FUNCTIONAL STATE OF THE PHOTOSYNTHETIC APPARATUS OF MISCANTHUS × GIGANTEUS UNDER DIFFERENT FERTILIZATION SYSTEMS AND POTASSIUM HUMATE FOLIAR APPLICATION
Abstract
The article presents the results of a five-year study (2021–2025) on the functional state of the photosynthetic apparatus of Miscanthus × giganteus cv. Osinnii Zoretsvit grown in the Forest-Steppe of Ukraine. The experiment was conducted at the educational farm of the National University of Life and Environmental Sciences of Ukraine in the Fastiv district of Kyiv region on podzolized chernozem. A two-factor design included three fertilization systems (control, Leonardite 100 kg/ha, and N₆₀P₁₆K₈₀) and four foliar application regimes of potassium humate. Plant response was assessed using a SPAD-502 chlorophyll meter, maximum quantum yield of photosystem II (Fv/Fm), and the performance index PI(abs) from the OJIP chlorophyll fluorescence test. Chlorophyll content increased from 34.2–44.3 SPAD units in 2021 to 42.5–55.0 units in 2025. The mineral fertilization system provided the highest SPAD values, reaching 50.9 units under double foliar application and exceeding the control by 29.2%. Potassium humate additionally increased SPAD by 8.5–10.2%. The Fv/Fm values ranged from 0.756 to 0.820, indicating a generally satisfactory condition of photosystem II. The organic system with Leonardite produced slightly higher mean Fv/Fm values (0.797) than the mineral system (0.790), which may reflect the stabilizing effect of humic substances on chloroplast membranes. PI(abs) proved to be the most sensitive indicator, varying from 2.59 to 3.75 relative units. Under the organic system combined with double foliar application, PI(abs) exceeded the control by 44.8%, confirming a marked positive effect of Leonardite humic acids on PSII reaction centres. The results confirm the effectiveness of combining soil fertilization and foliar biostimulant treatment to improve the photosynthetic activity and adaptive potential of miscanthus as a bioenergy crop.
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