The Production of Plantlets from Tissue Cultures of Brussels Sprout (Brassica oleracea L. var gemmifera D.C.)

Shoots were induced on callus derived from sprout sections andpetiole slices of an inbred parent line of Brussels sprout (Brassicaoleracea L. var gemmifera D.C.). The shoots, when excised andtransferred to fresh medium, enlarged and formed roots. Theseplantlets could be transferred to soil or their number increasedby a multiplication process involving the production of newshoots from the dormant lateral buds. Some of the plantletsderived from sprout callus were grown to maturity in the fieldand their morphology and chromosome number compared to seedgrown plants. There were no significant differences in sproutsize and stem diameter but there were significant differencesin plant shape. None of the plants in the field experiment showedpolyploidy. Plants derived from callus possessed an enhanced ability toform callus and redifferentiate when sections from these plantswere placed back on to nutrient medium.     

Diurnal Changes in Stem Diameter Depend Upon Variations in Water Content: Direct Evidence in Peach Trees

Three to five-year-old peach trees (Prunus persica (L.) Batschcv. ‘Maycrest’) grafted on P. ‘Damas 1869’grown in a sand trench were removed in the spring and grownhydroponically for several months. The system comprised twobalances continuously recording the mass of the nutrient solutionand that of the tree, so as to estimate transpiration and wateruptake rates separately. Diurnal variation in plant water content(transpiration minus water uptake) was observed, with rapidlydecreasing values when the solar radiation increased, whilethe reverse occurred when radiation decreased. Changes in stemdiameter were continuously recorded using linear variable differentialtransducers. Data collected over several days of contrastingclimatic conditions revealed that rapid changes in the stemdiameter occurred throughout the day and were closely relatedto plant water content. A lag-time not exceeding 10 min wasfound between changes in stem diameter and plant water content.These results are discussed in relation to the use of micromorphometricmethods to control irrigation in fruit trees. Moreover, we givevalues for the water stored in the shoots which may contributeto the transpiration stream. Key words: Prunus persica, stem shrinkage, plant water storage, water uptake, transpiration     

The Relationship between Stem Diameter and Water Potentials in Stems of Young Cabbage Plants

Continuous measurements were made of stem shrinkage, stem waterpotential (₃) and transpiration rate (T) in young, pot-growncabbage plants subjected to cycling evaporative demands. Sequencesof increasing evaporative demands induced increases in T anddecreases in both ₃ and stem diameter and conversely, wheneverevaporative demand decreased, T declined and ₃ and stem diameterrose. Over short periods, stem water potentials and stem shrinkagewere virtually parallel even when rapid oscillations were induced.Over longer periods the effects of growth were important comparedwith those of water stress on stem diameter when the moisturecontent of the soil was high. Growth, however, ceased when theplant was subjected to relatively mild water stress (₃ = –0.4MPa). Stem diameters, after correction for growth, were linearlyrelated to plant water potential. The results suggest that stemshrinkage and only a few calibration measurements might be usedto provide continuous estimates of water potentials in fieldcrops. Key words: Cabbage, Stem diameter, Stem water potential     

Water Balance in Cucumis Plants, Measured by Nuclear Magnetic Resonance, I

In this paper we demonstrate the study of plant water balanceby the non-invasive measurement of tissue water content andwater flow using proton nuclear magnetic resonance (NMR). Sapvelocity and flux were measured independently in the presenceof an excess of stationary tissue water. The instrumentationdescribed allows automated and unattended measurement of flow-and water content-variables in a well-defined region of theplant over periods of several days, with a time resolution betweensuccessive measurements of c. 5 s. Using this apparatus theeffect of changes in light intensity (day/night rhythm) andrelative humidity on stem tissue water content as well as onthe velocity and flux of xylem sap in the stem were investigatedin a cucumber plant. The results are in agreement with predictionsfrom a simple model for plant water balance, which is basedon water potential, flow rate and resistance to flow. As longas only transpiration is varied, flow rate and water content(or potential) are affected in opposite ways as demonstratedin this paper. In contrast, the model predicts that changesin uptake (resulting from changes in, for example, root resistance)will induce changes in water content and flow in the same direction.An experimental verification of this prediction is given ina subsequent paper, where, in addition, the NMR results arecompared to those obtained with a dendrometer. Key words: Water balance model, Cucumis sativus L., flow, water content, NMR, water balance measurement     

Effects of Chlorocholine Chloride and Water Regime on Growth, Yield, and Water Use of Spring Wheat

When CCC was applied as a spray to the leaves of Brassica oleraceaL. (Brussels sprout) grown in pots, plant height and mean internodelength were reduced. The effects appeared more slowly and wereless pronounced than those previously observed when CCC hadbeen applied to the soil; other differences were that root growthwas not inhibited, stem weight was only significantly reducedat the highest rate of application (2 per cent), and stomatalnumber per unit area of lower leaf epidermis was not affected.In common with soil applications, leaf thickness, stem diameter,and the percentage moisture contents of the leaves were allincreased by foliar applications.In a further experiment theprogress of wilting was observed in untreated plants and inplants treated with CCC applied either to the leaves or to thesoil. The rates of water loss and the moisture contents of theleaf laminae of the treated plants, after a period of wilting,were not significantly different from the controls. The treatedplants, however, looked less ‘wilted‘ for the changein angle of the leaf lamina to the stem was less and their leaveswere therefore held more upright.     

The Effects of Foliar Applications of (2-Chloroethyl)-trimethylammonium Chloride on Leaf Area and Dry Matter Production by the Brussels Sprout Plant

When CCC was applied as a spray to the leaves of Brassica oleraceaL. (Brussels sprout) grown in pots, plant height and mean internodelength were reduced. The effects appeared more slowly and wereless pronounced than those previously observed when CCC hadbeen applied to the soil; other differences were that root growthwas not inhibited, stem weight was only significantly reducedat the highest rate of application (2 per cent), and stomatalnumber per unit area of lower leaf epidermis was not affected.In common with soil applications, leaf thickness, stem diameter,and the percentage moisture contents of the leaves were allincreased by foliar applications.In a further experiment theprogress of wilting was observed in untreated plants and inplants treated with CCC applied either to the leaves or to thesoil. The rates of water loss and the moisture contents of theleaf laminae of the treated plants, after a period of wilting,were not significantly different from the controls. The treatedplants, however, looked less ‘wilted‘ for the changein angle of the leaf lamina to the stem was less and their leaveswere therefore held more upright.     

Oscillatory Transpiration of Avena Plants: Perturbation Experiments Provide Evidence for a Stable Point of Singularity

Oscillatory plant water regulation of young Avena plants was studied. The period of the oscillations was around 40 min. Pulse perturbations were given to plants showing oscillations in the transpiration rate. Perturbations consisted in temporary irradiance changes of the leaf or in water potential changes around the root. The effect of the pulse perturbations on the amplitude of the oscillations was recorded. The oscillations could be permanently halted after a perturbation of suitable magnitude given at a suitable phase of The oscillations. A subsequent perturbation could restart the oscillations again. By means of simulations it was shown that a feedback model For the transpiration oscillations could explain the experimental outcome if a non finearity of a special kind was incorporated. The circadian eclosion rhythm of Drosophila pseudoobscura and the petal rhythm of Kalanchoe blossfeldiana show many features in common with the experiments reported. Biophysically the present results indicate that the transpiration oscillations of Avena plants have a stable point of equilibrium or a stable point of singularity.     

The Comparative Drought Resistance of Landraces of Sorghum and Millet From Dry and Humid Regions

It may be that land-races of sorghum (Sorghum sp.) and millet[Pennisetum americanum (L.) Leeke] which evolved along geographicalgradients of rainfall in Africa and India, differ in their droughtresistance. Any physiological attributes found to be correlatedwith low rainfall might be important and effective characteristicsfor crop production in dry regions. Twenty land-races were chosen which evolved along geographicalgradients of rainfall, seven millets from India, six sorghumsfrom Mali, and seven sorghums from the Sudan. Races were evaluatedfor their growth potential and plant water relations under hydroponicsconditions in a growth chamber. A water stress treatment wasimposed by adding polyethylene glycol-8000 to the nutrient solution,giving a solute water potential of -0.5 MPa, compared with acontrol solution at 003 MPa. Drought resistance, in terms of relatively less growth inhibitionunder stress, was higher in races from dry regions than in racesfrom humid regions. Of all the physiological variables measured[carbon exchange rate, (CER), transpiration, transpiration ratio(CER/transpiration), leaf diffusive resistance, leaf water potentialand osmotic adjustment], only osmotic adjustment under stresswas generally correlated with average rainfall at each race'sorigin, indicating greater osmotic adjustment in land-racesfrom drier regions. Races with a greater capacity for osmoticadjustment were characterized by smaller plants with high ratesof transpiration and low rates of leaf senescence under stress. The carbon exchange rate per unit leaf area increased as liveleaf area decreased under stress due to leaf senescence. Thus,drought resistant races under stress tended to have lower CERper unit live leaf area (but not per plant) than susceptibleraces. Transpiration ratios under stress were lower in resistantthan in susceptible races, mainly because resistant races hadhigher transpiration. The results for the measured variables showed a general trendfor greater drought resistance in sorghum than in millet, indicatingthat the commonly observed adapation of the millets to dry environmentsmay be due to other factors, such as drought escape or heattolerance. Sorghum sp. Pennisetum americanum L. (Leeke), water stress, osmotic adjustment, photosynthesis, transpiration, evolution, drought resistance     

Oscillatory Transpiration in a Cotton Plant: I. EXPERIMENTAL CHARACTERIZATION

A resistance-capacitance model of oscillatory transpirationin a cotton plant under constant environmental conditions isdiscussed. Experimental results showed that a cotton plant exposedto a sudden and large evaporative demand went through a periodof self-adjustment indicating the presence of negative feedbackmechanisms. Phase relationships between flow in stem, leaf waterpotential, water uptake by roots, and leaf turgor were characterizedthrough the day. The water-based oscillatory processes are explainedthrough a proposed model based upon a hysteresis-controlledfunction and delayed coupling in the leaf component. The periodof oscillation was found to be about 30 min. Results also indicatedthat the lag between the transpiration and water uptake ratesduring oscillations was strongly influenced by the capacitanceof the leaf and that stem capacitance was negligible.     

Measured and Calculated Transpiration in Trifolium repens under Different Water Potentials

The relationship between transpiration measured gravimetrically,a generalized transpiration equation, and the ratio VPD/r_leafwas investigated in Trifolium repens plants subjected to varyingwater potentials. Dawn leaf water potential was measured witha pressure chamber, leaf diffusion resistance with a diffusionporometer, leaf temperatures with a thermistor, and relativehumidity with an aspirated psychrometer. During drought transpirationrates determined by both methods were quite similar particularlyat the lowest water potentials. After rewatering calculatedrates were somewhat higher than measured ones. It is concludedthat transpiration calculated by the indirect method is a usefuland reasonable estimate of transpiration for single plants undervarying water potentials.     

Measurement of Stem Water Potential of Young Plants Using a Hygrometer Attached to the Stem

A simple method was developed for the continuous non-destructivemeasurement of plant stem water potential. It consists of exposinga small section of stem vascular tissue and attaching to ita standard commercially available dewpoint hygrometer. Excellentagreement was found between water potential determined by thismethod, and that measured with a pressure-chamber on leavescovered with aluminium foil. Stem water potentials respondedwithin minutes to changes in radiation and transpiration. Resistanceto water uptake was found to be independent of the rate of transpiration.     

EFFECT OF SOIL MOISTURE STRESS ON THE CORRELATION BETWEEN HEAT PULSE VELOCITY AND TRANSPIRATION

(1) Heat pulse velocity in the stem of pine and sour orangeseedling, measured by HUBER's heat pulse method, was correlatedwith transpiration as measured by determining the humidity ofair passed over the plant. This correlation was used to calculatetranspiration under natural conditions, when heat pulse velocityonly was measured. (2) It was shown that heat pulse velocity at a given transpirationrate was related to water stress. When water potential in thesoil decreased heat pulse velocity increased in pine seedlingsand decreased in sour orange. This effect of water stress didnot appear to be completely reversible. Under conditions ofvery low water potential in the soil, no useful correlationbetween transpiration and heat pulse velocity could be obtainedeither in pine or sour orange. (3) From the data obtained it is concluded that transpirationcan be reliably calculated from heat pulse velocity measurementsonly if the calibration curve relating transpiration to heatpulse velocity is determined shortly beforehand under similarconditions of soil moisture and potential evapotranspiration.Even so, only total daily transpiration can be estimated bythis method. Hourly figures of transpiration are unreliable,especially under conditions conducive to low water potentialin the plant. (Received June 24, 1964; )     

Effects of Waterlogging on Water Relations of Actively-growing and Dormant Sitka Spruce Seedlings

Two-year-old Sitka spruce [Picea sitchensis (Bong.) Carr.] seedlings,either actively growing or dormant, were waterlogged in a growthroom at 15 °C. Shoot and root growth, transpiration andleaf water potential were observed. In actively-growing plants shoot extension continued after waterlogging,though at a reduced rate, and shoots of dormant plants brokebud and extended during the waterlogging period. Root growthwas suppressed by waterlogging in both types of plant. The 22day waterlogging treatment eventually killed the actively-growingplants but plants which were dormant at the time of waterloggingwere more tolerant. Changes in plant water relations after waterloggingwere entirely different depending on the condition of the plantswhen the soil was flooded. Dormant plants showed a gradual reductionin transpiration and increased water stress over the waterloggingperiod; after the soil was drained leaf water potential increasedto equal the value of control plants which had been maintainedin a freely drained condition, but transpiration did not increaseuntil root growth began. Actively-growing plants exhibited amore complex behaviour, characterized by a very rapid reductionin transpiration after waterlogging, accompanied by a briefperiod of water stress, followed by a period of increasing transpirationrate in the absence of water stress. Finally a second reductionin transpiration occurred and water stress increased as theseedlings died. The importance of the stage of activity of theroot system to the response of plants to waterlogging is discussed. Picea sitchensis (Bong.) Carr., Sitka spruce, waterlogging, water relations, dormancy, transpiration, water potential     

Quantitative analysis of transpiration stream dynamics in an intact cucumber stem by a heat flux control method

Water flux of transpiration stream in an intact stem of the 10 leaf stage cucumber plant (Cucumis sativus L. cv. Chojitsu-Ochiai) was measured by a novel system of heat flux control method with a resolution of 1 × 10⁻³ grams per second and a time constant of 1 minute; two heat flux control sensors were attached to the seventh internode and the stem base. The transpiration stream responded clearly to leaf transpiration and root water absorption when the plant was exposed to light, and the water flux at the stem base corresponded to the transpiration rate per plant in steady state. Root water absorption lagged about 10 minutes behind leaf transpiration. Dynamics of water fluxes were affected by the lag of water absorption in roots, and temporary water loss caused by rapid increase in leaf transpiration was buffered by about 5% of the water content in the stem.     

The Effects of Transverse Cuts Through the Stems of Transpiring Woody Plants on Water Transport and Stress in the Leaves

Both single and double-overlapping transverse cuts were madein the stems of transpiring plants (Acer pseudoplatanus andGossypium hirsutum) and their effects on transpiration and waterstress in the leaves studied. Water stress was measured as depressionof water potential and ß-gauge reading. Results weresimilar with both species. Single cuts severing as much as 90 per cent of the cross-sectionalarea of the stem had no detectable effect on the rate of transpirationor on stress in the leaves above. Double overlapping cross cutscaused irreversible increased stress and cessation of sap flowonly if their vertical separation was less than a critical distance,specific for each species and related to the respective vessellengths. Cuts separated by more than this distance had no detectableeffect on stress and transpiration rate was unchanged. Alsono effect was detected if a length of the stem between suchcuts was cooled to near-freezing temperature, or killed by steamtreatment. Patterns of heat pulse velocities showed how thesap was channelled around the cuts through remaining intactxylem.     

Transpiration Stream of Desert Species: Resistances and Capacitances for a C3, a C4, and a CAM Plant

Transpiration rates and water potentials of three sympatricdesert perennials, a C3 subshrub (Encelia farinosa), a C₄ bunchgrass(Hilaria rigida), and a CAM succulent (Agave deserti), wereanalysed using an electrical circuit analogue that includedresistances and capacitances for the leaves, stems, and roots.The water storage capability of the organs differed considerably,capacitance ranging over 1000-fold from the thin leaves of H,rigida to the massive leaves of A. deserti, although the capacitanceper unit volume varied only 1.9-fold. The diurnal changes inwater storage could support maximum transpiration rates of H.rigida for 4 min, E. farinosa for 7 min, and A. deserti for16 h. The time constant for equilibration of water from storageto the xylem ranged from 29 s for roots of H. rigida to 52 minfor leaves of A. deserti. Resistances for such movement wererelatively low for the succulent leaves of A. deserti and wereup to about 50-fold higher for the three organs of E. farinosa.Xylem resistances calculated using the Hagen-Poiseuille lawand measured xylem dimensions were 2.1- to 2.1-fold lower thanresistances estimated from observed water potential drops, adiscrepancy which is in agreement with other published data.Contrary to data on other plants, the xylem resistances in theroots and leaves of E. farinosa and H. rigida averaged only15% of the stem xylem resistance. Key words: Capacitance, Xylem resistance, Transpiration stream, Desert     

The Regulation of the Water Potential Gradient in the Host and Parasite Relationship betweenSorghum bicolorandStriga hermonthica

A glasshouse experiment was carried out to investigate the factorscontrolling the abstraction of xylem fluid from its host bythe parasiteStriga hermonthica(Scrophulariaceae).Strigahad amean daily transpiration rate far exceeding that of its hostsorghum (Sorghum bicolor), with infestation byStrigaalso shownto lower the transpiration rate of the host. Stopping the host'stranspiration was shown to decrease the transpiration rate ofthe parasite. Stopping the parasite's transpiration only gavean initial increase in the host's transpiration rate which wasnot sustained. The parasite had a lower water potential thanits host, values being -0.42 MPa and -0.23 MPa, respectively,and an accompanying higher osmotic pressure of 0.68 MPa against0.51 MPa for sorghum. Modifying the water potential gradientby bagging both partners together showed that the differentialin osmotic pressure and water potential was largely maintainedby the parasite's higher rate of transpiration. A favourablewater potential gradient towards the parasite still existedfollowing the cessation of transpiration, this being generatedby the haustorial resistance to hydraulic conductivity whichwas found to be some 1.5–4.5 times greater than that offeredby the parasite shoot. Both the high rate of transpiration andthe increased resistance across the haustoria would appear tobe necessary means to facilitate the diversion of host resourcesto the parasite.Copyright 1997 Annals of Botany Company Striga hermonthica; sorghum; water relations; haustorium; root parasite     

The Effect of Grain Number per Ear (Sink Size) on Source Activity and its Water-Relations in Wheat

Blum, A., Mayer, J. and Golan, G. 1988. The effect of grainnumber per ear (sink size) on source activity and its water-relationsin wheat.–J. exp. Bot. 39: 106–114. Work was done to evaluate the nature of sink-source relationshipsin wheat (Triticum aestivum L.), when the strength of the sinkwas modified by the removal of half of the grain from the earat about anthesis. The main hypothesis was that sink-sourcerelationship would be modified by water stress and that a weakersink would improve the drought resistance of the source. Two experiments were performed. The first experiment evaluatedthe effect of de-graining in two wheat varieties grown in thefield. The second experiment (in the greenhouse) evaluated theeffect of de-graining in plants subjected to water stress afteranthesis by immersing the root system in a solution of polyethyleneglycol (6000), as compared with non-stressed controls. In bothexperiments measurements were performed after de-graining toprovide data on leaf gas exchange, leaf water potential, osmoticadjustment of leaves and ears (greenhouse), the percent of stemweight loss as an index of stem reserve mobilization, finalroot weight (greenhouse) and ear weight components. De-graining caused a decrease in flag leaf stomatal conductance,carbon exchange rate (CER) and transpiration and an increasein flag leaf water potential. These effects were stronger withwater stress. De-graining did not affect osmotic adjustmentin the flag leaf but induced better adjustment in glumes andawns. De-graining decreased the percent of stem weight lossand increased final root weight, especially under drought stress. A weaker sink was, therefore, considered to improve plant droughtresistance in terms of the maintenance of higher leaf waterpotential, a larger root, a better osmotic adjustment in theear and, possibly, increased flag leaf longevity. The ‘cost’of this improved drought resistance was in reduced flag leafCER and reduced stem (and root?) reserve mobilization. Key words: Drought resistance, carbon exchange rate, stomata, transpiration, osmotic adjustment, leaf water potential, root, awns, yield     

Modification of Drought Resistance by Water Stress Conditioning in Acacia and Eucalyptus

Plants of Acacia and Eucalyptus species were grown under differentlevels of shading, nutrition, and irrigation to assess the effectof these factors on plant water use. Water use per unit of leaf(phyllode) area was affected only by the irrigation treatment,control plants that had received water daily using appreciablymore water than plants that had been repeatedly subjected towater stress. Water stress conditioning had little or no effecton plant height, leaf (phyllode) area, or minimum stomatal resistancein any of the species. Detailed study of the water stress conditioningof Eucalyptus robusta showed that controls used 46% more waterthan conditioned plants. Leaf area and plant height were unaffectedby conditioning. Control of transpiration was not due to stomatalfunctioning, both sets of plants operating with the same leafdiffusive resistance under conditions of ready water availability.Hydraulic conductivity of the intact root system was loweredby conditioning and it is suggested that this was due, at leastin part, to the effect that conditioning had on root xylem conductivity.Specific conductivity of stem sections was lowered by waterstress conditioning. Water stress avoidance was also associatedwith a more pronounced tendency for stomata to close prior towilting and with a higher level of leaf resistance which couldbe maintained at a low leaf water potential. Conditioned plantsexhibited drought tolerance in their ability to control lossof water from the leaf at lower leaf water potentials than thecontrols.     

Oscillations in stomatal conductance and plant functioning associated with stomatal conductance: Observations and a model

Summary Measurements of transpiration, leaf water content, and flux of water in a cotton plant exhibiting sustained oscillations, in stomatal conductance are presented, and a model of the mechanism causing this behaviour is developed. The dynamic elements, of the model are capacitors—representing the change of water content with water potential in mesophyll, subsidiary and guard cells—interconnected by resistances representing flow paths in the plant. Increase of water potential in guard cells causes an increase in stomatal conductance. Increase of water potential in the subsidiary cells has the opposite effect and provides the positive feed-back which can cause stomatal conductance to oscillate. The oscillations are shown to have many of the characteristics of free-running oscillations in real plants. The behaviour of the model has been examined, using an analogue computer, with constraints and perturbations representing some of those which could be applied to real plants in physiological experiments. Aspects of behaviour which have been simulated are (a) opening and closing of stomata under the influence of changes in illumination, (b) transient responses due to step changes in potential transpiration, root permeability and potential of water surrounding the roots, (c) the influence of these factors on the occurrence and shape of spontaneous oscillations, and (d) modulation of sustained oscillations due to a circadian rhythm in the permeability of roots.