The aim of the study was to measure the tensile strength of 4-year-old alfalfa leaves cultivated from seeds exposed to pre-sowing stimulation with He–Ne laser light for 1 or 5 min (designated respectively as F1 and F5) and alternating magnetic field with the exposure time of 1 or 5 min (respectively, L1 and L5). The leaves were measured in terms of blade length and width as well as petiole thickness prior to the tensile test. Measurements were also conducted to determine the chlorophyll fluorescence lifetime and content of photosynthetic pigments (chlorophyll a, b, a + b and carotenoids). The observed tensile strength was between 1.59 and 2.45 N. In the test group, the observed strength was lower in leaves collected from the top and central section of the stem but higher in the bottom part of the stem as compared to the control. The maximum increase of the tearing tensile force relative to the control (C) was observed for the L1 and F1 stimulation samples in leaves collected from the top and bottom parts of the stem, while the maximum decrease for that force was recorded for L5 leaves from the middle and top part of the stem. Chlorophyll fluorescence lifetimes and the overall content of photosynthetic pigments (chlorophyll a and b and carotenoids) were noticeably decreased in the leaves subjected to the stressors/stimulants applied. The results obtained for F1, L5 and, L1 stimulation revealed a decrease in fluorescence lifetimes. The content of photosynthetic pigments was also decreased under the influence of laser light stimulation (L1). This was a clear indication of plant ageing.