The Responses of Four Typical Annual Desert Species to Drought and Mixed Growth

Abstract

Soil desiccation is increasingly threatening the growth of vegetation in artificial forests at the margins of arid desert oases, where a variety of annual herbaceous plants coexist. It is important to understand the response of annual desert plants to droughts and mixed growth and the resulting patterns of change in photosynthetic and physiological properties. Our results showed that annual plants were primarily affected by drought stress, and the effect of interspecific competition was significant only under severe drought stress. In the sprouting stage, moderate drought increased seed germination rates, whereas severe drought stress decreased the germination rates. In the growth phase, the aboveground and belowground parts of annual herbaceous plants showed a synergistic response to drought. Under mild and moderate drought stress, annual herbaceous plants promoted photosynthesis by increasing chlorophyll content, thereby promoting plant stem growth. Following moderate and high drought, root vigor increased to maintain basic metabolic activities and annual herbaceous plants used the “shadow and avoid” response by increasing stem and root length to increase competitive ability. Under severe drought stress, planted seedling chlorophyll levels decreased, resulting in a simultaneous reduction in photosynthetic ability. The root growth of annual herbaceous plants depends on their photosynthesis ability but the decrease in biomass led to a decrease in root growth. The mixed habitat reduced the inhibition of seedling stem growth by drought stress and promoted plant growth.

1. Introduction

In the arid desert ecosystem, annual grasses and forbs are widely distributed in sandy and gravel deserts, which have an irreplaceable ecological function as pioneer plants in different stages of desert vegetation succession. As the constant layer of desert flora, annual plants can adjust their physiological and growth pattern according to the changes in abiotic and biotic conditions and complete their life history using limited rainfall [1]. The time of occurrence, duration, and intensity of precipitation have large variability in arid deserts, and long-term droughts occasionally occur after a precipitation event. Thus, the adaptation of desert annual plants to drought is one of the key factors determining population survival. Drought tolerance in annual desert plants, a crucial aspect of the flora of arid desert ecosystems [2], is particularly important for their growth. Annual grasses and forbs produce seeds that are scattered around the mother plant after maturity. In planted forests, the young plants often grow clustered together because of the relatively low wind speed in such plantation habitats [3]. Thus, annual desert plants also tend to agglomerate and compete for limited water resources [4]. A combination of abiotic and biotic factors affects the life history processes of germination, growth, and reproduction in annual plants, which means that annual plants tend to be distributed in bands along the drought gradient. Therefore, plants that live in arid desert ecosystems have developed a wide variety of adaptations in response to long-term drought, which are typically effective when responding to changes in the degree of drought stress and different species combinations.

Plant seeds germinate sequentially in mixed communities. After sprouting, grasses rapidly occupy the space, encroaching heavily on water, sunlight, and other resources for survival, with negative competitive effects on slower-germinating plants, which leads to a decrease in plant germination rate [5]. In order to compete for sunlight and avoid being physically overshadowed by their neighbors [6], plants in mixed communities direct much of their energy towards vertical growth so they can access more of the limited light resources [7,8,9]. Previous studies have investigated the response of herbaceous plants in mixed treatments and found that intercropping has a beneficial impact on plant height and grass biomass above- and belowground [10,11,12]. Other studies have explored the relationship between the invasive plant species Alternanthera philoxeroides and Wedelia trilobata and the native plant species A. sessilis and W. chinensis by setting up both mono- and mixed cultures. Their results indicated that congeners and allopatric plants will compete for the same resources. Typically, the closer the ecological niches of plants, the stronger their competitive effects; however, interspecific facilitation exists [13]. Wang investigated the competitive effects of herbaceous plants at different densities and found that plant height is significantly reduced in relation to biomass when different plants are grown together [14]. Guo et al. investigated how drought affects the physiological characteristics of the grasses Bothriochloa ischaemum (L.) Keng and Lespedeza davurica (Laxm.) Schindl. and found that mixed cropping facilitates photosynthesis and improves the efficiency of leaf water use on the Loess Plateau [15]. Li et al. demonstrated that both Hemarthria compressa and Cynodon dactylon show a degree of sensitivity and plasticity under water stress and found that an optimal species ratio exists under different water treatments [16]. When drought stress increases to a certain level, the relationship between different species’ roots changes from competition to facilitation [17].

The Hexi Corridor in the arid northwest of China is an important region for agricultural production. However, it is also the main wind erosion area in northwest China due to its aridity, low rainfall, frequent wind–sand activity, and severe oasis desertification [18]. The establishment of artificial sand-fixing vegetation in the transition zone of the desert oasis maintains a stable oasis environment. Shrubs and perennial herbs colonized sparsely planted forests after 50 years of establishment of artificial vegetation, while a large number of annual herbs became the dominant plants in the herbaceous layer of the artificial sand-fixing vegetation communities, playing a crucial role in controlling near-surface wind levels and keeping the sand surface stable [19]. The annual flora of the region consists mainly of annual grasses and forbs. Among them, the annual grasses Setaria viridis and Chloris virgata and the annual forbs Bassia dasyphylla and Halogeton arachnoideus are the dominant species. These annual plants and other shrubs have developed time-complementary sets of soil conservation functions. The desert shrubs do not germinate during early spring and the annual plants may make up for the reduced protective function of shrubs. In terms of spatial distribution, annual plants cover the bare surface of the desert ecosystem as much as possible to prevent soil erosion and recover vegetation productivity. It is predicted that precipitation will increase markedly in the middle and high latitudes of the Northern Hemisphere during the next 20–100 years [20], and fluctuations in precipitation will become more pronounced in the arid and semi-arid regions of northwest China [21,22]. This is evidenced by an increase in the number of heavy precipitation events, with increased precipitation amounts in single events and longer intervals between rainfall events in the summer [23,24,25]. Plant drought exposure due to changes in precipitation patterns and the combined effects of competition or facilitation from plant aggregation will impact the germination, physiology, growth and reproductive processes of plants in the region [26,27,28,29,30].

We conducted laboratory experiments to investigate the changes in germination, physiological indices, and growth of annual grasses and forbs in the transition zone of the desert oasis in the Hexi Corridor under different drought and mixed treatments. The aims were to determine the response patterns of the germination, physiological indices, and growth of annual grasses and forbs under different drought stresses and combinations of plant species treatments to explore the influence of drought stresses and mixed plant species on seedling survival and growth and to reveal the adaptation mechanisms of annual desert plants in arid regions.

2. Research Methods

2.1. Study Site and Experimental Materials

The seeds for the experiment were harvested in September 2019 from semi-fixed sand dunes near the Linze Inland River Basin Research Station of the Chinese Academy of Sciences in the western corridor of the river. The region has a temperate continental desert climate, with sparse and concentrated precipitation. The annual average precipitation is 116.8 mm, mainly in June–September; the annual evaporation is 2337.6 mm; the annual average temperature is 7.6 ℃, and the maximum temperature is 39.1 ℃. The sandstorm activity is mainly concentrated in March–May, with an average annual wind speed of 3.2 m/s and 15 days for strong wind days with a wind speed greater than 17 m/s. The natural vegetation on the edge of desert oasis is sparse, with low biological productivity and a simple vegetation layer structure. The annual plants gradually become the dominant plant group in herbaceous layer after shrub plantation, mainly including Setaria viridis, Chloris virgata, Bassia dasyphylla, Halogeton arachnoideus (Figure S1). These four annual plants have the widest distribution, the highest coverage and the largest biomass in the artificial sand fixation area and play a vital role in preventing sand erosion.

Seeds of the same species were randomly collected from 20–30 well-grown mature plant individuals and stored in a ventilated and dry room for later experiments. In this experiment, seed germination bags (with lengths of 35 cm and widths of 25 cm) were used for germination and seedling growth and four annual herbaceous plant seeds with full grains and of uniform size were selected for the experiment (Figure S2).

2.2. Experimental Design

The sand dunes in this study area are undulating, with annual plants clustered in the lowlands between the dunes. The topographic relief leads to differences in soil water effectiveness. Therefore, we used a two-factor treatment method in this experiment (Figure S3).

(1) Plant mixture treatment.

Four species of annual desert plants (2 Gramineae and 2 Chenopodiaceae) were selected. Six sowing treatments were set up and two plant combinations (with 25 individuals of each species) were sown in each germination bag, as follows: Setaria viridis mixed with Chloris virgata (M1), Bassia dasyphylla mixed with Halogeton arachnoideus (M2), Setaria viridis mixed with Bassia dasyphylla (M3), Setaria viridis mixed with Halogeton arachnoideus (M4), Chloris virgata mixed with Bassia dasyphylla (M5), and Chloris virgata mixed with Halogeton arachnoideus (M6).

(2) Drought stress treatment.

Five drought stress gradients were set up and different gradients of drought stress were simulated by adding different concentrations of PEG-6000 solution to the germination bags, as follows: control 0% (D0), mild drought 2% (D1), moderate drought 5% (D2), moderate to high drought 10% (D3), and severe drought 15% (D4). A total of 30 treatments were used (

Table 1. Treatments of the experiment.

Mixed Growing Treatments	Drought Treatments (PEG-6000)
	0%	2%	5%	10%	15%
Setaria viridis and Chloris virgata	M1D0	M1D1	M1D2	M1D3	M1D4
Bassia dasyphylla and Halogeton arachnoideus	M2D0	M2D1	M2D2	M2D3	M2D4
Setaria viridis and Bassia dasyphylla	M3D0	M3D1	M3D2	M3D3	M3D4
Setaria viridis and Halogeton arachnoideus	M4D0	M4D1	M4D2	M4D3	M4D4
Chloris virgata and Bassia dasyphylla	M5D0	M5D1	M5D2	M5D3	M5D4
Chloris virgata and Halogeton arachnoideus	M6D0	M6D1	M6D2	M6D3	M6D4

). The seeds were placed in a germination bag at a temperature of 20/25 ℃ and relative humidity of 50% for germination. After germination, the seeds were kept under an 8 h light/16 h darkness cycle with a light intensity of 4000 Lx. Five replicates were set up for each treatment, with 150 in total, and the experiments were conducted as follows. Plant mixtures were formed by mixing two species’ seeds and planting them uniformly in a slot formed by a paper core. To carry out the drought stress treatment, 20 mL of distilled water was added to saturate the paper core before sowing in the control group, and 20 mL of different concentrations of PEG-6000 solution were added to the other gradients. The laboratory temperature was maintained at 20–25 ℃ to ensure smooth germination of seeds. The number of germinations was recorded every 24 h until germination was complete for all seeds. Germination was defined as having occurred when the radicle reached 1–2 mm [31]. Following germination, 20 mL of distilled water was added to each germination bag to fully saturate the paper core to ensure smooth seed germination. After the seedlings had grown, the PEG-6000 solution was added to simulate drought stress tests, using the same drought stress gradient as above. After starting the stress treatment, seedling survival was recorded every 2 d until the end of the experiment. At the end of the experiment, the samples were stored in a low-temperature refrigerator at 4 °C.

2.3. Indicator Measurement and Methods

(1) Measurement of germination index: seed germination rate (germination rate = total number of germinated seeds/number of seeds for testing × 100%). Germination index (GI = ∑(Gt/Dt)), GI is the seed germination index, Gt is the number of germinations on a given day (t), and DT is the corresponding germination time (d) [32].

(2) Determination of physiological indicators: root vigor was determined using the TTC (2, 3, 5-triphenyltetrazolium chloride) method; chlorophyll content was determined by the 80% acetone method [33].

(3) Determination of morphological indices: seedling root length was read directly from the scale at the edge of the sprouting bag; seedling height was measured with a scale accurate to 0.01 cm; and the total biomass was calculated by fresh weight on a 1/1000 balance. The above indicators were measured three times and the average value was taken.

2.4. Data Analysis

All data were expressed as “mean ± standard deviation”. Two-way ANOVA SPSS 21.0 (IBM, Armonk, NY, USA) and Duncan’s multiple comparisons were used to analyze significant differences (p < 0.05). Graphs were created using Origin 2018 (OriginLab, Northampton, MA, USA) graphing software.

3. Results

3.1. Emergence Characteristics of Annual Herbs

3.1.1. Germination Rate

The germination rate of grasses was significantly affected by drought but was not significantly affected by mixed growth and interaction (Setaria viridis F = 11.151, df = 4, p < 0.001; Chloris virgata F = 8.454, df = 4, p < 0.001) (

Table 2. Two-way ANOVA analysis of the response of four annual desert plants to drought and mixed sowing interactions.

Plant Species	Indicators	Drought			Mixed Sowing			Drought and Mixed-Sowing Interactions
		df	F	p	df	F	p	df	F	p
Setaria viridis	Germination rate	4	11.151	<0.001	1	2.597	0.116	4	0.720	0.584
	Germination index		12.650	<0.001		0.352	0.557		0.576	0.682
	Root Vitality		71.170	<0.001		0.000	0.995		2.780	<0.05
	Chlorophyll		40.506	<0.001		1.114	0.298		3.879	<0.05
	Length of root		49.716	<0.001		4.857	<0.05		1.573	0.203
	Stem length		26.005	<0.001		11.187	<0.01		4.381	<0.01
Chloris virgata	Germination rate		8.454	<0.001		1.293	0.263		0.532	0.713
	Germination index		8.693	<0.001		0.075	0.786		1.379	0.261
	Root Vitality		56.609	<0.001		3.080	0.088		0.881	0.486
	Chlorophyll		28.974	<0.001		5.112	<0.05		7.121	<0.001
	Length of root		37.350	<0.001		0.806	0.375		0.122	0.974
	Stem length		10.538	<0.001		2.078	0.158		4.326	<0.01
Bassia dasyphylla	Germination rate		2.4300	0.063		36.737	<0.001		3.047	<0.05
	Germination index		2.193	0.075		13.271	<0.001		3.456	<0.05
	Root Vitality		5.905	<0.01		0.840	0.366		1.463	0.234
	Chlorophyll		20.578	<0.001		5.339	<0.05		1.287	0.294
	Length of root		7.746	<0.001		11.262	<0.01		3.811	<0.05
	Stem length		5.470	<0.01		0.874	0.356		0.483	0.748
Halogetonarachnoideus	Germination rate		1.795	0.152		19.267	<0.001		2.829	<0.05
	Germination index		1.814	0.148		21.133	<0.001		2.748	<0.05
	Root Vitality		18.906	<0.001		10.454	<0.01		1.738	0.164
	Chlorophyll		12.007	<0.001		0.231	0.634		0.894	0.478
	Length of root		6.710	<0.001		7.309	<0.05		0.172	0.951
	Stem length		6.525	<0.001		2.171	0.150		0.551	0.699

Note: D0, D1, D2, D3, D4, denote different drought stress gradients and M1, M2, M3, M4, M5, M6 denote different mixing patterns. Same as below.

). The germination rate of both graminoid species showed a decreasing trend with increasing drought stress, fluctuating and increasing at D2 and D3 and then decreasing to a minimum value at D4. In similar mixes (M1), the two graminoids decreased by 86.79% (Setaria viridis) and 52.94% (Chloris virgata) compared to the control values, while in mixes of graminoids with Chenopodiaceae (M3, M4, M5, and M6), the two graminoids decreased by 73.68%, 41.18%, 35.82% and 43.84%, respectively, compared to the control values (Figure 1).

Conversely, the germination rate of Chenopodiaceae was significantly affected by mixing (Bassia dasyphylla F = 36.737, df = 1, p < 0.001, Halogeton arachnoideus F = 19.267, df = 1, p < 0.001) and interaction (Bassia dasyphylla F = 3.047, df = 4, p < 0.05, Halogeton arachnoideus F = 2.829, df = 4, p < 0.05) but was not significantly affected by drought (Table 2). The germination rate of Chenopodiaceae (Bassia dasyphylla and Halogeton arachnoideus) reached the maximum in the similar mixed (M2) treatment: 37% and 20%, respectively. The germination rate of Chenopodiaceae was higher under each mixed treatment in the drought stress concentrations of D1 and D2, with the maximum germination rate of mistletoe ice Chenopodiaceae (37%) occurring under the combined effects of D1M2 and that of white-stemmed saltbush (44%) occurring under combination with D1M6 (Figure 1).

3.1.2. Germination Index

Gramineae germination index was significantly affected by drought but not by mixed growth or the interaction of the two treatments (Setaria viridis F = 12.650, df = 4, p < 0.001 and Chloris virgata F = 8.693, df = 4, p < 0.001) (Table 2). The germination index of both graminoid species decreased with increasing drought stress, trending upward at D2 and D3 and then decreasing to a minimum value at D4. In similar mixes (M1), the two graminoids decreased by 90.49% (Setaria viridis) and 70.98% (Chloris virgata) compared to the control values, while in mixes of graminoids with Chenopodiaceae (M3, M4, M5, and M6), the two graminoids decreased by 60.4%, 51.28%, 24.23% and 52.07%, respectively, compared to the control values (Figure 2).

The germination index of Chenopodiaceae was significantly affected by mixing (mistletoe F = 13.271, df = 1, p < 0.001, white-stemmed saltbush F = 21.133, df = 1, p < 0.001) and treatment interaction (Bassia dasyphylla F = 3.456, df = 4, p < 0.05, Halogeton arachnoideus F = 2.748, df = 4, p < 0.05) but was not significantly affected by drought (Table 2). The germination index of Chenopodiaceae (Bassia dasyphylla and Halogeton arachnoideus) reached a maximum in the similar mixed (M2) treatment, of 1.62 and 0.63, respectively. Furthermore, under mixed sowing after drought stress, the germination index of Chenopodiaceae was higher under each mixed treatment and the drought stress concentrations D1 and D3. Bassia dasyphylla reached a maximum germination rate (1.44) under D3M2, while Halogeton arachnoideus reached a maximum at D1M6 (1.62) (Figure 2).

3.2. Physiological Response of Annual Herbaceous Plants

3.2.1. Root System Vigor

Four annual herbaceous species were significantly affected by drought but were not significantly affected by mixed growth and interactions (Setaria viridis F = 71.170, df = 4, p < 0.001; Chloris virgata F = 56.609, df = 4, p < 0.001; Bassia dasyphylla F = 5.905, df = 4, p < 0.01; and Halogeton arachnoideus F = 18.906, df = 4, p < 0.001) (Table 2).

The growth rates of all four annual herbaceous species first increased and then decreased with increasing drought stress, reaching maximum values at D2 and D3 and minimum values at D4. In M1, Setaria viridis and Chloris virgata reached maximum values at D3 of 10.38 mg·(g·h)⁻¹ and 8.31 mg·(g·h)⁻¹, respectively; in M2, Bassia dasyphylla and Halogeton arachnoideus reached maximum values at D2 of 4.37 mg·(g·h)⁻¹, 4.81 mg·(g·h)⁻¹; in M3, Setaria viridis a maximum of 10.53 mg·(g·h)⁻¹ at D3 and Bassia dasyphylla reached a maximum of 5.37 mg·(g·h)⁻¹ at D2; in M4, Setaria viridis reached a maximum of 10.33 mg·(g·h)⁻¹ at D3 and Halogeton arachnoideus reached a maximum value of 4.26 mg·(g·h)⁻¹ at D2; In M5, Chloris virgata and Bassia dasyphylla reached a maximum value of 8.51 mg·(g·h)⁻¹ and 7.43 mg·(g·h)⁻¹ at D3, respectively; in M6, Chloris virgata reached a maximum value of 8.74 mg·(g·h)⁻¹ at D3 and Halogeton arachnoideus reached a maximum value of 5.49 mg·(g·h)⁻¹ at D2 (Figure 3).

3.2.2. Chlorophyll

Annual herbaceous chlorophyll was significantly affected by mixing and interaction with drought stress (Setaria viridis F = 40.506, df = 4, p < 0.001, Chloris virgata F = 28.974, df = 4, p < 0.001, Bassia dasyphylla F = 20.578, df = 4, p < 0.001, Halogeton arachnoideus F = 12.007 df = 1, p < 0.001) when mixed (Setaria viridis F = 5.112, df = 1, p < 0.05, Bassia dasyphylla F = 5.339, df = 1, p < 0.05) and at interactions (Setaria viridis F = 3.879, df = 4, p < 0.05, Chloris virgata F = 7.121, df = 4, p < 0.001) (Table 2).

Under normal water conditions (D0), the maximum chlorophyll of grasses (Setaria viridis and Chloris virgata) was 0.84 mg·(g·h)⁻¹ and 0.65 mg·(g·h)⁻¹, respectively, in the mixed treatment (M1), and the maximum chlorophyll of Chenopodiaceae (Bassia dasyphylla and Halogeton arachnoideus) was 0.56 mg·(g·h)⁻¹ and 0.43 mg·(g·h)⁻¹, respectively, in the mixed treatment (M5 and M6) with grasses (Chloris virgata). Under the post-drought stress interaction treatment, the chlorophyll of all four annual herbaceous species increased and then decreased with increasing drought stress, reaching maximum values at D1 and D2, and also at different class mixtures. Setaria viridis reached a maximum value of 10.53 mg·(g·h)⁻¹ at D3M3, Chloris virgata 8.74 mg·(g·h)⁻¹ at D3M6, Bassia dasyphylla 7.43 mg·(g·h)⁻¹ at D3M5, and Halogeton arachnoideus 7.43 mg·(g·h)⁻¹ at D3M5 (Figure 4).

3.3. Growth Response of Annual Herbaceous Plants

3.3.1. Root Length

Annual herbaceous plant root length was significantly affected by drought and mixed drought stress: Setaria viridis F = 49.716, df = 4, p < 0.001; Chloris virgata F = 37.350, df = 4, p < 0.001; Bassia dasyphylla F = 7.746, df = 4, p < 0.001; Halogeton arachnoideus F = 6.710, df = 4, p < 0.001. Mixed treatment: Setaria viridis F = 4.875, df = 1, p < 0.05, Bassia dasyphylla F = 11.262, df = 1, p < 0.01, Halogeton arachnoideus F = 7.309, df = 1, p < 0.05 (Table 2).

Under normal water conditions (D0), the root length of grasses (Setaria viridis and Chloris virgata) reached maximum values of 4.76 cm and 4.41 cm when mixed with Chenopodiaceae (Bassia dasyphylla and Halogeton arachnoideus) (M3 and M6), respectively. The root length of Chenopodiaceae (Bassia dasyphylla and Halogeton arachnoideus) also reached maximum values of 3.39 cm and 2.93 cm when mixed with grasses and Chenopodiaceae (M3 and M6), respectively. The root length of all four annual herbaceous species increased and then decreased with increasing drought stress, reaching a maximum at D2 and D3, including when different plant species were mixed. The maximum root length of Setaria viridis was 9.69 cm at D3M3, Chloris virgata 7.34 cm at D3M5, Bassia dasyphylla 4.34 cm at D3M5, and Halogeton arachnoideus 4.93 cm at D2M6 (Figure 5).

3.3.2. Stem Length

Annual herbaceous plant stem length was significantly affected by drought and mixing, and the interaction of the two. Drought stress: Setaria viridis F = 26.005, df = 4, p < 0.001; Chloris virgata F = 10.538, df = 4, p < 0.001; Bassia dasyphylla F = 5.470, df = 4, p < 0.01; Halogeton arachnoideus F = 6.525, df = 4, p < 0.001. Mixed treatment: Setaria viridis F = 11.187, df = 1, p < 0.01. Treatment interaction: Setaria viridis F = 4.381, df = 4, p < 0.01 and Chloris virgata F = 4.326, df = 4, p < 0.01 (Table 2).

Under normal water conditions (D0), the stem length of grasses (Setaria viridis and Chloris virgata) reached maximums of 8.20 cm and 6.44 cm, respectively, when mixed with the same species (M1), and the stem length of Chenopodiaceae (Bassia dasyphylla and Halogeton arachnoideus) reached the maximums of 5.41 cm and 4.74 cm, respectively, when mixed with grasses (Chloris virgata) in a single treatment (M5 and M6). The stem length of all four annual herbaceous plants increased and then decreased with increasing drought stress, reaching a maximum at D0 and D1, including when the plants were mixed with different species. The stem length of Setaria viridis reached a maximum value of 9.20 cm at D1M3, the Chloris virgate reached a maximum value of 7.44 cm at D1M6, the Bassia dasyphylla reached a maximum value of 5.41 cm at D0M5, and the Halogeton arachnoideus reached a maximum value of 4.74 cm at D0M6. Under severe drought stress, stem lengths decreased to the lowest value, and in the mixed species treatment (M1), the stem length of the two types of grass decreased by 39.15% (Setaria viridis) and 50.93% (Chloris virgata) compared with the control value. When the grasses were mixed with Chenopodiaceae (M3 and M5), the two types of grass decreased by 28.97% and 46.90%, respectively, compared to the control values (Figure 6).

4. Discussion

4.1. Germination Characteristics of Annual Herbaceous Plants

Seed germination is the starting point of the growth and development of higher plants and is a critical stage of plant life history [34,35]. The germination rate and germination index conditions can reflect the response of plant seed vigor to environmental stress to a certain extent [36]. In this study, the germination of Graminaceae was considerably affected by drought stress. Notably, with increasing drought stress, the germination rate and germination index of Graminaceae decreased and then increased, reaching a maximum value in moderate to high drought conditions and decreasing to a minimum value under severe drought stress. This is due to the fact that, with increasing drought stress, the internal membrane system of the seeds was damaged and germination was inhibited, inverting the original promoting effect of germination to an inhibiting effect [37]. In this study, the drought inhibition of grass germination was alleviated by mixing different plant species as, under severe drought stress, the mixing of different types of plants enhances the competition between plants and the competitive ability of grasses was better than that of Chenopodiaceae, increasing the germination rate and germination index.

4.2. Physiological Response of Annual Herbaceous Plants

Chlorophyll is the most important factor for plant photosynthesis as it absorbs light energy and transfers it to reaction centers. When plants face environmental stresses, chlorophyll content can reflect the assimilative capacity of the plant [38]. In this study, we found that the chlorophyll content of annual herbaceous plants was markedly affected by drought, mixing, and the interaction of the two. The chlorophyll content of the four annual herbaceous plants increased and then decreased under drought stress, indicating that mild and moderate drought stress can increase plant chlorophyll production but as the degree of drought stress increases, blocked chlorophyll synthesis or chlorophyll degradation can occur. Under drought stress, reduced levels of intracellular water in leaves and slower chlorophyll synthesis rates can affect ribosome formation, block protein synthesis, slow metabolism, inhibit chlorophyll biosynthesis, and decompose chlorophyll [39].

Root vigor is a basic physiological indicator reflecting the vital activity of the root system and affecting the uptake of soil water by plants [40]. In this study, we found that the root vigor of annual herbaceous plants was significantly affected by the degree of drought, increasing then decreasing with increased drought stress and reaching maximum vigor at moderate and relatively high drought levels. This indicates that an appropriate increase in drought stress can maintain high root vigor. The plant response to drought stress is accompanied by a synergistic aboveground and belowground response [41,42]. In this study, chlorophyll content increased under mild and moderate drought stress, promoting plant photosynthesis, while root vigor was strongest under moderate to high drought stress. Chlorophyll was more affected than root vigor by drought stress during moderate to high drought, indicating that under moderate drought stress annual herbaceous plants were able to promote photosynthesis. The root system is considerably more vigorous during moderate to high drought as the plant draws more water from the ground to maintain its basic metabolic activities.

4.3. Growth Response of Annual Herbaceous Plants

In this study, the stem length content of annual herbaceous plants was markedly affected by drought and species mixing. Mixing Chenopodiaceae with grasses promoted Chenopodiaceae stem growth and inhibited grass stem growth. It was shown that plants respond to environmental stress by adjusting their phenotypic characteristics in a direction that favors fitness to minimize adverse environmental effects on survival [43]. Stem length is one of the main functional traits reflecting plant growth and under competitive stress, plant height is generally suppressed, although some studies have shown that stem length increases, for instance, in the “shade avoidance” response [44]. We found that plants competed for light resources by increasing stem length to maximize stem length during mild drought. Under severe drought stress, plant mixing can reduce stem length inhibition and promote plant growth. It was shown that plants are able to adapt to drought and competition by adjusting their morphology, and that drought stress alters interspecific plant relationships [45].

Root length is the most significant morphological trait of the belowground part of the plant, determining the depth of soil through which the plant can extend and directly affecting the plant’s ability to access water [46]. We found that the root length of annual herbaceous plants was considerably affected by both drought and hybridization. Species mixing can promote root growth in annual herbs. It was shown that increasing root length is a way for plants to avoid or reduce competitive pressure [47], for instance by extending the plant root system to a soil depth that the competitor cannot reach [48,49,50,51]. In this study, root lengths grew considerably under the post-drought stress interaction in the mixed treatment, as root length elongation facilitates water acquisition, allowing the plant to gain a greater competitive advantage during the drought period [46]. Numerous studies have shown that as soil moisture content decreases, plant growth centers inevitably shift toward the root system, increasing root growth [52]. The present study found that the root growth of annual herbaceous plants under drought stress was not singularly promoted or inhibited but was related to the degree of stress, with moderate-to-high drought enhancing root length, while severe drought inhibited root growth. Severe drought markedly reduces the photosynthetic capacity of annual herbaceous plants, affecting plant growth and inhibiting root growth. The results of the study also confirmed that plant root growth depends on the rate of photosynthesis, and in drought conditions, the shortage of energy supply to the roots due to the decrease in the accumulation of photosynthetic products limits root growth [53].

5. Conclusions

In this study, we found that in arid desert areas, the seed germination of annual plants reached maximum rates under moderate drought. The annual desert plants maintained a normal internal growth environment during mild and moderate drought stress, primarily through physiological adjustments such as increasing the content of leaf proline, soluble protein, and soluble sugar content to maintain leaf osmotic pressure. Due to the physiological adjustment effect, the 100-grain weight and number of seeds were maintained at a high level during mild and moderate drought. While under severe drought stress, leaf malondialdehyde content increased rapidly, accelerated chlorophyll decomposition caused a decrease in soluble protein content, and osmoregulation reached its limit. Thus, effective plant growth was inhibited and plants initiated individual morphological changes, such as increasing root input and reducing above-ground stem input. Root extension reflects an “open source” strategy by which annual desert plants adapt to drought stress.

Under drought stress, chlorophyll content of annual desert plants increased under mild and moderate drought stress to promote photosynthesis, thereby promoting stem length growth, and considerably increasing root vigor under moderate-to-high drought, allowing plants to absorb more water from the ground to maintain their basic metabolic activities. Moreover, under competitive stress, annual plants were able to “shade” their competitors by increasing stem length, while further reducing competitive stress by increasing root length. Annual plant root growth depends on the aboveground photosynthetic supply. However, under severe drought stress, both chlorophyll and photosynthesis decrease, which limits root growth due to a decrease in energy supply to the roots as a result of decreased photosynthetic product accumulation.