In-Phase Cycling of Photosynthesis and Conductance at Saturating Carbon Dioxide Pressure Induced by Increases in Water Vapour Pressure Deficit

Bunce, J. A. 1987. In-phase cycling of photosynthesis and conductanceat saturating carbon dioxide pressure induced by increases inwater vapour pressure deficit.—J. exp. Bot. 38: 1413–1420. The leaf to air water vapour deficit was increased suddenlyfrom about 1·0 to 2·5 IcPa for single leaves ofsoybean (Glycine max L. Merr.) plants held at 30 °C, 2·0mmol m ^–2 s^–1 photosynthetic photon flux density(PPFD) and carbon dioxide pressures saturating to photosynthesis.After a lag of about 10 min, photosynthetic rate and stomatalconductance to water vapour began to decrease, and then cycledin phase with each other. The period of the cydes was about20 min. During these cycles the substomatal carbon dioxide pressurewas constant in the majority of leaves examined, and was alwaysabove saturation for photosynthesis. Epidermal impressions showedthat most stomata changed in aperture during the cycles, andthat very few were ever fully closed. Water potential measuredon excised discs changed by at most 0·1 MPa from theminima to the maxima in transpiration rate. In contrast, forleaves of sunflower (Helianthus animus L.) grown at low PPFD,the increase in VPD led to leaf wilting and decreased photosynthesis,followed by recovery of turgor and photosynthesis as stomatalconductance began to decrease. In these leaves photosynthesisand conductance then cycled approximately 180° out of phase.It is suggested that in soybeans decreased leaf conductanceinduced by high VPD provided a signal which decreased the rateof photosynthesis at carbon dioxide saturation by a mechanismthat was not related to a water deficit in the mesophyll. Key words: Photosynthesis, stomatal conductance, cycling, vapour pressure deficit     

Characterization of a wilty sunflower (Helianthus annuus L.) mutant II. Water relations, stomatal conductance, abscisic acid content in leaves and xylem sap of plants subjected to water deficiency

The response of w-1, a wilty sunflower (Helianthus annuus L.)mutant, to water stress is described in comparison with thecontrol line (W-1). Detached leaves of w-1 strongly dehydratedduring the first 30 min without significant changes in leafconductance, whereas W-1 responded rapidly to water loss byreducing stomatal aperture. After 2 h stress ABA increased slightlyin w-1, while W-1 leaves showed a 20-fold increase. When waterstress was imposed to potted plants by water withholding, w-1quickly dehydrated, and lost turgor, while W-1 maintained positiveturgor values for a longer period. Wild-type plants respondedto small changes in leaf water potential by accumulating ABAand by closing stomata, whereas in the mutant significant changesin ABA content and in stomatal conductance were found only atvery low water potentials. In another experiment in which waterwas withheld under high relative humidity, when soil water contentstarted to decrease W-1 rapidly closed stomata in the absenceof any change in leaf water status and the reduction in conductancewas paralleled by a rise in xylem sap ABA concentration. Bycontrast the mutant started to accumulate ABA in the xylem sapand to close stomata when soil water content and leaf waterpotential were dramatically reduced. The low endogenous ABAlevels and the inability to synthesize the hormone rapidly eitherin the leaves or in the roots seem to be responsible for thehigh sensitivity of w-1 to water stress. Key words: ABA, Helianthus annuus L, water relations, stomatal conductance, drought, wilty mutant     

Effect of Verticillium dahliae on Photosynthesis, Leaf Expansion and Senescence of Field-grown Sunflower

On the basis of known sunflower (Helianthus annuus L.) responsesto soil water deficit, it is proposed that the effect of thefungus Verticillium dahliae Klebahn on plant leaf area precedesand is greater than its effect on leaf photosynthesis and stomatalconductance. To test this hypothesis, we measured shoot andleaf area growth, leaf photosynthetic rate, stomatal conductanceand disease symptoms in a field experiment including hybridsof high (Sankol) and low (Dekasol 3900) susceptibility to V.dahliae. Plants inoculated with V. dahliae and controls werecompared. We also investigated the effect of V. dahliae on keycomponents of plant leaf area, leaf expansion and senescence,in inoculated and control plants of Sankol and Toba, a hybridof intermediate susceptibility to V. dahliae. Reduction in plantleaf area caused by V. dahliae was first detected 31 d afterinoculation (DAI), when visual symptoms of disease in inoculatedplants were slight (Sankol) or absent (Dekasol 3900). Reductionin leaf photosynthesis was first observed 66 DAI; stomatal conductanceand leaf dark respiration were both unaffected by V. dahliaeduring the whole experiment. In comparison with controls, V.dahliae reduced seasonal duration of plant leaf area by 25%in Dekalb 3900 and by 55% in Sankol, whereas the average reductionin leaf photosynthetic rate was 9%. In correspondence with thereduction in leaf area duration, inoculation reduced shoot drymatter of mature Sankol by 50%. In both experiments, less leafexpansion accounted for most of the early reduction in plantleaf area; as the disease progressed, increasing senescencealso contributed to reduced plant leaf area. It is concludedthat the response of sunflower to V. dahliae resembled the responseof the plant to soil water deficit: (1) plant leaf area, ratherthan leaf photosynthetic rate, accounted for the reduction ingrowth in mass; and (2) reduced leaf expansion early in theseason and faster leaf senescence in older plants accountedfor the decrease in plant leaf area. Copyright 2000 Annals ofBotany Company Helianthus annuus, Verticillium dahliae, allometry, apical dominance, drought, leaf expansion, leaf senescence, photosynthesis, stomatal conductance, growth     

Inhibition of Light-Stimulated Leaf Expansion by Abscisic Acid

Abscisic acid (ABA) applied to intact bean (Phaseolus vulgaris)leaves or to isolated leaf discs inhibits light-stimulated cellenlargement This effect may be obtained with 10^–4 molm^–3 ABA, but is more significant at higher concentrations.The inhibition of disc expansion by ABA is greater for discsprovided with an external supply of sucrose than for discs providedwith KC1, and may be completely overcome by increasing the KC1concentration externally to 50 mol m^–3. Decreased growthrate of ABA-treated tissue is not correlated with loss of solutesfrom growing cells, but is correlated with a decrease in cellwall extensibility. ABA does not prevent light-stimulated acidificationof the leaf surface, and stimulates the acidification of theexternal solution by leaf pieces. However, the capacity of thecell walls to undergo acid-induced wall loosening is diminishedby ABA-treatment. The possibility that ABA acts directly byinhibiting growth processes at the cellular level, or indirectlyby causing stomatal closure, is discussed. Key words: Phaseolus vulgaris, ABA, Inhibition, Leaf expansion     

The Measurement of Stomatal Responses to Stimuli in Leaves and Leaf Discs

A comparison has been made of stomatal responses in intact leaves,leaf discs supplied with water via their cut edges and leafdiscs floating on water. Xanthium pennsylvanicum leaf discswatered via their cut edges appeared to be more turgid thanintact leaves; this considerably slowed down the rate of stomatalopening but it slightly increased the final steady-state stomatalopening. When the water potential of such leaf discs was loweredby pre-treatment with mannitol solutions rates of stomatal openingincreased whereas maximum steady-state openings decreased. In tobacco leaf discs floating on water the stomata in contactwith water were wider open than those in contact with normalair and they did not respond to treatment with carbon dioxide-freeair. The rate of photosynthesis was severely reduced in tobaccoleaf discs floating with the lower epidermis on water, mostprobably owing to the slow rate of diffusion of carbon dioxidein water. By floating such discs on osmotica the degree of stomatalopening was increased, however, a response to treatment withcarbon dioxide-free air was still not measurable. It is postulatedthat, on account of the relative unavailability of carbon dioxidefrom the water, the carbon dioxide concentration in the substomatalcavities of the lower surface is abnormally low, irrespectiveof whether ordinary air or carbon dioxide-free air is availableto the upper surface. A comparison between porometer readings and measurements ofsiliconerubber impressions of stomatal pores taken from insidethe porometer cup confirmed that the silicone-rubber impressionmethod of assessing stomatal responses to stimuli has severelimitations, especially at small stomatal apertures.     

The Effect of Vapour Pressure Deficit on Carbon Isotope Discrimination in the Desert Shrub Larrea tridentata(Creosote Bush)

Larrea tridentata (creosote bush) seedlings were subjected tothree regimens of atmospheric humidity in a growth chamber experiment.Relative humidity was varied to achieve daytime vapour pressuredeficits (VPD) during growth of 29, 48 and 77 kPa. Photosyntheticgas exchange, carbon isotope composition and biomass productionwere measured after 8–10 weeks of treatment. Whereas stomatalconductance (g) declined linearly with increasing ambient VPD,CO₂ assimilation rate (A) was not measurably affected by changesin ambient VPD. This resulted in a decrease in intrinsic wateruse efficiency (ratio of CO₂ assimilation to stomatal conductance;A/_g) with increasing VPD. Leaf carbon isotope discrimination(A) was negatively correlated (r² = 088) with A/g ratios. Carbonisotope discrimination also correlated positively with ratiosof internal (C₁) to ambient (c_a) CO₂ levels determined by gasexchange measurements (c₁/c). The ratio of c₁ to c_a was lowerat higher VPD levels. Leaf biomass decreased with increasingambient VPD and correlated positively with. Root to leaf biomassratio increased at higher VPD levels and correlated negativelywith. Key words: Larrea tridentata, vapour pressure deficit, carbon isotope discrimination, intrinsic water-use efficiency     

Drought-induced Closure of Phaseolus vulgaris L. Stomata Precedes Leaf Water Deficit and any Increase in Xylem ABA Concentration

Young seedlings of two cultivars of Phaseolus vulgaris L. (cv.Cacahuate-72 and Michoacan-12A3) were subjected to a soil dryingtreatment. Stomata started to close before any leaf water deficitcould be detected. Soil drying promoted a small degree of ABAaccumulation in roots, but, at the time stomatal closure wasinitiated, at least in one cultivar, xylem ABA and bulk leafABA concentration were not enhanced. The mechanism of the stomatalresponse is discussed in terms of an as-yet unidentified regulatorof stomatal behaviour, redistribution of existing ABA in plants,and a high sensitivity of stomata to small changes in ABA concentration.     

Root hypoxia reduces leaf growth : role of factors in the transpiration stream

This study examined the potential role of restricted phloem export, or import of substances from the roots in the leaf growth response to root hypoxia. In addition, the effects of root hypoxia on abscisic acid (ABA) and zeatin riboside (ZR) levels were measured and their effects on in vitro growth determined. Imposition of root hypoxia in the dark when transpirational water flux was minimal delayed the reduction in leaf growth until the following light period. Restriction of phloem transport by stem girdling did not eliminate the hypoxia-induced reduction in leaf growth. In vitro growth of leaf discs was inhibited in the presence of xylem sap collected from hypoxic roots, and also by millimolar ABA. Disc growth was promoted by sap from aerated roots and by 0.1 micromolar ZR. The flux of both ABA and ZR was reduced in xylem sap from hypoxic roots. Leaf ABA transiently increased twofold after 24 hours of hypoxia exposure but there were no changes in leaf cytokinin levels.     

Effects of NORIN 10 and Tom Thumb Dwarfing Genes on Morphology, Physiology and Abscisic Acid Production in Wheat

The pleiotropic effects of three genetically related dwarfinggenes were investigated in near-isogenic lines of wheat. TheNORIN 10 semi-dwarfing alleles, Rht 1 and Rht 2, and the TomThumb allele, Rht 3, were assessed for effects on some vegetativemorphological and physiological characters. The Rht allelesaffected leaf size with a resultant decrease in leaf area ofthe whole plant. Rht 3, which had the most marked effects, reducedleaf area in young plants by as much as 30 per cent. Althoughflag leaf dimensions and stomatal distributions of the flagleaf were altered, the gene had no effect on its area, stomatalconductance or net CO₂ exchange rate. Comparisons of Rht andtall plants revealed no differences in the abscisic acid (ABA)levels of either turgid or partially dehydrated leaves. Triticum aestivum L., wheat, dwarfing genes, leaf structure, abscisic acid, stomatal conductance, CO₂, exchange, relative growth rate     

Osmotic Adjustment and Stomatal Response to Water Deficits in Maize

A pot experiment was carried out using five maize {Zea maysL.) cultivars under three soil moisture levels (MPa 0 to –0.05,–0.3 to –0.9 and –1.2 to –1.5) to investigatethe effects of water deficits on osmotic adjustment and stomatalconductance. The degree of leaf rolling and the sugar and nutrientconcentrations in leaf cell sap were measured. Leaf water potential and osmotic potential decreased and stomatalconductance decreased with increasing water deficits. Stomatalconductance correlated positively with leaf water potentialand osmotic potential. Degree of leaf rolling was lower in cultivarswhich maintained higher turgor. Osmotic adjustment of 0.08 to0.43 MPa was found under the lowest soil moisture level in fivecultivars used. Sugar and K were the major osmotic substancesin the maize plant. Sugar, K and Mg concentrations increasedunder water deficit, and correlated negatively with a decreasein osmotic potential. Key words: Zea mays L., leaf water relations, leaf rolling, osmotic adjustment, stomatal conductance, water deficit     

Quantitative analysis of the combined effects of temperature, evaporative demand and light on leaf elongation rate in well-watered field and laboratory-grown maize plants

The respective effects of meristem temperature, vapour pressuredeficit (VPD) and photosynthetic photon flux density (PPFD)on leaf elongation rate (LER) of maize, in the absence of waterdeficit in the soil have been quantified. This analysis wascarried out in a series of field experiments in northern andsouthern France over several seasons and years, and in growthchamber experiments. LER was measured with 10 min steps, togetherwith meristem temperature, VPD and PPFD at leaf level in threetypes of experiments: in growth chamber experiments with stepsin PPFD or VPD at constant meristem temperature, in growth chamberexperiments with several combinations of constant, but contrasting,PPFDs, VPDs and meristem temperatures, and in the field withfluctuating conditions, (i) When evaporative demand was low(night or day with low air VPD), LER was only linked to meristemtemperature, regardless of other climatic conditions, (ii) Lighthad no effect per se on LER in the range from 0 to 1500 molm^–2 s^–1 for time-scales longer than 2 h, providedthat its indirect effects on meristem temperature and on evaporativedemand were corrected (in the growth chamber) or taken intoaccount (in the field), and provided that cumulated PPFD overa weekly time-scale was compatible with field conditions, (iii)Evaporative demand sensed by growing leaves, as estimated bymeristem-to-air vapour pressure difference, markedly affectedLER in the range from 1–4 kPa, at all time-scales understudy, with a unique relationship in the growth chamber (constantconditions) and in the field (fluctuating conditions). Thiseffect was only observed when PPFD was high enough for stomatato open. The negative effect of evaporative demand on LER wasprobably not due to long distance root-to-shoot signalling,since soil was wet, calculated root water potential remainedclose to 0 MPa and concentration of ABA in the xylem sap wasvery low. Therefore, it is proposed to model maize LER witha two-step process, involving the calculation of the maximumLER at a given meristem temperature and then the calculationof the reduction in LER due to evaporative demand. Joint analysisof the whole set of data by using the two equations yieldeda r² of 0.75. This two-step process would be more accurate thanthe provision of LER from temperature only in cases where airVPD frequently exceeds 2 kPa. Key words: Leaf growth, light, evaporative demand, temperature, thermal time, water deficit, ABA, Zea mays L.     

Senescence-Associated Changes during Long-day-induced Leaf Senescence and the Nature of the Graft-transmissible Senescence Substance in Kleinia articulata

Schwabe, W. W. and Kulkarni, V. J. 1987. Senescence-associatedchanges during long-day-induced leaf senescence and the natureof the graft-transmissible senescence substance in Kleinia articulata.— J. exp. Bot. 38: 1741–1755. The long-day-induced senescence in Kleinia articulata leaveswas characterized by a loss in fresh and dry weight, in therate of leaf expansion and progressive loss of chlorophyll inthe detached rooted leaves. Ultrastructural examination of mesophyllcells of leaves from plants grown in continuous light showedthat osmiophilic globules accumulating in the chloroplasts werethe first visible sign of senescence in the organdies. Thesefirst signs of senescence could be detected in very young leavesof plants in continuous light, even before the leaves had expanded.Attempts were made to study the cause of this photoperiodicsenescence which, from previous work, appeared to involve agraft-transmissible substance. Leaves in continuous light showed reduced stomatal opening andextracts from them had very much higher activity in the Commelinastomatal closure assay (ABA-like activity ?) compared with non-senescingleaves grown in short days (8 h). However, even if all the activitywere due to ABA, this on its own does not appear to be the senescencesubstance because a much longer exposure to continuous lightwas required to induce irreversible senescence than to reachmaximum stomatal closure promoting activity in the bioassay.Moreover, severe water stress (high ABA?) did not lead to senescenceunless combined with continuous light or ethylene treatment.It is postulated that while ABA may play an important role inKleinia leaf senescence its lethal effect may not be realizedunless ethylene-induced membrane changes may synergisticallyassist. Key words: Leaf senescence, ABA, Daylength, stomatal movement, Kleinia     

Effects of Periodical Soil Drying and Leaf Water Potential on the Sensitivity of Stomatal Response to Xylem ABA

The study on the changes of stomatal sensitivity in relation to xylem ABA during periodical soil drying and the effect of leaf water status on the stomatal sensitivity has confirmed that xylem ABA concentration is a good indicator of soil water status around roots and the relation between xylem ABA concentration and predawn leaf water potential remained constant during the three consecutive soil drying cycles based on the slopes of the fitted lines. The sensitivity of stomata to xylem ABA increased substantially as the soil drying cycles progressed, and the xylem ABA concentration needed to cause a 50% decrease of stomatal conductance was as low as 550 mnoL/L in the next two soil drying cycle, as compared with the 750 nmol/L ABA in the first cycle of soil drying. The results using the split-root system showed that leaf water deficit significantly enhanced the stomatal response to xylem ABA and the xylem ABA concentration needed to cause a 50% decrease in stomatal conductance was 2 to 4 times smaller in the whole-root-drying treatment than those in the semi-root- drying treatment. These results suggested that the sensitivity of stomata to xylem ABA concentration is not a fixed characteristic.     

Stomatal Behaviours of Aspen (<Emphasis Type="Italic">Populus tremuloides</Emphasis>) Plants in Response to Low Root Temperature in Hydroponics

Hydroponic-grown seedlings of aspen (Populus tremuloides Michx.) were used to investigate how low root temperatures (5°C) affect stomatal conductance and water relations. An isohydric manner of the stomatal behaviour was found with the seedlings when their roots were subjected to the low temperature. Stomatal conductance rapidly and dramatically reduced in response to the low root temperature, while the xylem water potential did not significantly alter. Under the low root temperature, pH value of the xylem sap increased from 6.15 to 6.72 within the initial 4 h, while abscisic acid (ABA) concentration increased by the eighth hour of treatment. K⁺ concentration of the xylem sap significantly decreased within the 8th h and then reversed by the 24th h. The ion change was accompanied by a decrease and then an increase in the electrical conductivity, and an increase and then a decrease in the osmotic potential. The tempo of physiological responses to the low root temperature suggests that the rapid pH change of the xylem sap was the initial factor which triggered stomatal closure in low temperature-treated seedlings, and that the role of the more slowly accumulating ABA was likely to reinforce the stomatal closure. Xylem sap from the seedlings subjected low root temperature affected stomatal aperture on leaf discs when they were floated on the sap solution. The stomatal aperture correlated (P = 0.006) with the changed pattern of [K⁺] in the sap while the range of pH or ABA found in the xylem sap did not influence stomatal aperture of leaf discs in solution. The effect of xylem sap on stomatal aperture on leaf discs was different from on stomatal conductance in the intact seedlings. Comparison was made with previous study with the soil-grown seedlings.     

周期性土壤干旱和叶片水势对气孔响应木质部ABA灵敏度的影响

研究了周期性土壤干旱期间气孔对木质部ＡＢＡ响应的灵敏度的变化以及叶片水势对灵敏度的影响。实验结果证明了木质部ＡＢＡ浓度是反映根系周围土壤水分状况的一个指标的结论。土壤周期性干旱不影响木质部ＡＢＡ浓度对土壤水分状况的依赖关系,但显著地提高了气孔对木质部ＡＢＡ 响应的灵敏度。根据对实测数据的数学模拟结果显示,引起气孔导度下降５０％ 所需的木质部ＡＢＡ浓度从第一轮土壤干旱的７５０ ｎｍｏｌ／Ｌ降至第二轮土壤干旱的５５０ ｎｍｏｌ／Ｌ。分根实验的结果表明,叶片水分亏缺显著提高了气孔对木质部ＡＢＡ 的响应的灵敏程度,全根干旱中引起气孔导度下降５０ ％ 所需的木质部ＡＢＡ 浓度比半根干旱的小２ ～４ 倍。这表明,气孔对木质部ＡＢＡ响应的灵敏度不是一个固定的特性,可随植物生长环境及许多其他因素的变化而表现出很大的差异     

The Influence of Prior Water Stress and Abscisic Acid Foliar Spraying on Stomatal Responses to CO2, IAA, ABA, and Calcium in Leaves of Solanum melongena

The influence of a water stress or foliar ABA spraying pretreatmenton stomatal responses to water loss, exogenous ABA, IAA, Ca²⁺,and CO₂ were studied using excised leaves of Solanum melongena.Both pretreatments increased stomatal sensitivity of water loss,in the presence and absence of CO₂, but decreased stomatal sensitivityto exogenous ABA. CO₂ greatly reduced the effect of exogenouslyapplied ABA. IAA decreased leaf diffusion resistance for controland ABA sprayed leaves, but did not influence the LDR of previouslywater-stressed leaves. CA²⁺ did not influence LDR of any leavesof any treatments. Key words: Water stress, stomatal response, pretreatments     

Characterization of a wilty sunflower (Helianthus annuus L.) mutant: III. Phenotypic interaction in reciprocal grafts from wilty mutant and wild-type plants

The present study was conducted to evaluate phenotypic interactionin reciprocal grafts between wilty (w-1) sunflower mutant andnormal (W-1) plants. The w-1 genotype is a ‘leaky’ABA-deficient mutant, characterized by high stomatal conductance,in both light and dark conditions, and high transpiration rate. In well-watered conditions, mutant scions grafted on to normalrootstock (w-1/W-1) showed higher leaf relative water content,leaf water potential and ABA levels than those of control grafts(w-1/w-1). In addition, detached leaves of w-1/W-1 exhibitedlower water loss than w-1/w-1 grafts, while mutant rootstockdid not affect the transpiration rate of detached W-1 leaves.When drought stress was imposed to potted plants by withholdingwater, the mutant scions grafted on to normal roots showed apartial phenotypic reversion. A rapid stomatal closure and arise in ABA levels in response to a small decrease in leaf waterpotential was observed. By contrast, in w-1/w-1 grafts significantreductions in stomatal conductance and ABA accumulation weredetected only in conjunction with a severe water deficit. W-1scions on mutant stocks (W-1/w-1) maintained the normal phenotypeof control wild-type grafts (W1/W-1). Key words: ABA, grafting, Helianthus annuus, stomatal conductance, water relations, wilty mutant     

Stomata of Micropropagated DelphiniumPlants Respond to ABA, CO2, Light and Water Potential, but Fail to Close Fully

Morphological and physiological characteristics of micropropagatedplants of Delphinium cv. Princess Caroline were studied. Leavesproduced in vitro showed poor control of water loss which appearsto result from restricted responses by stomata and not frompoor cuticular development. Stomata of leaves produced in vitrowere larger and more frequent than those produced during acclimatization.Despite the fact that stomata from isolated epidermis of leavesproduced in vitro reduced their apertures when exposed to turgor-reducingtreatments, they did not close fully. This, together with highstomatal frequencies might explain the poor control of waterloss shown by intact leaves produced in culture when exposedto dry air. While leaves from acclimatized plants showed almostcomplete closure with ABA, low water potentials, darkness andCO₂, stomata from leaves produced in vitro reduced their apertureswhen exposed to those factors, but only to a limit. Therefore,stomata from leaves cultured in vitro seem to be partially functional,but some physiological or anatomical alteration prevents themfrom closing fully. Stomata from leaves produced in vitro wereparticularly insensitive to ABA which appears to be partly associatedwith the high cytokinin concentration in the culture medium.In the long-term, this stomatal insensitivity to ABA might contributeto plant losses when micropropagated plantlets are transferredto soil. Key words: Micropropagation, stomatal physiology, dehydration, PEG, ABA, BAP, darkness, CO₂, Delphinium     

Growth Rate and Water Relations of Citrus Leaf Flushes

Growth rates of seasonal leaf flushes of ‘Valencia’orange [Citrus sinensis (L.) Osbeck] were measured and waterrelations characteristics of young (new) and over-wintered (old)citrus leaves were compared. New flush leaves had lower specificleaf weights and lower midday leaf water potentials than comparablyexposed old leaves. Spring and summer flush new leaves had higherosmotic potentials than old leaves. These differences becamenon-significant as the new leaves matured. During summer conditions,water-stressed new leaves reached zero turgor and stomatal conductancealso began to decrease in them at higher leaf water potentialsthan in old leaves. Old leaves were capable of maintaining openstomata at lower leaf water potentials. Opened flowers and newflush leaves lost more water, on a dry weight basis, than flowerbuds, fruit or mature leaves. The results illustrate differencesin leaf water potential and stomatal conductance which can beattributed to the maintenance of leaf turgor by decreases inleaf osmotic potentials as leaves mature. These changes in citrusleaf water relations are especially important since water stressresulting from high water loss rates of new tissues could reduceflowering and fruit set. Citrus sinensis (L.) Osbeck, orange, Citrus paradisi Macf., grapefruit, growth rate, leaf water relations, osmotic potential, water potential, stomatal conductance     

Water relations and abscisic acid levels of watermelon as affected by rooting volume restriction

Rooting volume restriction (RVR) reduces shoot growth of plantsprovided with sufficient water or nutrients. The effects ofRVR on water status, abscisic acid (ABA) levels in leaves, roots,or xylem sap from detopped plants of watermelon [Citrullus lanatus(Thunb.) Matsum. and Nakai ‘StarBrite’] seedlingswere evaluated with five rooting volumes (18, 26, 36, 46, or80 cm3). Shoot water potential increased with increasing rootingvolume, with no difference between plants from 18 and 26 cm³cells or between plants from 36 and 46 cm³ cells. Stomatal conductancewas not consistently affected by RVR; at 10 and 20 DAE, stomatalconductance in plants grown in 36 cm³ cells was higher thanthat of plants grown in any other cell volume. Severe RVR (18and/or 26 cm³) tended to produce plants with higher ABA levelsin roots (15 DAE only), xylem sap (all dates), and leaves (5and 10 DAE). Plants grown in 18 and 26 cm³ cells had higherroot ABA levels than those from 46 and 80 cm³ cells at 15 DAE.Plants grown in 18 cm³ cells had the highest xylem sap ABA levelat all dates, but ABA levels did not differ among plants grownin the other cell volumes. Plants grown in 18 cm³ cells at 5DAE and 18 and 26 cm³ cells at 10 DAE also had higher leaf ABAlevels than those from other rooting volumes. The results suggestthat ABA may act as a signal for reduced growth of plants underRVR conditions. Key words: Abscisic acid, ABA, root signals, root volume restriction, water relations