The relation between cell size, ploidy level and nitrate concentration in lettuce

In two experiments the relation between cell size, ploidy level and nitrate concentration of genotypes of lettuce ( Lactuca saliva L.) was investigated. Artificial increase of cell size by application of gibberellic acid to young plants decreased the percentage dry weight and the concentration of organic acids in the plant sap and increased the nitrate concentration of the shoot. Application of chlormequat chloride caused the opposite effects. Tetraploid genotypes of lettuce had larger cells, a lower nitrate concentration and a higher concentration of organic solutes than diploid genotypes. Genotypical differences in nitrate concentration could not be explained by differences in cell size, but were consistently related to differences in concentration of organic acids in plant sap. The percentage dry weight of genotypes was positively correlated with the concentration of organic acids in plant sap. However, differences in nitrate concentration were also found among genotypes with comparable dry matter content. These differences could be caused by differences in partitioning of photosynthates over structural components and organic acids in the vacuoles. Mono-saccharides seem to be less important than organic acids for explaining genotypical differences in nitrate level, because the concentration of monosaccharides was much lower than that of organic acids and because a negative relation between the concentration of monosaccharides and of nitrate was not found in every case. Genotypical differences in nitrate accumulation were not related to differences in osmolarity.     

Nitrate Accumulation and Osmotic Regulation in Italian Ryegrass (Lolium multiflorum Lam.)

The role of nitrate accumulation was studied in osmotic adaptationof Italian ryegrass (Lolium multiflorum Lam. cv. Romo). Plantswere grown under different light intensities (68 and 143 W m^–2)and on Hoagland nutrient solution with a solute potential of0.035 MPa and –0.335 MPa. The solute potential of thenutrient solution was decreased by addition of NaCl, which wastaken up by the plant, and PEG 6000 which was not. At each growingcondition the solute potential of the expressed plant sap wasconstant during the experimental period of 19 d, but was decreasedby increase in light intensity and addition of osmotic substancesin the root medium. At sugar concentrations between 20 and 60mg per gram plant water a change in sugar concentration is osmoticallyfully compensated for by an opposite change in nitrate concentration.At sugar concentrations lower than 20 mg per gram plant waterthere is an overcompensation of nitrate. The overcompensationincrease is attributed to a concomitant decrease of other organicsolutes such as amino acids and organic acids. When NaG is addedto the nutrient solution, the nitrate concentration in the plantdecreases and the solute potential is decreased by uptake ofchloride. With PEG the nitrate content of the plant also decreased.The solute potential of the plant sap is decreased by accumulationof sugar, at least at the high light intensity. Key words: Lolium multiflorum, Nitrate accumulation, Light intensity, Osmotic regulation     

C and N utilization of two lettuce genotypes during growth under non-varying light conditions and after changing the light intensity

Growth and N-incorporation in two lettuce genotypes ( Lactuca sativa L. cv. Deci minor and cv. Grosse brune), which differ significantly in nitrate accumulation, were studied. Under constant environmental conditions cv. Deci minor produced more fresh and dry weight than cv. Grosse brune. Cultivar Deci minor also produced more fresh weight per mmol N absorbed than cv. Grosse brune, and contained less organic nitrogen in the dry matter, but accumulated more nitrate. As cv. Deci minor showed a higher fresh and dry weight production per mmol N absorbed than cv. Grosse brune, it used its nitrogen more efficiently.
When the light intensity was decreased, the growth of both cultivars decreased, and the fresh weight production per mmol N absorbed increased. After reduction of the light intensity, cv. Deci minor maintained a higher fresh weight production per N absorbed than before, whereas cv. Grosse brune returned to its original level. After decrease of the light intensity, an increased nitrate concentration in the cell sap was accompanied by a decreased concentration of organic compounds in both cultivars. The organic nitrogen level in the dry matter remained constant after the higher intensity was reduced. However, due to the decreased dry weight percentage, the demand for nitrogen for protein synthesis decreased on fresh weight basis.
It can be concluded that the two cultivars differ in their partition of C and N between dry matter and cell sap. Nitrate accumulation in preference to accumulation of organic compounds does not automatically result from a shortage of organic compounds. The high accumulation of nitrate of cv. Deci minor enables it to use more carbohydrates for structural growth than cv. Grosse brune.     

Storage of Osmotically Active Compounds in the Taproot of Daucus carota L.

The osmotic potential of cell sap from the storage root, lateralroots and shoots of carrot (Daucus carota L., cv. AmsterdamseBak) was calculated from the concentration of osmotically activecompounds in these tissues. The osmotic potential of the taprootdid not change with age prior to and during the storage of osmoticallyactive sugars, as sucrose and reducing sugars. The increased contribution of soluble sugars in the osmoticpotential was accompanied by a proportionally decreased contributionof potassium and organic acids. Before storage of soluble sugarsin the taproot occurred, potassium and organic acids contributed80% to the total osmotic value, in contrast with lateral roottissue, where potassium and nitrate were the main osmotic solutes.The concentration of osmotically active solutes was lower inlateral root tissue than in storage root tissue. Shoot tissueresembled taproot tissue before storage, in having potassiumand organic acids as the main osmotic solutes. The concentrationof osmotically active solutes was highest in shoot tissue andit increased towards the end of the experimental period. The calculated osmotic potentials were in good agreement withthe experimental values, as determined from the molecular depressionof the freezing point of cell sap. Storage of reducing sugarsand sucrose is discussed in relation to acid and neutral invertaseactivities. Key words: Daucus carota, Osmotic solutes, Sugar storage, Invertase activity     

Xylem Sap from Actinidia chinensis: seasonal Changes in Composition

Seasonal variation was followed in the content of total a-aminoacids, arginine, calcium, total carbohydrate, ß-galactosidaseactivity, magnesium, nitrate, phosphatase activity, phosphateand sulphate in vacuum-extracted xylem sap from Actinidia chinensisvar. hispida, the Chinese gooseberry or kiwifruit. There wasa marked increase in the concentration of most sap componentsjust prior to leaf emergence followed by a rapid decrease afterthe leaves had expanded. Experiments with excised extensionshoots showed that much of this spring-time increase in concentrationsof sap components was due to mobilization of nutrients withinthe shoot itself. Sap from the trunk and the older brancheschanged less in composition than did sap from the younger partsof the plant. The amplitude and direction of trends in concentrationof sap from the different parts of the plant varied with nutrientand with time. Analysis of vacuum-extracted xylem sap collectedduring periods of rapid transpiration from early summer onwardsgives a reliable indication of the composition of the transpirationstream. Actinidia chinensis, Chinese gooseberry, kiwifruit, storage reserves, xylem sap, amino acids, arginine, calcium, carbohydrate, ßgalactosidase, magnesium, nitrate, phosphatase, phosphate, potassium, sulphate     

Biochemical Changes in Root Exudate and Xylem Sap of Tomato Plants Infected with Meloidogyne incognita

Under two monoxenic culture techniques of growing plants (filter paper and silica sand cultures), sugar in root exudate from Meloidogyne incognita-infected tomato increased 133 to 836% over controls. In contrast, amino acids were moderately reduced 52 to 56%. Chromatographic analysis showed that galled root exudate contained three sugars, twelve amino acids, and three organic acids, whereas healthy root exudate contained four sugars, fifteen amino acids, and four organic acids. Polysaccharide was responsible for the large increase of sugars in galled root exudates. The concn and the absolute amount of total sugars in the infected plant xylem sap were greater than in healthy plant xylem sap up to 6 wk after inoculation, whereas amino acids were moderately lower than in controls throughout the test period. Chromatographic analysis showed that xylem sap from both healthy and infected plants at 4 wk after inoculation contained four sugars and five organic acids. We identified 18 and 17 amino acids in the healthy and infected plant xylem sap, respectively. The concn of sugar increased as the nematode inoculum increased at 2, 4 and 6 wk after inoculation. The amino acids in all samples from the infected plant moderately decreased with an increase of nematode inoculum. We suggest that changes in total sugars and amino acids, of infected plant xylem sap and root exudate are a probable mechanism by which tomato plants are predisposed to Fusarium wilt.     

Regulation of Growth at Steady-state Nitrogen Nutrition in Lettuce (Lactuca sativa L.): Interactive Effects of Nitrogen and Irradiance

The morphological and physiological adaptation of Lactuca sativaL. (‘Vegas’) to different irradiance levels andrates of nitrogen supply was analysed in such a way that effectsof irradiance were clearly distinguished from the effects ofnitrogen. Lettuce was grown in a glasshouse in aerated nutrientsolutions containing all necessary nutrients except nitrogen.Nitrogen was supplied in excess and at limiting rates in relationto plant growth to provide steady state nutrition. Shading theplants created the low irradiance level. The effects of nitrogensupply and irradiance on growth showed a marked interaction.Dry matter production decreased strongly with decreasing nitrogensupply at high irradiance, but decreased only slightly at lowirradiance. Nitrogen had no effect on radiation use efficiencyexcept for the lowest nitrogen treatment at high irradiance.The effect of nitrogen on growth was mainly mediated by itseffect on leaf area development and hence on light interception.Decreases in leaf area were due to decreases in specific leafarea and dry matter partitioning towards the leaves, while thedecrease in specific leaf area was the result of an increasein leaf dry matter percentage at low nitrogen supply. Dry matterand nitrogen partitioning, and nitrate concentration were closelyrelated to plant nitrogen concentration. Irradiance did notaffect these relationships. Irradiance influenced partitioningonly indirectly by affecting plant nitrogen concentration. Thedemand for organic nitrogen per unit leaf area was lower atlower irradiance. Organic nitrogen per unit leaf area appearedto be adjusted to the irradiance level, independently of thenitrogen supply, suggesting priority of nitrogen usage in photosynthesis.Copyright 2000 Annals of Botany Company Lactuca sativa L., lettuce, growth, irradiance, leaf area, nitrogen, radiation use efficiency, partitioning     

Endogenous Development of Adventitious Root Primordia in Lettuce Hypocotyls

Rudimentary adventitious root primordia were observed in optically-sectionedFeulgen-stained hypocotyls of 8-d-old lettuce seedlings germinatedin the dark and in a low light intensity environment. It issuggested that formation of these primordia may represent anaspect of normal development in the seedling. Adventitious, root primordia, hypocotyls, lettuce, Lactuca sativa     

Enhancement of Gibberellic Acid-stimulated Hypocotyl Elongation of Lettuce (Lactuca sativa L. cv. Grand Rapids) by Phlorizin and Light

The interaction of light, gibberellic acid (GA₃), and phlorizinin the growth of lettuce (Lactuca sativa L. cv. ‘GrandRapids’) hypocotyls was investigated. At all concentrationsof GA₃, phlorizin enhanced GA₃-induced growth at luminous intensitiesabove 50 ft-c (continuous light). Without GA₃, phlorizin hadno effect on hypocotyl growth in the light but it inhibitedgrowth in the dark. Both seedlings and hypocotyl sections respondedto phlorizin in the presence of GA₃. There was no iteractionbetween phlorizin and KCl. Water-growth was severly inhibitedby light. GA₃,-induced growth was slightly inhibited by light,and then only at luminous intensities above 50 ft-c. Thus, relativeto H₂O-growth, GA₃-induced growth increased with increasingluminous intensity up to 450 ft-c, where it reached saturation.It seems that a synergism may exist between light and GA₃ aswell as between phlorizin and GA₃. Lactuca sativa L, lettuce, hypocotyl elongation, gibberellic acid, phlorizin, light     

Sample preservation for determination of organic compounds: microwave versus freeze-drying

In search of a reliable drying method, which might be used evenunder field conditions, microwave drying was compared to freeze-dryingof plant material. Leaves of Ananas comosus and Avicennia germinansas well as buds and phloem of Acer pseudoplatanus were usedand checked for one or more of the following substances: sugars,sugar alcohols, organic and amino acids, total nitrogen, andglycinebetaine. With most samples good agreement was achieved between the twodrying methods. Only in the case of the Ananas comosus leaves,which exhibited low pH and high water content, did appreciabledifferences occur in organic and amino acids. Besides that,sucrose was the compound most susceptible to alterations, whichwas especially evident when leaves of Sambucus nigra were driedin the two different compartments (condenser compartment, dryingbell jar) of the freeze-dryer in use. For Ananas comosus leaf samples it was shown that microwavingcan also be used prior to extraction of tissue sap. Key words: Microwave, freeze-drying, drying method, tissue sap, organic solutes     

The Distribution of Nitrate Assimilation Between Root and Shoot in Vicia faba L.

Nitrate assimilation was examined in two cultivars (Banner Winterand Herz Freya) of Vicia faba L. supplied with a range of nitrateconcentrations. The distribution between root and shoot wasassessed. The cultivars showed responses to increased applied nitrateconcentration. Total plant dry weight and carbon content remainedconstant while shoot: root dry weight ratio, total plant nitrogen,total plant leaf area and specific leaf area (SLA) all increased.The proportion of total plant nitrate and nitrate reductase(NR) activity found in the shoot of both cultivars increasedwith applied nitrate concentrations as did NO₃^–: Kjeldahl-Nratios of xylem sap. The cultivars differed in that a greaterproportion of total plant NR activity occurred in the shootof cv. Herz Freya at all applied nitrate concentrations, andits xylem sap NO₃^–: Kjeldahl-N ratio and SLA were consistentlygreater. It is concluded that the distribution of nitrate assimilationbetween root and shoot of V. faba varies both with cultivarand with external nitrate concentration. Vicia faba L., field bean, nitrate assimilation, nitrate reductase, xylem sap analysis     

Influence of phloem transport, N-fertilization and ion accumulation on sucrose storage in the taproots of fodder beet and sugar beet

To investigate the factors governing the accumulation of sucroseand amino acids in the taproots of sugar beet, their contentswere measured in the leaves, phloem sap and the taproots ofsugar beet, fodder beet and a hybrid between both, grown oneither 3.0 or 0.5 mM nitrate. In the taproots the contents ofmalate, citrate and inorganic ions were also determined. Forthe high sucrose accumulation in sugar beet as compared to theother varieties three factors were found. (a) In sugar beet,less amino acids and more sucrose are taken up into the phloemthan in fodder beet. (b) In sugar beet, the sucrose and aminoacid syntheses are less sensitive to the nitrate concentrationsthat are required for optimal plant growth than in other varieties.In fodder beet, upon raising the nitrate concentration from0.5 mM to 3 mM, the synthesis and storage of sucrose is decreasedand that of amino acids increased. The corresponding valuesin sugar beet (0.5 mM) are similar to those in fodder beet andare not much affected by an increase of nitrate. (c) The sucroseaccumulation is limited by the accumulation of inorganic ionsin the taproots. The sucrose content in the taproots is negativelycorrelated to the total ion content. Whereas sucrose representstwo-third of all solutes in the taproots of sugar beet, it amountsto only one-third of the solutes in fodder beet taproots. Key words: Amino acids, Beta vulgans L, phloem sap, potassium, sucrose storage, sugar beet, taproots, transport     

Genotype-independent transformation of lettuce using Agrobacterium tumefaciens

The genetic manipulation of lettuce (Lactuca sativa) necessitatesa reliable and efficient, genotype-independent method of transformation.Thirteen lettuce cultivars have been assessed for their amenabilityto Agrobacterrum-mediated gene transfer linked to their tissueculture responsiveness, including callus induction and shootregeneration. A reliable protocol has been developed for theroutine production of transgenic plants for all 13 cultivarsinvestigated. Key words: Agrobacterium-mediated transformation, Lactuca sativa genotypes, lettuce     

Interaction of Moisture Stress and Three Phenolic Compounds on Lettuce Seed Germination

No interactions between water stress and three phenolic acids(p-coumaric, caffeic and ferulic acids) on lettuce (Lactucasativa L. var. Grand Rapids) seed germination were found. Probitanalysis indicated that mechanisms of action of water stressand the phenolic inhibitors were similar. The relative effectivenessof the compounds was p-coumaric > ferulic > caffeic. Nointeraction was found between p-coumaric and ferulic acid, whereasantagonism was found between caffeic acid and each of the othertwo phenolic acids. Lactuca sativa L., lettuce, germination, phenolic compounds, moisture stress, allelopathy, seed     

The Relationship between Sugar and Amino Acid Export to the Phloem in Young Wheat Plants

The relationship between amino acid and sugar export to thephloem was studied in young wheat plants (Triticum aestivumL. ‘Pro-INTA, Isla Verde’) using the EDTA-phloemcollection technique. Plants grown with a 16 h photoperiod showeda rapid decrease in the concentration of sugars and amino acidsin the phloem exudate from the beginning of the dark period.When plants grown with a 16 h photoperiod were kept in the darkfor longer than 8 h the free amino acid content in leaves andexudate (on a dry weight basis) increased continually throughoutthe 72 h of darkness. During the first 24 h of darkness thesugars in the phloem exudate decreased to 30% of the initialvalue, and returned to the control level when plants were returnedto light. When plants grown under low light intensity for 10d were transferred to high light intensity, they showed an increasein leaf sugar content (dry weight basis) after 3 d but therewere no differences in leaf free amino acid content (dry weightbasis) compared to low-light plants. The sugar concentrationin the phloem exudate was increased by higher light intensities,but there was no difference in the amino acid concentrationof the phloem exudate, and thus the amino acid:sugar ratio inthe phloem decreased in the high-light plants. The present resultssuggest that amino acids can be exported to the phloem independentlyof the export of sugars. Copyright 1999 Annals of Botany Company Sugar exudation, amino acid transport, nitrogen, phloem, transport, wheat, Triticum aestivum L.     

The Effects of Mechanically-induced Stress on the Growth of Water and Nutrient Deficient Lettuce and Cauliflower Seedlings

Mechanically-induced stress (MIS) applied by brushing the shootsof lettuce and cauliflower seedings with paper for 90 s eachday retarded the growth of water deficient and nutrient deficientseedlings as effectively as it did the growth of those wateredregularly or fed regularly with nutrient. The results are discussedboth in relation to how MIS might effect plant growth in thefield and to the possible use of stress treatments applied duringthe raising of transplants. Lactuca sativa L., lettuce, Brassica oleracea var, botrytis DC, cauliflower, mechanically-induced stress (MIS), water deficiency, nutrient deficiency, water potential     

Growth-limiting phosphate nutrition suppresses nitrate accumulation in greenhouse lettuce

Lettuce was grown in aerated nutrient solutions containing all necessary plant nutrients except phosphate. Phosphate was supplied at four different exponentially increasing rates of daily dosage. In addition, two levels of light and atmospheric CO2 were imposed. The four phosphate addition treatments resulted in different, fairly constant relative growth rates, whereas little effect was observed from light or CO2 levels. Growth reduction associated with limiting phosphate nutrition resulted in increases in root/shoot ratio, dry matter content and concentrations of sugars and organic acids. By contrast, the concentrations of nitrate in the shoots was decreased. The concentrations of nitrate and the sum of sugars and organic acids in the shoot showed a strong negative correlation. Two opposing mechanistic models concerning the effects of environmental conditions on nitrate accumulation in plants are discussed in the light of these results.     

On the Structure of Lettuce (Lactuca sativa L.) Endosperm during Germination

Endosperm cells of lettuce (Lactuca sativa L.) are characterizedby thick cell walls and dense cytoplasm which contains numerousprotein bodies. Other organelles such as nucleus, mitochondria,plastids and dictyosomes are typical of plant cells. Light andelectron microscopy reveal that before radicle emergence micropylarcells of endosperm tissue undergo drastic protoplast alterations.These alterations seem to be the only structural modificationsbefore rupturing of the tissue since the walls of the endospermcells seem to degrade only after radicle emergence. The differentialbehaviour of the micropylar area of the endosperm before radicleemergence and the observation that the micropylar cells remainmetabolically active long after radicle emergence while therest of the tissue is almost completely disintegrated, suggeststhat the endosperm cells of the micropylar area may have a roleother than being a main reserve site like the rest of the endosperm. Lactuca sativa L., endosperm structure, seed germination, lettuce     

Water relations link carbon and oxygen isotope discrimination to phloem sap sugar concentration in Eucalyptus globulus

A strong correlation was previously observed between carbon isotope discrimination (Delta(13)C) of phloem sap sugars and phloem sap sugar concentration in the phloem-bleeding tree Eucalyptus globulus Labill. (J. Pate, E. Shedley, D. Arthur, M. Adams [1998] Oecologia 117: 312-322). We hypothesized that correspondence between these two parameters results from covarying responses to plant water potential. We expected Delta(13)C to decrease with decreasing plant water potential and phloem sap sugar concentration to increase, thereby maintaining turgor within sieve tubes. The hypothesis was tested with analyses of E. globulus trees growing on opposite ends of a rainfall gradient in southwestern Australia. The Delta(13)C of phloem sap sugars was closely related to phloem sap sugar concentration (r = -0.90, P < 0.0001, n = 40). As predicted, daytime shoot water potential was positively related to Delta(13)C (r = 0.70, P < 0.0001, n = 40) and negatively related to phloem sap sugar concentration (r = -0.86, P < 0.0001, n = 40). Additional measurements showed a strong correspondence between predawn shoot water potential and phloem sap sugar concentration measured at midday (r = -0.87, P < 0.0001, n = 30). The Delta(13)C of phloem sap sugars collected from the stem agreed well with that predicted from instantaneous measurements of the ratio of intercellular to ambient carbon dioxide concentrations on subtending donor leaves. In accordance, instantaneous ratio of intercellular to ambient carbon dioxide concentrations correlated negatively with phloem sap sugar concentration (r = -0.91, P < 0.0001, n = 27). Oxygen isotope enrichment (Delta(18)O) in phloem sap sugars also varied with phloem sap sugar concentration (r = 0.91, P < 0.0001, n = 39), consistent with predictions from a theoretical model of Delta(18)O. We conclude that drought induces correlated variation in the concentration of phloem sap sugars and their isotopic composition in E. globulus.     

Nitrate Reduction in Solanum tuberosum L.: Development of Nitrate Reductase Activity in Field-grown Plants

Nitrate reductase activity (in vivo method, substrate non-limiting)in unshaded leaves from the top of the canopy has been determinedfor field-grown potato plants over the course of the growingseason. The pattern of change was almost identical for plantsreceiving no added fertilizer and those receiving 24 g N m^–2.Activity increased to a peak at about 90 days after plantingand declined thereafter. On a fresh weight basis activity wasalways higher in fertilized plants. Nitrate reductase activitywas positively and significantly correlated with leaf proteincontent in high N plants (r² = 0.71; P = 0.05), but poorly correlatedwith both the nitrate content of the leaf lamina and the nitrateconcentration in petiole sap. Up until 90 days after planting(mid-July) there appeared to be a positive relationship betweenincreased activity of nitrate reductase and solar radiation.However, results obtained over two seasons showed that the declinein activity after this time was not consistently linked witha fall in the level of solar radiation. Remobilization of reduced-Nand stored nitrate from leaves and stems accompanied this declinein nitrate reductase activity and in the latter part of theseason appeared to account for all of the N gained by growingtubers. In unfertilized plants nitrate-N accounted for 5 per cent orless of total plant N. Fertilized plants contained up to 25per cent nitrate-N. While nitrate availability limited growthin unfertilized plants, sub-optimal rates of nitrate assimilationin fertilized plants, particularly during the early stages ofpost-emergence growth, may contribute to inefficient use ofacquired nitrate. The carbohydrate status of leaf lamina and petiole sap weremodified by N supply. The soluble sugar and starch contentsof low N leaves were higher than in their high N counterparts.By contrast, the concentration of soluble sugars in petiolesap increased to a higher value in high N samples. Althoughsap sugar levels declined in both treatments towards the endof the season, N application delayed this decline for severalweeks. Solanum tuberosum, nitrate reductase, nitrate assimilation, senescence