This paper presents the results of measuring parameters that describe the photosynthetic activity of beech seedlings under suboptimal mineral nutrition. The aim of paper was to present the issue of mineral nutrition of forest trees to forestry practice through the discussion of the obtained results in the context of previous knowledge about the impact of mineral nutrition on the photosynthetic process of forest trees. In the early spring of 2019, beech seedlings originating from a natural mixed stand of sessile oak and common beech were transplanted into a sterile substrate (agroperlite). After transplanting, the seedlings were regularly watered (treated) with a complete nutrient solution (KO treatment), and nutrient solutions in which nitrogen (–N treatment), phosphorus (–P treatment), magnesium (–Mg treatment) or iron (–Fe treatment) were omitted. The following parameters were regularly measured on six seedlings within each of the five previously described treatments during the 2019 growing season: rate of photosynthesis (A), stomatal coonductance (g_s), intercellular concentration of CO₂ (c_i), relative chlorophyll content index (CCI) and the photosynthetic performance index<i> </i>(PI_ABS). All parameters were worse in seedlings from –N, –P and –Mg treatment compared to seedlings from KO treatment. Such a result is probably due to the physiological functions of N, P and Mg that participate in the structure of proteins and enzymes, energy turnover and storage, and the structure of chlorophyll, which together are necessary for the proper functioning of the overall photosynthetic process. Despite the important role of Fe in all life processes of plant, including photosynthesis, all measured parameters in seedlings from –Fe and KO treatments were very similar.<br>However, unlike N, P and Mg which belong to the group of macronutrients, Fe belongs to the group of micronutrients whose concentrations in plant tissue are relatively low. Therefore, it is possible to assume that the investigated seedlings from natural stand soil and/or from seed reserves during the initial phase of their development could absorb and/or retraslocate enough Fe for normal functioning throughout the growing season, regardless of subsequent treatment. Given such a results, initial monitoring of photosynthetic activity parameters will be continued over the next few growing seasons. In this period, it is planned to start monitoring the vegetative growth dynamics, leaf phenology and the development of visual symptoms under suboptimal mineral nutrition of beech seedlins in order to further approximate this issue to forestry practice.