Early Leaf Senescence Research Articles

Early leaf senescence is associated with an altered cellular redox balance in Arabidopsis cpr5/old1 mutants

Reactive oxygen species (ROS) are the inevitable by-products of essential cellular metabolic and physiological activities. Plants have developed sophisticated gene networks of ROS generation and scavenging systems. However, ROS regulation is still poorly understood. Here, we report that mutations in the Arabidopsis CPR5/OLD1 gene may cause early senescence through deregulation of the cellular redox balance. Genetic analysis showed that blocking stress-related hormonal signalling pathways, such as ethylene, salicylic acid, jasmonic acid, abscisic acid and sugar, did not affect premature cell death and leaf senescence. We took a bioinformatics approach and analysed publicly available transcriptome data of presymptomatic cpr5/old1 mutants. The results demonstrate that many genes in the ROS gene network show at least fivefold increases in transcripts in comparison with those of wild-type plants, suggesting that presymptomatic cpr5/old1 mutants are in a state of high-cellular oxidative stress. This was further confirmed by a comparative, relative quantitative proteomics study of Arabidopsis wild-type and cpr5/old1 mutant plants, which demonstrated that several Phi family members of glutathione s-transferases significantly increased in abundance. In summary, our genetic, transcriptomic and relative quantitative proteomics analyses indicate that CPR5 plays a central role in regulating redox balance in Arabidopsis.

Effects of growth temperature and winter duration on leaf phenology of a spring ephemeral (Gagea lutea) and a summergreen forb (Maianthemum dilatatum)

Effects of growth temperature and winter duration on leaf longevity were compared between a spring ephemeral, Gagea lutea, and a forest summergreen forb, Maianthemum dilatatum. The plants were grown at day/night temperatures of 25/20 degrees C and 15/10 degrees C after a chilling treatment for variable periods at 2 degrees C. The temperature regime of 25/20 degrees C was much higher than the mean air temperatures for both species in their native habitats. Warm temperature of 25/20 degrees C and/or long chilling treatment shortened leaf longevity in G. lutea, but not in M. dilatatum. The response of G. lutea was consistent with that reported for other spring ephemerals. Air temperature increases as the vegetative season progresses. The decrease in leaf longevity in G. lutea under warm temperature condition ensures leaf senescence in summer, an unfavorable season for its growth. This also implies that early leaf senescence could occur in years with early summers. Warm spring temperatures have been shown to accelerate the leafing-out of forest trees. The decrease in leaf longevity due to warm temperature helps synchronize the period of leaf senescence roughly with the time of the forest canopy leaf-out. Prolonged winter due to late snowmelt has been shown to shorten the vegetative period for spring ephemerals. The decrease in leaf longevity due to long chilling treatment would correspond with this shortened vegetative period.

Effects of cadmium on ultrastructure and antioxidative defense system in hyperaccumulator and non-hyperaccumulator ecotypes of Sedum alfredii Hance

Plant growth, ultrastructural and antioxidant adaptations and glutathione biosynthesis in Cd-hyperaccumulating ecotype Sedum alfredii Hance (HE) countering high Cd environment were investigated and compared with its non Cd-hyperaccumulating ecotype (NHE). Cadmium exposure resulted in significant ultrastructural changes in root meristem and leaf mesophyll cells of S. alfredii, but damage was more pronounced in NHE even when Cd concentrations were one-tenth of those applied to HE. Cadmium stress damaged chloroplasts causing imbalanced lamellae formation coupled with early leaf senescence. Histochemical results revealed that glutathione (GSH) biosynthesis inhibition led to overproduction of hydrogen peroxide (H 2O 2) and superoxide radical (O 2 −) in HE but not in NHE. Differences were noted in both HE and NHE for catalase (CAT), guaiacol peroxidase (GPX), ascorbate peroxidase (APX) and glutathione reductase (GR) activities under various Cd stress levels. No relationship was found between antioxidative defense capacity including activities of superoxide dismutase (SOD), CAT, GPX, APX and GR as well as ascorbic acid (AsA) contents and Cd tolerance in the two ecotypes of S. alfredii. The GSH biosynthesis induction in root and shoot exposed to elevated Cd conditions may be involved in Cd tolerance and hyperaccumulation in HE of S. alfredii H.

Arabidopsis VPS35, a Retromer Component, is Required for Vacuolar Protein Sorting and Involved in Plant Growth and Leaf Senescence

The retromer complex is responsible for retrograde transport, which is coordinated with anterograde transport in the secretory pathway including vacuolar protein sorting. Yeast VPS35 is a component of the retromer complex that is essential for recognition of specific cargo molecules. The physiological function of VPS35 has not been determined in vacuolar protein sorting in higher organisms. Arabidopsis thaliana has three VPS35 homologs designated VPS35a, VPS35b and VPS35c. We isolated four vps35 mutants (vps35a-1, vps35b-1, vps35b-2 and vps35c-1) and then generated four double mutants and one triple mutant. vps35a-1 vps35c-1 exhibited no unusual phenotypes. On the other hand, vps35b-1 vps35c-1 and the triple mutant (vps35a-1 vps35b-2 vps35c-1) exhibited severe phenotypes: dwarfism, early leaf senescence and fragmentation of protein storage vacuoles (PSVs). In addition, these mutants mis-sorted storage proteins by secreting them out of the cells and accumulated a higher level of vacuolar sorting receptor (VSR) than the wild type. VPS35 was localized in pre-vacuolar compartments (PVCs), some of which contained VSR. VPS35 was immunoprecipitated with VPS29/MAG1, another component of the retromer complex. Our findings suggest that VPS35, mainly VPS35b, is involved in sorting proteins to PSVs in seeds, possibly by recycling VSR from PVCs to the Golgi complex, and is also involved in plant growth and senescence in vegetative organs.

Study of Early Leaf Senescence in Arabidopsis thaliana by Quantitative Proteomics Using Reciprocal 14N/15N Labeling and Difference Gel Electrophoresis

Leaf senescence represents the final stage of leaf development and is associated with fundamental changes on the level of the proteome. For the quantitative analysis of changes in protein abundance related to early leaf senescence, we designed an elaborate double and reverse labeling strategy simultaneously employing fluorescent two-dimensional DIGE as well as metabolic (15)N labeling followed by MS. Reciprocal (14)N/(15)N labeling of entire Arabidopsis thaliana plants showed that full incorporation of (15)N into the proteins of the plant did not cause any adverse effects on development and protein expression. A direct comparison of DIGE and (15)N labeling combined with MS showed that results obtained by both quantification methods correlated well for proteins showing low to moderate regulation factors. Nano HPLC/ESI-MS/MS analysis of 21 protein spots that consistently exhibited abundance differences in nine biological replicates based on both DIGE and MS resulted in the identification of 13 distinct proteins and protein subunits that showed significant regulation in Arabidopsis mutant plants displaying advanced leaf senescence. Ribulose 1,5-bisphosphate carboxylase/oxygenase large and three of its four small subunits were found to be down-regulated, which reflects the degradation of the photosynthetic machinery during leaf senescence. Among the proteins showing higher abundance in mutant plants were several members of the glutathione S-transferase family class phi and quinone reductase. Up-regulation of these proteins fits well into the context of leaf senescence since they are generally involved in the protection of plant cells against reactive oxygen species which are increasingly generated by lipid degradation during leaf senescence. With the exception of one glutathione S-transferase isoform, none of these proteins has been linked to leaf senescence before.

Serpentine leafminer (Liriomyza trifolii) on potato (Solanum tuberosum): field observations and plant photosynthetic responses to injury

Serpentine leafminers, Liriomyza spp. (Diptera: Agromyzidae), are polyphagous insects that feed on numerous crops worldwide including potato. Recently, leafminer larvae (Liriomyza trifolii) have become an economically important pest of potato. The larvae eat the mesophyll of leaflets leaving long winding tunnels inside the leaflets. The photosynthetic effects of larval tunneling on the remaining leaf tissue are unknown. In 2003, physiological responses of potato to leafminer, L. trifolii were evaluated in Kearney, Nebraska, USA. The leaflets were examined 7 and 14 days post infestation for leaf area injury, photosynthetic rates and fluorescence. Leafminers caused up to 13% leaf area loss due to leafminer injury with no effect on the photosynthetic rates of the remaining leaf tissue thus having similar effects as other gross tissue removers. However, fluorescence measures revealed changes in the photosynthetic efficiency and depend of the type of injury, it may lead to early leaf senescence. Field monitoring of L. trifolii infestations showed that treatments with abamectin were effective in reducing leafminer numbers and had no immediate effect on beneficial parasitoid from Eulophidae family suggesting that abamectin is a good option for chemical control.

Ozone air pollution effects on tree-ring growth, 13C, visible foliar injury and leaf gas exchange in three ozone-sensitive woody plant species

We assessed the effects of ambient tropospheric ozone on annual tree-ring growth, delta(13)C in the rings, leaf gas exchange and visible injury in three ozone-sensitive woody plant species in southern Switzerland. Seedlings of Populus nigra L., Viburnum lantana L. and Fraxinus excelsior L. were exposed to charcoal-filtered air (CF) and non-filtered air (NF) in open-top chambers, and to ambient air (AA) in open plots during the 2001 and 2002 growing seasons. Ambient ozone exposures in the region were sufficient to cause visible foliar injury, early leaf senescence and premature leaf loss in all species. Ozone had significant negative effects on net photosynthesis and stomatal conductance in all species in 2002 and in V. lantana and F. excelsior in 2001. Water-use efficiency decreased and intercellular CO(2) concentrations increased in all species in response to ozone in 2002 only. The width and delta(13)C of the 2001 and 2002 growth rings were measured for all species at the end of the 2002 growing season. Compared with CF seedlings, mean ring width in the AA and NF P. nigra seedlings was reduced by 52 and 46%, respectively, in 2002, whereas in V. lantana and F. excelsior, ring width showed no significant reductions in either year. Although delta(13)C was usually more negative in CF seedlings than in AA and NF seedlings, with the exception of F. excelsior in 2001, ozone effects on delta(13)C were significant only for V. lantana and P. nigra in 2001. Among species, P. nigra exhibited the greatest response to ozone for the measured parameters as well as the most severe foliar injury and was the only species to show a significant reduction in ring width in response to ozone exposure, despite significant negative ozone effects on leaf gas exchange and the development of visible foliar injury in V. lantana and F. excelsior. Thus, significant ozone-induced effects at the leaf level did not correspond to reduced tree-ring growth or increased delta(13)C in all species, indicating that the timing of ozone exposure and severity of leaf-level responses may be important in determining the sensitivity of tree productivity to ozone exposure.

PHYSIOLOGICAL AND DEVELOPMENTAL EFFECTS OF O3ON COTTONWOOD GROWTH IN URBAN AND RURAL SITES

Previously we found that cloned cottonwood saplings (Populus deltoides) grew twice as large in New York, New York, USA, compared to surrounding rural environments and that soils, temperature, CO2, nutrient deposition, and microclimatic variables could not account for the greater urban plant biomass. Correlations between final season biomass and cumulative O3 exposures, combined with twofold growth reductions in an open-top chamber experiment provided strong evidence that higher cumulative O3 exposures in rural sites reduced growth in the country. Here, we assess the field gas exchange, growth and development, and allocation responses underlying the observed growth differences and compare them with isolated O3 responses documented in the open-top chamber experiment. Cottonwoods showed no visible foliar injury, reduced photosynthesis of recently expanded foliage, early leaf senescence, protective reduction in stomatal conductance, or compensatory allocation to shoot relative to root biomass for either the chamber or field experiment. Instead, O3-impacted chamber plants had significantly higher conductance and reduced photosynthesis of older foliage that led to reduced leaf area production and a twofold biomass reduction in the absence of visible injury. Rural-grown field plants showed the same pattern of significantly higher conductance in the absence of concomitant increases in photosynthesis that was indicative of a loss of stomatal control. Incremental changes in foliar production were also significantly inversely related to fluctuations in ambient O3 exposures. The similarity in biomass, gas exchange, phenological, and allocation responses between chamber and field experiments indicate that mechanisms accounting for reduced growth at rural sites were consistent with those in the open-top chamber O3 experiment. This study shows the limitation of visible symptoms as a sole diagnostic factor for documenting detrimental O3 impacts and points toward a new approach to show O3 impacts when visible injury is not present. Namely, O3-impacted vegetation showed an unusual inverse relationship of increased conductance with lower photosynthesis of older foliage that was indicative of a loss of stomatal control. This increased stomatal conductance of O3-impacted vegetation accentuates pollutant flux into affected foliage and has important implications for system water balance during warm, dry portions of the growing season when O3 concentrations are highest.

Nitrogen‐use efficiency: trade‐offs between N productivity and mean residence time at organ, plant and population levels

Summary Nitrogen‐use efficiency (NUEN) is often decomposed into the product of N productivity (AN) and the mean residence time of N (MRTN). Theory suggests a trade‐off between both components, but direct experimental evidence is still scarce. A field study with young trees of the evergreen Quercus ilex and the marcescent‐evergreen Quercus faginea was carried out to test this trade‐off through analysis of plant traits at organ, whole‐plant and population levels. Specific leaf area (SLA) was the main trait positively related to AN in Q. faginea. By contrast, greater litter production and consumption by caterpillars resulted in larger N losses and shorter MRTN in Q. faginea. Early leaf senescence in Q. faginea produced leaf litter with high N concentration that contributed significantly to N loss. Moreover, Q. ilex had higher plant survivorship. The inverse relationship between leaf longevity and SLA is probably a key component of the trade‐off between N losses and plant N productivity. Quercus faginea had greater N uptake from soil, linked to its longer specific root length of fine roots and greater biomass allocation to underground tissues. Smaller N losses in Q. ilex compensated for its smaller N uptake and allowed a similar N balance at whole‐plant level. Our results support the hypothesis of a trade‐off between AN and MRTN. Quercus ilex had a long MRTN, while Q. faginea has a high AN, and vice versa. The long MRTN in Q. ilex involves not only reduced N loss through long intrinsic leaf life span, but also resistance to harsh environmental factors and defence against herbivores. This suggests that a long MRTN is a potentially successful strategy in nutrient‐poor environments.

The impact of tropospheric O3 on leaf number duration and tuber yield of the potato (Solanum tuberosum L.) cultivars Bintje and Kardal

Ozone (O3) is a major phytotoxic air pollutant with the potential to cause severe yield losses in potato (Solanum tubersosum L.) and in other crops. The present study was aimed (i) to investigate the O3 sensitivity of two potato cultivars (Bintje and Kardal) in relation to their earliness in maturation, and (ii) to analyse possible O3 effects on potato tuber dry mass (DM) production in relation to leaf duration. The experiment was performed in the southwest of Sweden using open-top chamber (OTC) technique. The crops were exposed to three levels of O3: charcoal-filtered air (CF), non-filtered air (NF) and non-filtered air + 26 nmol mol−1 O3 (NF+). In addition, ambient air plots were used to monitor the impact of the OTC enclosure. Leaf number duration (LND) was calculated as the number of leaves integrated over the temperature sum accumulation during the season. In the present study, the O3 effects on potato were characterised by visible injury of the leaves and earlier leaf senescence. The haulm/plant DM ratio was significantly lower in NF+ compared to CF and NF. The conclusions were (i) that the early to intermediate maturing cultivar Bintje tended to suffer more from O3 injury than the late maturing cultivar Kardal, (ii) that this was due to a difference in the ability to compensate for haulm damage by the development of new leaves rather than to a difference in leaf O3 sensitivity, and (iii) that a rather low O3 exposure can induce a significant reduction in LND in potato. The O3 effects on tuber DM (−2% in NF+ and +31% increase in CF, both compared to NF) could not be statistically demonstrated, although there was a strong correlation between LND and tuber DM.

Glycerol-insensitive Arabidopsis mutants: gli1 seedlings lack glycerol kinase, accumulate glycerol and are more resistant to abiotic stress.

The aim of this study was to investigate the process of glycerol catabolism in germinating Arabidopsis seed. A genetic screen was performed to isolate glycerol-insensitive (gli) mutant seedlings. Three separate mutant loci were identified (gli1, gli2 and gli3). Of these, only gli1 is unable to utilise glycerol. Following germination, gli1 seedlings transiently accumulate glycerol derived from the breakdown of storage oil and are more resistant to hyperosmotic stress, salt stress, oxidative stress, freezing and desiccation. Enzyme assays revealed that gli1 lacks glycerol kinase activity. GLI1 mapped to chromosome 1 near the putative glycerol kinase gene NHO1. Mutations in this gene were identified in three independent gli1 alleles. A cDNA encoding GLI1 was cloned and its function was proven by complementation of an Escherichia coli glycerol kinase (glpK) deletion strain. Quantitative RT-PCR analysis showed that GLI1 is expressed in all tissues, but is transiently upregulated during early post-germinative growth and leaf senescence. These data show that glycerol kinase is required for glycerol catabolism in Arabidopsis and that the accumulation of glycerol can enhance resistance to a variety of abiotic stresses associated with dehydration.

Species differences in stomatal control of water loss at the canopy scale in a mature bottomland deciduous forest

In order to evaluate factors controlling transpiration of six common eastern deciduous species in North America, a model describing responses of canopy stomatal conductance ( G S) to net radiation ( R N), vapor pressure deficit ( D) and relative extractable soil water (REW) was parameterized from sap flux data. Sap flux was measured in 24 mature trees consisting of the species Carya tomentosa, Quercus alba, Q. rubra, Fraxinus americana, Liriodendron tulipifera, and Liquidambar styraciflua in a bottomland oak-hickory forest in the Duke Forest, NC. Species differences in model coefficients were found during the 1997 growing season. All species showed a reduction in G S with increasing D. R N influenced G S in the overstory shade intolerant L. styraciflua to a larger extent than the other species measured. In addition, despite a severe drought during the study period, only L. tulipifera showed a decline in G S with decreasing REW. The primary effect of the drought for the other species appeared to be early autumn leaf senescence and abscission. As a result, despite the drought in this bottomland forest accustomed to ample water supply, maximum daily transpiration (1.6 mm) and growing season transpiration (264 mm) were similar to a nearby upland forest measured during a year of above average precipitation. These results may aid in assessing differences in water use and the ability of bottomland deciduous species to tolerate alterations in the frequency or amount of precipitation. Results also suggest little variation in water use among forests of similar composition and structure growing in different positions in the landscape and subjected to large interannual variation in water supply.

Associations among fruit firmness, morphological traits and RAPD markers in the ‘firm’ tomato mutant

The Santa Clara Cultivar is widely known in the tomato-producing region of Vicosa (MG). Recently, tomato plants from the same cultivar with changes in morphological traits and post-harvest fruit characteristics have been identified. The inheritance study and the allelism test carried out by Schuelter et al. (2002) determined that a recessive gene with pleiotropic effects modified the expression of morphological traits, such as the color of stigma and fruits and the early leaf senescence. This gene, present in the ‘firm’ mutant ( frm ), was mapped on the tomato chromosome 10 region - the same region of the lutescent-2 ( l-2 ) gene. However, the identification and location of the genes that increase the firmness of ‘firm’ mutant fruits remain unknown. Therefore, the objective of this study was to analyze the inheritance of morphological and firmness characteristics of fruits which were modified by the mutation using RAPD markers. Results demonstrated that the genomic region comprising the l-2 gene increases the fruit firmness, explaining from 6.27 to 25.09% the phenotypic variation for this trait along the 15 day-period of storage. However, the AQ16 747 and AS8 622 markers, mapped at 11.67 and 21.67 cM from the mutation frm, indicated that the further they were located from this region, the smaller proportion of the phenotypic variation they had. Thus, we can conclude that that the genomic region flanking the l-2 gene also increases fruit firmness in the ‘firm’ mutant, a trait that has never been associated with this gene.

Effects of multiple applications of methyl jasmonate on fruit ripening, leaf gas exchange and vegetative growth in fruit trees

SummaryMultiple applications of methyl jasmonate (Me-J), at 10.mM, enhanced the rate of fruit ripening in peach ‘Redhaven’. The effect of Me-J on red colour development and fruit firmness was most pronounced in fruit treated six times with Me-J. Lower concentrations of Me-J had no effect of fruit colour. Me-J, at 10.mM, significantly reduced stomatal conductance, photosynthetic rate and transpiration rate of crabapple leaves, but had no effect on these parameters in peach leaves. The short-term inhibitory effects of Me-J on leaf gas exchange in crabapple persisted for up to 24 h. Fourteen applications of Me-J over a period of 28 days resulted in a 60% decrease in the concentration of chlorophyll in leaves. These Me-J applications also reduced the length of new branches, leaf number and leaf fresh weight by 65%, 31% and 47%, respectively. In peach, Me-J induced some leaf chlorosis and early leaf senescence within two weeks of the treatments which resulted in significant reductions in shoot length.

Rosetted siblings in F2 of a cross in pomegranate (Punica granatum L.) can be useful model for rosetting investigations

Environmentally and developmentally controlled rosette formation is welldocumented in certain crop species. But this report describes a geneticallyinduced rosetting observed in pomegranate (Punica granatum L.).Rosette genotypes were detected at a frequency of 0.12 in the F2 familiesof cv. Ganesh and a recessive rosette mutant clone of cv. Kabul Yellow. Therosetted seedlings had leaves almost touching the ground at emergence, whichwere closely set on a thick, compressed stem with narrow inter-nodes andinactive subapical meristem. They were similar in some certain respects tothe naturally occurring rosetting noticed in lettuce, spinach and lisianthus.The occurrence of rosette type was attributed to a recessive mutant gene ofKabul Yellow and was ascribed the gene symbol rg (rosette growth) whichwas found to alter a host of morphological traits in rosette siblings innursery and field. The foliage colour, pink of Ganesh and yellow of KabulYellow, served as visual marker in distinguishing segregating progeny intopink/yellow normal and rosette types in the nursery. A monogenic segregationin F2 for pink and yellow was evident both in normal and rosette siblings.When a single gene mutation for rosetting was considered a large portion ofthe rosette progeny in F2 was found missing. This has been explained on thebasis of two conditional lethal genes operating in rosette individuals.Moreover, the F2 and BC1 seeds had very low seed fertility indicatingthat F1 possibly carried heavy load of lethal genes. On transplantation,the rosette mutants started to grow tall and erect, like spinach, but had acompact habit with restricted branching and exhibited early leaf senescence,leaving a crown of bushy growth at the tip of almost naked, brittle shoots.Rosettes differed significantly from normal siblings for all the morphologicalcharacteristics studied. The normal progenies flowered and set fruits within13 months, the rosette ones almost failed to do so even after growing for 38months, although they showed relatively loose rosetting than in the nursery.However, one seedling produced 3 small flowers with rudimentary ovary.Moreover, it was observed that the rosette plants had a tendency to regressto normal state, since 9 out of the 31 rosette progeny produced axilary shootsthat had normal stem and leaf attributes. The individuals with both normal androsette shoots should serve the same purpose in research work, as do perfect isogeniclines. Based on the frequency at which the normal shoots appeared the role ofcryptic transposable elements (TEs) was suspected rather than back mutation.Rosette siblings with altered morphology reported here can be secured inabundant number along with the contrasting normal siblings by raising theF2, which provide a good opportunity to gain an insight in the control ofvarious plant developmental processes. Besides, it can be viewed as a valuablemodel system for fundamental research on physiological, biochemical andmolecular genetical bases of rosetting in crop plants.

Arabidopsis onset of leaf death mutants identify a regulatory pathway controlling leaf senescence.

The onset of leaf senescence is controlled by leaf age and ethylene can promote leaf senescence within a specific age window. We exploited the interaction between leaf age and ethylene and isolated mutants with altered leaf senescence that are named as onset of leaf death (old) mutants. Early leaf senescence mutants representing three genetic loci were selected and their senescence syndromes were characterised using phenotypical, physiological and molecular markers. old1 is represented by three recessive alleles and displayed earlier senescence both in air and upon ethylene exposure. The etiolated old1 seedlings exhibited a hypersensitive triple response. old2 is a dominant trait and the mutant plants were indistinguishable from the wild-type when grown in air but showed an earlier senescence syndrome upon ethylene treatment. old3 is a semi-dominant trait and its earlier onset of senescence is independent of ethylene treatment. Analyses of the chlorophyll degradation, ion leakage and SAG expression showed that leaf senescence was advanced in ethylene-treated old2 plants and in both air-grown and ethylene-treated old1 and old3 plants. Epistatic analysis indicated that OLD1 might act downstream of OLD2 and upstream of OLD3 and mediate the interaction between leaf age and ethylene. A genetic model was proposed that links the three OLD genes and ethylene into a regulatory pathway controlling the onset of leaf senescence.

The Arabidopsis Xylem Peptidase XCP1 Is a Tracheary Element Vacuolar Protein That May Be a Papain Ortholog

XCP1 is a xylem-specific papain-like cysteine peptidase in Arabidopsis. To determine whether XCP1 could be involved in tracheary element autolysis, promoter activity and localization of XCP1 were investigated using XCP1 promoter-beta-glucuronidase fusions and immunofluorescence confocal microscopy. A tracheary element expression pattern was detected for XCP1. Results from confocal microscopy and biochemical subcellular fractionation indicated that XCP1 was localized in the vacuole. Ectopic expression of XCP1 resulted in a reduction in plant size in some lines and early leaf senescence, as indicated by early loss of leaf chlorophyll. Reduced plant size was correlated with higher levels of XCP1, as shown by immunoblot and peptidase activity gel analyses. The XCP1 prodomain exhibits exceptionally high similarity (greater than 80%) to the prodomains of papain and other papain-like enzymes isolated from papaya (Carica papaya) laticifers when compared with all other reported papain-like enzymes. The potential for XCP1 and papain to perform common functions as catalysts of autolytic processing following cell death due to programmed suicide or to wounding is discussed.

Genetic Improvement in Short-Season Soybeans: II. Nitrogen Accumulation, Remobilization, and Partitioning.

Genetic improvement in yield is conditional on surmounting yield-limiting factors. Nitrogen (N) has been considered an important limiting factor to soybean [Glycine max (L.) Merr.] yield. The high demand for N by soybean seed was previously considered to lead to early leaf senescence through accelerated remobilization of N from the vegetative tissue. The consequent reduction in photosynthetic capacity was postulated to limit yield. The objectives of the current experiment were to determine the changes in N accumulation, remobilization, and partitioning associated with genetic yield improvement. Two groups of old, low-yielding ('Pagoda' and 'Mandarin Ottawa') and new, high-yielding ('Maple Glen' and 'OAC Bayfield') soybean cultivars of similar maturity were grown in side-by-side trials at the Elora Research Station, Ontario, in 1996 and 1997. Nitrogen and dry matter accumulation in leaf, stem + petiole, roots, and seeds were determined during the growing season. The newer cultivars had higher yields and higher seed N content. Contrary to the postulated association between leaf senescence and leaf N values, neither leaf N concentration nor leaf N content per unit leaf area (at R6) were association consistently with either yield or leaf area duration (LAD). Although most of the N in the seed was derived from N remobilized from vegetative tissue, the newer cultivars with their higher yields and LAD, remobilized no more N out of the vegetative tissue than did older, lower-yielding ones. The newer cultivars were distinct from their older counterparts in their ability to accumulate more N during the seed filling period (SFP). Genetic improvement of the short-season soybeans tested was a consequence of continued N accumulation during the SFP and was not due to differences in the genotype's capacity to remobilize or partition N to the seed.

Genetic Improvement in Short-Season Soybeans

Genetic improvement in yield is conditional on surmounting yield-limiting factors. Nitrogen (N) has been considered an important limiting factor to soybean [Glycine max (L.) Merr.] yield. The high demand for N by soybean seed was previously considered to lead to early leaf senescence through accelerated remobilization of N from the vegetative tissue. The consequent reduction in photosynthetic capacity was postulated to limit yield. The objectives of the current experiment were to determine the changes in N accumulation, remobilization, and partitioning associated with genetic yield improvement. Two groups of old, low-yielding (‘Pagoda’ and ‘Mandarin Ottawa’) and new, high-yielding (‘Maple Glen’ and ‘OAC Bayfield’) soybean cultivars of similar maturity were grown in side-by-side trials at the Elora Research Station, Ontario, in 1996 and 1997. Nitrogen and dry matter accumulation in leaf, stem + petiole, roots, and seeds were determined during the growing season. The newer cultivars had higher yields and higher seed N content. Contrary to the postulated association between leaf senescence and leaf N values, neither leaf N concentration nor leaf N content per unit leaf area (at R6) were association consistently with either yield or leaf area duration (LAD). Although most of the N in the seed was derived from N remobilized from vegetative tissue, the newer cultivars with their higher yields and LAD, remobilized no more N out of the vegetative tissue than did older, lower-yielding ones. The newer cultivars were distinct from their older counterparts in their ability to accumulate more N during the seed filling period (SFP). Genetic improvement of the short-season soybeans tested was a consequence of continued N accumulation during the SFP and was not due to differences in the genotype's capacity to remobilize or partition N to the seed.

Physio-Morphological Characters of F1 Hybrids of Rice (Oryza sativa L.) in Japonica-Indica Crosses: II. Heterosis for leaf area and dry matter accumulation

SummaryA pot experiment was conducted to investigate the heterosis for morphological characters and to examine the relationship among some characters at active tillering, flowering and dough ripe stages of 12 Fj hybrids from crosses between japonica and indica rice. Heterosis for plant height, number of tillers, green leaf area and dry matter accumulation per plant was positive at all stages. The intensity of heterosis was higher at the active tillering stage for number of tillers, leaf area and dry matter accumulation per plant. A significant positive relationship was found between tiller number and leaf area per plant at active tillering and flowering stages. Significant positive relationships between leaf area and dry matter accumulation, and between tiller number and dry matter accumulation per plant were observed at all stages. A significant positive relationship was found between leaf area per plant at an early stage and dry matter accumulation per plant at a later stage, suggesting that early development of leaf area is an important factor for higher dry matter accumulation in 1 hybrids. Although heterosis for percent dead leaf blade was positive at the flowering stage due to early leaf senescence in some Fx hybrids, a larger leaf area was also found in Fx hybrids at this stage and leaves of Fx hybrids remained green longer compared to parent cultivars up to the dough ripe stage.