化感物质香豆素对苜蓿种子萌发及幼苗初级氮同化的影响

香豆素类物质是苯丙酸内酯（环酯）类化合物，绝大部分高等植物通过次生代谢途径都能合成。研究表明，香豆素类物质是花椒体内最重要的化感物质，系统研究香豆素类物质的作用机理有助于理解和最终解决花椒连作障碍。本文通过研究香豆素对几种植物种子特别是苜蓿种子萌发、苜蓿幼苗初级氮同化的影响，从生理生化角度揭示香豆素的作用方式，为花椒连作障碍的解决和化感作用机制的深入理解提供依据。主要研究结果如下：1. 研究了香豆素对6 种常见作物种子萌发的影响，并对一组数据采用4个不同的指标进行评价，对生物测定化感作用中存在的问题进行了讨论。结果发现1.0mM的香豆素对采用的6 种作物的种子萌发均表现出一定的化感作用，4 个指标的敏感程度依次为S (发芽速度)＞AS(累积发芽速度)＞CRG(发芽指数)＞GT(最终发芽率)。种子萌发实验是化感作用研究中最重要、应用最广泛的生物测定方法之一，应根据不同的研究目的合理采用指标和实验方法。2. 采用培养皿试纸法进行种子萌发试验，研究了香豆素水溶液在苜蓿种子萌发过程中对其吸水、电导率及抗氧化保护酶活性的影响。结果表明，影响苜蓿种子发芽的香豆素浓度阀值为0.3mM。香豆素在1.0mM 的浓度下降低了苜蓿种子吸水阶段Ⅱ的吸水速度，使其外渗物质增多，电导率增大，并显著抑制了超氧化物歧化酶(SOD)、过氧化物酶(POD)、过氧化氢酶(CAT)的活性，同时种子体内丙二醛（MDA）的含量显著增大。高浓度香豆素破坏了膜的结构、影响了抗氧化保护酶的活性是香豆素降低苜蓿发芽率的原因之一，也可能是影响花椒-苜蓿间作的关键因素之一。3. 不同浓度（0、25 μM、50 μM、0.1 mM、1.0 mM）化感活性物质香豆素对10 日龄苜蓿幼苗初级氮同化的影响的结果表明25 µM～50 µM 的香豆素加快了苜蓿幼苗对硝态氮的吸收。高浓度的香豆素导致苜蓿根系和叶片内可溶性蛋白含量降低、鲜重减小、地下鲜重/地上鲜重（R/S）的比值升高，根系中初级氮同化的关键酶硝酸还原酶（NR）、谷氨酸胺合成酶（GS）、谷氨酸脱氢酶（GDH）的活性降低，叶片中NR、GS 的活性减低、叶绿素含量减少，而GDH 的活性升高。香豆素影响苜蓿幼苗氮代谢和氨同化的关键酶，导致体内养分的缺失是香豆素抑制苜蓿幼苗生长的机理之一。Coumarins are lactones of o-hydroxycinnamic acid, and are allelopathiccompounds that originate in the phenylpropanoid pathway. They are synthesized byalmost all higher plants. According to previous studies, coumarins were mostimportant allelochemicals in Chinese prickly ash. Systematically research of theeffect of coumarin could help to comprehend the continuous cropping impediment.The effects of coumarin on seed germination and primary nitrogen assimilation ofalfalfa were studied. The main results showed that:1. We compared four common germination indices (S, AS, CRG, GT)preciously calculated with the same date. The results showed that, at theconcentration of 1.0 mM, coumarin inhibited seeds germination. Among all indices,the S index was most sensitive, followed by the AS and CRG indices. Andsuggestions on the expression of bioassay results were also provided.2. At concentrations above 0.3 mM, coumarin inhibited seed germination in aconcentration-dependent manner. During seed imbibitionⅡ, coumarin at 1.0 mMsignificantly reduced the activities of superoxide dismutase (SOD), catalase (CAT),peroxidase (POD), while the content of malonyldialdehyde (MDA) in alfalfa seedssignificantly increased. The higher concentration coumarin destroyed structure ofmembrane and influenced activities of antioxidant enzymes, which might be one ofthe reasons that coumarin decreased germination rate of alfalfa, and one of the keyfactors influencing Chinese prickly ash-alfalfa intercropping.3. Alfalfa plants were exposed to different concentration of coumarin (0、25μM、50 μM、0.1 mM、1.0 mM) grown for 10 days on control medium. Coumarin, in the range of 25 μM～50 μM, significantly stimulated the net nitrate uptake.Increasing coumarin concentration led to a decrease of protein contents in theleaves and roots. The root to shoot (R/S) FW ratio was increased by increasingcoumarin concentration. Under high coumarin concentration, the activities of nitratereductase (NR) and glutamine synthetase (GS) were repressed in the roots andleaves. Glutamate dehydrogenase (GDH) was inhibited in the roots, while enhancedin the leaves. Chlorophyll contents in the leaves were also decreased under highcoumain concentration. Coumarin decreased alfalfa growth by (i) nutritionaldeficiencies shown by the decrease of nitrate, (ii) lowered N compound synthesisvia inhibition of nitrate reduction and ammonium assimilation.

Seasonal changes in the distribution of dissolved organic nitrogen in coastal and open-ocean waters in the North East Pacific: sources and sinks

The distribution of dissolved organic nitrogen (DON) and nitrate were determined seasonally (winter, spring and summer) during three years along line P, i.e. an E-W transect from the coast of British Columbia, Canada, to Station P (50degreesN, 145degreesW) in the subarctic North East Pacific Ocean. In conjunction, DON measurements were made in the Straits of Juan de Fuca and Georgia within an estuarine system connected to the NE Pacific Ocean. The distribution of DON at the surface showed higher values of 4-17 muM in the Straits relative to values of 4-10 muM encountered along line P, respectively. Along line P, the concentration of DON showed an inshore-offshore gradient at the surface with higher values near the coast. The equation for the conservation of DON showed that horizontal transport of DON (inshore-offshore) was much larger than vertical physical mixing. Horizontal advection of DON-rich waters from the coastal estuarine system to the NE Pacific Ocean was likely the cause of the inshore-offshore gradient in the concentration of DON. Although the concentration of DON was very variable in space and time, it increased from winter to summer, with an average build up of 4.3 muM in the Straits and 0.7 muM in the NE subarctic Pacific. This implied seasonal DON sources of 0.3 mmol N m(-2) d(-1) at Station P and 1.5 mmol N m(-2) d(-1) in the Straits, respectively. These seasonal DON accumulation rates corresponded to about 15-20% of the seasonal nitrate uptake and suggested that there was a small seasonal build up of labile DON at the surface. However, the long residence times of 180-1560 d indicated that the most of the DON pool in surface waters was refractory in two very different productivity regimes of the NE Pacific. (C) 2002 Elsevier Science Ltd. All rights reserved.

Nitrogen isotopic fractionation during a simulated diatom spring bloom: importance of N-starvation in controlling fractionation

N isotope fractionation (epsilon) was first determined during ambient NO3- depletion in a simulated diatom spring bloom. After 48 h of N-starvation, NH4+ was resupplied to the diatoms in small pulses to simulate grazer-produced N and then epsilon was determined. Large variations in epsilon values were observed: from 2.0-3.6 to 14-0 parts per thousand during NO3- and NH4+ uptake, respectively. This is the first study reporting an epsilon value as low as 0 to 2 parts per thousand for NH4+ uptake and we suggest that greater N demand after N-starvation may have drastically reduced NH3 efflux out of the cells. Thus the N status of the phytoplankton and not the ambient NH4+ concentration may be the important factor controlling epsilon, because, when N-starvation increased, epsilon values for NH4+ uptake decreased within 30 h. This study may thus have important implications for interpreting the delta(15)N of particulate N in nutrient-depleted regimes in temperate coastal oceans.

Nitrogen isotope fractionation during nitrate, ammonium and urea uptake by marine diatoms and coccolithophores under various conditions of N availability

Stable isotopes of N provide a new approach to the study of algal production in the ocean, yet knowledge of the isotope fractionation (epsilon) in various oceanic regimes is lacking. Here we report large and rapid changes in isotope composition (delta(15)N) of 2 coastal diatoms and 2 clones (open and coastal) of a coccolithophore grown in the simultaneous presence of nitrate, ammonium and urea under varying conditions of N availability (i.e. N-sufficiency and N-starvation followed by N-resupply) and hence different physiological states, During N-sufficiency, the delta(15)N of particulate organic N (PON) was well reproduced, using a model derived from Rayleigh distillation theory, with constant epsilon similar to that for growth on each individual N source. However, following N-resupply, the variations in delta(15)N(PON) could be well explained only in the case of the open ocean Emiliania huxleyi, with epsilon similar to N-sufficient conditions. It was concluded that the mechanism of isotope fractionation changed rapidly with N availability for the 3 coastal clones. However, in the case of E. huxleyi isolated from the Subarctic Pacific Ocean, no evidence of a change in mechanism was found, suggesting that perhaps open ocean species can quickly recover from N-depleted conditions.

Temporal and spatial variability in nitrogen uptake kinetics during harmful dinoflagellate blooms in the East China Sea

Temporal and spatial variability in the kinetic parameters of uptake of nitrate (NO3-), ammonium (NH4+), urea, and glycine was measured during dinoflagellate blooms in Changjiang River estuary and East China Sea coast, 2005. Karenia mikimotoi was the dominant species in the early stage of the blooms and was succeeded by Prorocentrum donghaiense. The uptake of nitrogen (N) was determined using N-15 tracer techniques. The results of comparison kinetic parameters with ambient nutrients confirmed that different N forms were preferentially taken up during different stages of the bloom. NO3- (V-max 0.044 h(-1); K-s 60.8 mu M-N) was an important N source before it was depleted. NH4+ (V-max 0.049 h(-1); K-s 2.15 mu M-N) was generally the preferred N. Between the 2 organic N sources, urea was more preferred when K. mikimotoi dominated the bloom (V-max 0.020 h(-1); K-s 1.35 mu M-N) and glycine, considered as a dominant amino acid, was more preferred when P. donghaiense dominated the bloom (V-max 0.025 h(-1); K-s 1.76 mu M-N). The change of N uptake preference by the bloom-forming algae was also related to the variation in ambient N concentrations. Published by Elsevier B.V.