Effect of source-sink relations and nitrogen nutrition on senescence and N remobilization in the flag leaf of wheat

The relation between the source-sink ratio and nitrogen nutrition on grain yield of wheat ( Triticum aestivum L. cv. Klein Chamaco) was studied in a greenhouse experiment. Plants were grown until anthesis in pots with soil fertilized with 0.16 mmol N per plant twice a week. At anthesis, all leaves but the flag leaf were excised in a group of plants. In another group the treatment consisted in a similar defoliation plus the longitudinal excision of half the ear, while a third group was left untouched as a control. At the same time, the N supply to half of the plants in each group was interrupted, while the other half continued receiving 16 m M N. The defoliated plants showed a longer functional life of the flag leaf than the control, retaining the chlorophyll, soluble proteins and total reduced nitrogen for a longer time. The ear-excised plants showed an intermediate behavior. The plants with the interrupted N supply showed a faster leaf senescence than the N supplied ones, and this coincided with an increase in the proteolytic activity and nitrogen transport to the ear. However there were no differences in ear weight between the two nitrogen treatments. It is concluded that leaves and ear compete for the nitrogen, and that a low level of carbohydrates in the flag leaf, due to a low source-sink ratio, delays leaf senescence.     

Changes in photosystem II function during senescence of wheat leaves

Analyses of chlorophyll fluorescence were undertaken to investigate the alterations in photosystem II (PSII) function during senescence of wheat ( Triticum aestivum L. cv. Shannong 229) leaves. Senescence resulted in a decrease in the apparent quantum yield of photosynthesis and the maximal CO₂ assimilation capacity. Analyses of fluorescence quenching under steady‐state photosynthesis showed that senescence also resulted in a significant decrease in the efficiency of excitation energy capture by open PSII reaction centers (F'_v/F'_m) but only a slight decrease in the maximum efficiency of PSII photochemistry (F'_v/F'_m). At the same time, a significant increase in non‐photochemical quenching (q_N) and a considerable decrease in photochemical quenching (q_P) were observed in senescing leaves. Rapid fluorescence induction kinetics indicated a decrease in the rate of Q_A reduction and an increase in the proportion of Q_B‐non‐reducing PSII reaction during senescence. The decrease in both F'_v/F'_m and q_P explained the decrease in the actual quantum yield of PSII electron transport ((φ_PSII). We suggest that the modifications in PSII function, which led to the down‐regulation of photosynthetic electron transport, would be in concert with the lower demand for ATP and NADPH in the Calvin cycle which is often inhibited in senescing leaves.     

不同小麦品种氮效率与氮吸收对氮素供应的响应及生理机制

以具有典型特征的不同氮效率小麦品种为材料,研究了低氮和高氮条件下小麦的生物学性状、生理参数和氮同化代谢酶活性.结果表明:低氮条件下,不同氮效率小麦品种根系干质量、茎叶干质量、植株氮累积量基本上为氮高效品种>中效品种>低效品种.低氮条件下,氮吸收高效品种（冀97-6360）的根系活跃吸附面积、TTC还原力、叶片硝酸还原酶活性和叶片NO₃^-含量最大;生理高效品种(石新5418)具有较高的叶片亚硝酸还原酶活性和谷氨酰胺合成酶活性,较低的植株全氮含量、叶片NO₃^-含量和硝酸还原酶活性.低氮条件下植株氮利用效率与氮吸收系数显著相关.不同小麦品种在高氮条件下的生物学性状、生理参数和氮同化代谢酶活性与低氮条件下不尽一致.     

Ammonium and amino acid metabolism in excised leaves of wheat (Triticum aestivum) senescing in the dark

Several parameters of amino acid metabolism were studied in detached primary leaves of wheat (Triticum aestivum L. cv. Castell) during a 14 day incubation period in the dark. Protein loss was accompanied by a 5-fold increase in the total amount of free amino acids during the first 4 days of the incubation period with asparagine being the most important. Beyond this stage a pronounced intracellular accumulation of ammonium occured. A gradual decrease in the levels of free amino acids and ammonium at the later stages of senescence could in part be accounted for by leakage from the leaves. Additionally, some nitrogen was lost due to ammonia volatilization. The rapid decay of the glutamine synthetase (GS; EC 6.3.1.2)-glutamate synthase (Fd-GOGAT; EC 1.4.7.1) system and the fast decline of glutamate-pyruvate transaminase (GPT; EC 2.6.1.2) activity appear to be predominant features of senescence in the dark. Decreasing Fd-GOGAT activity was slightly compensated by a small and temporary increase in the activity of NADH-GOGAT (EC 1.4.1.14). Glutamateoxalocetate transaminase (GOT: EC 2.6.1.1) activity, although declining continuously, proved to be much more persistent. Changes in glutamate dehydrogenase (GDH; EC 1.4.1.3) activity closely resembled the profile of ammonium evolution in the leaves and NADP-isocitrate dehydrogenase (IDH; EC 1.1.1.42) activity revealed a temporary maximum during the period of rapid increase in GDH activity. Increased activity of GDH could also be induced by exogenous ammonium. Ammonium accumulation could, at least partly, be caused by increased asparaginase (EC 3.5.1.1) activity which accompanied the rapid conversion of asparagine to aspartic acid. Asparagine aminotransferase (EC 2.6.1.14) activity declined sharply from the beginning of the senescence period. Although the activity profile of glutaminase (EC 3.5.1.2) was similar to that of asparaginase, glutamine was of little importance quantitatively and an analogous relationship between glutamine and glutamic acid could not be detected.     

施氮量和花后土壤含水量对小麦旗叶光合特性和产量的影响

在防雨池栽条件下,研究了施氮量和花后土壤含水量对小麦旗叶光合特性和产量的影响。结果表明,在相同土壤含水量下,小麦旗叶SPAD值、光合速率、Fv／Fo和Fv／Fm均表现为随着施氮量的增加而升高;增加施氮量有利于提高穗数,但过多（300kg／hm^2）或过少（150kg/hm^2）施氮均不利于穗粒数和干粒重的提高,而导致减产。在相同施氮量下,均表现为花后土壤含水量60％～70％处理的旗叶SPAD值、光合速率、Fv／Fo和Fv／Fm最高,40％～50％处理最低,80％～90％处理居中,花后土壤含水量过高（80％～90％）或过低（40％-50％）导致穗粒数减少,千粒重降低,最终使产量降低。表明花后土壤含水量过高或过低均影响小麦旗叶的光合特性降低,而适当增施氮肥可以改善旗叶的光合特性,增加粒重,提高产量。     

Degradation of proteins from thylakoid membranes in senescing wheat leaves at high temperature

This investigation determined whether thylakoid proteins would be degraded more rapidly or not in senescing wheat (Triticum aestivum L. em. Thell.) leaves concurrently exposed to high temperatures. Excised leaves were pulse-labelled with [³⁵S]-methionine for a 12 h period, and then incubated at 22,32 or 42°C for 0, 1, 2, or 3 d, before extracting a thylakoid enriched membrane sample. After electrophoretic separation, two prominent [³⁵S]-labelled protein bands were chosen for further analyses. Band A contained the D-1 thylakoid protein and band B contained thylakoid proteins of the light harvesting complex (LHCII) associated with photosystem II (PSII). Total protein, [³⁵S]-labelled protein, band A protein, and band B protein within the thylakoid enriched membrane samples were measured. Unlabelled thylakoid enriched membrane samples, extracted from leaves given similar treatments, were used to measure uncoupled whole-chain and photosystem II (PSII) electron transport and chlorophyll fluorescence. Accentuated decline in whole-chain and PSII electron transport, increasing F_o values, and decreasing F_max values were a result of high temperature injury in leaves treated at 42°C. None of the thylakoid enriched membrane protein fractions were degraded more rapidly in high-temperature treated leaves. Degradation of the total [³⁵S]-labelled membrane proteins and band B was inhibited by the 42°C treatment. The results indicate that high temperature stress may disrupt some aspects of normal senescence.     

Quantitative trait loci mapping of dark-induced senescence in winter wheat (Triticum aestivum)

In order to explore the genetics of dark-induced senescence in winter wheat(Triticum aestivum L.),a quantitative trait loci(QTL)analysis was carried out in a doubled haploid population developed from a cross between the varieties Hanxuan 10(HX)and Lumai 14(LM).The senescence parameters chlorophyll content(Chl a+b,Chl a,and Chl b),original fluorescence(Fo),maximum fluorescence level(Fm),maximum photochemical efficiency(Fv/Fm),and ratio of variable fluorescence to original fluorescence(Fv/Fo)were evaluated in the second leaf of whole three-leaf seedlings subjected to 7 d of darkness.A total of 43 QTLs were identified that were associated with dark-induced senescence using composite interval mapping.These QTLs were mapped to 20 loci distributed on 11 chromosomes:1B,1D,2A,2B,3B,3D,5D,6A,6B,7A,and 7B.The phenotypic variation explained by each QTL ranged from 7.5% to 19.4%.Eleven loci coincided with two or more of the analyzed parameters.In addition,14 loci co-located or were linked with previously reported QTLs regulating flag leaf senescence,tolerance to high light stress,and grain protein content(Gpc),separately.     

Effect of the nitrogen-containing salt on the content of cytokinins in detached wheat leaves

The dynamics of the cytokinin content in detached leaves of wheat (Triticum durum, cv. Bezenchukskaya 139) seedlings moistened with ammonium nitrate or water (control) was studied by immunoenzyme analysis. Leaf treatment with water was accompanied by a transient accumulation of cytokinins, maybe due to their release from their O-glucosylated forms. An increase in the contents of zeatin and its riboside after their initial decrease in detached leaves treated with ammonium nitrate could not occur due to their release from stored forms (nucleotides or O-glucosides) because the contents of zeatin and its riboside increased simultaneously with the content of stored cytokinins. The accumulation of isopentenyladenosine and zeatin nucleotide, which occurred simultaneously with an increase in the content of zeatin and zeatin riboside, permits a supposition that cytokinins can be synthesized in detached wheat leaves treated with ammonium nitrate.     

The regulation of nitrogen accumulation in the grain of wheat plants (Triticum aestivum)

Nitrogen accumulation in the ear of wheat plants ( Triticum aestivum L. cv. Klein Chamaco) during ear growth was studied under 4 experimental conditions. Plants were grown in pots with Perlite or soil, and fertilized with nutrient solutions. In one experiment the plants were grown in a greenhouse and supplied with high (16m M ) or low (1.6 m M ) N in the nutrient solutions until anthesis, and then with or without nitrogen supply until ripening. In a second experiment the plants were grown with high N supply until anthesis, and then for half of the plants light intensity was decreased by 50%, and at the same time. N supply was terminated for half of the plants within each light treatment. A third experiment was similar to the previous one, but was carried out in a growth cabinet under 20% of the maximal irradiance in the greenhouse. In a fourth experiment half the ear was excised at anthesis in half of the plants, and these plants were then supplied with or without nitrogen.
In all experiments there was a linear relation between the rate of N accumulation and the rate of ear growth. A wide range of final individual grain weights and N concentration was observed among the experiments. The same maximum N concentration was observed for all grain sizes, although the N concentration could be different between grains of the same size. The grain N concentration correlated with the rate of N accumulation per unit of ear weight increase during ear growth. It is suggested that in wheat plants there is a dependence of nitrogen transport on carbon transport to the ear, and to the ear, and that the final grain N concentration is determined by the N/C ratio exported from the vegetative tissues.     

Effect of cytokinins supplied via the xylem at multiples of endogenous concentrations on transpiration and senescence in derooted seedlings of oat and wheat

This study was conducted lo determine whether naturally occurring xylem cytokinins, when supplied to leaves via the xylem at approximately endogenous concentrations, increase transpiration and delay senescence in selected monocot species (oat and wheat). The concentrations of some of the major cytokinins (zeatin, dihydrozeatin, ciszeatin and their ribosides, the O-glucosides and nucleotides) were determined in the xylem exudate of oat and wheat seedlings by radioimmunoassay. Evidence is presented that the small volume of exudate (4–5 mm³) collected per plant was xylem sap in transit at the time of shoot excision. Using the data on cytokinin levels, the individual bases and ribosides (and a base/riboside mixture), at multiples of concentrations determined in xylem sap, were tested in transpiration and senescence bioassays. The individual O-glucosides (and mixtures of the O-glucosides) were similarly tested at (i) multiples of the molar concentrations of the corresponding bases and ribosides, and/or at (ii) multiples of the endogenous concentrations. Similarly, zeatin and dihydrozeatin nucleotides were tested at multiples of the molar concentration of zeatin riboside and, in some instances, at multiples of endogenous concentrations. Our results suggest that, at least in oat and possibly in wheat, zeatin-type bases, ribosides and O-glucosides supplied to the leaf in xylem sap are likely to play a role in regulating transpiration in vivo. O-glucosides in oat xylem sap may be important regulators of leaf senescence in the intact plant. The nucleotides were present in xylem sap at lower concentrations than most of the bases, ribosides and O-glucosides. The nucleotides appear likely to play a lesser role than the bases, riboside and O-glucosidcs in controlling transpiration and senescence in the intact plant.     

Mode of high temperature injury to wheat. II. Comparisons of wheat and rice with and without inflorescences

High temperature injury to wheat ( Triticum aestivum L.) during grain development is manifested as acceleration of senescence. Experiments were conducted to elucidate the mode of senescence and site of high temperature responses. Wheat (cv. Chris) and rice ( Oryza sativa L. cv. Newbonnet), which have C₃ photosynthesis but different temperature responses, were grown with and without inflorescences under three temperature regimes after anthesis. Plant growth and constituents associated with senescence were measured weekly until physiological maturity. Increasing temperatures from 25°C/15°C to 35°C/25°C day/night after anthesis decreased growth, leaf viability, chlorophyll and protein concentrations, and RuBP carboxylase activity and increased protease and RNase activities in wheat. Inflorescence removal increased vegetative weights and slowed most senescence processes more in wheat than in rice, but did not alter the course of high temperature responses. Results are interpreted as indicating that diversion of nutrients from roots by inflorescence sinks at normal temperatures and by increased respiration at high temperatures caused similar responses. Source and sink activities appeared to be regulated jointly, probably by cytokinins from roots, during senescence at normal and elevated temperatures.     

小麦幼苗叶片抗氰呼吸对轻度水分胁迫的响应

小麦幼苗经－０．５ＭＰa聚乙二醇（ＰＥＧ－６０００）溶液暗中渗透迫０、６、１２、１８、２４、３０、３６、４２、４８h,叶片的总呼吸速率（Ｖt)呈现先上升后降低的趋势,交替途径呼吸也表现出相同的变化模式。水分胁迫初期（０－１２h),交替途径容量（Ｖalt)、实际运行活性（ρValt)及运行系数（ρ值）均上升,此后（１８－４８h）逐渐下降,水分胁迫也影响了呼吸电子流在２条呼吸途径中的分配比例,胁迫初期的０－１２h内,流经交替途径的电子流增多,而流向细胞色素主路的电子流减少,但随着胁迫时间的延长,交替途径的贡献降低,而细胞色素主路的贡献增加,说明小麦叶片的抗氰呼吸在水分胁迫初期被诱导增加,而随着胁迫进行的延长又表现为下降。     

不同植物生长调节剂对小麦衰老及产量构成的调节效应

研究了叶面喷施GA3、NBT、6－BA、TIBA、PP333对小麦旗叶衰老过程中若干生理指标的影响,分析了它们对成产要素的不同作用。结果表明：不同生长调节剂可以以不同的方式调节小麦旗叶的衰老,影响小麦产量构成因素水平。其中6－BA和PP333能够提高单位面积穗数,明显延缓叶片衰老,增加了穗粒数和千粒重,使成产3因素间比例协调合理,产量较高;TIBA可以明显提高叶绿素含量并显著提高千粒重。GA3和NBT对延缓旗叶衰老有一定作用,但增产效果不显著。研究指出,根据小麦生育状况有选择性地使用生长调节剂,协调成产因素关系,是一个有效的增产途径。     

Effects of ozone on the photosynthetic apparatus and leaf proteins during leaf development in wheat

Leaves of Triticum aestivum cv. Avalon were grown in an atmosphere that contained 150 nmole mol^-1 ozone for 7h each day. After leaves had reached maximum size, the leaf blade was divided into three sections to provide tissue of different age, the youngest at the base of the blade and the oldest at the leaf tip. The ozone treatment was found to decrease significantly the light-saturated rate and quantum yield of CO₂ assimilation and the maximum quantum yield of photosystem II photochemistry in the oldest leaf section. No effects were found on the basal and middle sections of the leaf. These ozone-induced decreases in the photosynthetic parameters were associated with decreases in the efficiency of utilization of light for CO₂ assimilation at the photon flux density under which the leaves were grown. The depression in photosynthetic performance of tissue near the leaf tip was accompanied by large decreases in the contents of total, soluble and thylakoid proteins and chlorophyll. There was also found to be a preferential loss of ribulose-1,5-carboxylase-oxygenase. These ozone-induced changes in chlorophyll and protein contents and the photosynthetic activities of the leaf tissue were similar to changes normally associated with leaf senescence. Two-dimensional polyacrylamide gel analyses of leaf proteins demonstrated the loss of some minor, and unidentified, proteins, whilst another group of minor proteins appeared. It is concluded that daily exposure of the leaf to 150 nmol mol^-1 ozone for 7h had no effect on the development of the photosynthetic apparatus and its activities during leaf expansion, but it did promote the onset of premature senescence in fully expanded tissue that resulted in a loss of pigments, proteins and photosynthetic capacity and efficiency.     

叶部激素变化与小麦的整体衰老

旗叶中的叶绿素含量在抽穗—开花期达其最大值 ,然后迅速下降 ;丙二醛的含量逐渐上升 ;SOD活性逐渐下降。脱落酸的含量在灌浆的初期逐渐升高 ,中期达其最高值 ;玉米素在开花期达其峰值后迅速下降 ,旗叶的衰老与叶片中二激素间的浓度变化、平衡有关 ;根系活力在拔节—孕穗期达其高峰值 ,以后逐渐下降 ;根系活力在生育中、后期的下降与叶片中玉米素的下降、脱落酸的上升有直接关系 ,根系的早衰可能是引发小麦地上部衰老的重要原因之一。     

Influence of Nitrogen Supply on the Susceptibility of Wheat to Septoria tritici

Nitrogen (N) fertilization is required for achieving high yields in wheat (Triticum aestivum L.) but may enhance the development of Septoria tritici Rob. ex Desm. A study was undertaken to investigate how N supply influences the area under disease progress curve (AUDPC). Two field experiments were carried out in a typical argiudol in 1996 and 1997. Six cultivars were grown at two N treatments (0 and 100–150 kg N/ha) in a split plot design. Percentage of necrosis (severity) of the two upper leaves of each treatment was evaluated three times during the growing season. Height and heading date were also scored and related to the resistance level. In 1996, with weather conditions conducive to the disease, AUDPC values were higher in the fertilized treatment. In 1997, with insufficient rain immediately after inoculation, the disease only progressed faster under N fertilization in the flag leaf, which was exposed to conducive environmental conditions from its appearance. The effect of N fertilization was influenced by the cultivar characteristics, climatic and agronomic conditions. Knowledge that N fertilization promotes the development of S. tritici in conducive conditions will be useful for deciding management strategies of the cultivars and for optimizing conditions for the selection in breeding programmes.     

Changes in hydrogen peroxide homeostasis and cytokinin levels contribute to the regulation of shade-induced senescence in wheat leaves

In a previous work we demonstrated that the suppression of blue light in shaded leaves of wheat increases their senescence rate and the development of oxidative stress symptoms. In order to better understand the interaction between the oxidative metabolism and light spectral quality in the regulation of leaf senescence, we studied the evolution of H₂O₂ concentration, protein oxidation, proteolytic activity and cytokinin content in excised leaves, either illuminated (control, “C”) or shaded under blue (“B”, high blue light transmission) or green (“G”, very low blue light transmission) light filters. H₂O₂ concentration significantly increased during the first 9 h after treatment initiation, an effect that was consistently higher in treatments B and C. Leaves from these treatments showed lower chlorophyll and protein degradation rates, lower concentration of oxidized proteins, and maintained higher levels of the cytokinin isopentenyl-adenosine than those from treatment G. When moderate H₂O₂ concentrations were supplied during 6–9 h after the onset of the shade treatments, senescence rate in treatment G was delayed, while the opposite effect was observed in the presence of the H₂O₂ scavengers catalase and, to a lesser extent, dimethylthiourea. These effects were accompanied by an increment or a decrement, respectively, of catalase activity, suggesting that the early changes in H₂O₂ homeostasis in leaves from treatments B and C may contribute to the prevention rather than to the induction of further oxidative damage. Altogether our results show that the suppression of blue light transmission in shaded leaves act as a stress signal that increases their sensitivity to oxidative stress and accelerates cell death.     

条锈病对小麦(Triticum aestivum L.)叶片光合功能及光合功能蛋白D1表达的影响

测定了小麦(Triticum aestivum L.)感染小麦条锈病后的光合常数,以及叶绿素含量、类囊体膜光合电子传递速率和光合反应中心D1蛋白的变化.实验显示,条锈病侵染导致感病小麦叶片净光合速率与叶绿素含量降低;抗病小麦经侵染后净光合速率却有恢复过程,叶绿素含量先降后升.此外,感病小麦叶片被侵染后全链电子传递速率受到抑制,PSII电子传递速率的变化与全链电子传递速率的变化趋势相似,但PSI电子传递速率受到的影响较小;抗病小麦小麦叶片被侵染后电子传递速率所受影响较小.同时发现,病程中,感病和抗病小麦PSII的光合反应中心D1蛋白含量变化总是与PSII电子传递速率的变化类似,推测D1蛋白的表达量变化是引起PSII电子传递活性与全链电子传递速率变化的主要因素之一.