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THESIS LIBRAR Y Michigan 5m: Ummv‘w This is to certify that the thesis entitled SIMULTANEOUS INHIBITION OF TRANSLOCATION OF PHOTO- SYNTHATE AND OF THE FLORAL STIMULUS BY LOCALIZED LOW—TEMPERATURE TREATMENT IN THE SHORT-DAY PLANT PHARBITIS M presented by David Leon Kavon has been accepted towards fulfillment of the requirements for M. S. degree in Botany Major professor 0.7639 s i l ”3‘. ~.‘ A _ ‘ _.-. -—- » ~.__.~~.‘--a— OVERDUE FINES: 25¢ per day per item RETURNING LIBRARY MATERIALS: Place in book return to remove charge from circulation records J. ‘ nmwmmv‘rx. - u ‘1“ ~m..—:-—-—t-:-v- . — “as: ~ ’2» .mm .: \-¢: J: “’14-?" 7'...- \—-- -€7""FP£.¥‘. ‘t‘ SIMULTANEOUS INHIBITION OF TRANSLOCATION OF PHOTO- SYNTHATE AND OF THE FLORAL STIMULUS BY LOCALIZED LON-TEMPERATURE TREATMENT IN THE SHORT-DAY PLANT PHARBITIS NIL By David Leon Kavon A THESIS Submitted to Michigan State University in partial fulfillment of the requirements for the degree of MASTER OF SCIENCE Department of Botany and Plant Pathology l979 "2 ”/3 900 f I), ABSTRACT SIMULTANEOUS INHIBITION OF TRANSLOCATION OF PHOTO- SYNTHATE AND OF THE FLORAL STIMULUS BY LOCALIZED LON-TEMPERATURE TREATMENT IN THE SHORT-DAY PLANT PHARBITIS NIL By David Leon Kavon There is considerable evidence to indicate that the floral stimulus moves along with photosynthate in the phloem. In examining this phenomenon, the effect of cooling a localized region of the stem of the short-day plant Pharbitis nil Chois. was investigated. l4 The movement of both C-labeled assimilates and the floral stimulus was followed. Three methods were devised to apply low temperature to the translocation path. The initial method, using an ice-filled Plexi- glas trough involved a gentle bending of the stem. It was shown that the bending itself contributed to an inhibitory effect on translocation of assimilates as well as movement of the floral stimulus. An erect version of this cold block, eliminating the bend, was shown to apply mechanical pressure on the stem which also appeared to contribute to an inhibitory effect. The final apparatus, based exclusively on the circulation of low-temperature air around the stem zones, eliminated all mechanical contact with David Leon Kavon the translocation path. The effective low-temperature treatment simultaneously inhibited translocation of photosynthate and of the floral stimulus, thus further supporting the idea that the floral stimulus is transported concurrently with assimilates in the phloem. ACKNOWLEDGMENTS I am pleased to thank Jan Zeevaart, chairman of my guidance committee, for the time and effort he invested in this project. I sincerely appreciate the encouragement he offered on a continual basis during the course of my research. I would also like to thank Debbie Delmer, Andrew Hanson, and Hans Kende for serving on the guidance committee. Foremost, I feel immeasureable gratitude to my wife, Susie, whose patience was exceeded only by her wisdom and foresight. Finally, I think my daughter; Yedida, whose first year made this final year a veritable joy. ii TABLE OF CONTENTS LIST OF TABLES LIST OF FIGURES INTRODUCTION . Assimilate Translocation . . . Tissue of Assimilate Translocation . . Substances Transported in the Sieve Tubes Mechanism of Translocation Early Observations Solute Translocation Independent of Solvent Water Mass Flow Resistance to Flow. Other Theories . Role of Metabolism Anoxia Effects . Inhibitor Studies Low- Temperature Inhibition Transient inhibition and recovery Floral Stimulus Translocation Recognition of a Flowering Signal and a Transmissi- ble Floral Stimulus . . Floral Stimulus Movement . . Tissue of Floral Stimulus Transport . . Inhibition of Flowering by Noninduced Leaves . Other Factors Affecting Floral Stimulus Pro- duction and Movement . . . . Velocity of Floral Stimulus Movement . Attempts to Divert or Impede the Movement of the Floral Stimulus . . . . . . . . . Experimental Objectives MATERIALS AND METHODS . Growing Conditions . Low-Temperature Treatment I4C- Labeling of Assimilates . Movement of the Floral Stimulus m Page Page RESULTS AND DISCUSSION . . . . . . . . . . . . . 62 Labeling Method . . . . . 62 14C- Assimilate Detection by Rapid- -Scan Method . . . 62 Movement of Floral Stimulus Out of Cotyledons of Pharbitis Seedlings . . . 65 Effect of Leaf Size on Perception of the Inductive Conditions . 65 Effect of a Localized Cold Block on 14C- Photosynthate. Translocation . 7l Effect of a Localized Cold Block on 14C-Photosynthate Translocation and on Floral Stimulus Transport . . 7l Effect of an Erect, Localized Cold Block or Simulated- Ice Block on the Transport of the Floral Stimulus . 82 Effect of a Localized Cold Block, Based an Low- Temperature Air, on Translocation of I Photosynthate and on Transport of the Floral Stimulus . . . . . . . . . . . . 85 SUMMARY AND CONCLUSIONS . . . . . . . . . . . . 95 LITERATURE CITED . . . . . . . . . . . . . . . 97 iv TABLE LIST OF TABLES Effect of Leaf Size on the Perception of the Inductive Conditions Effect of Cold Block, Bending and Excision at Various Heights Above the Cold Block on Movement of the Floral Stimulus . . . . . . Effect of an Erect, Localized Cold Block or Simulated- Ice Block on the Transport of the Floral Stimulus . . . . . . . . . . . Effect of Localized Chilling of the Stem on Trans- location of the Floral Stimulus in Pharbitis njl_. Page 72 8] 83 9l LIST OF FIGURES Figure Page l. Devices used for localized chilling of Pharbitis _ stems . . . . . . . . . . . . . . . . 58 2. Distribution of 14C- -activity along Pharbitis stems after labeling of the source leaves By two differ- ent methods . . . . . . 63 3. Scan of 14C-activity along Pharbitis stems . . . . 66 4. Movement of floral stimulus out of cotyledons of Pharbitis seedlings . . . . . . . . . . . 69 14 5. Distribution of C-activity along Pharbitis stems in initial cold block after labeling of the source leaves . . . . . . . . . . . . . . . . 73 6. Distribution of 14C-activity along Pharabitis stems in initial cold block and in bent control after labeling of the source leaves . . . . . . . . 78 7. Distribution of 14C-activity along Pharbitis stems in cold block based on low-temperature air 7 h after labeling of the source leaves . . . . . . . . 86 8. Profile of distribution of 14C-activity along the stem of Pharbitis l2 h after labeling of the source leaf . . . . . . . . . . . . . . . . 89 vi INTRODUCTION Assimilate Translocation Tissue of Assimilate Translocation In the course of scientific inquiry, it is not uncommon to note observations whose significance becomes apparent only years thereafter. Such is the case with some early studies on translocation in plants. Stephen Hales (65) conducted girdling or ”ringing” experi- ments on shoots of a dwarf pear tree. In T727 he reported that where a ring of bark was removed both above and below a segment containing a thriving, leaf-bearing bud, by the end of the growing season this remaining ringlet swelled greatly at the bottom, yet not at the top. No swelling was observed if the segment contained no bud. He further correlated the extent of the bark swell at the bottom of the ringlet with the extent of leaf tissue produced by the adjoining, developing bud. While he neglected to interpret his results in terms of inter- ruption of assimilate flow, his experiment clearly demonstrated that such interruptions of the continuity of a transport apparatus results in the accumulation of the transported substances just above the region of interruption. Such experimental results later generated speculation that the site of the tissue responsible for the trans- location of photosynthate is in the bark and that removal of this tissue by girdling will result in the accumulation of photosynthate which causes the swelling. In the nineteenth century Hartig (73) described the sieve tube of the phloem and proposed that it is through this tissue that the basipetal transport of organic assimilates from the leaves occurs. Schneider-Orelli (l37) observed that the mobilization of the starch accumulated in apple leaves during the day is blocked in areas where the leaf-mining larvae stage of the moth Lyonetia clercella L. sever the phloem channels. This premise was strongly supported in l928 by convincing girdling experiments performed by Mason and Maskell (l09). They were able to demonstrate that the diurnal fluctuations of leaf sugar concentrations could be observed several hours later in the inner bark further down the stem, but not in the wood. They further showed that when the bark was removed, sugar translocation halted, whereas it continued when the bark and wood were merely separated. They also reported that translocation continued even into loosened flaps of bark. Fluorescent dyes were later used to visually ascertain that the sieve tubes of the phloem were, in fact, the channels of translocation. Schumacher (l38) applied a fluorescent dye to a Pelargonium leaf and was able to follow its transport down the petiole. Furthermore, by inducing callose-plug formation at the sieve plates, he was able to block further transport. The strongest evidence for movement of photosynthate in the phloem was presented once radioisotopes were available for biological research. Colwell (3l) was the first to employ autoradiography to study the movement of assimilates through the phloem. Bieleski (6) reported that following labeling with 14CO2 the cells which accumu- lated the most label were in the secondary phloem. The movement of 14C-assimilates in the sieve tubes of Cucurbita was shown by Webb and Gorham (I82). Cucurbita is an ideal plant to study translocation since in addition to the usual vascular bundles it contains isolated strands of sieve tubes and companion cells in the vascular region and cortex. While they showed that the sieve tubes do indeed carry translocate, it remained for Trip and Gorham (I63) and Schmitz and Willenbrink (l36) to show that cells other than the sieve tubes do not carry photosynthate. Another approach, which has become very useful for transloca- tion studies, is the aphid-feeding technique developed by Mittler (ll4,l15). Aphids feed on phloem sap by puncturing a single sieve tube with their stylets. Eventually they will exude a dr0plet of honeydew which is quite similar in chemical composition to the phloem sap. The honeydew can be analyzed or the aphid can be severed from the stylet and the sap collected directly. Aphids which are feeding on a plant recently photosynthesizing in the presence of 14CO2 will release honeydew containing radioactive sugars (l4), further confirming the sieve tubes as the translocation channels. Substances Transported in the Sieve Tubes There exist several methods by which sieve-tube contents can be studied. Tapping sieve-tube sap from bark does not work with many species, although the method reported by King and Zeevaart (96) using chelating agents to enhance phloem exudation from petioles makes it possible to collect the contents of phloem from more herbaceous species. There are other drawbacks to the tapping method, including delivery of only part of the sieve-tube contents, dilution or con- tamination by cut cells or metabolic changes by the sap enzymes. The aphid-stylet technique avoids most of these problems, but it pro- duces very small quantities for analysis. Analysis of the entire conducting bundle makes it difficult to differentiate between the content of the sieve tubes and the accompanying parenchyma and com- panion cells. Microautoradiography can be a helpful procedure and indirect evidence for transport is avilable if importing tissues can be shown to require specific compounds which they cannot synthesize from simpler, transported substances. While no single method is ade- quate for positive confirmation, a combination of methods may be accepted as proof. If a compound can be isolated from sieve-tube sap, will accumulate above a girdle and is part of a moving front of radioactive assimilates, then one may conclude that it travels in the phloem. Water is the most abundant and important substance trans- located in sieve tubes. Second only to water, carbohydrates--sugars and sugar alcohols--form the bulk of the transport substances in the phloem. An exception to this rule occurs in certain species of the Cucurbitaceae where nitrogenous compounds form the bulk of the trans- port substances, whereas carbohydrates are only l% of the fresh weight (l99). Zimmermann and Brown (208) classified three types of plants according to the form of carbohydrate translocated in the phloem: (a) species with sucrose as the major sugar, (b) species which contain large quantities of oligosaccharides of the raffinose family in addition to sucrose, and (c) species with considerable amounts of sugar alcohols such as mannitol, sorbitol, and dulcitol. Free hexoses are rarely found. All transport sugars are non reducing as noted by Trip et al. (l65). Reducing sugars appear to be excluded at the phloem loading step. Sucrose and sugar alcohols predominate probably because of their high solubility in water, their ease of synthesis from first stable products of photosynthesis, their ease in undergoing further metabolism by receiving cells, their protection from degradation in the sieve tubes,and their ability to be actively transported across membranes (l99). Except for species of the Cucurbitaceae, nitrogenous sub- stances are less abundant in sieve-tube sap than carbohydrates. In Salix Mittler (ll5) found the nitrogen content to range between 0.3-2.0 mg ml-L, varying with season. The protein of the phloem sap consists mainly of "P—protein,” a class of proteins with a filamentous and tubular structure. Although there had been speculation that it may be a contractile protein responsible somehow for phloem transport, all evidence now points against this (187). The amino acids aspara- gine, aspartate, glutamine, and glutamate are predominant in sieve- 'mnxasap(66). Serine, which is also common, is readily formed from 3-phosphoglycerate, an early photosynthetic product. Proline may be the most abundant amino acid transported in sieve tubes of some species at the end of the growing season following leaf-protein mobili- zation. Putrescine, canavanine, allantoin, allantoic acid and citrul- line have all been detected in some species, but are clearly less common . There are scattered reports of ether-soluble compounds in phloem exudate (l99), but no systematic study of lipids in sieve- tube sap has been reported as yet. Citric, oxalic, malic, tartaric, and other metabolically important organic acids have been identified by Peel and Weatherly (l27), but are minor components of the sieve-tube sap. Sugars, rather then any transported organic acids, undoubtedly serve as the basic skeletons for sink-tissue building materials. Mature sieve elements are cells which have lost their nuclei, yet there are some indications that nucleic acids or their components are transported in the sieve tubes. Both DNA and RNA levels have been measured in sieve-tube sap. AMP and ADP have also been detected but ATP concentrations are noted to be particularly high with concen- 1 (199). While free adenine trations ranging from 34.9-976 ug ml‘ has been detected, this may be the result of an enzymatic degradation of ATP to adenine. The ATP concentration in sieve tubes remains relatively constant and yet turns over rapidly. The companion cells are probably responsible for maintaining the ATP level. Uracil derivatives, which may function in transport-sugar biosynthesis, and cytosine derivatives have also been found in phloem exudate. Correlative control of developmental growth processes by growth regulators requires their transmission via a long-distance transport system. It is, therefore, not surprising that the known growth regulators are easily found in phloem exudate. IAA appears to follow the direction of assimilates in bulk flow. Bioassay activity of IAA-like substances in sieve-tube sap was first detected by Huber et al. (82). Some indole derivatives apparently cannot enter the sieve tubes. Hall and Baker (66) estimated the IAA concen- tration in sieve tubes to be 0.6 x lo-7 M. There are several possible roles for auxin transported in the phloem. Auxin may affect activa- tion of sieve tubes, initiate cambial activity in overwintered trees and influence sugar translocation in sieve tubes both laterally from storage tissue to sieve tube and longitudinally within the sieve tubes. Using the dwarf corn (d5) bioassy, Kluge et al. (97) found gibberellin—like activity in the phloem exudate of several species. The concentration was about 5le3 pg ml']. Similarly, gibberellin- like activity has been detected in the honeydew of aphids feeding on several plant species (80). In Salix_it was shown that the concentra- tion of gibberellin—like substances in sieve-tube sap was related to the day-length in which the plants were growing (80). Phillips and Cleland (l29) obtained evidence, using the soybean-callus bioassay, of cytokinin activity in honeydew of aphids feeding on Xanthium. The absence of such activity in honeydew of aphids feeding on a totally defined diet clearly showed that the plant is the source of the cytokinin-like substances. Three cytokinin-like substances from Ricinus phloem exudate were separated by Hall and Baker (66). Abscisic acid is known to markedly affect the transpirational status of plant tissues. Hoad (78) detected ABA in honeydew of aphids feeding on Salix viminalis. Its concentration was reported to be inversely related to plant daylength (9). Zeevaart (194) further demonstrated that not only ABA, but also its two metabolites, phaseic acid and dihydrophaseic acid are translocated in the phloem to the sink tissue, the shoot tips. Head (79) reported that under conditions of water stress the ABA level in lupin sieve-tube sap was very high when compared to the ABA levels in leaf,seed and pod tissues. The author suggested the possibility of active secretion of the hormone into the phloem from the leaf based on these results. Eschrich (42) reviewed the unidentified hormonal factors reported to be translocated in the phloem. Included in these factors are a cold-hardiness factor, a leaf factor necessary for successful graft unions, a factor for root assimilation of certain substances,and the floral stimulus. Many herbicides and synthetic growth regulators are phloem mobile. This mobility in the phloem is neither correlated with the water solubility of the herbicides, nor with the rate of their metabolic degradation, nor with their degree of chlorine substitu- tion. However, the presence of carboxyl groups on the molecules appears to be closely related to their phloem mobility. There are several exceptions whereby compounds are mobile but lack a carboyxl function. In such cases their products of degradation may be responsible for their mobility. Alar (B-995) and morphactins such as chlorfurenol contain carboxyl groups and are mobile in the phloem. AMO-l6l8 and CCC are not carboxylated, but their phloem mobility has not been investigated (l99). Nonautotrophic plant tissues, such as roots, require the supply of certain vitamins from leaf tissue. Based on tissue culture and girdling experiments Ziegler and Ziegler (ZOl) showed that thia- mine, niacin, pantothenic acid, B6-complex vitamins including pyri- doxine and its derivatives are transported in the phloem. Riboflavin, biotin, folic acid, vitamin B12 and “Crithidiafactor” may occur in phloem exudate, but only in very small amounts. Inositol, which functions in biosynthesis of galactose-containing oligosaccharides and as a pectin precursor can be found in phloem exudate in high con- centration. Ascorbic acid, dehydroascorbic acid and diketogulonic acid have also been detected in considerable concentration (20l). Numerous organic-phosphate compounds and phenolic compounds have been-detected in small quantities in sieve tubes and phloem exudate. The fact that they also accumulate above a girdle adds credence to the claim that they are phloem mobile and not simply contaminants from adjacent, damaged cells. Inorganic substances are also present in the phloem. Bukovac and Wittwer (l3) studied the mobility of mineral elements in the phloem and categorized them according to mobility. Potassium is the predominant cation in the phloem and other alkali metals such as sodium, rubidium and cesium are likewise phloem mobile with the exception of lithium. Magnesium is readily mobile in the phloem, while calcium, strontium and barium are not. Phosphate appears to be the predominant anion although chlor- ide is present in comparable concentration in Ricinus (66). The lO abundance of phosphate determines the low concentration of cations which form phosphate salts of low solubility. The abundance of potassium and phosphate appears to largely account for the generally alkaline pH of sieve-tube sap. Most recent studies indicate an absence of nitrate in phloem exudate. Heavy metal nutrients including manganese, iron, zinc, copper, molybdenum, cobalt, aluminum and titanium have all been detected in phloem exudates of some species and are generally classed as relatively mobile elements. Calcium, boron, and lead are particularly immobile elements. Calcium is an element important to plant growth and the Ca/K ratio in an organ is indicative of the balance between phloem supply and xylem supply to the importing tissue. Boron is likewise an essential nutrient which must be taken up in the xylem owing to its phloem immobility. Under very humid conditions, newly-developing leaves may become deprived of adequate boron. Lead is a significant air pollutant resulting from gasoline combustion, and as in the case of calcium, the phosphate salt is very insoluble in water and cannot enter the sieve tubes. While viruses are not normal constituents of phloem sap, certain virus particles, owing to their size, can occur in the sieve tubes, while others move from cell to cell via the plasmodesmata. Upon reviewing the many different substances of varying mole— cular weights, chemical substituents, charges and shapes which move ll along with water in the phloem, it becomes evident that the phloem transport mechanism is based on mass flow driven by an osmotic gradient. Mechanism of Translocation Earlngbservations Both the tissue of translocation and the mechanism of trans- location have been studied concurrently. De Vries (l73) in l855 felt that diffusion down a concentration gradient was much too slow a process to account for the quantity of material translocated per unit time and transverse sectional area. Having observed protoplasmic streaming in many species, he suggested that perhaps protoplasmic streaming might supply a driving force for translocation. Support for this view came from the work of Curtis (36) in l935. While he strongly advocated the protoplasmic streaming theory, the role, if any, of this phenomenon in phloem transport has yet to be demon- strated. More recently, Thaine (l54) has reported observing trans— cellular streaming in sieve tubes and Canny (l5) calculated that the energy required to drive protoplasmic streaming at phloem transport rates compares well with the experimental values of sugar transported per unit time. Nevertheless, the general objections are strong. The transcellular streaming observation has not been clearly confirmed. Esau et al. (39) argued that such transcellular streaming is highly improbable. Yet, like Thaine, Parker (l25) also observed some type of transcellular strands. Transcellular fibrillar or tubular mate- rial was observed by Evert and Murmanis (50), but these fibrils may 12 be very different from what Thaine observed (208). The velocity generally recorded for protoplasmic streaming is only lO-20% of the velocity of a radioactive front moving through the phloem. Lastly, applied substances should be distributed evenly throughout the plant if protoplasmic streaming is the motive force, yet observation simply does not bear this out. There were others who, unable to reconcile specific mass transfer with diffusion in the phloem, advocated the xylem as the tissue of transport. Birch-Hirschfeld (7) calculated that diffusion was too slow to account for transport and insofar as protoplasmic streaming did not accelerate transport in her experiments, she sug- gested the xylem as an alternative route. Dixon and Ball (37) also offered the xylem tracheae as the transport conduit since they assumed that the flow of a lO% sugar solution at 40 cm h-1 could not be accomplished in the phloem. Similar calculations made by Mason and Maskell (llO) showed that the observed specific mass transfer could not possibly be accounted for by diffusion, which encouraged specula- tion as to alternate mechanisms. In l927, Mfinch (l2l) described a physical system which, when applied to the plant, would seemingly account for many of the observa- tions in the translocation literature. He envisioned two osmotic cells connected via a channel through which a solute concentration gradient would be maintained. If one cell were at a higher concen- tration than the other, a mass flow would result provided water exchange and return could occur through the semipermeable cells. The l3 products of photosynthesis would maintain the high concentration in cells which would connect via the sieve tubes to meristematic or storage cells, with water returning through the xylem. In short, a gradient of decreasing turgor pressure would exist between a source region and a sink region. His proposal generated much interest, and its virtues are even currently the subject of debate in the litera- ture (144). Solute Translocation Independent of Solvent Water While any mass-flow theory requires the movement of solute molecules along with the solvent water, other theories espouse the’ transport of solutes, independent of the solvent water, by some active process. This differentiation can be useful in examining the experi- mental evidence. Protoplasmic streaming is one theory according to which the solutes move independently of the solvent water. The bases of and objections to this theory have already been revieWed (see pp. ll-l2). Van den Honert (l70) proposed that phloem transport may be explained in terms of rapid movement at interfaces along a concen- tration gradient. His proposal was based on the observation that oleate spreads rapidly at the interface between water and ether. There is no evidence to indicate that one may generalize from this case to the many different molecules translocated in the phloem. The theory put forward by Canny (l6) was based on Thaine's transcellular strand observation. He postulated that half of the l4 strands move in each direction and carry dissolved substances in equilibrium with the vacuolar solutes. The transport follows a con- centration gradient. It is essentially a diffusion theory accelerated by protoplasmic streaming or contractile proteins. The same objec- tions to the protoplasmic streaming theory also hold in this case in addition to questions regarding the nature of the proteins in the sieve tubes. Two important questions have been asked by researchers in an attempt to establish if solutes flow with the solvent water or inde— pendently and thus approach the mechanism of translocation. First, does simultaneous bidirectional transport occur within a single sieve tube? If it did occur, then obviously the solutes are moving inde- pendently of the solvent. Second, are there solutes which move in the same sieve tube at different velocities? If the answer is in the affirmative, then once again it would appear that solute and solvent must move independently. 3H-water and 14C-sugars Biddulph and Cory (5) found that both move together in a single conducting bundle. Trip and Gorham (l64) followed simultaneous movement of 14C-sugars and 3H—water in the phloem of squash plants and both were blocked by steam girdling. These and similar evidence would argue against bidirectional trans- port. However, Gage and Aronoff (54) were unable to detect 3H-water movement in the phloem. Furthermore, bidirectional movement in phloem has been reported by Ho and Peel (76), Peel et al. (178), Canny and Askham (l7),and Eschrich (4T). In interpreting the above l5 results, it is imperative to ascertain if: (a) the observed trans- port is occurring in the very same sieve tube; (b) the observed transport is not movement occurring in the same sieve tube, but from opposite directions to a common sink such as an aphid stylet; and (c) the 3H-water often used in such experiments exchanges through the sieve tubes resulting in dilution and, therefore, poor detection. These reservations render the interpretations inconclusive. The . entire question of bidirectional transport was addressed by Mac- Robbie (l07) and reviewed most recently by Eschrich (43). There exists much evidence for the nonspecificity of trans- : port in the phloem. Virus particles, various ions, herbicides, and ) hormones all seem to be carried together. The results of differential L translocation experiments, however, as was the case with bidirec- tional transport experiments, are subject to alternate explanations. When conducted over long periods of time, one may actually be dealing with both phloem and xylem circulation. Altered sink activities in inhibitor studies may account for apparent differential translocation. Cataldo et al. (l9) have demonstrated that 3H-water is very mobile and is subject to rapid local diffusional exchange and adsorption during translocation through the sieve tubes. This, too, can account for the appearance of differential tracer translocation. Mass Flow While the results of the simultaneous and bidirectional trans- port experiments are equivocal, most other evidence points to a mass- flow system. In Heracleum montegazzianum, the vascular strands lie 16 near the surface of the inner cavity of the petioles such that the phloem can be removed from the xylem and ground tissue. Ziegler (l98) surgically separated xylem from phloem, but removal from its osmotic environment complicates the picture. Respiration of the iso- lated bundles increased dramatically. Ziegler and Vieweg (200) iso- lated such a phloem strand and applied a heat pulse whose movement was detected by a thermocouple located some distance from the source. It seemed that the heat pulse was carried by mass flow. But Hoddinott and Gorham (8l) noted that even slight disturbances of the vascular strands of H, montegazzianum lead to a loss of translocation. Kursanov (99), studying the movement of radioactive tracer fronts as a measure of translocation velocity, recorded velocities of up to 200 cmh'1 which are clearly too rapid for diffusion and proba- bly indicate some form of mass flow. Zimmermann (207) studied translocation velocity by following the progress of oligosaccharide ratios in phloem exudate along the trunk of actively photosynthesizing trees. He, too, found that high velocities can be achieved, pointing again to a mass flow. In a very creative study, Walding (l74) demonstrated mass flow using a willow stem without leaves or roots. While the ends were submerged in water,32P was applied to a region of abraded bark near the upper end, and radioactivity was monitored some distance from the source. Little movement was noted over one week. Follow- ing the establishment of a feeding aphid further down the stem, in a few hours the honeydew from the aphid already contained radioactivity. l7 Apparently the turgor pressure had dropped as a result of the feeding and a mass flow resulted. Assuming that phloem transport is a solution flow, water would return to the leaves via the xylem when sugars are removed at a sink such as storage tissue in a tree. By enclosing a downward- pointing bark flap in plastic, it is possible to collect water pro— vided that transport continued. Continuation of phloem transport was confirmed by formation of wood on the cambium side (l2). That phloem transport occurs via a solution flow can be best concluded on the basis of the observed aphid stylet exudation rate and a few simple calculations. Typical values of exudation rate from the stylet of Longistigma gary9e_Harris inserted in a single sieve element 3 l 3 -l are 2-5 mm h- or about 0.00l nm sec . A typical sieve element has a diameter of 0.025 mm and a length of 0.35 mm--a volume of about 0.0002 mm3. Clearly, then, it must deliver five times its own volume per second. The only way to account for refilling five times per second is by a flow mechanism (207). On this point most are agreed in contrast to the controversy surrounding the mechanism providing the force for this flow. Zimmermann (203), in an attempt to order the evidence for a pressure—driven, mass-flow mechanism, established three criteria: (a) the sieve tube system must be semipermeable with respect to the apoplast. That is, the longitudinal walls must be lined with a semi- permeable membrane to prevent leakage of osmotically active molecules. Both entry and removal of these solutes must be the result of active l8 loading and unloading. (b) the sieve plates between elements must be permeable to the transported solutes. (c) the turgor must be positive in the direction of flow. In other words, a turgor gradient must exist from source to sink. The first criterion has been met numerous times. Currier et al. (34) demonstrated plasmolysis in sieve elements which indicates the presence of a semipermeable membrane. Weatherly et al. (l78) per- fused xylem tissue with increasing concentrations of a mannitol solu- tion. As the osmotic concentration was raised, the concentration in the adjacent sieve tubes also increased and the rate of exudation decreased reflecting the drop in turgor pressure. Clearly the walls are bound by some semipermeable membrane and the sieve elements act as osmotic cells. Weatherly et al. (178) further performed razor-incision experi- ments on a stem containing an exuding aphid stylet. Longitudinal cuts made a distance of two cm away from the stylet had very little effect when compared with transverse cuts made ten cm above the stylet which resulted in the immediate drop in exudation rate. These latter results provide support for meeting the second criterion. In other incision experiments Zimmermann (205) studied the concentration of sugar in sieve tubes and observed dilution of the sugars with time, apparently the result of dilution by water from the surrounding apop— last. In a later study (206) he was able to show, using double incision experiments, that exudate flows axially toward the incision from above and below. Thus both the criteria of lateral semipermea- bility and longitudinal permeability have been met. l9 There have been several attempts to answer the question of turgor gradients from source to sink. Huber et al. (82) obtained sieve-tube exudate by incision and measured in Quercus rubra L. a concentration gradient of 0.01 mole m"1 or about 0.2 atm m‘]. By defoliation treatments, Zimmermann (204) was able to manipulate the concentration gradient in sieve-tube exudate. Defoliation resulted in removal of the sugar source and lowering of the xylem tension. Both of these processes caused a decrease in the exudate concentration in the tree. The total molar concentration gradient, which was positive in the direction of flow prior to defoliation, disappeared when the leaves were removed, presumably when transport ceased. This would seem to indicate that turgor pressure is the driving force for translocation in the sieve tubes. Hammel (67) attempted to directly measure sieve-tube turgor pressure in red oak using a syringe-type manometer. He found a pressure gradient on the order of 0.2-0.4 atm m.1 which seems to match well the theoretical estimates for turgor-pressure gradients (208). Further confirmation of such values is needed before there is general acceptance of these data as definitely meeting the third criterion. Resistance to Flow If the driving force behind the solution flow of transloca- tion is turgor pressure, as posited by the pressure-flow hypothesis, then the resistance to flow in the sieve tubes must be of a magnitude which can be overcome by the turgor-pressure gradient. Furthermore, it must be small enough to account for the observed flow rates. 20 It is not possible to accurately calculate the resistance to flow in sieve tubes. At best, it can only be approached by the Hagen- Poiseuille equation which describes the theoretical steady-state laminar flow through ideal capillaries. It states the relation connecting the rate of change of pressure (dp) with distance (dx) along a long, circular, horizontal tube of radius (r) containing a fluid of viscosity (n) flowing at a volumetric rate (0) per unit time. dp 4 *-= -8nQ/Wr dx Accordingly, a sap solution of 20% sucrose, i.e., about 2 centipoise viscosity at room temperature can be driven by a pressure gradient of 0.l atm m'1 up to l000 cm h'l through a 30u diameter capillary. In considering the parameters of sieve tubes, however, they are neither long, nor consistantly circular or horizontal tubes whose ”radius,” viscosity and flow rate are continually changing. Since the radius term is raised to the fourth power, the equation becomes very sensitive to changes in the circular cross section. In short, they are nonideal capillaries. The above equation is based on a paraboloid flow with the peak velocity on the axis of the tube and an hypothetical zero velocity at the tube wall. However, such a paraboloid flow can result only when the tube length approaches 50 times its diameter (208). There are, however, modifications of the basic equation such that it may apply to nonideal capillaries (l23). They include a 21 coefficient for cross-section shape other than circular (for particu- lar shapes only), nonhorizontal tubes, the internal surface of the tube, and a calculation of the Reynolds Number, a dimensionless value which defines the flow as either turbulent or laminar. There are also modifications which take into account tubes having a vari- able cross section and even variable flow and viscosity under some conditions. The greatest obstacle in applying such equations to flow in sieve tubes is the presence of the sieve plates. The extent of resistance introduced by such structures is a matter of current con— troversy. In another case of resistance to flow, Mfinch (l22) found that in tracheids of Abig§_the hydraulic conductivity was l/3 that of ideal capillaries of the same diameter. This fact would require the tripling of the calculated pressure gradient in the Hagen- Poiseuille equation, if, in fact, the situations are analogous, i.e., bordered pits as compared with sieve pores. Since the question is one of structure particularly with respect to the sieve pores and their ifl_yiyg_condition, anatomical evidence is often cited in the support of a particular position. Insofar as artifacts are not uncommon in the anatomical literature, care must be exercised in the assessment of anatomical evidence. Exudation from phloem, which, in some cases, can be maintained for many hours, even in water-stressed tissue, is a clear indication that the sieve-tube contents are under pressure. Sudden release of this pressure causes a surging in the sieve tubes which is likely to 22 result in displacement of structural material and give an altered picture of the cell anatomy. Therefore, the success of an anatomical investigation, particularly of sieve pores, often rests with the fixation technique. The original chemical fixative for E.M. work, KMn04, has generally been replaced with buffered glutaraldehyde followed by 0504 post fixation. Evert (49) outlined several criteria for acceptable E.M. work in structure-function studies. Thaine (l57) claims that even the currently standard fixation procedure fails to preserve cytoplasmic strands in phloem exudate. Another criticism of fixation technique was posed by Kidwai and Robards (92). They noted that while most phloem exudate has been measured at about pH 8.0, the fixation buffer is often l.0-l.5 pH units lower, which may be criti- cal in terms of surface charge and membrane separation. Much of the work related to the question of the degree of occlusion of the sieve tubes in yiy9_was reviewed and critically assessed by Spanner (l44). It is largely on this point that the various theories hinge. If the sieve pores are open, then pressure flow is clearly a viable mechanism. If they are mostly occluded, the resistance to flow would be too large for the pressure generated. If, however, there is just some structural material, such as fila- ments, traversing the pores, the feasibility of a pressure-flow mechanism would depend on filament spacing (l77). Because the appearance of the pores varies according to Species (33), environ- mental condition of the tissue (6l), and fixation techniques (49), it is difficult to make any universal statement with our present 23 knowledge. This remains a major limitation to progress on conclu- sively establishing a mechanism of phloem translocation. Other Theories The hypotheses of Canny--accelerated diffusion, and Thaine- transcellular strands-~neither based on mass flow, have already been addressed. Other models, while based on a flow of solution propose some source, other than pressure,as the driving force of transloca- tion. Spanner (l43) pr0posed a theory based on electroosmotic flow. He envisions an electrically-polarized sieve plate maintained by a metabolically-driven ion circulation (of K+ ions, for example) with ions being recirculated via the companion cells. Bowling (10), using microelectrodes, measured a potential across the sieve plates of Vitis vinifera. This finding lends some support to such an electro- osmotic theory. Fensom (51) independently proposed the same type of electroosmotic theory. Since then he has withdrawn his electro- osmotic theory (52). Role of Metabolism While there is general agreement that translocation is dependent, at some point, upon metabolic energy, the pressure-flow theory, unlike others, proposes no metabolically generated force along the path. Rather, translocation is driven by activities at source and sink, i.e., vein loading and unloading, which are clearly metabolic (2). A recent report of Chamberlain and Spanner (29) 24 claimed apparent unloading despite chilling of the sink to 0°C. The authors followed 14C-transport from a mature Saxifraga sarmentosa via a stolen to a developing plantlet. Unfortunately, the radio- activity was expressed on a per segment basis, rather than on a weight-of-tissue basis. This could easily account for the apparent accumulation of coUnts in the relatively heavy plantlets. Loading and unloading are undoubtedly metabolically driven. There exists, then, a means of testing the validity of the assertion that path metabolism is not directly responsible for trans- location. There are several methods used to interrupt path meta- bolism. Anoxia, metabolic inhibitors and low-temperature treatments have all been used as probes. There are problems with these tech- niques which render their effects somewhat nonspecific or nonlocalized and the results of such studies equivocal. Anoxia Effects Treatment with nitrogen removes oxygen as the terminal electron acceptor for respiration, inhibiting oxidative phosphory- lation. While the objective is to decrease ATP levels, anoxia also may affect membrane permeability and electrical conductivity. In addition, some products of anaerobic metabolism, which are toxic, will eventually accumulate. There are other factors which may influence the effects of anoxia which include extent of 02 depletion, presence or absence of light, stomatal aperture, duration of treatment and capacity to remove toxic byproducts. 25 Mason and Phillis (111) found that 02-deprivation had no effect on phloem translocation until drastic conditions were imposed. Vernon and Aronoff (172) were also unable to detect inhibition with anoxia. Ullrich (168) confirmed these earlier results. Qureshi and Spanner (130), however, flushed Saxifraga stolons with nitrogen 137 over 20-30 cm lingths and obtained a reversible reduction in Cs and 14 C-translocation. Callose formation did not appear to be involved. A transient inhibition of assimilate transport was reported by Sij and Swanson (141). They suggested that perhaps the energy generation blocked by anoxia may be required only for sieve- tube maintenance as suggested earlier by Garner and Peel (56). Full recovery was obtained within one hour, while 2411of 02-deprivation resulted in further inhibition of translocation, probably a conse- quence of tissue damage. The available evidence generally seems to favor no direct role for path metabolism in phloem translocation. Inhibitor Studies Respiratory inhibitors and other metabolic poisons have also been used to study the role of path metabolism in phloem transloca- tion. Here the difficulties were twofold-~localization and non- specificity. Substances such as 2,4-dinitrophenol, azide and cyanide are mobile in phloem tissue and may not remain confined to the local— ized zone of application. If they affect either source or sink, translocation will surely be perturbed. Therefore, an apparent inhibition of translocation may not reflect any role of path meta- bolism. In addition, agents which alter the energy state of tissues 26 may also have effects which are nonspecific. For example, 2,4-dinitrophenol (DNP), is generally used because it uncouples phosphorylation from terminal electron transport, yet if it affects membrane permeability, leakage to the xylem may occur and the inhibi- tor may reach the leaf blade. In short, if an effect is observed, it still does not prove translocation to be dependent on path metabolic energy. Willenbrink (185) isolated the central vascular bundle of Pelargonium petioles and treated them with respiratory enzyme inhibi- tors. The observed inhibitory effect of cyanide was reversible. Here the objections of nonlocalization and nonsepcificity may hold. Duloy et al. (38) cited inhibitor results as consistently below 50% and probably due to either effects on source and/or sink or inhibi— tion of companion cell function. Ullrich (169) found that treatment with cyanide blocked translocation and that this treatment correlated with an increase in callose, although removal of the cyanide restored translocation deSpite the remaining high callose levels. Callose is a B-1-3 glucan, a structural carbohydrate, which is often associated with sieve pores, particularly when the phloem is cut. This response has suggested callose as a plugging material for damaged sieve tubes. It was reported by McNairn and Currier (112) that warming the hypo- cotyls of cotton to 40°-50°C over a 4 cm zone for 15 min resulted in callose formation and a halt in translocation. After several hours the callose diminished and translocation was restored. Canny (16) was unable to reproduce these results. Willenbrink (186) reported 27 blockage of phloem transport by cyanide treatment and suggested that either the path is actively transporting, or some cyanide-sensitive system is crucial to the maintenance of Open phloem tissue for normal translocation. Harel and Reinhold (72) re-examined the effect~ HH COLO BLOCK ‘5 H CB CONTROL —01 5 2162 - O L 011 WL . 5 H00 WARM BLOCK IO ’ Hwa CONTROL 10‘ JP 0 ’ 5 25 45 65 85 105 DISTANCE FROM LEAF (Cm) 88 chamber (WB) produced a 14C-assimilate transport profile identical to the 14 C-profile of the WB control group,the stems of which were not enclosed in a chamber (Figure 7B). This indicates that low temperature alone does block phloem translocation in Parbitis stems. It was further found that in Pharbitis the phloem transport system does not acclimate to the low temperature; at least trans- location did not resume during a 12-h test period (Figure 8). This is in contrast to some chilling-insensitive species such as Bgtp_ vulgaris (150). Thus, Pharbitis more closely resembles Phaseolus vulgaris, a chilling-sensitive species which at 3°C did not acclimate for at least 9 h (60). Low temperature applied to a 20-cm zone of the stem between the donor leaf and the receptor bud virtually eliminated the flower- ing response while the warm block, WB control and CB control treat- ments all resulted in the same flowering response (Table 4). This demonstrates that the low temperature inhibited transmission of the floral stimulus and that the presence of the chamber itself or the air turbulence did not interfere with translocation. This point is noteworthy in view of the preliminary experiments using the cold block of Figure 1A where gentle bending of the stems alone affected both the 14 C-assimilate transport profile and the flowering response using the erect, plastic-cup cold block where, presumably, the pressure of the ice on the stems contributed to the reduction in the flowering response. In light Of the varied and Often transient responses of translocation to low temperature (61), it is crucial to 89 Figure 8.--Profile of distribution of 14C-activity along the stem of Pharbitis 12 h after labeling the source leaf. The stem was exposed to low temperature for the duration of the 12 h period. Cross-hatched bar on the abscissa indicates the zone of the stem enclosed in the cold block. me- mg”l dry wt. 90 r I Or-\\ 1 20 1 m 1 1 4O 60 80 100 DISTANCE FROM LEAF (cm) IZO 91 TABLE 4.--Effect of localized chilling Of the stem on translocation of the floral stimulus in Pharbitis nil. Ten plants per group, except in cold-block treatment which had only 9 plants. Data of two separate experiments combined No. of Plants Treatment with No. of Plants F1°W°r BUdS With Terminal Flower Buds per Planta Flower Buds Cold block 4 0.9 i 0.39 0 CB Control 10 7.0 i 0.58 2 Warm block 10 7.8 i 0.71 2 W8 control 10 8.3 i 0.78 0 CB; donor leaf not removedb 10 7.0 e 0.94 0 Mean and standard deviation of the mean. bStem zone chilled for 6 h. 92 determine that the responses Observed are indeed caused by low tem- perature, and not by some additional factor(s). When the low- temperature block was removed after 6 h without excising the donor leaf, the flowering response was identical to that of the nontreated controls (Table 4) indicating that the low-temperature effect on the transport system for the floral stimulus was fully reversible. The small flowering response in the low-temperature treat- ments (Table 4) can probably be accounted for by the fact that the critical photoperiod of adult plants Of P, nil; cv. "Violet" is ca. 11 h (85), i.e., 5 h shorter than the inductive treatment employed in the present experiments. It has been shown that when the dis- tance between donor leaves and receptor buds was 100 cm, removal Of the donor leaves after 16 h of darkness resulted in the formation of one flower bud per plant (151). Likewise, in these experiments a sufficient amount of floral stimulus could have moved, prior to chilling of the stem, the 80-Cm distance between the donor leaves and receptor buds to evoke a small flowering response, since move- ment of the floral stimulus in Pharbitis is known to occur equally well in light and in darkness (94). A possible Objection which one might raise with regard to such cold-block experiments is that while chilling a thin stem to l-2°C, it is inevitable that the xylem water will also be cooled. If, upon reaching the source leaf, this water remains cool enough, it could hamper C02 fixation and/or vein loading (as noted in the Introduction, p.28). These effects would produce a similar 93 14C-assimilate profile to path translocation inhibition. Although the xylem-water temperature near the leaf was not measured by thermocouple, the distance from the top of the cold block to the source leaf was at least 30 cm, a distance over which the xylem water would most probably return to ambient temperature. More con- vincing, however, are the time-course results of Figures 5A-C and Figure 8 where the radioactive assimilates appear to accumulate above the cold block with time. If either C02 fixation or vein loading had been affected, progressive accumulation would not have resulted with time. It appears, therefore, that the phloem—path translocation is experiencing the inhibition. One final note of interest is that while the flowering data are presented as ”number of buds per plant,” this measure provides no clue to the sequence of vegetative or floral buds on the individual plants. It was a frequent Observation that vegetative and floral buds would alternate. Even if several floral buds were produced at successive nodes, reversion to the vegetative condition for one or more nodes was not uncommon. As many as three or four such alterna- tions was not an extraordinary occurrence. Such observations were made even in the case of plants which ultimately produced a terminal inflorescence, although they were more common in plants which even- tually reverted altogether to the vegetative state. A similar obser- vation was made by Imamura and Marushige (84): ”After a weak floral stimulus, the shoot apex gives rise to floral primordia in the axils of a few successive nodes. In rare cases one or two vegetative buds may be inserted between flowering nodes." While their observation 94 was made in response to weak induction, terminal inflorescence pro- duction, as noted in the present case is indicative of a strong induction. Nevertheless, these are undoubtedly related Observations. Floral differentiation is a complex developmental process. The explanation for the above observation is likely to become apparent only when more is understood Of the phenomenon of floral expression and of the stimulus by which it is triggered. SUMMARY AND CONCLUSIONS It is generally assumed that both the floral stimulus and assimilates are translocated concurrently in the phloem. This assump- tion is, among others,based on very similar velocities of translo- cation Of 14C-labeled assimilates and of the floral stimulus in Pharbitis (94), and on the correlation between distribution of 14C-activity following labeling of a Perilla donor leaf and the flowering response (95). In this present study three methods were devised for apply- ing localized low temperature to the translocation path. The first two methods indicated that mechanical effects contributed to the inhibition of translocation. The results of the last set Of experi- ments, in which a region of stem between the donor leaf and receptor bud was chilled without mechanical perturbation, showed that in 14C-labeled assimilates as well as the Pharbitis translocation Of flowering response were simultaneously inhibited by low temperature only. This inhibition is perhaps the result Of some physical block- age Of the sieve tubes. NO recovery of translocation was observed even up to 12 h with the cold block continually present. The inhibi- tory effect of low temperature on transport of the floral stimulus was shown to be fully reversible following removal Of the cold block. Yet, irrespective of the factor(s) which alone or in combination 95 96 14C-translocation achieved blockage, the correlation between extent of blockage and inhibition of flowering always held. Thus, these results give additional support to the contention that floral stimulus and photosynthate move together in the phloem. LITERATURE CITED 10. 11. 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