Combined Biostimulant Treatment Is Associated with PSII Energy Partitioning and Leaf Anatomy in Cucumber Leaves and Fruits

Plant biostimulants may alter photosynthetic energy use and leaf structure, but their effects on complementary PSII energy partitioning in both cucumber leaves and fruits remain insufficiently characterized. Using plant material from a commercial greenhouse trial previously reported for agronomic outcomes, we evaluated a combined CropBioLife®, KeyPlex 120®, and Key Eco Oil® application protocol relative to conventional management. Chlorophyll Content Index (CCI), chlorophyll fluorescence imaging, and leaf anatomical traits were assessed. Under low light (LL; 455 μmol photons m−2 s−1), the combined-treatment greenhouse showed higher ΦPSII than the conventional-control greenhouse by 22% in fruits and 50% in leaves; fruit ΦNPQ was 17% higher, whereas ΦNO was 12% lower in fruits and 26% lower in leaves. Under high light (HL; 973 μmol photons m−2 s−1), differences in ΦPSII were smaller, while leaf ΦNO remained 17% lower. CCI was 38% higher in fruits and 52% higher in leaves. In leaves, spongy parenchyma thickness increased from 110 ± 3.4 to 135 ± 4.1 μm, palisade parenchyma thickness from 75 ± 2.1 to 78 ± 1.8 μm, total leaf thickness from 230 ± 4.6 to 245 ± 5.2 μm, and intercellular air-space area from 420 ± 18 to 610 ± 22 μm2. The parallel physiological and anatomical differences are consistent with a possible functional association, but gas exchange, mesophyll conductance, ROS, and water-use efficiency were not measured in the present dataset. Because the two management regimes occupied separate, non-replicated greenhouses and received different crop-protection inputs, the findings should be interpreted as treatment-associated differences rather than causal effects attributable to individual products.