Silicon improves photosynthesis and strengthens enzyme activities in the C3 succulent xerophyte Zygophyllum xanthoxylum under drought stress.

One main strategic adaptive mechanism adopted by succulent xerophyte species, resistance to drought stress is absorbing and accumulating large amounts of sodium (Na+) from poor and dry soil which was stored in photosynthesizing branches as well as leaves as major osmoregulators, while still accumulating and storing a great deal of silicon (Si) in roots to resist to arid environments. To understand the possible adaptive strategies underlying how Si accumulation stimulates growth and ameliorates the adverse environmental impacts of drought stress on the C3 succulent xerophyte Zygophyllum xanthoxylum, plants grown for 3 weeks were suffered different K2SiO3 concentrations (1.5-7.5mM) (3-15mM KCl as control) treatments in sand culture experiments. Plants were also treated with different osmotic stresses caused by polyethylene glycol (PEG 6000) and drought stress (maintain water content about 30% of field water capacity) (30% of FWC) with or without additional 2.5mMK2SiO3 (5mMKCl as control) treatment in sand culture and pot experiments, respectively. We found that 2.5mMK2SiO3 (5mMKCl as control) resulted in optimal plant growth and alleviated adverse influences of drought stress on Z. xanthoxylum, by strengthening the activities of superoxide dismutase, peroxidase and catalase, reducing membrane lipid peroxidation and decreasing soluble sugar and free proline concentrations, concomitantly, increasing tissue water content, leaf area and chlorophyll a concentration. The result of ion analysis indicated that the Si absorption of Z. xanthoxylum was markedly induced by drought stress and that the 2.5mMK2SiO3 (5mMKCl as control) treatment significantly increased the aboveground and root Si concentration under different osmotic stresses and 30% of field water capacity compared with the drought and drought with 5mMKCl treatments. Although the K+ concentration in root in the drought with 2.5mMK2SiO3 treatment was no significant changes compared with the drought treatment, K+ concentration in aboveground and root in drought with 2.5mMK2SiO3 treatment were significantly decreased by 42% and 65.2% compared with drought with 5mMKCl treatment under 30% of FWC, indicating that Si replaced the function of K+, thus stimulating the growth and mitigating adverse effects of Z. xanthoxylum under water deficit. These findings showed that the positive roles of Si in the drought tolerance of Z. xanthoxylum might be due to the ability of plant to accumulate a great quantity of Si and utilize it as an osmoregulator to copy with water deficit, which was coupled with an obvious improvement in photosynthetic activity and anti-oxidative enzyme activities.