氮输入对植物光合固碳的影响研究进展

植物光合固碳(C)是生物固C的重要途径和生态系统C循环中的重要环节。在全球环境变化背景下,研究氮(N)输入对植物光合固C的影响,对于更好的认识C、N循环过程及生态系统对全球变化的响应过程等具有重要意义。N输入是否能够增加植物固C取决于生态系统类型以及生态系统的N饱和度;草原和湿地生态系统N输入的临界负荷值较高,干旱、半干旱荒漠地区较低;N输入可能改变植物光合固C在各器官的分配,主要由植物生理、自身生长节律和环境养分等决定。由于物种和生态系统类型的差异,N输入对植物固C的影响仍具有很大的不确定性,目前缺乏准确、定量表达N输入对生态系统光合和C同化物分配影响的数学表达方法和过程算法。未来应着重加强N输入下C同化物分配的生物地球化学模型和N、P富集下植物光合固C耦合模型研究,并应用同位素标记和分子生物学技术,从生态系统角度综合探讨N输入下植物光合固C的分配和转化特征。     

氮磷添加对内蒙古温带草地地上生物量的影响

干旱半干旱地区植物生长不仅受到水分的限制,同时也受到养分的限制.为进一步明确养分在多大程度上能促进半干旱区植物的生长,本研究在内蒙古温带草原开展了连续2年的养分添加控制试验,设置10和40 g·m^-2·a^-1的N添加水平以及10 g·m^-2·a^-1的P添加水平.结果表明： N添加能够显著促进植物生长,10和40 g N·m^-2·a^-1处理地上生物量在2012年分别较对照增加50.8%和65.9%,2013年增加71.6%和93.3%,2个N添加处理之间地上生物量无显著差异.与单独10和40 g·m^-2·a^-1的N添加处理相比,P添加后地上生物量在2012年分别提高98.4%和186.8%,2013年分别提高111.7%和141.4%.N添加普遍提高了3种植物功能群(禾本科、菊科和其他科)的地上生物量,但较对照差异不显著,而N、P同时添加显著提高了菊科植物的地上生物量.养分添加使植被盖度升高,从而改善了表层土壤的水分条件,这可能是N、P添加促进植物生长和提高降水利用效率的重要机制之一.     

Physiological responses of <Emphasis Type="Italic">Catharanthus roseus</Emphasis> to different nitrogen forms

The responses of carbon and nitrogen metabolisms in the medical plant Catharanthus roseus to the nitrogen solutions (N1, N2 and N3) containing different ratio of nitrate to ammonium (1:0, N1; 1:1, N2; 1:3, N3) were investigated here. The plants in N3 nitrogen solution were strongly inhibited in photosynthetic gas exchange and carbohydrate accumulation, reflecting the toxicity symptom of excess ammonium continuously accumulated in plants. The treatment with N2 nitrogen solution, however, displayed an obviously synergistic effect on plant growth and metabolisms in contrast to nitrate as the sole source. The short-term (7 days) exposure of plants to N2 nitrogen solution resulted in an increased shoot/root ratio, leaf mass ratio, and Pn, as well as the elevated levels of sucrose, glutamate, aspartate, proline and threonine. The plants in N2 nitrogen solution accumulated twofold catharanthine and vinblastine than did the plants in N1 or N3 nitrogen solution after the long-term incubation. Internal nitrate had an increased accumulation in the plants in N2 nitrogen solution compared to the counterparts. The supply of N2 or N3 nitrogen solution to plants for 7 days induced an over tenfold increase of ammonium in leaves as compared to the case using N1 nitrogen solution. The increased ammonium ion promoted the activities of NADH-dependent glutamate dehydrogenase (NADH-GDH) both in the leaf and root of plants. Under the ammonium-containing solution (N2 and N3 nitrogen solutions), there was a significantly increased activity for glutamine synthase (GS) in the root during experiment and for nitrate reductase (NR) in the leaf and root only after 21 days of treatment. The performed correlation analysis revealed a negative relation between soluble sugars and internal ammonium, whereas a positive correlation of alkaloid production with glutamate and aspartate.     

Responses of the functional traits in Cleistogenes squarrosa to nitrogen addition and drought

植物功能性状被广泛地用于研究植物对环境变化的响应。糙隐子草(Cleistogenes squarrosa)是内蒙古草原重要的C4物种,其功能性状是如何对水氮环境的变化做出响应的,还不十分清楚。该文采用盆栽实验的方法,进行氮添加(0,10.5,35.0和56.0 g·m–2·a–1)和降水(自然降水和70%平均月降水量)处理,研究糙隐子草整株性状、叶形态性状和叶生理性状对氮添加和干旱的响应。结果表明,氮添加显著影响了糙隐子草的整株性状,氮、水处理及它们的交互作用显著影响了糙隐子草的叶形态性状和叶生理性状。各功能性状对氮添加的响应格局在自然降水和干旱处理下是不同的。根深、茎生物量和茎叶比在干旱条件下低和中氮添加处理中较高,而在自然降水下无明显变化;比叶面积在干旱条件下随氮添加量的增加而增加,而在自然降水下无增加趋势;自然降水下,高氮添加显著刺激了光合速率和蒸腾速率,增加了水分利用效率,而在干旱条件下氮添加对它们没有显著影响;叶片单位面积的氮含量在自然降水下随氮添加量的增加有增加趋势,而在干旱条件下显著降低。在自然降水下,氮添加主要影响糙隐子草的叶形态和生理性状,而在干旱条件下,氮添加主要影响糙隐子草的整株性状和形态性状。总之,糙隐子草的功能性状对氮添加表现出明显的响应,响应格局在不同的水分条件下不同,反映了其对氮水环境变化的弹性适应。     

短期氮素添加和模拟放牧对青藏高原高寒草甸生态系统呼吸的影响

氮沉降和放牧是影响草地碳循环过程的重要环境因子,但很少有研究探讨这些因子交互作用对生态系统呼吸的影响。在西藏高原高寒草甸地区开展了外源氮素添加与刈割模拟放牧实验,测定了其对植物生物量分配、土壤微生物碳氮和生态系统呼吸的影响。结果表明:氮素添加显著促进生态系统呼吸,而模拟放牧对其无显著影响,且降低了氮素添加的刺激作用。氮素添加通过提高微生物氮含量和土壤微生物代谢活性,促进植物地上生产,从而增加生态系统的碳排放;而模拟放牧降低了微生物碳含量,且降低了氮素添加的作用,促进根系的补偿性生长,降低了氮素添加对生态系统碳排放的刺激作用。这表明,放牧压力的存在会抑制氮沉降对高寒草甸生态系统碳排放的促进作用,同时外源氮输入也会缓解放牧压力对高寒草甸生态系统生产的负面影响。     

Pulse additions of soil carbon and nitrogen affect soil nitrogen dynamics in an arid Colorado Plateau shrubland

Biogeochemical cycles in arid and semi-arid ecosystems depend upon the ability of soil microbes to use pulses of resources. Brief periods of high activity generally occur after precipitation events that provide access to energy and nutrients (carbon and nitrogen) for soil organisms. To better understand pulse-driven dynamics of microbial soil nitrogen (N) cycling in an arid Colorado Plateau ecosystem, we simulated a pulsed addition of labile carbon (C) and N in the field under the canopies of the major plant species in plant interspaces. Soil microbial activity and N cycling responded positively to added C while NH₄⁺–N additions resulted in an accumulation of soil NO₃⁻. Increases in microbial activity were reflected in higher rates of respiration and N immobilization with C addition. When both C and N were added to soils, N losses via NH₃ volatilization decreased. There was no effect of soil C or N availability on microbial biomass N suggesting that the level of microbial activity (respiration) may be more important than population size (biomass) in controlling short-term dynamics of inorganic and labile organic N. The effects of C and N pulses on soil microbial function and pools of NH₄⁺–N and labile organic N were observed to last only for the duration of the moisture pulse created by treatment addition, while the effect on the NO₃⁻–N pool persisted after soils dried to pre-pulse moisture levels. We observed that increases in available C lead to greater ecosystem immobilization and retention of N in soil microbial biomass and also lowered rates of gaseous N loss. With the exception of trace gas N losses, the lack of interaction between available C and N on controlling N dynamics, and the subsequent reduction in plant available N with C addition has implications for the competitive relationships between plants species, plants and microbes, or both.     

Responses of photosynthesis and carbohydrate-partitioning to limitations in nitrogen and water availability in field-grown sunflower*

Abstract. Sunflower plants (Helianthus annuus L., cv. CGL 208) were field-grown in adjacent plots of varying resource availability. Control plants received irrigation (on a 4–5 d interval) and high levels of fertilizer nitrogen. Nutrient-stress (N-stress) plants received control levels of irrigation but no nutrient amendments and were determined to be nitrogen-limited. Water-stress (H₂O-stress) plants received control levels of fertilizer nitrogen, but no irrigation after approximately 6 weeks of plant growth. Both stress treatments reduced maximum and diurnal net photosynthesis (A) but resulted in different physiological or biochemical adjustments that tended to maintain or increase A per unit of resource (nitrogen or water) in shortest supply while decreasing the ratio of A per unit of abundant resource. Nutrient-stress reduced total foliar nitrogen, foliar chlorophyll, and initial and total RuBPCase activities, thereby enhancing or preserving photosynthetic nitrogen-use efficiency (NUE), defined as the maximum A observed per unit of leaf nitrogen, relative to the control and H₂O-stress treatments. In addition, N-stress reduced photosynthetic water-use efficiency (WUE), defined as the ratio of A to stomatal conductance to water vapour (g). The slope of A versus g increased with H₂O-stress. In addition, sunflower plants responded to H₂O-stress by accumulating foliar glucose and sucrose and by exhibiting diurnal leaf wilting, which presumably provided additional improvements in photosynthetic WUE through osmoregulation and reduction of midday radiation interception respectively. Photosynthetic NUE was decreased by H₂O-stress in that control levels of total nitrogen, foliar chlorophyll, and RuBPCase activities were maintained even after mean diurnal levels of A had fallen to less than 50% of the control level. We conclude that field-grown sunflower manages a trade-off between photosynthetic WUE and NUE, increasing use efficiency of the scarce resource while decreasing use efficiency of the abundant resource.     

低氮诱导玉米幼苗叶片衰老过程中碳氮平衡的动态变化

叶片适时衰老对保证玉米产量有重要意义。本试验以玉米自交系PH6WC和PH4CV为研究对象,通过水培方法,设置低氮(0.04 mmol·L^-1,LN)和正常(4 mmol·L^-1,CK)氮素水平两种处理,在培养2、4、6和8 d后,对其幼苗第2和第3叶片表型、光合特性、叶片中氮素和糖分含量及碳氮比进行分析,旨在探究低氮胁迫下玉米幼苗叶片衰老过程中碳氮平衡的动态变化。结果表明: 与CK相比,LN造成两玉米自交系幼苗第2和第3叶片的面积、生物量、相对叶绿素含量、净光合速率、可溶性糖和淀粉含量均下降,而其氮物质生产能力均先后增加,但第2叶片的变化时间均早于第3叶片;在两叶片的各性状上,均为LN下PH6WC的变化幅度大于PH4CV,且仅幼苗叶片中的碳氮比在LN下显著提高;PH6WC的叶片衰老更快,PH4CV有更强的碳氮平衡能力,其叶片衰老相对滞后。综上,低氮会诱导玉米幼苗叶片衰老,高碳氮比具有促进叶片衰老的调控作用,低氮胁迫下幼苗叶片的碳氮平衡能力在两个玉米基因型间存在较大差异。     

水分和养分添加对羊草功能性状和地上生物量的影响

研究水分和养分添加对植物功能性状的影响, 对于揭示植物对环境变化的响应和适应规律至关重要。该文采用盆栽试验的方法, 进行不同水平水分处理(增水50%, 减水50%, 以498 mm降水量作为对照)和养分添加(无养分添加, 单施氮肥, 单施磷肥, 氮磷共施), 研究羊草(Leymus chinensis)的10种功能性状和地上生物量对水分和养分添加的响应。得出以下结论: (1)双因素方差分析结果表明, 水分主效应对羊草株高、分蘖数、茎生物量、叶生物量、叶面积、叶质量、净光合速率、蒸腾速率、水分利用效率存在显著影响; 养分主效应对羊草分蘖数、茎生物量、净光合速率、蒸腾速率、水分利用效率存在显著影响; 水分和养分的交互作用对羊草分蘖数、茎生物量、蒸腾速率、水分利用效率存在显著影响。(2)各功能性状对降水量的响应在不同养分添加水平是不同的, 分蘖数和叶面积在单施氮肥和氮磷共施条件下随降水量增加而增加, 而在无养分添加和单施磷肥条件下无显著变化; 茎生物量在无养分添加、单施氮肥和单施磷肥条件下随降水量增加而增加, 而在氮磷共施条件下无增加趋势; 比叶面积在单施氮肥条件下增水处理显著低于对照组, 而在其他养分添加条件下无明显变化。(3)短期氮磷处理显著影响羊草叶片光合生理性状, 而对叶形态性状影响不显著。(4)羊草地上生物量随降水量的增加呈现上升趋势, 并且在单施氮肥条件下, 增水处理使地上生物量达到最高, 为522.55 g·m ^-2。总之, 羊草的功能性状对降水量增加表现出明显的响应, 响应格局在不同养分条件下不同, 反映了其对水肥环境变化的适应。     

Global response patterns of plant photosynthesis to nitrogen addition: A meta‐analysis

A mechanistic understanding of plant photosynthetic response is needed to reliably predict changes in terrestrial carbon (C) gain under conditions of chronically elevated atmospheric nitrogen (N) deposition. Here, using 2,683 observations from 240 journal articles, we conducted a global meta‐analysis to reveal effects of N addition on 14 photosynthesis‐related traits and affecting moderators. We found that across 320 terrestrial plant species, leaf N was enhanced comparably on mass basis (N_mass, +18.4%) and area basis (N_area, +14.3%), with no changes in specific leaf area or leaf mass per area. Total leaf area (TLA) was increased significantly, as indicated by the increases in total leaf biomass (+46.5%), leaf area per plant (+29.7%), and leaf area index (LAI, +24.4%). To a lesser extent than for TLA, N addition significantly enhanced leaf photosynthetic rate per area (A_area, +12.6%), stomatal conductance (g_s, +7.5%), and transpiration rate (E, +10.5%). The responses of A_area were positively related with that of g_s, with no changes in instantaneous water‐use efficiency and only slight increases in long‐term water‐use efficiency (+2.5%) inferred from ¹³C composition. The responses of traits depended on biological, experimental, and environmental moderators. As experimental duration and N load increased, the responses of LAI and A_area diminished while that of E increased significantly. The observed patterns of increases in both TLA and E indicate that N deposition will increase the amount of water used by plants. Taken together, N deposition will enhance gross photosynthetic C gain of the terrestrial plants while increasing their water loss to the atmosphere, but the effects on C gain might diminish over time and that on plant water use would be amplified if N deposition persists.     

内蒙古草原不同基因组大小植物对氮水添加的响应

被子植物基因组大小的种间差异巨大,约为2400倍.基因组大小与植物从细胞核到个体水平的一系列性状密切相关,进而影响植物对环境变化的响应.作为水分和养分共同限制的生态系统,内蒙古草原植物群落对氮素、水分有效性变化的响应具有明显的种间差异,这种差异可能与种间基因组大小不同有关.本研究利用流式细胞术测定了内蒙古典型草原水分、氮素添加实验平台植物的基因组大小,研究了不同基因组大小植物地上净初级生产力(ANPP)和物种丰富度对水分、氮素添加及其交互作用的响应.结果表明:基因组大小显著影响了不同植物ANPP对水分的响应,小基因组植物ANPP对氮水添加响应更敏感,加水和氮水共同添加显著增加了小基因组植物ANPP,而大基因组植物ANPP对所有处理响应均不显著.加氮对大小基因组植物ANPP都无显著影响.大小基因组植物的物种丰富度对氮水添加的响应也均不显著.基因组大小影响内蒙古草原不同植物ANPP对水分增加的响应.作为植物细胞核水平上十分稳定且种间差异巨大的物种性状,将基因组大小引入生态学研究将对全球变化背景下生态系统结构与功能变化研究起到重要作用.     

外来入侵植物对土壤氮转化主要过程及相关微生物的影响

外来入侵植物不仅影响植物群落组成、生物多样性以及生态系统的结构和功能, 而且显著影响土壤氮(N)的转化过程。外来入侵植物对N循环影响的研究已成为入侵生态学的研究热点。N循环与凋落物的分解和养分释放有关, 外来入侵植物能够改变凋落物的组成与结构, 进而影响土壤的N转化过程。另外, 外来入侵植物的化感作用也会影响土壤N转化过程, 这些作用与土壤微生物的结构与功能变化密不可分。该文主要从凋落物分解与养分释放及外来入侵植物化感作用两个方面综述了外来入侵植物对土壤N转化的影响, 总结了外来入侵植物对土壤N转化相关土壤微生物(尤其是氨氧化细菌与氨氧化古菌)的影响, 探讨了土壤N转化对外来植物入侵的反馈, 并探讨了丛植菌根真菌与外来入侵植物的互相影响。     

降水量与氮添加对荒漠草原生态系统碳交换的影响

为深入了解降水格局改变和氮沉降增加对荒漠草原生态系统碳交换的影响机制,于2017年在宁夏荒漠草原设立了降水量变化（减少50%、减少30%、自然降水量、增加30%以及增加50%）和氮添加（0和5 g m^-2 a^-1）的野外试验,研究了2019年生长季（5-10月份）净生态系统碳交换（Net ecosystem carbon exchange,NEE）、生态系统呼吸（Ecosystem respiration,ER）和总生态系统生产力（Gross ecosystem productivity,GEP）的时间动态,分析了三者与植被组成以及土壤属性的关系。NEE、ER和GEP日动态和月动态均呈先增加后降低,NEE在整个生长季表现为净生态系统碳吸收。0和5 g m^-2 a^-1氮添加下,减少降水量显著降低了NEE、ER和GEP （P<0.05）,增加30%降水量显著提高了三者（P<0.05）。相同降水量条件下,氮添加不同程度地提高了NEE、ER和GEP,且其效应在增加50%降水量时较为明显。净生态系统碳吸收（-NEE）、ER和GEP与群落生物量、牛枝子（Lespedeza potaninii）以及草木樨状黄芪（Astragalus melilotoides）生物量正相关。三者亦随Patrick丰富度指数和Shannon-Wiener多样性指数的增加而增加。本文结果意味着,减少降水量降低了土壤水分和养分有效性、抑制了植物生长,从而降低了生态系统碳交换。适量增加降水量则可能通过提高土壤含水量、刺激土壤酶活性、调节土壤C ： N ： P平衡特征等途径,促进了植物生长和物种多样性,从而提高了生态系统碳汇功能;氮添加亦促进了生态系统碳交换,但其与降水的交互作用尚不明显,需通过长期观测进行深入探讨。     

Dirunal leaf movement, chlorophyll fluorescence and carbon assimilation in soybean grown under different nitrogen and water availabilities

Diurnal heliotropic leaf movements, photosynthetic gas exchange, and the ratio of variable fluorescence to maximum fluorescence (Fv/Fm) of unrestrained and horizontally restrained leaves from soybean (Glycine max cv. Cumberland) plants grown in two different water and two different nitrogen treatments were measured. Leaves of plants grown in low water or low nitrogen availability treatments displayed more pronounced diaheliotropism (solar tracking) in the afternoon and a longer period of paraheliotropism (light avoiding) at midday relative to those of well-watered, high-nitrogen-grown plants. Photosaturated photosynthetic rates and the photon flux required to saturate photosynthesis were reduced by water stress and nitrogen deficiency. Compared to horizontal leaves, irradiance on orienting leaves was nearer to the breakpoint of the photosynthetic light response curve, where photosynthesis is co-limited by ribulose biphosphate regeneration and carboxylation. This would increase the carbon return on investments of nitrogen into photosynthesis. A positive linear relationship between Fv/Fm and quantum yield of photosynthesis was measured. Leaves of low-nitrogen-grown plants had earlier and more prolonged reductions in Fv/Fm at midday compared to leaves of high nitrogen grown plants of the same water treatment. Within the same water and nitrogen treatment, horizontally restrained leaves had lower midday Fv/Fm in relation to orienting leaves. Nitrogen deficiency and water stress enhanced this difference such that horizontally restrained leaves of low water and low nitrogen grown plants had earlier and longer midday depressions in Fv/Fm.     

Effects of nitrogen addition on nitrogen metabolism and carbon reserves in the temperate seagrass Posidonia oceanica

The effect of repeated N additions on a dense, shallow meadow of Posidonia oceanica (L.) Delile in the NW Mediterranean was studied over a year. N was added biweekly both to the sediment and to the water column as ammonium and nitrate. The most obvious result of these additions was an overall increase in N content (% DW) in all tissues of fertilized plants; this increase was maximum in rhizomes, with values of 5% N reached, which confirmed the storage capacities of these organs.Fertilization affected the different N fractions in distinct ways. The free amino acid (FAA) concentration increased the most, particularly in rhizomes and roots, suggesting the function of these compounds for N storage and, probably, translocation. The non-soluble N fraction also increased greatly. The total soluble protein (TSP) and the inorganic N forms concentrations were less sensitive to fertilization, and only increased moderately in a few cases. N assimilation, assessed through in vivo glutamine synthetase (GS) activity, was maximum in leaves after the peak of growth, which coincided with the lowest N values in both control and fertilized plants. Thus assimilation was probably greatest at the period of highest N deficiency. Growth rates did not respond to N enrichment. Another clear effect of N addition was to decrease carbon reserves. In effect, the concentration of total non-structural carbohydrate (TNC) greatly decreased in rhizomes of fertilized plants, coinciding with the increase in FAA. We conclude that increased nitrogen availability can affect plant survival through the decrease in their carbon reserves, crucial for P. oceanica overwintering. This interaction between N and C metabolism helps to explain changes in benthic vegetation after steadily increasing coastal water eutrophication.     

氮添加对中国陆地植被地上-地下生物量分配的影响

大气氮沉降水平持续升高导致的外源氮输入增加,强烈影响了陆地生态系统的碳循环。目前,已有大量报道证实了氮沉降升高对全球陆地植被固碳的积极影响。虽然之前大部分研究将这一结果归因于光合作用增强导致的地上生物量增加,但最近的研究发现长期氮添加对植物地下根系的影响也同样重要。归纳整理了181篇公开发表的我国野外模拟氮沉降试验结果,采用整合分析(Meta-analysis)方法,定量评估了氮添加对我国陆地植被地上-地下生物量分配的影响特征和不同生态系统类型及施氮方式之间的影响差异。通过分析地上-地下生物量分配对氮添加的响应差异来探究植被碳增益对长期大气氮沉降增加的潜在响应机制。结果表明,氮添加显著增强了我国陆地植被的光合作用及碳固存,且植物碳增益在不同生态系统类型及施氮制度间有所差异。植物叶片的氮含量显著增加,使得叶片碳氮比及凋落物碳氮比显著降低,但并未显著影响细根的碳氮比。氮添加总体上显著提高了植物的净光合速率,但降低了光合利用效率。地上生物量,凋落物产量和根生物量平均分别显著增加了38%,17%和18%,总体上植物地上部分对氮添加的响应程度比地下部分更高。然而,不同生态系统类型的地上-地下生物...     

Changa in the structure and activity of a soil microbial community caused by inorganic nitrogen fertilization

The changes in the structure and activity of a soil microbial community caused by addition of moderate and high rates of the mineral nitrogen fertilizer (KNO₃) were studied in a laboratory incubation experiment. The structure of the microbial community was evaluated from the phospholipid fatty acid (PLFA) profile; specific growth rate of the microorganisms was determined by the method of the kinetics of substrate-induced respiration; the total pool of microbial carbon was estimated by the fumigation-extraction method. The amounts of nitrogen fertilizer applied in three treatments of the experiment were 0 (control), 100, and 2000 ??g N/g soil. Even in the absence of additional sources of organic carbon, a considerable portion of the added ¹⁵N (up to 74%) was immobilized. No significant increase in the amount of microbial carbon was observed during incubation. The specific growth rate of the microbial community in soil supplemented with glucose decreased twofold after addition of 2000 ??g N/g soil. In this treatment, the ratio of cyclic fatty acids to their monoenoic precursors also increased, indicating the adaptation of microbial cells to extremely high amounts of nitrogen fertilizer. Moreover, considerable changes in the structure of the soil microbial community, such as an increase in the ratio of fungalto bacterial markers and a decrease in the ratio between PLFA of gram-positive and gram-negative bacteria, were observed in the treatment with addition of 2000 ??g N/g soil. Our data clearly indicate that mineral nitrogen fertilization of soil under carbon limitation has a pronounced impact on the structure and activity of soil microbial communities.     

An engineered phosphoenolpyruvate carboxylase redirects carbon and nitrogen flow in transgenic potato plants

Phosphoenolpyruvate carboxylase (PEPC) plays a central role in the anaplerotic provision of carbon skeletons for amino acid biosynthesis in leaves of C3 plants. Furthermore, in both C4 and CAM plants photosynthetic isoforms are pivotal for the fixation of atmospheric CO2. Potato PEPC was mutated either by modifications of the N-terminal phosphorylation site or by an exchange of an internal cDNA segment for the homologous sequence of PEPC from the C4 plant Flaveria trinervia. Both modifications resulted in enzymes with lowered sensitivity to malate inhibition and an increased affinity for PEP. These effects were enhanced by a combination of both mutated sequences and pulse labelling with 14CO2 in vivo revealed clearly increased fixation into malate for this genotype. Activity levels correlated well with protein levels of the mutated PEPC. Constitutive overexpression of PEPC carrying both N-terminal and internal modifications strongly diminished plant growth and tuber yield. Metabolite analysis showed that carbon flow was re-directed from soluble sugars and starch to organic acids (malate) and amino acids, which increased four-fold compared with the wild type. The effects on leaf metabolism indicate that the engineered enzyme provides an optimised starting point for the installation of a C4-like photosynthetic pathway in C3 plants.     

Effects of nitrogen nutrition and root medium water potential on growth, nitrogen uptake and osmotic adjustment of rice

The effects of nitrogen (N) nutrition on growth, N uptake and leaf osmotic potential of rice plants (Oryza sativa L. ev. IR 36) during simulated water stress were determined. Twenty-one-day-old seedlings in high (28.6 × 10 ^?4M) and low (7.14 × 10 ⁴M) N levels were exposed to decreased nutrient solution water potentials by addition of polyethylene glycol 6000. The roots were separated from the solution by a semi-permeable membrane. Nutrient solution water potential was ?0.6 × 10⁵ Pa and was lowered stepwise to ?1 × 10⁵, ?2 × 10⁵, ?4 × 10⁵ and ?6 × 10⁵ Pa at 2-day intervals. Plant height, leaf area and shoot dry weight of high and low nitrogen plants were reduced by lower osmotic potentials of the root medium. Osmotic stress caused greater shoot growth reduction in high N than in low N plants. Stressed and unstressed plants in 7.14 × 10⁴M N had more root dry matter than the corresponding plants in 28.6 × 10⁴M N. Dawn leaf water potential of stressed plants was 1 × 10⁵ to 5.5 × 10⁵ Pa lower than nutrient solution water potential. Nitrogen-deficient water-stressed plants, however, maintained higher dawn leaf water potential than high nitrogen water-stressed plants. It is suggested that this was due to higher root-to-shoot ratios of N deficient plants. The osmotic potentials of leaves at full turgor for control plants were about 1.3 × 10⁵ Pa higher in 7.14 × 10^?4M than in 28.6 × 10^?4M N and osmotic adjustment of 2.6 × 10⁵ and 4.3 × 10⁵ Pa was obtained in low and high N plants, respectively. The nitrogen status of plants, therefore, affected the ability of the rice plant to adjust osmotically during water stress. Plant water stress decreased transpiration and total N content in shoots of both N treatments. Reduced shoot growth as a result of water stress caused the decrease in amount of water transpired. Transpiration and N uptake were significantly correlated. Our results show that nitrogen content is reduced in water-stressed plants by the integrated effects of plant water stress per se on accumulation of dry matter and transpiring leaf area as well as the often cited changes in soil physical properties of a drying root medium.     

Suboptimal nitrogen status sensitizes the photosynthetic apparatus in willow leaves to long term but not short term water stress

The response to drought was compared for willow plants of optimal leaf nitrogen content (100 N) and those of 86% of this content (86 N). Gas exchange measurements revealed that the carboxylation efficiency (CE) of photosynthesis was more sensitive to drought than the photosynthetic capacity in both N regimes. Since the leaf content of ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) was found to be much more resistant it is suggested that a decreased specific activity of Rubisco underlies the decreased CE. Although the rate of water consumption was the same for 86 N and 100 N plants the photosynthetic apparatus responded much more rapidly in the 86 N leaves. This increased sensitivity of 86 N leaves was not due to accelerated senescence as judged by comparison with parallel plants subjected to discontinued fertilization; the two categories of treatments resulted in the same loss of leaf nitrogen and Rubisco but drought induced a much more rapid photosynthetic depression. In contrast to the drought situation, 86 N and 100 N plants behaved similarly when compared under short term water stress. First, when single attached leaves were exposed to a sudden drop in air humidity the capacity of CO₂ uptake in both N regimes decreased about 20% over 10 min while the leaf water potential remained high. Second, in freely transpiring leaf discs cut from 86 N and 100 N leaves the same relationship between capacity of O₂ evolution and extent of dehydration was observed. The possible mechanisms underlying the increased susceptibility of 86 N leaves to drought is discussed; the water status of the roots not the leaves is suggested to be the determining factor.Abbreviations CE carboxylation efficiency - 100 N optimal nitrogen regime - 86 N suboptimal nitrogen regime with 86% of the optimal leaf nitrogen content, Rubisco ribulose-1,5-bisphosphate carboxylase/oxygenase