Translocation of Calcium in Relation to Tomato Fruit Growth

Regulation of the uptake and distribution of calcium in thetomato plant was investigated in plants grown in recirculatingnutrient solutions at electrical conductivities of 2,7,12 and17 millisiemens (mS). Despite an increased calcium content inthe nutrient solution at high conductivity (7–17 mS),the accumulation of calcium by fruit was progressively reducedby increasing salinity, particularly in the distal half. Theincidence of blossom-end rot in fruit (BER) also increased withsalinity. The uptake of water and ⁴⁵Ca by plants was substantially reducedin the high salinity treatment (17 mS) and, to a lesser extent,by high relative humidity (90 per cent r.h. at 20 °C). Further,the translocation of ⁴⁵Ca from roots to shoots was reduced byhigh salinity, while the percentage distribution of ⁴⁵Ca tothe apex was reduced by high humidity. Only approx. 2 per centof the ⁴⁵Ca taken up by a plant was imported by the truss. The uptake of ⁴⁵Ca and its distribution among pedicel, calyxand berry by detached fruit in 24 h showed that fruit from highsalinity plants had a reduced uptake and a lower accumulationof ⁴⁵ Ca in the berry than in the calyx. In addition, plants grown at high conductivity had a lower rateof xylem sap exudation from decapitated plants. The fruit ofthese plants had a smaller xylem cross-sectional area in thefruit pedicel and a smaller calyx than those of the low conductivitytreatment. Calcium, translocation, tomato, fruit, blossom-end rot     

Uptake and Transport of Calcium and the Possible Causes of Blossom-end Rot in Tomato

The possible causes of blossom-end rot (BER) in tomato fruitwere investigated by comparing the uptake of calcium by theroots, the distribution of ⁴⁵Ca within the fruit and the vascularbundle network in the fruit of susceptible cultivars (Calypsoand Spectra) with those of a less susceptible cultivar (Counter)grown in a range of salinities (electrical conductivity of 5,10 and 15 mS cm^–1). The daily calcium uptake rates at5 mS cm^–1 as estimated from the xylem exudation of thedecapitated stem stump in young plants of Calypso and old plantsof Spectra, were lower than that of Counter. The uptake of ⁴⁵Caby, and the transport to, the distal part of the detached fruitof susceptible cultivars, especially Calypso, were less thanin Counter at 10 mS cm^–1. The number of vascular bundlesin both proximal and distal fruit tissues was similar in allcultivars and was only slightly reduced by salinity. However,the number of bundles containing lignified xylem vessels, asdetected by safranin staining, was reduced substantially bysalinity, particularly in Calypso. The estimated area of thefruit tissue served by individual xylem bundles in the BER susceptiblefruit grown at high salinity was greater than in Counter. Theincidence of BER in all trusses was linearly related to theproduct of average daily irradiance and daily temperature throughoutthe year. Temperature appears to be the major environmentalfactor which induces BER, regardless of cultivars and salinitytreatment. The most likely causes of BER in susceptible cultivarsare the interactions of (a) light and temperature on fruit enlargement,(b) inadequate xylem tissue development in the fruit and (c)competition between leaves and fruit for the available Ca. Key words: Lycopersicon esculentum, calcium transport, susceptibility to blossom-end rot, root exudation, xylem     

Effects of environment on the uptake and distribution of calcium in tomato and on the incidence of blossom-end rot

Studies of Ca uptake and distribution in relation to environmental variables were used to relate Ca status of tomato fruit to blossom-end rot (BER) incidence. Ca uptake was highly correlated with solar radiation and root temperature. The rate of Ca uptake decreased linearly with increasing salinity. High humidity reduced Ca import by the leaves but increased that by the fruit. While total plant dry weight was reduced more than fruit dry weight by salinity, total Ca uptake and the Ca content of the fruit were decreased similarly. Thus, the concentration of calcium in the fruit was substantially reduced by salinity. The distal half of the fruit contained less Ca than the proximal half. The lowest % Ca was found in the distal placenta and locular tissues, where BER first develops. The incidence of BER was often stimulated more by high salinity achieved with the addition of major nutrients than with NaCl. The cause of BER is usually an interaction between the effects of irradiance and ambient temperature on fruit growth and the effects of environmental stress on calcium uptake and distribution within the whole plant.     

Effects of Diurnal Changes in the Salinity of the Nutrient Solution on the Accumulation of Calcium by Tomato Fruit

Tomato fruit grown in diurnally fluctuating salinities (8 mScm^–1 during the day and 3 mS cm^–1 at night; 8/3mS cm^–1), accumulated the same amount of dry matter andmagnesium (Mg) as those in constant 3 or 8 mS cm^–1, butan intermediate amount of calcium (Ca). Raising the salinityof the nutrient solution by enriching with macronutrients orby adding NaCl had similar effects. The uptake of ⁴⁵Ca by tomato plants during the day was greaterthan at night and was reduced by salinity in both periods. Whilethe uptake of ⁴⁵Ca by 8/3 mS plants at night was similar tothat of 3 mS plants, the daily uptake was less than that in3 and 5.5 mS plants. The Ca content of tomato fruit increased with truss number at3 and 5.5 mS cm^–1 but not at 8/3 and 8 mS cm^–1.Within the same truss, the distal fruit had a lower Ca contentbut higher Mg content than the proximal fruit. The reductionin Ca content of the distal fruit at 8/3 mS cm^–1 was similarto that at 5.5 mS cm^–1. The Ca content of the tissue atthe distal end of the 8/3 mS fruit was lower than that of the5.5 mS fruit. Similarly, the distribution of ⁴⁵Ca to the distalhalf of the detached 8/3 mS fruit was less than that of 5.5mS fruit. A reduced uptake and inadequate distribution of Cato the truss and to the distal end of the 8/3 mS fruit werethe main causes of these differences. Lycopersicon esculentum(Mill.), tomato, fruit, calcium, magnesium, diurnal salinity     

Parthenocarpic Fruit Growth Reduces Yield Fluctuation and Blossom-end Rot in Sweet Pepper

The aim of this work was to investigate whether parthenocarpicfruit growth could avoid flushing, i.e. an irregular yield pattern,in sweet pepper. Plants were grown in a greenhouse compartmentfrom April until August. Half of the plants were grown withouta fruit set treatment (control), whereas parthenocarpic fruitswere allowed to develop on the other plants by preventing self-pollinationand applying auxin to the stigma. For node positions 3 to 17,fruit set per node varied between 21 and 55% for control plants[coefficient of variation (CV) = 11%], whereas auxin-treatedplants showed much less variation in fruit set (41–57%;CV = 5%) and average fruit set was higher. In agreement withfruit set, fruit yield was also much more regular in the auxin-treatedplants. Fruit fresh yield varied between 0.2 and 1.0 kg m^-2forcontrol plants (CV = 20%), and between 0.4 and 0.8 kg m^-2forauxin-treated plants (CV = 9%). Results showed that developingseeds in sweet pepper fruits are the main cause of the abortionof new flowers, and irregular fruit set and yield. Parthenocarpicfruit growth resulted in flatter, 30% smaller fruits, becauseof a reduction in fruit growth rate; the duration of fruit growthwas 1 week longer than for fruits from control plants. Parthenocarpicfruits were hardly affected by blossom-end rot (BER) with only1% of fruits being affected compared to 31% in the control.Total dry mass production was the same for treated and controlplants; however, in auxin-treated plants, 50% of the total drymass was allocated to the fruits, compared to 58% in controlplants. Copyright 2001 Annals of Botany Company Abortion, auxin, BER, blossom-end rot, Capsicum annuum L., flushing, fruit set, irregular yield pattern, parthenocarpy, sweet pepper     

Mechanisms Involved in Calcium Deficiency Development in Tomato Fruit in Response to Gibberellins

Although gibberellins (GAs) have been shown to induce development of the physiological disorder blossom-end rot (BER) in tomato fruit (Solanum lycopersicum), the mechanisms involved remain largely unexplored. BER is believed to result from calcium (Ca) deficiency, but the relationship between Ca content and BER incidence is not strong. Our objectives were to better understand how GAs and a GA biosynthesis inhibitor affect BER development in tomato fruit. Tomato plants of two BER-susceptible cultivars, ‘Ace 55 (Vf)’ and ‘AB2,’ were grown in a greenhouse environment and subjected to Ca-deficiency conditions. Plants were treated weekly during fruit growth and development with 300 mg L^?1 GA₄₊₇, 300 mg L^?1 prohexadione-calcium (Apogee^®, a GA biosynthesis inhibitor), or water beginning 1 day after flower pollination. GA₄₊₇ treatment induced an increase in BER incidence in both cultivars up to 100%, whereas ‘Ace 55 (Vf)’ and ‘AB2’ plants treated with Apogee did not show BER incidence. The number of functional xylem vessels was higher in the placental and pericarp tissue of tomato fruit treated with Apogee at the early stages of fruit growth. Treatment with Apogee also increased fruit pericarp Ca concentration. GA₄₊₇ treatment enhanced the expression of the putative CAX and Ca-ATPase genes, that code for proteins involved in Ca movement into storage organelles. The lowest water-soluble apoplastic Ca concentration and the highest membrane leakage values were observed in the pericarp of GA₄₊₇-treated fruit. These results suggest that GAs consistently reduced fruit Ca uptake and water-soluble apoplastic Ca concentration, leading to leakier plasma membranes and an increase in BER development in fruit tissue of both tomato cultivars.     

Blossom-end rot in relation to growth rate and calcium content in fruits of sweet pepper (Capsicum annuum L.)

The relative importance of growth rate and calcium concentration in sweet pepper fruits (Capsicum annuum L.) for the induction of blossom-end rot (BER) was investigated in (1) four pollination treatments in one cultivar, (2) four cultivars with the same fruit load and (3) three fruit load treatments in four cultivars. For fruits with the same pollination treatment those eventually developing BER had a higher initial fruit growth rate than those not developing BER. Within the same experiment both the growth rate of the young fruit and BER increased with the number of seeds. The Ca concentration of the pericarp in mature fruits was negatively related to both fruit size and BER incidence. Differences in levels of BER between different pollination experiments could not be explained solely by differences in growth rate of the young fruit, but related to different Ca concentrations in the mature fruits. In the spring, but not in the summer, cultivars more susceptible to BER had a larger final size but lower Ca concentration in the young fruit than the resistant ones. By lowering the fruit load in the summer both the final fruit size and the BER incidence increased, but the Ca concentrations of both proximal and distal pericarp in the young fruit of all cultivars were not consistently affected. Despite a correlation between growth rate and low Ca concentration in the fruit, the incidence of BER may only be predicted from separate effects of fruit growth and of Ca concentration of fruit. The data indicated that at a higher growth rate a higher Ca concentration is required to prevent the induction of BER. The usefulness of the total Ca concentration of the fruit for determining the critical Ca concentration in the induction of BER is discussed.Key words: Capiscum annuum L., sweet pepper, blossom-end rot, calcium, growth rate, pollination, fruit load.      

Regulation of the Partitioning of Dry Matter and Calcium in Cucumber in Relation to Fruit Growth and Salinity

The regulation of the partitioning of dry matter and calciumin relation to fruit growth was investigated in cucumber plantsgrown in the salinity range of 3-8 mS cm^-1 in NFT (NutrientFilm Culture), with or without a fruit pruning treatment. Thedry weight gain of the plants was proportional to the outdoorintegral irradiance, with a common daily rate of 1 g MJ^-1 m^-2in two crops grown under summer (18 MJ m^-2 d^-1) and autumn (7MJ m^-2 d^-1) conditions. Within the salinity range studied, thereduction of plant dry weight was 9% mS^-1 cm^-1. However, fruitdry weight was only reduced at salinities above 5·5 mScm^-1, although the daily dry matter accumulation by fruit, asa percentage of total dry matter accumulation, was increased.Salinity reduced the dry matter accumulation in the young shootproportionally more than in the fruit. Although the total plantCa content was reduced by 13% mS^-1 cm^-1, the Ca content of theyoung shoot was reduced by 16·6%, compared to 11% inthe fruit. Pruning fruit reduced neither plant dry weight norCa uptake. The growth of the remaining fruit, and to a lesserdegree of the young shoot, accounted for all surplus assimilates.Thus, fruit were the dominant sinks for assimilates whilst themature leaves were the strongest sinks for Ca. Nevertheless,the fruit sustained the capacity to import Ca better than theyoung shoot, when supplies of both assimilates and Ca were reducedby high salinity.Copyright 1994, 1999 Academic Press Cucumber, Cucumis sativus L., salinity, fruit pruning, dry matter and calcium     

Water deficit, root demography, and the causes of internal blackening in field-grown tomatoes:(Lycopersicon esculentum Mill.)

An internal blackening disorder may cause substantial losses in the value of tomatoes grown for processing. The disorder resembles an internal form of blossom-end rot and appears to be more common in dry seasons. In an experiment to test whether the internal blackening is caused by water deficit and whether it is indeed blossom-end rot, plots of cv. Cannery Row were irrigated to keep the soil moisture deficit <50 mm and others were sheltered from rain and not irrigated from early flowering onwards. Shoot growth (total and fruit dry mass) was measured destructively at intervals, and root growth and death nondestructively using minirhizotrons. There was a greater incidence of internal blackening and blossom-end rot, and lower Ca concentrations, in the fruit of non-irrigated plants. Root growth and root death were accelerated in these plants around the time that internally-blackened fruit were set. Although the internal blackening syndrome shared some features with blossom-end rot some differences were apparent in this experiment. It is suggested that internal blackening could have resulted from increased root competition for photosynthate, leading to abnormal seed development. Root turnover was appreciable (30–40% of the roots survived 28 days). This suggests there may be substantial errors in contemporary models of dry matter partitioning in tomato crops.     

Environmental Effects on the Diurnal Accumulation of 45Ca by Young Fruit and Leaves of Tomato Plants

Diurnal uptake and distribution of ⁴⁵Ca in young fruiting tomatoplants were assessed 12 or 24 h after ⁴⁵Ca was applied to thenutrient solution at the beginning of either the light (12 h)or the dark (12 h) period. During the experiment, the salinityof the nutrient solution (measured as electrical conductivity,EC) was either 2·5 or 17 mS cm^–1 and the relativehumidity (measured as vapour pressure deficit, VPD) was either0·2 or 0·6 kPa The uptake of ⁴⁵Ca by a tomato plant over 12 h was higher inthe light than in the dark but the difference was less at lowhumidity. More ⁴⁵Ca was transported from the roots to the shootin the light than in the dark. More than half of the ⁴⁵Ca inthe shoot was accumulated by the stem; the proportion of ⁴⁵Cain the stem was greater in the dark and was further enhancedby high humidity to more than 80% of the ⁴⁵Ca in the shoot.The accumulation of ⁴⁵Ca by the fruit truss in the dark wasgreater than in the light in all experimental conditions. Underlow humidity the accumulation of ⁴⁵Ca by young leaves was similarin both light and dark. In high humidity there was considerablyless accumulation of ⁴⁵Ca by the young leaves in the dark The uptake of ⁴⁵Ca continued over 24 h but the transport of⁴⁵Ca to individual organs in the second 12 h period was affectedby both light and humidity. Some of the ⁴⁵Ca accumulated byyoung leaves and fruit in the second period appears to havebeen derived from ⁴⁵Ca released from the xylem wall along thetransport pathway in the stem The roles of root pressure and transpiration in the diurnalaccumulation of calcium in young fruit and leaves are discussed Calcium, diurnal translocation, tomato, young fruit and leaves     

The suppression of salinity-associated oxygen radicals production, in pepper (Capsicum annuum) fruit, by manganese, zinc and calcium in relation to its sensitivity to blossom-end rot

We investigated the possibility that oxidative stress contributes to blossom-end rot (BER) initiation in bell pepper ( Capsicum annuum L.) grown under high salinity. Pepper plants (cv. Mazurka, Rijk Zwaan, the Netherlands) were grown in a greenhouse and irrigated with nutrient solution made up with either desalinated water (control — rising from E.C. 1.9 to 2.4 dS m⁻¹) or saline water (salinity – rising from E.C. 3.2 to 7.0 dS m⁻¹). Irrigation was by a circulation system. BER symptoms were observed throughout the experiment but were highly enhanced in the salinity–grown plants during the spring and summer. The fruit calcium concentration was not affected by salinity, but manganese concentrations in both leaves and fruits were significantly reduced under these conditions. Under salinity there was an enhancement of apoplast reactive oxygen species (ROS) production, which was partly a result of increase in NAD(P)H oxidase activity in the pericarp of pepper fruit at the stage that it was most sensitive to BER. Apoplast ROS production and extracted NAD(P)H oxidase activity were inhibited by manganese, zinc and to a lesser extent by calcium. These cations also negated the enhancement of ROS production caused by incubation of fruit pericarp discs in NaCl solutions. Manganese, zinc and calcium also inhibited NAD(P)H oxidase activity, extracted following their infiltration into fruit pericarp discs. The results suggest that generation and scavenging of oxygen free radicals in the apoplast may contribute to the appearance of BER symptoms in pepper fruits under saline conditions. It is suggested that manganese may serve as antioxidant in pepper fruit and that manganese addition to peppers grown under salinity may alleviate BER symptoms in the fruits.     

Effect of potassium nutrition of three tomato varieties on incidence of blossom-end rot

Summary Three tomato varieties, Amberley Cross, VF-145 and VF-13L (the last two reported to show K-deficiency symptoms independent of the amount of K applied) were grown in sand with three concentrations of applied nutrient K at a constant high level of Ca. There was a varietal difference in the K concentration and total K uptake into the plants. The Mg concentrations in the fruit were unaffected by K nutrition but the concentrations fell in the leaves of all varieties when the nutrient K was increased above 0.28 meq/l.While the concentration of Ca in the leaves of Amberley Cross was not significantly reduced by raising the concentration of K in the nutrient feed, there was a reduction in the varieties VF-145 and VF-13L. Maximum concentrations of Ca were present in fruit of all varieties receiving 2 meq K⁺/l, and that present in fruit of Amberley Cross was significantly higher than in fruit from either VF-145 or VF-13L. Comparing the varieties in the 10.2 meq K⁺/l treatment, fruit of VF-13L contained the lowest concentration and total amount of Ca, and had the highest incidence of blossom-end rot. VF-13L was the most susceptible to blossom-end rot, particularly in the highest K treatment, while Amberley Cross was the only variety free of symptoms in all three nutrient treatments.     

Abscisic acid triggers whole-plant and fruit-specific mechanisms to increase fruit calcium uptake and prevent blossom end rot development in tomato fruit

Calcium (Ca) uptake into fruit and leaves is dependent on xylemic water movement, and hence presumably driven by transpiration and growth. High leaf transpiration is thought to restrict Ca movement to low-transpiring tomato fruit, which may increase fruit susceptibility to the Ca-deficiency disorder, blossom end rot (BER). The objective of this study was to analyse the effect of reduced leaf transpiration in abscisic acid (ABA)-treated plants on fruit and leaf Ca uptake and BER development. Tomato cultivars Ace 55 (Vf) and AB2 were grown in a greenhouse environment under Ca-deficit conditions and plants were treated weekly after pollination with water (control) or 500 mg l(-1) ABA. BER incidence was completely prevented in the ABA-treated plants and reached values of 30-45% in the water-treated controls. ABA-treated plants had higher stem water potential, lower leaf stomatal conductance, and lower whole-plant water loss than water-treated plants. ABA treatment increased total tissue and apoplastic water-soluble Ca concentrations in the fruit, and decreased Ca concentrations in leaves. In ABA-treated plants, fruit had a higher number of Safranin-O-stained xylem vessels at early stages of growth and development. ABA treatment reduced the phloem/xylem ratio of fruit sap uptake. The results indicate that ABA prevents BER development by increasing fruit Ca uptake, possibly by a combination of whole-plant and fruit-specific mechanisms.     

Role of pectin methylesterases in cellular calcium distribution and blossom-end rot development in tomato fruit

Blossom-end rot (BER) in tomato fruit (Solanum lycopersicum) is believed to be a calcium (Ca(2+) ) deficiency disorder, but the mechanisms involved in its development are poorly understood. Our hypothesis is that high expression of pectin methylesterases (PMEs) increases Ca(2+) bound to the cell wall, subsequently decreasing Ca(2+) available for other cellular functions and thereby increasing fruit susceptibility to BER. The objectives of this study were to evaluate the effect of PME expression, and amount of esterified pectins and Ca(2+) bound to the cell wall on BER development in tomato fruit. Wild-type and PME-silenced tomato plants were grown in a greenhouse. At full bloom, flowers were pollinated and Ca(2+) was no longer provided to the plants to induce BER. Our results show that suppressing expression of PMEs in tomato fruit reduced the amount of Ca(2+) bound to the cell wall, and also reduced fruit susceptibility to BER. Both the wild-type and PME-silenced fruit had similar total tissue, cytosolic and vacuolar Ca(2+) concentrations, but wild-type fruit had lower water-soluble apoplastic Ca(2+) content and higher membrane leakage, one of the first symptoms of BER. Our results suggest that apoplastic water-soluble Ca(2+) concentration influences fruit susceptibility to Ca(2+) deficiency disorders.     

The relationship between tomato fruit growth,incidence of blossom-end rot and phytohormone content as affected by sink/source ratio

Blossom-end rot is generally considered a calcium-related physiological disorder. The results of the previous studies show that several factors such as plant conditions can be effective on the blossom-end rot incidence. Therefore, the present study was undertaken to investigate the effect of the sink/source ratio on the incidence of the blossom-end rot of two greenhouse tomato (Solanum lycopersicum L.) cultivars: ‘Grandella’ and ‘Isabella’. To this end, four treatments were applied: saving one fruit per truss (1F), two fruits per truss (2F), three fruits per truss (3F), and no fruit pruning (control). The results showed that the tomato cultivar ‘Isabella’ was more susceptible to the blossom-end rot than ‘Grandella’. Decreasing the sink/source ratio increased the incidence of the blossom-end rot and the relative fruit growth rate. The correlation between the blossom-end rot incidence and the relative fruit growth rate showed that the fruit growth rate could be regarded as an important factor in the incidence of this disorder. Endogenous auxin and cytokinin concentrations acted as the regulators of the fruit growth rate and influenced it. Slowing down the relative growth rate by keeping proper sink/source ratio based on tomato cultivar is, therefore, an effective, cheap and healthy way to control the incidence of the blossom-end rot, especially in organic farming.     

A cellular hypothesis for the induction of blossom-end rot in tomato fruit

BACKGROUND: The incidence of blossom-end rot (BER) is generally associated with a calcium (Ca) deficiency in the distal portion of tomato fruits. The visible symptom is a necrotic lesion, which is presumed to be a consequence of cell death and the subsequent leakage of solutes into the extracellular space. Environmental factors that affect either fruit cell expansion or Ca delivery to the distal portion of the fruit influence the occurrence of BER. However, since no absolute, critical fruit Ca concentration for the occurrence of BER has been identified, it is now important to define the role of Ca in fruit cell physiology and to seek the cause of BER at the cellular level. HYPOTHESIS: Here, it is suggested that BER is initiated by a cellular dysfunction in the distal portion of a young fruit during rapid cell expansion. It is proposed that insufficient Ca(2+) is available for critical apoplastic and cytoplasmic functions when the cellular Ca demand imposed by vacuolation exceeds the Ca delivery to an expanding cell. A local Ca deficiency, therefore, may result in aberrant intracellular Ca(2+) signals, a weakening of cell walls and a loss of cellular integrity. Ultimately it may lead to cell death and the visible symptoms of BER. Several experimental strategies are suggested to confirm the occurrence of aberrant Ca(2+) concentrations in cells contributing to BER. PERSPECTIVE: Many genetic and genomic resources are becoming available for tomato. Ultimately, these will allow genes affecting the occurrence of BER to be identified. Such knowledge will inform breeding strategies to eliminate BER. In the meanwhile, increasing the apoplastic Ca concentration in susceptible fruit tissue should provide a simple and reliable, practical solution for the prevention of BER in tomatoes. It is suggested that current horticultural practices, such as the manipulation of the mineral composition of the feed or the growth environment, are not completely effective in reducing BER because they affect apoplastic Ca concentration in fruit tissue indirectly. Therefore, spraying Ca directly onto young fruits is recommended for the prevention of BER.     

Effects of Inhibitors of Protein Synthesis on Transmembrane Auxin Transport in Cucurbita pepo L. Hypocotyl Segments

Pretreatment of 2?0 mm segments of etiolated zucchini (Cucurbitapepo L.) hypocotyl with cycloheximide (CH) or 2-(4-methyl-2,6-dinitroanilino)-N-methylpropionamide(MDMP) eliminated the stimulation by N-1-naphthylphthalamicacid (NPA) of net uptake of [1-¹⁴C]indol-3yl-acetic acid ([1-¹⁴C]IAA),but had relatively little effect on the net uptake of IAA inthe absence of NPA. The efflux of [1-¹⁴C]IAA from preloadedsegments was not substantially affected by inhibitor pretreatmentin the absence of NPA, but CH pretreatment significantly inhibitedthe reduction of efflux caused by NPA. Pretreatment with CHor MDMP did not affect net uptake by segments of the pH probe[2-¹⁴C]5,5-dimethyl-oxazolidine-2,4-dione ([2-¹⁴C]DMO), or thenet uptake of [¹⁴C]-labelled 3-O-methylglucose ([¹⁴C]3-0-MeGlu),suggesting that neither inhibitor affected intracellular pHor the general function of proton symporters in the plasma membrane.Both compounds reduced the incorporation of label from [³⁵S]methionineinto trichloroacetic acid (TCA)-insoluble fractions of zucchinitissue, confirming their inhibitory effect on protein synthesis. The steady-state association of [³H]IAA with microsomal vesiclesprepared from zucchini hypocotyl tissue was enhanced by theinclusion of NPA in the uptake medium. The stimulation by NPAof [³H]IAA association with microsomes was substantially reducedwhen the tissue was pretreated with CH. However, CH pretreatmentdid not affect the level of high affinity NPA binding to themembranes indicating that treatments did not result in lossof NPA receptors. It is suggested that the auxin transport site on the effluxcarrier system and the receptor site for NPA may reside on separateproteins linked by a third, rapidly turned-over, transducingprotein. Key words: Auxin carriers, auxin efflux, Cucurbita pepo, phytotropin receptors     

Changes of Cell Wall Composition and Polymer Size in Primary Roots of Cotton Seedlings Under High Salinity

The aim of this study was to investigate changes in cell wallchemical composition and polymer size in the root tip of intactcotton seedlings (Gossypium hirsutum L. cv. Acala SJ-2) grownin saline environments, in order to relate the interaction betweenhigh salinity and root growth to possible changes in cell wallmetabolism. Cotton seedlings were grown in modified Hoagland nutrient solutionwith various combinations of NaCl and CaCl₂. Cell walls werefractionated into four fractions (pectin, hemicellulose 1 and2, cellulose), and analysed for their total sugar content, neutralsugar composition and size of polysaccharides. At 1 mol m^–3Ca, 150 mol m^–3 NaCl resulted in a significant increasein the cell wall uronic acid content, but a reduction in cellulosecontent on a per unit dry weight basis. Supplemental Ca overcamethe inhibitory effect of high Na on cellulose content. The neutralsugar composition of the cell wall fractions showed no majorchanges caused by varied Na/Ca ratios. Determinations of polysaccharidepolymer size showed that high Na at 1 mol m^–3 Ca led toan increase in the amount of polysaccharides of intermediatemolecular size and a decrease in that of small size in the hemicellulose1 fraction, indicating a possible inhibition of polysaccharidedegradation by high Na. This change was not observed in the10 mol m^–3 Ca treatments. The results reveal a relationshipbetween the effects of high salinity on root growth and cellwall metabolism, particularly in regard to cellulose biosynthesis Key words: Gossypium hirsutum, salinity, root, cell wall     

The regulation of K+ influx into roots of rye (Secale cereale L.) seedlings by negative feedback via the K+ flux from shoot to root in the phloem

The uptake of K⁺ by plant roots is matched to the demand forK⁺ for growth. The growing shoot must communicate its K⁺ requirementto the root. It has been suggested that this might be effectedby varying the amount of K⁺ retranslocated from the shoot tothe root via the phloem. It is predicted that less K⁺ is returnedto the roots in K⁺-deficient plants and that this promotes compensatoryK⁺ uptake from the external medium. These experiments addressthis hypothesis. Rye (Secale cereale L.) was grown hydroponically in completenutrient solutions containing either 100 aM or 400 µMK⁺. Plant development, shoot fresh weight (FW) and plant drymatter accumulation did not differ between seedlings grown atthese K⁺ concentrations. However, root FW was lower in seedlingsgrown in solutions containing 100 µM K⁺, which resultedin a greater shoot/root FW ratio. Seedlings from both treatmentshad similar shoot K⁺ concentrations, but the root K⁺ concentrationof seedlings grown In solutions containing 100 µM K⁺ wasless than their counterparts grown at 400 µM K⁺. When assayed at the same K⁺ concentration, unidirectional K⁺(⁸⁶Rb⁺) influx into 14-d-old seedlings grown with 100 µMK⁺ in the nutrient solution was greater than that into seedlingsgrown with 400 µM K⁺ in the nutrient solution, indicatingan increased K⁺ influx capacity in the former. Furthermore,K⁺ (⁸⁶Rb⁺) influx into seedlings grown and assayed at 100 µMK⁺ was greater than that into seedlings grown and assayed at400 µM K⁺. Since net K⁺ uptake was lower in the seedlingsgrown at 100 µM K⁺, this indicates a greater unidirectionalK⁺ efflux from roots in solutions containing 100 µM K⁺. An empirical model, based on the immobility of calcium in thephloem, was used to describe quantitatively K⁺ fluxes in seedlings14 d after sowing. As primary data, the composition of xylemsap and the accumulation of elements in root and shoot tissueswere determined. Xylem sap was collected either as root-pressureexudate or from excised roots immersed in nutrient solutionand subjected to a pneumatic pressure of 0.4 MPa. The K:Ca ratioin these saps differed, and led to contrasting conclusions concerningthe effect of K⁺ nutrition on the recirculation of K⁺. Basedon the K:Ca ratio in the sap obtained following the applicationof pneumatic pressure, which is thought to resemble that ofintact transpiring plants, it was calculated that the K⁺ fluxfrom the shoot to the root was higher in seedlings maintainedin solutions containing higher K⁺ concentrations. This suggeststhat a negative feedback mechanism based on K⁺ recirculationfrom the shoot to the root via the phloem could be a primarysignal decreasing K⁺ influx. Key words: K⁺ influx, K⁺ recirculation, regulation, root, rye, Secale cereale L