The Exogenous Application of Brassinosteroids to Zea mays (L.) Stressed by Long-Term Chilling Does Not Affect the Activities of Photosystem 1 or 2

The effect of various concentrations of exogenously applied 24-epibrassinolide (E) and 2α,3α,17β-trihydroxy-5α-androstan-6-one (A) on the activities of Photosystem 1 and the Hill reaction, the contents of photosynthetic pigments, and the growth of plants was examined in young maize (Zea mays L.) plants subjected to long-term chilling stress or grown in normal-temperature conditions. Neither the activity of Photosystem 1 nor the Hill reaction activity of plants was in any way affected by the treatment with brassinosteroids (BRs), which suggests that the photosynthetic complexes of thylakoid membranes are not the primary site of the influence of BRs on photosynthesis. An extremely low (10⁻¹⁴ M) concentration of A applied to the nonstressed plants significantly increased the length of their 4th to the 7th leaves and their height, as well as the contents of chlorophylls a and b and total carotenoids. However, under chilling conditions, this positive effect was significant for the chlorophyll content only and higher concentrations of BRs (10⁻¹², 10⁻¹⁰, 10⁻⁸ M) usually had no effect at all.