The Relationship between the Cell Electrical Potential Difference and Salt Uptake in the Roots of Helianthus annuus

The effect of ringing the stem on the electrical potential difference(PD) in the root cortical cells of H. annuus was studied. PDand salt transport were followed simultaneously. By ringingit was possible to separate the PD from K⁺, , and Cl^– uptake and H⁺ efflux. The uptake of phosphatehowever was found to be closely connected with a component ofthe PD. It was concluded that there is an electrogenic pumpfor phosphate in these roots which generates 60–80 mV.     

Relationship between Water Potential and Leaf and Inflorescence Initiation in Helianthus annuus

The total number of leaves produced by the primary stem of Helianthus annuus L. cv. Pole Star and cv. Peredovik, was reduced from 29 leaves to 21–22 leaves when water stress ranging from ?2000 to ?3000 kPa was imposed over the period from day 10 to day 20 after sowing, by withholding water from the soil. Extending the stress period by 10 days did not cause any further significant reduction in leaf number. Examination of the shoot apex showed that the leaf initiation rate was depressed by low water potential, the greatest effect occurring in the range 0 to ?1000 kPa. It was established that inflorescence initiation commenced about 18 days after planting in control plants and this was delayed by only about 4 days by low water potentials in the range ?1000 to ?3000 kPa.     

The Effect of D-Glucose and Other Sugars on the Trans-root Potential of Zea Mays

The addition of D-glucose and certain other sugars to the bathingsolutions of young excised maize roots (Zea mays L., var. Pioneer)gives rise to rapid changes in the electrical potential differencebetween the bathing solution and the xylem fluid. The resultssuggest the presence of a fairly non-specific sugar transportsystem in the plasma membranes of the root epidermal cells inwhich the hydroxyl group in the I-carbon position of n-glucoseis cntically involved. D-Mannose, 2-deoxy-D-glucose, and 2-deoxy-D-galactose have adelayed but more profound effect on this potential, reducingit to a much smaller value. The subsequent addition of adequateamounts of D-glucose restores this potential to about its formervalue, suggesting that these three compounds interfere withthe supply of endogenous glucose or glucose-derived productswithin the root.     

Polyamines in Helianthus annuus L. during Germination under Salt Stress

The level of the three main polyamines putrescine, spermidine, and spermine and the biosynthetic enzyme arginine decarboxylase (ADC) decreased in Helianthus annuus L. seedlings subjected to increasing (50, 100, and 150 mm) NaCl concentrations. The pattern of polyamines in control plants increased during the initial 72 h and then reached a plateau. The putrescine level showed an increase of 370% after 72 h of development. The lower salt treatment slightly diminished the overall polyamine content. The highest NaCl concentration (150 mm) induced a strong putrescine diminution (from 381 to 78.9 nmol g⁻¹ FW) at 72 h whereas a small decrease in ADC activity was detected. ODC was detected in neither control nor treated plantlets during the experimental period. The level of spermidine also decreased, but the magnitude of the decay was less pronounced than putrescine. The fact that ODC was not detected and ADC activity followed a pattern similar to that of putrescine led us to suppose that the variation in putrescine content could be attributed entirely to the decrease in ADC activity. α-Difluoromethylarginine and α-difluoromethylornithine (ADC and ODC inhibitor, respectively) did not inhibit but delayed the onset of germination of sunflower seeds, and α-difluoromethylornithine increased the content of spermidine and spermine. The present data suggest that polyamines could be involved in the germination process of H. annuus seeds and in response to salt stress. Received April 14, 1997; accepted July 10, 1997     

The Effects of Root Pruning on the Water Relations of Helianthus annuus L.

Four-week-old Helianthus annuus plants, grown in both soil andliquid culture, were root pruned at the point of root attachmentto the stem. Transpiration, leaf water potential and leaf conductivitywere monitored for several days after pruning. Pruning loweredtranspiration and leaf conductivity in amounts proportionalto the amount of pruning. In some experiments pruning causeda slight lowering of leaf water potential, while in others nopruning effect could be found. The effects of pruning varieddepending upon culture conditions, with greater effects beingfound in soil and unaerated liquid culture than in aerated liquidculture. Soil water potential did not appear to have a stronginfluence on the magnitude of the pruning effect. The effectsof root pruning are less than would be predicted by an Ohm'sLaw analysis of flow; possible reasons for this are discussed.     

Water Stress in Sunflower (Helianthus annuus L.): I. Effect on Plant Development

Development of sunflower plants grown in pots under controlledconditions was monitored at frequent intervals. Two separatewater stresses, which reached a minimum of –1.7 MPa, wereimposed at specified developmental stages on two separate batchesof plants. Leaf initiation and leaf unfolding were significantlyreduced by water stress. Inflorescence primordium initiation,involucre bract formation, disc and disc floret developmentwere not affected. The mitotic index was reduced by water stressin unfolding leaves from 5 to 0.5 per cent. In other leaves,the mitotic index was reduced to a lesser extent. This reducedmitotic index helps explain reduction in leaf development. Thetime between seed germination and inflorescence primordium initiationremained relatively constant despite the reduced leaf numberassociated with water stress. The discussion evaluates the complicationsthat this may cause in the comparison of leaves in the upperhalf of the leaf profile. Helianthus annuus L., sunflower, water stress, mitotic index, leaf development     

The Effect of Gravity Compensation on Growth and Cell Wall-loosening Enzymes in Helianthus annuus Hypocotyls

The effect of gravity compensation by the clinostat on the elongation, weight, and activity of two cell wall-loosening enzymes (cellulase and β-1,3-glucanase) in Helianthus annuus hypocotyls was examined. Gravity compensation increases elongation (28.1 %) and weight (18.3 %). The activity of cellulase extracted from the apical sections is raised, but there is no significant effect on β-1,3-glucanase. The relationship between gravity compensation, changes in auxin level, and function of these two enzymes in respect to elongation is discussed.     

Photophosphorylation in Attached Leaves of Helianthus annuus at Low Water Potentials

The in situ response of photophosphorylation and coupling factor activity to low leaf water potential (ψ_L) was investigated using kinetic spectroscopy to measure the flash-induced electrochromic absorption change in attached sunflower (Helianthus annuus L. cv IS894) leaves. The electrochromic change is caused by the formation of an electric potential across the thylakoid membrane associated with proton uptake. Since depolarization of the thylakoid membrane following flash excitation is normally dominated by proton efflux through the coupling factor during ATP formation, this measurement can provide direct information about the catalytic activity of the coupling factor. Under low ψ_L conditions in which a clear nonstomatal limitation of net photosynthesis could be demonstrated, we found a strong inhibition of coupling factor activity in dark-adapted leaves which was probably caused by an increase in the energetic threshold for the activation of the enzyme at low ψ_L. While this result supported earlier in vitro findings, we further discovered that the light-dependent reduction of coupling factor reversed any observable effect of low ψ_L on the energetics of activation or on photophosphorylation competence. Furthermore, coupling factor was reduced, even in severely droughted sunflower, almost immediately upon illumination. Based on these measurements, we conclude that the nonstomatal limitation of photosynthesis observed by us and others in droughted plants cannot be explained by impaired coupling factor activity.     

The Effect of Phloem Ringing on Root Growth and Potassium Uptake by Helianthus annuus

The effect of phloem ringing on the uptake and transport ofpotassium by the roots of 4 week old sunflower plants has beeninvestigated. Ringing caused a rapid decline in both K⁺ uptakeand its transport (⁸⁶Rb tracer) to the shoot. The rate of rootelongation and the levels of sucrose in the root showed paralleldecreases after ringing. Measurement on isolated roots indicatedthat the effect of ringing the stem on K⁺ uptake by the rootswas confined to the apical 10 mm that included the extensionzone. It is postulated that the decline in potassium uptakeand transport, brought about by ringing, is due to the severanceof the sucrose supply which stops root growth. Key words: Roots, Growth, Salt uptake     

Diffusible gibberellins and phototropism in Helianthus annuus

Summary Endogenous gibberellins were obtained in agar from the lower cut surface of upright sunflower shoot-tips. Exposure to unilateral light of the tips standing on agar, with the lower cut ends bisected by a vertical glass barrier at right angles to incident light, resulted in approximately 8 times the quantity of gibberellins moving into the agar below the shaded side than into the agar below the illuminated side. These results are similar to those reported earlier for gibberellins and geotropism in sunflower shoots, and suggest than the development of both light-and gravity-induced growth curvatures involve an asymmetry in gibberellin distribution across elongating internodes.     

Effect of the ferredoxin electron donor on sunflower (Helianthus annuus) desaturases

Ferredoxins are proteins that participate in photosynthesis and in other processes that require reducing equivalents, such as the reduction of nitrogen or fatty acid desaturation. Two classes of ferredoxins have been described in plants: light-regulated photosynthetic ferredoxins and heterotrophic ferredoxins whose activity is not influenced by light. Genes encoding the two forms of ferredoxin have been cloned and characterized in developing sunflower cotyledons. Here, these genes were overexpressed in Escherichia coli and they were purified by ion exchange and size exclusion chromatography to study their capacity to supply electrons to two different sunflower desaturases: soluble stearoyl-ACP desaturase from sunflower cotyledons, and membrane bound desaturase FAD7 expressed in yeast. In both cases photosynthetic ferredoxin was the form that promoted the strongest desaturase activity.     

A Relationship between Phosphate Uptake and Membrane Electrical Potential in Excised Roots of Helianthus annuus

Phosphate uptake by excised roots of sunflower (Helianthus annuus)was determined by the disappearance of phosphate from the externalsolution and by the accumulation of phosphate labelled with³²P. Over a 24 h period it was observed that net phosphate uptakedeclined to zero whilst uptake of ³²P continued unabated. Theelectrical PD of the cortical cell membranes declined in parallelwith net phosphate uptake and it was found that both could berestored by creating a pH gradient across the plasmalemma. Itwas concluded that net phosphate uptake was responsible fora component of the membrane PD of the root cortical cells. Key words: Roots, Phosphate, Membranes     

Characteristics of action potentials in Helianthus annuus

The action potentials induced by nondamaging electrical stimuli in 16- to 22-day-old plants of Helianthus annuus were examined. Typical recordings are presented. Mean values of their amplitudes and conduction velocities in the stem, the strength-duration relation, the 'all-or-none' law and the refractory periods have been determined. The amplitude and velocity of propagation were essentially identical in the upward and downward direction, but greater in the upper than in the lower half. In 'electrically active' plants, the rheobase value is 2 V, the minimum period for stimulation is 1.8 s. and the chronaxie 2.3 s. It is noted that the excitability level between similar plants on the same day and in the same plant on different days is highly variable and undergoes periodic changes.     

A Theory for Circumnutations in Helianthus annuus

A theory is given for circumnutations in plants, especially hypocotyls of Helianthus annuus, which were used as experimental material The theory is based on the lateral auxin transport, which arises when a gravitational force component acts on the plant. With a suitable time delay between stimulus and response, oscillations or circumnutations should arise. It is possible to describe these oscillation phenomena by the solutions of a differential equation, derived in this paper. The time delay has a central role in this equation. The time delay is assumed to be identical with the geotropic reaction time for the hypocotyls. The ratio between the periodic time for the circumnutations and the reaction time for geotropic curvatures was found to be approximately constant in the temperature region investigated (namely 15–40°C), which supports the theory. Different methods of recording the circumnutations were used, 8 mm film camera technique being the most frequently employed. The introduction of a weighting function for describing the plants “memory” of the stimulation makes it possible to relate the periodic time of the circumnutations to the reaction time for geotropic curvatures. The necessity of this weighting function as well as of the time delay in the equations is emphasized. An explanation of the “Fünfphasen-bewegungen”, reported in the literature, is presented.     

Water Relations of Sunflower (Helianthus annuus) Shoots during Exposure of the Root

The influence of root anoxia on the water balance of hydroponicallygrown sunflower plants was investigated in a controlled environmentroom. Leaf water potentials declined within 1 h of the onsetof anaerobic treatment of the roots. The severity of the waterstress during the initial 8 h of treatment was dependent onthe ambient humidity of the aerial environment. Recovery ofshoot water balance occurred by the beginning of the secondfull day of treatment, without stomatal closure. After 3 d of continuous anaerobic treatment, plant responsein the same experiment followed one of two patterns. In thefirst, there were declines in leaf water content, leaf waterpotentials, lamina expansion, transpiration rates and stomatalconductance. Osmotic adjustment occurred but it was not sufficientto prevent loss of turgor and the plants wilted at day 4 oftreatment. Complete rehydration took place after 6 d of treatmentif the stem was severed (under water) just below the cotyledons,indicating that shoot dehydration was a result of xylem occlusionat the root/shoot interface. Sections of the stem in this regionshowed xylem vessels to be discoloured and plate-like structuresobstructing the lumen. In the other pattern of response, plants did not develop a morenegative leaf water potential (except for an initial periodimmediately following treatment initiation) throughout the 6-danaerobic treatment period. There were no signs of water deficitalthough other symptoms such as precocious leaf senescence andepinasty occurred concomitantly with those in the first groupof plants that developed long-term water stress. Changes in shoot water relations are discussed in terms of ourearlier observations on anoxia-induced changes in the hydraulicconductivity and selectivity of the roots. Possible explanationsare offered for inter-plant variations in response to root anoxia. Key words: Anaerobiosis, water relations, xylem blockage     

Effect of Mechanical Stress on Sunflower (Helianthus annuus L.) Hypocotyl Growth

Etiolated seedlings of Sunflower (Helianthus annuus L.) weresubjected to mechanical stress by longitudinally compressingthe hypocotyl with approx. 0·05 N force (equivalent to0·025 MPa for a 1·6 mm diameter hypocotyl). Thisrelatively low compressive stress effected an increase in relativegrowth rates (RGR) of the hypocotyl for a period of 1-2 h, followingwhich RGR returned to the pretreatment rate. RGR was also increasedby an equivalent compressive stress treatment (0·025MPa) for 4 h in water or in 10^-10 mol l^-1 IAA. These resultsare discussed in the context of a possible role for mechanicallyinduced stress in the initiation and maintenance of nutationalgrowth movements.Copyright 1994, 1999 Academic Press Helianthus annuus, Sunflower, hypocotyl growth, mechanical stress, seedling growth, nutational growth movements, circumnutation     

The Relative Magnitude of the Partial Resistances to Transpirational Water Movement in Sunflower (Helianthus annuus L.)

The effects on leaf water status of cooling entire root systemsor stem and petiolar segments were examined using hydroponicsunflower plants. Leaf water potential (^I) decreased by up to7 x 10⁵ Pa when root temperature was reduced to 10 °C orbelow; complete recovery occurred subsequently provided freezingwas avoided. Leaf water status was unaffected by cooling stemor petiolar segments unless freezing occurred, when severe irreversiblewater stress developed above the cooled zone. The leaves belowthe cooled zone were unaffected, demonstrating that the stressdid not originate from transmitted effects on the roots. Subsequent measurements using small heads of water applied toexcised petioles demonstrated that petiolar resistance was low(c. 0.04 Pa s cm_–4) except in immature and senescent petioleswhere resistance was up to 10 times larger. This trend reflectedthe developmental stage and condition of the xylem. Abrupt increases in evaporative demand, obtained by rapid reductionof relative humidity from 100% to 60 or 40%, induced transientdecreases in of approximately 4.5 and 2.5 x 10⁵ Pa, respectively,which were accompanied by simultaneous large increases in stomatalresistance. No simultaneous transient effects were observedin the stem xylem, demonstrating that the factor responsiblefor the formation of the transient foliar stress resides withinthe lamina. The results are discussed in relation to the relative magnitudeof the various partial resistances to transpirational watermovement.     

Effect of atmospheric pressure on sunflower (Helianthus annuus L.) protoplast division

Summary Sunflower protoplasts were cultured in liquid medium under high atmospheric pressure (0.2 to 0.6 MPa) and the plating efficiency, cell wall synthesis and microtubule organization were assessed. In 7-day-old cultures under a pressure of 0.4 MPa and above, the division rate was strongly reduced by more than 60% as compared to the control. Although most of the protoplasts had begun to regenerate a new cell wall they were unable to complete this process. Pressure also had an inhibitory effect on microtubule synthesis. The percentage of protoplasts showing a disassembled cortical network of microtubules was significantly increased up to 60% of the population. These effects were reversible: when protoplasts were transferred to normal pressure most of them rapidly recovered their capacity to divide and afterwards developed normally. Culturing protoplasts under a pressurized atmosphere revealed to be a good model system for studying cortical microtubule dynamics.Abbreviations BSA bovine serum albumin - PBS phosphate buffered saline - TBS tris buffer saline - MT(s) microtubule(s)     

Effect of drought, on transport and metabolism of abscisic acid in aeroponically grown Helianthus annuus

The effect of drought on transport and metabolism of radioactive abscisic acid (ABA) in roots and shoots of sunflower ( Helianthus annuus L. cv. Russian) was observed. Radioactivity from ABA moved freely all over the plants. Young shoot tissues, such as the growing apical bud or axillary buds released from apical dominance, were strong sinks for ABA. Mature tissues were effective exporters. Drought-induced alterations in the pattern of transport of radioactivity do not appear to be a major factor in the control of drought-induced changes in ABA levels. Metabolism of ABA occurred in all organs examined in stressed and unstressed plants. Labelled ABA and its metabolites moved in the xylem. Drought altered the quantity of radioactive metabolites and reduced the amount of radioactive ABA in extracts from the stressed plants.