Silicon improves the growth of cucumber under excess nitrate stress by enhancing nitrogen assimilation and chlorophyll synthesis

Silicon (Si) can increase salt tolerance of plants, and previous studies have focused on NaCl stress; whereas in protected facilities, nitrate (but not NaCl) accumulation is one of the major causes of secondary soil salinization. However, information on Si's effect on plant growth under nitrate stress is very limited, and the underlying mechanism is unknown. Here, we investigated Si's effect on plant growth, nitrogen assimilation and chlorophyll synthesis in cucumber. Cucumber seedlings ('Jinyou 1') were subjected to 200 mM nitrate stress without or with addition of 2 mM Si. The results showed that root application, but not foliar application of Si, could improve cucumber growth under nitrate stress. Root addition of Si increased photosynthetic rate and decreased oxidative damage of stressed plants. Under nitrate stress, Si addition decreased the accumulation of nitrate, nitrite and ammonium, and promoted the activities of nitrate reductase, nitrite reductase, glutamine synthase, glutamine-2-oxoglutarate aminotransferase and glutamate dehydrogenase in leaves. The concentrations of glutamic acid, 5-aminolevulinic acid, porphobilinogen and uroporphyrinogen III were increased under nitrate stress, while these were decreased by added Si. Added Si increased the levels of chlorophyll and its precursors (protoporphyrin IX, Mg-protoporphyrin IX and protochlorophyllide), and expressions of genes encoding enzymes in chlorophyll synthesis (CHLH, POR and CAO) under nitrate stress. These results suggest that Si could improve cucumber growth under nitrate stress by enhancing nitrogen assimilation and chlorophyll synthesis, and imply an application of Si fertilizer in solving secondary soil salinization in protected facilities.