The effects of different nitrogen sources on camptothecin content and related gene expression in Camptotheca acuminata seedlings

Camptotheac acuminata Decne is a unique tree species in China with an important secondary metabolite,camptothecin(CPT),used in the treatment of cancer.Nitrogen(N) is an important element that affects plant growth and the accumulation of CPT.Reports on the effect of N on CPT synthesis from a genetic perspective are scarce.To explore the effects of different N sources and levels on CPT synthesis in C.acuminata,two-year-old seedlings were fertilized with different concentrations of pure ammonium sulphate,source of ammonium N(NH_4~+-N),and potassium nitrate for nitrate N(NO_3~--N).Concentrations of 2.5,5,7.5,and10 g pot~(-1) NH_4~+-N and NO_3~--N were used.The results showed that 7.5 g NH_4~+-N and NO_3~--N treatments were best for growth and fresh weight of leaves.Compared with the other treatments,the CPT content,tryptophan synthase and tryptophan decarboxylase activities,and expression of the CaTSB and CaTDCl genes under the 2.5 g NH4+-N and NO_3~--N treatments peaked significantly at 30 days.However,the expression of CaTDC2 surpassed that of the other two genes at 60 days.Therefore,compared with NH4+-N source,the NO_3~--N source was more beneficial for growth,and NO_3~--N was better for CPT yield.Consequently,leaves of C.acuminata treated with 2.5 g NO_3~--N could be harvested after 30 days to obtain maximum CPT content.CaTDC1 is more closely linked to CPT synthesis.The results of this study improved the production of CPT in C.acuminata via fertilization.     

Absorption and assimilation of nitrate and ammonium ions by jack pine seedlings

Jack pine (Pinus banksiana Lamb.) seedlings were grown in a shaded or unshaded light regime with either NO(3) (-)- or NH(4) (+)-N as the sole N source. After three months, seedlings grown with NH(4) (+)-N were larger than seedlings grown with NO(3) (-)-N. Irradiance had a greater effect on growth of ammonium-fed seedlings than on growth of nitrate-fed seedlings.At all times from 6 to 24 h following incorporation of (15)N, soluble, insoluble, and total (15)N contents of shoots and roots were higher in ammonium-fed seedlings than in nitrate-fed seedlings. The pattern of (15)N accumulation in shoots was similar to that in roots. After 6 and 24 h of (15)N incorporation, unshaded, ammonium-fed seedlings had 8.8 and 2.8 times greater total (15)N contents, respectively, than unshaded, nitrate-fed seedlings. In response to shading, ammonium-fed seedlings increased their total uptake of (15)N per unit root weight, whereas nitrate-fed seedlings did not. No nitrate or (15)NO(3) (-) was detected in any plant tissue. Nitrate-fed plants had higher NH(4) (+), Asp, and Gln concentrations in needles and higher gamma-aminobutyric acid and Arg concentrations in stems. Accumulation of (15)N in roots was not affected by the pH of the (15)N solution or by the N source fed to the seedlings before the period of (15)N incorporation. Thus NO(3) (-) transport into roots, rather than its reduction or transport within the plant, seems to be the factor limiting the growth of jack pine supplied with NO(3) (-)-N as the sole N source.     

Nitrate content, amino acid composition and growth of yellow birch seedlings in response to light and nitrogen source

Yellow birch (Betula alleghaniensis Britt.) seedlings were grown for three months in a greenhouse at two radiant flux densities-full light (FL) and 50% shade (LL)-and with three nitrogen sources- ammonium only (NH(4) (+)), nitrate only (NO(3) (-)) and a 1:1 mixture of ammonium and nitrate (NH(4) (+)/NO(3) (-))-in a completely randomized factorial design. The total biomass of seedlings grown under low light (LL) did not vary significantly with nitrogen source; although NO(3) (-)-treated seedlings were smaller and had a significantly lower (P 相似文献   

Influence of a high Mn supply on Norway spruce (Picea abies (L.) Karst.) seedlings in relation to the nitrogen source

Effects of 3, 25, 100, 200 and 800 microM Mn on biomass and pigment, starch and nitrate concentrations were studied in Norway spruce (Picea abies (L.) Karst.) seedlings grown with either NO(3) (-) or NH(4) (+) as the sole nitrogen source. After 77 days of exposure to 800 microM Mn, shoot growth had ceased in about 50% of the seedlings independently of the N source. Despite high Mn concentrations in roots and shoots of the Mn-treated seedlings, no visible symptoms of Mn toxicity were evident. The rate of root elongation was decreased by treatment with >/= 200 microM Mn when N was supplied as NO(3) (-), but not when it was supplied as NH(4) (+). This difference could be attributed to the higher Mn concentrations in root tips of the NO(3) (-)-grown seedlings compared with the NH(4) (+)-grown seedlings. In Mn-treated seedlings, the concentration of Mg, and to a lesser extent that of Ca, decreased. Depletion of these elements might account for the observed growth depression. Potassium concentrations were similar in the control and Mn-treated seedlings. Treatment of seedlings with 800 microM Mn for 50 days led to several physiological changes: starch accumulated, the concentrations of nitrate and phenolic compounds increased, pigment concentrations decreased, and in vivo nitrate reductase activity in roots was reduced.     

Growth response of cottonwood roots to varied NH4:NO3 ratios in enriched patches

Maximization of short-rotation forest plantation yield requires frequent applications of nutrients, especially nitrogen (N). Whole-plant growth is known to be sensitive to the proportion of ammonium to nitrate (NH4:NO3). However, the extent to which N form affects root growth, branching and morphology is poorly understood, and these variables may have substantial impacts on plant nutrient and water acquisition. We used rooted cuttings of cottonwood (Populus deltoides Bartr. ex Marsh.) to investigate the effect of various NH4:NO3 ratios on root growth in N-enriched patches. A sand culture study with split-root systems was carried out in which 1-3% of the total root system of each cutting was supplied with 2 mM N at NH4:NO3 ratios of 0:100, 20:80, 40:60, 60:40, 80:20 and 100:0 (molar basis), with the rest of the plant supplied with 0 mM N, resulting in the whole plant becoming N deficient. During the experiment, whole-plant growth was unaffected by the treatments. Of the NH4:NO3 ratios tested, greatest total root length, specific root length, and root N concentration of roots in enriched patches occurred in the 20:80 NH4:NO3 treatment. The largest response of roots in enriched patches was third- and fourth-order root production. We conclude that N form has a profound effect on root development and morphology in enriched patches.     

Regulation of organic and inorganic nitrogen uptake in Scots pine (Pinus sylvestris) seedlings

Plants possess regulatory mechanisms that enhance nitrogen (N) uptake under conditions of spatial and temporal variation in N availability. Study of regulatory mechanisms has focused almost exclusively on the uptake of inorganic N sources (i.e., ammonium (NH4+), nitrate (NO3-). Several lines of evidence, however, suggest that amino acids may constitute a potential source of N for a number of plant species, including conifers. In the present study, we investigated the uptake of amino acids and inorganic N in Scots pine (Pinus sylvestris L.) seedlings grown at different N concentrations. We compared the uptake rate of the individual N sources using U-[13C2], [15N]-glycine, U-[13C6], [15N4]-arginine, 15NH4, or 15NO3, and tested the short-term effect of N supply on the uptake rate of glycine, arginine and in field-grown Scots pine seedlings. Our data indicate that Scots pine seedlings can absorb substantial amounts of N in the form of intact arginine and glycine molecules. The data also suggest that Scots pine seedlings down-regulate their uptake of NH4+-N and arginine-N, but not of glycine-N in response to increased endogenous N concentrations.     

Influence of elevated CO2 and mycorrhizae on nitrogen acquisition: contrasting responses in Pinus taeda and Liquidambar styraciflua

An understanding of root system capacity to acquire nitrogen (N) is critical in assessing the long-term growth impact of rising atmospheric CO2 concentration ([CO2]) on trees and forest ecosystems. We examined the effects of mycorrhizal inoculation and elevated [CO2] on root ammonium (NH4+) and nitrate (NO3-) uptake capacity in sweetgum (Liquidambar styraciflua L.) and loblolly pine (Pinus taeda L.). Mycorrhizal treatments included inoculation of seedlings with the arbuscular mycorrhizal (AM) fungus Glomus intraradices Schenck & Smith in sweetgum and the ectomycorrhizal (EM) fungus Laccaria bicolor (Maire) Orton in loblolly pine. These plants were then equally divided between ambient and elevated [CO2] treatments. After 6 months of treatment, root systems of both species exhibited a greater uptake capacity for NH4+ than for NO3-. In both species, mycorrhizal inoculation significantly increased uptake capacity for NO3-, but not for NH4+. In sweetgum, the mycorrhizal effect on NO3- and NH4+ uptake capacity depended on growth [C02]. Similarly, in loblolly pine, the mycorrhizal effect on NO3- uptake capacity depended on growth [CO2], but the effect on NH4+ uptake capacity did not. Mycorrhizal inoculation significantly enhanced root nitrate reductase activity (NRA) in both species, but elevated [CO2] increased root NRA only in sweetgum. Leaf NRA in sweetgum did not change significantly with mycorrhizal inoculation, but increased in response to [CO2]. Leaf NRA in loblolly pine was unaffected by either treatment. The results indicate that the mycorrhizal effect on specific root N uptake in these species depends on both the form of inorganic N and the mycorrhizal type. However, our data show that in addressing N status of plants under high [CO2], reliable prediction is possible only when information about other root system adjustments (e.g., biomass allocation to fine roots) is simultaneously considered.     

氮肥对盆栽欧洲水青冈生物量和营养分配的影响(英文)

调查了施加氮肥(15NH4和15NO3)处理后在两个连续生长季内欧洲水青冈(Fagus sylvatica L.)幼苗地上部分和地下部分的生物量和营养元素分配。盆栽欧洲水青冈幼苗培养于温室大棚内,培养土样取自相邻的三种林分:欧洲水青冈,挪威赤松,欧洲水青冈-赤松混交林。结果表明,氮肥(15N)处理对欧洲水青冈营养元素分配没有显著影响,施加氮素形式决定自身流入植物库的情况。欧洲水青獭收氮素主要以硝态氮的形式,因此,尽管植物体内保存的硝态氮和氨态氮并没有统计差异,但是叶片中硝态氮明显减少。施加硝态氮对欧洲水青冈氮素恢复的影响要大于施加氨态氮。与欧洲水青冈茎、粗根相比,优质根系对氮素(15N)固定是一个缓慢过程。表8图1参40。     

Uptake and distribution of calcium and phosphorus in beech (Fagus sylvatica) as influenced by aluminum and nitrogen

We studied the effects of excess nitrogen added as nitrate (NO(3) (-)) or ammonium (NH(4) (+)), or both, on mineral nutrition and growth of beech (Fagus sylvatica L.) plants grown at pH 4.2 in Al-free nutrient solution or in solutions containing 0.1 or 1.0 mM AlCl(3). A high external concentration of NH(4) (+) increased the concentration of nitrogen in roots, stems and leaves. The root/shoot dry weight ratio was less in plants grown in the presence of NH(4) (+) than in plants grown in the presence of NO(3) (-). The concentration of phosphorus in the roots was increased and the concentration of potassium in all parts of the plant was decreased by NH(4) (+). A high external concentration of NO(3) (-) caused a decrease in phosphorus concentrations of the root, stem and leaf. Uptake of (45)Ca(2+) by roots was reduced in the presence of high concentrations of NH(4) (+) or NO(3) (-), and a combination of high concentrations of nitrogen and aluminum further reduced the uptake of (45)Ca(2+). Uptake of phosphate ((32)P) and concentrations of phosphorus in root and shoot were increased when plants were grown in the presence of 0.1 mM Al. Exposure to 1.0 mM Al, however, reduced the concentration of phosphorus in roots and shoots and the reduction was greater when plants were grown in the presence of a high external NO(3) (-) concentration. Aluminum binds to roots, and plants grown in the presence of 1.0 mM Al had a slightly higher concentration of aluminum in roots than plants grown in the presence of 0.1 mM Al, whereas the concentration of Al in the shoot was increased 2 to 3 times in plants exposed to 1.0 mm Al. Furthermore, the effects of 1.0 mM Al on uptake of other macronutrients were quite different from the effects of 0.1 mM Al. We conclude that 0.1 mM Al facilitates uptake and transport of phosphorus in beech and that between 0.1 and 1.0 mM Al there is a dramatic change in the effects of Al on uptake and transport of divalent cations and phosphorus.     

Effects of nitrogen source on growth and activity of nitrogen-assimilating enzymes in Douglas-fir seedlings

Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco) seedlings were grown for 68 days in a growth chamber in nutrient solutions with ammonium, nitrate or ammonium nitrate as the nitrogen source. Among the nitrogen sources tested, whole-seedling biomass, relative growth rate (RGR), root and shoot elongation, and number of lateral roots, were greatest in seedlings grown with ammonium. In the absence of nitrogen, plant growth and formation of lateral roots were poor. Initially, glutamine synthetase, NAD-glutamate dehydrogenase and aspartate aminotransferase activities were high in young roots and shoots, but all three enzymatic activities decreased after one month of culture. In root apices, glutamine synthetase and aspartate aminotransferase activities were higher than NAD-glutamate dehydrogenase activity. Enzymatic activities were often higher in ammonium-fed seedlings than in seedings supplied with the other forms of nitrogen. Activities of all three enzymes were significantly reduced in seedlings grown in the absence of nitrogen. The beneficial effect of ammonium is discussed on the basis of its involvement in the assimilation pathways of Douglas-fir.     

Family variation in nutritional and growth traits in Douglas-fir seedlings

Nitrogen (N) uptake and utilization in seedlings of six full-sib families of coastal Douglas fir (Pseudotsuga menziesii (Mirb.) Franco) known to differ in growth rate were assessed at the whole plant and root levels. Seedlings were grown in soil or aeroponically with high and low nutrient availability. Consistent family differences in growth rate and N utilization index were observed in both soil and aeroponic culture, and high-ranking families by these measures also had greater net N uptake in soil culture. Two of the three families found to be fast-growing in long-term field trials exhibited faster growth, higher N utilization indices and greater net N uptake at the seedling stage. Mean family net influx of ammonium (NH4+) and efflux of nitrate (NO3-) in the high- and low-nutrient treatments were significantly correlated with measures of mean family biomass. The high-nutrient availability treatment increased mean net fluxes of NH4+ and NO3- in roots. These results indicate that efficiency of nutrient uptake and utilization contribute to higher growth rates of trees. Nutrient-related traits should be considered in tree breeding programs, as the indications are that assessments may be made at an early stage.     

不同形态氮素配比对马大杂种相思无性系幼苗生长的影响

在温室内采用不同氮形态配比的营养液,水培研究9个马大杂种相思（Acacia mangium×A.au-riculiformis Cunn.ex Bench）无性系幼苗的生长状况,结果表明,NO3^-∶NH4＋值对马大相思生长及生物量的影响均达到极显著水平。当NO3^-∶NH4^＋为25∶75时,马大相思幼苗的苗高、生物量干质量及生物量鲜重积累均达到最大。而NO3^-∶NH4^＋为75∶25时,根长净生长量最大,但抑制了苗高的生长;在供氮总量不变的条件下,NO3^-∶NH4^＋为25∶75时最有利于马大杂种相思幼苗的生长。     

Ammonium and nitrate acquisition by plants in response to elevated CO2 concentration: the roles of root physiology and architecture

We examined changes in root system architecture and physiology and whole-plant patterns of nitrate reductase (NR) activity in response to atmospheric CO2 enrichment and N source to determine how changes in the form of N supplied to plants interact with rising CO2 concentration ([CO2]). Seedlings of Betula alleghaniensis Britt. and Pinus strobus L., which differ in growth rate, root architecture, and the partitioning of NR activity between leaves (Betula) and roots (Pinus), were grown in ambient (400 microl l(-1)) and elevated (800 microl l(-1)) [CO2] and supplied with either nitrate (NO3-) or ammonium (NH4+) as their sole N source. After 15 weeks of growth, plants were harvested and root system architecture, N uptake kinetics, and NR activity measured. Betula alleghaniensis responded to elevated [CO2] with significant increases in growth, regardless of the source of N. Pinus strobus showed no significant response in biomass production or allocation to elevated [CO2]. Both species exhibited significantly greater growth with NH4+ than with NO3-, along with lower root:shoot biomass ratios. Betula showed significant increases in total root length in response to elevated [CO2]. However, root N uptake rates in Betula (for both NO3- and NH4+) were either reduced or unchanged by elevated [CO2]. Pinus showed the opposite response to elevated [CO2], with no change in root architecture, but an increase in maximal uptake rates in response to elevated [CO2]. Nitrate reductase activity (on a mass basis) was reduced in leaves of Betula in elevated [CO2], but did not change in other tissues. Nitrate reductase activity was unaffected by elevated [CO2] in Pinus. Scaling this response to the whole-plant, NR activity was reduced in elevated [CO2] in Betula but not in Pinus. However, because Betula plants were larger in elevated [CO2], total whole-plant NR activity was unaffected.     

Effects of pretreatment conditions on ammonium and nitrate uptake by Douglas-fir seedlings

Effects of pretreatment solutions containing varying concentrations of calcium, potassium, ammonium and nitrate were evaluated by measuring fluxes of the same ions during a subsequent 4-hour uptake by Douglas-fir seedlings. Maximal rate of ammonium uptake (11 microeq g(-1) root dry weight h(-1)) was about 5 times faster than that of nitrate (2 microeq g(-1) root dry weight h(-1)). Ammonium uptake was most rapid after pretreatment with low potassium levels and was unaffected by ammonium pretreatment. Nitrate uptake was most rapid after pretreatment with high levels of nitrate and low levels of potassium. Calcium uptake was greater when nitrate replaced ammonium as the N source. High calcium pretreatment levels depressed subsequent calcium uptake or resulted in calcium release in both ammonium and nitrate experiments. Potassium efflux occurred with both N sources, but the release was less during nitrate uptake than during ammonium uptake. Efflux of potassium is probably associated with the high potassium status of the seedlings and the exchange between potassium and other cations. High levels of potassium in the pretreatment solutions enhanced potassium efflux and caused a reduction in the subsequent rates of uptake of both ammonium and nitrate. Length of pretreatment and seedling size generally did not affect uptake rates.     

Hebeloma crustuliniforme facilitates ammonium and nitrate assimilation in trembling aspen (Populus tremuloides) seedlings

This study examined the role of ectomycorrhizal associations in nitrogen assimilation of Populus tremuloides seedlings. Seedlings were inoculated with Hebeloma crustuliniforme and compared with non-inoculated plants. Nitrogen-metabolizing enzymatic properties were also determined in H. crustuliniforme grown in sterile culture. The seedlings and fungal cultures were subjected to nitrogen treatments (including NO??, NH?? and a combination of NO???+?NH??) for 2 months to examine the effects on growth, nitrogen-assimilating enzyme activities and xylem sap concentrations of NH?? and NO??. Seedlings were also provided for 3 days with 1?N-labeled NH?? and NO??, and leaf and root 1?N content relative to total nitrogen was measured. Both NO?? and NH?? were effective in supporting seedling growth when either form was provided separately. When NO?? and NH?? were provided together, seedling growth decreased while enzymatic assimilation of NH?? increased. Additionally, nitrogen assimilation in inoculated seedlings was less affected by the form of nitrogen compared with non-inoculated plants. Fungal ability to enzymatically respond to and assimilate NH?? combined with aspen's enzymatic responsiveness to NO?? was likely the reason for efficient assimilation of both nitrogen forms by mycorrhizal plants.     

模拟酸雨对杜英幼苗生长及养分元素含量的影响

模拟不同酸雨浓度(pH2.5、4.0、5.6)对杜英幼苗进行喷淋控制实验,研究不同酸梯度条件下杜英幼苗的生长及养分元素含量特征,结果表明:(1)对照组(pH7.0)全株干重约为酸处理组的2倍,酸雨处理对植株生长产生抑制作用,其中对茎部影响最为明显。(2)与pH2.5和pH5.6相比,pH4.0处理组全株干重最大,且根冠比最低,说明植株对pH4.0酸雨有较好的适应性。该处理下,植株体内氮、磷元素含量均较高,钾、镁元素含量在茎中较高,在根中较低。(3)随着酸处理组pH值的降低,植株叶片中钾、硫含量呈明显降低趋势,铁含量呈明显增高趋势;茎中铁含量呈明显增高趋势;根中锌含量呈明显降低趋势。(4)酸雨对杜英幼苗体内硫元素含量有一定的抑制作用,且pH 5.6处理组叶、根中的硫含量显著高于pH 2.5和pH 4.0,即酸雨带来的硫输入在植株体内非但没有出现硫富集现象,反而产生淋失现象。     

不同N源对兴安落叶松苗木生长,养分吸收及离子平衡的影响(英文)

本文研究了不同N源对落叶松苗木干物重、养分吸收及离子平衡的影响。结果表明:施铵态N的苗木干物重小于对照处理;施硝铵态N的苗木生长与对照和施硝态N的苗木差别不大,但较施铵态N的苗木生长要好。铵态N与硝铵态N处理苗木地上部的阳离子总浓度受N水平的影响很小,而硝态N处理时则阳离子浓度增加;各处理对地上部阴离子浓度的影响均不大。在根部除硫酸根与钙离子外,阳离子与阴离子浓度均变化不大。硝态N处理的苗木具有较高的C-A值;△(C-A)/△Norg值在硝态N处理的苗木中随N供应量的增加而略有变化,但铵态N处理则明显下降;硝铵态N处理苗木的△(C-A)/△Norg值界于铵态N处理与硝态N处理之间。     

Free amino acids and protein in Scots pine seedlings cultivated at different nutrient availabilities

Scots pine (Pinus sylvestris L.) seedlings of a provenance from northern Sweden were cultivated hydroponically for 7 weeks in a climate chamber. The nutrient solution contained either 2.5 (low-N) or 50 (high-N) mg N l(-1) with other essential elements added in a fixed optimal proportion to the nitrogen. After 5 and 7 weeks, the seedlings were analyzed for growth, total nitrogen and other essential nutrients, protein and free amino acids. Low-N seedlings grew more slowly and had higher root/shoot ratios than high-N seedlings. With respect to total nitrogen, the effect of the lower nutrient supply was mainly on the nitrogen content of the whole plant and the allocation of nitrogen among tissues, not on tissue nitrogen concentration. This was also the case for potassium, phosphorus, calcium and magnesium. The proportions by weight among these macronutrients in the whole seedlings were similar in both nutrient regimes. The proportion and concentration of sulfur were significantly lower in low-N seedlings than in high-N seedlings, because of a lower net uptake of sulfur than of other macronutrients. The shoot, needles and stem of low-N seedlings had higher concentrations of free amino acids and lower concentrations of protein than the shoot, needles and stem of high-N seedlings. Arginine dominated the pool of free amino acids in the low-N seedlings, whereas glutamine predominated in the high-N seedlings. We conclude that Scots pine seedlings accumulated soluble nitrogen as arginine when net protein synthesis was limited by factors other than nitrogen availability. Nutritional imbalance, as revealed by growth characteristics and a suboptimal proportion and concentration of sulfur in the seedlings, probably affected synthesis of S-amino acids, resulting in the diversion of assimilated nitrogen to arginine instead of protein.     

Effects of nitrogen source and defoliation on growth and biological dinitrogen fixation of Gliricidia sepium seedlings

Effects of four N sources and two defoliation treatments on growth and nitrogenase activity of Gliricidia sepium (Jacq.) Walp seedlings were studied in a greenhouse. All nutrients were supplied in irrigation water to the sterile growing medium. The N sources were: (1) 100 mg l(-1) of N supplied as NO(3) (-) (high-NO(3) (-)), (2) 50 mg l(-1) of N supplied as NO(3) (-) and inoculation with Rhizobium spp. medium-NO(3) (-)), (3)100 mg l(-1) of N supplied as NH(4)NO(3), and (4) inoculation with Rhizobium spp without mineral N (N(2)). At 35 weeks after sowing, mean total biomass was 130.5, 50.5, 22.9 and 17.4 g seedling(-1) in the NH(4)NO(3), N(2), medium-NO(3) (-) and high-NO(3) (-) treatments, respectively. The root/shoot ratio was high in all of the N treatments (1.73-2.77) because the seedlings had big taproots. The medium-NO(3) (-) treatment completely inhibited nodulation, whereas seedlings in the N(2) treatment were profusely nodulated. At 32 weeks after sowing, groups of seedlings in the N(2) and high-NO(3) (-) treatments were subjected to 50 or 100% defoliation. Closed-chamber acetylene reduction assays of intact root systems were conducted to compare nitrogenase activity at 7, 14 and 28 days after defoliation (DAD). At 7 and 14 DAD, nitrogenase activity of completely and partially defoliated seedlings was about 10 and 60%, respectively, of that of undefoliated controls. At 28 DAD, nitrogenase activity of completely defoliated seedlings was twice the predefoliation value, whereas nitrogenase activity of partially defoliated seedlings was only 87% of the predefoliation value. Recovery of nitrogenase activity was strongly correlated with foliage regrowth in the completely defoliated seedlings, but not in the partially defoliated seedlings. Abundant belowground C and N reserves in the large taproot probably contributed to the rapid recovery from defoliation. Accumulation of belowground biomass may also improve defoliation tolerance of mature trees.     

不同氮素施肥方法对尾巨桉苗期生长和光合生理特性的影响

采用3种不同的氮素施肥处理(平均施肥、直线施肥和指数施肥)对尾巨桉苗木进行施肥处理,结果显示：指数施肥获得的苗木苗高和地径均大于其他施肥处理;指数施肥下尾巨桉苗木的叶、茎、根和地上部分干质量和整株生物量均显著大于其他施肥处理,指数施肥获得了更多的苗木生物量;3种施肥处理下,指数施肥的苗木净光合速率 Pn、气孔导读 Gs 和蒸腾速率 Tr 均为最大值,且显著大于其他处理。因此,3种施肥方法中,指数施肥方法最利于尾巨桉苗木的生长和光合作用。