Effects Of Soil Compaction Stress On Carbon And Nitrogen Metabolism Of Cucumber

Soil compaction stress is one of the major problems facing soil or greenhouse soil,especially in arid and semi-arid areas. Soil compaction stress can damage soil structure anddeteriorate environment for root growth, thus having negative effect on total plant growth anddevelopment. Cucumber is one of the major vegetables cultivated in open field andgreenhouse, with its shallow root systerm mainly distributing in the0—20cm soil layer, isvery sensitive to the degree of soil compaction.In this work, the soils with1.25（loose soil, CK） and1.55（compacted soil） g/cm3bulkdensity was experimented on growing cucumber（Cucumis sativus L.） plants in pots. Effects ofsoil compaction stress on plant growth, photosynthesis, carbohydrate metabolism and nitrogenmetabolism of cucumber cultivar ‘Jinchun4’ at different growth stages were studied forinvestigating injuring mechanism to cucumber growth under soil compaction stress. Theresults showed:1. Soil compaction stress greatly decreased total root length, root surface area, root forksand root tips, thereby hampering root elongation and lateral root emergence remarkably atdifferent growth stages. Root radial growth was found to be positively affected by soilcompaction stress, therefore, root average diameter was observed to be significantly increasedat different growth stages. Root activity and respiratory rate were strikingly reduced incompacted soil. Plant height, stem diameter, leaf number, leaf area, economic yield,shootbiological yield, root biological yield and root/shoot ratio were significantly reduced incompacted soil. In a word, soil compaction stress greatly hampered shoot and root growth ofcucumber.2. Under soil compaction stress, net photosynthetic rate （P_n）, stomatal conductance （G_s）and transpiration rate （T_r） of cucumber leaves were decreased significantly while intercellularCO2concentration （C_i） was increased markedly at different growth stages. Thereby,photosynthesis was hampered according to non-stomatal limitation. The stem flow wasreduced in plants grown in compacted soil. Contents of chlorophyll a，chlorophyll b，chlorophyll（a+b） and carotenoid decreased markedly in compacted soil. Meanwhile, Soil compaction stresssignificantly decreased the maximum fluorescence（F_m），variable fluorescence （F_v），electron transport activity of PSâ…¡reaction center（F_m/F_o）, potential photochemical efficiency of PSⅡ（F_v/F_o）, maximalphotochemical efficiency of PSⅡ（F_v/F_m）, while improving the initial fluorescence （F_o） markedly, thestructure of PSâ…¡ was destroyed. As a result, photochemical activity of PSâ…¡was constrained andphotosynthetic rate was decreased.3. In cucumber leaves, the activity of sucrose phosphate synthase （SPS） was decreasedsignificantly while sucrose synthase （SS）, acid invertase （AI） and alkaline invertase （NI）activities were promoted remarkably in compacted soil treatment. Furthermore, contents ofsucrose, glucose, fructose and starch were also increased markedly. Synthesis and export forsucrose in leaves were restrained. In roots, the activities of SPS, AI and NI were decreasedsignificantly while SS activity was obviously promoted under soil compaction stress.Moreover, contents of sucrose, glucose, fructose were also increased strikingly while starchcontent was almost constant. In fruit，the activities of SPS, AI and NI were decreasedsignificantly, both of sucrose synthesis and decomposition were hampered in compacted soil.In addition, contents of sucrose, glucose, fructose were also decreased strikingly while starchcontent was promoted under the stress. Thus, cucumber quality was decreased.4. Soil compaction stress had severely adverse effects on nitrogen metabolism incucumber leaves and roots. Nitrate reductase （NR）, glutamine synthetase （GS） and glutamatesynthase （GOGAT） activities were considerably reduced by soil compaction stress whileglutamate dehydrogenase （GDH） was stimulated. NO₃^-, free amino acids and soluble proteincontents decreased significantly in response to soil compaction stress, whileNH₄⁺concentration increased substantially.