Effects Of Exogenous Silicon On Growth And Physiological Characteristics Of Continuous-cropped Cucumber Seedlings

There are features of a high degree of intensification, high multiple cropping indices, and often involve mono-cropped production under protectd cultivation for vegetable production, which leads to continuous-cropped obstacles increasing year by year. Continuous-cropped obstacle phenomenon has become urgent in the current facility vegetable production to be solved. With the increasing of continuous-cropped, which often result in physical and chemical properties of soil degradation, soil borne diseases and pests increased, decreased the yield and quality of vegetables, seriously restrict the sustainable development of vegetable production.Cucumber (Cucumis sativus L.) as one of the widely greenhouse vegetables in China, which is particularly serious on the phenomenon of continuous-cropped obstacles. Silicon (Si) is the second most abundant element on the Earth’surface and plays a key role in growth, development, nutrient absorption and defense response in plants under stress conditions. In this paper, effects of silicon on growth, photosynthesis, chlorophyll fluorescence, antioxidant enzymes, phenolic compounds were studied, which can explore the physiological mechanism of silicon alleviating continuous-cropped obstacle of cucumber seedlings (’Xintai Mici’and ’Jinyan4’) and provide a new theoretical foundation and an effective way by Si improving plant growth and overcoming continuous-cropped obstacles. The main results obtained are as follows:1. Within1-3mmol·L-1Si concentrations, electrolyte leakage (EL) and malondialdehyde (MDA) content in leaves were significantly decreased, while the contents of chlorophyll a (Chla), chlorophyll b (Ch1b), carotenoids (Car), chlorophyll (a+b) and photo synthetic rate (Pn) in leaves were significantly improved, moreover, the activities of superoxidase (SOD), peroxidase (POD), catalase (CAT) and ascorbic acid peroxidase (APX) were also significantly increased. These findings indicated that exogenous Si could enhance the capacity of scavenging active oxygen species and improve photosynthesis, protect cucumber seedlings from the lipid peroxidation, promote the increasing of plant height, stem diameter and dry mass accumulation of cucumber seedlings and increase resistance to continuous-cropped cucumber obstacle. Compared with low Si concentration, excessive Si resulted in higher EL and MDA, but they were still lower than that in control. At the same time, antioxidant enzymes activity and photosynthesis were also decreased by excessive Si, resulting in injury of cucumber seedlings. These results indicated that the effect of exogenous Si was dependent on the concentration of Si. Excessive Si treatment would decrease the resistance to continuous-cropped cucumber obstacle. The optimal silicon concentration was2mmol-L-1.2. Under continuous-cropped cucumber soil, EL and MDA content in leaves of’Xintai Mici’(XT) cucumber seedlings were lower than’Jinyan NO.4’(JY), while there was a contrary trend on the activities of SOD, POD, CAT, APX and photo synthetic and fluorescent parameters in leaves between the two cultivars. These findings suggested that the resistance of XT was stronger than JY. After spraying Si, EL and MDA content in two cultivars leaves of continuous-cropped cucumber seedlings were decreased, while improved the activities of SOD, POD, CAT, APX and the contents of Chlorophyll a (Chl a), Chlorophyll b (Chl b) and Carotenoids (Car), maintained the integrity of the chloroplast and mitochondrial ultrastructure, as well as the whole membrane system; enhanced the net photo synthetic rate (Pn), PSII potential fluorescence efficiency (Fv/Fo), maximal photochemistry efficiency of PSII (Fv/Fm), actual photochemical efficiency of PSII (ΦPSII), and non-photochemical fluorescence quenching coefficient (NPQ), which promoted the increasing of plant growth. The results indicated that exogenous Si could increase the resistance of cucumber seedlings to continuous-cropped obstacle by reducing the lipid peroxidation and improving the photo synthetic capacity. The interaction effect of cultivars and Si concentration was significant on EL, SOD and NPQ of cucumber seedling leaves, and there were no significant effects on other indexes.3. Soluble sugar and Pro contents in leaves of XT cucumber seedlings were lower than JY, while there was a contrary trend on the contents of total phenolic contents, flavonoid contents, lignin contents under continuous-cropped soil. Spraying Si could reduced soluble sugar and Pro contents in leaves of two cultivars continuous-cropped cucumber seedlings, increased the contents of total phenolic, flavonoid contents, lignin contents, which promoted the increasing of biomass. These results showed that exogenous Si could increase cucumber seedlings tolerance to continuous-cropped obstacle by reducing the leaves contents of osmotic adjustment substances and improving Si and the phenolic compounds contents. The interaction effect of cucumber seedlings on Pro contents is very significantly between cultivars and silicon concentration, no significant effect on other indexes.4. The effects of cultivars on root fresh weight, plant height, soluble sugar contents, photosynthesis pigment and net photosynthetic rate were higher than Si concentration, while the effects of Si concentration of cucumber seedling on dry weight, shoot fresh weight, leaf area, EL, MDA and Pro content, POD, CAT, APX activities, fluorescence parameters, also total phenolic, flavonoids, lignin contents and leaves, stems, roots silicon contents were higher than that of cultivars. The results showed that cultivars is the key factor to root fresh weight, plant height, soluble sugar content, photosynthesis pigment, net photosynthetic rate, and Si concentration is the key factor to determine the dry weight, fresh weight, leaf area, EL, MDA and Pro contents, POD, CAT, APX activities, fluorescence parameters, total phenolic, flavonoids, lignin contents and leaves, stems, roots silicon contents.