Natural variation in the regulation of leaf senescence and relation to other traits in Arabidopsis

Leaf senescence results in the recycling of nutrients, thereby providing resources required for growth and reproduction. In this study, the effect of day-length on leaf senescence in eight different Arabidopsis thaliana ecotypes was determined and the relationship between senescence and other morphological and life history traits was analysed. A significant variation in the start and extent of leaf senescence depending on the genetic background and the response to day-length was found. Whereas senescence of early flowering ecotypes was accelerated by long days, no effect of day-length on senescence could be found in late flowering Kas-1 plants. Senescence in the different ecotypes was associated with other traits, such as floral transition, the total number of fruits, the total number of leaves and the maximum chlorophyll content. Plants that bolted early also senesced early, produced fewer leaves, accumulated less chlorophyll, but produced more fruits. The present results indicate that senescence may be a key component in the trade-off between investment in photosynthetic capacity and reproduction. The relationship between senescence and other traits was maintained independent of whether differences in senescence were caused by genetic (ecotype) or environmental (day-length) variation, suggesting that genetic and environmental factors affect these traits through common regulatory pathways.     

小麦根系活力变化与叶片衰老的研究

对小麦根系活力、叶片叶绿素含量和光合速率变化的田问实验结果表明,根系活力的衰退早且快于叶片叶绿素含量和光合速率的衰退;根系活力和叶片光合速率呈正相关关系(r=0．8006)．拔节期追施氮肥,改善根际营养条件后,叶片的光合速率、根系活力明显提高,叶片中ABA含量降低,ZRs升高,SOD活性增加,MDA的高峰期比对照推迟8～10d,说明根系活力变化与地上部叶片的衰老密切相关,改善根际营养,提高生育前期的根系活力有助于延缓地上部分的衰老．     

The relation between nitrogen deficiency and second leaf senescence in wheat plants

Life span of the second leaf of wheat(Triticum aestivum L., cv. Grana) plants was studied from day 8 to day 50 of plant age in a variant with nitrogen (+N) and in a variant in which plant senescence was induced by the omission of nitrogen from the nutrient solution (−N). Seed protein was the sole source of nitrogen for these plants. Specific leaf mass (SLM) in the −N variant, and specific leaf area (SLA), the mass of fresh leaf, soluble protein content and total nitrogen content in the +N variant peaked by day 22 of plant age (that is by day 19 of leaf age). Dry matter content, leaf length and leaf area, and SLM in the +N variant peaked by day 29 of plant age (that is by day 26 of leaf age). The ontogeny of the second leaf in the variant with enhanced senescence was shorter by at least 14 days. Plants from this variant showed typical symptoms of N deficiency, that is yellowing of leaves, tip burn, and lack of tillering. However, the growth and biochemical characters studied did not indicate an earlier onset of the senescence of the second leaf of −N plants. Both +N and −N variants reached their peaks (with the exception of an earlier peak by day 12 in case of total nitrogen content in the −N variant) on the same day of leaf age. Thus the first part of the leaf life span from leaf growth initiation to full expansion was of the same length in both the control and N-def icient plants. The stage of the proper senescence of the second leaf of −N plants was very short; the leaf completely died away within 7 days after senescence onset.     

Natural senescence of the root cortex of spring wheat in relation to susceptibility to common root rot (Cochliobolus sativus) and growth of a free-living nitrogen-fixing bacterium

Summary Natural senescence of the root cortex was assessed by nuclear staining, for cultivars and chromosome substitution lines of spring wheat known to differ in (1) susceptibility to common root rot, (2) total rhizosphere populations and (3) ability to support growth of a free-living nitrogen-fixing bacterium.Together, three root rot susceptible wheat lines showed significantly more cortical senescence than did three resistant lines; the susceptible lines also support larger rhizosphere populations. The wheat line that supports growth of a nitrogen-fixing bacterium showed significantly less cortical death than did any other line. Substitution of chromosome pairs 5B or 5D between the parent cultivars Rescue and Cadet substantially altered the amount of root cortex death, which is thus genetically determined. It is suggested thatCochliobolus sativus and other weak parasites benefit from early natural senescence of the root cortex, and that the degree of susceptibility or resistance of wheat lines to common root rot is at least partly determined by differences in cortical senescence.     

Characterisation of grain yield and nitrogen level in relation to flag leaf senescence of wheat varieties

Three wheat cultivars ( Triticum aestivum L.), Splendeur, Hobbit and Maris Huntsman grown in pots were compared. Especially when compared to Splendeur, the flag leaf senesced most rapidly in Maris Huntsman, which presented the most rapid loss of moisture, chlorophyll and nitrogen. The uptake of exogenous nitrogen during the post-anthesis period was lower in the rapidly than in the slowly senescing variety. A higher concentration of free amino nitrogen in the flag leaf at a given sampling date was associated with a lower percentage decrease of soluble proteins at the following date. Acid proteinase activity in the flag leaf was inversely related to moisture percentage and free amino nitrogen level, but unrelated to the nitrogen loss of the flag leaf. Acid proteinase activity in the flag leaf was directly related to grain nitrogen percentage, but inversely related to grain yield. Grain yield was also directly related to the mean soluble protein content of the flag leaf through senescence.     

Genotypic variation in nitrogen utilization efficiency in oilseed rape is related to the coordination of leaf senescence and root N uptake during reproductive stage

Plant and Soil - Due to their low nitrogen utilization efficiency (NUtE) oilseed rape (OSR) crops require large quantities of nitrogen (N) fertilizer to achieve acceptable economic yields. To...     

渍水对小麦扬麦5号旗叶和根系衰老的影响

1　引　　言生育中后期渍水是长江中下游麦区小麦高产稳产的主要限制因子[13 ,16] .该区由于普遍实行稻麦多熟种植制度 ,前茬水稻使土壤浸水时间过长 ,土壤粘重 ,排水困难 ,透气性差而造成湿害 ;另外 ,该区常年麦季降雨量 5 0 0～ 80 0ｍｍ (浙江省可达 10 0 0ｍｍ)多集中于小麦生长的中后期 ,大大超过了小麦正常需水量 (35 0～ 4 5 0ｍｍ) ,从而加剧渍害[5] .　　研究表明 ,渍水小麦株高、地上部干重、分蘖数、主茎绿叶片数、绿叶面积等都受到影响[1,12 ,16] ,叶片光合速率、气孔导度、细胞间隙ＣＯ2 浓度下降[8] ,ＲｕＢＰＣＯ活性降低[14…     

Molecular regulation of leaf senescence

Leaf senescence is a process of programmed cell death, which is induced in an age-dependent manner and by various environmental cues. The mechanisms that regulate the induction and progression of leaf senescence remain unclear because of their complexity. However, recent genetic and reverse-genetic approaches have identified key components of the regulation of leaf senescence and have revealed glimpses of the underlying molecular mechanisms.     

Cytokinin biochemistry in relation to leaf senescence. VIII. Translocation, metabolism and biosynthesis of cytokinins in relation to sequential leaf senescence of tobacco

Cytokinin bases (zeatin and dihydrozeatin) and ribosides (zeatin riboside and dihydrozeatin riboside) were identified as major cytokinins in tobacco xylem sap by radioimmunoassay. When ³H-labelled zeatin riboside or dihydrozeatin riboside were supplied to tobacco plants via the xylem, leaves of differing maturity did not differ appreciably in level of radioactivity or in metabolism of the cytokinin. The major metabolites of zeatin riboside in leaves were adenine, adenosine and adenine nucleotides, whereas that of dihydrozeatin riboside was dihydrozeatin 7-glucoside. Incorporation of [¹⁴C]adenine into zeatin was evident in upper green leaves. indicating that young leaves have the capacity to synthesize cytokinins in situ. In contrast, fully expanded green leaves and senescing tobacco leaves exhibited little or no incorporation of [¹⁴C]adenine into cytokinins. This difference in cytokinin biosynthetic capacity may contribute to the differing cytokinin levels in leaves of different matirity, and may participate in control of sequential leaf senescence in tobacco.     

Hormonal regulation of leaf senescence in Lilium

In addition to floral senescence and longevity, the control of leaf senescence is a major factor determining the quality of several cut flowers, including Lilium, in the commercial market. To better understand the physiological process underlying leaf senescence in this species, we evaluated: (i) endogenous variation in the levels of phytohormones during leaf senescence, (ii) the effects of leaf darkening in senescence and associated changes in phytohormones, and (iii) the effects of spray applications of abscisic acid (ABA) and pyrabactin on leaf senescence. Results showed that while gibberellin 4 (GA(4)) and salicylic acid (SA) contents decreased, that of ABA increased during the progression of leaf senescence. However, dark-induced senescence increased ABA levels, but did not affect GA(4) and SA levels, which appeared to correlate more with changes in air temperature and/or photoperiod than with the induction of leaf senescence. Furthermore, spray applications of pyrabactin delayed the progression of leaf senescence in cut flowers. Thus, we conclude that (i) ABA plays a major role in the regulation of leaf senescence in Lilium, (ii) darkness promotes leaf senescence and increases ABA levels, and (iii) exogenous applications of pyrabactin inhibit leaf senescence in Lilium, therefore suggesting that it acts as an antagonist of ABA in senescing leaves of cut lily flowers.     

Cytokinin biochemistry in relation to leaf senescence. VII. Endogenous cytokinin levels and exogenous applications of cytokinins in relation to sequential leaf senescence of tobacco

The cytokinin complex in tobacco leaves of various maturities was characterized by radioimmunoassay and mass spectrometry. Zeatin was the major base, whereas zeatin riboside was identified as the main riboside. in leaves of all maturities studied. Relative to upper younger leaves, the basal yellow leaves had reduced levels of both cytokinin bases and ribosides. Exogenous applications of dihydrozeatin and zeatin to detached tobacco leaves in amounts sufficient to delay senescence, elevated cytokinin base and riboside levels 2–5 fold. Presenescent and senescent leaves of intact plants showed quantitatively similar changes in cytokinin content. which therefore appear to be of significance in control of senescence. When supplied exogenously, the principal cytokinin bases found to occur in tobacco leaves (zeatin and dihydrozeatin) were markedly more effective than auxins and gibberellic acid in retarding senescence. Localised application of cytokinins to leaf blades of detopped plants was much less effective than application to intact plants. The cytokinin induced senescence retardation in tobacco leaves was independent of effects on directed metabolite transport. Evidence that endogenous levels of active cytokinins in intact tobacco leaves are involved in control of sequential leaf senescence is discussed.     

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.     

Photosynthesis and leaf-nitrogen dynamics during leaf senescence of tropical maize cultivars in hydroponics in relation to N efficiency in the field

The selection process of nitrogen (N)-efficient cultivars during plant breeding could be simplified by a specification of secondary plant traits that are decisive for N efficiency. It was shown that leaf senescence under N deprivation of sixteen tropical maize cultivars in a short-term nutrient solution experiment was related to leaf senescence and grain yield under N deficiency (N efficiency) in field experiments. In this study we investigated if a quantification of leaf- and plant-N flows by ¹⁵N labelling can improve the evaluation of genotypic differences in leaf senescence in short-term experiments. Cultivars differed in leaf-N content prior to senescence; however, this appeared to have no significant impact on the development of leaf senescence. N import into senescing leaves was not related to total plant N uptake, but seems to have been regulated by leaf-inherent factors. Leaf N remaining in the leaf seems to have comprised inefficiently remobilized leaf N, at least during early senescence stages. Photosynthetic rate and chlorophyll contents at early senescence stages depended on additional factors to leaf-N content. Nevertheless, all parameters used to characterize leaf senescence were related to leaf senescence at anthesis in field experiments. However, only photosynthetic rate during late leaf senescence reflected cultivar differences in leaf senescence during reproductive growth and N efficiency in field experiments.     

The relation between nitrogen deficiency and leaf senescence

Because the mobilization of nitrogen resulting from nutritional nitrogen deficiency is also prominent during leaf senescence, the characteristics of these two syndromes were compared. Oat plants ( Avena sativa L. cv. Victory) were raised on a nutrient solution, complete except for nitrogen supply (i.e., with only the seed protein as nitrogen source), and the senescence of their leaves was compared with that of controls grown on a full nutrient solution. The N-deficient plants flowered after forming only 4 leaves and each set a single seed. The nitrogen lack affected the content of chlorophyll somewhat more than the content of the amino acids or protein nitrogen. However, spraying the plants with kinetin solution was able to retain 20–30% of the chlorophyll and protein. During senescence, the chlorophyll appears to be less stable in the N-deficient leaves than in the controls, while the protein is somewhat more stable than in the controls. Also, when the detached leaves from N-deficient plants senesced in white light or in darkness, kinetin delayed their senescence almost as effectively as that of control leaves. Most strikingly, the stomata of N-deficient leaves after detachment and floating on water were largely closed in light, just as in senescence, but could be partially induced to open by kinetin treatment. Since stomatal closure has earlier been shown to cause senescence, the characteristic syndrome of foliar nitrogen deficiency is concluded to be partly that of senescence.     

小麦叶片气孔性状与产量和抗旱性的关系

以小麦旱选10号/鲁麦14 DH群体为试材,对干旱胁迫和正常灌溉条件下花后10 d和20d旗叶中部气孔密度、气孔大小、气孔导度、光合速率、蒸腾速率与产量和抗旱系数的关系进行相关和通径分析.结果表明:在两种水分条件下,花后10 d气孔密度、气孔大小、气孔导度、光合速率、蒸腾速率与产量和抗旱系数的相关性大多不显著;但花后20 d与千粒重呈极显著正相关,与穗粒数、单株产量和抗旱系数的相关性仍大多不显著.气孔导度、光合速率和蒸腾速率是影响单株产量和抗旱系数的主要因素,不仅对单株产量和抗旱系数的直接作用较大,间接作用也较大;气孔密度与气孔大小对单株产量和抗旱系数的直接作用和间接作用均较小.在两种水分条件下,花后10 d和20 d,气孔密度与气孔长度之间,气孔长度与气孔宽度、气孔导度、光合速率和蒸腾速率之间的相关性均显著或极显著,但气孔密度、气孔长度与气孔导度、光合速率和蒸腾速率间的相关性在不同水分条件和不同生育阶段存在差异,表明水分条件、生长发育阶段对这些性状之间的相关性影响较大.通过育种手段以叶片气孔密度和气孔大小为选择目标,来改善小麦气孔导度、光合速率和蒸腾速率,进而提高产量并不总是一种有效手段.     

Genetic analysis of root morphological traits in wheat

Traits related to root architecture are of great importance for yield performance of crop species, although they remain poorly understood. The present study is aimed at identifying the genomic regions involved in the control of root morphological traits in durum wheat (Triticum durum Desf.). A set of 123 recombinant inbred lines derived from the durum wheat cross of cvs. ‘Creso’ × ‘Pedroso’ were grown hydroponically to two growth stages, and were phenotypically evaluated for a number of root traits. In addition, meta-(M)QTL analysis was performed that considered the results of other root traits studies in wheat, to compare with the ‘Creso’ × ‘Pedroso’ cross and to increase the QTL detection power. Eight quantitative trait loci (QTL) for traits related to root morphology were identified on chromosomes 1A, 1B, 2A, 3A, 6A and 6B in the ‘Creso’ × ‘Pedroso’ segregating population. Twenty-two MQTL that comprised from two to six individual QTL that had widely varying confidence intervals were found on 14 chromosomes. The data from the present study provide a detailed analysis of the genetic basis of morphological root traits in wheat. This study of the ‘Creso’ × ‘Pedroso’ durum-wheat population has revealed some QTL that had not been previously identified.     

Nucleotide pools in leaf and root tissue of tobacco plants: Influence of leaf senescence

The content of the different ribonucleotides, nucleoside diphosphate sugars, NAD⁺ and NADP⁺ was determined in the leaves and roots of single plants of Nicotiana tabacum L. cv. Samsun using a new procedure. The determination makes use of extraction with HClO₄ followed by extract purification and separation by a combination of anion-exchange and reversed phase high performance liquid chromatography. The largest pools were the adenine nucleotides and the uracil nucleotides with nucleoside diphosphate sugars as the main fraction. Leaf senescence was accompanied by a strong reduction of the nucleotide content of the leaves, but the adenine nucleotide-derived energy charge remained at a high level and even increased at the later stages of senescence. This indicates a balanced metabolism also in the older leaves and excludes the possibility that leaf senescence is triggered by the energy charge.     

Grain filling in relation to monocarpic senescence of wheat in varying source-sink ratios

Flag leaf removal at any stage of grain growth hastened senescence (reduction in chlorophyll content) of the sterile glumes whereas a removal of the latter did not alter senescence of the former. Kernel mass, grain mass per ear, harvest index and sink activity reduced more conspicuously by the removal of glumes than by the flag leaf removal. Removal of grains hastened senescence of the glumes only, although protein content increased at a later grain removal in the majority of source organs. Moisture contents of the developing grains were mostly reduced by the removal of either the flag leaf or the glumes, though flag leaf removal at anthesis affected it the most. Protein levels of the developing grains reduced at harvest only when the glumes were removed at anthesis but sugar levels in later phases always decreased irrespective of the time of removal and nature of the source organs.