Partitioning of nitrogen and biomass at a range of N-addition rates and their consequences for growth and gas exchange in two perennial grasses from inland dunes

This paper describes the effects of nitrgen supply on the partitioning of biomass and nitrogen of Agrostis vinealis (L.) Schreber and Corynephorus canescens (L.) Beauv., two perennial grasses of dry, nutrient-poor inland dunes, and their consequences for growth and gas exchange. At a given plant nitrogen concentration (PNC) the two species allocate the same relative amount of dry matter and nitrogen to their leaves. However, A. vinealis allocates more dry matter and nitrogen to its roots and less to its above-ground support tissue than C. canescens . Both the leaf weight ratio and leaf nitrogen ratio increase with increasing PNC. Despite species-specific differences in growth form and leaf morphology, the leaf area ratio and specific leaf area of the two species are similar, both at high and low PNC. At intermediate nitrogen supply, and thus intemediate PNC, however, A. vinealis has a higher leaf area ratio and specific leaf area than C. canescens .
The two species exhibit a similar positive relationship when either the rate of net photosynthesis or the rate of shoot respiration are compared to the leaf nitrogen concentration, all expressed per unit leaf weight. The rate of net photosynthesis per unit Jeafnitrogen (PNUE) of the two species increases with decreasing leaf nitrogen concentration per unit leaf weight. C. canescens has a higher PNUE at low, and a lower PNUE at high leaf nitrogen concentration per unit leaf weight than A. vinealis . At non-limiting nitrogen supply, A. vinealis has a higher nitrogen productivity and net assimilation rate and a similar PNC and leaf area ratio as compared to C. canescens , which explains the higher relative growth rate (RGR_max) of A. vinealis. At growth-limiting nitrogen supply C. canescens achieves a similar relative growth rate at a lower PNC than A. vinealis.     

Transpiration, and nitrogen uptake and flow in two maize (Zea mays L.) inbred lines as affected by nitrogen supply

BACKGROUND AND AIMS: The influence of two nitrogen (N) levels on growth, water relations, and N uptake and flow was investigated in two different inbred lines of maize (N-efficient Zi330 and N-inefficient Chen94-11) to analyse the differences in N uptake and cycling within a plant. METHODS: Xylem sap from different leaves of the inbred lines cultured in quartz sand was collected by application of pressure to the root system. Plant transpiration was measured on a daily basis by weighing five pots of each of the treatments. KEY RESULTS: N-efficient Zi330 had a higher relative growth rate and water-use efficiency at both high (4 mm) and low (0.08 mm) N levels. At a high N level, the amount of N taken up was similar for the two inbred lines; the amount of N transported in the xylem and retranslocated in the phloem was slight greater in Chen94-11 than in Zi330. At a low N level, however, the total amount of N taken up, transported in the xylem and retranslocated in the phloem of Zi330 was 2.2, 2.7 and 2.7 times more, respectively, than that of Chen94-11. Independent of inbred line and N level, the amounts of N transported in the xylem and cycled in the phloem were far more than that taken up by roots at the same time. Low N supply shifted NO(3)(-1) reduction towards the roots. The major nitrogenous compound in the xylem sap was NO(3)(-1), when plants grew at the high N level, while amino acid-N was predominant when plants grew at the low N level. CONCLUSIONS: The N-efficient maize inbred line Zi330 had a higher ability to take up N and cycle N within the plant than N-inefficient Chen94-11 when grown under N-deficiency.     

Biomass production and nitrogen contents of the CAM plants Kalanchoe daigremontiana and K. tubiflora in cultures with different nitrogen and water supply

Summary Two CAM plant species (Kalanchoe daigremontiana and K. tubiflora) were cultivated in pure cultures with two different levels of nitrogen and water supply. A comparison of the plant dry weight showed that the productivity was severely reduced under low nitrogen or/and water conditions. Additionally the proportion of the different organs contributing to the total biomass, and thus the ratio of root/shoot dry weight shifted substantially. At the same time the production of leaf buds in ratio to leaf biomass was increased. Concentrations of organic nitrogen and nitrate in the shoots were drastically reduced under low nitrogen or/and water conditions, but organic nitrogen concentration in the roots remained relatively high. The leaf conductance over a day was investigated only for K. daigremontiana, and decreased with reduced water and nitrogen supply. The results indicate that CAM plants do not have the predicted high nitrogen use efficiency. Under environmental stress they change from biomass production towards provisions for life preservation.     

Gas exchange characteristics and nitrogen relations of two Mediterranean root hemiparasites:Bartsia trixago andParentucellia viscosa

Plant height, light-saturated rates of photosynthesis (A _max) and foliar nitrogen concentration (N ₁) were measured forBartsia trixago under field conditions in Mallorca. All three variables were postively correlated, and were also positively related to the abundance of nitrogen-fixing legumes in the associated vegetation (putative host species).A _max forB. trixago ranged from 7.7 to 18.8 mol m^-2 s^-1; similar rates were measured for a second hemiparasiteParentucellia viscosa, and both species were within the range of rates measured for six putative hosts (10.6–19.2 mol m^-2 s^-1). Fertilization of unattachedB. trixago plants with inorganic nitrogen (ammonium nitrate) elicited neither the growth nor the photosynthetic responses observed in plants considered to be parasitic on legumes and in receipt of an enriched organic nitrogen supply. Both hemiparasites had high diurnal leaf conductances (g _s) (469–2291 mmol m^-2 s^-1) and were at the upper end of the range of those measured in putative hosts (409–879 mmol m^-2 s^-1). In contrast with the latter, high nocturnal rates ofg _s were also recorded for the two hemiparasites (517–1862 mmol m^-2 s^-1). There was no clear relationship between eitherA _max orN ₁ and eitherg _s, transpiration (E) or water use efficiency (A _max/E) inB. trixago plants. The economics of water loss appear to be independent of both the supply of nitrogen from the host and autotrophic carbon fixation.     

Impact of nitrogen supply on growth,steviol glycosides and photosynthesis in Stevia rebaudiana Bertoni

This work investigated the agronomic, physiological and biochemical response of Stevia rebaudiana Bertoni grown under different nitrogen (N) rates. A pot trial in open air conditions was set up in 2012 with the aim to evaluate the effect of four N rates on the biometric and productive characteristics, steviol glycoside (SG) content as well as on leaf gas exchanges, chlorophyll fluorescence, photosynthetic pigments, Rubisco activity and N use efficiency. N deficiency caused a decrease in leaf N content, chlorophylls and photosynthetic CO₂ assimilation, resulting in a lower dry matter accumulation as well as in reduced SG production. The application of 150 kg N ha^? 1 seems to be the most effective treatment to improve rebaudioside A (Reb A) content, Reb A/stevioside ratio, photosynthetic CO₂ assimilation, stomatal conductance, N use efficiency, ribulose-1,5-bis-phosphate carboxylase/oxygenase (Rubisco) and PSII efficiency. The results demonstrate that by using an appropriate N rate it is possible to modulate the SG biosynthesis, with a significant increase in the Reb A content and in the ratio between Reb A and stevioside. This finding is of great relevance in order to obtain a raw material designed to meet consumer needs and bio-industry requirements for high-quality, Reb A content, and safe and environmentally friendly products.     

Biomass production and nitrogen content of C3- and C4- grasses in pure and mixed culture with different nitrogen supply

Summary Two C₃ grasses (Hordeum vulgare L., Avena sativa L.) and two C₄ grasses (Panicum miliaceum L., Panicum crus-galli L.) were cultivated in standard soil in the open air in pure cultures and in various mixed cultures at low and high nitrogen fertilization levels. After three months the dry weight, length and nitrogen content of the aboveground and below-ground parts of the plants and the shoot/root ratios were determined. Hordeum vulgare was the most successful species irrespective of the nitrogen fertilization level, and also exhibited in most cases the highest nitrogen concentrations. Panicum miliaceum, on the other hand, was the species least able to compete. The production of biomass was reduced in cultures growing under nitrogen starvation conditions, this phenomenon being more pronounced with respect to the C₄ than to the C₃ species. The decrease in the production of biomass at low N conditions was most drastic with Panicum crus-galli, the species with the lowest nitrogen content and thus assumed to be best adapted to nitrogen starvation conditions. In cultures growing at low nitrogen fertilization levels the shoot/root ratios of all species.shifted in favour of an increasing root proportion. The extent of this shift, however, differed from species to species.     

Effect of nitrogen application on growth and photosynthetic nitrogen use efficiency in two ecotypes of wild strawberry, Fragaria chiloensis

The relationships between increasing nitrogen fertilization and growth, maximum CO₂ assimilation and the initial slope of the CO₂ response curve were studied in 2 ecotypes of wild strawberry, Fragaria chiloensis (L.) Duchn. Nitrogen accumulation of CA11, an ecotype from a low-nutrient dune site, was greater at all nitrogen concentrations than that of RCP37, an ecotype from a higher-nutrient strand site. Maximum CO₂ assimilation, total Rubisco activity, dry weight, and initiation of leaves and crowns were higher in CAI1 than RCP37 as nitrogen treatment was increased from 0 to 200 mg ^l-1, whereas these parameters were lower in CAl1 when fertilized at 300 mg T¹, but not in RCP37. The mean leaf area of CA11 was greater than RCP37 when grown with no supplemental nitrogen, but mean leaf area of the 2 lines was similar under nitrogen fertilization. Maximum CO₂ assimilation and carboxylation efficiency increased with increasing leaf nitrogen in both clones. At equivalent concentrations of leaf nitrogen, RCP37 had higher CO₂ assimilation and carboxylation efficiency than CA11 and the difference between the 2 clones increased as ieaf nitrogen increased. Thus, RCP37 had a higher photosynthetic nitrogen use efficiency than CA11. However, at a given applied nitrogen level, CA11 allocated more nitrogen to a unit of leaf area so that photosynthetic rates were higher than RCP37, except at the highest application of 300 mg ^l-1. The high nitrogen accumulation capacity and resource allocation to fruiting structures (crowns) in CA11 leads us to suggest that this clone may possess genes that could increase fruit yield in cultivated strawberry.     

不同施氮量下灌水量对小麦耗水特性和氮素分配的影响

研究了不同施氮量条件下灌水量对高产小麦耗水特性和氮素分配利用的影响。设置4个施氮水平:0kg·hm-2(N0)、120kg·hm-2(N1)、210kg·hm-2(N2)和300kg·hm-2(N3),在每个施氮水平下设置4个灌水量处理:不浇水(W0)、底墒水+拔节水(W1)、底墒水+拔节水+开花水(W2)、底墒水+拔节水+开花水+灌浆水(W3),每次灌水量60mm。结果表明:(1)在N0水平下W0处理日耗水量以拔节至开花期最高,在N1水平下,拔节至开花期日耗水量与开花至成熟期的无显著差异。同一施氮水平下,小麦开花后总耗水量、耗水模系数和日耗水量随灌水量的增加而提高,但产量随灌水量的增加先升高后降低。(2)同一施氮水平下,成熟期W1处理20—140cm各土层土壤含水量低于W2和W3处理,140—200cm土层土壤含水量与W2处理无显著差异;W1处理0—40cm土层土壤硝态氮含量及植株氮素在籽粒中的分配比例高于W2和W3处理,100—140cm土层土壤硝态氮含量及植株氮素在营养器官中的分配量和分配比例低于W2和W3处理。表明灌溉底墒水和拔节水的W1处理,促进了小麦对20—140cm土层土壤水的吸收利用,减少了土壤硝态氮向100cm以下土层的淋溶,而且有利于营养器官中氮素向籽粒的再分配,水分和氮素利用效率较高。(3)在试验条件下,施纯氮210kg·hm-2、灌溉底墒水和拔节水的N2W1处理,籽粒产量最高,水分利用效率和氮素利用效率较高,可供生产中参考。     

Changes in gas exchange characteristics and water use efficiency of mangroves in response to salinity and vapour pressure deficit

Summary Measurements were made of the photosynthetic gas exchange properties and water use efficiency of 19 species of mangrove in 9 estuaries with different salinity and climatic regimes in north eastern Australia and Papua New Guinea. Stomatal conductance and CO₂ assimilation rates differed significantly between species at the same locality, with the salt-secreting species, Avicennia marina, consistently having the highest CO₂ assimilation rates and stomatal conductances. Proportional changes in stomatal conductance and CO₂ assimilation rate resulted in constant and similar intercellular CO₂ concentrations for leaves exposed to photon flux densities above 800 mol·m^-2·s^-1 in all species at a particular locality. In consequence, all species at the same locality had similar water use efficiencies. There were, however, significant differences in gas exchange properties between different localities. Stomatal conductance and CO₂ assimilation rate both decreased with increasing salinity and with increasing leaf to air vapour pressure deficit (VPD). Furthermore, the slope of the relationship between assimilation rate and stomatal conductance increased, while intercellular CO₂ concentration decreased, with increasing salinity and with decreasing ambient relative humidity. It is concluded from these results that the water use efficiency of mangroves increases with increasing environmental stress, in this case aridity, thereby maximising photosynthetic carbon fixation while minimising water loss.Contribution No. 459 from the Australian Institute of Marine Science     

Leaf photosynthesis, plant growth and nitrogen allocation in rice under different irradiances

The photosynthetic rates and various components of photosynthesis including ribulose-1,5-bisphosphate carboxylase (Rubisco; EC 4.1.1.39), chlorophyll (Chl), cytochrome (Cyt) f, and coupling factor 1 (CF₁) contents, and sucrose-phosphate synthase (SPS; EC 2.4.1.14) activity were examined in young, fully expanded leaves of rice (Oryza sativa L.) grown hydroponically under two irradiances, namely, 1000 and 350 μmol quanta · m⁻² · s⁻¹, at three N concentrations. The light-saturated rate of photosynthesis measured at 1800 μmol · m⁻² · s⁻¹ was almost the same for a given leaf N content irrespective of growth irradiance. Similarly, Rubisco content and SPS activity were not different for the same leaf N content between irradiance treatments. In contrast, Chl content was significantly greater in the plants grown at 350 μmol · m⁻² · s⁻¹, whereas Cyt f and CF₁ contents tended to be slightly smaller. However, these changes were not substantial, as shown by the fact that the light-limited rate of photosynthesis measured at 350 μmol · m⁻² · s⁻¹ was the same or only a little higher in the plants grown at 350 μmol · m⁻² · s⁻¹ and that CO₂-saturated photosynthesis did not differ between irradiance treatments. These results indicate that growth-irradiance-dependent changes in N partitioning in a leaf were far from optimal with respect to N-use efficiency of photosynthesis. In spite of the difference in growth irradiance, the relative growth rate of the whole plant did not differ between the treatments because there was an increase in the leaf area ratio in the low-irradiance-grown plants. This increase was associated with the preferential N-investment in leaf blades and the extremely low accumulation of starch and sucrose in leaf blades and sheaths, allowing a more efficient use of the fixed carbon. Thus, morphogenic responses at the whole-plant level may be more important for plants as an adaptation strategy to light environments than a response of N partitioning at the level of a single leaf. Received: 23 February 1997 / Accepted: 8 May 1997     

Herbivory alters plant carbon assimilation,patterns of biomass allocation and nitrogen use efficiency

Herbivory can trigger physiological processes resulting in leaf and whole plant functional changes. The effects of chronic infestation by an insect on leaf traits related to carbon and nitrogen economy in three Prunus avium cultivars were assessed. Leaves from non-infested trees (control) and damaged leaves from infested trees were selected. The insect larvae produce skeletonization of the leaves leaving relatively intact the vein network of the eaten leaves and the abaxial epidermal tissue. At the leaf level, nitrogen content per mass (N_mass) and per area (N_area), net photosynthesis per mass (A_mass) and per area (A_area), photosynthetic nitrogen-use efficiency (PNUE), leaf mass per area (LMA) and total leaf phenols content were measured in the three cultivars. All cultivars responded to herbivory in a similar fashion. The N_mass, A_mass, and PNUE decreased, while LMA and total content of phenols increased in partially damaged leaves. Increases in herbivore pressure resulted in lower leaf size and total leaf area per plant across cultivars. Despite this, stem cumulative growth tended to increase in infected plants suggesting a change in the patterns of biomass allocation and in resources sequestration elicited by herbivory. A larger N investment in defenses instead of photosynthetic structures may explain the lower PNUE and A_mass observed in damaged leaves. Some physiological changes due to herbivory partially compensate for the cost of leaf removal buffering the carbon economy at the whole plant level.     

干旱胁迫对不同施氮水平麻疯树幼苗光合特性及生长的影响

采用盆栽控水的方法,研究了干旱胁迫（连续干旱0 d,5 d,10 d,…,45 d）对不同施氮水平(对照 0 kg N·hm^-2、低氮 96 kg N·hm^-2、中氮 288 kg N·hm^-2、高氮 480 kg N·hm^-2)麻疯树幼苗光合特性及其生长的影响.结果表明: 随干旱胁迫强度的增加,各施氮水平麻疯树幼苗叶片相对含水量、苗高生长量、地径生长量、叶面积、净光合速率、蒸腾速率和气孔导度均降低,且各水分处理间差异极显著(P<0.01);随干旱时间的延长,叶绿素含量和水分利用效率表现出先升高后降低的趋势,而胞间CO₂浓度呈先降低后升高的趋势.正常供水时,施氮处理均不同程度提高了麻疯树幼苗的光合能力,促进了麻疯树幼苗的生长,且施氮量越高效果越好;干旱条件下,氮素营养对植株光合能力和生长的影响与干旱程度和施氮水平有关.轻度干旱时,提高施氮水平对植株光合能力和生长具有明显的促进作用;中度干旱时,中氮的促进作用明显高于其他施氮水平;严重干旱时,低氮的促进效果最好,高氮的促进作用减弱并逐渐转向抑制.     

毛乌素沙地中间锦鸡儿幼苗生长、气体交换和叶绿素荧光对模拟降水量变化的响应

选择毛乌素沙地优势灌木中间锦鸡儿为研究对象,人为控制4种降水量水平来测定中间锦鸡儿幼苗的形态,。生物量干重,气体交换和叶绿素荧光参数,结果表明,不同施水量显著影响幼苗株高,基径,叶数,叶面积,生物量干重,净光合速率,蒸腾速率,气孔导度以及水分利用效率,157．5mm和630mm施水的幼苗以上参数显著低于472．5mm施水的幼苗,157．5mm和630mm施水量的幼苗PSIII光化学效率和最大荧光与初始荧光比显著小于472．5mm施水量的幼苗;不同施水量对幼苗生物量干重分配有显著影响;随着施水量增加,根冠生物量干重比显著减少。     

Growth, gas exchange and water use efficiency of the facultative hemiparasite Rhinanthus minor associated with hosts differing in foliar nitrogen concentration

The facultative hemiparasitic angiosperm Rhinanthus minor was grown on 11 different host species, and in the absence of a host. The height of R. minor parasitising the legume Trifolium pratense exceeded that of unattached plants by more than an order of magnitude, with performance on grasses and non-legume dicotlyedonous hosts between these two extremes. Light saturated rates of photosynthesis in R. minor on different hosts ranged from 1. 5 to 22. 5 μmol g⁻¹ dry weight min⁻¹, and were positively correlated with growth. Foliar nitrogen concentrations in the parasite exceeded those of the hosts. The former were positively related to light saturated rates of photosynthesis, and inversely related to photosynthetic nitrogen use efficiency. There was no relationship between leaf nitrogen concentration and water use efficiency in R. minor. The data are discussed in relation to studies of nitrogen and water use in mistletoes.     

氮肥运筹对小麦产量、氮素利用效率和光能利用率的影响

连续2年在西南冬麦区的重庆、仁寿、广汉、西昌4个地点,开展3种施氮水平(每公顷纯氮0、120、180 kg,简写为N₀、N₁₂₀、N₁₈₀)和3种氮肥分配模式(N_A:底肥100%;N_B:底肥70%+苗肥30%;N_C:底肥60%+拔节肥40%)的田间试验,监测小麦花后冠层叶片SPAD值、群体光合速率(CAP)、光能利用等生理参数和籽粒产量,计算氮素利用效率、光能利用率等.结果表明: 随施氮水平增加,小麦上三叶SPAD值、CAP、光合有效辐射(PAR)截获率和产量均呈增加趋势,而氮肥农学利用效率、生产效率、吸收效率和利用效率呈降低趋势.氮肥后移的增效作用因施氮水平而异,SPAD于N₁₈₀增效明显,而CAP于N₁₂₀增效明显,不同氮肥管理模式的光能利用率因地点而异.氮肥后移能明显提高小麦氮肥农学效率、生产效率、吸收效率和氮素表观回收率,但氮肥利用效率则略有减少.氮肥后移效果N_C总体优于N_B处理.不同地点比较,广汉的SPAD值、CAP、PAR截获率、氮肥利用参数较高,其产量也相应最高;西昌的产量、SPAD值及氮素利用效率较高,但其光能利用率和CAP较低;重庆和仁寿的SPAD值、光能利用率及氮素利用效率均较低,其产量也最低.小麦生物产量与各地点的籽粒产量、CAP、SPAD值和PAR累积截获量均呈显著或极显著的正相关关系.表明不同生态区域增施氮肥都能促进小麦增产,氮肥后移可进一步优化产量结构、改善氮肥和光能利用效率,但存在年份和地点差异,各地需要制定有针对性的氮肥管理模式.     

Model of gas exchange and diffusion in legume nodules

A mathematical model is described which allows the estimation of rates of O₂, CO₂, N₂, and H₂ exchange from legume nodules under steady state conditions of N₂ fixation. Calculated rates of gas exchange under defined conditions of nodule size, relative growth rate (RGR), specific total nitrogenase activity (TNA), nitrogenase electron allocation coefficient (EAC), uptake-hydrogenase activity (HUP) and nature of the N export product compared favorably with experimentally-obtained rates reported in the literature. Therefore the model was used to predict the effects of varying each of these nodule characteristics on the rates of gas exchange, and on the apparent respiratory cost (CO₂/NH₃) and sucrose cost (sucrose consumed/NH₃) of N₂ fixation.The model predicted that, all other characters being equal, ureide-producing nodules would consume 8% less sucrose per N fixed than asparagine-producing nodules, but would display an apparent respiratory cost which would be 5% higher than that in asparagine-producing nodules. In both ureide-producing and asparagine-producing nodules, the major factor affecting the apparent respiratory cost of N₂ fixation was predicted to be EAC, followed by TNA, nodule RGR and nodule size. The relative importance of HUP in improving the apparent respiratory cost of N₂ fixation was predicted to be largely dependent upon its potential role in the regulation of EAC. Abbreviations: See Appendix 1.     

Effects of sand burial on survival, growth, gas exchange and biomass allocation of Ulmus pumila seedlings in the Hunshandak Sandland, China

BACKGROUND AND AIMS: In the last decade, the number of young plants of Ulmus pumila in the Hunshandak Sandland has decreased sharply because of severe sand burial, and their ecological protective function has been weakened. In order to develop an understanding of the tolerance of U. pumila to sand burial and to suggest reasonable measures to protect the sparse-elm-grassland ecosystem, the effects of burial on the survival, growth, photosynthesis and biomass allocation in U. pumila were studied. METHODS: Seedlings were buried at five different depths in pot experiments: no burial (control), partial burial (33 % and 67 % stem height), and complete burial (100 % and 133 % stem height). Growth analyses and measurements of photosynthesis were carried after the plants had been uncovered. KEY RESULTS: All the plants survived partial burial, but about 30 % and 80 % of the seedlings died as a result of the 100 % and 133 % sand burial treatments, respectively. The numbers of newly produced leaves and branches, and the height of the stems of the seedlings in the 33 % and 67 % burial treatments during the period of the experiment were significantly greater than those in the control. Furthermore, net photosynthetic rate, transpiration rate and water use efficiency were also elevated by the partial burial, but not affected by burial time. This might be attributed to the increased root length, which improved water acquisition. The biomass and biomass allocation of the seedlings were significantly changed by the burial treatments and burial times. The biomass was enhanced by partial burial but was reduced by complete burial at each burial time. However, the biomass allocation was not significantly changed by the 33 % and 67 % sand burial treatments 2 or 4 weeks following the burial. CONCLUSIONS: Ulmus pumila was shown to be tolerant to partial sand burial, but must be protected from complete burial.     

Comparative study of water uptake and photosynthetic gas exchange between scrub and fringe red mangroves,Rhizophora mangle L.

Summary The red mangrove (Rhizophora mangle L.) occurs frequently in both scrub and fringe mangrove forests. Our previous study demonstrated that individuals of this mangrove species growing in scrub and fringe forests differ significantly in both morphological and physiological characteristics. To further characterize physiological differences between scrub and fringe mangroves, we compared their differences in water uptake and photosynthetic gas exchange during different seasons. In the wet season (June–October, 1990), scrub mangroves showed lower D and ¹⁸O values of stem water than fringe mangroves, indicating more usage of rain-derived freshwater. In the dry season (Jan–April, 1991), however, scrub mangroves utilized the same water source as fringe mangroves, reflected by their similar D and ¹⁸O values of stem water. Consistently, there were significant differences in predawn water potentials between scrub and fringe mangroves in the wet season (October 1990) with higher values for scrub mangroves, but no significant differences in the dry season (January 1991). Higher elevation in the scrub forest seems to be the major factor responsible for the shift of water sources in scrub mangroves. On Apr. 27 and Aug. 8, 1990, scrub mangroves showed lower CO₂ assimilation rate, stomatal conductance, and intercellular CO₂ concentration than fringe mangroves. There were no differences in these gas exchange characteristics on the other two measuring dates: Oct. 17, 1990 and Jan. 11, 1991. Instantaneous water use efficiency was significantly higher for scrub mangroves than for fringe mangroves on three of the four sampling dates. Similarly, leaf carbon isotope discrimination of scrub mangroves was always significantly lower than that of fringe mangroves, indicating higher long-term water use efficiency. Higher water use efficiency in scrub mangroves is a result of stomatal limitation on photosynthesis, which may entail considerable carbon cost to the plants.     

Effects of a nitrogen fixing plant Vigna radiata on growth,leaf stomatal gas exchange and hydraulic characteristics of the intercropping Juglans regia seedlings

采用砂培方法, 在温室内将一年生核桃(Juglans regia)嫁接苗木和绿豆(Vigna radiata)进行间作, 研究绿豆对核桃苗木生长、水分平衡和光合特性的影响。该研究设有5种处理, 即: 对照(核桃单作, 正常供应氮素); 核桃单作, 不添加氮素; 核桃绿豆间作, 不添加氮素; 核桃绿豆间作, 正常供应氮素; 绿豆单作, 不添加氮素。结果显示: 种植绿豆可以增加土壤氮含量和核桃茎内氮含量, 但对核桃叶和根系中的氮含量影响不明显。种植绿豆显著增加不施氮核桃的高生长和直径生长, 但降低了正常供氮核桃的生长。无论种植绿豆与否, 不供氮处理降低了核桃的总叶面积, 提高了根冠比。核桃叶片气孔气体交换对各处理的响应和生长有相同的趋势。缺氮显著降低了核桃叶柄在中午的导水率、提高了导水损失率; 种植绿豆显著提高不供氮核桃的导水率而且明显降低了其导水损失率。然而, 种植绿豆使正常供氮的核桃降低了导水率, 加剧了导水损失率。同时, 绿豆受到间作的竞争压力, 产量和生物量有所下降。由研究结果可知, 在贫瘠的土壤上, 固氮植物绿豆改善了间作的核桃的氮营养, 有益于核桃木质部发育、水分平衡以及光合代谢。但是在氮充足的土壤中, 种植绿豆反而降低了核桃的水分供应, 影响其气体交换和生长。