植物根系吸水模型研究进展

植物根系吸水模型是当前生态水文和陆面过程建模领域最为活跃的研究方向,是研究流域水文、生态、环境以及水资源可持续利用等科学问题中最为关键的部分,研究植物根系吸水的物理和生理机制及其影响因素,是建立植物根系吸水模型的基础.本文通过对植物根系吸水模型研究的回顾,讨论了水分和盐分胁迫在根系吸水中的作用、根系吸水的多维模型、根系吸水在陆面过程中的作用等问题,指出植物根系吸水模型发展所面临的问题并展望了未来的发展方向.     

Compensatory function for water transport by adventitious roots of <Emphasis Type="Italic">Ipomoea pes-caprae</Emphasis>

To determine the role of adventitious roots in supplying water to Ipomoea pes-caprae (L.) Sweet (Convolvulaceae), we examined the effects of water deficit on water uptake and the growth patterns of leaves and shoots. After stopping the water supply from the primary root or adventitious roots, the water-uptake rate of the other root system increased steeply within 90–100 min to a level of 90% of the pretreatment water-uptake rate of the whole plant. Thus, the primary and adventitious roots can compensate for a decrease in the water-uptake rate of the whole plant caused by dehydration. The continuous growth of leaves and shoots after dehydration suggests that an increase in the water-uptake rate by either root system can support plant growth, although the growth rates of immature leaves in plants with no water supply from the primary or adventitious roots were lower than in controls. We conclude that the water supply from adventitious roots contributes to the survival and growth of plants, and will be important for vegetative propagation.     

Differences in the cellular zinc content and 5′-nucleotidase activity of normal and acrodermatitis enteropathica (AE) fibroblasts

The acrodermatitis enteropathica (AE) mutation affects zinc (Zn) metabolism in human fibroblasts. We hypothesize that the mutation affects the cell Zn content, which subsequently affects the activity of various zinc-dependent enzymes, such as 5′-nucleotidase. Therefore, normal and AE fibroblasts were grown in normal medium containing physiological levels of Zn (16 Μmol/L) for ∼24 h. The medium was replaced by normal medium (16 Μmol/L Zn), Zn-depleted medium (1.5 Μmol/L Zn), or Zn-supplemented medium (200 Μmol/L Zn) for another 24 h. Regardless of the Zn concentration of the growth medium, the AE fibroblasts contained significantly less Zn than normal fibroblasts grown in comparable medium. Nevertheless, growth of the fibroblasts in 200 Μmol/L Zn medium significantly increased the cell Zn content fourfold of both normal and AE fibroblasts. The activity of 5′-nucleotidase in the AE fibroblasts grown in 16 Μmol/L Zn or 1.5 Μmol/L Zn medium was also significantly lower than in normal fibroblasts. Changing the growth medium from 16 Μmol/L Zn to 1.5 Μmol/L Zn medium did not affect the activity of the enzyme in either genotype. Cells grown in 200 Μmol/L Zn medium exhibited threefold greater 5′-nucleotidase activity in AE fibroblasts, but had no affect on enzyme activity in normal cells. In summary, altering the cell Zn content of normal fibroblasts did not result in a significant change in their 5′ -nucleotidase activity. However, AE fibroblasts grown in 200 Μmol/L Zn medium exhibited recovery of their 5′-nucleotidase activity to normal levels. These results support the hypothesis that the AE mutation affects the cellular Zn content. The lower cell Zn content subsequently affects the activity of 5′-nucleotidase.     

Targeted mapping of ESTs linked to the adult plant resistance gene Lr46 in wheat using synteny with rice

The gene Lr46 has provided slow-rusting resistance to leaf rust caused by Puccinia triticina in wheat (Triticum aestivum), which has remained durable for almost 30 years. Using linked markers and wheat deletion stocks, we located Lr46 in the deletion bin 1BL (0.84–0.89) comprising 5% of the 1BL arm. The distal part of chromosome 1BL of wheat is syntenic to chromosome 5L of rice. Wheat expressed sequence tags (ESTs) mapping in the terminal 15% of chromosome 1BL with significant homology to sequences from the terminal region of chromosome 5L of rice were chosen for sequence-tagged site (STS) primer design and were mapped physically and genetically. In addition, sequences from two rice bacterial artificial chromosome clones covering the targeted syntenic region were used to identify additional linked wheat ESTs. Fourteen new markers potentially linked to Lr46 were developed; eight were mapped in a segregating population. Markers flanking (2.2 cM proximal and 2.2 cM distal) and cosegregating with Lr46 were identified. The physical location of Lr46 was narrowed to a submicroscopic region between the breakpoints of deletion lines 1BL-13 [fraction length (FL)=0.89–1] and 1BL-10 (FL=0.89–3). We are now developing a high-resolution mapping population for the positional cloning of Lr46.     

Utilization of Sediment CO2 by Selected North American Isoetids

The photosynthetic uptake of root-zone CO₂ was determined forEriocaulon septangulare, Gratiola aurea, Isoetes macrospora,Littorella uniflora var. americana and Lobelia dortmanna aspart of a study of the photosynthetic carbon economy of submergedaquatic isoetids. The pH and dissolved inorganic carbon (DIC)of the sediment interstitial water in four Wisconsin lakes reflectedthe water column character, where the DIC increased with depthin the sediment to concentrations five to ten times those ofthe water column. Sediment free CO₂ concentrations were 5–50times those in the water column and were similar at all sites(about 05–1.0mM CO₂ in the root-zone). In ‘pH-drift’studies these plants were unable to take up HCO₂^–. Laboratory determinations of the carbon uptake from the rootand shoot-zones were made for all five species. These experimentsshowed that CO₂ in the root-zone accounted for 65–95 percent of external carbon uptake for the five species. For G.aurea and E. septangulare, root-zone CO₂ was > 85 per centof carbon uptake. Carbon, CO₂, photosynthesis, sediment, isoetid, Eriocaulon septangulare, Gratiola aurea, Isoetes macrospora, Littorella uniflora, Lobelia dortmanna     

Development of simple sequence repeat markers specific for the Lr34 resistance region of wheat using sequence information from rice and Aegilops tauschii

Hexaploid wheat (Triticum aestivum L.) originated about 8,000 years ago from the hybridization of tetraploid wheat with diploid Aegilops tauschii Coss. containing the D-genome. Thus, the bread wheat D-genome is evolutionary young and shows a low degree of polymorphism in the bread wheat gene pool. To increase marker density around the durable leaf rust resistance gene Lr34 located on chromosome 7DS, we used molecular information from the orthologous region in rice. Wheat expressed sequence tags (wESTs) were identified by homology with the rice genes in the interval of interest, but were monomorphic in the ‘Arina’ × ‘Forno’ mapping population. To derive new polymorphic markers, bacterial artificial chromosome (BAC) clones representing a total physical size of ∼1 Mb and belonging to four contigs were isolated from Ae. tauschii by hybridization screening with wheat ESTs. Several BAC clones were low-pass sequenced, resulting in a total of ∼560 kb of sequence. Ten microsatellite sequences were found, and three of them were polymorphic in our population and were genetically mapped close to Lr34. Comparative analysis of marker order revealed a large inversion between the rice genome and the wheat D-genome. The SWM10 microsatellite is closely linked to Lr34 and has the same allele in the three independent sources of Lr34: ‘Frontana’, ‘Chinese Spring’, and ‘Forno’, as well in most of the genotypes containing Lr34. Therefore, SWM10 is a highly useful marker to assist selection for Lr34 in breeding programs worldwide.     

A study of modified forms of the Lr19 translocation of common wheat

 Following the induction of allosyndetic pairing between the Thinopyrum-derived Lr19 translocation in ‘Indis’ wheat and homoeologous wheat chromatin, eight suspected recombinants for the Lr19 region were recovered. These selections were characterised for marker loci that were previously used to construct a physical map of the Lr19 segment. At the same time near-isogenic lines were developed for some of the selected segments and tested for seedling leaf-rust resistance in order to confirm the presence of Lr19. It appeared that three of the four white-endosperm selections do not possess Lr19 and only one, 88M22-149, is a true Lr19 recombinant. The resistance gene in the three non-Lr19 selections resides on chromosome 6B, appears to derive from ‘Indis’, and was selected unintentionally during backcrossing. The pedigree of ‘Indis’ is suspect and it is believed that the Lr19 translocation in ‘Indis’ is in reality the Th. ponticum-derived (T4) segment rather than being of Th. distichum origin as was believed earlier. The white-endosperm recombinant, 88M22-149, retained the complete Lr19 resistance and was apparently re-located to chromosome arm 7BL in a double-crossover event. 88M22-149 has lost the Sd1 gene and often shows strong self-elimination in translocation heterozygotes. This effect may result from additional gametocidal loci or from an altered chromosome structure following re-location of the segment. 88M22-149 in fact contains a duplicated region involving the Wsp-B1 locus. Three selections had partially white endosperms and expressed Lr19 and other Thinopyrum marker alleles. Polymorphisms for the available markers confirmed that the translocated segment in at least one of them had been shortened through recombination with chromosome arm 7DL. Further markers need to be studied in order to determine whether the translocation in the remaining two partially white recombinants had also undergone recombination with wheat. The eighth selection has yellow endosperm and appears to self-eliminate in certain translocation heterozygotes. No evidence of recombination could be found with the markers used. If the latter selections are in fact recombinants they may prove useful in attempts to unravel the complex segregation distortion mechanism. Received: 8 August 1996 / Accepted: 10 January 1997     

Benefits of alternate partial root-zone irrigation on growth, water and nitrogen use efficiencies modified by fertilization and soil water status in maize

Alternate partial root-zone irrigation (APRI) is a new water-saving technique and may improve crop water use efficiency without much yield reduction. We investigated if the benefits of APRI on biomass accumulation, water and nitrogen use efficiencies could be modified by different soil fertilization and watering levels in pot-grown maize (Zea mays L. cv. super-sweet No 28, a local variety). Three irrigation methods, i.e. conventional irrigation (CI), alternate partial root-zone irrigation (APRI, alternate watering on both sides of the pot) and fixed partial root-zone irrigation (FPRI, fixed watering on one side of the pot), two watering levels, i.e. water deficit (W₁, 45–55% of field capacity) and well-watered (W₂, 70–80% of field capacity), and two N fertilization levels, i.e. no fertilization and fertilization, were designed. Results showed that APRI and FPRI methods led to more reduction in transpiration than in photosynthesis, and thus increased leaf water use efficiency (leaf WUE, i.e. the ratio of leaf net photosynthetic rate to transpiration rate). Compared to the CI treatment, APRI and FPRI increased leaf WUE by 7.7% and 8.1% before the jointing stage and 3.6% and 4.2% during the jointing stage, respectively. Under the fertilization and well-watered conditions, APRI treatment saved irrigation water by 38.4% and reduced shoot and total dry masses by 5.9% and 6.7%, respectively if compared to the CI treatment. APRI also enhanced canopy WUE (defined as the amount of total biomass per unit water used) and nitrogen (N) apparent recovery fraction (Nr, defined as the ratio of the increased N uptake to N applied) by 24.3% and 16.4%, respectively, indicating that effect of APRI can be better materialized under appropriate fertilization and water supply. Responsible Editor: Rana E. Munns     

A mathematical model for describing light-response curves in <Emphasis Type="Italic">Nicotiana tabacum</Emphasis> L.

A modified exponential model was used to describe light-response curves of Nicotiana tabacum L. The accuracies of an exponential model, a nonrectangular hyperbola model, a rectangular hyperbola model, a modified rectangular hyperbola model and the modified exponential model were evaluated by Mean square error (MSE) and Mean absolute error (MAE). The tests MSE and MAE of the modified exponential model were the lowest among the five models. The light saturation point (LSP) obtained by the exponential model, the nonrectangular hyperbola model and the rectangular hyperbola model were much lower than the measured values, and the maximum net photosynthetic rates (P _max) calculated from these models, were greater than the measured values. P _max at LSP of 1,077 μmol m⁻² s⁻¹ calculated by the modified exponential model was 12.34 μmol(CO₂) m⁻² s⁻¹, which was more accurate than the values obtained from the modified rectangular hyperbola model. The results show that the modified exponential model is superior to other models for describing light-response curves.     

Adhesion of Lactobacillus reuteri strain Lr1 to equine epithelial cells and competitive exclusion of Clostridium difficile from the gastro-intestinal tract of horses

Lactobacillus reuteri Lr1, isolated from healthy horses, remained viable after 2 h at pH 2.0 and in the presence of 1.5 % (w/v) bile. Strain Lr1 survived passage through the equine gastro-intestinal tract (GIT). However, no viable cells of L. reuteri Lr1 were detected on the third day after administration, suggesting that the strain did not colonise the GIT for longer than two days. Strain Lr1 adhered to non-viable, but not to viable, buccal epithelial cells in vitro. Adherence of strain Lr1 to buccal epithelial cells increased 25 % after treatment of the bacterial cells with pepsin. Treatment with pronase prevented the adhesion to epithelial cells. This suggested that specific proteins on the cell surface of L. reuteri Lr1 are involved in adhesion to epithelial cells. Strain Lr1 aggregated with Clostridium difficile C6, isolated from the GIT of a horse that died from severe colic. Adherence of C. difficile C6 to epithelial cells declined from 60 % to 3 % when challenged with L. reuteri Lr1 and the number of viable clostridia decreased tenfold during dosage. Red blood cell, haemoglobin and haemocrit levels were significantly (P ≤ 0.05) lower after dosage with L. reuteri Lr1. Cholesterol and glucose levels were mildly elevated for one day during dosage, but decreased significantly thereafter to levels similar than before dosage. Genes encoding adhesion to collagen, production of aggregation substances, cytolysin and β hemolysin III, resistance to vancomycin A, B and C, and gelatinase activity were not detected, suggesting that L. reuteri Lr1 is a potential probiotic that may be used to control C. difficile cell numbers in the GIT.     

Effects of the Temperature of the Rooting Zone on the Growth and Development of Roots of Potato (Solanum tuberosum)

In solution culture the effect of optimal (20 °C) and supra-optimal(30 °C) root-zone temperatures on root growth and root morphologyof six potato clones (Solanum tuberosum L spp) was studied Growthwas compared with sweet potato (Ipomoea batatas L) and cassava(Manihot esculenta Crantz) Significant genotypical differencesin the responses of potato roots to supra-optimal temperatureswere observed, indicating the potential for selecting heat tolerantpotato clones In both heat tolerant and heat sensitive clones,the size of the root system was reduced by supra-optimal root-zonetemperature This was principally a result of decreased numberand length of lateral roots The first symptom of heat damagewas a reduction in the rate of cell division, followed by cessationof root elongation Bending of the apical root-zone togetherwith the formation of root hairs on the inner (concave) andlateral roots on the outer (convex) side were other symptoms,these coincided with the loss of starch granules in the root-cap Potato, genotypical differences, root morphology, root growth, temperature, Solanum tuberosum L     

Effect of root applied 24-epibrassinolide on carbohydrate status and fermentative enzyme activities in cucumber (<Emphasis Type="Italic">Cucumis sativus</Emphasis> L.) seedlings under hypoxia

The effects of 24-epibrassinolide (EBR) added to nutrient solution on growth of cucumber (Cucumis sativus L.) under root-zone hypoxia were investigated. Cucumber seedlings were hydroponically grown for 8 days in normoxic and hypoxic nutrient solutions with and without addition of EBR at 1 μg l⁻¹. EBR exerted little influence on plant performance in the normoxic nutrient solution, while the chemical alleviated root-zone hypoxia-induced inhibition of root and shoot growth and net photosynthetic rate (Pn). EBR added to hypoxic nutrient solution caused an increase in the concentration of fructose, sucrose, and total soluble sugars in the roots but not in the leaves. Root-zone hypoxia enhanced the activities of lactate dehydrogenase (LDH), alcohol dehydrogenase (ADH), and pyruvate decarboxylase in the roots. Interestingly, EBR further enhanced ADH activity but lowered LDH activity in hypoxic roots. These results suggest that EBR added to hypoxic nutrient solution may stimulate the photosynthate allocation down to roots and the shift from lactate fermentation to alcohol fermentation in hypoxic roots, resulting in the increase in ATP production through glycolysis and the avoidance of cytosolic acidosis and eventually enhanced tolerance of cucumber plants to root-zone hypoxia.     

Occurrence of wheat leaf rust (Puccinia triticina) races and virulence changes in Slovakia in 1994–2004

In 1995–2004 we investigated leaf rust virulence in Slovakia on Thatcher near isogenic lines (NILs) with genes Lr1, Lr2a, Lr2b, Lr2c, Lr3a, Lr9, Lr10, Lr11, Lr15, Lr17, Lr19, Lr21, Lr23, Lr24, Lr26 and Lr28. According to reaction of leaf rust isolates resistance genes Lr9 and Lr19 were completely effective to all examined pathotypes in all years. The resistance genes Lr24 and Lr28 were also completely effective to all examined pathotypes till the year 2001. In the year 2001 we detected 20% and 10% virulent isolates on NILs Lr24 and Lr28, respectively. According to the reaction of investigated isolates from the territory of Slovakia on NILs, resistance genes Lr2c, Lr3a, Lr11, Lr17, Lr21, Lr23 and Lr26 were mostly ineffective. During the 1994–2004 period we detected 16 races of leaf rust (races 2, 2SaBa, 6, 6SaBa, 12, 12SaBa, 14, 14SaBa, 57, 57SaBa, 61, 61SaBa, 62SaBa, 77, 77SaBa, 77/57SaBa). The most frequently determined races were 61SaBa and 77SaBa, which occurred in all years. Among frequently determined races we can assign race 12SaBa as well. According to the field tests in 2001–2004 good resistance to leaf rust was displayed by the cvs Arida (Lr13, Lru), Eva (Lr3, Lru) and Solara (Lru).     

Mapping genes Lr53 and Yr35 on the short arm of chromosome 6B of common wheat with microsatellite markers and studies of their association with Lr36

The rust resistance genes Lr53 and Yr35, transferred to common wheat from Triticum dicoccoides, were reported previously to be completely linked on chromosome 6B. Four F ₃ families were produced from a cross between a line carrying Lr53 and Yr35 (98M71) and the leaf rust and stripe rust susceptible genotype Avocet “S” and were rust tested using Puccinina triticina pathotype 53-1,(6),(7),10,11 and Puccinia striiformis f. sp. tritici pathotype 110 E143 A+. The homozygous resistant lines produced infection types of “;1−” and “;N” to these pathotypes, respectively. The Chi-squared tests indicated goodness-of-fit of the data for one leaf rust gene and one stripe rust gene segregation. Linkage analysis using this population demonstrated recombination of 3% between the genes. Microsatellite markers located on the short arm of chromosome 6B were used to map the genes, with the markers cfd1 and gwm508 being mapped approximately 1.1 and 4.5 cM, respectively, proximal to Lr53. Additional studies of the relationship between Lr36, also located on the short arm of chromosome 6B, and Lr53 indicated that the two genes were independent.     

Identification of microsatellite markers linked to leaf rust resistance gene <Emphasis Type="Italic">Lr25</Emphasis> in wheat

The leaf rust resistance gene Lr25, transferred from Secale cereale L. into wheat and located on chromosome 4B, imparts resistance to all pathotypes of leaf rust in South-East Asia. In an F₂-derived F₃ population, created by crossing TcLr25 that carries the gene Lr25 for leaf rust resistance with leaf rust-susceptible parent Agra Local, three microsatellite markers located on the long arm of chromosome 4B were found to be linked to the Lr25 locus. The donor parent TcLr25 is a near-isogenic line derived from the variety Thatcher. The most virulent pathotype of leaf rust in the South-East Asian region, designated 77–5 (121R63-1), was used for challenging the population under artificially controlled conditions. The marker Xgwm251 behaved as a co-dominant marker placed 3.8 cM away from the Lr25 locus on 4BL. Two null allele markers, Xgwm538 and Xgwm6, in the same linkage group were located at a distance of 3.8 cM and 16.2 cM from the Lr25 locus, respectively. The genetic sequence of Xgwm251, Lr25, Xgwm538, and Xgwm6 covered a total length of 20 cM on 4BL. The markers were validated for their specificity to Lr25 resistance in a set of 43 wheat genetic stocks representing 43 other Lr genes.     

Identification of the allelic state of the <Emphasis Type="Italic">Lr34</Emphasis> leaf rust resistance gene in soft winter wheat cultivars developed in Ukraine

The distribution of alleles at the Lr34 locus associated with leaf rust resistance has been studied in soft winter wheat (Triticum aestivum L.) cultivars developed in Ukraine. To determine the allelic state of the Lr34 locus, codominant molecular marker cssfr5 has been used. Cultivars with the revealed Lr34(+) and Lr34(−) alleles have been identified as potentially resistant or susceptible, respectively. A collection of 81 cultivars from the main breeding centers of Ukraine has been examined; the Lr34(+) allele has been revealed in 44% of the tested cultivars. The obtained results have been compared with general data on the leaf rust resistance of wheat cultivars from different countries.     

Fine scale genetic and physical mapping using interstitial deletion mutants of <Emphasis Type="Italic">Lr34</Emphasis><Emphasis Type="Italic">/Yr18</Emphasis>: a disease resistance locus effective against multiple pathogens in wheat

The Lr34/Yr18 locus has contributed to durable, non-race specific resistance against leaf rust (Puccinia triticina) and stripe rust (P. striiformis f. sp. tritici) in wheat (Triticum aestivum). Lr34/Yr18 also cosegregates with resistance to powdery mildew (Pm38) and a leaf tip necrosis phenotype (Ltn1). Using a high resolution mapping family from a cross between near-isogenic lines in the “Thatcher” background we demonstrated that Lr34/Yr18 also cosegregated with stem rust resistance in the field. Lr34/Yr18 probably interacts with unlinked genes to provide enhanced stem rust resistance in “Thatcher”. In view of the relatively low levels of DNA polymorphism reported in the Lr34/Yr18 region, gamma irradiation of the single chromosome substitution line, Lalbahadur(Parula7D) that carries Lr34/Yr18 was used to generate several mutant lines. Characterisation of the mutants revealed a range of highly informative genotypes, which included variable size deletions and an overlapping set of interstitial deletions. The mutants enabled a large number of wheat EST derived markers to be mapped and define a relatively small physical region on chromosome 7DS that carried Lr34/Yr18. Fine scale genetic mapping confirmed the physical mapping and identified a genetic interval of less than 0.5 cM, which contained Lr34/Yr18. Both rice and Brachypodium genome sequences provided useful information for fine mapping of ESTs in wheat. Gene order was more conserved between wheat and Brachypodium than with rice but these smaller grass genomes did not reveal sequence information that could be used to identify a candidate gene for rust resistance in wheat. We predict that Lr34/Yr18 is located within a large insertion in wheat not found at syntenic positions in Brachypodium and rice. W. Spielmeyer and R. P. Singh contributed equally to the study through the “Thatcher” and “Lalbahadur” genetic stocks, respectively.     

Leaf tip necrosis, molecular markers and β1-proteasome subunits associated with the slow rusting resistance genes Lr46/Yr29

Resistance based on slow-rusting genes has proven to be a useful strategy to develop wheat cultivars with durable resistance to rust diseases in wheat. However this type of resistance is often difficult to incorporate into a single genetic background due to the polygenic and additive nature of the genes involved. Therefore, markers, both molecular and phenotypic, are useful tools to facilitate the use of this type of resistance in wheat breeding programs. We have used field assays to score for both leaf and yellow rust in an Avocet-YrA × Attila population that segregates for several slow-rusting leaf and yellow rust resistance genes. This population was analyzed with the AFLP technique and the slow-rusting resistance locus Lr46/Yr29 was identified. A common set of AFLP and SSR markers linked to the Lr46/Yr29 locus was identified and validated in other recombinant inbred families developed from single chromosome recombinant populations that segregated for Lr46. These populations segregated for leaf tip necrosis (LTN) in the field, a trait that had previously been associated with Lr34/Yr18. We show that LTN is also pleiotropic or closely linked to the Lr46/Yr29 locus and suggest that a new Ltn gene designation should be given to this locus, in addition to the one that already exists for Lr34/Yr18. Coincidentally, members of a small gene family encoding β-1 proteasome subunits located on group 1L and 7S chromosomes implicated in plant defense were linked to the Lr34/Yr18 and Lr46/Yr29 loci.     

Cloning and characterization of the lactate dehydrogenase genes from<Emphasis Type="Italic">Lactobacillus</Emphasis> sp. RKY2

Lactic acid is an environmentally benign organic acid that could be used as a raw material for biodegradable plastics if it can be inexpensively produced by fermentation. Two genes (IdhL andIdhD) encoding the L-(+) and D-(−) lactate dehydrogenases (L-LDH and D-LDH) were cloned fromLactobacillus sp., RKY2, which is a lactic acid hyper-producing bacterium isolated from Kimchi. Open reading frames ofIdhL for andIdhD for the L and D-LDH genes were 962 and 998 bp, respectively. Both the L(+)- and D(−)-LDH proteins showed the highest degree of homology with the L- and D-lactate dehydrogenase genes ofLactobacillus plantarum. The conserved residues in the catalytic activity and substrate binding of both LDHs were identified in both enzymes.     

Scion and Rootstock Effects on ABA-mediated Plant Growth Regulation and Salt Tolerance of Acclimated and Unacclimated Potato Genotypes

Tolerance of salt stress in potato (Solanum tuberosum L.) increased when the plants were pre-exposed to low concentrations of salt (salt acclimation). This acclimation was accompanied by increased levels of abscisic acid (ABA) in the shoot. To further study the role of roots and shoots in this acclimation process, reciprocal grafts were made between a salt-tolerant (9506) and salt-sensitive ABA(−) mutant and its ABA(+) normal sibling potato genotype. The grafted plants were acclimated with 75 or 100 mM NaCl for 3 weeks and then exposed to 150–180 mM NaCl, depending on the salt tolerance of the rootstock. After 2 weeks of exposure to the salt stress, the acclimated and unacclimated plants were compared for physiologic and morphologic parameters. The response to the salt stress was strongly influenced by the rootstock. The salt-tolerant 9506 rootstock increased the salt tolerance of scions of both the ABA-deficient mutant and its ABA(+) sibling. This salt tolerance induced by the rootstock was primarily modulated by salt acclimation and manifested in the scion via increased plant water content, stem diameter, dry matter accumulation, stomatal conductivity, and osmotic potential, and is associated with a reduction in leaf necrosis. There was also a pronounced scion effect on the rootstock. Using 9506 as a scion significantly increased root fresh and dry weights, stem diameter, and root water content of ABA(−) mutant rootstocks. Specific evidence was found of the role of exogenous ABA in the enhancement of water status in grafted plants under salt stress beyond that of grafting alone. This was verified by more positive stomatal conductivity and upward water flow in ABA-treated grafted and nongrafted plants and the absence of upward water flow in nontreated grafted plants through NMR imaging. Grafting using either salt-tolerant scions or rootstocks with inherently high ABA levels may positively modify subsequent responses of the plant under salt stress.