Relationships Between Nitrogen Content and Net Gas Exchange Components of a Cotton Leaf During Ontogeny

Relationships between leaf nitrogen (N) content and leaf gas exchange components of a single cotton (Gossypium hirsutum L.) leaf subtending the fruit during ontogeny were investigated under field conditions. A 20-d old leaf exhibited the highest physiological activity characterized by net photosynthetic (PN) and transpiration (E) rates, stomatal conductances to CO2 exchange (gsCO2) and water vapor transfer (gsH2O), and nitrogen (N) content. With the advent of leaf senescence, the gas exchange rates declined as exhibited by the 30-, 40-, and 60-d old leaves. Regression analysis indicated close relationships between gsCO2 and PN, and gsH2O and E as the leaves advanced towards senescence. Both PN and gsCO2 were related to N as they declined with leaf age. Thus, the declines in PN were associated with stomatal closure and removal of N during leaf ontogeny.     

Changes of Photosynthetic Characteristics in Relation to Leaf Senescence in Two Maize Hybrids with Different Senescent Appearance

A field experiment was conducted to investigate the changes in chlorophyll (Chl) and nitrogen (N) contents, ribulose-1,5-bisphosphate carboxylase/oxygenase (RuBPCO) and phosphoenolpyruvate carboxylase (PEPC) contents and PEPC activity, and the photon-saturated net photosynthetic rate (P _Nsat), and their relationships with leaf senescence in two maize hybrids with different senescent appearance. One stay-green (cv. P3845) and one earlier senescent (cv. Hokkou 55) hybrid were used in this study, and we found that Chl and N contents and the P _Nsat in individual leaves of P3845 were greater than those in corresponding leaves of Hokkou 55 at the successive growth stages. In addition, larger contents of RuBPCO and PEPC, and a greater activity of PEPC were observed in P3845. Due to the lower rates of decrease of Chl, RuBPCO, and PEPC amounts per unit of N, and the lower net C translocation rate per unit of N in the stay-green hybrid, leaf senescence was delayed in comparison to the earlier senescent hybrid.     

Mineral Depletion and Leaf Senescence in Soya Bean as Influenced by Foliar Nutrient Application During Seed Filling

Senescence, as judged by the time courses of leaf lamina photosynthesis,soluble protein and chlorophyll contents, was studied in relationto mineral redistribution in field-grown soya beans [Glycinemax (L.) Merr] to investigate the hypothesis that the depletionof nutrients m the leaves by the developing seeds is the causeof soya bean senescence. A mineral nutrient solution was appliedto the canopy during the seed-filling period, and the effectson senescence and mineral depletion of the leaves were determinedin three cultivars, at two leaf positions, weekly from beginningof seed filling through physiological maturity. The onset of senescence occurred shortly after the beginningof rapid seed filling Photosynthetic rate declined about 60per cent within 3 weeks. Protein dropped by 52 per cent andchlorophyll by 48 per cent over the same period. Foliar nutrient application, at a rate previously shown to givesignificant yield increases in soya beans, increased the concentrationsof N, P and K in the leaf laminae, but tended only to delaytheir decline and failed to either delay the onset or alterthe course of senescence. The results of this experiment seem to indicate that, undernormal growth conditions, the events of senescence in the soyabean are not causally related to the N, P or K concentrationsof the leaf laminae Glycme max (L.) Merr., soya bean, nitrogen, phosphorus, potassium, leaf protein, chlorophyll, photosynthesis, foliar nutrient application, mineral depletion, leaf senescence     

Association of red coloration with senescence of sugar maple leaves in autumn

We evaluated the association of red coloration with senescence in sugar maple (Acer saccharum Marsh.) leaves by assessing differences in leaf retention strength and the progression of the abscission layer through the vascular bundle of green, yellow, and red leaves of 14 mature open-grown trees in October 2002. Computer image analysis confirmed visual categorization of leaves as predominantly green, yellow or red, and chemical quantification of leaf pigment concentrations verified that leaf color reflected underlying differences in leaf biochemistry. Significantly lower chlorophyll concentrations within red and yellow leaves indicated that senescence was more advanced in leaves from these color categories relative to green leaves. Among leaf types, only red leaves contained high concentrations of anthocyanins. There were significant differences in leaf retention capacity among color categories, with the petioles of green leaves being the most firmly attached to twigs, followed by red and then yellow leaves. Microscopic analysis indicated that yellow leaves had the most advanced extension of the abscission layer through the vasculature, with green and red leaves having significantly less abscission layer progression than yellow. A more limited progression of the abscission layer through vascular bundles may be evidence of delayed leaf senescence that could extend resorption of mobile leaf constituents. Together, results from this study suggest an association between leaf anthocyanin content and functional delays in senescence.     

Both xanthophyll cycle-dependent thermal dissipation and the antioxidant system are up-regulated in grape (Vitis labrusca L cv Concord) leaves in response to N limitation

One-year-old grapevines (Vitis labrusca L. cv. Concord) were supplied with 0, 5, 10, 15, or 20 mM nitrogen (N) in a modified Hoagland's solution twice weekly for 4 weeks. As leaf N decreased in response to N limitation, leaf chlorophyll (Chl) decreased linearly whereas leaf absorptance declined curvilinearly. Compared with high N leaves, low N leaves had lower quantum efficiency of PSII as a result of both an increase in non-photochemical quenching (NPQ) and an increase in closure of PSII reaction centres at midday under high photon flux density (PFD). Both the xanthophyll cycle pool size on a Chl basis and the conversion of violaxanthin (V) to antheraxanthin (A) and zeaxanthin (Z) at noon increased with decreasing leaf N. NPQ was closely related to A+Z expressed either on a Chl basis or as a percentage of the xanthophyll cycle pool. As leaf N increased, superoxide dismutase (SOD) activity on a Chl basis decreased linearly; activities of catalase (CAT) and glutathione reductase (GR) on a Chl basis increased linearly; activities of ascorbate peroxidase (APX), monodehydroascorbate reductase (MDAR) and dehydroascorbate reductase (DHAR) expressed on the basis of Chl decreased rapidly first, then gradually reached a low level. In response to N limitation, the contents of ascorbate (AsA), dehydroascorbate (DAsA), reduced glutathione (GSH), and oxidized glutathione (GSSG) increased when expressed on a Chl basis, whereas the ratios of both AsA to DAsA and GSH to GSSG decreased. It is concluded that, in addition to decreasing light absorption by lowering Chl concentration, both xanthophyll cycle-dependent thermal energy dissipation and the antioxidant system are up-regulated to protect low N leaves from photo-oxidative damage under high light.     

Proteomic analysis of residual proteins in blades and petioles of fallen leaves of Brassica napus

Brassica napus L. is an important crop plant, characterised by high nitrogen (N) levels in fallen leaves, leading to a significant restitution of this element to the soil, with important consequences at the economic and environmental levels. It is now well established that the N in fallen leaves is due to weak N remobilisation that is especially related to incomplete degradation of foliar proteins during leaf senescence. Identification of residual proteins in a fallen leaf (i.e. incompletely degraded in the last step of the N remobilisation process) constitutes important information for improving nutrient use efficiency. Proteome analysis of the vascular system (petioles) and blades from fallen leaves of Brassica napus was performed, and the 30 most abundant residual proteins in each tissue were identified. Among them, several proteins involved in N recycling remain in the leaf after abscission. Moreover, this study reveals that some residual proteins are associated with energy metabolism, protection against oxidative stress, and more surprisingly, photosynthesis. Finally, comparison of blade and petiole proteomes show that, despite their different physiological roles in the non‐senescing leaf, both organs redirect their metabolism in order to ensure catabolic reactions. Taken together, the results suggest that a better degradation of these leaf proteins during the senescence process could enable improvements in the N use efficiency of Brassica napus.     

Changes in the Levels of Chlorophyll and Light-Harvesting Chlorophyll a/b Protein of PS II in Rice Leaves Aged under Different Irradiances from Full Expansion through Senescence

Effects of irradiance on changes in the amounts of chlorophyll(Chl) and light-harvesting chlorophyll a/b protein of PS II(LHCII) were examined in senescing leaves of rice (Oryza sativaL.). Results of treatments at two irradiances (100% and 20%natural sunlight) were examined after the full expansion ofthe 13th leaf throughout the course of senescence. With 20%sunlight, the Chl content decreased only a little during leafsenescence, while with 100% sunlight it decreased appreciably.Similarly, the amount of LHCII protein during treatment with20% sunlight remained almost constant. However, the ratio ofChl a/b during the shade treatment decreased significantly andthe rate of decrease was greater than during the full-sunlighttreatment. The ratio of Chl a/b for Chl a and b bound to LHCIIwas about 1.2, irrespective of leaf age or irradiance treatment.When the amounts of Chl bound to LHCII were calculated fromthe total leaf content of Chl and the ratio of Chl a/b, assuminga ratio of Chl a/b bound to LHCII of 1.2, they were well correlatedwith the amounts of LHCII protein. Changes in the amounts of LHCII synthesized during the two irradiancetreatments were examined using an ¹⁵ tracer. Incorporation of¹⁵N into LHCII declined dramatically during both treatmentsfrom full expansion through senescence, suggesting that therewas little synthesis of LHCII protein during that time. In addition,the amount of LHCII synthesized during senescence was lowerduring the shade treatment than during the 100% sunlight treatment.These results indicate that the absence of an apparent changein levels of LHCII with shade treatment during senescence wascaused by the very low rate of turnover of LHCII protein. (Received June 17, 1992; Accepted September 28, 1992)     

Tannins and nitrogen dynamics in mangrove leaves at different age and decay stages (Jiulong River Estuary,China)

Changes in the total phenolics, condensed tannins (CT), protein-precipitable phenolics content and protein precipitation capacity were determined on mangrove leaves representing a range of maturation and senescence in the Jiulong River Estuary, Fujian, China. The results showed that the total phenolics, extractable condensed tannins, total condensed tannins, protein-precipitable phenolics content and protein precipitation capacity in young leaves were higher than those in mature and senescent leaves. With leaf maturation and senescence, total pehnolics decreased. The rapid loss of phenolics observed following senescence and abscission can be ascribed to leaching and degradation. Protein-bound CT and fibre-bound CT tended to increase with leaf maturity, and decreased with senescence. Protein-bound CT and fibre-bound CT increased with leaf decomposition, with CT binding more strongly to protein than to fibre. The increases in nitrogen contents and ash free caloric values, and declines in total phenolics and total condensed tannins of mangrove leaf litter suggests that partially decomposed mangrove detritus is a palatable heterotrophic substrate, and thus may be an important source of matter and energy for the estuarine food web. Handling editor: K. Martens     

Carbon/Nitrogen Imbalance Associated with Drought-Induced Leaf Senescence in Sorghum bicolor

Drought stress triggers mature leaf senescence, which supports plant survival and remobilization of nutrients; yet leaf senescence also critically decreases post-drought crop yield. Drought generally results in carbon/nitrogen imbalance, which is reflected in the increased carbon:nitrogen (C:N) ratio in mature leaves, and which has been shown to be involved in inducing leaf senescence under normal growth conditions. Yet the involvement of the carbon/nitrogen balance in regulation of drought-induced leaf senescence is unclear. To investigate the role of carbon/nitrogen balance in drought-induced senescence, sorghum seedlings were subjected to a gradual soil drought treatment. Leaf senescence symptoms and the C:N ratio, which was indicated by the ratio of non-structural carbohydrate to total N content, were monitored during drought progression. In this study, leaf senescence developed about 12 days after the start of drought treatment, as indicated by various senescence symptoms including decreasing photosynthesis, photosystem II photochemistry efficiency (Fv/Fm) and chlorophyll content, and by the differential expression of senescence marker genes. The C:N ratio was significantly enhanced 10 to 12 days into drought treatment. Leaf senescence occurred in the older (lower) leaves, which had higher C:N ratios, but not in the younger (upper) leaves, which had lower C:N ratios. In addition, a detached leaf assay was conducted to investigate the effect of carbon/nitrogen availability on drought-induced senescence. Exogenous application of excess sugar combined with limited nitrogen promoted drought-induced leaf senescence. Thus our results suggest that the carbon/nitrogen balance may be involved in the regulation of drought-induced leaf senescence.     

Relationship between Photosynthetic Activity and Chlorophyll Content in an Isolated Quercus Ilex L. Tree during the Year

The relationship between chlorophyll (Chl) content and net photosynthetic rate (PN) in an isolated Quercus ilex tree, growing inside Villa Pamphili Park in Rome, was explored. The highest PN was in March, May, and September (10.1 mol m-2 s-1, maximum rate). PN decreased by 65 % (with respect to the yearly maximum) when leaf temperature reached 34 °C, and by 50 % when leaf temperature was 9 °C. The highest Chl contents were in April, October [1.47 g kg-1 (d.m.), maximum value], and December. The lowest Chl content was found in July (0.78 g kg-1). The decrease of PN in July was in close connection with the decrease of Chl content. On the contrary, the high Chl content during winter did not correspond with PN of this season. Discordances between Chl content and PN over the year influenced the regression analysis, which although positive did not show very high correlation coefficients (r = 0.7). The high Chl (a+b) content during most of the year indicated that the photosynthetic apparatus remained basically intact also during stress periods.     

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。     

Influence of Potassium Deficiency on Photosynthesis,Chlorophyll Content,and Chloroplast Ultrastructure of Cotton Plants

In cotton (Gossypium hirsutum L.) grown in controlled-environment growth chamber the effects of K deficiency during floral bud development on leaf photosynthesis, contents of chlorophyll (Chl) and nonstructural saccharides, leaf anatomy, chloroplast ultrastructure, and plant dry matter accumulation were studied. After cotton plants received 35-d K-free nutrient solution at the early square stage, net photosynthetic rate (P _N) of the uppermost fully expanded main-stem leaves was only 23 % of the control plants receiving a full K supply. Decreased leaf P _N of K-deficient cotton was mainly associated with dramatically low Chl content, poor chloroplast ultrastructure, and restricted saccharide translocation, rather than limited stomata conductance in K-deficient leaves. Accumulation of sucrose in leaves of K-deficient plants might be associated with reduced entry of sucrose into the transport pool or decreased phloem loading. K deficiency during squaring also dramatically reduced leaf area and dry matter accumulation, and affected assimilate partitioning among plant tissues.     

The Anticyclic Timing of Leaf Senescence in the Parasitic Plant Viscum album Is Closely Correlated with the Selective Degradation of Sulfur-Rich Viscotoxins

Leaf senescence and abscission have been studied in the semi-parasitic plant mistletoe (Viscum album). Leaf senescence and abscission occur in the summer, when the metabolic activity of the host has reached its maximum. In contrast with their hosts, mistletoes selectively degrade only one major leaf protein during leaf senescence, the sulfur-rich viscotoxin, whereas most of the remaining leaf proteins are lost during abscission. The changes in viscotoxin content are paralleled by changes in the concentration of the corresponding mRNA. Shortly before the onset of leaf senescence, the mRNA for viscotoxin has disappeared from the leaves. The anticyclic timing of leaf senescence and the degradation of only one major leaf protein seems to reflect an adaptation of the parasite to its habitat.     

砂仁不同叶位叶片的光合作用和氧化胁迫

衰老时砂仁叶片Pmax降低,这与叶片Gs、Chi含量和可溶性蛋白质含量的降低有关.随着叶片的衰老,NPQ、AQY、F/Fm、φPsIl和qp均降低,热耗散减少,光抑制加剧,衰老后期出现光破坏.但这些参数下降的幅度均小于Pmax下降幅度.光暗反应失衡,活性氧生成增加.衰老初期(老化)叶片MDA含量没有升高,衰老中后期叶片MDA含量显著升高,表明老化叶片能有效地耗散或清除活性氧,衰老叶片则不能,尽管其sOD、APX和POD等抗氧化酶活力显著升高.上述结果表明砂仁叶片老化与氧化胁迫关系不大,衰老与氧化胁迫密切相关.     

Phosphate Deficiency in Maize. V. Mobilization of Nitrogen and Phosphorus within Shoots of Young Plants and its Relationship to Senescence

The effects of phosphorus (P) deprivation on the changes inthe contents of reduced nitrogen (N) and P in the shoots ofyoung maize plants (Zea mays L.) were investigated for up to30 days after planting (DAP). P deprivation decreased the freshweight and the contents of Chl, total P, esterified P, acid-solubleP_i, total reduced N, soluble protein N, and insoluble proteinN in the entire shoot. Thirty DAP, in the fifth leaf blade,which was expanding, P deprivation greatly decreased the concentrationof acid-soluble P_i but had a smaller effect on the concentrationof esterified P and little or no effect on the concentrationsof Chl, total reduced N, soluble protein N, and insoluble proteinN. By contrast, in the fully expanded second leaf blade, P deprivationgreatly decreased the contents of acid-soluble P_i, esterifiedP, Chl, total reduced N, soluble protein N, and insoluble proteinN at 30 DAP. It is concluded that P deprivation induced theearly initiation and accelerated remobilization of N from oldleaf blades. Since this phenomenon is a critical feature ofleaf senescence, it is suggested that the P status of leavesis involved in the regulation of leaf senescence in maize. (Received March 23, 1995; Accepted June 19, 1995)     

The expression patterns of SAG12/Cab genes reveal the spatial and temporal progression of leaf senescence in Brassica napus L. with sensitivity to the environment

Despite a high nitrate uptake capacity, the nitrogen use efficiency (NUE) of oilseed rape is weak due to a relatively low N remobilization from vegetative (mostly leaves) to growing parts of the plant. Thus, this crop requires a high rate of N fertilization and leaves fall with a high N content. In order to reduce the rate of N fertilization and to improve the environmental impact of oilseed rape, new genotypes could be selected on their capacity to mobilize the foliar N. Various indicators of leaf senescence in oilseed rape were analysed during plant growth, as well as during senescence induced by N deprivation. Metabolic changes in leaves of increasing age were followed in N-supplied and N-deprived rosettes by measuring chlorophyll, total N, and soluble protein contents. Similarly, the expression of genes known to be up-regulated (SAG12) or down-regulated (Cab) during leaf senescence was monitored. The amount of soluble proteins per leaf was a better indicator of leaf senescence than chlorophyll or total N content, but was not evaluated as an accurate indicator under conditions of N deprivation. On the other hand, up-regulation of SAG12 concomitantly with down-regulation of Cab in the leaf revealed the spatial and temporal progression of leaf senescence in oilseed rape. This study shows, for the first time at the whole plant level, that the SAG12/Cab gene expressions match the sink/source transition for N during both developmental and nutrient stress-induced leaf senescence.     

Source-sink manipulation effects on postanthesis photosynthesis and grain setting on spike in winter wheat

Source-sink manipulation could regulate the net photosynthetic rate (PN) of winter wheat after anthesis, however, the direction and magnitude of the regulation varied with time after anthesis. The PN was significantly increased by source reduction at the initial time of grain filling, but sink reduction had little influence on the PN, which suggested that the sink (spike) limitation did not occur at this time. Source-sink relation markedly affected PN during rapid grain filling. The PN was increased by source reduction and decreased by sink reduction significantly, which indicated that PN was closely associated with the change of source or sink size. The effect of source-sink manipulation on PN had some relationship with the occurrence of plant senescence at the time of late grain filling. Source reduction accelerated the senescence and dropped the PN, meanwhile, sink reduction delayed the senescence and promoted the PN. A direct relation between the effect of source-sink manipulation on PN and stomatal limitation was not found. Removing one quarter of leaves (RQ) had little influence on spike development after anthesis. In this case there was enough compensation in source production through photosynthesis. Removing one half of leaves (RH) made grain mass per spike and mass of grains lowered, especially the grain mass in the top and base positions of spike declined markedly. The source supply was grain-limiting. Removing one quarter of spikelets (RS) was beneficial to grain-setting in the remaining spikelets, leading to the increase of grain mass. Thus promoting the source supply of photosynthates after anthesis is of major importance for grain to set and to develop.     

Nitrogen recycling and remobilization are differentially controlled by leaf senescence and development stage in Arabidopsis under low nitrogen nutrition

Five recombinant inbred lines (RILs) of Arabidopsis (Arabidopsis thaliana), previously selected from the Bay-0 x Shahdara RIL population on the basis of differential leaf senescence phenotypes (from early senescing to late senescing) when cultivated under nitrogen (N)-limiting conditions, were analyzed to monitor metabolic markers related to N assimilation and N remobilization pathways. In each RIL, a decrease of total N, free amino acid, and soluble protein contents with leaf aging was observed. In parallel, the expression of markers for N remobilization such as cytosolic glutamine synthetase, glutamate dehydrogenase, and CND41-like protease was increased. This increase occurred earlier and more rapidly in early-senescing lines than in late-senescing lines. We measured the partitioning of (15)N between sink and source leaves during the vegetative stage of development using (15)N tracing and showed that N remobilization from the source leaves to the sink leaves was more efficient in the early-senescing lines. The N remobilization rate was correlated with leaf senescence severity at the vegetative stage. Experiments of (15)N tracing at the reproductive stage showed, however, that the rate of N remobilization from the rosettes to the flowering organs and to the seeds was similar in early- and late-senescing lines. At the reproductive stage, N remobilization efficiency did not depend on senescence phenotypes but was related to the ratio between the biomasses of the sink and the source organs.