莴苣授粉前后柱头与花柱中钙的分布变化

莴苣柱头呈两裂片状,有接受花粉的接受面和非接受面之分。授粉前后,柱头接受面乳突细胞的细胞壁中贮存丰富的细小钙颗粒,而非接受面的表皮细胞壁中的钙颗粒很少。在花柱组织中钙的分布具有明显特征,在同一水平上,钙主要分布在引导组织的质外体中,如细胞壁和细胞间隙中,而在引导组织外的薄壁组织中钙主要分布在的细胞内液炮和细胞壁中以及维管束导管内。在花柱不同水平上,花柱中的钙呈现出梯度分布,顶端各组织中的钙颗粒较少,基部各组织中的钙颗粒较多。授粉后1h花柱基部组织的钙颗粒增多,钙梯度分布现象增强。花柱引导组织中的钙梯度分布很可能是吸引花粉管向下生长的原因。讨论了莴苣花柱引导组织中钙梯度分布特征和花粉管在其体内生长的关系。     

Absorption, translocation and metabolism of -DOPA in barnyardgrass and lettuce: their involvement in species-selective phytotoxic action

-DOPA (-3,4-Dihydroxyphenylalanine) is one of the most highly active allelochemicals. -DOPA is exuded from the roots of velvetbean [Mucuna pruriens (L.) DC. var. utilis] into soil, and causes growth inhibition of other species. In order to highly clarify the phytotoxic mechanism of -DOPA, its effect on 32 species was surveyed, and absorption, translocation and metabolism in a tolerant species, barnyardgrass (Echinochloa crus-galli L.), and susceptible species, lettuce (Lactuca sativa L. cv. Great lakes 366), were examined at the germination stage. The species tested showed distinctly different responses to -DOPA, with root elongation being more suppressed than that of shoots. Barnyardgrass was 77-fold more tolerant than lettuce based on the GR₅₀ values determined 5 days after treatment. Absorption of -DOPA in barnyardgrass and lettuce increased continuously during a 5 day exposure period, however,barnyardgrass absorbed a larger amount of -DOPA than lettuce. The translocation of radioactivity derived from ¹⁴C--DOPA to shoots was greater in lettuce than in barnyardgrass 3 and 5 days after treatment. Although the -DOPA absorbed was metabolized in the roots of both species, the percentage of radioactive ¹⁴C--DOPA increased in lettuce continuously but decreased in barnyardgrass over 5 days. In lettuce roots, a continuous increase of -DOPA but not other metabolites was observed. However, the concentration of -DOPA was higher in barnyardgrass roots compared with lettuce roots throughout the exposure period. These results suggest that -DOPA itself is the active form, and the species-selective phytotoxicity of -DOPA is at least partly due to metabolism and not due to absorption or translocation.     

Mg2+ accelerates plastoquinone reduction in chloroplasts

The transient fluorescence quenching induced by the addition of a small amount of an oxidant to illuminated chloroplasts can be used to estimate the rate of electron transported by the oxidant. Using this technique, it is found that the reduction of plastoquinone by the primary acceptor of Photosystem II is sensitive to salt depletion.     

Temporal and spatial spread of Lettuce mosaic virus in lettuce crops in central Spain: factors involved in Lettuce mosaic virus epidemics

Lettuce mosaic virus (LMV) is transmitted by aphid vectors in a nonpersistent manner as well as by seeds. The virus causes severe disease outbreaks in commercial lettuce crops in several regions of Spain. The temporal and spatial patterns of spread of LMV were studied in autumn 2002 in the central region of Spain. Symptomatic lettuce (var. Cazorla) plant samples were collected weekly, first at the seedling stage from the greenhouse nursery and later outdoors after transplantation. The exact position of symptomatic plants sampled in the field was recorded and then material was tested by enzyme‐linked immunosorbent assay to assess virus infection. Cumulative spatial data for infected plants at different growth stages were analysed using spatial analysis by distance indices. For temporal analysis, the monomolecular, Gompertz, logistic and exponential models were evaluated for goodness of fit to the entire set of disease progress data obtained. The results indicated that the disease progress curve of LMV epidemics in the selected area is best described by a Gompertz model and that the epidemic follows a polycyclic disease progression. Our data suggest that secondary cycle of spread occurs when noncolonising aphid species land on the primary infected plants (probably coming from infected seed) and move to adjacent plants before leaving the crop. The role of weeds growing close to lettuce fields as potential inoculum sources of virus and the aphid species most likely involved in the transmission of LMV were also identified.     

Growth Inhibitors of Lettuce Seedlings From Bacillus cereus EJ-121

The ethyl acetate extract of the Bacillus sp. EJ-121 culture broth exhibited growth inhibitory activity on a lettuce (Lactuca sativa L.) seedlings assay. Bacillus sp. EJ-121 was identified as Bacillus cereus by the morphological characteristic and nucleotide sequence of the 16S rDNA. The bioassay-guided fractionation of the ethyl acetate extract led to the isolation of two compounds. Their structures were deduced by spectroscopic methods and determined as sodium vanillate (1) and 2-aminobenzoic acid (2). Both compounds 1 and 2 inhibited more than 90% of root length at 50 ppm (0.26 and 0.36 mM, respectively) while they had a limited effect on shoot growth at the same concentration level. Roots and shoots of lettuce seedlings showed severe deterioration at 100 ppm. In order to study the fundamental structure–activity relationship, several structurally related benzoic acid derivatives were also assayed. The existence of a polar carboxyl moiety seemed to be responsible for the stronger activity.     

Kinetic and regulatory properties of plant ADP-glucose pyrophosphorylase genetically modified by heterologous expression of potato<Emphasis Type="Italic"> upreg</Emphasis> mutants in vitro and in vivo

In this study, the uses of the mutated genes, upreg1 and upreg2, encoding upregulated ADP-glucose pyrophosphorylase (AGPase) large subunits with increased enzymatic activity, to improve crop yield productivity was evaluated in vitro and in planta. For in vitro examination, wild type and upregs were co-expressed with three different AGPase small subunit genes from potato and perilla to produce nine AGPase isoforms. In kinetic experiments, 3-Phosphoglycerate increased the V _max and decreased the K _M for the recombinant AGPase. Regardless of the specific small subunit, Upreg-type AGPases had much larger increases in enzymatic activity with concomitant decreases in values as compared to the wild type enzyme. Transformation of lettuce with the upreg1 gene altered the regulatory properties of leaf AGPase. AGPases from transgenic lettuce showed greater 3-PGA activation and lower Pi inhibition than was observed for wild type AGPase. Fresh weights of the aerial parts of transgenic plants were larger than non-transgenic controls. Based on these results, upreg mutant genes could be used for the genetic improvement of plant AGPases other than potato and effectively increase crop yield productivity.     

Fungal antagonists of the plant pathogen Rhizoctonia solani: selection, control efficacy and influence on the indigenous microbial community

A broad spectrum of fungal antagonists was evaluated as potential biocontrol agents (BCAs) against the soil-borne pathogen Rhizoctonia solani using a new combination of in vitro and in vivo assays. The in vitro characterisation of diverse parameters including the ability to parasitise mycelium and to inhibit the germination of Rhizoctonia sclerotia at different temperatures resulted in the selection of six potential fungal antagonists. These were genotypically characterised by their BOX-PCR fingerprints, and identified as Trichoderma reesei and T. viride by partial 18S rDNA sequencing. When potato sprouts were treated with Trichoderma, all isolates significantly reduced the incidence of Rhizoctonia symptoms. Evaluated under growth chamber conditions, the selected Trichoderma isolates either partly or completely controlled the dry mass loss of lettuce caused by R. solani. Furthermore, the antagonistic Trichoderma strains were active under field conditions. To analyse the effect of Trichoderma treatment on indigenous root-associated microbial communities, we performed a DNA-dependent SSCP (Single-Strand Conformation Polymorphism) analysis of 16S rDNA/ITS sequences. In this first assessment study for Trichoderma it was shown that the pathogen and the vegetation time had much more influence on the composition of the microbiota than the BCA treatment. After evaluation of all results, three Trichoderma strains originally isolated from Rhizoctonia sclerotia were selected as promising BCAs.     

Wilt and vascular root rot (<i>Pythium tracheiphilum</i>) of lettuce in Bahía Blanca,Argentina

A new disease of lettuce has been observed in the surroundings of Bahía Blanca, Argentina. The symptoms include dwarfing, general chlorosis, wilting, root rot, and leaf blight, sometimes followed by plant death. Pythium tracheiphilum Matta was found to be consistently associated with those symptoms. The morphological and morphometric characteristics of this oomycete are described. Isolates obtained from field-infected lettuce plants were inoculated to lettuce and caused similar symptoms as those found in natural infections. Seedling emergence was also severely affected following experimental inoculation. Koch’s Postulates were fulfilled by recovering the fungus from inoculated plants. The new disease shows a high destructive potential but currently has a limited prevalence and incidence.     

Interpreting genotype-by-environment interaction using redundancy analysis

Summary Methods for the interpretation of genotype-by-environment interaction in the presense of explicitly measured environmental variables can be divided into two groups. Firstly, methods that extract environmental characterizations from the data itself, which are subsequently related to measured environmental variables, e.g., regression on the mean or singular value decomposition of the matrix of residuals from additivity, followed by correlation, or regression, methods. Secondly, methods that incorporate measured environmental variables directly into the model, e.g., multiple regression of individual genotypical responses on environmental variables, or factorial regression in which a genotype-by-environment matrix is modelled in terms of concomitant variables for the environmental factor. In this paper a redundancy analysis is presented, which can be derived from the singular-value decomposition of the residuals from additivity by imposing the restriction on the environmental scores of having to be linear combinations of environmental variables. At the same time, redundancy analysis is derivable from factorial regression by rotation of the axes in the space spanned by the fitted values of the factorial regression, followed by a reduction of dimensionality through discarding the least explanatory axes. Redundancy analysis is a member of the second group of methods, and can be an important tool in the interpretation of genotype-by-environment interaction, especially with reference to concomitant environmental information. A theoretical treatise of the method is given, followed by a practical example in which the results of the method are compared to the results of the other methods mentioned.     

PCR-based fingerprinting using AFLPs as a tool for studying genetic relationships in Lactuca spp.

AFLP markers were evaluated for determining the phylogenetic relationships Lactuca spp. Genetic distances based on AFLP data were estimated for 44 morphologically diverse lines of cultivated L. sativa and 13 accessions of the wild species L. serriola, L. saligna, L. virosa, L. perennis, and L. indica. The same genotypes were analyzed as in a previous study that had utilized RFLP markers. The phenetic tree based on AFLP data was consistent with known taxonomic relationships and similar to a tree developed with RFLP data. The genetic distance matrices derived from AFLP and RFLP data were compared using least squares regression analysis and, for the cultivar data, by principal component analysis. There was also a positive linear relationship between distance estimates based on AFLP data and kinship coefficients calculated from pedigree data. AFLPs represent reliable PCR-based markers for studies of genetic relationships at a variety of taxonomic levels.