Light is a fundamental environmental factor that governs plant growth, development, and metabolic activities. In particular, the intensity and quality of light directly influence leaf morphology, tissue organization, and physiological performance. Tea plants, especially the large-leaf variety Camellia sinensis var. assamica, are known to thrive under diffuse light conditions, making them suitable for intercropping with shade-providing structures. In recent years, the integration of photovoltaic (PV) technology with agriculture—termed “agrivoltaics”—has attracted considerable attention. Solar panels installed above tea plantations can reduce direct solar radiation while generating renewable energy. However, the extent to which different configurations of solar panels affect the leaf anatomical structure of tea plants remains poorly understood. In this study, we investigated the effects of four shading treatments imposed by different types of solar panels (flexible PV with 35 mm spacing, fixed-axis, flexible PV with 100 mm spacing, and horizontal single-axis) and a non-shaded control on the leaf anatomical parameters of C. sinensis var. assamica. Our objective was to identify the optimal shading level provided by solar panels that can maintain or improve leaf structure for enhanced tea quality and yield. We measured leaf thickness (LT), palisade tissue thickness (PT), spongy tissue thickness (ST), cuticle thickness (CT), and calculated the mesophyll loosening ratio (MLR), stomatal density (SD), veinlet density (VD), single leaf epidermal cell area (LEA), palisade-to-spongy tissue ratio, leaf area (LA), and specific leaf area (SLA). Our results demonstrate that shading from solar panels induces significant structural adjustments, and a moderate shading level (approximately 60% relative light intensity) appears most beneficial.
Materials and Methods
Study Site and Plant Material
The experiment was conducted in a photovoltaic organic tea plantation at Pu’er University, located in the Simao District of Pu’er City, Yunnan Province, China (altitude 1200–1300 m). The region has a subtropical monsoon climate with an average annual temperature of 17.7–19.3°C, extreme maximum of 37.7°C, and extreme minimum of 3.4°C. Annual sunlight hours average 2159.7 h, and annual precipitation ranges from 1100 to 2780 mm. The soil is acidic yellow loam. We selected 19–20-year-old Camellia sinensis var. assamica plants (rootstock: Yunkang 10; scion: Yunhuang 1) as experimental materials.
Shading Treatments
Four shading treatments were implemented using different configurations of solar panels installed approximately 10 cm above the tea canopy: T1 – flexible PV panels with 35 mm spacing; T2 – fixed-axis PV panels; T3 – flexible PV panels with 100 mm spacing; T4 – horizontal single-axis PV panels. A non-shaded control (CK) was also included. The relative light intensity (RLI) at the leaf surface was measured using a TES-1334 ALIGHTMETER, with CK set as 100% full light. The results are summarized in Table 1.
| Treatment | RLI (%) |
|---|---|
| T1 | 23.24 ± 0.001 e |
| T2 | 29.80 ± 0.007 d |
| T3 | 36.33 ± 0.003 c |
| T4 | 59.65 ± 0.005 b |
| CK | 100.00 ± 0.001 a |
Different lowercase letters indicate significant differences at p < 0.05.
Leaf Sampling and Anatomical Measurements
In early June 2024, we marked healthy plants and sampled the third fully expanded leaf from the apex (30 leaves per treatment). Leaves were immediately fixed in FAA solution (70% ethanol 450 mL + glacial acetic acid 25 mL + formalin 25 mL) with 25 mL glycerol added to prevent shrinkage. For cross-section analysis, freehand sections were made avoiding the midrib, stained, and observed under a Leica DM2500 microscope at 20× and 40× magnifications. Twelve replicates per treatment were measured. Using ImageJ software, we quantified leaf thickness (LT), cuticle thickness (CT), palisade tissue thickness (PT), and spongy tissue thickness (ST). The palisade-to-spongy ratio (PT/ST) and mesophyll loosening ratio (MLR = (ST + PT)/LT) were calculated. For stomatal characteristics, epidermal peels were prepared from the middle portion of leaves (avoiding major veins), and images were captured at 40×. Stomatal density (SD) was counted per mm², and single epidermal cell area (LEA) was measured. To determine veinlet density (VD), leaf pieces of 1 cm² were cleared in 5% NaOH at 70°C until tissues were transparent, then stained and photographed. Veinlet length was measured using ImageJ and density calculated as total length per area. Leaf area (LA) was determined by weighing paper cutouts of leaf outlines and comparing with a known area standard. Specific leaf area (SLA) was calculated as LA (cm²) divided by dry weight (g) after oven-drying at 65°C to constant mass. All data were analyzed using ANOVA in SPSS 27.0, with Tukey’s HSD test for post-hoc comparisons.
Results
Leaf Cross-Sectional Anatomy
Table 2 presents the effects of solar panels shading on leaf transverse structures. Spongy tissue thickness (ST) ranged from 138.50 µm (T2) to 154.75 µm (T1), but no significant differences were detected among treatments (p > 0.05). Leaf thickness (LT) was significantly lower in T2 (226.10 µm) than in T1, T3, and T4, but all treatments were comparable to the control (230.99 µm). Cuticle thickness (CT) decreased markedly under all shading treatments compared to CK (35.71 µm). The smallest CT was observed in T1 (17.72 µm). Palisade tissue thickness (PT) was significantly reduced in T1 (66.83 µm), T2 (66.63 µm), and T3 (64.37 µm) relative to CK (76.89 µm), whereas T4 (74.65 µm) did not differ from CK. These results indicate that solar panels shading mainly affects the cuticle and palisade layer, while spongy tissue and overall leaf thickness are more resilient.
| Treatment | Spongy tissue thickness (ST) | Leaf thickness (LT) | Cuticle thickness (CT) | Palisade tissue thickness (PT) |
|---|---|---|---|---|
| T1 | 154.75 ± 8.34 a | 233.92 ± 2.83 a | 17.72 ± 0.83 d | 66.83 ± 3.46 b |
| T2 | 138.50 ± 7.95 a | 226.10 ± 2.17 b | 34.26 ± 0.26 b | 66.63 ± 2.63 b |
| T3 | 149.43 ± 3.06 a | 233.28 ± 1.03 a | 32.82 ± 0.22 c | 64.37 ± 1.66 b |
| T4 | 141.86 ± 4.89 a | 235.78 ± 0.90 a | 34.04 ± 0.07 b | 74.65 ± 1.98 a |
| CK | 138.55 ± 4.68 a | 230.99 ± 1.13 ab | 35.71 ± 0.22 a | 76.89 ± 2.96 a |
Different letters indicate significant differences (p < 0.05).
Tissue Organization Ratios
The ratio of palisade to spongy tissue (PT/ST) and the mesophyll loosening ratio (MLR) are shown in Table 3. PT/ST was significantly lower in T1 (0.43), T2 (0.49), and T3 (0.43) compared to CK (0.57), while T4 (0.54) did not differ from CK. The MLR values in T1 (0.66) and T3 (0.65) were significantly higher than in CK (0.60), indicating a looser mesophyll structure under heavy shading.
| Treatment | PT/ST | MLR (%) |
|---|---|---|
| T1 | 0.43 ± 0.01 c | 0.66 ± 0.01 a |
| T2 | 0.49 ± 0.02 bc | 0.61 ± 0.01 b |
| T3 | 0.43 ± 0.01 c | 0.65 ± 0.01 a |
| T4 | 0.54 ± 0.03 ab | 0.60 ± 0.01 b |
| CK | 0.57 ± 0.04 a | 0.60 ± 0.02 b |
Veinlet Density
Veinlet density (VD) showed a non-monotonic response to shading (Table 4). T3 had the highest VD (6.84 µm), significantly greater than CK (6.73 µm) and T4 (6.72 µm). T1 and T2 were intermediate and not different from CK. This suggests that moderate shading (T3) may enhance vascular development, while severe or very light shading does not.
| Treatment | VD (µm) |
|---|---|
| T1 | 6.75 ± 0.005 bc |
| T2 | 6.79 ± 0.005 ab |
| T3 | 6.84 ± 0.005 a |
| T4 | 6.72 ± 0.045 c |
| CK | 6.73 ± 0.005 bc |
Stomatal and Epidermal Characteristics
As shown in Table 5, single leaf epidermal cell area (LEA) was significantly larger in T1 and T2 than in T3 and T4. T4 did not differ from CK. Stomatal density (SD) ranged from 105.87 (T1) to 115.19 (T4) stomata per mm². T1 had significantly lower SD than T3 and T4, but all treatments were statistically similar to CK. Thus, shading from solar panels influences epidermal cell expansion more than stomatal density.
| Treatment | LEA (µm²) | SD (stomata·mm⁻²) |
|---|---|---|
| T1 | 890.39 ± 11.92 a | 105.87 ± 2.09 b |
| T2 | 880.78 ± 16.25 ab | 109.81 ± 1.53 ab |
| T3 | 780.11 ± 20.81 d | 113.93 ± 2.55 a |
| T4 | 820.48 ± 15.14 cd | 115.19 ± 2.27 a |
| CK | 843.58 ± 11.70 bc | 111.07 ± 2.88 ab |
Leaf Area and Specific Leaf Area
Table 6 demonstrates that leaf area (LA) was significantly larger under heavy shading (T1: 100.80 cm²) compared to other treatments. T2 (83.24 cm²), T3 (76.80 cm²), T4 (71.18 cm²), and CK (78.51 cm²) were not significantly different from each other, except T2 was larger than T4. Specific leaf area (SLA) was significantly higher in all shading treatments (140–147 cm²·g⁻¹) compared to CK (118.12 cm²·g⁻¹). No differences were found among the four shaded groups, indicating that SLA responds rapidly to the presence of shading regardless of its intensity.
| Treatment | Leaf area (cm²) | SLA (cm²·g⁻¹) |
|---|---|---|
| T1 | 100.80 ± 3.47 a | 147.22 ± 2.18 a |
| T2 | 83.24 ± 3.55 b | 140.72 ± 2.33 a |
| T3 | 76.80 ± 3.32 bc | 144.07 ± 1.67 a |
| T4 | 71.18 ± 2.63 c | 142.71 ± 2.17 a |
| CK | 78.51 ± 1.50 bc | 118.12 ± 2.01 b |
Discussion
Our study demonstrates that shading from solar panels induces significant plasticity in the leaf anatomical traits of Camellia sinensis var. assamica. The reduction in cuticle thickness (CT) under all shading treatments is a typical response to lower irradiance: a thinner cuticle reduces reflection of diffuse light, allowing more photons to reach the photosynthetic mesophyll. This is consistent with previous work on other shade-tolerant species. The concurrent increase in leaf area (LA) and specific leaf area (SLA) under shading (especially in T1) further enhances light capture per unit dry mass. These morphological adjustments are particularly important under solar panels because they compensate for the reduced total light energy available to the understory tea plants.
Interestingly, the palisade tissue thickness (PT) decreased significantly in T1, T2, and T3, while spongy tissue (ST) remained stable. This shift in tissue proportion (lower PT/ST ratio) likely improves light penetration through the leaf by creating a looser mesophyll structure (higher MLR). A more developed spongy layer with larger intercellular spaces facilitates CO₂ diffusion and increases the chance of light scattering, which can boost photosynthesis under low-light conditions. Our observation that MLR was highest in T1 and T3 supports this interpretation. However, excessive loosening may also increase water loss and reduce photosynthetic efficiency due to greater internal shading. Therefore, a balance is needed.
Veinlet density (VD) showed a unimodal response: it increased from CK to T3 and then decreased slightly in T4 and T1. This pattern may reflect an optimal investment in vascular tissue under moderate shade. Higher VD improves water and nutrient supply to the mesophyll, which can support larger leaf area. But under very heavy shade (T1), leaf area expansion may outpace vein development, leading to a lower VD. Our results suggest that T3 (flexible PV with 100 mm spacing, ~36% RLI) triggers the greatest vein investment.
Stomatal density (SD) did not differ significantly between shaded and unshaded leaves, except that T1 had a slightly lower SD than T3 and T4. However, epidermal cell area (LEA) increased significantly in T1 and T2, indicating that cells expanded but the number of stomata per unit area remained relatively constant. This implies that the stomatal index (stomata per epidermal cell) might have decreased under heavy shade. This strategy allows the leaf to maintain stomatal conductance flexibility while avoiding the cost of producing extra guard cells in a low-light environment.
Among all treatments, T4 (horizontal single-axis solar panels, 60% RLI) appeared to provide the most favorable shading level. In T4, cuticle thickness was reduced but not as drastically as in T1; palisade thickness remained similar to CK; the PT/ST ratio was not significantly different from CK; and leaf area was only slightly reduced relative to heavier shading. Moreover, T4 maintained a high SLA and did not suffer from excessive mesophyll loosening. These traits suggest that T4 leaves are optimally adapted to utilize diffuse light while retaining structural integrity. In contrast, T1 (23% RLI) caused excessive thinning of the cuticle and palisade, which could compromise leaf protection and photosynthetic capacity in the long term.
The integration of solar panels in tea plantations offers a dual benefit: renewable energy generation and improved microclimate for tea growth. Our anatomical evidence indicates that moderate shading (~60% RLI from horizontal single-axis panels) can maintain healthy leaf structure while potentially enhancing tea quality through increased diffuse light utilization. Further studies should investigate the physiological responses, such as photosynthesis and secondary metabolite accumulation, to confirm the optimal PV configuration for both yield and quality.
Several limitations of this study should be noted. First, we only measured anatomical parameters at a single time point; seasonal dynamics might reveal additional adaptive changes. Second, the solar panels used in our experiment have different spectral compositions that could influence leaf development beyond simple light intensity. Future work should consider the spectral quality of transmitted light under different PV types. Nevertheless, our results provide a solid foundation for designing agrivoltaic systems tailored to tea cultivation.
Conclusion
Shading by solar panels significantly alters the leaf anatomical structure of Camellia sinensis var. assamica. Key changes include reduced cuticle and palisade thickness, increased leaf area and specific leaf area, and modified veinlet density and epidermal cell size. Among the tested configurations, horizontal single-axis solar panels (T4, 60% relative light intensity) induced moderate adjustments that preserved palisade thickness and maintained a balanced mesophyll organization. This level of shading appears optimal for promoting tea leaf development under agrivoltaic systems. Our findings support the practical application of solar panels in tea gardens to achieve “one land, multiple products” goals, contributing to sustainable agriculture and renewable energy production in tropical regions.