The Growth and Carbon Economy of Selection Lines of Lolium perenne cv. S23 with Differing Rates of Dark Respiration: 1. Grown as Simulated Swards During a Regrowth Period

Simulated swards of each of two selection lines of Lolium perennecv. S23 with ‘fast’ and ‘slow’ ratesof ‘mature tissue’ respiration were establishedin growth rooms at 20/15 °C day/night temperatures and studiedover four successive regrowth periods of 46, 30, 26 and 53 daysduration. The ‘slow’ line outyielded the ‘fast’,both in harvestable shoot (above a 5 cm cut) and in root andstubble. Its advantage increased over successive regrowth periodsto 23 per cent (total biomass). Gas analysis measurements onthe entire communities (including roots), during the final regrowthperiod, showed that the ‘slow’ line had a 22–34per cent lower rate of dark respiration per unit dry weight.This enabled it to maintain its greater mass of tissue for thesame cost in terms of CO₂ efflux per unit ground area. Halfthe extra dry weight produced by the ‘slow’ line,relative to the ‘fast’, could be attributed to itsmore economic use of carbon. The rest could be traced to a 25per cent greater tiller number which enabled the ‘slow’line to expand leaf area faster (though not at a greater rateper tiller), intercept more light and fix more carbon, earlyin the regrowth period. Lolium perenne L., ryegrass, respiration, maintenance respiration, tiller production, simulated swards, canopy photosynthesis, carbon economy     

The Role of the Coleoptile Apex in Controlling Organ Elongation: I. THE EFFECTS OF DECAPITATION AND APICAL INCISIONS

It has been widely accepted for over 50 years that the elongationrate of a coleoptile is dependent on the supply of auxin fromthe apex. The original coleoptile decapitation experiments whichprovided support for this view have been repeated but the measurementsof coleoptile elongation were made with greater temporal andspatial precision. The experiments confirm that Avena and Zeacoleoptile elongation is retarded by decapitation but the locationand timing of the growth rate changes are not consistent withthe hypothesis that decapitation reduces growth rate solelyby removing the major supply of auxin. Evidence is presentedthat wounding is the prime cause of the effects of decapitation.Data are also presented showing that the recovery of growthrate of coleoptiles after decapitation or wounding is not dearlyassociated with any events near the cut surface and hence thetraditional explanation of this phenomenon (‘regenerationof the physiological tip’) is misleading. Key words: Coleoptile, decapitation, apex     

Patterns of Root Respiration Associated with the Induction of Aluminium Tolerance in Phaseolus vulgaris L.

Aluminium (Al) tolerance in an Al-tolerant cultivar of Phaseolusvulgaris L. (‘Dade’) was found to be an inducibletrait. Upon exposure to 10 µM Al, the rate of root elongationwas inhibited in comparison to controls. During the following72 h, the rate of elongation returned to levels comparable tocontrols. In contrast, root elongation of an Al-sensitive cultivar(‘Romano’) did not recover after exposure to Al.In Dade, the resumption of root elongation following exposureto Al was accompanied by increased rates of root respiration,whereas respiration rates slowly declined over the 72 h treatmentperiod in Romano. When partitioned into growth and maintenanceexpenditures, a larger proportion of root respiration of Dadeexposed to Al was allocated to maintenance processes, potentiallyreflecting diversion of energy to metabolic pathways that offsetthe adverse effects of Al toxicity. Romano did not show sucha pattern and respiration associated with both growth and maintenancewas reduced after exposure to Al. Root and shoot growth of bothcultivars were also measured to determine the effects of long-term(21 d) exposure to 10 µM Al. Dade plants exposed to Alexhibited enhanced growth in comparison to controls, whereasRomano plants were characterized by reduced shoot and root growth.Modelling the time-course of root respiration and measuringthe long-term growth responses to Al is a valuable method ofelucidating respiratory costs of stress tolerance. Key words: Aluminium, differential tolerance, maintenance respiration, Phaseolus vulgaris, root respiration     

The Growth and Carbon Economy of Selection Lines of Lolium perenne cv. S23 with Differing Rates of Dark Respiration: 2. Grown as Young Plants from Seed

Young plants of two selection lines of Lolium perenne cv. S23with ‘fast’ and ‘slow’ rates of ‘maturetissue’ respiration were individually grown from seed,together with plants of S23, their common parent, in 9.2 cmpots in a controlled environment at 20/15 °C day/night temperatures. No significant differences were found between the genotypesin leaf extension and tiller production during this early stageof their growth. They did differ however, by an average of 26%,in the rate of dark respiration of fully expanded leaf laminae.The use of a simple model demonstrated that such a differencein respiration could alone account for the different rates ofdry matter production shown by the selection lines when grownas young crops from seed. Possible penalties of ‘slow’respiration are also considered. Lolium perenne L., ryegrass, respiration, maintenance respiration, stimulated swards, leaf growth, tiller production, carbon economy     

Effects of Selection for Dark Respiration Rate on Enzyme Genotypes in Lolium perenne

Two lines of perennial ryegrass (Lolium perenne) that had previouslybeen selected for ‘fast’ and ‘slow’rates of leaf dark respiration were examined to determine whetherselection had differentiated allelic or genotypic frequenciesof five polymorphic enzyme loci, and determine whether respirationrates differed among genotypes. The lines were differentiatedfor genes coding for phosphoglucose isomerase (PGI) and UDP-glucosepyrophosphorylase (UDP). Respiration rates were heterogeneousamong UDP and 6-phosphogluconate dehydrogenase (6PGD) genotypes.Founder effects may have produced the genetic differentiationof lines at the PGI locus as we detected no differences amonggenotypes for respiration rates. The genetic differentiationamong lines for UDP was consistent with differences in darkrespiration among genotypes. Although we detected large differences(75%) in dark respiration rate among 6PGD genotypes in the fastrespiring line, selection did not differentiate the lines atthis locus. Lolium perenne L., perennial ryegrass, enzyme polymorphisms, dark respiration rate     

Biophysics of the growth responses of pea roots to changes in penetration resistance

Two experiments were performed in simplified soil-less systems to study how roots respond to changes in mechanical impedance. In the first the increases in root force and diameter that occur when a pea root was impeded mechanically inside a hole with rigid conical walls were determined. The experiment was performed at 8°C and at 25°C, and the root growth pressures generated were calculated during periods of 12, 24, 48 and 72 hours. The maximum growth pressures generated were approximately the same at both temperatures, although the maximum pressure was achieved approximately twice as quickly at 25°C than at 8°C, being reached within 15–20 hours. In the second set of experiments a new technique was developed to measure simultaneously the elongation rate and the force exerted by the roots of seedlings grown in moist air. A constant force was exerted by a force transducer on a pea radicle using a system of pulleys, and the elongation rate of the pea root was monitored using a linear variable differential transformer (LVDT). The changes in root elongation rate were recorded that occurred in response to increases and decreases in the applied force. Root elongation rate decreased by more than 50% within 30 min of increasing the applied force by 100 mN. A similarly fast, but smaller increase in growth rate occurred when the force was removed. The interpretation of results from both studies will be discussed in terms of a modified form of the Lockhart model of growth.     

Mycorrhizal Response of Two Tomato Genotypes Relates to their Ability to Acquire and Utilize Phosphorus

The purpose of this study was to determine how a plant's responseto colonization by mycorrhizal fungi relates to its abilityto acquire and utilize phosphorus for growth and reproduction.Two tomato genotypes previously found to be either responsive(‘LA1709’) or unresponsive (‘large cherry’)to mycorrhizal colonization during early vegetative growth wereexamined in detail. Plants were grown at four levels of addedphosphate or with mycorrhizal inoculum. Vegetative and reproductivegrowth, phosphorus uptake and root length density were measuredduring the course of plant development. Mycorrhizal symbiosissignificantly increased above-ground dry mass, root length,phosphorus content and yield under low phosphorus conditionsin ‘LA1709’, while it had less effect on these characteristicsin ‘large cherry’. When uninfected, however, ‘LA1709’grew and reproduced poorly unless high amounts of phosphoruswere added to the soil, while ‘large cherry’ grewwell under very low phosphorus conditions. This was because‘large cherry’ had significantly higher root lengthdensities than ‘LA1709’, enabling plants from thisgenotype to explore more soil volume and acquire greater amountsof phosphorus when grown without mycorrhizal fungi in low phosphorussoil. ‘Large cherry’ also had higher phosphorususe efficiency and allocated a greater proportion of phosphorusto reproduction when uninfected than ‘LA1709’. Itappears traits that affect a plant's ability to acquire andutilize phosphorus efficiently for growth and reproduction canalso affect its response to mycorrhizal colonization in tomato.Copyright1998 Annals of Botany Company. Tomato,Lycopersicon esculentum,mycorrhiza,Glomus etunicatum,phosphorus, reproduction, lifespan.     

The growing outer epidermal wall: design and physiological role of a composite structure

Background: The cells of growing plant organs secrete an extracellular fibrouscomposite (the primary wall) that allows the turgid protoplaststo expand irreversibly via wall-yielding events, which are regulatedby processes within the cytoplasm. The role of the epidermisin the control of stem elongation is described with specialreference to the outer epidermal wall (OEW), which forms a ‘tensileskin’. Novel Facts: The OEW is much thicker and less extensible than the walls ofthe inner tissues. Moreover, in the OEW the amount of celluloseper unit wall mass is considerably greater than in the innertissues. Ultrastructural studies have shown that the expandingOEW is composed of a highly ordered internal and a diffuse outerhalf, with helicoidally organized cellulose microfibrils inthe inner (load-bearing) region of this tension-stressed organwall. The structural and mechanical backbone of the wall consistsof helicoids, i.e. layers of parallel, inextensible cellulosemicrofibrils. These ‘plywood laminates’ containcrystalline ‘cables’ orientated in all directionswith respect to the axis of elongation (isotropic material).Cessation of cell elongation is accompanied by a loss of order,i.e. the OEW is a dynamic structure. Helicoidally arranged extracellularpolymers have also been found in certain bacteria, algae, fungiand animals. In the insect cuticle crystalline cutin nanofibrilsform characteristic ‘OEW-like’ herringbone patterns. Conclusions: Theoretical considerations, in vitro studies and computer simulationssuggest that extracellular biological helicoids form by directedself-assembly of the crystalline biopolymers. This spontaneousgeneration of complex design ‘without an intelligent designer’evolved independently in the protective ‘skin’ ofplants, animals and many other organisms.     

Freezing tolerance, protein composition, and abscisic acid localization and content of pea epicotyl, shoot, and root tissue 3

The freezing tolerance of many plants, such as pea (Pisum sativum),is increased by exposure to low temperature or abscisic acidtreatment, although the physiological basis of this phenomenonis poorly understood. The freezing tolerance of pea shoot tips,root tips, and epicotyl tissue was tested after cold acclimationat 2C, dehydration/rehydration, applications of 10^–4M abscisic acid (ABA), and deacclimation at 25C. Tests wereconducted using the cultivar ‘Alaska’, an ABA-deficientmutant ‘wil’, and its ‘wildtype’. Freezinginjury was determined graphically as the temperature that caused50% injury (T₅₀) from electrical conductivity. Endogenous ABAwas measured using an indirect enzyme-linked immunosorbant assay,and novel proteins were detected using 2-dimensional polyacrylamidegel electrophoresis. The maximum decrease in T₅₀ for root tissuewas 1C for all genotypes, regardless of treatment. For ‘Alaska’shoot tips and epicotyl tissue, exogenous ABA increased thefreezing tolerance by –1.5 to –4.0C, while coldtreatment increased the freezing tolerance by –7.5 to–14.8C. Cold treatment increased the freezing toleranceof shoot tips by –9 and –15C for ‘wil’and ‘wild-type’, respectively. Cold acclimationincreased endogenous ABA concentrations in ‘Alaska’shoot tips and epicotyls 3- to 4-fold. Immunogold labeling increasednoticeably in the nucleus and cytoplasm of the epicotyl after7 d at 2C and was greatest after 30 d at the time of maximumfreezing tolerance and soluble ABA concentration. Cold treatmentinduced the production of seven, three, and two proteins inshoot, epicotyl, and root tissue of ‘Alaska’, respectively.In ‘Alaska’ shoot tissue, five out of seven novelproteins accumulated in response to both ABA and cold treatment.However, only a 24 kDa protein was produced in ‘wil’and ‘wild-type’ shoot and epicotyl tissues aftercold treatment. Abscisic acid and cold treatment additivelyincreased the freezing tolerance of pea epicotyl and shoot tissuesthrough apparently independent mechanisms that both resultedin the production of a 24 kDa protein. Key words: Pisum sativum, cold acclimation, immuno-localization     

Seasonal Changes in the Physiology of S24 Perennial Ryegrass (Lolium perenne L.): 3. Partition of Assimilates between Root and Shoot during the Transition from Vegetative to Reproductive Growth

The partition of ¹⁴C labelled current assimilates to root insimulated swards of Lolium perenne cv. S24 was measured duringthe transition from vegetative growth in autumn to reproductivegrowth in spring under close to natural conditions of lightand temperature. Assimilate partitioning was also measured in‘established’ swards cut three times during thegrowing season and in vegetative ‘seedling’ swardsgrowing in autumn and in spring. All measurements were madewhen the swards had achieved more than 90 per cent light interception,and all swards were abundantly supplied with water and mineralnutrients. During autumn there was a gradual decrease in the proportionof assimilates partitioned to the roots in both the ‘established’and the ‘seedling’ swards. In the established swards,partition to roots was low over winter, increased during earlyspring, but decreased dramatically, later in the spring, whenstem elongation began. In contrast, in the unvernalized vegatativeseedling swards in spring, partition to roots remained high. The seasonal pattern of assimilate partitioning is consideredin relation to changes in the natural environment and the rateat which the crop fixed carbon in photosynthesis. A decreasein the proportion of assimilates partitioned to roots duringlate spring was significant in increasing the production ofshoot at that time but seasonal differences in partition contributedvery little to the marked differences in shoot growth betweenthe spring and autumn crop. Lolium perenne L., perennial ryegrass, partition of assimilates, flowering     

The Developmental Anatomy of the Root System in Yam Bean, Pachyrhizus erosus Urban

Investigations revealed that the anatomy of the primary radicularroot of yam bean (Pachyrhizus erosus L.) was typically dicotyledonousexcept that the xylem was not completely developed centripetally.Most of the roots had tetrarch xylem, although a few triarchand pentarch roots were also observed. In both tuberous andnon-tuberous roots, secondary thickening occurred by the formationof the meristematic vascular cambium which formed secondarytissues in a normal fashion. Subsequently, tuberization wasinitiated in the secondary xylem by the development of anomalous‘secondary’ cambia from parenchyma cells surroundingvessel elements. Anomalous ‘secondary’ cambia alsodeveloped from parenchyma cells not associated with vessels.Subsequently, anomalous ‘tertiary’ cambia differentiatedfrom tissues produced by the anomalous ‘secondary’cambia. Activities of these anomalous cambia resulted in theproduction of parenchyma storage cells and were chiefly responsiblefor the growth of the mature tuber. Pachyrhizus erosus L., yam bean, tuberous root, anatomy, anomalous ‘secondary’ cambia, anomalous ‘tertiary’ cambia, centripetal xylem development     

Shoot Elongation in Scots Pine: Diurnal Variations and Response to Temperature

Kanninen, M. 1985. Shoot elongation in Scots pine: Diurnal variationsand response to temperature.—J. exp. Bot. 36: 1760–1770. Time series analysis is used to study the diurnal variationsin the rate of shoot elongation of Scots pine (Pinus sylvestrisL.) seedlings, and its response to temperature. The shoot elongation of three 5-year-old Scots pine seedlingswas measured at 2 h intervals over a 12 d period in June 1979.Ambient temperature, measured with a thermocouple, was continuouslyrecorded during the same period. The measurements were carriedout in the field. Time series analysis of the data was carried out by estimatedauto-correlation and partial auto-correlation functions. A multiplicative‘seasonal’ model was applied to both the input andoutput series to ‘prewhiten’ the data set. Serialcross-correlation analysis of the prewhitened series was usedto identify the structure of the transfer function model betweentemperature and growth rate. There was a phase shift between the air temperature and shootextension. The cross-correlation function peaked at 2 h lagvalue. The influence of temperature on the growth rate was adequatelydescribed by a first-order transfer function model. The characteristicfeature of the model was a time constant of 3.3 h and a basetemperature of 6.3 °C for the response of shoot elongationto temperature. Key words: Scots pine, Pinus sylvestris L., shoot elongation, temperature response, time-series analysis     

Injury to Rice Plants Caused by Complete Submergence; A Contribution by Ethylerie (Ethene)

Complete submergence of rice plants (Oryza sativa L. cv. ‘IR42’)in dilute nutrient solution for 3–6 d almost stopped theaccumulation of dry matter, depressed soluble carbohydrate concentrationby over 75% and promoted chlorosis in fully expanded leaves.Increase in fresh weight by the shoots was not impaired. Extensionby the youngest visible leaf was stimulated. Extension by thenext leaf to appear was retarded by submergence. These growthresponses to submergence were associated with a 1-5-fold increasein the partial pressure of endogenous ethylene (ethene). Applying ethylene (0.3–0.35 Pa) in the gas-phase to non-submergedplants reproduced some, but not all, of these effects of submergence.Thus, greater leaf extension and chlorosis of submerged plantscould be attributable to accumulated ethylene but neither theslow relative growth rate nor the decreased extension of leavesemerging after the start of submergence could be so attributed. Two cultivars (‘FR13A’ and ‘Kurkaruppan’)already known to tolerate submergence, differed little fromsubmergence-intolerant ‘IR42’ in their relativegrowth rate and soluble carbohydrate concentration during submergence.However, their underwater leaf extension was less than in ‘IR42’and chlorosis was much less prevalent, especially in ‘FR13A’.Similarly, ethylene supplied to non-submerged plants was a lesseffective promotor of leaf extension and chlorosis in the twosubmergence tolerant cultivars. Application of 1.0 kPa carbondioxide in the gas-phase prevented the chlorosis response toethylene. The results indicate that accumulated ethylene is a likely causeof fast leaf extension and chlorosis in submergence intolerantforms of rice, particularly when amounts of dissolved carbondioxide are minimal. Key words: Oryza sativa L., aeration, ethylene (ethene), stress-tolerance     

Root system growth and nodule establishment on pea (Pisum sativum L.)

Development of the root system, appearance of nodules, and relationshipsbetween these two processes were studied on pea (Pisum sativumL., cv. Solara). Plants were grown in growth cabinets for 4weeks on a nitrogen—free nutrient solution inoculatedwith Rhizobium leguminosarum. Plant stages, primary root length,distance from the primary root base to the most distal first-orderlateral root, and distance from the root base to the most distalnodule, were recorded daily. Distribution of nodules along theprimary root and distribution of laterals were recorded by samplingroot systems at two plant stages. Primary root elongation ratewas variable, and declined roughly in conjunction with the exhaustionof seed reserves. First-order laterals appeared acropetallyon the primary root. A linear relationship was found betweenthe length of the apical unbranched zone and root elongationrate, supporting the hypothesis of a constant time lag betweenthe differentiation of first-order lateral's primordia and theiremergence. Decline of the primary root elongation rate was precededby a reduction in density and length of first-order laterals.Nodules appeared not strictly but roughly acropetally on theprimary root. A linear relationship was found between the lengthof the apical zone without nodule and root elongation rate,supporting the hypothesis of a constant time lag between infectionand appearance of a visible nodule. A relationship was foundbetween the presence/absence of nodules on a root segment andthe root elongation rate between infection and appearance ofnodules on the considered root segment. Regulation of both processesby carbohydrate availability, as a causal mechanism, is proposed. Key words: Pisum sativum L, root system, nodules     

Homeostasis in Nitrogenous Uptake/Assimilation by the Green Alga Platymonas (Tetraselmis) striata (Prasinophyceae)

When nitrate, ammonium, or urea were the sole nitrogen sourcesfor the growth of Platymonas striata there was a short initiallag period, after which uptake/assimilation occurred at a constantrate. Glycine ‘uptake’ was however non-linear withtime. Addition of ammonium or urea markedly inhibited nitrate‘uptake’ whereas although glycine was initiallytotally inhibitory, nitrate ‘uptake’ recommencedabout 1 h after addition, but at a greatly reduced rate. Ammoniumtotally inhibited urea ‘uptake’, but did not inhibitglycine ‘uptake’. When ammonium, urea, or glycinewere present at the same time as nitrate the total rate of nitrogen'uptake/assimilation was within 40% (or closer) of that of nitratenitrogen ‘uptake’ alone. In the first two casesthis reflected a total switching off of nitrate ‘uptake’whilst the new substrate was assimilated at a rate similar tothe initial nitrate rate, whereas in the case of glycine, bothsubstrates were used simultaneously. Once the concentrationof any of the inhibitory nitrogen sources had been reduced toa low level the ‘uptake’/assimilation of nitraterecommenced. It is suggested that the ability of P. striatato maintain a more or less constant rate total nitrogen ‘uptake’/assimilationirrespective of the mixture of utilizable nitrogen sources presentedto it, is not due to controls at the level of the entry of substrateinto the cell, but to intracellular actions at the enzyme level.The relative constancy of, and high rate of ‘uptake’,irrespective of nitrogen source and over a wide range of extracellularconcentrations, is tentatively considered to be due to saturationof the assimilatory mechanisms under the conditions employed;all the sources lead, either directly or indirectly, to theproduction of amino groupings for entry into nitrogenous metabolism.This homeostasis of nitrogen ‘uptake’/assimilationwould clearly be of considerable benefit to the alga in nature,enabling it to maximize growth in changing environmental conditions. Platymonas striata, nitrogen update, nitrogen assimilation     

The Initiation, Growth, and Emergence of Leaf Primordia in Fragaria

The leaf initiation rate in Fragaria vesca (var. ‘RoyalSovereign’) has been compared with the elongation rateof the leaf primordia at different seasons. Certain conceptionsof growth correlation within the bud are presented. Experimentson the nature of elongation and emergence of primordia are described,and the causes of emergence are discussed.     

Physiology of Flowering in Saccharum: I. DAYLENGTH CONTROL OF FLORAL INITIATION AND DEVELOPMENT IN S. SPONTANEUM L.

Flowering in two clones of Saccharum spontaneum L. is controlledby photoperiod. The earliest stages of development, ‘induction’and ‘initiation of the inflorescence axis primordium’(IAP) were optimally promoted under intermediate days of 12h 30 min, while the subsequent stage ‘initiation of inflorescencebranch primordia’ (IBP) was inhibited by days longer than13 h. The following stage ‘initiation of spikelet primordia’(ISP) showed a quantitatively intermediate response with anoptimum photoperiod of 9 h to 11 h. The elongation of the differentiatedinflorescence was found to be only slightly sensitive to photoperiodsof 13 h or longer in one of the clones. Unfavourable photoperiodsat stages following induction resulted in the arrest or delayof inflorescence development and when these were given duringthe IAP and IBP stages, reversion to the vegetative conditioncommonly occurred.     

CORRECTIONS, VOLUME 29

Part 1, under the frontispiece portrait of Dr. N. B. Eales,the words ‘President 1948–1951’ should havebeen added. Page 103, line 49, for ‘Newton Collection’ read‘Norman Collection (Canon Norman)’. 185, line 37, for ‘capillaris’ read ‘capillacca’. 188, Table 1, for ‘bemoralis’. read ‘nemoralis’. 188, Table 2, for ‘Cochlicella acuta (Müll)? ventrosa(Fér.)’ read ‘Cochlicella ventrosa (Fér.)’. 191, line 24, for ‘araheo-’ read ‘archeo-’.     

Influence of Flooding on Net CO2Assimilation, Growth and Stem Anatomy of Annona Species

A series of experiments was conducted to assess net CO₂assimilationand growth responses to waterlogging of grafted and seedlingtrees in the genus Annona. Seedlings of A. glabra, A. muricataandA. squamosa L., and scions of ‘Gefner’ atemoya(A. squamosaxA. cherimola Mill.), ‘49-11’ (‘Gefner’atemoyaxA. reticulata L.), ‘4-5’ (‘Priestley’atemoyaxA. reticulata), A. reticulata grafted onto either A.glabra, A. reticulata orA. squamosa rootstocks were floodedfor up to 60 d. Soil anaerobiosis occurred on the third dayof flooding. Seedlings ofA. glabra and A. muricata, and thescions ‘49-11’, ‘Gefner’ atemoya, andA. reticulata grafted onto A. glabra rootstock were consideredflood tolerant based on their ability to survive and grow inflooded conditions. Scions of the normally flood-sensitive A.reticulata, ‘Gefner’ atemoya, and ‘49-11’tolerated root waterlogging when grafted onto the flood-tolerantspecies, A. glabra. In contrast, flooding of A. squamosa seedlingsand rootstocks, and A. reticulata rootstocks greatly reducedgrowth and net CO₂assimilation rates, and resulted in 20–80%tree mortality. Stem anatomical responses to long-term flooding(12 continuous months) were assessed in seedlings of A. glabraand A. muricata, and trees of ‘49-11’ grafted ontoA. glabra. Flooded trees developed hypertrophied stem lenticels,particularly in A. glabra, and enlarged xylem cells resultingin thicker stems with reduced xylem density. Flooding did notincrease air spaces in pre-existing xylem near the pith or inxylem tissue that was formed during flooding. Thus, flood tolerancedid not involve aerenchyma formation in the stem. Copyright1999 Annals of Botany Company Flood tolerance, net CO₂assimilation, photosynthesis, stem anatomy, shoot growth, anaerobiosis, Annonaceae.     

Pressure and Solute Potentials in Stomatal Cells of Tradescantia virginiana

The postulated mechanical advantage of subsidiary cells overguard cells has been estimated using leaves of Tradescantiavirginiana. The turgor pressures of subsidiary cells were adjusted to bezero or maximal by plasmolytic treatments, and the resultingstomatal apertures were measured. The ‘mechanical advantage’was calculated from two mathematical models which define itas an ‘antagonism ratio’. The discussion deals withmethods of preparing the tissue, the validity of the plasmolytictreatments, and the function of the antagonism ratio in relationto the Spannungsphase