| Cucumber (Cucumis sativus L.) is one of the important vegetables in world and widely planted in greenhouse. Cucumber roots are shallowly distributed and show preference for fertilier, but no tolerance to excess fertilier, so cucumber can be easily affected by salinization. Soil salinization has been a main barrier in protected cucumber production. The main stress anion is NO3-and the main cation is Ca2+. The previous studies mostly concentrated on the physiological mechanism of NaCl stress on plants, but the repots of vegetable under Ca(NO3)2stress are few. Therefore, this experiment was desighed to compare the56mmol·L-1Ca(NO3)2and84mmol·L-1NaCl on the damage and the difference in physiological and biochemical characteristics of cucumber seedlings, cultivar ’Jinchun2’(with less salt resistance)was used to carry out our experiment via hydroponics. The main results were presented as follows.The results showed that the fresh and dry biomass weight was significantly decreased under both salt stress, but the damage degree that under NaCl stress exceeded Ca(NO3)2stress. The root activity, K+content in root, K+/Na+ratio, root-shoot ratio(R/T)were significantly decreased,but the root relative membrane permeability increased significantly under NaCl stress, suggesting that NaCl stress inhibited seedlings growth mainly by restraining root growth; However, the effects of Ca(NO3)2stress on the root relative membrane permeability, K+content, and the ratio of K+/Na+were less than NaCl, and the root activity and root-shoot ratio(R/T)were significantly increased, but the leaf water content and osmotic adjustment was less than NaCl stress. Soluble sugar is the main osmotic adjustment substance in NaCl stress, but soluble protein in Ca(NO3)2stress. The results showed that both treatments caused damage to cucumber seedlings plant, and iso-osmotic NaCl stress brought about heavier damage than Ca(NO3)2stress. Ca(NO3)2stress inhibited growth mainly by inducing physiological drought and NaCl mainly by destroying root plasma membrane structure.Under Ca(NO3)2treatment the total root biomass declined by29.23%and the root length decreased significantly but the lateral root density and average root diameter increased obviously, root tips become short but swollen, the whole root grows like clusters and the root activity increased significantly; Meanwhile, under NaCl treatment the total root biomass significantly decrease by43.08%and the decrease in total root length is less than the isotonic Ca(NO3)2stress, but the lateral roots density and average root diameter decreased obviously, root become slender, and the root activity decreased significantly. In conclusion, both salt stress inhibited the root growth and change the root architecture, and the damage of NaCl stress on root is greater than the isotonic Ca(NO3)2stress.The chlorophyll content was decreased by both salt stress, and the decreased degree of NaCl stress was bigger than that of Ca(NO3)2stress. The ultra-structural analysis showed that under Ca(NO3)2stress most of the chloroplasts were out of the cell wall and the number of grana thylakoids were decreased; whereas under NaCl stress the structure of chloroplast displayed deformation, while the grana lamellae become inflation and the hungry particles increased significantly. The net photosynthesis rate (Pn), stomatal conductance (Gs), PSII photochemical quantum yield (Fv/Fm), PSII actual photochemical efficiency (ΦPSII) and photochemical quenching (qP) were higher under Ca(NO3)2stress than those of NaCl stress. Intercellular CO2concentration (Ci) was decreased by the Ca(NO3)2stress, while was increased by the NaCl stress. Thus, we concluded that the stomatal limitation was a dominating factor of Pn decrease under Ca(NO3)2stress and non-stomatal factors was a dominating factor of Pn reduction under NaCl stress.Under NaCl stress, the K+、Mg+contents decreased significantly while Na+content increased significantly in roots、stems and leaves, the Ca2+content decreased in leaves but accumulated substantially in roots and stems; under Ca(NO3)2stress, Ca2+content increased while K+and Mg2+contents decreased in the roots、stems and leaves, and the degree of Mg2+content decreased was greater than that of K+. Trace elements test shows that NaCl stress aggravated the accumulation of Fe24、Cu2+and Zn2+in roots, while inhibited the absorption of Mn2+in roots; Ca(NO3)2stress promoted the accumulation of Mn2+and Zn2+in roots but inhibited Cu2+absorption. In summary, plant growth was inhibited by NaCl stress through increasing the Na+content of leaves, decreasing the K+、Ca2+contents of leaves and promoting the excessive accumulation of Cu2+in roots, while isotonic Ca(NO3)2stress inhibited plant growth mainly by increasing the Ca2+content in leaves, decrease the Mg2+content of leaves and promote the excessive accumulation of Mn2+in root. |
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