Influence of two types of biochars on the photosynthetic apparatus of prickly-seeded spinach (Spinacia oleracea L.)

Aim. To compare the effect of two biochars from different raw materials and their concentrations in soil on the main plant photosynthesis processes. Methods. Photosynthetic activity of prickly-seeded spinach plants (Spinacia oleracea L.), hybrid Corvair F1, was measured under controlled conditions in a pot experiment in a growth chamber (24–26 oC, light 150 μmol photons m–2 s–1 for 16 h per day; substrate humidity 60 % of full moisture capacity) at the stage of the fourth true leaf development (BBCH 14) using a portable fluorometer (MultispeQ v1.0), recording the following parameters: quantum efficiency of photosystem II (φII), quantum yield of non-photochemical quenching of chlorophyll (φNPQ), fraction of light energy lost due to unregulated processes (φNO), qL – fraction of open photosystem II; Fv′/Fm′ – maximum quantum efficiency of photosystem II, ECSt – capacity of ATP synthase; gH+ – proton conductivity; vH+ – steady-state proton flux. The two biochars used originated from the aboveground biomass of Miscanthus plants (Bch1, variants D2-4) and the sewage sludge of municipal sewage treatment plants (Bch2, variants D5-7) in the amount of 1 % (D2; D5), 3 % (D3; D6), 5 % (D4; D7) from the dry mass of a heavy loamy low-humus chernozem. Control plants were grown in soil without biochar. The data were statistically processed using R and RStudio with ANOVA, Kruskal-Wallis, Tukey’s HSD test and Principal Component Analysis (PCA). The measurements were conducted using ten plants per variant. Results. When the biochars Bch1 and Bch2 were applied, they influenced the photosynthetic properties of plants, including the chlorophyll content. Bch1 did not significantly increase the relative chlorophyll content (SPAD) in spinach leaves, while Bch2 significantly increased SPAD (by 17–19 %). The presence of biochar in the soil positively changed the temperature differential (TD) of the leaves, which indicated transpiration and marked the water supply of plants. The leaves of variants D3 (Bch1, 3 %) and D7 (Bch2, 5 %) were characterized by the most significant negative TD, the hydration of which, compared to the control, was higher by 3 and 1.7 %, respectively. The study of primary photosynthetic processes by chlorophyll fluorescence induction showed that both biochars generally had a positive effect on photosynthetic activity, particularly at 3 % addition on the photosystem II quantum efficiency (φII) and the maximum quantum yield in photosynthesis (Fv′/Fm′). Non-photochemical quenching without dark adaptation (NPQt) was 35–39 % lower in variants with Bch1, indicating more efficient use of light energy for photochemical processes, which may indicate that this biochar may contribute to reduced light energy dissipation and increased photosynthetic efficiency. In general, both types of biochar, reduced the loss of light energy and increased the photosynthesis efficiency by 3–7 %, thus indicating that they may be used in practice to stimulate photosynthesis and yield of Spinacia oleracea L. Conclusions. Adding both types of biochar to the typical heavy loamy low-humus chernozem in the amount of 1–5 % increased the photochemical efficiency and a 17–39 % decrease in non-photochemical quenching of chlorophyll fluorescence in spinach plants. The increase by 3–7 % in the maximum quantum yield and by 6–9 % in the quantum efficiency of photosystem II, along with lower values of φNPQ and NPQt compared to the control, indicate a higher efficiency of photochemical processes in plants grown in soil with added biochar. Future field studies should confirm if this increased photosynthesis is still present and leads to healthier plants and increased yield.