Increasing the resistance of the hollyhock plant to cold stress: silicon or selenium

It is well-known that freezing affects crop growth worldwide and significantly reduces crop production in the affected regions. The study objective was to investigate the effect of sodium silicate and sodium selenite individually or in combination on Alcea rosea plants grown under cold temperature stress. In this work, hollyhock plants were grown with sodium selenite and sodium silicate to 1/2 Hoagland solution, including 1) control, 1 mM sodium sulfate, 2) 1 mM sodium silicate 3), 0.03 mM sodium selenite, 4) 1 mM sodium silicate + 0.03 mM sodium selenite and exposed to nonfreezing (20 °C) and freezing temperature (−4 °C). Nutritional elements, photosynthesis parameters, electrolyte leakage, malondialdehyde, water content, osmolytes, and antioxidant enzymes were determined. The application of Si + Se effectively enhanced the Si and Se content of leaves (54% and 164%) compared to control plants. The combined Si and Se addition increased phenols, water-soluble sugars, proline, chlorophyll, net photosynthesis (Pn), and stomatal size under cold stress. Moreover, Si and Se in combination in stressed plants inhibited electrolyte leakage and malondialdehyde (MDA) by increasing the activities of enzymes such as superoxidase dismutase (SOD), ascorbate peroxidase (APX), and catalase (CAT). The maximal to minimal values of essential oil content, relative water content (RWC), and stomatal density depended on the treatment: Si + Se > Si = Se. By increasing K content, phenol, proline, and water-soluble sugars, Si + Se > Si > Se application decreased the adverse effects of cold stress on the dry weight of the hollyhock.