Transport processes in stimulated and non-stimulated leaves of Mimosa pudica

Summary Using energy-dispersive X-ray microanalysis, the concentrations of ions, especially potassium and chlorine, were determined in different tissues of primary and tertiary pulvini of Mimosa pudica. It was shown that stimulating the leaf was followed by ion displacements which were most striking in the outer extensor cells, resulting in turgor loss. Since Ca concentration remains relatively constant in cell walls of collapsed cells, the changes of K concentration are best described by the K:Ca ratio. After stimulation the K:Ca ratio dropped in the outer extensor of the primary pulvinus from 775.3 to 2.37 in the cytoplasm, and from 542.2 to 9.25 in the cell wall. Changes in chlorine content were less striking in the primary pulvinus. The KCl ratios in some cases were lower than 1.0, which indicates that Cl content can increase, while K content is diminished. In the non-stimulated tertiary pulvini the outer extensor cells show high concentrations of Cl, but much lower Cl concentrations were found after stimulation. In contrast to the primary pulvinus the K content of the tertiary pulvini is very low. In the vascular tissues of both primary and tertiary pulvini stimulation is followed by a release of K and Cl out of the sieve element cytoplasm into the apoplast. K then appears accumulated in the cell walls of the collenchymatous tissue. These displacements lead to the assumption that the collenchymatous apoplast temporarily functions as a reservoir for K and to a lesser extent for Cl. With regard to the mechanism of leaf movement after stimulation, the accumulation of ions in the apoplast seems to be initiated by the decrease of water potential triggered by an apoplastic accumulation of unloaded sucrose (Fromm and Eschrich 1988a). The resulting turgor release in the outer extensor is accompanied by an efflux of ions.Supported by the Deutsche Forschungsgemeinschaft     

Specialized formations of endoplasmic reticulum in parenchyma cells ofMimosa pudica L.

Summary Parenchyma cells ofMimosa pudica display close associations between two or more cisternae of the endoplasmic reticulum. These associations form simplified types of lamellar bodies in which inner paired lamellae have lost their ribonucleoprotein granules and are separated by a dense layer.     

Plasmalemmal,voltage-dependent ionic currents from excitable pulvinar motor cells ofMimosa pudica

Summary Plasmalemmal ionic currents from excitable motor cells of the primary pulvinus ofMimosa pudica were investigated by patch-clamp techniques. In almost all of the enzymatically isolated protoplasts, a delayed rectifier potassium current was activated by depolarization, while no currents were detected upon hyperpolarization. This sustained outward current was reversibly blocked by Ba and TEA and serves to repolarize the membrane potential. Outward single channel currents that very likely underly the macroscopic outward potassium current had an elementary conductance of 20 pS. In addition, in a few protoplasts held at hyperpolarized potentials, depolarization-activated transient inward currents were observed, and under current clamp, action potential-like responses were triggered by depolarizing current injections or by mechanical perturbations. The activation characteristics of both inward currents and spikes showed striking similarities compared to those of action potentialsin situ.     

Phloem unloading following reactivation in predarkened mature maize leaves

Mature leaf blades of 48-h predarkened maize plants (Zea mays L. cv. Prior) were excised, and treated apically as the source (light, normal air) and basally as the sink (light or dark, air without CO₂). After providing the source portion with ¹⁴CO₂, the sink portions were harvested after 2, 7 or 14 h by freezing with liquid nitrogen, grinding, and freeze-drying. Extracts, fractionated by ionexchange resins into neutral, basic and acid fractions, were chromatographed on thin cellulose layers, and autoradiographed. Identification of labeled compounds was carried out by co-chromatography with authentic labeled substances. Activities of enzymes pertaining to the metabolism of sucrose were checked. Results show that the source supplies sucrose to the sink, where it is unloaded and metabolized by acid invertase (EC 3.2.1.26) in both the light and the dark. Starch appearing in the sink only in the light, after 7 h of re-illumination, yields labeled glucose upon hydrolysis. Although sucrose-phosphate synthetase (EC 2.4.1.14) is active in sinks and in isolated vascular-bundle fragments, it remains questionable whether sucrose unloaded from sieve tubes is metabolized by a method other than inversion. Sucrose synthetase (EC 2.4.1.13) was found to be inactive. Obviously, the main metabolite of unloaded sucrose is glucose-6-phosphate, giving access to the glycolytic pathway. The main difference between the sinks in the light and the dark is the lack of labeled glycine and serine in the dark. This indicates that in the light decarboxylation of glycine yields CO₂, which is recycled photosynthetically.Abbrevations Glc1P glucose-1-phosphate - Glc6P glucose-6-phosphate - TLC thin-layer chromatography - UDPGlc uridine 5-diphosphate glucose     

Reactivation of phloem export in mature maize leaves after a dark period

Mature leaves of corn plants (Zea mays L. cv. Prior) which were darkened for 48 h contain neither bundle-sheath starch nor glucose, and their sucrose content is below 5 M. In such leaves phloem export has ceased. When re-illuminated, photosynthetic sucrose production starts without delay, but the sucrose: glucose ratio is 1.25:1. Obviously, most of the new-formed sugar is utilized locally. Labeling with ¹⁴CO₂ has shown that phloen export starts 30 to 40 min after the onset of photosynthesis, when the sucrose: glucose ratio has increased to 13:1. The first newly formed starch can be detected when phloem export is reactivated. Glucose content remains constantly low af about 2 M for at least 2 h, and it never exceeds 10 M. Radioactivity in the exporting veins is about five times higher after 2 to 7 h of re-illumination than in the 14-h-day plant. Therefore, phloem export is either intensified during the period of reactivation or exported assimilates are partly unloaded along their way. Comparison of photosynthetic activity of equal-sized leaf strips has shown that both accumulation of photosynthates and radioactivity of exporting veins are about three times higher in the detached strip than in the strip which remained attached to the mother plant.     

Photoassimilate-transport characteristics of nonchlorophyllous and green tissue in variegated leaves of Coleus blumei Benth

The nonchlorophyllous (albino) tissue of mature C. blumei leaves is a sink for photoassimilate. Transport from the green to the albino region of the same leaf was inhibited by cold and anoxia. When the green tissue of mature leaves was removed, the remaining albino portion imported labeled translocate from other mature leaves in the phloem. Photoassimilate unloading in the albino region of mature leaves was studied by quantitative autoradiography. The unloading was inhibited by cold but not by anoxia. No labeled photoassimilate could be detected in the free space of mature albino tissue by compartmental efflux analysis as phloem unloading proceeded in a N₂ atmosphere, indicating that unloading, may occur by a symplastic pathway as it apparently does in sink leaves of other species. The minor veins of mature albino leaf tissue did not accumulate exogenous [¹⁴C]sucrose. Minor veins of green tissue in the same leaves accumulated [¹⁴C]sucrose but, in contrast to other species studied to date, this accumulation was insensitive to the inhibitor p-chloromercuribenzensulfonic acid (PCMBS).In its capacity to import and unload photoassimilate, and in the inability, of the minor veins to accumulate exogenous sucrose, the albino region of the mature C. blumei lamina differs from mature albino tobacco leaves and darkened mature leaves of other species. This, together with evidence indicating that phloem loading in C. blumei and other species may occur by different routes and with different sensitivity to PCMBS, indicates that the mechanism of transfer of photoassimilates between veins and surrounding tissues, and the mechanism of the sink-source transition, may not be the same in the leaves of all species. It is speculated that the unusual properties of the C. blumei leaf may be a consequence of the presence, in the minor veins, of intermediary cells, large companion cells connected to the bundle sheath by abundant plasmodesmata.Abbreviation PCMBS p-chloromercuribenzenesulfonic acid     

A morphometric analysis of the phloem-unloading pathway in developing tobacco leaves

A morphometric analysis of developing leaves of Nicotiana tabacum L. was conducted to determine whether imported photoassimilates could be unloaded by symplastic transport and whether interruption of symplastic transport could account for termination of import. Five classes of veins were recognized, based on numbers of cells in transverse section. Photoassimilate is unloaded primarily from Class III veins in tissue nearing the end of the sink phase of development. Smaller veins (Class IV and V) do not transport or unload photoassimilate in sink tissue because the sieve elements of these veins are immature until after the tissue stops importing. In Class III veins the sieve element-companion cell (SE-CC) complexes are surrounded by phloem parenchyma which abuts the bundle sheath. Along the most obvious unloading route, from SE-CC complex to phloem parenchyma to bundle sheath to mesophyll cells, the frequency of plasmodesmata at each interface increases. To determine whether this pattern of plasmodesmatal contact is consistent with symplastic unloading we first demonstrated, by derivation from Fick's law that the rate of diffusion from a compartment is proportional to a number N which is equal to the ratio of surface area to volume of the compartment multiplied by the frequency of pores (plasmodesmata) which connect it to the next compartment. N was calculated for each compartment within the vein which has the SE-CC complex as its center, and was shown to be statistically the same in all cases except one. These observations are consistent with a symplastic unloading route. As the leaf tissue matures and stops importing, plasmodesmatal frequency along the unloading route decreases and contact area between cells also decreases as intercellular spaces enlarge. As a result, the number of plasmodesmata between the SE-CC complex and the first layer of mesophyll cells declines in nonimporting tissue to 34% of the number found in importing tissue, indicating that loss of symplastic continuity between the phloem and surrounding cells plays a role in termination of photoassimilate unloading.Abbreviation SE-CC sieve element-companion cell     

Phloem unloading in tobacco sink leaves: insensitivity to anoxia indicates a symplastic pathway

Phloem unloading in transition sink leaves of tobacco (Nicotiana tabacum L.) was analyzed by quantitative autoradiography. Detectable levels of labeled photoassimilates entered sink leaves approx. 1 h after source leaves were provided with ¹⁴CO₂. Samples of tissue were removed from sink leaves when label was first detected and further samples were taken at the end of an experimental phloem-unloading period. The amount of label in veins and in surrounding cells was determined by microdensitometry of autoradiographs using a microspectrophotometer. Photoassimilate unloaded from first-, second-and third-order veins but not from smaller veins. Import termination in individual veins was gradual. Import by the sink leaf was completely inhibited by exposing the sink leaf to anaerobic conditions, by placing the entire plant in the cold, or by steam-girdling the sink-leaf petiole. Phloem unloading was completely inhibited by cold; however, phloem unloading continued when the sink-leaf petiole was steam girdled or when the sink leaf was exposed to a N₂ atmosphere. Compartmental efflux-analysis indicated that only a small percentage of labeled nutrients was present in the free space after unloading from sink-leaf veins in a N₂ atmosphere. The results are consistent with passive symplastic transfer of photoassimilates from phloem to surrounding cells.Symbol VI radio of ¹⁴C in veins and interveinal tissue     

Chloride ion efflux during an action potential in the main pulvinus ofMimosa pudica

The shortening, action potential and Cl⁻-efflux of the excised lower half cortex in the main pulvinus ofMimosa pudica were simultaneously recorded. The mean values±(S.E.) for Cl⁻-efflux and shortening were 183±18 picomoles/mg fresh weight/impulse and 87.0±2.2 μm, respectively.     

Phloem turgor and the regulation of sucrose loading in Ricinus communis L.

The influence of plant water relations on phloem loading was studied in Ricinus communis L. Phloem transport was maintained in response to bark incisions even at severe water deficits. Water stress was associated with a net increase in the solute content of the sieve tubes, which resulted in maintenance of a positive phloem turgor pressure _p. There was a significant increase in solute flux through the phloem with decreasing xylem water potential (). In addition, sugar uptake by leaf discs was examined in media adjusted to different water potentials with either sorbitol (a relatively impermeant solute) or ethylene glycol (a relatively permeant solute). The limitations in this experimental system are discussed. The results nevertheless indicated that sucrose uptake can be stimulated by a reduction in cell _p, but that it is little affected by cell or solute potential _s. On the basis of these data we suggest that sucrose loading is turgor-pressure dependent. This may provide the mechanism by which transport responds to changes in sink demand in the whole plant.Abbreviations water potential - _s solute potential - _p pressure potential     

The shortening and action potential of the cortex in the main pulvinus ofMimosa pudica

The shortening and action potential of the upper and lower cortices of the main pulvinus ofMimosa pudica were recorded simultaneously. The shortening and action potential were observed only in the lower cortex. The extensibility and the excitability of the cortex are discussed.     

Production and diurnal utilization of assimilates in leaves of spinach (Spinacia oleracea L.) and barley (Hordeum vulgare L.)

Rates of CO₂ fixation during the light period and the rates of CO₂ release during the night period were measured using mature leaves from 39- to 49-d-old spinach (Spinacia oleracea L., US Hybrid 424; grown in 9 h light, 15 h darkness, daily) and mature leaves from 21-d-old barley (Hordeum vulgare L., cv. Apex; grown in 14 h light, 10 h darkness, daily). At certain times during the light and dark periods leaves were harvested for assay of their contents of soluble carbohydrates, starch, malate and the various amino acids. Evaluation of the results of these measurements shows that in spinach and barley leaves 46% and 26%, respectively, of the carbon assimilated during the light period is deposited in the leaves for export during the night period. Taking into account the carbon consumption in the source leaves by dark respiration, it is evaluated that rates of assimilate export during the light period from spinach and barley leaves [38 and 42 atom C · (mg Chl)^–1 · h^–1] are reduced in the dark period to 16 atom C · (mg Chl)^–1 · h^–1 in both species. The calculated C/N ratios of the photoassimilates exported during the dark period were 0.029 and 0.015 for spinach and barley leaves, respectively.This work was supported by the Deutsche Forschungsgemeinschaft. We thank Dr. Dieter Heineke for stimulating discussions and Mrs. Petra Hoferichter and Mrs. Marita Feldkämper for their technical assistance.     

Plasmodesmal widening accompanies the short-term increase in symplasmic phloem unloading in pea root tips under osmotic stress

Summary Root tips ofPisum sativum seedlings were exposed to 350 mM mannitol, which was shown to effect a transient but dramatic increase in phloem unloading, and investigated by electron microscopy. After chemical fixation and embedding, extremely thin sections of the root extension zone were examined. Outer, inner, and desmotubule diameters of 830 primary plasmodesmata in transverse walls of cortical cells were measured. Statistical analysis indicated that the majority of plasmodesmata had no neck constriction during osmoregulation. Compared to controls, a highly significant increase in mean plasmodesmata diameter was found, but the desmotubule diameter remained unchanged. Both loss of neck constriction and widening of the cytoplasmic sleeve indicate an increase in effective passage area of plasmodesmata. Spokes between plasma membrane and desmotubule were preserved. Continued exposure of the root tips to mannitol led to a return to control values for plasmodesmal diameters. In contrast to these responses, plasmolysis of cortical cells by 1,000 mM sucrose, diminishing phloem unloading, was accompanied by a reduction in those plasmodesmata classified as open. This is the first report showing a correlation between the ultrastructure of plasmodesmata and the rate of symplasmic transport. The role of the different plasmodesmal components in controlling the passage area of symplasmic transport is discussed.     

Ultrastructure,plasmodesmatal frequency,and solute concentration in green areas of variegated Coleus blumei Benth. leaves

The photosynthetic tissue of green portions of variegated Coleus blumei leaves consists primarily of palisade and spongy parenchyma cells as well as bundle-sheath cells. The moderate numbers of plasmodesmata connecting these cells may be sufficient to provide a symplastic pathway for assimilates moving toward the minor veins. The minor veins, however, are unusual in having two sets of phloem-loading cells which have little symplastic continuity with one another: one consisting of large, peripherally located intermediary cells, and a second set made up of smaller, usually more internal companion cells, both sets having their associated sieve-tube members. The intermediary cells are connected to vascular-parenchyma and bundle-sheath cells by unique branched plasmodesmata which are particularly abundant at the bundle-sheath interface. In addition, numerous plasmodesmata-pore connections occur between the intermediary cells and their associated sieve-tube members. Neither the intermediary cells nor their sieve-tube members plasmolyze when treated with 1.4 M mannitol, whereas mesophyll and vascular-parenchyma cells plasmolyze at 0.5 M and bundle-sheath cells at 0.6 M mannitol. By contrast, the companion cells and their associated sieve-tube members are symplastically isolated from the bundle-sheath cells and the sieve-tube-intermediary-cell complexes, and share few plasmodesmata with the vascular-parenchyma cells. Moreover, the companion cells plasmolyze at 1.1 M mannitol and their sieve tubes at 1.3 M. The intermediary-cell-sieve-tube complex thus appears to be structurally equipped to load assimilates entirely via the symplast, while the sieve-tube-companion-cell complex is probably loaded from the apoplast.Abbreviation ER endoplasmic reticulum     

Raffinose oligosaccharide concentrations measured in individual cell and tissue types in Cucumis melo L. leaves: implications for phloem loading

Raffinose, stachyose, and galactinol are synthesized in intermediary cells (specialized companion cells) of the minor-vein phloem of cucurbits. To better understand the role of these carbohydrates and the regulation of their synthesis and transport, we measured the concentrations of each of the components of the raffinose oligosaccharide synthetic pathway in mesophyll and sieve element-intermediary cell complexes (SE-ICCs) in the leaves of melon (Cucumis melo L. cv. Hale's Best Jumbo). These concentrations are consistent with a polymer-trapping mechanism for phloem loading, with sucrose diffusing from mesophyll into intermediary cells and being made into raffinose and stachyose, which are too large to diffuse back to the mesophyll. To determine carbohydrate concentrations, we developed a method involving microdissected tissues. Blind endings of areoles, and mesophyll surrounding these veins, were separately removed from lyophilized leaf tissue. Carbohydrates were quantitated by high-performance liquid chromatography with pulsed amperometric detection. A small amount of mesophyll remained attached to the blind endings; the carbohydrate contribution of these cells to the vein sample was eliminated by subtraction, based on the amount of chlorophyll. Volumes of cells and subcellular compartments were calculated by morphometric analysis and were used to calculate carbohydrate concentrations. Assuming no subcellular compartmentation, the additive concentration of sugars in the SE-ICCs of minor veins is about 600 mM. Stachyose and raffinose concentrations are about 330 mM and 70 mM, respectively, in SE-ICCs; concentrations of these sugars are much lower in mesophyll (0.2 and 0.1 mM). This is consistent with the view that stachyose and raffinose are unable to pass through the plasmodesmata between intermediary cells and bundle-sheath cells. Sucrose levels appear to be higher in the SE-ICC (about 130mM) than in the mesophyll (about 10 mM), but if compartmentation is taken into account the gradient for sucrose is probably downhill from mesophyll to intermediary cells. Flux through plasmodesmata between the bundle sheath and intermediary cells was calculated and was found to be within the range of values of flux through plasmodesmata reported in the literature.Abbreviations BS-IC bundle sheath-intermediary cell - PC plasmodesmatal channel - SE-ICC sieve element-intermediary cell complex - SEL size exclusion limit We would like to thank Gayle Volk, Philip Laible, Canan Inan, Esther Gowan, Richard Medville, Nathan Wilson, Jessica Plant, and Steven Boese for their help, Thomas Owens, M.V. Parthasarathy, and Ian Merwin for use of equipment, and Nancy Haritatos for suggestions. This research was supported by U.S. Department of Agriculture Competitive Grant 94-37306-0351 (R.T.), the Swiss National Foundation (F.K.), and a NSF/DOE/USDA Cornell Plant Science Center fellowship (E.H.).     

Wall yield threshold and effective turgor in growing bean leaves

The rate of cell enlargement depends on cell-wall extensibility (m) and on the amount of turgor pressure (P) which exceeds the wall yield threshold (Y). The difference (P-Y) is the growth-effective turgor (P _e). Values of P, Y and P _ehave been measured in growing bean (Phaseolus vulgaris L.) leaves with an isopiestic psychrometer, using the stress-relaxation method to derive Y. When rapid leaf growth is initiated by light, P, Y and P _eall decrease. Thereafter, while the growth rate declines in maturing leaves, Y continues to decrease and P _eactually increases. These data confirm earlier results indicating that the changes in light-stimulated leaf growth rate are primarily controlled by changes in m, and not by changes in P _e. Seedlings incubated at 100% relative humidity have increased P, but this treatment does not increase growth rate. In some cases Y changes in parallel with P, so that P _eremains unchanged. These data point out the importance of determining P _e, rather than just P, when relating cell turgor to the growth rate.Abbreviations and symbols FC fusicoccin - m wall extensibility - P turgor pressure - P _e effective turgor - RH relative humidity - Y yield threshold - _w water potential - _s osmotic potential     

Osmoregulation and the control of phloem-sap composition in Ricinus communis L.

Phloem-sap composition was studied in plants of Ricinus communis L. grown on a waterculture medium. The sap possessed a relatively high K⁺:Na⁺ ratio and low levels of Ca²⁺ and free H⁺. Sucrose and K⁺ (together with its associated anions) accounted for 75% of the phloem-sap solute potential (_s). In plants kept in continuous darkness, a decrease in phloem-sap sucrose levels over 24h was accompanied by an increase in K⁺ levels. Measurements of phloem-sap _s and xylem water potential () indicated that this resulted in a partial maintenance of phloem turgor pressure _p. In darkness there was also a marked decrease in the malate content of the leaf tissue, and it is possible that organic carbon from this source was mobilized for export in the phloem. The results support the concept of the phloem sap as a symplastic phase. We interpret the increase in K⁺ levels in the phloem in darkness as an osmoregulatory response to conditions of restricted solute availability. This reponse can be explained on the basis of the sucrose-H⁺ co-transport mechanism of phloem loading.Abbreviations water potential - _s solute potential - _p pressure potential     

Distribution and frequency of plasmodesmata in relation to photoassimilate pathways and phloem loading in the barley leaf

Large, intermediate, and small bundles and contiguous tissues of the leaf blade of Hordeum tvulgare L. ‘Morex’ were examined with the transmission electron microscope to determine their cellular composition and the distribution and frequency of the plasmodesmata between the various cell combinations. Plasmodesmata are abundant at the mesophyll/parenchymatous bundle sheath, parenchymatous bundle sheath/mestome sheath, and mestome sheath/vascular parenchyma cell interfaces. Within the bundles, plasmodesmata are also abundant between vascular parenchyma cells, which occupy most of the interface between the sieve tube-companion cell complexes and the mestome sheath. Other vascular parenchyma cells commonly separate the thick-walled sieve tubes from the sieve tube-companion cell complexes. Plasmodesmatal frequencies between all remaining cell combinations of the vascular tissues are very low, even between the thin-walled sieve tubes and their associated companion cells. Both the sieve tube-companion cell complexes and the thick-walled sieve tubes, which lack companion cells, are virtually isolated symplastically from the rest of the leaf. Data on plamodesmatal frequency between protophloem sieve tubes and other cell types in intermediate and large bundles indicate that they (and their associated companion cells, when present) are also isolated symplastically from the rest of the leaf. Collectively, these data indicate that both phloem loading and unloading in the barley leaf involve apoplastic mechanisms.     

Effects of glycine on dark-and ligh-induced pulvinar movements and modifications of proton fluxes in the pulvinus of Mimosa pudica during glycine uptake

Glycine (1–50 mM) increases the rate of the dark-induced (scotonastic) movements and decreases the amplitude and the rate of the light-induced (photonastic) movements of the secondary pulvini of Mimosa pudica leaves. The uptake of glycine is accompanied by a long-lasting dose-dependent increase in the alkalinity of the bathing medium of the excised pulvini. The data are in agreement with a H⁺-glycine co-transport mechanism within the pulvinar cells. Fusicoccin (50 M), known to promote H⁺–K⁺ exchange, antagonizes the effects of glycine on the movements and the alkalization of the bathing medium of the excised pulvini. The present results argue for the hypothesis that proton fluxes mediate the scotonastic and photonastic pulvinar movements.Abbreviations Gly glycine - FC fusicoccin - P₁ primary pulvinus - P₂ secondary pulvinus     

Control of photosynthesis by the carbohydrate level in leaves of the C4 plant Amaranthus edulis L.

Photosynthesis was studied in relation to the carbohydrate status in intact leaves of the C₄ plant Amaranthus edulis. The rate of leaf net CO₂ assimilation, stomatal conductance and intercellular partial pressure of CO₂ remained constant or showed little decline towards the end of an 8-h period of illumination in ambient air (340 bar CO₂, 21% O₂). When sucrose export from the leaf was inhibited by applying a 4-h cold-block treatment (1°C) to the petiole, the rate of photosynthesis rapidly decreased with time. After the removal of the cold block from the petiole, further reduction in photosynthetic rate occurred, and there was no recovery in the subsequent light period. Although stomatal conductance declined with time, intercellular CO₂ partial pressure remained relatively constant, indicating that the inhibition of photosynthesis was not primarily caused by changes in stomatal aperture. Analysis of the leaf carbohydrate status showed a five- to sixfold increase in the soluble sugar fraction (mainly sucrose) in comparison with the untreated controls, whereas the starch content was the same. Leaf osmotic potential increased significantly with the accumulation of soluble sugars upon petiole chilling, and leaf water potential became slightly more negative. After 14 h recovery in the dark, photosynthesis returned to its initial maximum value within 1 h of illumination, and this was associated with a decline in leaf carbohydrate levels overnight. These data show that, in Amaranthus edulis, depression in photosynthesis when translocation is impaired is closely related to the accumulation of soluble sugars (sucrose) in source leaves, indicating feedback control of C₄ photosynthesis. Possible mechanisms by which sucrose accumulation in the leaf may affect the rate of photosynthesis are discussed with regard to the leaf anatomy of C₄ plants.Abbreviations and symbols A net CO₂ assimilation rate - Ci intercellular CO₂ partial pressure - PEP phosphoenolpyruvate - RuBP ribulose-1,5-bisphosphate - water potential - osmotic pressure