| Drought and nutrition deficiency are the main factors of limiting plant growth andecological restoration in arid and semiarid areas of China.In order to detect the soil moisture and nitrogen availability to the influence of the plantgrowth and development, one-year-old Robinia pseudoacacia were planted individually inpots to a randomized complete block design and exposed to three soil water contents (suitablesoil water content, medium drought, and severe drought, corresponding to soil water contentsbetween70~75%,50~55%and30~35%of field water capacity, respectively) as well as tolow N (no N applied to substrate) and high N (supplied with154mg N kg-1DW soil) levelsfrom mid-May to late August, respectively. We compared the differences of differenttreatments in growth condition, photosynthetic characteristic, transport of the reducednitrogen in the xylem and phloem, water use efficiency, membrane permeability andantioxidant enzyme activities, and tried to discuss whether N supply could promote earlygrowth and development of R. pseudoacacia seedlings under drought environment. Aim ofthis study is to provide theoretical reference for vegetation recovery in arid and semiaridregions of Northwest China. The results showed that:(1) Under low N condition, drought stress inhibited leaf area of R. pseudoacaciaseedlings. With the drought stress degree intensifying, each part of the leaf, stem and rootbiomass accumulation was declined, however, root/shoot ratio was increased. These effectsbehaved significantly under severe drought stress. Under high N condition, leaf area andbiomass of R. pseudoacacia seedlings showed continuous decline significantly with thereduction of soil water availability, but root/shoot ratio declined under moderate droughtstress and increased under severe drought stress. Compared to low N condition, N supplyenabled biomass and leaf area to increase significantly under suitable moisture and moderatedrought stress, but did not increase, and even showed decreasing trend under severe droughtstress. (2) Drought stress made the content of photosynthetic pigments decrease in the leaves,resulting in declined photosynthesis. Under low N condition, with the reduction of soilmoisture content, transpiration rate (Tr), stomatal conductance (Gs), and intercellular CO2concentration (Ci) of R. pseudoacacia seedling leaves decreased, but stomatal limitation value(Ls) showed an increasing trend, resulting in a decreased net photosynthetic rate (Pn), whichwas mainly caused by stomatal limitation after analysis. However, the increase in soil nitrogenavailability did not change the reduced trend of photosynthesis of R. pseudoacacia. Under thesame water condition, the increase in N availability made photosynthetic pigment contents inthe leaves of R. pseudoacacia seedlings significantly increased. Meanwhile, Tr, Gsand Ciincreased, but Lsdecreased, which making Pnincreased. This indicated that N supply couldsignificantly enhance photosynthetic capacity of R. pseudoacacia seedling and compensatefor the reduction of photosynthesis by water stress.(3) According to the analysis of photosynthetic light response curve, we obtained thatmaximum net photosynthetic rate (Amax), apparent quantum yield (AQY) and light saturationpoint (LSP) were decreased while dark respiration rate (Rd) and light compensation point(LCP) were increased with the reduction of soil water availability, which narrowed the rangeof photosynthetically active radiation (PAR), photosynthetic capacity of R. pseudoacaciaseedlings was decreased under drought stress regardless of N conditions. Compared to low Ncondition, however, Amax, LCP and were significantly enhanced under suitable moisture andmoderate drought stress, but showed no remarkable change under severe drought stress afterN supply. Therefore, only when water and nitrogen reasonable match can the photosyntheticactivity of mesophyll cells of R. pseudoacacia seedlings enhance.(4) The leaf instantaneous WUE (WUEi) of diurnal variation showed a bimodal curve,and the peak was at9:00and17:00respectively. With the reduction of soil water contents,WUEiand whole tree WUE (WUEb) of R. pseudoacacia seedlings were both increased underlow N condition, but decreased under high N condition. Compared to low N condition, Nsupply could enhance WUE significantly under suitable moisture and moderate drought stress,and do not consume more soil water into the price, but decrease WUE under severe droughtstress.(5) Drought stress made the relative conductivity (RC) and malondialdehyde (MDA)content of R. pseudoacacia leaves increase significantly regardless of N conditions, whichindicating that membrane permeability increased and membrane lipid peroxidation intensified.Under the same water condition, N supply RC decrease significantly and MDA content alsodecrease in a degree, which indicated that the increase of soil nitrogen availability couldenable cell membrane structure and function of the R. pseudoacacia leaves to remain relatively stable, so that the vulnerability of R. pseudoacacia seedlings reduced greatly, whichenhanced its drought resistance.(6) Under low N condition, drought stress made the superoxide dismutase (SOD) andperoxidase (POD) activities in leaves of R. pseudoacacia seedlings decrease significantly, butno significant change in catalase (CAT) activity, which indicated that SOD and POD of R.pseudoacacia were extremely sensitive to drought stress, thus CAT was the major reactiveoxygen species scavenger under drought stress and low N condition. Under high N condition,with the reduction of soil water availability, SOD activity showed no significant change, whileCAT activity decreased, however, POD activity was increased under moderate drought stress,but decreased significantly under severe drought stress, which indicated that SOD and PODwere the major reactive oxygen species scavenger under drought stress and high N condition.Compared to low N condition, N supply could improve SOD, POD and CAT activities of R.pseudoacacia seedling leaves under moderate and severe drought stress drought stress, andprotection enzymes activities showed significant increase (P<0.05) especially under moderatedrought stress.(7) Total amino compound contents in the xylem and the phloem of R. pseudoacaciaseedlings remained unaffected by the drought treatment. However, the composition of aminocompound and content of individual amino compounds changed remarkably. In the phloem ofR. pseudiacacia, drought favored amino compounds with a low C/N ratio, in particular Argcontaining four moles N per Mol increased significantly, indicating a reduced investment of Cinto root in C/N reallocation from shoot to root.In summary, the rational N supply under moderate drought stress could significantlyenhance photosynthetic capacity, maintain the relative stability of cell membrane structure andfunction and improve the active oxygen scavenging activities of key enzymes, therebyenhancing the drought resistance of R. pseudoacacia. However, N fertilization under severedrought stress had no promoting effect on the growth of R. pseudoacacia seedlings. |
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