## 1 Introduction  
The world population is constantly increasing, which has caused an increase in world food demand. Therefore, the optimisation of food production, including yield size, is essential. One of the methods of increasing yield in modern, sustainable agriculture is the enhancement of germination rate and the improvement of vigour of seedlings. There are various methods of the pre-treatment of seeds to stimulate germination and seedlings growth, including thermal treatment [1], seed priming [2], seed coating [3], nonthermal plasma treatment [4], laser irradiation [5], or gamma irradiation [6]. One of promising techniques is the exposure of seeds to an electromagnetic field (EMF) or magnetic field (MF). The effect of MF or EMF on biological objects can relate to various phenomena such as the behaviour of magnetic particles, paramagnetic elements, and free radicals, the induction of currents in these objects, the change in the energy of intra-atomic and inter-atomic interactions, and the change in water component behaviour [7]. MFs affect plant photosynthetic function and enzyme activity [8]. Generally, scientific reports indicate positive results of treating seeds with an electromagnetic or magnetic field. Electromagnetic seed treatments can improve germination rate, growth, and plant yield. This effect is caused by the positive impact of the EMF on some biological processes such as the increase in enzymes activity and seed coat membrane integrity [9] or the change in lipid peroxidation [10]. It was proven that the magnetic field treatment of sunflower seeds improves the growth and yield of plants [11]. The positive effect of a stationary and homogeneous magnetic field in the range of 2.23-3.72 mT on the germination time of tritticale (_X Triticoscale Wittrack_) seeds was reported [12]. The increase in germination rate and seedling length was observed after the pre-treatment of radius (_Raphanus sativus_ L.) seeds with the use of a stationary magnetic field (8 mT and 20 mT) [13]. However, some negative effects of electromagnetic seed treatment were also reported in the literature. [PERSON] and co-authors [14] used a weak magnetic field for the pre-treatment of _Myrtus communis_ L. seeds and observed a reduction in the number and speed of germination when compared to the control [14]. The negative effects on the germination of P. _elongata_ were caused by the exposure of dry seeds to a magnetic field [15]. The accumulation of reactive oxygen species in broad bean (_Vicia faba_ L.) was also reported [16]. [PERSON] observed the negative effect of a 200 mT magnetic field on cucumber germination and seedlings vigour [17]. The research reports concerning the influence of magnetic fields on seeds concentrated mainly on germination rate. A significantly smaller number of reports focused on the growth of seedlings and yields. There is limited scientific reports describing the influence of magnetic fields on cucumber (_Cucumis sativus_ L.) seeds, their germination, as well as the further growth of plants and yield. The cucumber is one of the most popular vegetables in the world. The global world production of cucumber in the year 2019 was about 87,805 million tonnes. Poland is one of the biggest producers of cucumber in the European Union with almost ten thousand hectares under cucumber cultivation in the year 2019. Only a few studies relating to the pre-sowing magnetic field treatment of cucumber seeds have been reported so far. The MF pre-treatment of cucumber seeds with 0.2 T and 0.45 T increased seed germination rate, seedling emergence rate, above-ground biomass, and leaf area [8]. The positive effect of magnetisation with a certain magnetic intensity on cucumber (_Cucumis sativus_ L. var. Beit Alpha) seeds germination, seedlings biomass, and vigour was reported by [PERSON] [17]. The significant increase in percentage and speed of germination of cucumber seeds exposed to MF strengths from 50 to 250 mT for 1, 2, and 3 h was described by [PERSON] and co-authors [18].