The Effect of Drip Irrigation Quota on Biochemical Activities and Yield-Related Traits in Different Drought-Tolerant Maize Varieties

Abstract

Drip irrigation has a close relationship with the growth and development of maize grains and yield formation in semiarid areas. To explore the response mechanism of grain yield formation to drip irrigation quotas, a 2-year pond planting experiment was conducted under controlled conditions, by using two maize varieties with differences in drought resistance as experimental materials. Six treatments were set up, including CK1 (drought-resistant variety, 500 mm), T1 (drought-resistant variety, 350 mm), T2 (drought-resistant variety, 200 mm), CK2 (drought-sensitive variety, 500 mm), T3 (drought-sensitive variety, 350 mm), and T4 (drought-sensitive variety, 200 mm). The changes in maize grain filling characteristics, related hormones, enzyme activity related to starch synthesis, sugars and amino acids contents, and yield were analysed. The results showed that 100-grain weight at different filling times, filling rate, average filling rate, auxin, cytokinin, acid sucrose invertase, sucrose synthase, starch synthase, and adenosine diphosphate glucose pyro phosphorylase activities in maize grains decreased and the abscisic acid content and content of various amino acids and sugars in grains increased with the decrease in drip irrigation quota. The percentage of changes in drought-sensitive maize varieties was relatively high. The maize yield decreased with the decrease in drip irrigation quota. In summary, there was no significant difference in grain filling characteristics, hormone content, starch synthesis enzyme activity, and yield between maize treated with T1 (drought-resistant variety, 350 mm) and the control treatment. This effectively maintained grain growth and yield formation, achieving the goal of water saving and stable yields.

1. Introduction

Along with rice and wheat, maize provides at least 30% of the food calories for over 4.5 billion people in 94 developing countries, making it their preferred staple food [1]. Currently, the maize-planting area in 125 developing countries is close to 100 million hectares, making it one of the three most widely planted crops among 75 countries [2]. The stable increase in maize production plays an important role in promoting the socioeconomic development of countries around the world [3]. The uneven distribution of natural precipitation in the arid and semiarid areas where maize is grown, coupled with the increasingly evident phenomenon of climate warming, drying, and seasonal droughts [4], can easily hinder the ability of maize roots to absorb water and nutrients [5], slow leaf physiological metabolism [6], and growth and development processes [7], and ultimately lead to a decrease in grain yield [8]. Agricultural irrigation is an important way to maintain crop yields in arid and semiarid regions [9]. With the advancement of technology, traditional furrow irrigation and flood irrigation have gradually faded out of use due to water resource waste. Drip irrigation has become one of the main ways for maize to replenish water in semiarid regions. On the one hand, its advantage is that it can directly transport nutrients and water to the vicinity of the root zone of maize, which can make reasonable use of water and nutrients to promote the growth and development of the aboveground parts of the plant and grains [10]. On the other hand, drip irrigation can save water resources, reduce water evaporation, improve the irrigation water utilization rate [11], and protect the fragile agricultural ecological environment in semiarid areas. Therefore, analysing the growth and development status of corn grains under drip irrigation conditions is crucial for evaluating the production capacity of maize in semiarid areas.

The filling process of maize kernels is a process of starch synthesis and accumulation produced by a series of enzyme catalysis reactions. The activity of starch synthase is an important indicator to judge the limit of grain filling, and the amount of starch accumulation in the grain is directly related to maize grain yield [12]. Research has shown that optimizing the drip irrigation mode can significantly improve the activity of starch synthase in the late stage of maize grain filling, increase starch accumulation in maize grains, and achieve the goals of water conservation and yield increase [13]. As regulators of the growth and development of maize kernels, hormones play a regulatory role in starch synthesis [14]. Numerous studies have shown that hormones in grains can participate in the stress response between crops and environmental factors [15,16]. Therefore, by analysing the changes in hormone content in grains under different drip irrigation conditions, the regulatory mechanism of hormones on grain growth and development under drip irrigation conditions can be further explored. Amino acids, as the basic component of proteins, are crucial for maintaining normal cellular metabolism [17]. The amino acid content in maize kernels is closely related to their growth and development [18], and the impact of different soil moisture conditions on amino acid content is not the same. Research has shown that moderate water deficit can increase the content of essential amino acids in crop seeds [19]. When soil water deficit was severe, some researchers found that the increase in essential and nonessential amino acid content in soybean seeds may be related to quality traits [20]. However, some researchers found that as soil water deficit intensified, the amino acid content in maize seeds significantly decreased, and it was most sensitive to the drought generated during the jointing period, which was influenced by the growth environment and the combination of varieties [19]. Soluble carbohydrates such as sucrose, fructose, and glucose in grains can participate in the physiological metabolism of plants [21]. Some studies have found that soil moisture deficiency can lead to changes in sucrose content in grains, affecting the transportation of photosynthetic products and regulating the metabolic processes within crop cells [22]. Fructose is the main storage form of carbohydrates, serving as a nutrient storage medium and regulating plant physiological metabolism [23]. The changes in sugar content in grains were related to treatment time and stress level under different soil moisture conditions [24].

The author’s previous research showed [7,25] that setting reasonable irrigation quotas in the semiarid area of Jilin Province can ensure the photosynthesis and growth of maize leaves and improve the water use efficiency and yield of maize while reducing water resource input. Currently, researchers have paid less attention to the growth and development status of different drought-tolerant maize varieties during grain filling process under shallow drip irrigation conditions. However, the growth and development status of grains is closely related to the evaluation of maize yield formation in semiarid areas. In view of this, two maize varieties with significant differences in drought resistance were selected here to study the effects of drip irrigation quotas on the grain filling characteristics, related hormones, starch synthase activity, and main amino acid and sugar contents of different drought-resistant maize varieties. The regulatory mechanism of different drip irrigation quotas on grain growth and development was analysed. The research results can provide a theoretical basis for the water-saving and efficient cultivation of maize in semiarid areas.

2. Materials and Methods

2.1. Plant Materials

The conventional maize varieties Fumin 985 (FM985) and Xiangyu 218 (XY218) grown in Northeast China were used as the plant materials, with average growth periods of 128 and 132 days, and with the average required growing degree days (GDD) of 1510.56 and 1435.38 °C∙d in the year of 2021 and 2022, respectively. FM 985 is a drought-sensitive variety, while XY 218 is a drought-resistant variety. The selection of the varieties was obtained by screening the drought resistance of 41 conventional maize varieties planted in the region in the past 10 years during the seedling and full growth stages [26].

2.2. Experimental Site and Its Features

The experiment was carried out in the city of Gongzhuling, in the highly efficient crop water nursery of Agricultural Science of Jilin Province, Jilin Province, from 2021 to 2022 (east longitude 124.81°, north latitude 43.52°). The nursery had a continental monsoon climate in the northern temperate zone, with a frost-free period of approximately 144 days. The effective accumulated temperatures ≥10 °C in 2021 and 2022 were 3135.50 and 3087.50 °C, respectively, and the daily average temperatures during the whole growth period (May to October) were 20.49 ± 4.59 and 20.18 ± 4.43 °C, respectively, and the precipitation amounts during the whole growth period (May to October) were 560.40 and 660.50 mm, respectively. Before the implementation of the test, the content of soil organic matter in 0–40 cm was 22.40 g∙kg⁻¹, the content of available nitrogen was 75.70 mg∙kg⁻¹, the content of available phosphorus was 42.10 mg∙kg⁻¹, the content of available potassium was 120.30 mg∙kg⁻¹, the pH value was 6.43, and the soil type was black soil.

2.3. Experimental Design

There was a fully open and movable rainproof shed in the nursery. When there is external rainfall, the rainproof shed could be completely closed by two motors on both sides within 15 min to avoid external precipitation. There were multiple pools formed by cement walls on the sides and bottom of the rainproof shed, which can maintain the soil moisture in the pool from external influences (Figure 1). The experiment adopted the split plot design. Three drip irrigation quotas, 500 mm, 350 mm, and 200 mm, were set with a total of 6 treatments, each repeated 3 times, for a total of 18 ponds. The treatment names are detailed in

Table 1. List of Test Treatment.

Treatment	Varieties	Irrigation Quota (mm)
CK1	XY 218 (Drought-tolerant variety)	500
T1		350
T2		200
CK2	FM 985 (Drought-sensitive variety)	500
T3		350
T4		200

. The setting of a 500 mm drip irrigation quota as CK treatment was due to the average rainfall of the entire maize growth period in the region in the past 10 years (480 mm). The sowing dates were 6 May 2021 and 10 May 2022, and the harvest dates were 27 September 2021 and 28 September 2022. The pond was evenly ridged with a width of 60 cm and a planting density of 60,000 plants∙hm⁻². Each pond had an area of 24 m², a length of 4 m, and a soil depth of 1.5 m. Drip irrigation was used for irrigation. The drip irrigation belt was placed 5 cm on one side of the planting belt, and a water meter was set near the inlet of each pool to control the amount of irrigation each time. The two-year irrigation schedule is detailed in

Table 2. Drip irrigation system in the whole growth period of maize.

Irrigation Quota (mm)	Irrigation Times	Irrigation Interval Days (d)	Single Irrigation Amount (mm)
500	1st–2nd	15	35
	3rd–5th	10	35
	6th–8th	10	40
	9th–13th	7	38
	14th	-	15
350	1st–2nd	15	25
	3rd–5th	10	24
	6th–8th	10	29
	9th–13th	7	26
	14th	-	11
200	1st–2nd	15	17
	3rd–5th	10	12
	6th–8th	10	16
	9th–13th	7	15
	14th	-	7

.

2.4. Measurement of Biochemical and Morphological Traits

2.4.1. Grain Filling Rate

Fifty maize plants with basically the same growth before maize spinning were marked in each pond. The first sampling was conducted on the 5th day after the flowering of maize, with a sampling interval of 10 days. In 2021, sampling began on 3rd August and ended on 22nd September, and in 2022, sampling began on 1st August and ended on 19th September, with an average of 6 samples taken. During each sampling, 3 maize plants in the pond were selected, and the middle kernels of the ears were taken, dried in a drying oven at 80 °C to constant weight, an electronic balance was used to measure the dry weight of 100 kernels, and the grain filling rate and average filling rate were calculated based on the dry weight of the kernels [27].

2.4.2. Hormone Content and Starch Synthesis Related Enzyme Activity in Grains

At the maize-grain-forming stage (R2), filling stage (R3), and milk ripening stage (R4) in the year of 2021 and 2022, three maize plants with basically the same growth in each pond were selected, the grains in the middle of the ear were taken, and the enzyme-linked immunosorbent assay kit manufactured by Meimian Biotechnology Co., Ltd., Jinan, China was used to determine the content of cytokinin (CTK), abscisic acid (ABA), auxin (IAA), acid invertase, sucrose synthase starch synthase, and adenosine diphosphate glucose pyrophosphorylase activities [28].

2.4.3. Sugar and Amino Acid Content in Grains

In the R3 stage of 2021, grains in the middle of the spike were selected, and high-performance liquid chromatography tandem mass spectrometry (Shimadzu LC20AD—API 3200MD TRAP, Shimadzu Instruments Suzhou Co., Ltd., Suzhou, China) was used to determine the content of 20 conventional protein amino acids in the grains using the amino acid kit (MSLAB-45+AA) manufactured by Beijing Mass Spectrometry Medical Research Co., Ltd., Beijing China; in liquid phase conditions, the chromatographic column used MSLab 45+AA-C18 (150 × 4.6 mm 5 µm, Beijing Mass Spectrometry Medical Research Co., Ltd., Beijing, China) [29]. High-performance liquid chromatography tandem mass spectrometry (HPLC—MS/MS) was used to determine the content of 20 sugars using Shimadzu LC20AD—API 3200MD TRAP (Shimadzu Instruments Suzhou Co., Ltd., Suzhou, China) Innoval-NH2 (250 × 4.6 mm 5 µm, Beijing Mass Spectrometry Medical Research Co., Ltd., Beijing, China) was used as the chromatographic column in the liquid phase condition [30].

2.4.4. Yield and Its Components

At the maturity stage of maize (R6), a 10 m² area without sampling was selected for each plot, and yield was measured at 15.5% moisture content. For each treatment, 10 ears with basically the same size were selected for seed testing to obtain 100 grain weight, number of ears, ear length, and ear diameter [31].

2.5. Statistical Analysis

The mapping and analysis of all test data were performed by Excel 2016 and SAS 9.0 data processing systems (EMBL, North Carolina State University, Raleigh, NC, USA). All data in the table and figure are the average of 3 repetitions. The data were analyzed using the LSD method of one-way ANOVA (p < 0.05) to determine the significance between different treatments. The data were determined by using the two-way ANOVA analysis of variance program in SAS 9.0 data processing systems to perform variance analysis on the data of yield and its components.

3. Results

3.1. Grain Filling Characteristics

The drip irrigation quota significantly affected the 100-grain weight of maize at different filling times, and decreased with the decrease in drip irrigation quota. The 100-grain weights of the T2, T3, and T4 treatments were significantly lower than those of the CK1 and CK2 treatments (

Table 3. The effect of different drip irrigation quota on dry weight of 100 kernels.

Year	Treatments	Times after Anthesis (d)
		5	15	25	35	45	55
2021	CK1	3.84 ± 0.30 a	10.44 ± 0.21 a	22.02 ± 0.51 a	31.05 ± 1.83 a	34.95 ± 0.75 a	37.13 ± 0.28 a
	T1	3.63 ± 0.31 ab	9.71 ± 0.43 ab	19.98 ± 0.89 b	30.15 ± 0.18 a	33.93 ± 0.97 a	36.13 ± 0.30 a
	T2	3.03 ± 0.23 c	8.47 ± 0.23 c	17.04 ± 0.46 d	25.98 ± 0.65 bc	28.24 ± 0.55 c	29.34 ± 1.03 c
	CK2	3.94 ± 0.03 a	10.51 ± 0.37 a	22.18 ± 0.24 a	30.93 ± 0.51 a	34.09 ± 0.71 a	36.97 ± 0.52 a
	T3	3.53 ± 0.18 b	9.19 ± 0.36 b	18.15 ± 0.88 c	27.59 ± 0.38 b	30.68 ± 0.73 b	31.81 ± 0.77 b
	T4	2.93 ± 0.15 c	7.79 ± 0.14 d	16.04 ± 0.55 e	24.80 ± 0.59 c	26.46 ± 0.60 d	27.77 ± 0.52 d
2022	CK1	3.58 ± 0.19 a	8.63 ± 0.29 a	19.08 ± 0.55 a	26.22 ± 0.13 a	30.55 ± 1.10 a	33.70 ± 0.99 a
	T1	3.32 ± 0.38 ab	8.19 ± 0.57 a	18.53 ± 0.32 b	24.37 ± 0.89 ab	28.15 ± 1.13 ab	31.79 ± 0.79 ab
	T2	2.74 ± 0.26 c	7.31 ± 0.47 bc	16.46 ± 0.62 d	22.01 ± 0.49 c	24.98 ± 1.07 c	27.15 ± 0.76 c
	CK2	3.54 ± 0.13 a	8.51 ± 0.40 a	19.28 ± 0.35 a	26.32 ± 0.28 a	30.46 ± 0.86 a	32.87 ± 0.93 a
	T3	2.91 ± 0.12 bc	7.84 ± 0.40 b	17.75 ± 0.63 c	22.89 ± 0.34 b	27.10 ± 0.50 b	28.86 ± 0.55 bc
	T4	2.56 ± 0.09 c	6.91 ± 0.22 c	15.92 ± 0.41 d	20.82 ± 0.23 c	24.01 ± 0.90 c	25.63 ± 0.51 d

Note: CK1—drought-tolerant variety with 500 mm; T1—drought-tolerant variety with 350 mm; T2—drought-tolerant variety with 200 mm; CK2—drought-sensitive variety with 500 mm; T3—drought-sensitive variety with 350 mm; T4—drought-sensitive variety with 200 mm. Values are means ± SD, n = 3. Different lowercase letters indicate significant differences at p < 0.05.

). Starting 25 days later, the 100-grain weight of the T2, T3, and T4 treatments was significantly lower than that of T1 treatment. There was no significant difference in 100-grain weight between the T1 treatment and CK1 and CK2 treatments at different time stages. Compared with the above two treatments, the percentage of decrease in 100-grain weight of drought-sensitive varieties was higher. Compared with the CK2 treatment, the T3 and T4 treatments showed average decreases of 12.15% and 22.94%, respectively.

The grain-filling rate of all treatments showed a parabolic trend (Figure 2), reaching its peak at 25 days. The CK1 and CK2 treatments maintained a relatively high filling rate during the grain-filling process, which was significantly lower than the T1 treatment at the two growth stages of 35 days in 2021 and 55 days in 2022. This may be because the grain-filling rate of T1 treatment was significantly lower than that of CK1 and CK2 treatments at 25 days and 35 days in 2021 and 2022, respectively, and the increase in grain filling rate was delayed compared to CK1 and CK2 treatments. At other growth stages, the grain filling rate decreased with the decrease in drip irrigation quota. At 25 days, the T2, T3, and T4 treatments significantly decreased compared to the CK1 and CK2 treatments.

As the drip irrigation quota decreased, the average grain filling rate of both maize varieties showed a decreasing trend (Figure 3), and those of the T2, T3, and T4 treatments were significantly lower than those of the CK1, T1, and CK2 treatments. The percentage of decrease in the average grain filling rate of drought-sensitive varieties was relatively high. Compared with that of the CK2 treatment, the average grain filling rate of T3 and T4 treatments decreased by 14.55% and 25.45% and 12.24% and 22.45%, respectively, in 2 years.

3.2. Grain Hormone Content

The CTK content in the grains of the two maize varieties showed a significant decrease with the decrease in drip irrigation quota (Figure 4). The drought-sensitive varieties had a higher percentage of decrease, and compared with that of the CK2 treatment, the CTK content in grains of the T3 and T4 treatments showed an average decrease of 5.91% and 6.60% at each growth stage. There was no significant difference in the CTK content between the T1 treatment and the CK1 and CK2 treatments in the R2 and R4 stages.

The IAA content in grains showed the same trend as CTK (Figure 5), and there was no significant difference between the T1 treatment and the CK1 and the CK2 treatments. Compared with that of the CK1 treatment, the content of the T1 treatment only decreased by 2.45% at each growth stage.

The ABA content showed an opposite trend to the above two indicators (Figure 6), and the ABA content in the CK1 and CK2 treatments was significantly lower than that in the T2, T3, and T4 treatments. The ABA content in drought-sensitive varieties increased as the drip irrigation quota decreased. Compared with that of the CK2 treatment, the ABA content of the T3 and T4 treatment significantly increased by 8.63% and 13.86%, respectively, at each growth stage.

3.3. Enzyme Activity Related to Starch Synthesis in Grains

The activity of four starch-synthesis-related enzymes decreased with the decrease in drip irrigation quota (Figure 7), and there was no significant difference in the activity of the four enzymes among the CK1, T1, and CK2 treatments. Compared with CK1 and CK2, the activities of acid sucrose invertase, sucrose synthase, starch synthase, and adenosine diphosphate glucose pyrophosphorylase in the T2, T3, and T4 treatments decreased significantly, with the highest percentage of decrease in drought-sensitive varieties. Compared with the CK2 treatment, acid invertase, sucrose synthase, starch synthase, adenosine diphosphate glucose pyrophosphorylase, and adenosine diphosphate glucose pyrophosphorylase in the T3 and T4 treatments decreased by 7.33% and 14.85%, 9.56% and 16.28%, and 10.11% and 17.97%, respectively, on average, in each growth stage. There was no significant difference in the activity of four starch-synthesis-related enzymes between the T1 treatment and the CK1 and CK2 treatments.

3.4. Content of Multiple Amino Acids in Grains

The content of different amino acids in the grains of the two maize varieties increased with the decrease in drip irrigation quota (

Table 4. The effect of different drip irrigation quota on content of various amino acids in grains.

Indexes	Treatments
	CK1	T1	T2	CK2	T3	T4
Leucine content (μg∙g−1)	108.78 e	137.65 c	172.49 a	120.35 d	156.39 b	173.53 a
Valine content (μg∙g−1)	120.40 d	169.49 b	203.39 a	136.04 c	184.14 ab	213.74 a
Threonine content (μg∙g−1)	111.62 d	153.69 bc	208.55 a	144.56 c	182.13 ab	202.36 a
Tryptophan content (μg∙g−1)	19.61 d	25.83 b	33.68 a	24.30 c	29.26 ab	34.44 a
Lysine content (μg∙g−1)	57.59 e	71.62 d	125.90 b	71.79 d	93.04 c	144.26 a
Methionine content (μg∙g−1)	28.32 c	37.12 b	43.15 a	32.65 c	40.19 ab	46.18 a
Phenylalanine content (μg∙g−1)	73.57 d	89.41 c	105.94 ab	80.97 c	97.33 b	119.41 a
Isoleucine content (μg∙g−1)	69.36 c	78.44 b	92.35 a	67.71 c	88.19 a	96.64 a
Total essential amino acid content (μg∙g−1)	589.25 e	763.55 c	985.46 a	678.37 d	870.67 b	1030.55 a
Glycine content (μg∙g−1)	22.31 c	36.45 b	49.88 a	27.58 c	41.78 ab	53.16 a
Alanine content (μg∙g−1)	655.80 d	774.22 bc	977.59 a	716.97 c	843.84 b	994.89 a
Serine content (μg∙g−1)	74.76 c	95.93 b	116.86 a	86.31 c	102.01 ab	122.67 a
Proline content (μg∙g−1)	269.75 d	323.43 c	491.48 a	283.95 c	385.64 b	485.33 a
Cysteine content (μg∙g−1)	-	-	-	-	-	-
Asparagine content (μg∙g−1)	68.36 d	85.01 c	118.67 a	75.48 cd	96.77 b	121.18 a
Aspartic acid content (μg∙g−1)	69.92 d	83.15 b	108.95 a	79.67 c	94.99 ab	102.38 a
Glutamine content (μg∙g−1)	-	-	-	-	-	-
Glutamic acid content (μg∙g−1)	303.17 c	389.21 b	498.35 a	334.64 c	423.49 b	501.13 a
Histidine content (μg∙g−1)	27.48 c	32.47 b	47.37 a	30.18 bc	37.89 ab	45.24 a
Arginine content (μg∙g−1)	47.74 e	66.76 c	89.74 a	57.01 d	77.01 b	89.28 a
Tyrosine content (μg∙g−1)	65.49 d	84.58 bc	98.59 ab	74.75 c	92.49 b	107.03 a
Total non-essential amino acids content (μg∙g−1)	1604.79 e	1971.21 c	2602.43 a	1766.54 d	2195.90 b	2628.17 a
Total amino acids content (μg∙g−1)	2194.04 d	2643.76 c	3340.06 a	2308.91 d	2943.51 b	3635.00 a
Percentage of essential amino acids in total amino acids (%)	26.85 a	27.92 a	27.46 a	27.74 a	28.39 a	28.17 a

Note: CK1—drought-tolerant variety with 500 mm; T1—drought-tolerant variety with 350 mm; T2—drought-tolerant variety with 200 mm; CK2—drought-sensitive variety with 500 mm; T3—drought-sensitive variety with 350 mm; T4—drought-sensitive variety with 200 mm. Values are means ± SD, n = 3. Different lowercase letters indicate significant differences at p < 0.05.

). Compared with the CK1 treatment, the total content of eight essential amino acids in the T1 and T2 treatments increased by 29.58% and 67.23%, respectively. Compared with that of the CK2 treatment, the total content of eight essential amino acids in the T3 and T4 treatments increased by 28.35% and 51.92%, respectively, and that of drought-tolerant maize varieties increased by a higher percentage. Cysteine and glutamine were not detected in all treatments. Compared with the CK1 treatment, the total content of 12 non-essential amino acids in the T1 and T2 treatments increased by 22.83% and 62.17%, respectively, and that in the T3 and T4 treatment increased by 24.31% and 48.77%, respectively, compared with the CK2 treatment. The ratio of essential amino acids to total amino acids of two maize varieties was the highest in the T1 and T3 treatments with a 350 mm irrigation quota, but there was no significant difference among the six treatments.

3.5. Multiple Sugar Contents in Grains

Under different drip irrigation quotas, the contents of sucrose (58.97%), fructose (15.07%), and glucose (12.19%) in maize kernels accounted for a higher proportion of the total content of 20 sugars, and pine alcohol was not detected in all treatments (

Table 5. The effect of different drip irrigation quota on content of various sugars in grains.

Indexes	Treatments
	CK1	T1	T2	CK2	T3	T4
Erythrin content (μg∙g−1)	31.86 c	36.67 bc	53.51 a	20.60 d	40.89 b	57.37 a
Fructose content (μg∙g−1)	10,561.25 c	11,240.31 b	13,351.85 a	9546.35 d	12,930.73 a	13,605.31 a
Xlopyranose content (μg∙g−1)	76.05 c	80.27 c	136.43 a	58.70 d	116.39 b	147.77 a
Rhamnose content (μg∙g−1)	43.19 d	54.18 c	71.53 a	32.22 e	64.59 b	75.94 a
Glucose content (μg∙g−1)	8127.93 c	9444.44 b	10,781.53 a	7887.60 c	10,355.03 a	10,701.19 a
Sorbitol and Mannitol content (μg∙g−1)	1144.78 c	1220.16 bc	1553.71 a	953.72 d	1314.78 b	1525.76 a
Pineol content (μg∙g−1)	-	-	-	-	-	-
Galactolipin content (μg∙g−1)	311.13 c	438.89 b	564.89 a	283.17 c	496.92 ab	583.83 a
Seminose content (μg∙g−1)	5155.04 c	5497.36 b	6642.98 a	4683.30 d	5888.89 ab	6330.17 a
Fucose content (μg∙g−1)	6.79 c	7.98 bc	10.09 ab	5.39 d	8.85 b	13.45 a
Arabinose content (μg∙g−1)	201.75 b	204.42 b	244.44 a	191.52 c	229.71 a	251.94 a
Saccharose content (μg∙g−1)	41,860.47 c	45,001.13 b	52,220.25 a	37,092.98 d	49,257.25 ab	56,706.25 a
Lactobiose content (μg∙g−1)	1324.48 c	1372.36 c	1577.78 a	1212.62 d	1437.98 b	1536.57 a
Raffinose content (μg∙g−1)	147.78 c	159.44 b	177.61 a	130.51 d	169.96 a	179.21 a
Stachyose content (μg∙g−1)	8.64 c	10.09 bc	15.08 a	7.04 d	12.73 b	15.70 a
Xylitol content (μg∙g−1)	15.74 c	17.23 bc	30.36 a	13.93 d	21.85 b	32.54 a
Inositol content (μg∙g−1)	56.83 c	68.64 b	79.89 a	49.35 c	74.63 ab	80.87 a
D-ribose content (μg∙g−1)	77.75 b	84.89 b	118.47 a	64.89 c	111.04 a	125.29 a
Maltose content (μg∙g−1)	910.82 d	1082.53 c	1518.65 a	658.78 e	1235.19 b	1546.04 a
Trehalose content (μg∙g−1)	88.70 d	115.55 c	190.74 a	75.33 e	142.25 b	197.16 a
Total content of 20 sugars (μg∙g−1)	70,150.97 c	76,136.55 b	89,339.79 a	62,968.00 d	83,909.65 ab	93,712.35 a

Note: CK1—drought-tolerant variety with 500 mm; T1—drought-tolerant variety with 350 mm; T2—drought-tolerant variety with 200 mm; CK2—drought-sensitive variety with 500 mm; T3—drought-sensitive variety with 350 mm; T4—drought-sensitive variety with 200 mm. Values are means ± SD, n = 3. Different lowercase letters indicate significant differences at p < 0.05.

). The 19 sugar contents in maize kernels showed an increasing trend with the decrease in drip irrigation quota, and the percentage of increase in drought-sensitive varieties was higher than that in drought-tolerant varieties. Compared with that of the CK1 treatment, the percentage increases in sucrose, fructose, and glucose contents in the T1 and T2 treatments were 7.50% and 24.75%, 6.43% and 26.42%, and 16.20% and 32.65%, respectively. Compared with that of the CK2 treatment, the percentage increases in sugar content in the T3 and T4 treatments were 32.79% and 52.88%, 35.45% and 42.52%, and 31.28% and 35.67%, respectively. The total content of 19 sugars also showed similar changes. Compared with that the CK1 treatment, the total content of 19 sugars in the T1 and T2 treatments significantly increased by 8.53% and 27.35%, respectively, while that of the T3 and T4 treatments significantly increased by 33.26% and 48.82%, respectively, compared to that of the CK2 treatment.

3.6. Yield and Yield Components

The impact of varieties on yield, 100-grain quality, ear length, and ear diameter were significant (

Table 6. The effect of different drip irrigation quota on maize yield and its components.

Year	Treatments	Yield (kg·hm−2)	100-Grain Mass (g)	Grain Number per Spike	Spike Length (cm)	Spike Diameter (cm)
2021	CK1	10,689.17 ± 256.69 a	33.76 ± 1.78 a	544.67 ± 31.07 a	18.87 ± 0.68 a	5.14 ± 0.08 a
	T1	10,206.89 ± 344.41 a	32.43 ± 0.69 a	528.44 ± 30.48 ab	18.17 ± 0.78 a	5.01 ± 0.05 ab
	T2	9308.24 ± 377.85 b	27.88 ± 0.22 bc	490.00 ± 15.53 bc	16.50 ± 0.85 b	4.82 ± 0.06 cd
	CK2	10,569.42 ± 114.26 a	32.80 ± 1.25 a	539.56 ± 16.61 a	18.03 ± 0.55 a	5.10 ± 0.06 a
	T3	9664.58 ± 277.12 b	28.14 ± 1.40 b	501.89 ± 14.66 bc	16.83 ± 0.90 b	4.93 ± 0.11 bc
	T4	8530.81 ± 386.16 c	25.87 ± 0.98 c	479.47 ± 18.37 c	15.77 ± 0.83 b	4.71 ± 0.10 d
2022	CK1	10,527.15 ± 329.62 a	33.53 ± 1.60 a	531.89 ± 13.19 a	17.67 ± 0.47 a	5.16 ± 0.15 a
	T1	10,163.94 ± 522.60 ab	32.11 ± 1.06 a	514.56 ± 20.63 a	17.23 ± 0.83 ab	5.01 ± 0.14 ab
	T2	8880.17 ± 367.47 c	27.51 ± 0.51 c	469.78 ± 14.75 bc	16.30 ± 0.40 cd	4.77 ± 0.10 bc
	CK2	10,692.88 ± 181.37 a	34.52 ± 1.62 a	538.00 ± 21.74 a	17.60 ± 0.62 ab	5.19 ± 0.12 a
	T3	9657.74 ± 254.88 b	29.48 ± 0.66 b	488.67 ± 13.01 b	16.77 ± 0.55 bc	4.87 ± 0.07 bc
	T4	8332.13 ± 482.19 c	27.98 ± 0.56 c	455.56 ± 20.25 c	15.83 ± 0.65 d	4.64 ± 0.15 c
Source of variance	Degree of freedom	p > F
Varieties (V)	1	0.003	0.001	0.073	0.008	0.037
Drip irrigation quota (Q)	2	<0.0001	<0.0001	<0.0001	<0.0001	<0.0001
Year (Y)	1	0.321	0.061	0.046	0.050	0.791
V × Q	2	0.514	0.920	0.669	0.400	0.385
V × Y	1	0.442	0.010	0.841	0.169	0.938
Q × Y	2	0.061	0.002	0.291	0.706	0.385
V × Q × Y	2	0.902	0.894	0.899	0.842	0.740

Note: CK1—drought-tolerant variety with 500 mm; T1—drought-tolerant variety with 350 mm; T2—drought-tolerant variety with 200 mm; CK2—drought-sensitive variety with 500 mm; T3—drought-sensitive variety with 350 mm; T4—drought-sensitive variety with 200 mm. Values are means ± SD, n = 3. Different lowercase letters indicate significant differences at p < 0.05. p > F-mean square value.

). The drip irrigation quota had a significant impact on all five indicators, and the year only had a significant impact on the number of grains per ear, but not on other indicators. The variety and drip irrigation quota were key factors in regulating maize yield and its components, while the year had no effect on maize yield and its components. The interaction between year and drip irrigation quota, as well as the interaction between year and drip irrigation quota and variety, had no significant impact on yield, 100-grain quality, number of grains per ear, ear length, and ear diameter. This indicated that drip irrigation quotas in different years and years, as well as the interaction between drip irrigation quotas and varieties, had the same impact trend on the above five indicators, and their response regulation was similar. The interaction between varieties and drip irrigation quotas, as well as the interaction between varieties and years, had no significant impact on yield, grain number per ear, ear length, and ear diameter. This indicated that the impact trend of drip irrigation quotas between different varieties and the interaction between different varieties and years on the above four indicators is consistent. Table 6 shows that with the decrease in drip irrigation quota, yield, 100-grain quality, number of grains per ear, ear length, and ear diameter all showed a decreasing trend. Among them, drought-sensitive varieties had a higher percentage of decline in the above indicators. Taking yield as an example, compared to that of the CK1 treatment, the yield of the T1 and T2 treatments decreased by 4.51% and 12.92%, and 3.45% and 15.65%, respectively, in 2 years, while that of the T3 and T4 treatments decreased by 8.56% and 19.29%, and 9.68% and 22.08%, respectively, compared to that of the CK2 treatment. The 100-grain quality, number of grains per ear, ear length, and ear diameter also showed similar changing trends. There was no significant difference in various indicators between the T1 treatment and the CK1 and CK2 treatments.

3.7. Correlation Analysis

The correlation analysis of the main data in this article is shown in Figure 8. It can be seen that the drip irrigation quota was significantly negatively correlated with the abscisic acid content, the total essential amino acid, the total amino acids, and the total content of 20 sugars in the maize grain, and was significantly positively correlated with other indicators (Figure 8). There was a significant negative correlation between maize yield and abscisic acid content, total essential amino acid, total amino acids, and total of 20 sugars in maize grains, and there was a significant positive correlation between maize yield and other indicators.

4. Discussion

The filling period is the most important for the formation of maize yield, and maize grain filling is the process of transporting photosynthetic material products to the grains [32]. Staged soil moisture deficiency can have adverse effects on the grain filling process of maize [14]. This study showed that a decrease in drip irrigation quota resulted in a decrease in the 100-grain weight of grains. The 100-grain weight of the T2, T3, and T4 treatments was significantly lower than that of the CK1 and CK2 treatments at all time stages. The CK1 and CK2 treatments maintained a relatively high filling rate during the grain-filling process. At the peak filling rate of 25 days, the T2, T3, and T4 treatments significantly decreased compared to CK1 and CK2 treatments. After 35 days of the T1 treatment, the percentage decrease in the filling rate compared to the CK1 and CK2 treatments was very small, indicating that drought-tolerant maize varieties alleviate the damage caused by insufficient drip irrigation quotas by increasing the grain filling time in the later filling stage. This is basically consistent with the research results of previous studies [33,34]. In addition, it was found that compared with drought-tolerant varieties, drought-sensitive varieties showed a higher percentage decrease in the average grain filling rate when the drip irrigation quota decreased, indicating that drought-sensitive varieties were inhibited during the grain filling process.

Plant hormones plays a regulatory role in endosperm development and grain filling in Poaceae crops [35]. This study showed that the CTK and IAA contents in the grains of two maize varieties decreased with decreasing drip irrigation quota. The percentage of decrease in drought-sensitive varieties was higher than that in drought-tolerant varieties. The higher IAA and CTK contents in the T1 treatment can promote the transport of photosynthetic products to the grains, forming a larger “sink”. Under mild soil water deficit, the impact on grain filling was relatively small, which is similar to the study of Liang et al. [36] on wheat. ABA, as an important crop growth hormone, is of great significance for crops to resist stress [37]. The decrease in drip irrigation quotas led to a significant increase in ABA content in grains, while drought-sensitive varieties had a higher percentage of increase, indicating that the growth and development of drought-sensitive varieties were inhibited. This was basically consistent with the research of Wang et al. [14] on maize and Zhang et al. [33] on wheat. The decrease in drip irrigation quota also resulted in a decrease in the activity of four starch synthesis related enzymes. Drought-sensitive varieties had a higher percentage of decrease. Acid sucrose invertase can regulate sugar accumulation and utilization in grains, maintain sucrose balance in the “sink”, and regulate plant ageing and grain maturity [38], while sucrose synthase regulates starch and cellulose synthesis and grain reproductive growth, regulates sucrose metabolism ability, and affects crop grain growth and development [39]. The results of this study were basically consistent with those of Liu et al. [39]. Drought-sensitive varieties had poor sucrose metabolism ability during the filling stage under drought stress, which inhibited the grain filling process and duration. Starch synthesis in maize grains is the result of a series of enzyme catalysis, and starch synthase and adenosine diphosphate glucose pyrophosphorylase indirectly control the speed of enzyme catalysis in grains [40], affecting the speed of starch synthesis in grains. This study found that the decrease in drip irrigation quota significantly reduced the activities of starch synthase and ADP glucose pyrophosphorylase in grains, and starch synthesis was inhibited.

Amino acids and sugars, as important osmotic regulators in plant organisms, are more sensitive to drought stress [41]. The results of the study showed that the contents of different amino acids and sugars in the grains of two maize varieties increased with the decrease in drip irrigation quota, indicating that the decrease in drip irrigation quota caused maize to suffer from water stress. Amino acids and sugars were added to maize grains to reduce cell membrane peroxidation, thereby alleviating stress damage caused by an insufficient drip irrigation quota. Furthermore, in the grains of different drought-resistant maize varieties, the changes in amino acid and sugar contents caused by the decrease in drip irrigation quota were not the same. Drought-resistant maize varieties were found to have a higher percentage of increase in amino acid content, while drought-sensitive varieties were found to have a higher percentage of increase in sugar content. This may be due to genetic factors [17], and there is a complementary trend between sugar and amino acids in the grains under stress.

In the production of maize in semiarid areas, irrigation is the key to increasing maize yield. In addition, the summer rainy and hot season in semiarid areas has a high potential for crop yield [4,15]. Reasonable irrigation is very important for improving crop yield and protecting the ecological environment. Previous studies [7,27] have found that reducing irrigation volume appropriately during drip irrigation and maintaining a certain soil water deficit can achieve water-saving and yield-increase effects. This study found that when the drip irrigation quota of drought-sensitive varieties decreased, the percentage of decline in yield and its constituent factors was higher, and the varieties had a significant impact on the above indicators. However, there was no significant difference between the T1 treatment and the CK1 and CK2 treatments. Under the condition of reducing the drip irrigation quota by 150 mm, the reduction in maize yield was relatively small. From the perspective of saving water resources and increasing economic benefits, using drought-resistant maize varieties and using a 350 mm drip irrigation quota can achieve water-saving and stable yield goals, which has certain application value. However, this study was completed under controlled conditions and did not clarify the water-saving and yield increasing effects of different drought-tolerant maize varieties under different natural precipitation years. In the future, research will be conducted in a field environment to explore the formation of more optimized irrigation systems based on different types of varieties.

5. Conclusions

In summary, the decrease in drip irrigation quota affects the grain filling process and starch synthesis of two different drought-tolerant maize varieties, resulting in the decrease of the grain filling rate, average filling rate, IAA and CTK content in the grain, and the activities of acid sucrose invertase, sucrose synthase, starch synthase, and adenosine diphosphate glucose pyrophosphorylase decrease, and the increase of the ABA content, the content of various amino acids and sugars in grains. Furthermore, the decrease in drip irrigation quota resulted in a higher percentage of changes in the above indicators in drought-sensitive maize varieties compared to drought-resistant maize varieties. The T1 treatment (drought-resistant variety, 350 mm) showed no significant differences compared to the control treatment in terms of grain-filling characteristics, hormone content, starch synthesis enzyme activity, and yield. It effectively maintained grain growth and yield formation, achieving the goal of water-savings and stable yields. The research results have important theoretical and practical value for maize production and drought-resistant maize variety breeding under drip irrigation conditions in semiarid areas.