Thylakoid Nanoarchitecture and Photosystem II Energy Partitioning in Response to Treatment with Huminatrin Concentrate “Boron”

Photosynthetic acclimation involves coordinated changes in the structural organization and functional activity of the photosynthetic apparatus. However, the structural basis underlying enhanced photosynthetic activity following biostimulant treatment under non-stress conditions remains insufficiently understood. In the present study, we investigated the effects of exogenous application of the humic-boron fertilizer Huminatrin Concentrate “Boron” on the structural and functional organization of the photosynthetic apparatus in spring rapeseed (Brassica napus L.) cv. Siberian. Thylakoid and granum architecture was characterized using transmission electron microscopy and quantitative morphometric analysis, whereas photosynthetic function was assessed using pulse-amplitude-modulated (PAM) chlorophyll fluorescence. Huminatrin treatment was associated with changes in granum organization, including a reduction in the stacking repeat distance (SRD), as well as increases in the apparent electron transport rate (ETR) and the effective quantum yield of photosystem II (PSII) photochemistry (Y(II)). Redistribution of PSII excitation energy was accompanied by a decrease in the regulatory quantum yield in PSII (Y(NPQ)) and an increase in the non-regulated energy loss in PSII (Y(NO)). These changes occurred without evidence of pronounced PSII photoinhibition, indicating that the observed changes in PSII photochemistry were not a response to experimentally induced stress. Structural and functional changes were accompanied by increases in plant and leaf biomass. Taken together, these findings indicate that leaf biostimulation is associated with coordinated remodeling of thylakoid architecture and PSII excitation-energy partitioning under the experimental conditions studied.