic characteristic parameters, root fresh biomass, plant fresh biomass, and fruit fresh biomass during the three growth periods were analyzed (Table 4). It was found that fruit fresh biomass was positively correlated with the root fresh biomass, total fresh biomass, and Pn in FP. The root fresh biomass had a positive correlation with the total fresh biomass and Pn in FP. A structural equation analysis of the effect of Pn on fruit fresh biomass was also performed (Figure 3). The results showed that the total effects of Pn in FP, FSP, and MP on fruit fresh biomass were 0.55, 0.09, and −0.05, respectively. The root fresh biomass had a total effect of 0.58 on Figure 2. Fresh biomass accumulation and distribution by the plastic covering method, drip irrigation pipe density, and lower limit of irrigation. Note: F, H, and N are full, half, and no plastic covering, respectively. T 1 , T 3/4 , and T 1/2 are one pipe per one row, three pipes for four rows, and one pipe for two rows, respectively. L 60 , L 70 , and L 80 are 60%, 70%, and 80% field water capacity, respectively. a (b,c), a' (b',c'), A' (B',C') and A (B, C) mean the significant difference (p < 0.05) of root, plant, fruit fresh and total biomass of different treatments, respectively.

Simple Correlation Analysis and Structural Equation Model
The correlations of the photosynthetic characteristic parameters, root fresh biomass, plant fresh biomass, and fruit fresh biomass during the three growth periods were analyzed (Table 4). It was found that fruit fresh biomass was positively correlated with the root fresh biomass, total fresh biomass, and P n in FP. The root fresh biomass had a positive correlation with the total fresh biomass and P n in FP. A structural equation analysis of the effect of P n on fruit fresh biomass was also performed ( Figure 3). The results showed that the total effects of P n in FP, FSP, and MP on fruit fresh biomass were 0.55, 0.09, and −0.05, respectively. The root fresh biomass had a total effect of 0.58 on the fruit fresh Agriculture 2020, 10, 84 9 of 15 biomass. The variables in this model can account for 89% of the variation in the fruit fresh biomass of muskmelon. Note: * stands for significant correlation, and ** stands for extremely significant correlation. Pn is net CO 2 assimilation rate. FP, FSP, and MP mean flowering period, fruit swelling period, and mature period, respectively.
Agriculture 2020, 10, x; doi: FOR PEER REVIEW www.mdpi.com/journal/agriculture the fruit fresh biomass. The variables in this model can account for 89% of the variation in the fruit fresh biomass of muskmelon.

Soil Urease Activity
The soil urease activity under three drip irrigation pipe densities showed a significant difference in each period (Figure 4). In FP, T1/2, and T3/4 increased soil urease activity by 34.71% and 35.28% compared to T1 , respectively. In FSP, the soil urease activity of T1/2 was not significantly different from that of T3/4 and T1. However, T1 increased soil urease activity by 32.93% compared to T3/4. In MP, T1/2 and T3/4 increased soil urease activity by 22.26% and 56.90% compared to T1, respectively.

Soil Urease Activity
The soil urease activity under three drip irrigation pipe densities showed a significant difference in each period (Figure 4). In FP, T 1/2 , and T 3/4 increased soil urease activity by 34.71% and 35.28% compared to T 1 , respectively. In FSP, the soil urease activity of T 1/2 was not significantly different from that of T 3/4 and T 1 . However, T 1 increased soil urease activity by 32.93% compared to T 3/4 . In MP, T 1/2 and T 3/4 increased soil urease activity by 22.26% and 56.90% compared to T 1 , respectively.

Half Plastic Covering was Beneficial to Pn and Fresh Biomass
In this study, the half plastic covering was able to keep Pn at a high level during the whole growth period (Table 2), but it enhanced Gs (Table S3) and decreased Ci (Table S3) in the fruit swelling period, compared to the other two covering methods. This is different from the findings that a semimulch covering can significantly improve Ci, possibly due to the adoption of a straw covering [25]. Many studies have observed that a decrease of Pn is determined by the non-stomatal limitations when Pn and Gs decrease with a decrease of Ci [26]. However, there are few clear studies that explain why Pn and Gs increase with a decrease of Ci. The increase in the photosynthetic activity of mesophyll cells may enhance CO2 consumption, resulting in an increase of Pn and decrease of Ci [27]. The half plastic covering might have enhanced the photosynthetic activity of mesophyll cells and increased Pn, which consumed more leaf intercellular CO2 and then correspondingly reduced Ci. The decrease of Ci promoted the opening of the leaves' stoma and increased Gs, whereas the increased Gs enhanced the absorption of more CO2 by the leaves and improved Pn. A previous study found that the Pn and Gs of soybean increased with a decrease of Ci, which is similar to the results of our study. However, their study added exogenous silicon into the soil, and this was able to enhance the light capturing ability of leaves and improve Pn [28].
In addition, the half plastic covering can significantly promote root-soil-microorganism interactions and enhance the nutrient uptake by plant roots [29], which will indirectly affect and improve Pn. In this study, it was found that the half plastic covering increased the total fresh biomass by 5.65% and 24.15%, respectively, compared with the full covering and no plastic covering. The increase in total fresh biomass was due to Pn improvement. On the contrary, the vigorous photosynthetic product output also stimulates the enhancement of Pn. Our previous studies have found that a half plastic covering can maintain the soil and air connection and avoid anaerobic factors from increasing in the soil. In addition, there was no difference in the soil temperature between half the plastic covering and the full plastic covering. At the same time, the half plastic covering also created a more uniform distribution of 0-60 cm soil moisture in the root-zone, resulting in better soil moisture and temperature environment [30]. This may be the internal mechanism in soil by which a half plastic covering ultimately improves Pn, but the related changes in the photosynthetic physiological characteristics of leaves need to be studied further.

Half Plastic Covering was Beneficial to Pn and Fresh Biomass
In this study, the half plastic covering was able to keep Pn at a high level during the whole growth period (Table 2), but it enhanced Gs (Table S3) and decreased Ci (Table S3) in the fruit swelling period, compared to the other two covering methods. This is different from the findings that a semi-mulch covering can significantly improve Ci, possibly due to the adoption of a straw covering [25]. Many studies have observed that a decrease of Pn is determined by the non-stomatal limitations when Pn and Gs decrease with a decrease of Ci [26]. However, there are few clear studies that explain why Pn and Gs increase with a decrease of Ci. The increase in the photosynthetic activity of mesophyll cells may enhance CO 2 consumption, resulting in an increase of Pn and decrease of Ci [27]. The half plastic covering might have enhanced the photosynthetic activity of mesophyll cells and increased Pn, which consumed more leaf intercellular CO 2 and then correspondingly reduced Ci. The decrease of Ci promoted the opening of the leaves' stoma and increased Gs, whereas the increased Gs enhanced the absorption of more CO 2 by the leaves and improved Pn. A previous study found that the Pn and Gs of soybean increased with a decrease of Ci, which is similar to the results of our study. However, their study added exogenous silicon into the soil, and this was able to enhance the light capturing ability of leaves and improve Pn [28].
In addition, the half plastic covering can significantly promote root-soil-microorganism interactions and enhance the nutrient uptake by plant roots [29], which will indirectly affect and improve Pn. In this study, it was found that the half plastic covering increased the total fresh biomass by 5.65% and 24.15%, respectively, compared with the full covering and no plastic covering. The increase in total fresh biomass was due to Pn improvement. On the contrary, the vigorous photosynthetic product output also stimulates the enhancement of Pn. Our previous studies have found that a half plastic covering can maintain the soil and air connection and avoid anaerobic factors from increasing in the soil. In addition, there was no difference in the soil temperature between half the plastic covering and the full plastic covering. At the same time, the half plastic covering also created a more uniform distribution of 0-60 cm soil moisture in the root-zone, resulting in better soil moisture and temperature environment [30]. This may be the internal mechanism in soil by which a half plastic covering ultimately improves Pn, but the related changes in the photosynthetic physiological characteristics of leaves need to be studied further.

The Smaller Drip Irrigation Pipe Density was Beneficial to Pn
Previous studies have shown that drip irrigation pipe density significantly affects soil moisture, temperature, and permeability [31,32]. A smaller drip irrigation pipe density could produce a more uniform distribution of soil moisture and temperature around the root system and create better soil permeability, which would improve the soil's enzymatic activity [33]. In this study, one pipe for two rows was conducive to maintaining higher soil urease activity during the muskmelon growth period (Figure 4). High activity of soil urease can promote the conversion and absorption of soil nitrogen and affect a plant's net CO 2 assimilation [34]. Our determination also found that one pipe for two rows increased the nitrogen content in muskmelon plant stems by 10.31% and 2.26% compared to one pipe for one row and three pipes for four rows, respectively (Table S4). This may be the real reason why a smaller drip irrigation pipe density (one pipe for two rows) significantly improved Pn. A previous study of our research group also found that an uneven soil moisture distribution was created by a larger drip irrigation pipe density (one pipe for one row) [34]. The water supply method of one pipe for one row was able to more easily deliver soil moisture to the root system and led to greater moisture in the local soil of the root area. These changes might be more beneficial to the soil moisture exchange in the continuous system of "soil-plant-atmosphere". Therefore, using one pipe for one row increased the T r of leaves. However, using one pipe for one row also decreased the soil's urease activity in the flowering period, while that in the mature period was relatively higher. Overall, this would not be conducive to the improvement of plant nitrogen absorption and photosynthetic rate [35]. The effect of using three pipes and four rows on soil urease activity was similar to that when using one pipe and two rows. In addition, the greater uniformity of soil moisture distribution did not lead to high moisture content in the local soil around the roots but might delay the moisture exchange rate in the "soil-plant-atmosphere" continuous system, resulting in a plant Tr lower than that when using one pipe for one row and one pipe for two rows.

Lower Limit of Irrigation Significantly Affected Pn during Fruit Swelling Period
Plants regulate CO 2 absorption by adjusting the Gs of leaves during the photosynthetic process [36]. A value of 80%F could maintain a high level of Gs, which is conducive to CO 2 absorption and Pn improvement. However, 80%F resulted in the highest Pn and the lowest Ci during the fruit swelling period. The increased Pn of the mesophyll cells consumed more intercellular CO 2 , resulting in a lower value of Ci [28]. In this study, the three lower limits of irrigation had no significant difference on Pn in the flowering and mature period but had a significant effect on Pn during the fruit swelling period. Because the fruit swelling period is a key period for the synthesis of muskmelon photosynthetic products and because plants need considerable water to grow, larger drought stresses under 60%F caused the plants to close more stomata of their leaves, which reduced transpiration [37]. The measured values also showed that the Gs and the Tr of 60%F were the smallest during the fruit swelling period, yielding the lowest value of Pn for 60%F. However, the Ci value of 60%F was higher than that of 70%F and 80%F due to the slow intercellular CO 2 consumption resulting from the minimum Pn of 60%F. The moderate drought stresses created by 70%F might fundamentally meet the water demand of the muskmelon in the fruit swelling period. This might mean that 70%F also had larger Gs and Tr values, with only the secondary Pn at 80%F. In addition, a medium level Pn of 70%F might cause its intercellular CO 2 consumption rate to be higher than that of 60%F but lower than that of 80%F, resulting in a medium level of Ci.
A study found that the muskmelon Pn was the highest in the early growth period when the soil moisture was 60% to 80%F [38]. This finding is similar to the results of this study. However, this finding came from a pot-based experiment; there is currently no study on the Gs and the whole growth period of melon. Another study showed that a larger lower limit of irrigation significantly improved the Pn, Gs, and Tr of melon in the partial growth period, which is consistent with the results of this study. However, the photosynthetic characteristics of melon leaves during the whole growth period were not monitored [39].

Determination of the Combination of Different Mulching Drip Irrigation Measures
Biomass accumulation and yield improvement are the basic purposes of agricultural production. In this study, the root fresh biomass and Pn in FP and FSP promoted fruit fresh biomass (Figure 3). The muskmelon yield was positively correlated with the fruit fresh biomass, root fresh biomass, and total biomass (Table S5). The half plastic covering was found to promote the stem diameter of muskmelon plants, which was able to accelerate the nutrient exchanges between roots and plants and increase photosynthetic product accumulation [40]. Therefore, the half plastic covering significantly increased the muskmelon's fresh biomass and yield ( Figure S1), in addition to keeping Pn at a high level. The medium and low drip irrigation pipe density (one pipe for two rows and three pipes for four rows) was able to help distribute more photosynthetic products into fruits ( Figure 2) and to increase yield ( Figure S1). Moderate drought stress (70%F) enhanced yield as a result of the proportion of the fruits' fresh biomass increase. There was no significant difference in the muskmelon yield between low drought stress (80%F) and 70%F, even though 80%F promoted a greater distribution of photosynthetic products to the plants. The reason for this result may be that the relatively sufficient water supply (80%F) was able to reduce the heat dissipation ratio of the light energy absorbed by the leaves, increase the energy conversion ratio in the optical system [41], and improve the photosynthetic products and yields. However, a relatively scarce water supply (60%F) had the opposite effect on the absorption and utilization of light energy by leaves, as the yield was relatively reduced. Therefore, the optimal combination of a half plastic covering and one pipe for two rows and the sub-optimal combination of a half plastic covering and three pipes for four rows should be considered. The lower limit of irrigation should be 70%F.

Conclusions
The photosynthetic characteristics, biomass accumulation, and distribution of muskmelon were significantly affected by the plastic covering method, drip irrigation pipe density, and lower limit of irrigation. The half plastic covering maintained the Pn and Gs at a high level during the whole growth period and increased fruit biomass and yield. One pipe for two rows promoted the distribution of photosynthetic products into fruits and increased yield. Using three pipes for four rows was also beneficial to distribute more photosynthetic products into fruits and increase yield. A soil of 70%F could basically meet the water needs of muskmelon and enhanced yield due to an increase in the fruits' fresh biomass. In general, the optimal combination of a half plastic covering and one pipe for two rows, and the sub-optimal combination of a half plastic covering and three pipes for four rows, as well as a lower limit of irrigation of 70%F, are recommended in muskmelon cultivation, as they can improve Pn, the photosynthetic product distribution, and yield.