Real Time Studies of Carbon- 11 Translocation in Moonflower: III. FURTHER EXPERIMENTS ON THE EFFECTS OF A NITROGEN ATMOSPHERE, WATER STRESS, AND CHILLING; AND A QUALITATIVE THEORY OF STEM TRANSLOCATION

Translocation of naturally loaded carbon- 11 is studied in themoonflower Ipomoea alba L. and data are presented to show that:(i) source leaf export is momentarily inhibited by a suddenlyapplied water stress; (ii) pathway translocation appears toproceed by a MÜnch-like mechanism, but in a porous mediumrather than in an array of parallel, only slightly leaky pipes;and (iii) sink unloading seems to proceed normally in a nitrogenatmosphere. A qualitative model which incorporates these findingsis presented.     

Translocation of Assimilates Within and Between Potato Stems

Three aspects of translocation in potato were examined: (i)translocation within stems (ii) translocation between individualstems of a plant (iii) translocation between tubers followinginjection of ¹⁴C sucrose into a single daughter tuber. Assimilatesexported from single leaves of evenly illuminated potato stemsremained confined to the same side of the stem as the sourceleaf in a pattern consistent with the internal arrangement ofvascular bundles in the stem, and tubers borne on stolons verticallybelow the source leaf contained higher concentrations of ¹⁴Cthan those on the opposite side. Consequently ¹⁴C import intothe tubers bore little relationship to tuber growth rates. However,alteration of source/sink relations by pruning stems to a singlesouce leaf resulted in an even distribution of ¹⁴C throughoutthe vascular bundles of the stem and ¹⁴C import into the tubersbore a stronger relationship to tuber growth rates than to thephyllotactic relationship of the tubers with the source leaf. Labelling one stem of a potato plant resulted in little or nomovement of ¹⁴C into tubers on other unlabelled stems. However,removal of the unlabelled stems at ground level induced a significantmovement of ¹⁴C from the labelled stem to the tubers on unlabelledstems, this movement occurring via the mother tuber. Shadingthe unlabelled stems had less effect than stem removal. ¹⁴C sucrose injected into single daughter tubers was translocatedto other tubers on the same stem and also to tubers on a secondstem at the opposite end of the mother tuber. The sucrose wasconverted to starch in these tubers. The results favour the view that each potato stem functionsas an independent unit with potential for assimilate redistributionwithin a stem but with little or no carbon exchange occurringbetween stems, unless under severely altered source/sink patterns. Assimilates, ¹⁴C, autoradiography, potato (Solanum tuberosum L.), tuber growth     

Effects of Root Pruning on Translocation of Photosynthates in Phaseolus vulgaris L

Root pruning increased the level of ethanol soluble sugars inred kidney bean plants (Phaseolus vulgaris L. ) grown in aeratednutrient solution. However, the concentration gradient of thesesugars down the stem and its translocation velocity remainedunchanged. Removal of 50% of the roots had no effect on thetotal photosynthates exported from source leaves but the finaldistribution pattern of photosynthates was altered; less movingtoward the upper plant parts, and accumulation occurring inthe lower stems. Translocation velocity of photosynthates towardthe upper plant parts was drastically reduced by root pruning. Key words: Phaseolus vulgaris, Photosynthate translocation, Root pruning     

The Effect of Calcium Starvation on Assimilate Partitioning and Mineral Distribution of the Phloem

Populus plants were grown in a medium lacking calcium and exposedto ¹⁴CO₂. In contrast to plants in the complete nutrient medium,the percentage amount of ¹⁴C-assimilates increased in the leavesof calcium-deficient plants and decreased in the stem and theroots. When plants were grown without potassium or magnesiumno differences in the amount of ¹⁴C-label occurred in comparisonwith plants in the complete nutrient medium. Translocation wasrecorded by microautoradiography. It was observed that considerableamounts of labelled photoassimilates were unloaded from thephloem in the middle part of the stem in plants of the completenutrient medium. In contrast, during calcium starvation ¹⁴C-labelwas restricted to the phloem of the stem. In addition, the concentrationsof magnesium and phosphorus showed a remarkable increase instem sieve tubes of calcium-deficient plants. When sieve tubesof source leaves from Populus, barley and maize were comparedwith those of sink leaves, the latter showed higher calciumconcentrations. The results suggest that calcium is a necessaryfactor in the regulation of phloem translocation. Key words: Calcium deficiency, phloem translocation, sieve element loading and unloading, X-ray microanalysis     

Studies on the Movement of Solutes between the Sieve Tubes and Surrounding Tissues3: I. INTERFERENCE BETWEEN SOLUTES AND RATE OF TRANSLOCATION MEASUREMENTS

Certain aspects of the secretion of solutes into, and removalfrom, the sieve tubes of isolated stem segments and rooted cuttingsof Salix viminalis have been studied. Sieve-tube sap was obtainedeither as honeydew from whole individuals or via the severedstylets of the aphid Tuberolachnus salignus (Gmelin). It was shown that interference occurred between the chemicallyunrelated solutes, sucrose and the cations potassium and rubidium.On raising the potassium concentration in the sieve-tube sapby passing a solution of this ion through the xylem, the sucroseconcentration declined. When the sucrose concentration fellover a period of days due to respiratory loss of carbohydratesfrom an isolated stem segment, a concomitant rise in eitherthe potassium or rubidium level in the sap occurred. When a solution of sodium was passed through the xylem, theconcentration of this ion in the sieve-tube sap rose, whilstthat of potassium fell at first, but later rose higher thanits initial value, indicating that both antagonism and synergycan occur between these ions. On introducing both these cationsinto the xylem simultaneously, more sodium than potassium wastaken up by the segment, though the increase in the sodium concentrationin the sieve-tube sap was less than that of the potassium. Perfusingthe xylem with a calcium solution had no effect upon the concentrationof potassium in the sieve tube. It has been shown that the rate of translocation of a solutealong the sieve tube, as measured by the two colony technique,depends upon the rate of removal of this solute from the sievetube. The amount of such lateral loss from the sieve tube isrelated to the potential gradient for a solute between the sievetube and surrounding cells.     

Effect of Source-to-sink Ratio on Partitioning of Dry Matter and14C-photoassimilates in Wheat during Grain Filling

Increasingly, wheat (Triticum aestivumL.) is being grown intropical environments, but there is inadequate information aboutthe physiological processes limiting yield. In this investigation,the source:sink ratio was manipulated to examine the performanceof source-sink interactions after anthesis and the factor(s)limiting grain filling in tropical conditions. Plants of threewheat cultivars, Cuba C-204, Candeias and IAC-60, were artificiallymodified to give different source:sink ratios. The treatmentswere: I, Control; II, all spikelets on one side of the spikeremoved; III, all spikelets removed except the four centralspikelets of the spike; and IV, flag leaf blade removed. Thedistribution of dry matter between kernels and stem internodeswas analysed at harvest in all three cultivars. Partitioningof¹⁴C-photoassimilates was measured on three occasions afteranthesis in the cultivar Cuba C-204. Modifications of source:sinkratio led to different patterns of allocation of dry matterbetween cultivars and sowing dates. The reduction in sink sizein treatment II produced no significant change in the mass pergrain in the January sowing, but this was enhanced in two cultivarsin the November sowing. In treatment III, both mass per grainand translocation of¹⁴C-photoassimilates declined, apparentlydue to feedback inhibition of photosynthesis. The participationof stem reserves in grain filling and the existence of genotypicdifferences in response to availability of photoassimilateswere corroborated. The pattern of partitioning of dry matterobserved in plants in this investigation suggests a source limitation,particularly during the November sowing. This pattern differedmarkedly from that in other studies, most of which have beenmade in temperate areas.Copyright 1999 Annals of Botany Company Photoassimilates, sink, source, partitioning, grain filling, wheat.     

Partial recovery of grape energy metabolism upon aeration following anaerobic stress

The metabolic changes of mature grape berries during periodsof anoxia and upon return to air were examined using two cultivars.Glutamate declined during anoxia, together with a correspondingaccumulation of -aminobutyrate (GABA). The reverse occurredduring a 24 h period of air. Total adenine nucleotide (AdN),ATP level, and adenylate energy charge (AEC) all declined duringperiods of anoxia but showed a reversible pattern upon subsequentaeration. However, aerobic recovery of metabolite levels wasnot observed when the duration of anoxia at 30°C exceeded6 d. Accumulated ethanol during anoxia (up to 0.22 M) triggered afurther increase in the rate of ethanol synthesis when stressedberries were returned to air. Ethanol may be the principal determinantgoverning the ability of grapes to withstand and recover fromanoxic stress. We propose that the imbalance between aerobicand fermentative pathways may be due to the ability of ethanolto impair mitochondrial membrane function and uncouple oxidativephosphorylation, the rate of anaerobic respiration being insufficientto meet energy requirements. Key words: Grape, energy metabolism, anaerobic stress, aeration, ethanol production     

Translocation into mature leaves

Vigna sesquipedalis Fruw. was used to investigate whether translocationcould take place into leaves older than the source. In controlplants translocation was from the third to fifth leaf only,but when the second leaf was infested with Aphis craccivoraKoch there was movement also into that leaf. (Received July 5, 1973; )     

Role of water and electrolyte influxes in anoxic plasma membrane disruption

The role of water and electrolyte influxes in anoxia-inducedplasma membrane disruption was investigated using rabbit proximal tubule suspension. The results indicated that normal proximal tubule(PT) cells have a great capacity for expanding cell volume in responseto water influx, whereas anoxia increases the susceptibility to waterinflux-induced disruption, and this was attenuated by glycine. However,resistance of anoxic plasma membranes to water influx-induced stress isnot lost, although their mechanical strength was diminished, comparedwith normoxic membranes. Anoxic membranes did not disrupt under anintra-to-extracellular osmotic difference as great as 150 mosM.Potentiating or attenuating water influx by incubating PT cells inhypotonic or hypertonic medium, respectively, during anoxia, did notaffect anoxia-induced membrane disruption. After the transmembraneelectrolyte concentration gradient was eliminated by a"intracellular" buffer or by permeabilizing the plasma membraneto molecules <4 kDa using -toxin, anoxia still caused furthermembrane disruption that was prevented by glycine or low pH. Theseresults demonstrate that 1) water ornet electrolyte influxes are probably not a primary cause foranoxia-induced membrane disruption and2) glycine could prevent the plasmamembrane disruption during anoxia independently from its effect ontransmembrane electrolyte or water influxes. The present data support abiochemical rather than a mechanical alteration of the plasma membraneas the underlying cause of membrane disruption during anoxia.

     

THE RELATIONSHIP OF GIBBERELLIN AND AUXIN IN PLANT GROWTH

No synergism was found between IAA and gibberellin in the Avenucurvature test and this bioassay thus measures changes in diffusibleauxin resulting from gibberellin treatment and not a synergisticaction of the gibberellin on the curvature response to auxin.Gibberellin treatment causes an increase in diffusible auxinfrom the stem apex of dwarf pea (Pisum sativum L. var. LittleMarvel) 24 to 48 hours before the elongation response in thestem. The increase in diffusible auxin in the stem apex of Centaureacyanus L. var. Blue Boy occurs four to six days before the boltingresponse to gibberellin treatment under short days. The stemtissues of both the dwarf pea and Centaurea show an elongationresponse to IAA when the IAA is applied in a manner simulatingthe stem apex. Thus the growth of the dwarf pea and the boltingof Centaurea brought about by treatment with gibberellin aredependent on an increase in diffusible auxin. ¹Present address: Biological Institute, College of General Education,University of Tokyo, Komaba, Meguro, Tokyo.     

Translocation in Plants Possessing Supernumerary Phloem: II. THE INCLUDED PHLOEM OF BOUGAINVILLEA GLABRA CHOISY AND THE BICOLATERAL BUNDLES OF CUCUMBER (CUCUMIS SATIVUS L.)

The translocation path in the included phloem of Bougainvilleaand in the bicolateral bundles of cucumber was studied by exposingyoung branches to ¹⁴CO₂ and detecting the radioactive compoundsby autoradiography. In Bougainvillea, the structure and functionof the phloem system is comparatively uncomplicated and uniform.All phloem bundles, i. e. those which are located in parenchymatoustissue of the central zone and those embedded in the secondaryxylem, become labelled. Exogenous IAA was translocated in thebundles, but the exact mode of translocation was not ascertained.Apical dominance was not affected by girdling. The implicationof this fact is discussed with respect to the translocationof the auxin that determines the correlative inhibition involvedin apical dominance. In cucumber the inner phloem became labelled throughout theplant to a lesser extent than the outer phloem. However, inthe petiole of the assimilating leaf the intensity of the labelwas the same in both inner and outer phloem, although the innerphloem has fewer elements. Below the treated leaf the innerphloem translocated less than the outer phloem. Above this leafthe inner phloem was entirely unlabelled     

The Response of Groundnut (Arachis hypogaea L.) to Timing of Irrigation: II. 14C-PARTITIONING AND PLANT WATER STATUS

Finite quantities of water were applied at different growthstages of groundnut stands (Arachis hypogaea L.) grown in controlledenvironment glasshouses. Soil moisture deficits were imposedbetween sowing and pod initiation or between pod initiationand final harvest by withholding or applying water. Effectson assimilate production and partitioning and plant water relationswere examined. Leaves were the primary sites of ¹⁴CO₂ fixation, though theircontribution generally declined late in the season, whereasfixation by stems was initially low but increased sharply whenstress was released in the late-irrigated stands. ¹⁴C-fixationby stem apices and pegs also rose sharply following irrigationof the late-stressed stands. Leaves were the primary source of assimilates, though translocationtended to decrease as the season progressed, even in the late-irrigatedstands. Stems were initially the major sinks, but their sinkactivity disappeared almost completely when stress was releasedin the late-irrigated stands. Assimilate import by stem apicesdeclined progressively and pod sink activity was negligiblein the late-stressed stand, but both increased markedly whenearly-season stress was released. Leaf water status showed marked diurnal variation, whereas pegsshowed less variation and maintained much higher turgor levels,largely because of their lower solute potentials. Marked osmoticadjustment occurred in expanding but not in mature leaves, allowingthem to maintain higher turgor levels during periods of severestress. This adjustment was rapidly lost when stress was released.The observed changes in assimilate production and partitioningpreceded detectable changes in bulk turgor levels. Implications for growth, development and yield are discussed. Key words: Groundnut, irrigation, partitioning, water status     

Modulation of stress proteins and apoptotic regulators in the anoxia tolerant turtle brain

Freshwater turtles survive prolonged anoxia and reoxygenation without overt brain damage by well-described physiological processes, but little work has been done to investigate the molecular changes associated with anoxic survival. We examined stress proteins and apoptotic regulators in the turtle during early (1 h) and long-term anoxia (4, 24 h) and reoxygenation. Western blot analyses showed changes within the first hour of anoxia; multiple stress proteins (Hsp72, Grp94, Hsp60, Hsp27, and HO-1) increased while apoptotic regulators (Bcl-2 and Bax) decreased. Levels of the ER stress protein Grp78 were unchanged. Stress proteins remained elevated in long-term anoxia while the Bcl-2/Bax ratio was unaltered. No changes in cleaved caspase 3 levels were observed during anoxia while apoptosis inducing factor increased significantly. Furthermore, we found no evidence for the anoxic translocation of Bax from the cytosol to mitochondria, nor movement of apoptosis inducing factor between the mitochondria and nucleus. Reoxygenation did not lead to further increases in stress proteins or apoptotic regulators except for HO-1. The apparent protection against cell damage was corroborated with immunohistochemistry, which indicated no overt damage in the turtle brain subjected to anoxia and reoxygenation. The results suggest that molecular adaptations enhance pro-survival mechanisms and suppress apoptotic pathways to confer anoxia tolerance in freshwater turtles.     

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

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

Relationship between Coleoptile Elongation and Alcoholic Fermentation in Rice Exposed to Anoxia. II. Cultivar Differences

The relationship between coleoptile elongation and survivalvs. alcoholic fermentation of rice under anoxia is examinedusing eight cultivars differing in submergence tolerance. Anoxiawas imposed on either 1 or 4 d aerated seeds either by N₂ flushingsubmerged tissues or by incubating tissues in stagnant deoxygenatedagar at 0·1% w/v; the latter simulated the stagnant conditionsof waterlogged soil. Two cultivars that were most tolerant tosubmergence also had the greatest tolerance to anoxia, whilea submergence intolerant cultivar was also intolerant to anoxia. Coleoptile growth under anoxia was related to rates of ethanolsynthesis (R_E), however differences between growth during anoxiaand survival after anoxia indicated that post-anoxic injurymay also be important in rice seeds exposed to anoxia. The correlationbetween coleoptile growth and R_E measured on a tissue basisusing intact seeds was r² = 0·67 among six varietiesover 0-3 d anoxia. This correlation improved to about r² = 0·85using R_E of (embryos plus coleoptiles) over 0-3 d, or coleoptilesat 3 d after anoxia. Coleoptile growth of individual seeds wasusually poorly correlated to R_E in these cultivars at 2-3 dafter anoxia. When coleoptiles of similar lengths were obtainedfrom different cultivars using 4 d aerated seeds, there weredifferences in R_E and coleoptile growth which were related tocoleoptile growth during 3 or 5 d anoxia, either on a tissue(r² = 0·85) or a fresh weight basis (r² = 0·70-0·97respectively). Results are discussed in relation to factorswhich may limit ethanol synthesis in rice exposed to anoxiaand the importance of growth to the survival of seeds and matureplants during submergence in the natural environment.Copyright1994, 1999 Academic Press Anoxia, ethanol, alcoholic fermentation, Oryza sativa L., rice, submergence     

Translocation Profiles of [11C] and [13N]-Labelled Metabolites after Assimilation of 11CO2 and [13N]-Labelled Ammonia Gas by Leaves of Helianthus annuus L. and Lupinus albus L.

Tracer amounts of atmospheric [¹³N]-Iabelled ammonia gas, wereabsorbed by leaves of Lupinus albus and Helianthus annuus inboth the light and the dark. Exogenous [¹³N]-ammonia was onlyabsorbed in the dark when the feeding occurred shortly aftera period of illumination and the tissue was not depleted ofits carbohydrate reserves (e.g. starch). Incorporation of the[¹³N]-ammonia appeared to occur via the leaf glutamine synthetase/glutamatesynthase (GS/GOGAT) cycle since 2.0 mol m^–3 MSX, an inhibitorof the GS reduced uptake in both the light and dark. Photosyntheticincorporation of ¹¹CO₂ was not affected by this treatment Therate of movement of [¹³N]-assimilates in the petiole of attachedleaves of Helianthus and Lupinus was similar to that of the¹¹Cl-photo assimilates. Export of both [¹³N] and [¹¹C]-Iabelledassimilates from the leaf and movement in the petiole in boththe light and the dark was inhibited by source leaf anoxia (i.e.nitrogen gas). Translocation was re-established at the samerate when the feed leaf was exposed to gas containing more than2% O₂ which permitted dark respiration to proceed. After aninitial feeding of either ¹¹CO₂ or [¹³N]-ammonia at ambient(21%) O₂ exposure of the source leaf to 2% O2, or 50% O² didnot alter the rates of translocation, indicating that changesin photosynthetic activity in the source leaf due to photorespiratoryactivity need not markedly alter, at least during the shortperiod, the loading and translocation of either [¹¹C ] or [¹³N]-labelledleaf products. Key words: Translocation, CO₂, NH₃, Leaves, Helianthus annuus, Lupinus albus     

The calanoid copepod Acartia italica Steuer, phenomenon in the small saline Lake Rogoznica (Eastern Adriatic coast)

Lake Rogoznica, near ibenik, Croatia, is a salt coastal lakewith a maximum depth of about 15 m. During most of the year,this small, naturally eutrophied lake is highly stratified,with hypoxia/anoxia occurring in the bottom layer. Total anoxia,with the presence of hydrogen sulphide, was recorded on September27, 1997. At that moment, massive mortality of all planktonicand benthic organisms was observed. After the anoxia, Acartiaitalica, the only planktonic copepod species of the lake, re-establishedquickly. Before anoxia, the population dynamics of the A.italicapopulation depended primarily on predator/prey relationships.However, in the post-anoxic period, nutrient–phytoplankton–copepodrelationships became more important, as there was no longerany predation pressure. From all accounts, it would appear thatonly A.italica is adapted to the extreme conditions which appearfrom time to time in the lake, and that this species has animportant role in the functioning of the lake ecosystem. Acartiaitalica was very important for the gradual normalization oflife in the lake after total anoxia.     

Translocation of Carbohydrate in Cotton: Movement to the Fruiting Bodies

The translocation of ¹⁴C-labelled assimilate was followed frommain stem leaves on the cotton plant (Gossypium hirsutum) tobolls on the lower sympodia. From the upper leaves the assimilatedescends the stem in well-defined strands in the phloem. Wheresympodia originate close to these strands some of the assimilateis distributed to the bolls on those sympodia. There is evidencethat the involucre, but not the green boll wall, contributesto boll nutrition by assimilation of external CO₂.     

Developmental regulation of anoxic stress tolerance in maize

Anoxia associated with flooding stress is detrimental to plant growth and productivity. When maize seedlings 2 to 7 d old were exposed to anoxic stress, 3-d-old seedlings were found to have much lower tolerance than 2-d-old seedlings. Ninety per cent of 2-d-old seedlings survived 72 h of anoxic stress compared with 0% of the 3-d-old seedlings. Since 2-d-old isolated root tips survived anoxic stress better than 3-d-old tips, the anoxic tolerance of 2-d-old seedlings was independent of the translocation of nutrient reserves from the endosperm to the root. The addition of glucose to the medium improved the anoxia tolerance of 2-d-old seedlings by 25% but had no effect on 3-d-old seedlings. Acclimation by pre-cxposure to 4% oxygen and pre-treatment with 100mmol m^?1 abscisic acid (ABA) improved the anoxia tolerance of 3-d-old seedlings by 2- and 4-fold, respectively. However, acclimation and ABA treatment had no effect on 2-d-old seedlings. The results indicate that anoxia tolerance in maize is develop-mentally regulated. The mechanism of anoxia tolerance innate to 2-d-old seedlings was inducible in 3-d-old seedlings by acclimation or treatment with ABA.     

Studies on the Expansion of the Leaf Surface: IV. THE CARBON AND PHOSPHORUS ECONOMY OF A LEAF

The fixation, utilization, and translocation of carbon and thenet import and export of phosphorus by three leaves of Cucumissativus over the course of their lives were measured in a controlledenvironment. The rate of photosynthesis of a leaf followed a regular dailypattern, rising to a maximum during the first 2 hrs. of thelight period and subsequently falling. Dark respiration wasusually highest at the beginning of the dark period and fellthroughout it. The daily rate of photosynthesis per unit areaof a leaf fell during its later life partly as a result of shadingby upper leaves and also because of an independent age factor.The rate of dark respiration per unit area was high in veryyoung leaves but fell rapidly with age. The amount of phosphorus in each leaf reached a maximum beforethe leaf had reached its maximum dry weight. There was thensubstantial net loss of phosphorus from the leaf. The changing function of each leaf as a sink or source of carbohydrateand mineral nutrients was determined. Four stages were recognized:(1) early development from inception until some time after unfolding,when the leaf was dependent upon imported carbohydate; (2) aperiod of rapid expansion, associated with a high rate of uptakeor mineral nutrients, during which translocation of assimilatedcarbon from the leaf was most rapid; (3) a time of decliningrates of growth, photosynthesis and export of carbon, associatedwith substantial loss of phosphorus; (4) finally, a short sensescentphase with net loss of CO₂, terminating in the death of theleaf.