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Article

Spatial Proximity to Perennial Groundcover Triggers Shade Avoidance Responses in Corn

by
Amina Moro
1,2,*,
A. Susana Goggi
1,2,
Ken J. Moore
1,
Shui-zhang Fei
1,3 and
Amy Kaleita
4
1
Department of Agronomy, Iowa State University, Ames, IA 50011, USA
2
Seed Science Center, Iowa State University, Ames, IA 50011, USA
3
Department of Horticulture, Iowa State University, Ames, IA 50011, USA
4
Department of Agricultural and Biosystems Engineering, Iowa State University, Ames, IA 50011, USA
*
Author to whom correspondence should be addressed.
Agronomy 2026, 16(7), 729; https://doi.org/10.3390/agronomy16070729
Submission received: 17 February 2026 / Revised: 18 March 2026 / Accepted: 24 March 2026 / Published: 31 March 2026
(This article belongs to the Section Innovative Cropping Systems)
Browse Figures
Versions Notes

Abstract

Perennial groundcover (PGC) systems integrate perennial grasses with annual crops such as corn (Zea mays L.) to provide continuous soil cover and enhance soil health. However, the proximity to groundcover vegetation can alter light quality perceived by developing seedlings, inducing shade avoidance response (SAR), a phytochrome-mediated developmental response that modifies plant architecture and may compromise yield. Identifying the distance at which SAR is initiated and the extent to which management practices modulate this response is critical for optimizing PGC systems. This growth chamber study aimed to (1) identify the distance at which SAR occurs in corn seedlings, (2) determine whether the thiamethoxam seed treatment mitigates SAR expression, and (3) compare hybrid physiological responses to PGC-induced SAR. The experiment was arranged in a randomized complete block design with four replications across three periods and included two corn hybrids (P1185, P1197), two seed treatments (untreated and thiamethoxam at 0.25 mg seed−1), and four perennial ryegrass (Lolium perenne L.) distances [0, 6, 25 cm, and a control (no-grass)]. Reduced red to far-red light ratios associated with closer proximity to ryegrass induced SAR responses. Corn plants at 6 cm from PGC exhibited significant stem and height elongation beginning at 8 days after planting (DAP), followed by reduced growth by 14 DAP, confirming an early SAR response. Plants grown at 0 cm exhibited reduced height and growth compared to other distances at all growth stages. Hybrid responses differed, and Hybrid P1197 showed enhanced stem elongation, a characteristic SAR response. The thiamethoxam seed treatment did not mitigate SAR. These results indicate that SAR causes stem elongation without altering root or shoot biomass.

1. Introduction

The stationary nature of plants requires an adaptive mechanism for survival in their environment, which involves sensing competition before it becomes detrimental to growth [1]. Changes in light quality are reliable indicators used by plants to detect competition, particularly a reduction in the red to far-red (R:FR) light ratio [2] caused by selective absorption of red wavelengths and reflection of far-red light by photosynthetic tissues. Changes in R:FR ratio are perceived by the phytochrome photoreceptor system, with phytochrome B (phyB) playing a major role in regulating seedling growth in response to changes in light quality [3,4]. Under far-red-enriched light conditions, phyB is inactivated, resulting in the activation of phytochrome-interacting factors (PIFs) that regulate hormonal pathways controlling cell elongation and organ development [5,6].
The activation of these signaling pathways induces morphological and physiological responses collectively described as the shade avoidance response (SAR). Some of the common physiological changes in plants due to SAR include increased elongation of the mesocotyl or hypocotyl, leading to stem elongation, and reduced leaf expansion [7]. A prolonged exposure of the plant to such conditions alters biomass partitioning that favors shoot growth and reduces root development [1,8,9]. While these responses may temporarily enhance light capture, they often compromise seedling development and early vigor. Shade avoidance response is most observed in seedlings, and its effects can influence seedling establishment and subsequent growth trajectories [10], particularly in managed cropping systems, where uniform emergence and early vigor are critical for achieving yield potential [11,12].
In corn, SAR can affect early vegetative development and stand uniformity due to reduced seedling vigor [13]. Yield in corn is driven by the total number of plants per hectare [14]; population size is important, and SAR can cause a reduction in plant population density, ultimately leading to a decrease in yield [14]. The exposure to low R:FR conditions in corn during early growth has been associated with increased stem elongation, reduced stem diameter, altered leaf angle, and changes in node and leaf development [15,16]. These morphological changes in corn seedlings can increase susceptibility to plant lodging and reduce the efficiency of resource allocation during early growth stages. Plants exhibiting SAR typically experience phenological delays, which postpone flowering and shorten the time available for seed filling, ultimately impacting reproductive success [17]. Breeding efforts in corn have reduced sensitivity to excessive elongation through the selection for improved canopy architecture, including more upright leaves and reduced plant height, which promote light distribution within dense stands [18]. However, corn seedlings remain responsive to light quality cues, particularly during early development, when phytochrome-mediated signaling strongly influences growth patterns [19]. Despite extensive research on SAR in high-density crop stands, relatively little is known about the distance at which corn seedlings perceive neighboring vegetation and initiate SAR, especially in systems where competition arises from different species, such as groundcover, rather than in-row crop neighbors.
Thiamethoxam is a neonicotinoid insecticide widely used as a seed treatment to protect seedlings from early-season insect pests. In addition to its insecticidal activity, the thiamethoxam seed treatment has been reported to influence early seedling physiology, including increased seedling vigor, enhanced root and shoot growth, and improved tolerance to environmental stress during early development [20,21]. The thiamethoxam seed treatment enhanced antioxidant capacity, improved carbohydrate metabolism, and increased efficiency of early nutrient uptake [22]. In corn, thiamethoxam-treated seeds have been associated with improved emergence and early biomass accumulation under stressful conditions, including competition from weeds [21]. However, the extent to which thiamethoxam influences plant responses specifically associated with light quality-mediated competition, such as SAR, remains unclear.
Our study investigated the influence of the thiamethoxam seed treatment on the SAR in two corn hybrids with different SAR sensitivities, specifically focusing on the critical distance at which neighbor detection triggers morphological changes. We hypothesized that: (i) corn seedlings grown in close proximity to a groundcover were expected to exhibit SAR, resulting in increased stem elongation and decreased stem thickness; (ii) the thiamethoxam seed treatment would mitigate seedling growth by improving seedling tolerance to early competition stress [22]; and (iii) hybrid performance and physiological responses to PGC-induced SAR would differ among genotypes. To test these hypotheses, a growth chamber experiment was conducted in which corn seedlings were planted at varying distances from a perennial ryegrass groundcover (PGC) with the objective of investigating the effects of light quality signaling on corn plants placed at different distances from the PGC. The experiment was designed to isolate the aboveground light signaling from belowground root interactions and competition by preventing grass–corn root and corn–corn root interactions and thus isolate SAR responses from other stresses.

2. Materials and Methods

2.1. Experimental Layout

This study was conducted in a temperature- and light-controlled growth chamber at the Seed Science Center, Iowa State University, Ames, IA, USA. The experiment was designed as a randomized complete block design (RCBD), with four blocks per period, and it was repeated across three periods. For each period, four benches (blocks) were placed inside the growth chamber. The treatments included two corn hybrids, P1185 and P1197, known to differ in their response to shade [23,24], a seed treatment of thiamethoxam at 0.25 mg/seed (T1), and an untreated seed (T0). The final factor was the distance between the corn plants and perennial ryegrass. Three distances of 0, 6, and 25 cm separated the corn plant from the PGC, along with a control corn plant growing without PGC competition. The total number of experimental units was 16 per block, randomly placed on each bench, for a total of 64 experimental units per period (Figure 1). Each experimental unit consisted of 3 corn plants, with 1 plant in a separate pot, and the data were collected on these corn plants. The data from these three corn plants were averaged, and these average values represented each experimental unit.

2.2. Hybrids

Two 111-day relative maturity yellow dent corn hybrids (Corteva Agriscience, Johnston, IA, USA) with contrasting responses to PGC were selected based on yield trial data collected at the Bruner Farm (Boone County, IA, USA), which revealed a clear hybrid × PGC interaction [23,24,25]. Hybrid P1197, commercialized in 2014, is a high-yielding genotype that has set world corn yield records under favorable, high-input conditions [26]. It exhibits moderate plant height, lower ear placement, strong stalk strength, good stay-green, and resistance to northern corn leaf blight [27], but it showed greater yield sensitivity when grown with PGC, indicating strong competitive responsiveness. Hybrid P1185, commercialized in 2015 [28], contrasts with P1197 by exhibiting shorter stature, lower ear height, excellent root strength, and tolerance to mid-season brittle snap, and it maintained more stable yield under PGC. These hybrids represent contrasting shade avoidance response phenotypes.

2.3. Seed Treatment of Seed Corn

Thiamethoxam was hand-applied by an expert from Syngenta (Basel, Switzerland) in the form of Cruiser 5FS, which contains the active ingredient thiamethoxam at 47.6% by weight (0.599 kg L−1) and other ingredients at 52.4%. The formulation was designed to achieve a thiamethoxam treatment concentration of 0.25 mg per seed (20 g per bag). This commercial treatment rate was used for protecting the emerging seedlings from early-season insect pests. The seed treatment’s mode of action was systemic, and thus the active ingredient was absorbed into the plant and distributed throughout its tissues.

2.4. Establishment of Grasses

Perennial ryegrass (‘Top Gun II’) seeds by Barenbrug USA (Tangent, OR, USA) were established in 6.35 × 6.35 cm cells aligned in a single row. Each row consists of 12 cells, and the total length was 25.4 cm. The rye grasses were grown for 4 weeks before being used in the experiment. Sungro soil (Agawam, MA, USA) was used in the study. The cells were filled with soil, and a hole was created at the center of each cell. The seeds of the perennial rye grass were placed at the center and covered gently with a thin layer of soil. The grass strips were watered every other day after planting until the grass emerged, then switched to daily watering. The grass was dense enough to cover the entire soil surface before being used for the experiment. New grass was grown for each period. Growth chamber settings were maintained at 24 °C with a 16-h photoperiod and an 8-h dark period. The lighting system was configured to provide 100% output of blue, red, far-red, and broad-spectrum white light to mimic natural daylight.

2.5. Establishment of Corn Seedlings

Individual corn seeds were planted at a depth of 3.5 cm in a 6 cm × 6 cm pot. Each experimental unit consisted of three separate containers of corn, two strips of grass measuring 25 cm in length, and a tray measuring 25 cm by 50 cm (Figure 2). The three corn pots were placed on a 25 × 50 cm growing tray. Two grass strips were positioned at assigned distances from the corn containers of (0, 6 cm, and 25 cm) along the tray. So, each edge of the tray had already established grasses with three pots of corn plants in the middle. This prevented corn grass root interaction and corn-to-corn root interaction. After setting up the experiment, the grasses were uniformly trimmed to 11.5 cm to standardize canopy interference. The corn was watered every other day, while the grass was watered every day to maintain non-limiting moisture conditions.

2.6. Water Regime

The amount of water applied to each pot was determined using the following procedure. Each 6 × 6 cm pot was saturated and allowed to drain freely, and the maximum water-holding capacity was measured as 218 mL per pot. Published crop evapotranspiration (ETc) data indicate that corn uses approximately 760–890 m3 ha−1 of water from planting to the V6 stage [26]. Scaled to the surface area of a 6 × 6 cm pot, this corresponds to an estimated 360–380 mL of cumulative water requirement from planting to the V6 stage. Based on growth and water demand, watering was applied incrementally to maintain soil moisture near field capacity. From emergence to V3, plants received 30 mL every 2 days; from V3 to V5, irrigation was increased to 50 mL every 2 days; and from V5 to V6, plants received 80 mL every other day. This schedule ensured a consistent moisture supply throughout the early vegetative growth period. Perennial ryegrass rows were watered daily to prevent wilting.

2.7. Data Collection

The corn plants were monitored every 24 h, and seedling emergence was recorded when the seedling height reached 2.5 cm above the soil. Once all seedlings had emerged, plant height from the soil to the tip of the highest leaf (plant height) and the plant height from the soil to the collar of the fully developed leaf (stalk length) were measured every other day until the V6 stage, when the experiment was terminated. Leaf chlorophyll content was assessed using a SPAD meter [29]. Chlorophyll content per leaf was measured 3 times within each period. A fully developed leaf was selected, and measurements were taken at the base, mid-length, and tip, then averaged for the individual plant. The data for all three plants in each experimental unit were recorded. The average of the three plants represents the value assigned to each experimental unit. The stem diameter was measured using a digital Vernier caliper. The stem diameter was measured above the soil surface and below the first leaf for each plant, and an average for all three plants was recorded for each experimental unit.
Light quality was quantified using a LI-180 Portable Spectrometer (LI-COR Biosciences, Lincoln, NE, USA), which measures spectral photon flux density across wavelengths. The sensor was positioned approximately 1 m above the corn canopy and oriented downward to capture the integrated spectral environment of each experimental unit. This measurement includes both incident solar radiation and radiation reflected and scattered from the PGC and soil surface. Because vegetation reflects proportionally more far-red than red light, reflected radiation from the PGC can reduce the R:FR ratio of the crop light environment, a key signal regulating SAR. Thus, these measurements were used to characterize how proximity to the PGC modifies the spectral light environment experienced by the crop canopy rather than to quantify only incoming sunlight.
Each period was terminated when the corn plants reached the V6 stage. Shoot biomass was collected by cutting the shoot above the soil, then drying it for 72 h at 80 °C and weighing each plant shoot. The dry weight from the three plants was averaged for each experimental unit. The root biomass was obtained by shaking and washing the roots to remove all the soil. The roots were dried for 72 h at 80 °C to determine their dry weight. The root dry weights from the three plants were added together and averaged to represent each experimental unit.

2.8. Statistical Analysis

The data were analyzed using SAS, version 9.4. Generalized linear mixed-effects models were fit using PROC GLIMMIX. The emergence data were analyzed assuming a binomial distribution with restricted pseudo-likelihood (RSPL) estimation. Continuous traits, including chlorophyll content (SPAD), stem diameter, and root and shoot biomass, were analyzed assuming a Gaussian distribution and fit using restricted maximum likelihood (REML).
Fixed effects included the hybrid, seed treatment (T), perennial groundcover (PGC) distance, days after planting (treated as a repeated measure), and all possible two-way and three-way interactions among these factors. Random effects included period and block nested within period to account for the experimental design structure.
For repeated measurements across days, an autoregressive [AR(1)] covariance structure was specified to account for temporal correlation among observations within experimental units. The models were fit using restricted maximum likelihood (REML).
Least-squares means (LS-means) were estimated for significant fixed effects and interactions. Mean separations were conducted using Fisher’s least significant difference (LSD) test at the 5% significance level (α = 0.05). For significant interactions, simple effects were examined using the SLICE option to evaluate effects at each level of the slicing variable.

3. Results

3.1. Emergence (Days After Planting)

Appendix A, Table A1 presents the sources of variation and p-values for the statistical analysis from all variables measured. The statistical analysis for emergence measured in days after planting revealed that the hybrid (p = 0.0212) and distance to PGC (p = 0.0074) had significant effects on corn plant emergence, whereas seed treatment did not. There was a moderate and significant interaction between the hybrid and PGC distance (p = 0.0379). Across treatments, Hybrid P1185 showed a faster emergence (mean = 4.14 days) compared to Hybrid P1197 (4.26 days).
The seed treatment with thiamethoxam (T = 1) had no effect (p = 0.106) on seedling emergence (mean = 4.15 days) when compared to the untreated control (4.24 days).
The emergence was recorded when coleoptile elongation reached 2.5 cm above the soil surface, and the result clearly shows that the PGC distance significantly influenced emergence timing (p = 0.0074). Corn planted at 6 cm (4.32 days) and 0 cm (4.24 days) from the PGC strips emerged late, while the emergence was earlier at 25 cm (4.0 days) and for the control plants (4.16 days) (Appendix A, Table A1).
The analysis revealed a significant interaction between the hybrid and PGC distance, indicating a distinct genotypic response difference to the proximity of the PGC (Figure 3). Hybrid P1185 emerged the earliest at 25 cm (B) and in the control (B), but it was slower at 0 cm (A) and 6 cm (A). In contrast, Hybrid P1197 showed no significant difference among treatments (C). Although the analysis showed no interaction between the seed treatment and PGC distance (p = 0.1829), the emergence was consistently earlier for untreated seeds across all distances, even though the differences were not significant. Untreated seeds emerged within 4.15 days compared with 4.24 days for untreated seeds (Appendix A, Table A2).

3.2. Corn Height and Stalk Length

The hybrid and PGC distance had highly significant effects on corn height and stalk length (p < 0.0001 for both traits) (Appendix A, Table A1). The interactions between the hybrid and PGC distance and the hybrid and days from planting were not significant (p = 0.28) (Appendix A, Table A1), indicating that both hybrids responded similarly across distances and growth stages. There was significant interaction between the PGC distance and the number of days from planting (p = 0.0012) (Appendix A, Table A1). Each PGC distance performed differently with each growth stage. Plants at a 6-cm distance showed rapid growth in the early stage compared to other distances, and growth slowed down at later developmental stages of the corn plant (Figure 4 and Figure 5).
Mean comparisons from hybrid by PGC interaction revealed that distance to PGC had a significant effect on stalk length (Hybrid P1185: p = 0.0014; Hybrid P1197: p = 0.0026) and plant height (Hybrid P1185: p = 0.0006; Hybrid P1197: p = 0.0005), confirming that each hybrid responded differently to PGC proximity.
Although both hybrids show similar growth patterns, pairwise comparisons revealed no significant differences between plants at 6 cm, 25 cm, and the control distance for Hybrid P1185). In contrast, Hybrid P1197 exhibited a difference in growth at different PGC distances. Plants positioned at 6 cm distance were taller in both stalk and height than the control and at 25 cm distance on days 8 to 12 (Figure 4 and Figure 5). Growth in plants at 6 cm began to slow down on days 13 to 14, as indicated by the recorded corn plant height and stalk length (Figure 4 and Figure 5). Plants positioned at 25 cm distance and the control plants were taller at V5–V6. Plants positioned at 0 cm distance from the PGC show slower growth throughout the experiment than plants at all other distances (Figure 4 and Figure 5).

3.3. Stem Diameter

The thiamethoxam seed treatment had a significant effect on stem diameter (p = 0.0003). Although the main effects of the hybrid and PGC distance were not significant, a significant two-way interaction between the seed treatment and PGC distance was detected (p = 0.0368; Appendix A, Table A1), indicating that the effect of distance on stem diameter depended on seed treatment. Across all hybrids, distances, and sampling periods, untreated plants exhibited a greater mean stem diameter (6.24 mm) than untreated plants (5.99 mm) (p = 0.0027).
Although the overall effect of the PGC distance was not statistically significant at a 5% level of probability, a statistical difference was observed at a 10% level (p = 0.0622). Further analysis from PGC seed treatment indicates that treated plants had significantly smaller stem diameters at 0 cm (5.67 mm) and control (5.83 mm) compared with untreated plants (6.20 mm and 6.45 mm, respectively), with differences of approximately 0.38–0.58 mm. However, at 25 cm and 6 cm, stem diameters did not differ significantly between treated and untreated plants (p > 0.05). The mean comparison grouping indicated that the 0 cm and the control treatments formed a separate group (D) from the 6 cm and 25 cm (C) and (B), respectively (Figure 6).

3.4. Chlorophyll Content (SPAD) of the Corn Plants

The thiamethoxam seed treatment and hybrid type had a significant impact on the SPAD values of the corn leaves (p = 0.0001 and p = 0.0002, respectively) (Appendix A, Table A1, Figure 7). Hybrid P1197 exhibited significantly greater mean leaf chlorophyll content (41.9) than Hybrid P1185 (38.9) (p = 0.001). Similarly, seed treatment with thiamethoxam resulted in a significantly lower SPAD value (p = 0.0001) compared to untreated seed (Figure 7). Although the PGC distance did not have a significant effect on SPAD (p = 0.578). (Appendix A, Table A1), the analysis revealed a higher four-way interaction among hybrid, the thiamethoxam seed treatment, PGC distance, and days (p = 0.0427) (Appendix A, Table A1). This indicates that SPAD responses were influenced by hybrid, seed treatment, and days from planting within the PGC distances. The thiamethoxam seed treatment affected chlorophyll content across sampling times, particularly at 6, 12, and 14 days after planting. There was an increase in the chlorophyll content in leaves from days 6 to 12, followed by a decline on day 14. Across all PGC distances, the SPAD values were statistically similar (range = 39.7–40.2), confirming that there was no measurable suppression of chlorophyll formation due to PGC proximity. The SPAD values increased significantly over time, peaking at 12 DAP (41.9) before slightly declining by 140 DAP (38.6). The hybrid by DAP effect was highly significant (p = 0.0001) (Appendix A, Table A1), indicating that hybrids accumulated chlorophyll differently over time. Hybrid P1197 exhibited higher SPAD values than Hybrid P1185 at all sampling times, with the largest difference observed 12 days after planting (+1.5 units relative to Hybrid P1185). Both hybrids followed a similar pattern, with an increase in chlorophyll content up to 12 days after planting, followed by a slight decline at 14 DAP. However, Hybrid P1197 maintained a consistently greener canopy. Figure 7 shows an interaction between the seed treatment and PGC distance, indicating that treated seeds had lower SPAD values that were statistically similar across all the distances, while there was variation among distance across untreated seeds.

3.5. Red:Far-Red (R:FR) Ratio

The statistical analysis revealed significant main effects of the seed treatment (p = 0.0001), PGC distance (p = 0.0001), and days from planting (p = 0.0001) on the measured R:FR, while the hybrid had no significant overall effect (p = 0.5982) (Appendix A, Table A1). Seed treatment consistently increased values across all levels of the PGC distance over time. The dense canopy from the grasses strongly influenced the light quality, with plants located at 0 and 6 cm from the PGC strip exhibiting significantly lower values than those at 25 cm and the control (Figure 8). The quality R:FR ratio within the plant canopy declined over time, with days after planting showing a highly significant effect (p < 0.0001) (Appendix A, Table A1).
Further analysis confirmed that the PGC distance effects were significant at days 5, 10, and 15 (p < 0.0001 for all). At later days, plants at 25 cm and the control maintained the highest R:FR ratio, while those at 0 and 6 cm remained significantly lower. The RR:FR ratio at these two distances further decreased at later sampling dates due to the closure of the corn canopy.
Seed treatment exhibited significant interactions with the PGC distance (p = 0.0013). The treated plants consistently showed lower values than untreated plants (Figure 8), although the magnitude of treatment benefit varied across distances and sampling dates. The strongest seed treatment effect was observed at the 6 cm PGC distance (Figure 8). Although 0 cm distance recorded the lower R:FR ratio, there was no significant difference between treated and untreated plants (Figure 8).
The hybrid by PGC distance interaction was not statistically significant, however, further analysis revealed significant differences for the control treatment (p = 0.0171). Under noncompetitive conditions, Hybrid P1185 exhibited a significantly greater R:FR ratio (3.24) compared with Hybrid P1197 (2.73), indicating differences in canopy structure and light interception. In contrast, at distances where plants experienced competition from the PGC strip (0, 6, and 25 cm), hybrids did not differ significantly (p > 0.15).

3.6. Root-to-Shoot Ratio

The root-to-shoot ratio was not significantly influenced by the hybrid type (p = 0.73), thiamethoxam seed treatment (p = 0.21), or PGC distance (p = 0.85), and no significant interactions among these factors were detected (Appendix A, Table A1).
Mean root-to-shoot ratio for hybrids was similar, with Hybrid P1185’s ratio = 1.168 and Hybrid P1197’s ratio = 1.191 (p = 0.73). Means comparisons confirmed no significant difference (p = 0.7275). Likewise, seed treatment did not significantly influence the root-to-shoot ratio (p = 0.207), with untreated plants averaging a root-to-shoot ratio of 1.179 and treated plants averaging 1.176, which were not statistically not different.

4. Discussion

4.1. Corn Emergence (DAP)

Seedling emergence and early plant morphological development are strongly influenced by both genetic factors and the environmental conditions surrounding the seedling, including light, nutrient reserves in the seed, and soil moisture [30]. During the heterotrophic phase, the mobilization of seed nutrient reserves is critical for fueling initial growth until the seedling establishes photosynthetic autonomy [31,32].
Corn plants located closest to the PGC (0 cm and 6 cm) emerged significantly later than plants at the 25 cm distance and the control (no grasses), indicating that the suppressive effect of the PGC on early seedling establishment is highly localized and decreases rapidly with increasing distance [25,33].
In corn–PGC systems, surrounding vegetation modifies both the quantity and quality of light reaching the soil surface. Green leaves absorb R light while reflecting and transmitting FR wavelengths, reducing the R:FR ratio near the PGC [34]. Plants detect this change through phytochrome photoreceptors, where a low R:FR ratio signals nearby vegetation and can influence early seedling growth processes [5,9]. At the 25 cm distance, seedlings are sufficiently removed from the reflective grass canopy, allowing the light environment to resemble the control and resulting in earlier emergence.
The PGC can modify the soil temperature, thereby reducing emergence timing [35]. Because seed germination and emergence depend on thermal time accumulation [36], cooler soil conditions near the PGC may slow seedling development. Shading from the PGC canopy at 0 cm and 6 cm likely reduced localized soil temperatures compared with the more exposed soil in the 25 cm and control treatments.
Hybrid differences were evident across treatments, indicating inherent variation in early vigor, mesocotyl elongation capacity, or seed metabolic activity [37,38]. Hybrid differences in SAR sensitivity likely contributed to the significant interaction between the hybrid and PGC distance. Hybrid P1185 responded favorably to moderate far-red enrichment at 6 cm, exhibiting accelerated emergence, whereas Hybrid P1197 performed best in the control rows. This supports the idea that SAR sensitivity varies among genotypes and that it can influence emergence patterns under modified light quality environments [39].
The seed treatment with thiamethoxam did not significantly affect emergence, consistent with its limited role in processes directly tied to mesocotyl elongation or phytochrome signaling [20]. Although untreated seeds emerged slightly earlier, the emergence timing was largely determined by spatial cues and genetic potential rather than seed treatment.
For producers using multispecies frameworks such as the PGC–corn system, maintaining a distinct, vegetation-free crop zone is critical to ensuring uniform emergence and robust early seedling establishment [33]. Adequate spatial separation mitigates the localized suppressive effects of altered light quality and reduced soil temperatures associated with the groundcover canopy without compromising the broader soil conservation benefits of the system [40]. Our results highlight significant differential responses among corn hybrids suggests that selecting hybrids characterized by strong early-season vigor or reduced shade avoidance sensitivity could substantially improve crop performance and yield stability in PGC-based agroecosystems [35].

4.2. Corn Stalk Length and Plant Height

The PGC distance had an impact on corn height and stalk length. The corn morphology is sensitive to even small spatial shifts in canopy structure [41,42], which modify the early-season light environment and alter development across growth stages [43]. This reversal observed after the initial elongation phase illustrates a common trade-off in SAR [12]. Hybrid P1185’s growth was less affected by the PGC distance, suggesting either lower SAR responsiveness or a more conservative growth strategy in response to spatial competition [44]. Genetic variation plays a critical role in regulating the SAR in corn, particularly influencing the degree of sensitivity to morphological changes such as stem elongation and reduced leaf thickness [45].
These findings have significant implications for multispecies corn systems. In intercropping environments, where neighboring vegetation alters light quality [46], selecting corn hybrids with reduced sensitivity to the SAR can mitigate excessive early stem elongation and promote stable plant architecture [47]. Under low R:FR light conditions, seedlings that strongly express SAR tend to elongate rapidly; this rapid vegetative growth can increase susceptibility to late-season lodging, potentially compromising the effective plant population [48]. Alternatively, from an agronomic management perspective, establishing a dedicated planting zone via strip tillage can minimize early-season canopy interference, thereby preserving a more favorable light environment during critical crop establishment phases [49].

4.3. Stem Diameter

The thiamethoxam seed treatment significantly reduced corn stem diameter, with untreated plants exhibiting substantially greater radial thickness than their treated counterparts. This suggests that thiamethoxam may impose subtle physiological and metabolic costs that limit structural growth. Such effects are likely mediated through alterations in hormonal signaling pathways, shifts in carbon allocation, or the disruption of early rhizosphere microbial interactions that are critical for robust plant development [50]. Furthermore, the significant interaction between seed treatment and PGC distance indicates that the phytotoxic or metabolic impacts of thiamethoxam are highly context-dependent. Specifically, these negative effects become more pronounced under conditions where seedlings are simultaneously responding to competitive environmental cues—such as the reduced R:FR light ratios near the PGC canopy, which trigger the SAR [20]. Under these low R:FR conditions, the insecticidal treatment appears to exacerbate the plant’s natural SAR tendency to prioritize vertical internode elongation at the expense of radial growth.
Spatial competition from the PGC resulted in elongated corn with thin, mechanically weak stems, possibly due to reallocation of carbohydrates toward vertical growth [51]. However, these differences were not statistically significant. The effect was modest, and hybrid differences were minimal, indicating that stem thickening is a relatively stable trait across genotypes used in this study.

4.4. Chlorophyll Content (SPAD) of Corn Leaves

The absence of PGC distance effect indicates that proximity to the PGC strip did not reduce chlorophyll accumulation during early growth. Chlorophyll formation is tightly regulated by nitrogen availability, leaf expansion rate, and plastid development [52]. The lack of distance effects suggested that resource acquisition, particularly nitrogen, remained adequate across all spatial positions within the PGC system at this stage [41].
Hybrid P1197 consistently maintained higher SPAD values than Hybrid P1185. This indicates that this hybrid has superior chlorophyll biosynthetic capacity, more efficient nitrogen assimilation, or greater stability of chlorophyll–protein complexes [53,54]. Higher SPAD values are often associated with enhanced photosynthetic efficiency, larger chloroplast density per leaf area, and improved early vigor traits that may offer an advantage under moderate competitive stress [54,55].
Contrary to expectations, seeds treated with thiamethoxam did not exhibit improved SPAD values; rather, they demonstrated a reduction in apparent leaf chlorophyll, suggesting the treatment failed to enhance early physiological performance under the conditions of this study. While thiamethoxam is widely documented to stimulate early plant vigor in various crops [21], these physiological benefits are highly dependent on environmental contexts and developmental stages. One plausible explanation for the observed reduction is that the insecticide induced mild phytotoxicity or metabolic stress in the seedlings, which may have temporarily inhibited chlorophyll biosynthesis or accelerated its degradation. Furthermore, seed-applied xenobiotics can alter early metabolic activity, potentially diverting energetic resources toward enzymatic detoxification pathways at the expense of chlorophyll production [56]. Alternatively, the treatment may have influenced leaf morphology, such as specific leaf area or thickness, which can confound SPAD meter readings without necessarily reflecting a proportionate decrease in absolute chlorophyll concentration [57]. Consequently, the reduced SPAD values observed in thiamethoxam-treated plants likely reflect transient, short-term physiological adjustments rather than a chronic detriment to overall plant growth.

4.5. Red:Far-Red (R:FR) Ratio

Proximity to the PGC strip substantially altered the R:FR ratio, thereby affecting light quality around the corn seedlings. PGC, like most plants in a similar competitive situation, absorbed photosynthetically active red light while reflecting far-red wavelengths [9,10]. This shift in the R:FR ratio is sensed by the phytochrome system of the corn seedlings, functioning as an early proximity signal that can trigger SAR even in the absence of direct canopy shading [3]. This was evident in our study, as we observed that corn plants at closer PGC distances exhibit SAR.
The reduced light quality around the corn seedling, caused by PGC at a 6 cm distance, resulted in the strongest expression of SAR plant morphology observed in the experiment. The increase in corn stalk height around V2–V4 is consistent with the established SAR, wherein the deactivation of phytochrome B (phyB), the primary red light receptor in corn, releases the inhibition of stem elongation [39,58]. As the PGC strip reflects far-red light toward the base of the corn seedlings, the resulting low R:FR ratio shifts phyB to its inactive state, triggering a hormonal change that prioritizes vertical growth over lateral development [59].
At 0 cm distance, the physical obstruction caused by dense PGC may have created a severe light-limited environment. While low R:FR ratios typically trigger elongation, the SAR is a resource-intensive process that requires a minimum threshold of photosynthetically active radiation (PAR) to provide the carbohydrates necessary for rapid cell expansion [60,61].
Under the close proximity of the 0 cm treatment, the seedlings likely encountered shaded conditions rather than simple proximity signals. When total light intensity (PAR) falls below a critical threshold, the energy-demanding elongation of the SAR is suppressed because the plant lacks the carbon assimilates required to fuel rapid cell expansion [10].
These findings highlighted the importance of managing groundcover proximity in PGC–corn systems to maintain a favorable light environment during early seedling development [62]. To optimize this, producers should prioritize establishing a clean or adequately buffered planting zone between the crop rows and the PGC strip. This spatial separation minimizes reductions in light quality, thereby preventing the premature activation of the SAR in the corn [12]. Close proximity to dense groundcover not only triggers SAR but can also attenuate overall photosynthetically active radiation (PAR) below the thresholds required to sustain robust early vegetative growth [10]. Therefore, precise spatial management, whether through optimized strip widths or targeted mechanical suppression near the crop row, is critical to ensure optimal light interception, support vigorous seedling development, and safeguard overall crop productivity.

4.6. Root-to-Shoot Ratio

Early-stage biomass allocation in corn remained constant under this study. The root-to-shoot ratio was stable across hybrids, seed treatment levels, and PGC distances. None of these factors altered the carbon partitioning strategy of the seedlings during the period evaluated [63,64].
Early vegetative growth prioritizes the coordinated expansion of both shoot and root systems, regulated by endogenous hormonal balances, particularly auxin, cytokinin, and gibberellin [65]. During early development, corn typically maintains a relatively fixed proportional allocation of biomass, unless stress becomes severe enough to alter hormonal signaling networks [66]. In this study, despite the competitive environment near the PGC strip, the overall spatial competition intensity may not have reached the threshold necessary to trigger a shift in biomass allocation.

5. Conclusions

This study demonstrates that light quality varied with distance from the PGC, with reduced R:FR ratios at closer PGC distances, inducing SAR in corn at 6 cm. These light-mediated signals reduced emergence at closer PGC proximities for Hybrid P1185 and promoted stem elongation, evident in Hybrid P1197, without affecting early biomass accumulation. Genotypic differences were evident: Hybrid P1197 was more sensitive to light quality signaling, whereas Hybrid P1185 exhibited delayed emergence but lower sensitivity to reduced R:FR ratios. The seed treatment with thiamethoxam did not modify corn responses to light quality or improve the physiological growth of the plants. In our experiments, the root–shoot ratio remained constant, likely because plants were terminated at the V6 vegetative stage. Breeding for reduced SAR or tolerance to low R:FR light ratios may improve corn performance in PGC systems. Future studies should assess whether early light-induced architectural changes influence later growth and final yield.

Author Contributions

Conceptualization, A.M., A.S.G. and K.J.M.; methodology, A.M., K.J.M. and A.S.G.; validation, A.M., A.S.G., K.J.M., S.-z.F. and A.K.; formal analysis, A.M. and K.J.M.; investigation, A.M.; resources, A.S.G. and K.J.M.; data curation, A.M., K.J.M. and A.S.G.; writing—original draft preparation, A.M.; writing—review and editing, A.M., A.S.G., K.J.M., S.-z.F. and A.K.; visualization, A.M.; supervision, A.S.G. and K.J.M.; funding acquisition, A.S.G. and K.J.M. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by Regenerating America’s Working Landscapes to Enhance Natural Resources and Public Goods through Perennial Groundcover (RegenPGC). RegenPGC is supported by the Agriculture and Food Research Initiative Competitive Grant No. 2021-68012-35923 from the USDA National Institute of Food and Agriculture. Any opinions, findings, conclusions, or recommendations expressed in this publication are those of the author(s) and do not necessarily reflect the view of the U.S. Department of Agriculture.

Data Availability Statement

The original contributions presented in this study are included in the article. Further inquiries may be directed to the author.

Acknowledgments

We thank Roger Hintz and Patrick Galland for assistance with experimental materials, as well as Philip Rockson, Samuel Soetan, and George Obeng-Akrofi for assistance with experimental setup and data collection.

Conflicts of Interest

The authors declare no conflict of interest.

Abbreviations

The following abbreviations are used in this manuscript:
PGCperennial groundcover
SARshade avoidance response
DAPdays after planting
CGcover crops

Appendix A

Table A1. The table shows probability > F (type III tests) for fixed effects across traits of corn grown with perennial groundcover (PGC) systems. Values of <0.05 indicate statistically significant effects.
Table A1. The table shows probability > F (type III tests) for fixed effects across traits of corn grown with perennial groundcover (PGC) systems. Values of <0.05 indicate statistically significant effects.
EffectEmergenceHeightStalk LengthSPADStem
Diameter		Red:Far-RedRootShoot
Hybrid0.0212<0.00010.0011<0.0002ns(ns)nsns
T (seed treatment)0.1060nsns<0.00010.0003<0.0001nsns
PGC distance (PGC)0.0074<0.0001<0.00010.5780.0622<0.0001--
Days-<0.0012<0.0001<0.0001<0.0001<0.0001--
Hybrid × Tnsnsns0.0049nsnsnsns
Hybrid × PGC0.0379ns0.00330.00830.0033<0.0001nsns
T × PGCDISns0.0664nsns0.0370.0013nsns
Hybrid × T × PGCns0.0882nsnsns<0.0001nsns
Hybrid × days-nsnsns0.0216ns--
Days × PGC -nsnsns0.076ns--
Days × T-nsns0.01890.0640.0001--
T × PGC × daysnsnsnsns0.02070.0586 --
Hybrid × T × PGC × daysnsnsns0.0427ns<0.0001--
Symbols: p < 0.05 indicates a significant effect. ns = not significant (p > 0.10); - = not applicable.
Table A2. Least-squares means (LS-means) of corn emergence across hybrids, PGC distances, and T (seed treatments).
Table A2. Least-squares means (LS-means) of corn emergence across hybrids, PGC distances, and T (seed treatments).
FactorLevelLS-Means
Hybrid14.262
24.135
PGC Distance0 cm4.246
25 cm4.066
6 cm4.323
Control4.160
T04.155
14.243

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Figure 1. The layout of the study in a growth chamber. Randomized complete block design with four replications. Each experimental unit consists of a tray with three pots of corn placed at one of the four distances (control, 0, 6, and 25 cm) from the PGC grass.
Figure 1. The layout of the study in a growth chamber. Randomized complete block design with four replications. Each experimental unit consists of a tray with three pots of corn placed at one of the four distances (control, 0, 6, and 25 cm) from the PGC grass.
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Figure 2. An experimental unit consisting of two strips of grass and three pots of single corn plants. The corn and grass root zones were physically isolated using separate containers on a 25 × 50 cm tray, with three corn pots and grass strips positioned at assigned distances (0, 6, and 25 cm) along the tray edges to establish different aboveground competition patterns.
Figure 2. An experimental unit consisting of two strips of grass and three pots of single corn plants. The corn and grass root zones were physically isolated using separate containers on a 25 × 50 cm tray, with three corn pots and grass strips positioned at assigned distances (0, 6, and 25 cm) along the tray edges to establish different aboveground competition patterns.
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Figure 3. Least-squares means (LS-means) of corn emergence, measured in days after planting, as influenced by perennial groundcover (PGC) distance and hybrid. The Y-axis represents the mean number of days to emergence, while the X-axis shows PGC distances (0, 6, 25 cm, and control). Hybrid 1 (P1185) and Hybrid 2 (P1197) are represented by blue and orange bars, respectively. Higher bars indicate delayed emergence. Different letters above the bars indicate significant differences among treatment combinations (p < 0.05); bars sharing the same letter are not significantly different.
Figure 3. Least-squares means (LS-means) of corn emergence, measured in days after planting, as influenced by perennial groundcover (PGC) distance and hybrid. The Y-axis represents the mean number of days to emergence, while the X-axis shows PGC distances (0, 6, 25 cm, and control). Hybrid 1 (P1185) and Hybrid 2 (P1197) are represented by blue and orange bars, respectively. Higher bars indicate delayed emergence. Different letters above the bars indicate significant differences among treatment combinations (p < 0.05); bars sharing the same letter are not significantly different.
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Figure 4. Time-series trends of mean corn height (cm) across perennial groundcover (PGC) distances. The Y-axis represents the mean plant height (cm) measured at multiple days after planting, and the X-axis represents days from planting. Colored lines denote PGC distances: 0 cm (blue), 6 cm (green), 25 cm (orange), and the control (red). Error bars indicate 95% confidence intervals of the least-squares means estimated from the mixed-effects model.
Figure 4. Time-series trends of mean corn height (cm) across perennial groundcover (PGC) distances. The Y-axis represents the mean plant height (cm) measured at multiple days after planting, and the X-axis represents days from planting. Colored lines denote PGC distances: 0 cm (blue), 6 cm (green), 25 cm (orange), and the control (red). Error bars indicate 95% confidence intervals of the least-squares means estimated from the mixed-effects model.
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Figure 5. Time-series trends of mean corn stalk length (cm) across perennial groundcover (PGC) distances. The Y-axis represents the mean stalk length (cm) measured at multiple days after planting, and the X-axis represents days from planting. Colored lines denote PGC distances: 0 cm (blue), 6 cm (green), 25 cm (orange), and the control (red). Error bars indicate 95% confidence intervals of the least-squares means estimated from the mixed-effects model.
Figure 5. Time-series trends of mean corn stalk length (cm) across perennial groundcover (PGC) distances. The Y-axis represents the mean stalk length (cm) measured at multiple days after planting, and the X-axis represents days from planting. Colored lines denote PGC distances: 0 cm (blue), 6 cm (green), 25 cm (orange), and the control (red). Error bars indicate 95% confidence intervals of the least-squares means estimated from the mixed-effects model.
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Figure 6. Least-squares means (LS-means) of corn stem diameter (mm) as affected by perennial groundcover (PGC) distance and seed treatment. The Y-axis represents the mean stem diameter (mm), while the X-axis shows PGC distances (0, 6, 25 cm, and control). The orange and blue bars represent the thiamethoxam seed treatment with untreated (T0) and treated (T1) seeds, respectively. Different letters above the bars indicate significant differences among treatment combinations (p < 0.05); bars sharing the same letter are not significantly different.
Figure 6. Least-squares means (LS-means) of corn stem diameter (mm) as affected by perennial groundcover (PGC) distance and seed treatment. The Y-axis represents the mean stem diameter (mm), while the X-axis shows PGC distances (0, 6, 25 cm, and control). The orange and blue bars represent the thiamethoxam seed treatment with untreated (T0) and treated (T1) seeds, respectively. Different letters above the bars indicate significant differences among treatment combinations (p < 0.05); bars sharing the same letter are not significantly different.
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Figure 7. Least-squares means (LS-means) of chlorophyll content (SPAD values) as influenced by perennial groundcover (PGC) distance and seed treatment. The Y-axis represents the mean SPAD values, while the X-axis shows PGC distances (0, 6, 25 cm, and control). The orange and blue bars represent thiamethoxam seed treatments with untreated seeds (T0) and treated seeds (T1), respectively. Different letters above the bars indicate significant differences among treatment combinations (p < 0.05); bars sharing the same letter are not significantly different.
Figure 7. Least-squares means (LS-means) of chlorophyll content (SPAD values) as influenced by perennial groundcover (PGC) distance and seed treatment. The Y-axis represents the mean SPAD values, while the X-axis shows PGC distances (0, 6, 25 cm, and control). The orange and blue bars represent thiamethoxam seed treatments with untreated seeds (T0) and treated seeds (T1), respectively. Different letters above the bars indicate significant differences among treatment combinations (p < 0.05); bars sharing the same letter are not significantly different.
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Figure 8. Least-squares means (LS-means) of red:far-red light ratio as affected by perennial groundcover (PGC) distance and seed treatment. The Y-axis represents the mean red:far-red ratio, while the X-axis shows PGC distances (0, 6, 25 cm, and control). Orange and blue bars represent untreated (T0) and treated (T1) seeds with thiamethoxam, respectively. Different letters above the bars indicate significant differences among treatment combinations (p < 0.05); bars sharing the same letter are not significantly different.
Figure 8. Least-squares means (LS-means) of red:far-red light ratio as affected by perennial groundcover (PGC) distance and seed treatment. The Y-axis represents the mean red:far-red ratio, while the X-axis shows PGC distances (0, 6, 25 cm, and control). Orange and blue bars represent untreated (T0) and treated (T1) seeds with thiamethoxam, respectively. Different letters above the bars indicate significant differences among treatment combinations (p < 0.05); bars sharing the same letter are not significantly different.
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MDPI and ACS Style

Moro, A.; Goggi, A.S.; Moore, K.J.; Fei, S.-z.; Kaleita, A. Spatial Proximity to Perennial Groundcover Triggers Shade Avoidance Responses in Corn. Agronomy 2026, 16, 729. https://doi.org/10.3390/agronomy16070729

AMA Style

Moro A, Goggi AS, Moore KJ, Fei S-z, Kaleita A. Spatial Proximity to Perennial Groundcover Triggers Shade Avoidance Responses in Corn. Agronomy. 2026; 16(7):729. https://doi.org/10.3390/agronomy16070729

Chicago/Turabian Style

Moro, Amina, A. Susana Goggi, Ken J. Moore, Shui-zhang Fei, and Amy Kaleita. 2026. "Spatial Proximity to Perennial Groundcover Triggers Shade Avoidance Responses in Corn" Agronomy 16, no. 7: 729. https://doi.org/10.3390/agronomy16070729

APA Style

Moro, A., Goggi, A. S., Moore, K. J., Fei, S.-z., & Kaleita, A. (2026). Spatial Proximity to Perennial Groundcover Triggers Shade Avoidance Responses in Corn. Agronomy, 16(7), 729. https://doi.org/10.3390/agronomy16070729

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