In modern horticulture the use of biostimulators for obtaining the maximum yields and qualities of the crops is favoured. Pentakeep®-V, the fertilizer containing 5-aminolevulinic acid (ALA) is currently recommended. The ALA is recognized as biostimulator of different physiological processes. It increases the photosynthetic efficiency, impact on the nitrogen metabolism – increases activity of nitrate reductase, decreases the content of nitrates. The flowering, nectar production and sugar content by flower position and after foliar application of Pentakeep®-V in Hosta Tratt. ‘Krossa Regal’ were examined in 2012–2013. The plants were sprayed with water solutions of Pentakeep®-V in concentrations: – 0 (control – distilled water), 0.02, 0.04, and 0.06%. Nectar was extracted from 24-hs flowers, using pipette method. Nectar amount and concentration was strongly influenced by floral position in the raceme, and fertilizer application. The total amount of nectar produced per flower declined along the inflorescence, starting from the bottom positioned. Pentakeep®-V fertilizer, depending on concentrations had a variety of effects on flower traits and nectar production. The application of 0.06% of Pentakeep®-V increased the number of flowers per inflorescence, however only in one year of the study. Pentakeep®-V in 0.02 and 0.04% concentrations increased the flower size, with respect to the perianth width as well as improved nectar production. The 0.06% of Pentakeep®-V application suppressed both the flower size and nectar production. These results might be explained by the stimulation of chlorophyll synthesis and CO₂ absorption in lower dosages of ALA and reduced photosynthetic efficiency with excess dosages of ALA.

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